Abstract

Severe sepsis, defined as sepsis with acute organ dysfunction, is associated with high morbidity and mortality rates. The development of novel therapies for sepsis is critically dependent on an understanding of the basic mechanisms of the disease. The pathophysiology of severe sepsis involves a highly complex, integrated response that includes the activation of a number of cell types, inflammatory mediators, and the hemostatic system. Central to this process is an alteration of endothelial cell function. The goals of this article are to (1) provide an overview of sepsis and its complications, (2) discuss the role of the endothelium in orchestrating the host response in sepsis, and (3) emphasize the potential value of the endothelium as a target for sepsis therapy.

Introduction

Sepsis is the most common cause of death among hospitalized patients in noncoronary intensive care units. Thus, an important goal in critical care medicine is to develop novel therapeutic strategies that will impact favorably on patient outcome. Unfortunately, the pathophysiology of severe sepsis remains poorly defined. While it is generally accepted that sepsis-associated mortality is related to the host response and involves a multitude of cell types, inflammatory mediators, and coagulation factors, clinical studies have largely failed to identify an effective therapeutic target. Future advances in sepsis therapy will require a better understanding of how the individual components of the host response interact. The endothelium plays a critical role in mediating the sepsis phenotype. This article provides an overview of sepsis and its complications, discusses the role of the endothelium in orchestrating the host response in sepsis, and emphasizes the potential value of the endothelium as a target for sepsis therapy.

Overview of the sepsis continuum

Definition

Sepsis and its sequelae represent a continuum in clinical-pathologic severity. However, it is one with definable phases that characterize patients at risk for increased mortality.1 The American College of Chest Physicians and the Society of Critical Care Medicine established a set of definitions to facilitate early detection and treatment of sepsis and to standardize patient requirements for research protocols.2Infection is defined as an inflammatory response to microorganisms or the invasion of normally sterile host tissue by those organisms. Sepsis represents the systemic inflammatory response to infection and is manifested by 2 or more of the systemic inflammatory response syndrome (SIRS) criteria (eg, changes in body temperature, tachycardia, tachypnea and/or hyocapnia, and changes in the number and/or immaturity of white blood cells). Severe sepsis is sepsis complicated by organ dysfunction. Septic shock (hypotension despite adequate fluid resuscitation) is a subcategory of severe sepsis. At the end of the spectrum is multiple organ dysfunction syndrome (MODS), defined as the presence of altered organ function in an acutely ill patient such that homeostasis cannot be maintained without intervention.

Epidemiology

In 1990, the Centers for Disease Control and Prevention reported an estimate of 450 000 cases of septicemia per year in the United States, with over 100 000 deaths.3 Recently, Angus et al estimated that 750 000 cases of severe sepsis occur per year, with a mortality rate of 28.6%.4 Other studies suggest that 28-day mortality rates of severe sepsis may be 50% or greater.5 According to Angus et al, the mortality rate due to severe sepsis represented 9.3% of all deaths in the United States in 1995.4 The average cost per case was $22 100, with annual costs of $16.7 billion nationally. Importantly, the incidence of severe sepsis was projected to increase by 1.5% per year.4 Taken together, these data suggest that severe sepsis is a serious public health concern.

Clinical manifestations

The most common clinical findings in sepsis are related to SIRS (eg, fever, tachycardia, tachypnea, and leukocytosis) and organ dysfunction (eg, acute lung injury, acute respiratory distress syndrome, shock). Laboratory markers of inflammation include high circulating levels of interleukin 6 (IL-6), IL-8, and tumor necrosis factor alpha (TNF-α).6-8 Activation of the coagulation cascade is most often manifested by increased D-dimer levels (≈ 100% patients) and decreased levels of circulating protein C (≥ 90% patients).9-11 In contrast, less than half of patients with sepsis meet the definition of disseminated intravascular coagulation (DIC),1,12,13 a syndrome that is characterized by widespread activation of coagulation, fibrin deposition, and thrombotic occlusions and/or bleeding.14,15 

Pathophysiology

There are several important themes in sepsis pathophysiology. First, it is the host response, rather than the nature of the pathogen, that primarily determines patient outcome. Second, monocytes and endothelial cells play a central role in initiating and perpetuating the host response. Third, sepsis is associated with the concomitant activation of the inflammatory and coagulation cascades. Finally, in a concerted effort to fend off and eliminate pathogens, the host response may inflict collateral damage on normal tissues, resulting in pathology that is not diffuse, but rather remarkably focal in its distribution. Each of these themes will be discussed in turn.

The importance of the host response.

Several findings point to the importance of host factors in determining outcomes in patients with severe sepsis. First, despite the prompt implementation of appropriate antibiotic therapy, sepsis mortality remains high, in the range of 28% to 50%. Second, patients with culture-positive and culture-negative sepsis or septic shock have comparable mortality rates.1,5 Third, administration of anti–endotoxin antibodies in large, clinical trials did not improve survival.16,17 Last, there is a direct correlation between the number of SIRS criteria and mortality rate, and there is a stepwise increase in mortality rates in the spectrum of SIRS, sepsis, severe sepsis, and septic shock.1 Clearly, the success of future therapies will rely on the ability to adequately target the host response.

Role of the monocyte and endothelial cell in mediating the host response.

Monocytes, tissue macrophages, other myeloid-derived cells, and to some extent endothelial cells, are the cornerstones of the innate immune response. As a first line of defense, these cells recognize invading pathogens through pattern recognition receptors that interact with conserved microbial structures.18-25 The interaction between pathogens and host cells results in the initiation of inflammatory and coagulation cascades (Figure1). These pathways yield soluble mediators that function in autocrine or paracrine loops to further activate the monocyte/tissue macrophage and/or endothelium.

Fig. 1.

The role of the monocyte and endothelium in mediating the host response to infection.

LPS and/or other pathogen-associated properties activate pathogen recognition receptors (or toll-like receptors) on monocytes, tissue macrophages, and endothelial cells, leading to the release of inflammatory mediators and tissue factor (with subsequent activation of coagulation). Together with products of the contact system (eg, kinins) and complement cascade (eg, C5a) (not shown), inflammatory mediators function in autocrine and paracrine loops to further activate the monocyte and local endothelium (dotted line, left, shows paracrine pathway). The various components of the coagulation cascade serve not only to activate their downstream substrate (leading to fibrin formation) but also to trigger protease-activated receptors on the surface of a variety of cell types, including the endothelium (broken line, right). The combined effects of LPS, inflammatory mediators, and serine proteases on the endothelium may result in significant phenotypic modulation (not shown). CAM indicates cell adhesion molecules; PAF, platelet activating factor; NO, nitric oxide; ROS, reactive oxygen species; MIP-2, macrophage inflammatory protein 2.

Fig. 1.

The role of the monocyte and endothelium in mediating the host response to infection.

LPS and/or other pathogen-associated properties activate pathogen recognition receptors (or toll-like receptors) on monocytes, tissue macrophages, and endothelial cells, leading to the release of inflammatory mediators and tissue factor (with subsequent activation of coagulation). Together with products of the contact system (eg, kinins) and complement cascade (eg, C5a) (not shown), inflammatory mediators function in autocrine and paracrine loops to further activate the monocyte and local endothelium (dotted line, left, shows paracrine pathway). The various components of the coagulation cascade serve not only to activate their downstream substrate (leading to fibrin formation) but also to trigger protease-activated receptors on the surface of a variety of cell types, including the endothelium (broken line, right). The combined effects of LPS, inflammatory mediators, and serine proteases on the endothelium may result in significant phenotypic modulation (not shown). CAM indicates cell adhesion molecules; PAF, platelet activating factor; NO, nitric oxide; ROS, reactive oxygen species; MIP-2, macrophage inflammatory protein 2.

Activation of the inflammatory and coagulation pathways.

It is widely accepted that the inflammatory response plays an important role in mediating the sepsis phenotype. Pathogens promote the early activation of the contact system (factor XII, prekallikrein, and high-molecular-weight kininogen) and the complement cascade, and induce the rapid release of inflammatory mediators from a number of cell types (eg, monocytes and endothelial cells), changes that correspond to the clinical designation of SIRS. Simultaneously, endogenous antiinflammatory pathways are activated, which serve to dampen the inflammatory response.26-30 The latter process has been termed the compensatory anti-inflammatory response syndrome.27 Ideally, these 2 phases are coordinated to defend the host against invasion by pathogens. However, an excessive or sustained inflammatory response, an inadequate anti-inflammatory response, or perhaps an uncoupling of these 2 phases may contribute to tissue damage and death.

Besides activating the inflammatory system, pathogens also trigger the clotting cascade.31 During sepsis, tissue factor (TF) expression on the surface of circulating monocytes and tissue macrophages is up-regulated, resulting in activation of the extrinsic pathway, thrombin generation, and fibrin formation. Fibrin not only stabilizes platelet plugs, but may also play an important role in immobilizing pathogens on the surface of the leukocyte, facilitating their engulfment and disposal. Blood coagulation is initiated through the extrinsic pathway and is amplified through the intrinsic pathway by mechanisms that involve cross-talk and feedback.31-35 The clotting cascade is composed of a series of linked reactions in which a serine protease, once activated, is free to activate its downstream substrate. These reactions occur on activated phospholipid membranes and in some cases are accelerated by the presence of cofactors (factors VIIIa and Va). For every procoagulant response there is a natural anticoagulant reaction. Tissue factor pathway inhibitor (TFPI) controls the extrinsic pathway,36 antithrombin III (ATIII)–heparan neutralizes the serine proteases in the cascade,37 the thrombomodulin (TM)/protein C/protein S mechanism inactivates cofactors Va and VIIIa,38 and plasmin degrades preformed fibrin. Hemostasis represents a finely tuned balance between procoagulant and anticoagulant forces. Not only is there activation of the extrinsic pathway in sepsis, but there is also an attenuation of natural anticoagulant responses (eg, reduction in circulating levels of protein C and ATIII, decreased expression of TM on the surface of endothelial cells, impaired fibrinolysis).31,39-42 The resulting shift toward a procoagulant state results in excessive thrombin generation, fibrin formation, and consumption of clotting factors.

Once activated, the inflammatory and coagulation pathways interact with one another to further amplify the host response (Figure 1). For example, inflammatory mediators induce the expression of TF on the surface of circulating monocytes, tissue macrophages, neutrophils, and possibly some subsets of endothelial cells.43-49Conversely, serine proteases are capable of interacting with protease-activated receptors on the surfaces of monocytes and endothelial cells, leading to activation and additional inflammation.50,51 For example, thrombin signaling in endothelial cells results in changes in cell shape,52 cell permeability,53 proliferative response,54 and leukocyte adhesion.55-58 The latter changes are mediated in large part by the ability of thrombin to induce the expression of E-selectin,59 P-selectin,55,57 intercellular adhesion molecule 1 (ICAM-1),56,58 and vascular cell adhesion molecule 1 (VCAM-1).58,60 In addition, thrombin signaling in endothelial cells has been shown to induce the secretion of von Willebrand factor (VWF),61 increase the expression of protease-activated receptor 1 (PAR-1) mRNA,62 and stimulate the release of soluble mediators, including platelet-activating factor (PAF),63IL-8,59,64 monocyte chemoattractant protein 1 (MCP-1),65 growth factors, and matrix metalloproteinases.66 TF/VIIa complex and factor Xa may also bind to protease-activated receptors and trigger a proinflammatory response.67-70 Finally, fibrin(ogen) has been shown to interact with endothelial cells, leading to a number of phenotypic changes including increased expression of IL-8.71,72 The cross-talk between inflammatory and coagulation pathways contributes to the potentially explosive host response to sepsis.

Focal expression of sepsis phenotype.

A consistent feature of the pathologic lesions in severe sepsis and MODS is the focal nature of their distribution. Typically, patients only develop dysfunction in a limited number of organs. The endothelium is an important determinant of the focal response in sepsis. As discussed below, the endothelium displays remarkable heterogeneity in health and disease states, integrating systemic changes in inflammation and coagulation in ways that differ from one organ to the next.

Role of the endothelium in orchestrating the host response in sepsis

Endothelial cell activation and dysfunction

The endothelium is a truly pervasive organ; the human body contains approximately 1013 endothelial cells, weighing 1 kg and covering a surface area of 4000 m2 to 7000 m2.73 Among other functions, the endothelium mediates vasomotor tone, regulates cellular and nutrient trafficking, maintains blood fluidity, contributes to the local balance in proinflammatory and anti-inflammatory mediators, participates in generation of new blood vessels, and undergoes programmed cell death.74-76 Importantly, each of these activities is differentially regulated in space and time (a phenomenon that has been variably termed endothelial cell heterogeneity or vascular diversity).31,75,77-79 

Under normal conditions, endothelial cells are highly active, constantly sensing and responding to alterations in the local extracellular environment, as might occur in the setting of transient bacteremia, minor trauma, and other common daily stresses. In other words, endothelial cell activation occurs as a normal adaptive response, the nature and duration of which depends not only on the type of stimulus, but also on the spatial and temporal dynamics of the system.80 For example, at any given time, the endothelial cells of a vein and artery may have distinct response patterns to a common systemic signal, while at any given site, the response will vary from one moment to the next, according to health and state of the whole organism. Therefore, endothelial cell activation is not an all-or-nothing response, nor is it necessarily linked to disease. Instead, endothelial cell activation represents a spectrum of response and occurs under both physiologic and pathophysiologic conditions.

Any response of the endothelium that benefits the host may be deemed functional, physiologic, or adaptive. For example, when pathogens invade a tissue, endothelial cells are induced locally to release inflammatory mediators, to recruit leukocytes, and to promote clotting as a means of walling off the infection. During this process, endothelial cells may undergo necrosis or apoptosis as tissue is reabsorbed and repaired. When viewed at the level of the single cell, necrosis and/or apoptosis are the ultimate expression of dysfunction. However, when considered in the larger context of host defense, the local loss of endothelium is part of a larger coordinated, adaptive response. Perhaps a fitting analogy is group altruism or group selection, an evolutionary mechanism of cooperation in animals, in which group-level positive effects outweigh the individual-level negative effects. The term endothelial cell dysfunction is better reserved for cases in which the endothelial cell response, whether local or systemic, represents a net liability to the host. For example, in severe sepsis, there is an excessive, sustained, and generalized activation of the endothelium. Without artificial organ support, virtually all patients with severe sepsis would die from their disease. In other words, most of these individuals have crossed the threshold from an adaptive to a maladaptive response. In so far as the endothelium contributes to the severe sepsis phenotype, its behavior may be characterized as dysfunctional.

Endothelial response in severe sepsis

Sepsis may induce phenotypic modulation of the endothelium by a number of different mechanisms. In some cases, pathogens directly infect intact endothelial cells.81 More commonly, components of the bacterial wall (eg, lipopolysaccharide [LPS]) activate pattern recognition receptors on the surface of the endothelium.22-25 Finally, a myriad of host-derived factors activate endothelial cells, including complement, cytokines, chemokines, serine proteases, fibrin, activated platelets and leukocytes, hyperglycemia, and/or changes in oxygenation or blood flow (see Table 1 and Figure 2 for an expanded list of host-derived mediators).

Table 1.

Treatment according to target(s)

Therapies that target the host-pathogen interaction Reference no. 
 Antibiotics* 221   
 Anti-endotoxin antibodies 16, 17, 222, 223  
 Toll-like receptor or CD14 antagonists 224  
Mediator-specific therapies that target inflammation  
 Anti–tumor necrosis factor antibodies 225, 226  
 Platelet-activating factor acetylhydrolase 227 
 Bradykinin antagonists 228, 229  
 Anti–factor XII antibodies 230  
 Prostaglandin antagonists 228, 231, 232 
 Anti–high mobility group 1 antibodies 233-235  
 Anti-C5a antibodies 236, 237  
 Granulocyte-macrophage colony-stimulating factor inhibition 238, 239  
 Macrophage inhibitory factor inhibition 240, 241  
 Anti-inflammatory agents (eg, interleukin 10) 242-244  
 Receptor antagonists  
  Interleukin-1r antagonist 12, 245  
  Soluble tumor necrosis factor receptor 13  
  Platelet-activating factor receptor antagonist 246, 247  
  C5a receptor antagonists 248 
  Protease-activated receptor antagonists 249, 250 
Mediator nonspecific therapies that target inflammation  
 CI-esterase inhibitor 49, 251, 252 
 Antioxidants 253, 254  
 Glucocorticoids1-153 148, 255-257 
 Activated protein C1-153 10, 11  
 Tissue factor pathway inhibitor 161, 258, 290  
 Antithrombin III 145, 259 
Therapies that target cell-cell interactions  
 Antiplatelet agents 260-262  
 Antiadhesion strategies 153, 263 
 
Therapies that target coagulation        
 
 Antithrombin agents  
  Heparin 157 
  Hirudin 45  
  Antithrombin III 145, 259 
 Inhibition of extrinsic pathway  
  Anti–tissue factor antibodies 264  
  Factor VII inhibition 162, 265 
  Tissue factor pathway inhibitor 161, 258, 290 
 Inhibition of cofactor activity  
  Activated protein C1-153 10, 11  
 Enhanced fibrinolysis  
  Tissue plasminogen activator 266  
  Activated protein C1-153 10, 11 
Miscellaneous targets, amenable to inhibition  
 Apoptosis 139  
 Transcription factors (eg, NF-κB) 188, 267  
 Ubiquitin-proteasome pathway 189 
 Mechanical stretch, barotrauma1-153 147, 268  
 Fever 269 
 Hyperglycemia1-153 149, 270  
 Elastase 271, 272  
 Poly (ADP-ribose) synthetase (PARS) 273  
 Poly (ADP-ribose) polymerase 1 274, 275  
 Nitric oxide 209, 210, 213  
 P38 MAPK 196, 198  
 PKCδ, ζ 199, 276  
Miscellaneous targets, amenable to enhancement  
 Oxygenation 173, 277-280 
 Blood flow and hemodynamic forces1-153 150, 281-284 
 Phospholipid oxidation products 142  
 Soluble lectin-like domain of TM 285  
 VEGF/Angiopoietin balance 286 
Therapies that target the host-pathogen interaction Reference no. 
 Antibiotics* 221   
 Anti-endotoxin antibodies 16, 17, 222, 223  
 Toll-like receptor or CD14 antagonists 224  
Mediator-specific therapies that target inflammation  
 Anti–tumor necrosis factor antibodies 225, 226  
 Platelet-activating factor acetylhydrolase 227 
 Bradykinin antagonists 228, 229  
 Anti–factor XII antibodies 230  
 Prostaglandin antagonists 228, 231, 232 
 Anti–high mobility group 1 antibodies 233-235  
 Anti-C5a antibodies 236, 237  
 Granulocyte-macrophage colony-stimulating factor inhibition 238, 239  
 Macrophage inhibitory factor inhibition 240, 241  
 Anti-inflammatory agents (eg, interleukin 10) 242-244  
 Receptor antagonists  
  Interleukin-1r antagonist 12, 245  
  Soluble tumor necrosis factor receptor 13  
  Platelet-activating factor receptor antagonist 246, 247  
  C5a receptor antagonists 248 
  Protease-activated receptor antagonists 249, 250 
Mediator nonspecific therapies that target inflammation  
 CI-esterase inhibitor 49, 251, 252 
 Antioxidants 253, 254  
 Glucocorticoids1-153 148, 255-257 
 Activated protein C1-153 10, 11  
 Tissue factor pathway inhibitor 161, 258, 290  
 Antithrombin III 145, 259 
Therapies that target cell-cell interactions  
 Antiplatelet agents 260-262  
 Antiadhesion strategies 153, 263 
 
Therapies that target coagulation        
 
 Antithrombin agents  
  Heparin 157 
  Hirudin 45  
  Antithrombin III 145, 259 
 Inhibition of extrinsic pathway  
  Anti–tissue factor antibodies 264  
  Factor VII inhibition 162, 265 
  Tissue factor pathway inhibitor 161, 258, 290 
 Inhibition of cofactor activity  
  Activated protein C1-153 10, 11  
 Enhanced fibrinolysis  
  Tissue plasminogen activator 266  
  Activated protein C1-153 10, 11 
Miscellaneous targets, amenable to inhibition  
 Apoptosis 139  
 Transcription factors (eg, NF-κB) 188, 267  
 Ubiquitin-proteasome pathway 189 
 Mechanical stretch, barotrauma1-153 147, 268  
 Fever 269 
 Hyperglycemia1-153 149, 270  
 Elastase 271, 272  
 Poly (ADP-ribose) synthetase (PARS) 273  
 Poly (ADP-ribose) polymerase 1 274, 275  
 Nitric oxide 209, 210, 213  
 P38 MAPK 196, 198  
 PKCδ, ζ 199, 276  
Miscellaneous targets, amenable to enhancement  
 Oxygenation 173, 277-280 
 Blood flow and hemodynamic forces1-153 150, 281-284 
 Phospholipid oxidation products 142  
 Soluble lectin-like domain of TM 285  
 VEGF/Angiopoietin balance 286 

References are not all-inclusive, but rather draw on a selection of basic, preclinical, early clinical and/or phase 3 clinical studies, as well as selected reviews.

*

Rapid institution of appropriate antibiotic therapy remains a mainstay in therapy of patients with severe sepsis.

Phase 3 clinical trials have been conducted and demonstrated no survival benefit.

The major physiologic role of Factor XII is not to mediate coagulation activation, but rather to increase the rate and extent of prekallikrein activation, resulting in generation of bradykinin, increased profibrinolytic activity and inhibition of thrombin-mediated platelet activation.287-289 

F1-153

Phase 3 clinical trials have been conducted and shown to improve survival. The degree to which treatment-related attenuation of endothelial cell activation contributed to overall benefit is unknown.

Fig. 2.

The endothelium as a therapeutic target.

An understanding of the endothelial response to pathogens provides a foundation for therapeutic design. For purposes of illustration and discussion, the temporal sequence of events is depicted from left to right. In sepsis, the endothelium is activated by LPS-mediated engagement of the toll-like receptor (TLR4) or by the interaction of inflammatory mediators (IL-6, TNF-α, IL-1, kinins, and C5a are shown) with their respective receptors (drawn as a single representative receptor). At the same time (or later during the sepsis cascade), the endothelium may be conditioned by other environmental factors, such as hypoxia, low blood flow, changes in temperature, acid-base/electrolyte abnormalities, and/or hyperglycemia. The interaction of extracellular mediators with their receptors leads to activation of downstream signaling pathways (including MAPK and PKC), which in turn promote posttranscriptional changes in cell function or alter gene expression profiles through a number of transcription factors, including NF-κB, GATA-2, and AP-1. The up-regulation of cell adhesion molecules on the surface of the endothelium (P-selectin, E-selectin, VCAM-1, and ICAM-1 are shown) promotes increased adhesion, rolling, and transmigration of circulating leukocytes. Leukocyte-endothelial interactions further modulate the phenotype of these cells. The release of cytokines from the endothelium results in additional activation of monocytes and endothelial cells. Increased expression of procoagulants (eg, TF) and/or reduced expression of anticoagulants (eg, TM, EPCR) promote increased thrombin generation and fibrin formation. Various components of the coagulation pathway (including serine proteases, fibrin, and platelets) may signal directly in the endothelium through protease-activated receptors (PAR-1 is shown). Changes in the expression of proapoptotic and antiapoptotic genes (along with a multitude of posttranscriptional changes) may result in a shift in balance toward programmed cell death. During the process of activation, NADPH oxidase may induce the formation of reactive oxygen species (ROS), nitric oxide (NO) is released, and cell permeability is increased. In keeping with the theme of spatial and temporal dynamics, the relative activity of the various pathways will vary between different endothelial cells and from one moment to the next. Not shown are the critical interactions between the endothelium and underlying extracellular matrix and parenchymal cells. Temp indicates temperature; ICAM-1, intracellular adhesion molecule 1; VCAM-1, vascular cell adhesion molecule; EC, endothelial cell; TF, tissue factor; TM, thrombomodulin; EPCR, endothelial protein C receptor; NO, nitric oxide; PGI2, prostacyclin. Receptors are labeled in light font.

Fig. 2.

The endothelium as a therapeutic target.

An understanding of the endothelial response to pathogens provides a foundation for therapeutic design. For purposes of illustration and discussion, the temporal sequence of events is depicted from left to right. In sepsis, the endothelium is activated by LPS-mediated engagement of the toll-like receptor (TLR4) or by the interaction of inflammatory mediators (IL-6, TNF-α, IL-1, kinins, and C5a are shown) with their respective receptors (drawn as a single representative receptor). At the same time (or later during the sepsis cascade), the endothelium may be conditioned by other environmental factors, such as hypoxia, low blood flow, changes in temperature, acid-base/electrolyte abnormalities, and/or hyperglycemia. The interaction of extracellular mediators with their receptors leads to activation of downstream signaling pathways (including MAPK and PKC), which in turn promote posttranscriptional changes in cell function or alter gene expression profiles through a number of transcription factors, including NF-κB, GATA-2, and AP-1. The up-regulation of cell adhesion molecules on the surface of the endothelium (P-selectin, E-selectin, VCAM-1, and ICAM-1 are shown) promotes increased adhesion, rolling, and transmigration of circulating leukocytes. Leukocyte-endothelial interactions further modulate the phenotype of these cells. The release of cytokines from the endothelium results in additional activation of monocytes and endothelial cells. Increased expression of procoagulants (eg, TF) and/or reduced expression of anticoagulants (eg, TM, EPCR) promote increased thrombin generation and fibrin formation. Various components of the coagulation pathway (including serine proteases, fibrin, and platelets) may signal directly in the endothelium through protease-activated receptors (PAR-1 is shown). Changes in the expression of proapoptotic and antiapoptotic genes (along with a multitude of posttranscriptional changes) may result in a shift in balance toward programmed cell death. During the process of activation, NADPH oxidase may induce the formation of reactive oxygen species (ROS), nitric oxide (NO) is released, and cell permeability is increased. In keeping with the theme of spatial and temporal dynamics, the relative activity of the various pathways will vary between different endothelial cells and from one moment to the next. Not shown are the critical interactions between the endothelium and underlying extracellular matrix and parenchymal cells. Temp indicates temperature; ICAM-1, intracellular adhesion molecule 1; VCAM-1, vascular cell adhesion molecule; EC, endothelial cell; TF, tissue factor; TM, thrombomodulin; EPCR, endothelial protein C receptor; NO, nitric oxide; PGI2, prostacyclin. Receptors are labeled in light font.

The endothelium responds in ways that differ according to the nature of the pathogen, host genetics, underlying comorbidity, age, gender, and the location of the vascular bed.82-91 Endothelial cells may undergo structural changes, including nuclear vacuolization, cytoplasmic swelling, cytoplasmic fragmentation, denudation, and/or detachment.92 Functional changes are more common and include shifts in the hemostatic balance, increased cell adhesion and leukocyte trafficking, altered vasomotor tone, loss of barrier function, and programmed cell death.

Procoagulant properties.

Inflammatory mediators may interact with endothelial cells to induce a net procoagulant phenotype. Under in vitro conditions, the addition of LPS and/or cytokines to endothelial cells has been shown to decrease synthesis of TM, tissue-type plasminogen activator and heparan, to increase expression of TF and plasminogen activator inhibitor 1 (PAI-1), and to generate procoagulant microparticles.76,93-96 The extent to which these changes occur in the intact endothelium is not entirely clear. In a recent study of patients with meningococcemia, TM levels were reduced in dermal microvessels, an effect that would be predicted to yield decreased levels of activated protein C.42 In a mouse model of endotoxemia, the administration of LPS resulted in reduction in total tissue TM antigen in the lung and brain, but not in the kidney,41 suggesting that sepsis-associated changes in TM expression may vary between organs. While sepsis is associated with increased levels of PAI-1,84,97 an endothelial source of PAI-1 has not been established. With few exceptions,46,47sepsis studies have consistently failed to demonstrate TF in the intact endothelium.

When the endothelium is viewed in the context of its native environment, additional properties emerge that may contribute to a procoagulant state. For example, activated endothelial cells attract platelets, monocytes, and neutrophils—cells that are capable of initiating or amplifying coagulation. Endothelial activation may result in translocation of cell surface phospholipids that enhance binding of coagulation complexes. Endothelial cells undergoing apoptosis may express an increasingly procoagulant phenotype.98 The development of a low blood-flow state in sepsis, whether secondary to reduced cardiac output, vasoconstriction, or occlusive lesions, may reduce clearance of activated serine proteases, thus promoting additional clotting.

As with other properties of the endothelium, the hemostatic balance is differentially regulated between vascular beds.31,75,77,99In a mouse model of endotoxemia, the systemic administration of LPS resulted in organ-specific deposition of fibrin in the kidney and adrenal gland.100 In another study, LPS administration resulted in detectable levels of fibrin in the lung, but not the brain, of wild-type mice.41 Still others have shown that LPS injection in wild-type mice yields increased fibrin levels in the kidney, liver, and myocardium, but not the lung.101 In a baboon sepsis model, the administration of lethal doses of E coli resulted in increased fibrin deposition in the marginal zone and sinusoids of the spleen, the hepatic sinusoids, the glomeruli, and peritubular vessels of the kidney, but little or no fibrin in the portal vessels of the liver, cerebral cortex, skin, myocardium, or aorta.47 The discrepant patterns of fibrin deposition in the above studies may be related to differences in the species/strain being analyzed, the type of sepsis model, and/or the nature of the fibrin assays. Nevertheless, when taken together, the data are consistent in demonstrating an association between sepsis and organ-specific coagulation.

In genetic mouse models of hypercoagulability, sepsis results in an accentuated shift in the hemostatic balance. For example, in mice that carry a TM gene mutation that disrupts TM-dependent activation of protein C, LPS administration resulted in higher levels of fibrin deposition in the lung and kidney but not the brain, compared with wild-type mice.41 In heterozygous ATIII-deficient mice, LPS challenge resulted in increased deposition of fibrin in the kidney, liver, and heart.101 These studies demonstrate the importance of underlying genetics in modulating the sepsis phenotype.

Proadhesive properties.

The endothelium responds to inflammatory mediators by expressing adhesion molecules on the cell surface, including P-selectin, E-selectin, ICAM-1, and VCAM-1. Collectively, these alterations result in increased rolling, strong adherence, and transmigration of leukocytes into underlying tissue. These changes are not universal, but rather occur locally in certain organs and segments of the vascular loop.102-108 Activated endothelial cells also recruit increased numbers of platelets to the blood vessel wall.109-113 The importance of adhesion molecules in mediating the sepsis phenotype is supported by studies in knock-out mice.114-116 

Vasomotor properties.

Vasomotor tone is regulated by a combination of endothelial-dependent and endothelial-independent mechanisms. Endothelial cells produce vasoactive molecules that regulate arteriolar tone and contribute to blood pressure control. These include the vasodilators (nitric oxide [NO] and prostacyclin) and the vasoconstrictors (endothelin, thromboxane A2, and platelet-activating factor).117 In sepsis, activated endothelium undergoes site-specific changes that impact the net balance of vasoconstrictor and vasodilatory properties.118 

Increased permeability.

In the intact vasculature, the endothelium forms a continuous, semipermeable barrier that varies in integrity and control for different vascular beds.119 A central feature of the endothelium in sepsis is an increased permeability or loss of barrier function, resulting in a shift of circulating elements and tissue edema. TNF-α induces an increase in endothelial cell permeability both in vitro and in vivo.120-123 Under in vitro conditions, thrombin also increases endothelial cell permeability, while TNF-α and thrombin act synergistically to induce barrier dysfunction in vitro.124 Redistribution of fluid from the intravascular to the extravascular compartment may contribute to hypovolemia, hemoconcentration, and stasis of blood flow.

Endothelial cell apoptosis.

Endothelial cell apoptosis is a highly regulated process.125 Normally, only a small percentage (< 0.1%) of endothelial cells are apoptotic. Under in vitro conditions, certain pathogens are capable of inducing endothelial cell apoptosis.126 The incubation of cultured endothelial cells with LPS has been reported to induce apoptosis in some, but not all, studies.126-129 LPS has been shown to up-regulate the Bcl-2 homologue, A1, and the zinc finger protein, A20, in cultured endothelial cells.130 The sepsis cascade involves a large number of other mediators that may induce endothelial cell apoptosis, including TNF-α, IL-1, interferon, oxygen free radicals, and hypoxia.125,129,131 The interaction between circulating cells and the endothelium may further augment proapoptotic signaling. For example, LPS-activated monocytes promote programmed cell death in endothelial cells by a combination of TNF-α–dependent and –independent mechanisms.132 

Endothelial cell apoptosis results in an accentuated proinflammatory response. For example, under in vitro conditions, apoptotic endothelial cells mediate IL-1–dependent paracrine induction of ICAM-1 and VCAM-1,133 increased production of reactive oxygen species (ROS), increased procoagulant activity,98 decreased production of prostacyclin,134 and activation of complement.135 In addition, endothelial cells undergoing apoptosis demonstrate increased binding to nonactivated platelets.136 

In a mouse model of endotoxemia, intraperitoneally delivered LPS resulted in widespread apoptosis of the endothelium.137 In other studies, the intravenous administration of LPS in mice was shown to induce endothelial cell apoptosis in the lung, but not the liver, pointing to organ-specific differences in programmed cell death.138,139 

Local versus systemic activation of the endothelium

The innate host response evolved as a locally operative mechanism to eradicate pathogens and necrotic tissue.140The endothelium orchestrates the local response by promoting the adhesion and transmigration of leukocytes, inducing thrombin generation and fibrin formation, altering local vasomotor tone, increasing permeability, and triggering programmed cell death.118 The activation of coagulation serves a number of potential roles, including the walling off of pathogens, the activation of protease-activated receptors, and the extravascular stimulation of macrophage chemokine expression.141 Normally, local and systemic negative feedback mechanisms are activated, dampening the response at distal sites.140,142 Compartmentalization of the innate immune response limits collateral damage to the host and preserves integrity and adaptability of uninvolved endothelium. Hence, the endothelium as a whole is not locked into a single response but remains poised to deal with other insults. When the host response generalizes, it escapes the well-developed local checks and balances and results in a dysregulated, undirected inflammatory response. Under these conditions, widespread involvement of endothelium and monocytes/tissue macrophages, together with the more generalized activation of inflammation and coagulation, may lead to SIRS and MODS.

Link between endothelial cell dysfunction and MODS

Despite an increasing appreciation that inflammatory and coagulation cascades are activated in severe sepsis, little is known about the mechanisms that ultimately lead to organ dysfunction and death. The inflammatory and coagulation pathways and the various cell types are so tightly coupled that they cannot and should not be viewed as discrete entities in severe sepsis. Activation of the inflammatory cascade impacts the coagulation pathway, and vice versa. Activated monocytes affect the endothelium, and the reverse is also true. Dysfunction of any one organ has a downstream effect on all other organs. Therefore, the host response to sepsis is highly integrated, and the whole is far greater than the sum of its constituent parts (Figure 3).

Fig. 3.

The complexity of the host response to infection.

The host response to infection involves a wide array of cells and soluble mediators, which include but are not limited to monocytes, endothelial cells, and platelets and components of the complement, inflammatory, and coagulation cascades. Rather than showing the detailed connections, this figure is intended to convey the interdependent, redundant, and pleiotropic nature of the host response. Several factors modulate the phenotype, including the type of pathogen, and host factors such as genetic make-up, age, gender, and the health of other organ systems (liver and kidney are shown as examples). Normally, the host mechanisms are highly coordinated in both space and time to defend the host against pathogens. However, when the response is disproportionate to the threat (eg, excessive, sustained, or poorly localized), then the balance of power shifts in favor of the pathogen, resulting in the sepsis phenotype. Given the highly integrated and nonlinear nature of this response, it will be difficult to identify a single component whose therapeutic modulation will short-circuit the sepsis cascade and improve outcome. As long as the complexity of the sepsis response remains outside our grasp the best hope for therapeutic advances will depend on broad base targeting, in which multiple components are targeted at the same time.

Fig. 3.

The complexity of the host response to infection.

The host response to infection involves a wide array of cells and soluble mediators, which include but are not limited to monocytes, endothelial cells, and platelets and components of the complement, inflammatory, and coagulation cascades. Rather than showing the detailed connections, this figure is intended to convey the interdependent, redundant, and pleiotropic nature of the host response. Several factors modulate the phenotype, including the type of pathogen, and host factors such as genetic make-up, age, gender, and the health of other organ systems (liver and kidney are shown as examples). Normally, the host mechanisms are highly coordinated in both space and time to defend the host against pathogens. However, when the response is disproportionate to the threat (eg, excessive, sustained, or poorly localized), then the balance of power shifts in favor of the pathogen, resulting in the sepsis phenotype. Given the highly integrated and nonlinear nature of this response, it will be difficult to identify a single component whose therapeutic modulation will short-circuit the sepsis cascade and improve outcome. As long as the complexity of the sepsis response remains outside our grasp the best hope for therapeutic advances will depend on broad base targeting, in which multiple components are targeted at the same time.

Based on these considerations, how can we fairly assess the endothelium's role in mediating the sepsis phenotype? Available evidence suggests that the function of the endothelium is altered in severe sepsis in ways that differ from one site of the vascular tree to another. These changes, while part of a larger, integrated host response, may help to initiate and perpetuate site-specific cycles of inflammation, coagulation, and cellular interactions that ultimately lead to microvascular occlusion, hypoxia, and organ dysfunction. To argue that the endothelium plays a more or less central role compared with the monocyte, or that inflammation is more or less important than the coagulation cascade in sepsis pathogenesis is misguided. Perhaps a more productive line of reasoning is as follows: the endothelium is a critical, but not the sole, component of the host response to sepsis; the endothelium is strategically located between blood and underlying tissue; the endothelium is a highly malleable and flexible cell layer; therefore, the endothelium is a potentially valuable target for sepsis therapy.

The endothelium as a therapeutic target

Therapeutic perspectives

Over the past decade, enormous resources have been expended on sepsis trials, with more than 10 000 patients enrolled in over 20 placebo-controlled, randomized phase 3 clinical trials.143,144 Most of these therapies have failed to reduce mortality in patients with severe sepsis, including antiendotoxin, anticytokine, antiprostaglandin, antibradykinin, and anti-PAF strategies, ATIII, and TFPI.143,145,146At the time of this writing, a total of 5 phase 3 clinical trials have demonstrated improved survival in critically ill patients or patients with severe sepsis. These include the use of low tidal volume ventilation,147 activated protein C,11low-dose glucocorticoids,148 intensive insulin therapy,149 and early goal-directed therapy.150 

Strategies for targeting the endothelium

In principle, there are 2 strategies for attenuating the endothelial response in sepsis. One is to target nonendothelial components of the host response, including soluble mediators or other cell types (eg, leukocytes, platelets), which negatively modulate endothelial cell function. The other is to target endothelial components (eg, cell surface receptors, signaling pathways, transcriptional networks, or endothelial cell gene products) that are involved in mediating the sepsis phenotype (Figure 2; Table 1). The targets that are listed in Table 1 are derived from a combination of basic and clinical studies. While a number of these therapies have reached phase 3 clinical trials, others are in preclinical or early-phase clinical stages. The extent to which these latter targets will translate into clinical efficacy remains to be seen.

Antimediator therapy.

Several efforts have been made to target LPS or inflammatory mediators that directly activate endothelial cells either at the level of the extracellular factor or its receptor. In large phase 3 clinical trials, the use of specific antimediator therapy has consistently failed to improve survival in patients with severe sepsis.146,151,152 

Antiadhesion therapy.

The interaction of circulating cells with the endothelium is likely to play an important role in the host response to infection. Several strategies have been used to inhibit leukocyte-endothelial cell interactions in animal models of sepsis, including the use of monoclonal antibodies.153 Moreover, nonactivated platelets roll on stimulated endothelium, in a process that involves P-selectin and E-selectin,154,155 suggesting that therapy aimed at these cell adhesion molecules may also attenuate platelet-endothelial interactions. Activated platelets have been shown to adhere to the endothelium through a GPIIbIIIa-dependent mechanism,156pointing to a potential role for GPIIbIIIa inhibitors in sepsis. At the present time, antiadhesion therapy in sepsis remains investigational.

Anticoagulant therapy.

Several natural anticoagulant molecules have been studied in nonhuman primate models of sepsis. Heparin and active-site-blocked factor Xa inhibited the activation of coagulation, but did not protect against organ dysfunction or mortality.157,158 These results suggest that activation of the coagulation cascade is not in and of itself sufficient to induce mortality in this syndrome. In contrast to agents that inhibit thrombin activity or thrombin generation, the administration of active-site-blocked factor VIIa, ATIII, activated protein C, or TFPI blocked activation of the coagulation and inflammatory pathways, reduced organ damage, and prevented lethality in a baboon model of sepsis.159-162 The anti-inflammatory effect of these agents is related, at least in part, to their ability to block protease-activated receptor–mediated signaling and/or to activate protective pathways within the endothelium.163-165 Together with the results of the failed anticytokine/antimediator trials, these data suggest, but by no means prove, that mortality in severe sepsis is linked to the combined activation of the coagulation and proinflammatory pathways.

The therapeutic potential of activated protein C was evidenced in the recent phase 3 Protein C Worldwide Evaluation in Severe Sepsis (PROWESS) trial, in which the administration of human recombinant activated protein C (drotrecogin alfa [activated]) to patients with severe sepsis resulted in reduced mortality.11 A total of 1690 patients with a diagnosis of severe sepsis were randomized to receive either drotrecogin alfa (activated) or placebo. There was a statistically significant reduction in 28-day all-cause mortality (24.7% vs 30.8% in the treatment and placebo groups, respectively,P < .005).11 The PROWESS trial is the first published clinical trial to demonstrate a survival benefit in patients with severe sepsis.

In contrast to the promising results of preclinical and early phase 1/2 studies, phase 3 clinical trials with ATIII or TFPI failed to demonstrate improved survival in patients with severe sepsis.145,290 One possible explanation for these findings relates to study design. For example, patients in the phase 3 clinical trials may have received suboptimal doses of ATIII and/or TFPI. Moreover, in the ATIII study, a potential benefit of the drug may have been obscured by the concomitant administration of heparin.166,167 An alternative explanation is that activated protein C has unique biologic effects that set it apart from ATIII and TFPI in humans with severe sepsis (as distinct from the baboon model of sepsis). Indeed, while TFPI and ATIII are likely to indirectly exert their anti-inflammatory effect through protease-activated receptors (to date, there is no evidence of an ATIII receptor), activated protein C binds to and activates a unique receptor, the endothelial protein C receptor (EPCR), which is expressed on the surface of endothelial cells and possibly monocytes. The interaction between activated protein C and its receptor has been implicated in its profound anti-inflammatory and antiapoptotic function (see “Note added in proof”).164 

Antiapoptosis therapy.

As discussed earlier, apoptosis may play a critical role in mediating the sepsis phenotype. Interestingly, inhibition of apoptosis represents a common thread in established sepsis therapies. For example, activated protein C has been shown to inhibit apoptosis in cultured endothelial cells by mechanisms that may include transcriptional down-regulation of the proapoptotic genes calreticulin and TRMP-2, and induction of the antiapoptotic genes A1 Bcl-2 homolog and inhibitor of apoptosis (IAP) homolog B.164 The maintenance of blood flow and hence shear stress may be an important inhibitor of endothelial cell apoptosis,168 and the benefits of early goal-directed therapy may reflect, at least in part, the protective effect of hemodynamics on endothelial cell function.150Hyperglycemia has been reported to promote endothelial cell apoptosis.169,170 Moreover, insulin promotes Akt-dependent endothelial cell survival.171 In light of these findings, it is interesting to speculate that intensive insulin therapy and tight blood glucose control in critically ill patients has a protective (prosurvival) effect on the endothelium.149 

Hypoxia has been shown to induce programmed cell death in endothelial cells, thus emphasizing the importance of maintaining adequate oxygenation.172,173 Other antiapoptotic strategies that may warrant consideration include statins,174antioxidants, growth factors,175 and caspase inhibitors.139 

Transcription factors as targets.

Several transcription factors in the endothelium have been implicated in the host response to infection, including NF-κB,25,176-179 epithelium-specific Ets factor-1 (ESE-1),180 activator protein-1 (AP-1),181-183 GATA-2,60,184,185 and Egr-1.186 In addition, LPS administration in rodents has been shown to down-regulate DNA-binding activity of Sp1 and AP-2.187 

Of the various transcription factors involved in mediating the sepsis phenotype, NF-κB has received the most attention as a potential therapeutic target. In a mouse model of endotoxemia, the intravenous somatic gene transfer with IκBα resulted in increased survival.177 In a rat model of sepsis, the systemic administration of pyrrolidine dithiocarbamate inhibited NF-κB–mediated gene expression of TNF-α, cyclooxygenase-2 (COX-2), and ICAM-1.188 More selective NF-κB inhibitors, such as the antibacterial peptide PR39, may hold greater promise.189 

ATIII and activated protein C have each been shown to inhibit NF-κB activation of endothelial cells.163,164 A recent study demonstrated that low-dose glucocorticoids reduce mortality in patients with severe sepsis.148 The beneficial effects of steroids may be related, in part, to an attenuation of NF-κB activity.184,190 

As an important caveat, NF-κB has been shown to attenuate TNF-α–mediated apoptosis of endothelial cells, perhaps through the induction of cytoprotective proteins such as IAPs, Bcl-2–like factors, and A20.191 Moreover, the selective blockade of NF-κB sensitized endothelial cells to the proapoptotic effects of TNF-α.192 These observations suggest that NF-κB may play a protective role during the sepsis continuum and underscore the need for caution in developing anti–NF-κB therapies.

Signaling pathways as targets.

The p38 mitogen-activated protein kinase (MAPK) signaling pathway is believed to play an important role in mediating proinflammatory responses and endothelial cell apoptosis.175,193,194 Mice that are null for MAPKAP kinase 2, a downstream p38 MAPK target, demonstrate increased resistance to LPS, an effect that is explained by reduced TNF-α production.195 Several studies have targeted p38 MAPK signaling in animal models, with mixed results.196,197 Interestingly, in a human model of endotoxemia, the oral administration of a new p38 MAPK inhibitor reduced cytokine production and leukocyte responses.198The extent to which the treatment impacted on endothelial cell dysfunction was not addressed.

Novel and atypical isoforms of protein kinase C (PKC) have also been implicated endothelial cell activation. Under in vitro conditions, thrombin-mediated induction of ICAM-1 in endothelial cells is dependent on a PKC-δ–NF-κB signaling pathway, whereas TNF-α–mediated stimulation of ICAM-1 involves PKC-ζ–NF-κB.56,199 Thrombin stimulation of VCAM-1 in endothelial cells is mediated by PKC-δ–NF-κB and PKC-ζ–GATA-2 signaling pathways.200 PKC-ζ has also been shown to mediate TNF-α stimulation of NADPH oxidase–derived ROS in endothelial cells.201 Compared with wild-type mice, LPS administration to PKC-ζ−/− mice resulted in significantly less NF-κB activation in the lung, but not the liver.202 These latter results suggest that the PKC-ζ isoform plays an important role in mediating the host response in select organs and may represent a valuable target for site-specific therapy in severe sepsis.

Nitric oxide synthase (NOS) inhibitors.

Sepsis is associated with increased inducible NOS (iNOS) activity and decreased endothelial NOS (eNOS) activity.203-205 However, the relative role of iNOS and eNOS in mediating the sepsis phenotype remains unclear. In genetic mouse models, the absence of iNOS or eNOS does not significantly alter the sepsis phenotype.206,207 Indeed, the chronic overexpression of eNOS in the endothelium of mice resulted in increased resistance to LPS-induced hypotension and death.208 In some studies, the use of NOS inhibitors yielded beneficial results,209-211 whereas other studies reported the opposite findings.212 In a rabbit model of sepsis, the administration of L-arginine, but not L-NAME (N(G)-nitro-L-arginine methyl ester), attenuated LPS-mediated endothelial cell injury.213 LPS-mediated induction of platelet-endothelial interactions in mice has been shown to be attenuated by NO donor and exacerbated by NOS inhibitor or eNOS deficiency, suggesting a beneficial effect of eNOS-derived NO.214 Further work is required before considering NOS inhibition therapy in sepsis.

Therapeutic challenges

Many reasons have been postulated to explain the long history of failed clinical trials in sepsis. These include inapplicability of results from animal models of sepsis, nonuniformity of supportive care, heterogeneity in patient populations, confounding effects of cointervention, inappropriate timing, and poor choice of outcome measures.143,146,151,152,166,215,216 An underemphasized explanation relates to the complexity of the host response. These themes are important to consider when approaching the endothelium as a therapeutic target.

Timing.

Sepsis represents a continuum in clinical and pathologic severity. In sepsis trials, the choice of inclusion and exclusion criteria may significantly influence the outcome. For example, at one end of the spectrum, the inclusion of low-risk patients may hide an otherwise beneficial response. In these individuals, the adverse effects of treatment (eg, anticoagulant-mediated bleeding) may outweigh any small benefit. Another important consideration is the adaptive nature of the host response. As long as the overall response is protective (eg, during the early stages of the sepsis continuum), targeted therapy may have no effect, or even a negative impact on survival.144 At the other end of the spectrum, patients who present with late-stage disease may be relatively resistant to therapy. Sepsis-induced cascades that were once amenable to therapeutic intervention may no longer be responsive. When designing therapies that target the endothelium, it will be important to define the optimal timing and spectrum of disease severity.

Complexity of the host response.

Traditionally, reductionist approaches have been applied to an understanding of sepsis pathophysiology. Indeed, the vast majority of basic studies in this field have focused on isolated and specific mechanisms of the host response. These data have given rise to linear models of pathophysiology, which in turn have guided the choice of therapeutic targets. The notion that the various components of the host response are aligned in series predicts that the attenuation of any one component (eg, TNF-α) will abort the sepsis cascade. This “domino-type model” is giving way to a more realistic paradigm of nonlinear complexity, in which the various cell types, inflammatory mediators, coagulation factors, cell surface receptors, intracellular signaling pathways, transcription factors, and genes interact as part of spatially and temporally coupled networks.80,217,218The nonlinear dynamics of the host response may help to explain, at least in part, the disappointing results of single-target therapy trials in severe sepsis.

One way to deal with the inherent complexity of the host response is to develop broad-base targeting schemes in which multiple components are attenuated at once, for example the inflammatory and coagulation cascades. It is perhaps by casting a wider “therapeutic net” that activated protein C succeeded, where so many other agents before it have failed. Although the precise mechanism(s) by which drotrecogin alfa (activated) improved survival in these patients is not known, several lines of evidence point to a multifaceted role of this agent in inhibiting the proinflammatory and procoagulant response, promoting fibrinolysis, and attenuating the activation of endothelial cells and white blood cells.38,219,220 

An alternative approach is to target a nonredundant component of the host response that is central to the initiation and perpetuation of the sepsis phenotype. Examples might include a single function of the endothelium (eg, apoptosis), or a single transcription factor (eg, NF-κB). However, in designing such strategies, it is important that we acknowledge the unpredictable behavior of complex nonlinear systems and readjust our expectations accordingly. While in theory the host response to infection (for any one patient at a single time point) may be modeled by a highly complex series of nonlinear equations, these formulas are not only elusive, but are likely to be exquisitely sensitive to initial conditions. As a result, single-component targeting may not only fail to modulate the host response, but may have unintended, deleterious consequences. An important scientific challenge for the 21st century is to learn how to leverage nonlinear interactions for mechanistic and therapeutic gain. Future progress in understanding complex networks will rely both on improved readouts and more complete statistical and mathematical tools, including advanced clustering techniques, other data mining and pattern recognition strategies, Bayesian techniques, differential equations, and simulation tools. By studying and embracing more realistic biologic models that involve complex networks, we may improve our capacity to reconfigure the host response in our favor.

Implications for clinical trials.

In clinical trials, patients may be alike, but they are never identical. From a therapeutic standpoint, what may save one patient may actually harm another. Moreover, a therapeutic intervention that benefits a given patient at one moment in time may be deleterious at another point in time. Thus, the optimal therapy for sepsis is highly patient and time dependent. However, until we can better characterize the complex behavior of the host response, we are restricted to classic randomized control trial design, in which a single intervention is tested in a heterogeneous group of patients. An important goal, which can be achieved only through large clinical trials, is to identify subgroups of patients that appear to benefit from treatment. This information may then be used to design new preclinical/clinical studies. Such an approach should help to reduce patient heterogeneity (or noise) and to develop more tailored therapy, for exampl, against one or another component of the endothelial response or toward specific vascular bed(s).

Conclusions

Despite new information about the pathophysiology and treatment of severe sepsis, this disorder continues to be associated with an unacceptably high mortality rate. Future breakthroughs will require a conceptual shift that emphasizes relationships between the various mediators and cells involved in host response. The endothelium is key in initiating, perpetuating, and modulating the host response to infection. Additional studies promise to provide new insight into the endothelium, not as an isolated mechanism of sepsis pathophysiology, but rather as the coordinator of a far more expansive, spatially and temporally orchestrated response.

I thank Derek Angus, John Marshall, Wes Ely, and Ary Goldberger for their helpful input.

Prepublished online as Blood First Edition Paper, January 23, 2003; DOI 10.1182/blood-2002-06-1887.

Supported in part by National Institutes of Health grants HL 60585-03, HL 63609-02, and HL 65216-02.

References

References
1
Rangel-Frausto
MS
Pittet
D
Costigan
M
et al
The natural history of the systemic inflammatory response syndrome (SIRS). A prospective study.
JAMA.
273
1995
117
123
2
Bone
RC
Balk
RA
Cerra
FB
et al
Definitions for sepsis and organ failure and guidelines for the use of innovative therapies in sepsis. The ACCP/SCCM Consensus Conference Committee. American College of Chest Physicians/Society of Critical Care Medicine.
Chest.
101
1992
1644
1655
3
Centers for Disease Control
Increase in national hospital discharge survey rates for septicemia—United States, 1979-1987.
JAMA.
263
1990
937
938
4
Angus
DC
Linde-Zwirble
WT
Lidicker
J
et al
Epidemiology of severe sepsis in the United States: analysis of incidence, outcome, and associated costs of care.
Crit Care Med.
29
2001
1303
1310
5
Brun-Buisson
C
Doyon
F
Carlet
J
et al
Incidence, risk factors, and outcome of severe sepsis and septic shock in adults. A multicenter prospective study in intensive care units. French ICU Group for Severe Sepsis.
JAMA.
274
1995
968
974
6
Damas
P
Canivet
JL
de Groote
D
et al
Sepsis and serum cytokine concentrations.
Crit Care Med.
25
1997
405
412
7
Thijs
LG
Hack
CE
Time course of cytokine levels in sepsis.
Intensive Care Med.
21(suppl 2)
1995
S258
S263
8
Pinsky
MR
Vincent
JL
Deviere
J
et al
Serum cytokine levels in human septic shock. Relation to multiple-system organ failure and mortality.
Chest.
103
1993
565
575
9
Yan
SB
Helterbrand
JD
Hartman
DL
Wright
TJ
Bernard
GR
Low levels of protein C are associated with poor outcome in severe sepsis.
Chest.
120
2001
915
922
10
Bernard
GR
Ely
EW
Wright
TJ
et al
Safety and dose relationship of recombinant human activated protein C for coagulopathy in severe sepsis.
Crit Care Med.
29
2001
2051
2059
11
Bernard
GR
Vincent
JL
Laterre
PF
et al
Efficacy and safety of recombinant human activated protein C for severe sepsis.
N Engl J Med.
344
2001
699
709
12
Opal
SM
Fisher
CJ
Jr
Dhainaut
JF
et al
Confirmatory interleukin-1 receptor antagonist trial in severe sepsis: a phase III, randomized, double-blind, placebo-controlled, multicenter trial. The Interleukin-1 Receptor Antagonist Sepsis Investigator Group.
Crit Care Med.
25
1997
1115
1124
13
Fisher
CJ
Jr
Agosti
JM
Opal
SM
et al
Treatment of septic shock with the tumor necrosis factor receptor:Fc fusion protein. The Soluble TNF Receptor Sepsis Study Group.
N Engl J Med.
334
1996
1697
1702
14
Levi
M
ten Cate
H
Disseminated intravascular coagulation.
N Engl J Med.
341
1999
586
592
15
Levi
M
Pathogenesis and treatment of disseminated intravascular coagulation in the septic patient.
J Crit Care.
16
2001
167
177
16
McCloskey
RV
Straube
RC
Sanders
C
Smith
SM
Smith
CR
Treatment of septic shock with human monoclonal antibody HA-1A. A randomized, double-blind, placebo-controlled trial. CHESS Trial Study Group.
Ann Intern Med.
121
1994
1
5
17
Angus
DC
Birmingham
MC
Balk
RA
et al
E5 murine monoclonal antiendotoxin antibody in gram-negative sepsis: a randomized controlled trial. E5 Study Investigators.
JAMA.
283
2000
1723
1730
18
Opal
SM
Huber
CE
Bench-to-bedside review: toll-like receptors and their role in septic shock.
Crit Care.
6
2002
125
136
19
Janeway
CA
Jr
Medzhitov
R
Innate immune recognition.
Annu Rev Immunol.
20
2002
197
216
20
Medzhitov
R
Toll-like receptors and innate immunity.
Nature Rev Immunol.
1
2001
135
145
21
Triantafilou
M
Triantafilou
K
Lipopolysaccharide recognition: CD14, TLRs and the LPS-activation cluster.
Trends Immunol.
23
2002
301
304
22
Faure
E
Thomas
L
Xu
H
et al
Bacterial lipopolysaccharide and IFN-gamma induce toll-like receptor 2 and toll-like receptor 4 expression in human endothelial cells: role of NF-kappa B activation.
J Immunol.
166
2001
2018
2024
23
Henneke
P
Golenbock
DT
Innate immune recognition of lipopolysaccharide by endothelial cells.
Crit Care Med.
30
2002
S207
S213
24
Zhang
FX
Kirschning
CJ
Mancinelli
R
et al
Bacterial lipopolysaccharide activates nuclear factor-kappaB through interleukin-1 signaling mediators in cultured human dermal endothelial cells and mononuclear phagocytes.
J Biol Chem.
274
1999
7611
7614
25
Zhao
B
Bowden
RA
Stavchansky
SA
Bowman
PD
Human endothelial cell response to gram-negative lipopolysaccharide assessed with cDNA microarrays.
Am J Physiol Cell Physiol.
281
2001
C1587
C1595
26
Bone
RC
Grodzin
CJ
Balk
RA
Sepsis: a new hypothesis for pathogenesis of the disease process.
Chest.
112
1997
235
243
27
Bone
RC
Sir Isaac Newton, sepsis, SIRS, and CARS.
Crit Care Med.
24
1996
1125
1128
28
Oberholzer
A
Oberholzer
C
Moldawer
LL
Sepsis syndromes: understanding the role of innate and acquired immunity.
Shock.
16
2001
83
96
29
van der Poll
T
van Deventer
SJ
Cytokines and anticytokines in the pathogenesis of sepsis.
Infect Dis Clin North Am.
13
1999
413
426
30
Pinsky
MR
Immune balance in critically ill patients.
Arch Immunol Ther Exp.
48
2000
439
442
31
Aird
WC
Vascular bed-specific hemostasis: role of endothelium in sepsis pathogenesis.
Crit Care Med.
29
2001
S28
S35
32
Osterud
B
Rapaport
SI
Activation of factor IX by the reaction product of tissue factor and factor VII: additional pathway for initiating blood coagulation.
Proc Natl Acad Sci U S A.
74
1977
5260
5264
33
Bauer
KA
Kass
BL
ten Cate
H
Hawiger
JJ
Rosenberg
RD
Factor IX is activated in vivo by the tissue factor mechanism.
Blood.
76
1990
731
736
34
ten Cate
H
Bauer
KA
Levi
M
et al
The activation of factor X and prothrombin by recombinant factor VIIa in vivo is mediated by tissue factor.
J Clin Invest.
92
1993
1207
1212
35
Gailani
D
Broze
GJ
Jr
Factor XI activation in a revised model of blood coagulation.
Science.
253
1991
909
912
36
Broze
GJ
Jr
Tissue factor pathway inhibitor.
Thromb Haemost.
74
1995
90
93
37
Rosenberg
RD
Biochemistry of heparin antithrombin interactions, and the physiologic role of this natural anticoagulant mechanism.
Am J Med.
87
1989
2S
9S
38
Esmon
CT
Protein C anticoagulant pathway and its role in controlling microvascular thrombosis and inflammation.
Crit Care Med.
29
2001
S48
S51
discussion 51-42.
39
Vervloet
MG
Thijs
LG
Hack
CE
Derangements of coagulation and fibrinolysis in critically ill patients with sepsis and septic shock.
Semin Thromb Hemost.
24
1998
33
44
40
White
B
Perry
D
Acquired antithrombin deficiency in sepsis.
Br J Haematol.
112
2001
26
31
41
Weiler
H
Lindner
V
Kerlin
B
et al
Characterization of a mouse model for thrombomodulin deficiency.
Arterioscler Thromb Vasc Biol.
21
2001
1531
1537
42
Faust
SN
Levin
M
Harrison
OB
et al
Dysfunction of endothelial protein C activation in severe meningococcal sepsis.
N Engl J Med.
345
2001
408
416
43
Osterud
B
Flaegstad
T
Increased tissue thromboplastin activity in monocytes of patients with meningococcal infection: related to an unfavourable prognosis.
Thromb Haemost.
49
1983
5
7
44
Collins
PW
Noble
KE
Reittie
JR
et al
Induction of tissue factor expression in human monocyte/endothelium cocultures.
Br J Haematol.
91
1995
963
970
45
Pernerstorfer
T
Hollenstein
U
Hansen
JB
et al
Lepirudin blunts endotoxin-induced coagulation activation.
Blood.
95
2000
1729
1734
46
Todoroki
H
Nakamura
S
Higure
A
et al
Neutrophils express tissue factor in a monkey model of sepsis.
Surgery.
127
2000
209
216
47
Drake
TA
Cheng
J
Chang
A
Taylor
FB
Jr
Expression of tissue factor, thrombomodulin, and E-selectin in baboons with lethal Escherichia coli sepsis.
Am J Pathol.
142
1993
1458
1470
48
Carson
SD
Johnson
DR
Consecutive enzyme cascades: complement activation at the cell surface triggers increased tissue factor activity.
Blood.
76
1990
361
367
49
Jansen
PM
Eisele
B
de Jong
IW
et al
Effect of C1 inhibitor on inflammatory and physiologic response patterns in primates suffering from lethal septic shock.
J Immunol.
160
1998
475
484
50
Coughlin
SR
How the protease thrombin talks to cells.
Proc Natl Acad Sci U S A.
96
1999
11023
11027
51
Coughlin
SR
Thrombin signalling and protease-activated receptors.
Nature.
407
2000
258
264
52
Vouret-Craviari
V
Boquet
P
Pouyssegur
J
Van Obberghen-Schilling
E
Regulation of the actin cytoskeleton by thrombin in human endothelial cells: role of Rho proteins in endothelial barrier function.
Mol Biol Cell.
9
1998
2639
2653
53
Malik
AB
Fenton
JW
2nd
Thrombin-mediated increase in vascular endothelial permeability.
Semin Thromb Hemost.
18
1992
193
199
54
Tsopanoglou
NE
Maragoudakis
ME
On the mechanism of thrombin-induced angiogenesis. Potentiation of vascular endothelial growth factor activity on endothelial cells by up-regulation of its receptors.
J Biol Chem.
274
1999
23969
23976
55
Sugama
Y
Tiruppathi
C
Offakidevi
K
et al
Thrombin-induced expression of endothelial P-selectin and intercellular adhesion molecule-1: a mechanism for stabilizing neutrophil adhesion.
J Cell Biol.
119
1992
935
944
56
Rahman
A
Anwar
KN
True
AL
Malik
AB
Thrombin-induced p65 homodimer binding to downstream NF-kappa B site of the promoter mediates endothelial ICAM-1 expression and neutrophil adhesion.
J Immunol.
162
1999
5466
5476
57
Lorant
DE
Patel
KD
McIntyre
TM
et al
Coexpression of GMP-140 and PAF by endothelium stimulated by histamine or thrombin: a juxtacrine system for adhesion and activation of neutrophils.
J Cell Biol.
115
1991
223
234
58
Kaplanski
G
Marin
V
Fabrigoule
M
et al
Thrombin-activated human endothelial cells support monocyte adhesion in vitro following expression of intercellular adhesion molecule-1 (ICAM- 1; CD54) and vascular cell adhesion molecule-1 (VCAM-1; CD106).
Blood.
92
1998
1259
1267
59
Kaplanski
G
Fabrigoule
M
Boulay
V
et al
Thrombin induces endothelial type II activation in vitro: IL-1 and TNF- alpha-independent IL-8 secretion and E-selectin expression.
J Immunol.
158
1997
5435
5441
60
Minami
T
Aird
WC
Thrombin stimulation of the vascular cell adhesion molecule-1 promoter in endothelial cells is mediated by tandem nuclear factor-kappa B and GATA motifs.
J Biol Chem.
276
2001
47632
47641
61
Vischer
UM
Barth
H
Wollheim
CB
Regulated von Willebrand factor secretion is associated with agonist-specific patterns of cytoskeletal remodeling in cultured endothelial cells.
Arterioscler Thromb Vasc Biol.
20
2000
883
891
62
Ellis
CA
Malik
AB
Gilchrist
A
et al
Thrombin induces proteinase-activated receptor-1 gene expression in endothelial cells via activation of Gi-linked Ras/mitogen-activated protein kinase pathway.
J Biol Chem.
274
1999
13718
13727
63
Zimmerman
GA
McIntyre
TM
Prescott
SM
Production of platelet-activating factor by human vascular endothelial cells: evidence for a requirement for specific agonists and modulation by prostacyclin.
Circulation.
72
1985
718
727
64
Ueno
A
Murakami
K
Yamanouchi
K
Watanabe
M
Kondo
T
Thrombin stimulates production of interleukin-8 in human umbilical vein endothelial cells.
Immunology.
88
1996
76
81
65
Colotta
F
Sciacca
FL
Sironi
M
et al
Expression of monocyte chemotactic protein-1 by monocytes and endothelial cells exposed to thrombin.
Am J Pathol.
144
1994
975
985
66
Duhamel-Clerin
E
Orvain
C
Lanza
F
Cazenave
JP
Klein-Soyer
C
Thrombin receptor-mediated increase of two matrix metalloproteinases, MMP-1 and MMP-3, in human endothelial cells.
Arterioscler Thromb Vasc Biol.
17
1997
1931
1938
67
Rottingen
JA
Enden
T
Camerer
E
Iversen
JG
Prydz
H
Binding of human factor VIIa to tissue factor induces cytosolic Ca2+ signals in J82 cells, transfected COS-1 cells, Madin-Darby canine kidney cells and in human endothelial cells induced to synthesize tissue factor.
J Biol Chem.
270
1995
4650
4660
68
Camerer
E
Huang
W
Coughlin
SR
Tissue factor- and factor X-dependent activation of protease-activated receptor 2 by factor VIIa.
Proc Natl Acad Sci U S A.
97
2000
5255
5260
69
Camerer
E
Kataoka
H
Kahn
M
Lease
K
Coughlin
SR
Genetic evidence that protease-activated receptors mediate factor Xa signaling in endothelial cells.
J Biol Chem.
277
2002
16081
16087
70
Bono
F
Schaeffer
P
Herault
JP
et al
Factor Xa activates endothelial cells by a receptor cascade between EPR- 1 and PAR-2.
Arterioscler Thromb Vasc Biol.
20
2000
E107
E112
71
Gorlatov
S
Medved
L
Interaction of fibrin(ogen) with the endothelial cell receptor VE-cadherin: mapping of the receptor-binding site in the NH2-terminal portions of the fibrin beta chains.
Biochemistry.
41
2002
4107
4116
72
Qi
J
Goralnick
S
Kreutzer
DL
Fibrin regulation of interleukin-8 gene expression in human vascular endothelial cells.
Blood.
90
1997
3595
3602
73
Wolinsky
H
A proposal linking clearance of circulating lipoproteins to tissue metabolic activity as a basis for understanding atherogenesis.
Circ Res.
47
1980
301
311
74
Cines
DB
Pollak
ES
Buck
CA
et al
Endothelial cells in physiology and in the pathophysiology of vascular disorders.
Blood.
91
1998
3527
3561
75
Gross
PL
Aird
WC
The endothelium and thrombosis.
Semin Thromb Hemost.
26
2000
463
478
76
Bombeli
T
Mueller
M
Haeberli
A
Anticoagulant properties of the vascular endothelium.
Thromb Haemost.
77
1997
408
423
77
Rosenberg
RD
Aird
WC
Vascular-bed-specific hemostasis and hypercoagulable states.
N Engl J Med.
340
1999
1555
1564
78
Stevens
T
Rosenberg
R
Aird
W
et al
NHLBI workshop report: endothelial cell phenotypes in heart, lung, and blood diseases.
Am J Physiol Cell Physiol.
281
2001
C1422
C1433
79
Gerritsen
ME
Functional heterogeneity of vascular endothelial cells.
Biochem Pharmacol.
36
1987
2701
2711
80
Aird
WC
Endothelial cell dynamics and complexity theory.
Crit Care Med.
30
2002
S180
S185
81
Volk
T
Kox
WJ
Endothelium function in sepsis.
Inflamm Res.
49
2000
185
198
82
Cariou
A
Chiche
JD
Charpentier
J
Dhainaut
JF
Mira
JP
The era of genomics: impact on sepsis clinical trial design.
Crit Care Med.
30
2002
S341
S348
83
Yamamoto
K
Shimokawa
T
Yi
H
et al
Aging accelerates endotoxin-induced thrombosis: increased responses of plasminogen activator inhibitor-1 and lipopolysaccharide signaling with aging.
Am J Pathol.
161
2002
1805
1814
84
Mavrommatis
AC
Theodoridis
T
Economou
M
et al
Activation of the fibrinolytic system and utilization of the coagulation inhibitors in sepsis: comparison with severe sepsis and septic shock.
Intensive Care Med.
27
2001
1853
1859
85
Kumar
A
Short
J
Parrillo
JE
Genetic factors in septic shock.
JAMA.
282
1999
579
581
86
Mira
JP
Cariou
A
Grall
F
et al
Association of TNF2, a TNF-alpha promoter polymorphism, with septic shock susceptibility and mortality: a multicenter study.
JAMA.
282
1999
561
568
87
Westendorp
RG
Langermans
JA
Huizinga
TW
Verweij
CL
Sturk
A
Genetic influence on cytokine production in meningococcal disease.
Lancet.
349
1997
1912
1913
88
Arbour
NC
Lorenz
E
Schutte
BC
et al
TLR4 mutations are associated with endotoxin hyporesponsiveness in humans.
Nat Genet.
25
2000
187
191
89
Arnalich
F
Lopez-Maderuelo
D
Codoceo
R
et al
Interleukin-1 receptor antagonist gene polymorphism and mortality in patients with severe sepsis.
Clin Exp Immunol.
127
2002
331
336
90
Hermans
PW
Hibberd
ML
Booy
R
et al
4G/5G promoter polymorphism in the plasminogen-activator-inhibitor-1 gene and outcome of meningococcal disease. Meningococcal Research Group.
Lancet.
354
1999
556
560
91
Nadel
S
Helping to understand studies examining genetic susceptibility to sepsis.
Clin Exp Immunol.
127
2002
191
192
92
Vallet
B
Wiel
E
Endothelial cell dysfunction and coagulation.
Crit Care Med.
29
2001
S36
S41
93
Bevilacqua
MP
Pober
JS
Majeau
GR
et al
Recombinant tumor necrosis factor induces procoagulant activity in cultured human vascular endothelium: characterization and comparison with the actions of interleukin 1.
Proc Natl Acad Sci U S A.
83
1986
4533
4537
94
Combes
V
Simon
AC
Grau
GE
et al
In vitro generation of endothelial microparticles and possible prothrombotic activity in patients with lupus anticoagulant.
J Clin Invest.
104
1999
93
102
95
Moore
KL
Andreoli
SP
Esmon
NL
Esmon
CT
Bang
NU
Endotoxin enhances tissue factor and suppresses thrombomodulin expression of human vascular endothelium in vitro.
J Clin Invest.
79
1987
124
130
96
Schleef
RR
Bevilacqua
MP
Sawdey
M
Gimbrone
MA
Jr
Loskutoff
DJ
Cytokine activation of vascular endothelium. Effects on tissue-type plasminogen activator and type 1 plasminogen activator inhibitor.
J Biol Chem.
263
1988
5797
5803
97
Green
J
Doughty
L
Kaplan
SS
Sasser
H
Carcillo
JA
The tissue factor and plasminogen activator inhibitor type-1 response in pediatric sepsis-induced multiple organ failure.
Thromb Haemost.
87
2002
218
223
98
Bombeli
T
Karsan
A
Tait
JF
Harlan
JM
Apoptotic vascular endothelial cells become procoagulant.
Blood.
89
1997
2429
2442
99
Weiler-Guettler
H
Christie
PD
Beeler
DL
et al
A targeted point mutation in thrombomodulin generates viable mice with a prethrombotic state.
J Clin Invest.
101
1998
1983
1991
100
Yamamoto
K
Loskutoff
DJ
Fibrin deposition in tissues from endotoxin-treated mice correlates with decreases in the expression of urokinase-type but not tissue-type plasminogen activator.
J Clin Invest.
97
1996
2440
2451
101
Yanada
M
Kojima
T
Ishiguro
K
et al
Impact of antithrombin deficiency in thrombogenesis: lipopolysaccharide and stress-induced thrombus formation in heterozygous antithrombin-deficient mice.
Blood.
99
2002
2455
2458
102
Eppihimer
MJ
Wolitzky
B
Anderson
DC
Labow
MA
Granger
DN
Heterogeneity of expression of E- and P-selectins in vivo.
Circ Res.
79
1996
560
569
103
Panes
J
Perry
MA
Anderson
DC
et al
Regional differences in constitutive and induced ICAM-1 expression in vivo.
Am J Physiol.
269
1995
H1955
H1964
104
Mulligan
MS
Vaporciyan
AA
Miyasaka
M
Tamatani
T
Ward
PA
Tumor necrosis factor alpha regulates in vivo intrapulmonary expression of ICAM-1.
Am J Pathol.
142
1993
1739
1749
105
Lopez
S
Prats
N
Marco
AJ
Expression of E-selectin, P-selectin, and intercellular adhesion molecule-1 during experimental murine listeriosis.
Am J Pathol.
155
1999
1391
1397
106
Bauer
P
Lush
CW
Kvietys
PR
Russell
JM
Granger
DN
Role of endotoxin in the expression of endothelial selectins after cecal ligation and perforation.
Am J Physiol Regul Integr Comp Physiol.
278
2000
R1140
R1147
107
Lush
CW
Cepinskas
G
Sibbald
WJ
Kvietys
PR
Endothelial E- and P-selectin expression in iNOS-deficient mice exposed to polymicrobial sepsis.
Am J Physiol Gastrointest Liver Physiol.
280
2001
G291
G297
108
Henninger
DD
Panes
J
Eppihimer
M
et al
Cytokine-induced VCAM-1 and ICAM-1 expression in different organs of the mouse.
J Immunol.
158
1997
1825
1832
109
Sheu
JR
Hung
WC
Wu
CH
et al
Reduction in lipopolysaccharide-induced thrombocytopenia by triflavin in a rat model of septicemia.
Circulation.
99
1999
3056
3062
110
Shibazaki
M
Kawabata
Y
Yokochi
T
et al
Complement-dependent accumulation and degradation of platelets in the lung and liver induced by injection of lipopolysaccharides.
Infect Immun.
67
1999
5186
5191
111
Shibazaki
M
Nakamura
M
Endo
Y
Biphasic, organ-specific, and strain-specific accumulation of platelets induced in mice by a lipopolysaccharide from Escherichia coli and its possible involvement in shock.
Infect Immun.
64
1996
5290
5294
112
Katayama
T
Ikeda
Y
Handa
M
et al
Immunoneutralization of glycoprotein Ibalpha attenuates endotoxin-induced interactions of platelets and leukocytes with rat venular endothelium in vivo.
Circ Res.
86
2000
1031
1037
113
Tsujikawa
A
Kiryu
J
Yamashiro
K
et al
Interactions between blood cells and retinal endothelium in endotoxic sepsis.
Hypertension.
36
2000
250
258
114
Munoz
FM
Hawkins
EP
Bullard
DC
Beaudet
AL
Kaplan
SL
Host defense against systemic infection with Streptococcus pneumoniae is impaired in E-, P-, and E-/P-selectin-deficient mice.
J Clin Invest.
100
1997
2099
2106
115
Steeber
DA
Tang
ML
Green
NE
et al
Leukocyte entry into sites of inflammation requires overlapping interactions between the L-selectin and ICAM-1 pathways.
J Immunol.
163
1999
2176
2186
116
Matsukawa
A
Lukacs
NW
Hogaboam
CM
et al
Mice genetically lacking endothelial selectins are resistant to the lethality in septic peritonitis.
Exp Mol Pathol.
72
2002
68
76
117
Wanecek
M
Weitzberg
E
Rudehill
A
Oldner
A
The endothelin system in septic and endotoxin shock.
Eur J Pharmacol.
407
2000
1
15
118
McCuskey
RS
Urbaschek
R
Urbaschek
B
The microcirculation during endotoxemia.
Cardiovasc Res.
32
1996
752
763
119
Stevens
T
Garcia
JG
Shasby
DM
Bhattacharya
J
Malik
AB
Mechanisms regulating endothelial cell barrier function.
Am J Physiol Lung Cell Mol Physiol.
279
2000
L419
L422
120
Ferro
T
Neumann
P
Gertzberg
N
Clements
R
Johnson
A
Protein kinase C-alpha mediates endothelial barrier dysfunction induced by TNF-alpha.
Am J Physiol Lung Cell Mol Physiol.
278
2000
L1107
L1117
121
Ferro
TJ
Gertzberg
N
Selden
L
Neumann
P
Johnson
A
Endothelial barrier dysfunction and p42 oxidation induced by TNF-alpha are mediated by nitric oxide.
Am J Physiol.
272
1997
L979
L988
122
Goldblum
SE
Ding
X
Campbell-Washington
J
TNF-alpha induces endothelial cell F-actin depolymerization, new actin synthesis, and barrier dysfunction.
Am J Physiol.
264
1993
C894
C905
123
Johnson
J
Meyrick
B
Jesmok
G
Brigham
KL
Human recombinant tumor necrosis factor alpha infusion mimics endotoxemia in awake sheep.
J Appl Physiol.
66
1989
1448
1454
124
Tiruppathi
C
Naqvi
T
Sandoval
R
Mehta
D
Malik
AB
Synergistic effects of tumor necrosis factor-alpha and thrombin in increasing endothelial permeability.
Am J Physiol Lung Cell Mol Physiol.
281
2001
L958
L968
125
Stefanec
T
Endothelial apoptosis: could it have a role in the pathogenesis and treatment of disease?
Chest.
117
2000
841
854
126
Hotchkiss
RS
Tinsley
KW
Swanson
PE
Karl
IE
Endothelial cell apoptosis in sepsis.
Crit Care Med.
30
2002
S225
S228
127
Bannerman
DD
Sathyamoorthy
M
Goldblum
SE
Bacterial lipopolysaccharide disrupts endothelial monolayer integrity and survival signaling events through caspase cleavage of adherens junction proteins.
J Biol Chem.
273
1998
35371
35380
128
Choi
KB
Wong
F
Harlan
JM
et al
Lipopolysaccharide mediates endothelial apoptosis by a FADD-dependent pathway.
J Biol Chem.
273
1998
20185
20188
129
Messmer
UK
Briner
VA
Pfeilschifter
J
Tumor necrosis factor-alpha and lipopolysaccharide induce apoptotic cell death in bovine glomerular endothelial cells.
Kidney Int.
55
1999
2322
2337
130
Hu
X
Yee
E
Harlan
JM
Wong
F
Karsan
A
Lipopolysaccharide induces the antiapoptotic molecules, A1 and A20, in microvascular endothelial cells.
Blood.
92
1998
2759
2765
131
Polunovsky
VA
Wendt
CH
Ingbar
DH
Peterson
MS
Bitterman
PB
Induction of endothelial cell apoptosis by TNF alpha: modulation by inhibitors of protein synthesis.
Exp Cell Res.
214
1994
584
594
132
Lindner
H
Holler
E
Ertl
B
et al
Peripheral blood mononuclear cells induce programmed cell death in human endothelial cells and may prevent repair: role of cytokines.
Blood.
89
1997
1931
1938
133
Hebert
MJ
Gullans
SR
Mackenzie
HS
Brady
HR
Apoptosis of endothelial cells is associated with paracrine induction of adhesion molecules: evidence for an interleukin-1 beta-dependent paracrine loop.
Am J Pathol.
152
1998
523
532
134
Mitra
D
Jaffe
EA
Weksler
B
et al
Thrombotic thrombocytopenic purpura and sporadic hemolytic-uremic syndrome plasmas induce apoptosis in restricted lineages of human microvascular endothelial cells.
Blood.
89
1997
1224
1234
135
Tsuji
S
Kaji
K
Nagasawa
S
Activation of the alternative pathway of human complement by apoptotic human umbilical vein endothelial cells.
J Biochem (Tokyo).
116
1994
794
800
136
Bombeli
T
Schwartz
BR
Harlan
JM
Endothelial cells undergoing apoptosis become proadhesive for nonactivated platelets.
Blood.
93
1999
3831
3838
137
Haimovitz-Friedman
A
Cordon-Cardo
C
Bayoumy
S
et al
Lipopolysaccharide induces disseminated endothelial apoptosis requiring ceramide generation.
J Exp Med.
186
1997
1831
1841
138
Fujita
M
Kuwano
K
Kunitake
R
et al
Endothelial cell apoptosis in lipopolysaccharide-induced lung injury in mice.
Int Arch Allergy Immunol.
117
1998
202
208
139
Kawasaki
M
Kuwano
K
Hagimoto
N
et al
Protection from lethal apoptosis in lipopolysaccharide-induced acute lung injury in mice by a caspase inhibitor.
Am J Pathol.
157
2000
597
603
140
Munford
RS
Pugin
J
Normal responses to injury prevent systemic inflammation and can be immunosuppressive.
Am J Respir Crit Care Med.
163
2001
316
321
141
Smiley
ST
King
JA
Hancock
WW
Fibrinogen stimulates macrophage chemokine secretion through toll-like receptor 4.
J Immunol.
167
2001
2887
2894
142
Bochkov
VN
Kadl
A
Huber
J
et al
Protective role of phospholipid oxidation products in endotoxin-induced tissue damage.
Nature.
419
2002
77
81
143
Cohen
J
Guyatt
G
Bernard
GR
et al
New strategies for clinical trials in patients with sepsis and septic shock.
Crit Care Med.
29
2001
880
886
144
Eichacker
PQ
Parent
C
Kalil
A
et al
Risk and the efficacy of antiinflammatory agents: retrospective and confirmatory studies of sepsis.
Am J Respir Crit Care Med.
166
2002
1197
1205
145
Warren
BL
Eid
A
Singer
P
et al
Caring for the critically ill patient. High-dose antithrombin III in severe sepsis: a randomized controlled trial.
JAMA.
286
2001
1869
1878
146
Zeni
F
Freeman
B
Natanson
C
Anti-inflammatory therapies to treat sepsis and septic shock: a reassessment.
Crit Care Med.
25
1997
1095
1100
147
ARDSNET
Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. The Acute Respiratory Distress Syndrome Network.
N Engl J Med.
342
2000
1301
1308
148
Annane
D
Sebille
V
Charpentier
C
et al
Effect of treatment with low doses of hydrocortisone and fludrocortisone on mortality in patients with septic shock.
JAMA.
288
2002
862
871
149
van den Berghe
G
Wouters
P
Weekers
F
et al
Intensive insulin therapy in the critically ill patients.
N Engl J Med.
345
2001
1359
1367
150
Rivers
E
Nguyen
B
Havstad
S
et al
Early goal-directed therapy in the treatment of severe sepsis and septic shock.
N Engl J Med.
345
2001
1368
1377
151
Natanson
C
Esposito
CJ
Banks
SM
The sirens' songs of confirmatory sepsis trials: selection bias and sampling error.
Crit Care Med.
26
1998
1927
1931
152
Marshall
JC
Clinical trials of mediator-directed therapy in sepsis: what have we learned?
Intensive Care Med.
26
2000
S75
S83
153
Harlan
JM
Winn
RK
Leukocyte-endothelial interactions: clinical trials of anti-adhesion therapy.
Crit Care Med.
30
2002
S214
S219
154
Frenette
PS
Johnson
RC
Hynes
RO
Wagner
DD
Platelets roll on stimulated endothelium in vivo: an interaction mediated by endothelial P-selectin.
Proc Natl Acad Sci U S A.
92
1995
7450
7454
155
Frenette
PS
Moyna
C
Hartwell
DW
et al
Platelet-endothelial interactions in inflamed mesenteric venules.
Blood.
91
1998
1318
1324
156
Bombeli
T
Schwartz
BR
Harlan
JM
Adhesion of activated platelets to endothelial cells: evidence for a GPIIbIIIa-dependent bridging mechanism and novel roles for endothelial intercellular adhesion molecule 1 (ICAM-1), alphavbeta3 integrin, and GPIbalpha.
J Exp Med.
187
1998
329
339
157
Coalson
JJ
Benjamin
B
Archer
LT
et al
Prolonged shock in the baboon subjected to infusion of E coli endotoxin.
Circ Shock.
5
1978
423
437
158
Taylor
FB
Jr
Chang
AC
Peer
GT
et al
DEGR-factor Xa blocks disseminated intravascular coagulation initiated by Escherichia coli without preventing shock or organ damage.
Blood.
78
1991
364
368
159
Emerson
TE
Jr
Fournel
MA
Redens
TB
Taylor
FB
Jr
Efficacy of antithrombin III supplementation in animal models of fulminant Escherichia coli endotoxemia or bacteremia.
Am J Med.
87
1989
27S
33S
160
Taylor
FB
Jr
Chang
A
Esmon
CT
et al
Protein C prevents the coagulopathic and lethal effects of Escherichia coli infusion in the baboon.
J Clin Invest.
79
1987
918
925
161
Creasey
AA
Chang
AC
Feigen
L
et al
Tissue factor pathway inhibitor reduces mortality from Escherichia coli septic shock.
J Clin Invest.
91
1993
2850
2856
162
Welty-Wolf
KE
Carraway
MS
Miller
DL
et al
Coagulation blockade prevents sepsis-induced respiratory and renal failure in baboons.
Am J Respir Crit Care Med.
164
2001
1988
1996
163
Oelschlager
C
Romisch
J
Staubitz
A
et al
Antithrombin III inhibits nuclear factor kappaB activation in human monocytes and vascular endothelial cells.
Blood.
99
2002
4015
4020
164
Joyce
DE
Gelbert
L
Ciaccia
A
DeHoff
B
Grinnell
BW
Gene expression profile of antithrombotic protein c defines new mechanisms modulating inflammation and apoptosis.
J Biol Chem.
276
2001
11199
11203
165
Souter
PJ
Thomas
S
Hubbard
AR
et al
Antithrombin inhibits lipopolysaccharide-induced tissue factor and interleukin-6 production by mononuclear cells, human umbilical vein endothelial cells, and whole blood.
Crit Care Med.
29
2001
134
139
166
Crowther
MA
Marshall
JC
Continuing challenges of sepsis research.
JAMA.
286
2001
1894
1896
167
Opal
SM
Kessler
CM
Roemisch
J
Knaub
S
Antithrombin, heparin, and heparan sulfate.
Crit Care Med.
30
2002
S325
S331
168
Dimmeler
S
Haendeler
J
Rippmann
V
Nehls
M
Zeiher
AM
Shear stress inhibits apoptosis of human endothelial cells.
FEBS Lett.
399
1996
71
74
169
Baumgartner-Parzer
SM
Wagner
L
Pettermann
M
et al
High-glucose–triggered apoptosis in cultured endothelial cells.
Diabetes.
44
1995
1323
1327
170
Du
XL
Sui
GZ
Stockklauser-Farber
K
et al
Introduction of apoptosis by high proinsulin and glucose in cultured human umbilical vein endothelial cells is mediated by reactive oxygen species.
Diabetologia.
41
1998
249
256
171
Hermann
C
Assmus
B
Urbich
C
Zeiher
AM
Dimmeler
S
Insulin-mediated stimulation of protein kinase Akt: a potent survival signaling cascade for endothelial cells.
Arterioscler Thromb Vasc Biol.
20
2000
402
409
172
Stempien-Otero
A
Karsan
A
Cornejo
CJ
et al
Mechanisms of hypoxia-induced endothelial cell death. Role of p53 in apoptosis.
J Biol Chem.
274
1999
8039
8045
173
Matsushita
H
Morishita
R
Nata
T
et al
Hypoxia-induced endothelial apoptosis through nuclear factor-kappaB (NF- kappaB)-mediated bcl-2 suppression: in vivo evidence of the importance of NF-kappaB in endothelial cell regulation.
Circ Res.
86
2000
974
981
174
Stefanec
T
The effects of statins on mortality rates among bacteremic patients [letter].
Clin Infect Dis.
34
2002
1158
175
Gratton
JP
Morales-Ruiz
M
Kureishi
Y
et al
Akt down-regulation of p38 signaling provides a novel mechanism of vascular endothelial growth factor-mediated cytoprotection in endothelial cells.
J Biol Chem.
276
2001
30359
30365
176
Denk
A
Goebeler
M
Schmid
S
et al
Activation of NF-kappa B via the Ikappa B kinase complex is both essential and sufficient for proinflammatory gene expression in primary endothelial cells.
J Biol Chem.
276
2001
28451
28458
177
Bohrer
H
Qiu
F
Zimmermann
T
et al
Role of NFkappaB in the mortality of sepsis.
J Clin Invest.
100
1997
972
985
178
Oitzinger
W
Hofer-Warbinek
R
Schmid
JA
et al
Adenovirus-mediated expression of a mutant IkappaB kinase 2 inhibits the response of endothelial cells to inflammatory stimuli.
Blood.
97
2001
1611
1617
179
Bohuslav
J
Kravchenko
VV
Parry
GC
et al
Regulation of an essential innate immune response by the p50 subunit of NF-kappaB.
J Clin Invest.
102
1998
1645
1652
180
Rudders
S
Gaspar
J
Madore
R
et al
ESE-1 is a novel transcriptional mediator of inflammation that interacts with NF-kappa B to regulate the inducible nitric-oxide synthase gene.
J Biol Chem.
276
2001
3302
3309
181
Martin
T
Cardarelli
PM
Parry
GC
Felts
KA
Cobb
RR
Cytokine induction of monocyte chemoattractant protein-1 gene expression in human endothelial cells depends on the cooperative action of NF-kappa B and AP-1.
Eur J Immunol.
27
1997
1091
1097
182
Ahmad
M
Theofanidis
P
Medford
RM
Role of activating protein-1 in the regulation of the vascular cell adhesion molecule-1 gene expression by tumor necrosis factor-alpha.
J Biol Chem.
273
1998
4616
4621
183
Hipp
MS
Urbich
C
Mayer
P
et al
Proteasome inhibition leads to NF-kappaB-independent IL-8 transactivation in human endothelial cells through induction of AP-1.
Eur J Immunol.
32
2002
2208
2217
184
Simoncini
T
Maffei
S
Basta
G
et al
Estrogens and glucocorticoids inhibit endothelial vascular cell adhesion molecule-1 expression by different transcriptional mechanisms.
Circ Res.
87
2000
19
25
185
Umetani
M
Mataki
C
Minegishi
N
et al
Function of GATA transcription factors in induction of endothelial vascular cell adhesion molecule-1 by tumor necrosis factor-alpha.
Arterioscler Thromb Vasc Biol.
21
2001
917
922
186
Yan
SF
Fujita
T
Lu
J
et al
Egr-1, a master switch coordinating upregulation of divergent gene families underlying ischemic stress.
Nat Med.
6
2000
1355
1361
187
Ye
X
Liu
SF
Lipopolysaccharide regulates constitutive and inducible transcription factor activities differentially in vivo in the rat.
Biochem Biophys Res Commun.
288
2001
927
932
188
Liu
SF
Ye
X
Malik
AB
Inhibition of NF-kappaB activation by pyrrolidine dithiocarbamate prevents in vivo expression of proinflammatory genes.
Circulation.
100
1999
1330
1337
189
Gao
Y
Lecker
S
Post
MJ
et al
Inhibition of ubiquitin-proteasome pathway-mediated I kappa B alpha degradation by a naturally occurring antibacterial peptide.
J Clin Invest.
106
2000
439
448
190
De Bosscher
K
Vanden Berghe
W
Vermeulen
L
et al
Glucocorticoids repress NF-kappaB-driven genes by disturbing the interaction of p65 with the basal transcription machinery, irrespective of coactivator levels in the cell.
Proc Natl Acad Sci U S A.
97
2000
3919
3924
191
Bach
FH
Hancock
WW
Ferran
C
Protective genes expressed in endothelial cells: a regulatory response to injury.
Immunol Today.
18
1997
483
486
192
Zen
K
Karsan
A
Stempien-Otero
A
et al
NF-kappaB activation is required for human endothelial survival during exposure to tumor necrosis factor-alpha but not to interleukin-1beta or lipopolysaccharide.
J Biol Chem.
274
1999
28808
28815
193
Herlaar
E
Brown
Z
p38 MAPK signalling cascades in inflammatory disease.
Mol Med Today.
5
1999
439
447
194
Marin
V
Farnarier
C
Gres
S
et al
The p38 mitogen-activated protein kinase pathway plays a critical role in thrombin-induced endothelial chemokine production and leukocyte recruitment.
Blood.
98
2001
667
673
195
Kotlyarov
A
Neininger
A
Schubert
C
et al
MAPKAP kinase 2 is essential for LPS-induced TNF-alpha biosynthesis.
Nat Cell Biol.
1
1999
94
97
196
Badger
AM
Bradbeer
JN
Votta
B
et al
Pharmacological profile of SB 203580, a selective inhibitor of cytokine suppressive binding protein/p38 kinase, in animal models of arthritis, bone resorption, endotoxin shock and immune function.
J Pharmacol Exp Ther.
279
1996
1453
1461
197
van den Blink
B
Juffermans
NP
ten Hove
T
et al
p38 mitogen-activated protein kinase inhibition increases cytokine release by macrophages in vitro and during infection in vivo.
J Immunol.
166
2001
582
587
198
Branger
J
van den Blink
B
Weijer
S
et al
Anti-inflammatory effects of a p38 mitogen-activated protein kinase inhibitor during human endotoxemia.
J Immunol.
168
2002
4070
4077
199
Rahman
A
Anwar
KN
Malik
AB
Protein kinase C-zeta mediates TNF-alpha-induced ICAM-1 gene transcription in endothelial cells.
Am J Physiol Cell Physiol.
279
2000
C906
C914
200
Minami
T
Abid
MR
Zhang
J
et al
Thrombin stimulation of vascular adhesion molecule-1 in endothelial cells is mediated by protein kinase C (PKC)-delta-NF-kappa B and PKC-zeta-GATA signaling pathways.
J Biol Chem.
278
2003
6976
6984
201
Frey
RS
Rahman
A
Kefer
JC
Minshall
RD
Malik
AB
PKCzeta regulates TNF-alpha-induced activation of NADPH oxidase in endothelial cells.
Circ Res.
90
2002
1012
1019
202
Leitges
M
Sanz
L
Martin
P
et al
Targeted disruption of the zetaPKC gene results in the impairment of the NF-kappaB pathway.
Mol Cell.
8
2001
771
780
203
Scott
JA
Mehta
S
Duggan
M
Bihari
A
McCormack
DG
Functional inhibition of constitutive nitric oxide synthase in a rat model of sepsis.
Am J Respir Crit Care Med.
165
2002
1426
1432
204
Liu
SF
Adcock
IM
Old
RW
Barnes
PJ
Evans
TW
Differential regulation of the constitutive and inducible nitric oxide synthase mRNA by lipopolysaccharide treatment in vivo in the rat.
Crit Care Med.
24
1996
1219
1225
205
Ermert
M
Ruppert
C
Gunther
A
et al
Cell-specific nitric oxide synthase-isoenzyme expression and regulation in response to endotoxin in intact rat lungs.
Lab Invest.
82
2002
425
441
206
Laubach
VE
Shesely
EG
Smithies
O
Sherman
PA
Mice lacking inducible nitric oxide synthase are not resistant to lipopolysaccharide-induced death.
Proc Natl Acad Sci U S A.
92
1995
10688
10692
207
Shesely
EG
Maeda
N
Kim
HS
et al
Elevated blood pressures in mice lacking endothelial nitric oxide synthase.
Proc Natl Acad Sci U S A.
93
1996
13176
13181
208
Yamashita
T
Kawashima
S
Ohashi
Y
et al
Resistance to endotoxin shock in transgenic mice overexpressing endothelial nitric oxide synthase.
Circulation.
101
2000
931
937
209
Avontuur
JA
Boomsma
F
van den Meiracker
AH
de Jong
FH
Bruining
HA
Endothelin-1 and blood pressure after inhibition of nitric oxide synthesis in human septic shock.
Circulation.
99
1999
271
275
210
Szabo
C
Southan
GJ
Thiemermann
C
Beneficial effects and improved survival in rodent models of septic shock with S-methylisothiourea sulfate, a potent and selective inhibitor of inducible nitric oxide synthase.
Proc Natl Acad Sci U S A.
91
1994
12472
12476
211
Okamoto
I
Abe
M
Shibata
K
et al
Evaluating the role of inducible nitric oxide synthase using a novel and selective inducible nitric oxide synthase inhibitor in septic lung injury produced by cecal ligation and puncture.
Am J Respir Crit Care Med.
162
2000
716
722
212
Cobb
JP
Natanson
C
Hoffman
WD
et al
N omega-amino-L-arginine, an inhibitor of nitric oxide synthase, raises vascular resistance but increases mortality rates in awake canines challenged with endotoxin.
J Exp Med.
176
1992
1175
1182
213
Wiel
E
Pu
Q
Corseaux
D
et al
Effect of L-arginine on endothelial injury and hemostasis in rabbit endotoxin shock.
J Appl Physiol.
89
2000
1811
1818
214
Cerwinka
WH
Cooper
D
Krieglstein
CF
Feelisch
M
Granger
DN
Nitric oxide modulates endotoxin-induced platelet-endothelial cell adhesion in intestinal venules.
Am J Physiol Heart Circ Physiol.
282
2002
H1111
H1117
215
Cohen
J
Adjunctive therapy in sepsis: a critical analysis of the clinical trial programme.
Br Med Bull.
55
1999
212
225
216
Dinarello
CA
Abraham
E
Does blocking cytokines in sepsis work?
Am J Respir Crit Care Med.
166
2002
1156
1157
217
Marshall
JC
Complexity, chaos, and incomprehensibility: parsing the biology of critical illness.
Crit Care Med.
28
2000
2646
2648
218
Seely
AJ
Christou
NV
Multiple organ dysfunction syndrome: exploring the paradigm of complex nonlinear systems.
Crit Care Med.
28
2000
2193
2200
219
Grinnell
BW
Joyce
D
Recombinant human activated protein C: a system modulator of vascular function for treatment of severe sepsis.
Crit Care Med.
29
2001
S53
S60
220
Esmon
CT
New mechanisms for vascular control of inflammation mediated by natural anticoagulant proteins.
J Exp Med.
196
2002
561
564
221
Kollef
MH
Sherman
G
Ward
S
Fraser
VJ
Inadequate antimicrobial treatment of infections: a risk factor for hospital mortality among critically ill patients.
Chest.
115
1999
462
474
222
Opal
SM
Yu
RL
Jr
Antiendotoxin strategies for the prevention and treatment of septic shock. New approaches and future directions.
Drugs.
55
1998
497
508
223
Lynn
WA
Anti-endotoxin therapeutic options for the treatment of sepsis.
J Antimicrob Chemother.
41(suppl A)
1998
71
80
224
Verbon
A
Dekkers
PE
ten Hove
T
et al
IC14, an anti-CD14 antibody, inhibits endotoxin-mediated symptoms and inflammatory responses in humans.
J Immunol.
166
2001
3599
3605
225
Abraham
E
Anzueto
A
Gutierrez
G
et al
Double-blind randomised controlled trial of monoclonal antibody to human tumour necrosis factor in treatment of septic shock. NORASEPT II Study Group.
Lancet.
351
1998
929
933
226
Cohen
J
Carlet
J
INTERSEPT: an international, multicenter, placebo-controlled trial of monoclonal antibody to human tumor necrosis factor-alpha in patients with sepsis. International Sepsis Trial Study Group.
Crit Care Med.
24
1996
1431
1440
227
Zimmerman
GA
McIntyre
TM
Prescott
SM
Stafforini
DM
The platelet-activating factor signaling system and its regulators in syndromes of inflammation and thrombosis.
Crit Care Med.
30
2002
S294
S301
228
Fink
MP
Therapeutic options directed against platelet activating factor, eicosanoids and bradykinin in sepsis.
J Antimicrob Chemother.
41(suppl A)
1998
81
94
229
Fein
AM
Bernard
GR
Criner
GJ
et al
Treatment of severe systemic inflammatory response syndrome and sepsis with a novel bradykinin antagonist, deltibant (CP-0127). Results of a randomized, double-blind, placebo-controlled trial. CP-0127 SIRS and Sepsis Study Group.
JAMA.
277
1997
482
487
230
Jansen
PM
Pixley
RA
Brouwer
M
et al
Inhibition of factor XII in septic baboons attenuates the activation of complement and fibrinolytic systems and reduces the release of interleukin-6 and neutrophil elastase.
Blood.
87
1996
2337
2344
231
Bernard
GR
Wheeler
AP
Russell
JA
et al
The effects of ibuprofen on the physiology and survival of patients with sepsis. The Ibuprofen in Sepsis Study Group.
N Engl J Med.
336
1997
912
918
232
Fink
MP
Prostaglandins and sepsis: still a fascinating topic despite almost 40 years of research.
Am J Physiol Lung Cell Mol Physiol.
281
2001
L534
L536
233
Wang
H
Yang
H
Czura
CJ
Sama
AE
Tracey
KJ
HMGB1 as a late mediator of lethal systemic inflammation.
Am J Respir Crit Care Med.
164
2001
1768
1773
234
Wang
H
Bloom
O
Zhang
M
et al
HMG-1 as a late mediator of endotoxin lethality in mice.
Science.
285
1999
248
251
235
Fiuza C, Bustin M, Talwar S, et al. Inflammatory promoting activity of HMGB1 on human microvascular endothelial cells. Blood. Prepublished on November 27, 2002, as DOI 10.1182/blood-2002-05-1300.
236
Laudes
IJ
Chu
JC
Sikranth
S
et al
Anti-c5a ameliorates coagulation/fibrinolytic protein changes in a rat model of sepsis.
Am J Pathol.
160
2002
1867
1875
237
Czermak
BJ
Sarma
V
Pierson
CL
et al
Protective effects of C5a blockade in sepsis.
Nat Med.
5
1999
788
792
238
Basu
S
Dunn
AR
Marino
MW
et al
Increased tolerance to endotoxin by granulocyte-macrophage colony- stimulating factor-deficient mice.
J Immunol.
159
1997
1412
1417
239
Hamilton
JA
GM-CSF in inflammation and autoimmunity.
Trends Immunol.
23
2002
403
408
240
Lue
H
Kleemann
R
Calandra
T
Roger
T
Bernhagen
J
Macrophage migration inhibitory factor (MIF): mechanisms of action and role in disease.
Microbes Infect.
4
2002
449
460
241
Baugh
JA
Bucala
R
Macrophage migration inhibitory factor.
Crit Care Med.
30
2002
S27
S35
242
Selzman
CH
Shames
BD
Miller
SA
et al
Therapeutic implications of interleukin-10 in surgical disease.
Shock.
10
1998
309
318
243
Howard
M
Muchamuel
T
Andrade
S
Menon
S
Interleukin 10 protects mice from lethal endotoxemia.
J Exp Med.
177
1993
1205
1208
244
van der Poll
T
Jansen
PM
Montegut
WJ
et al
Effects of IL-10 on systemic inflammatory responses during sublethal primate endotoxemia.
J Immunol.
158
1997
1971
1975
245
Fisher
CJ
Jr
Opal
SM
Lowry
SF
et al
Role of interleukin-1 and the therapeutic potential of interleukin-1 receptor antagonist in sepsis.
Circ Shock.
44
1994
1
8
246
Dhainaut
JF
Tenaillon
A
Le Tulzo
Y
et al
Platelet-activating factor receptor antagonist BN 52021 in the treatment of severe sepsis: a randomized, double-blind, placebo-controlled, multicenter clinical trial. BN 52021 Sepsis Study Group.
Crit Care Med.
22
1994
1720
1728
247
Dhainaut
JF
Tenaillon
A
Hemmer
M
et al
Confirmatory platelet-activating factor receptor antagonist trial in patients with severe gram-negative bacterial sepsis: a phase III, randomized, double-blind, placebo-controlled, multicenter trial. BN 52021 Sepsis Investigator Group.
Crit Care Med.
26
1998
1963
1971
248
Huber-Lang
MS
Riedeman
NC
Sarma
JV
et al
Protection of innate immunity by C5aR antagonist in septic mice.
Faseb J.
16
2002
1567
1574
249
Lindner
JR
Kahn
ML
Coughlin
SR
et al
Delayed onset of inflammation in protease-activated receptor-2-deficient mice.
J Immunol.
165
2000
6504
6510
250
Vogel
SM
Gao
X
Mehta
D
et al
Abrogation of thrombin-induced increase in pulmonary microvascular permeability in PAR-1 knockout mice.
Physiol Genomics.
4
2000
137
145
251
Giebler
R
Schmidt
U
Koch
S
Peters
J
Scherer
R
Combined antithrombin III and C1-esterase inhibitor treatment decreases intravascular fibrin deposition and attenuates cardiorespiratory impairment in rabbits exposed to Escherichia coli endotoxin.
Crit Care Med.
27
1999
597
604
252
Caliezi
C
Zeerleder
S
Redondo
M
et al
C1-inhibitor in patients with severe sepsis and septic shock: beneficial effect on renal dysfunction.
Crit Care Med.
30
2002
1722
1728
253
Armour
J
Tyml
K
Lidington
D
Wilson
JX
Ascorbate prevents microvascular dysfunction in the skeletal muscle of the septic rat.
J Appl Physiol.
90
2001
795
803
254
Heller
AR
Groth
G
Heller
SC
et al
N-acetylcysteine reduces respiratory burst but augments neutrophil phagocytosis in intensive care unit patients.
Crit Care Med.
29
2001
272
276
255
Cronin
L
Cook
DJ
Carlet
J
et al
Corticosteroid treatment for sepsis: a critical appraisal and meta-analysis of the literature.
Crit Care Med.
23
1995
1430
1439
256
Bradley
C
Steroids in sepsis—more effective than activated protein C?
Intensive Crit Care Nurs.
17
2001
242
244
257
Carlet
J
From mega to more reasonable doses of corticosteroids: a decade to recreate hope.
Crit Care Med.
27
1999
672
674
258
Abraham
E
Reinhart
K
Svoboda
P
et al
Assessment of the safety of recombinant tissue factor pathway inhibitor in patients with severe sepsis: a multicenter, randomized, placebo-controlled, single-blind, dose escalation study.
Crit Care Med.
29
2001
2081
2089
259
Eisele
B
Lamy
M
Thijs
LG
et al
Antithrombin III in patients with severe sepsis. A randomized, placebo-controlled, double-blind multicenter trial plus a meta-analysis on all randomized, placebo-controlled, double-blind trials with antithrombin III in severe sepsis.
Intensive Care Med.
24
1998
663
672
260
Pu
Q
Wiel
E
Corseaux
D
et al
Beneficial effect of glycoprotein IIb/IIIa inhibitor (AZ-1) on endothelium in Escherichia coli endotoxin-induced shock.
Crit Care Med.
29
2001
1181
1188
261
Parker
RI
IIb/IIIa or not IIb/IIIa, that is adhesion.
Crit Care Med.
29
2001
1286
1287
262
Taylor
FB
Coller
BS
Chang
AC
et al
7E3 F(ab')2, a monoclonal antibody to the platelet GPIIb/IIIa receptor, protects against microangiopathic hemolytic anemia and microvascular thrombotic renal failure in baboons treated with C4b binding protein and a sublethal infusion of Escherichia coli.
Blood.
89
1997
4078
4084
263
Ferri
LE
Swartz
D
Christou
NV
Soluble L-selectin at levels present in septic patients diminishes leukocyte-endothelial cell interactions in mice in vivo: a mechanism for decreased leukocyte delivery to remote sites in sepsis.
Crit Care Med.
29
2001
117
122
264
Taylor
FB
Jr
Chang
A
Ruf
W
et al
Lethal E coli septic shock is prevented by blocking tissue factor with monoclonal antibody.
Circ Shock.
33
1991
127
134
265
Biemond
BJ
Levi
M
ten Cate
H
et al
Complete inhibition of endotoxin-induced coagulation activation in chimpanzees with a monoclonal Fab fragment against factor VII/VIIa.
Thromb Haemost.
73
1995
223
230
266
Zenz
W
Bodo
Z
Zobel
G
Fanconi
S
Rettenbacher
A
Recombinant tissue plasminogen activator restores perfusion in meningococcal purpura fulminans.
Crit Care Med.
26
1998
969
971
discussion 972-963.
267
Umetani
M
Nakao
H
Doi
T
et al
A novel cell adhesion inhibitor, K-7174, reduces the endothelial VCAM-1 induction by inflammatory cytokines, acting through the regulation of GATA.
Biochem Biophys Res Commun.
272
2000
370
374
268
Frank
JA
Gutierrez
JA
Jones
KD
et al
Low tidal volume reduces epithelial and endothelial injury in acid-injured rat lungs.
Am J Respir Crit Care Med.
165
2002
242
249
269
Hasday
JD
Bannerman
D
Sakarya
S
et al
Exposure to febrile temperature modifies endothelial cell response to tumor necrosis factor-alpha.
J Appl Physiol.
90
2001
90
98
270
Chettab
K
Zibara
K
Belaiba
SR
McGregor
JL
Acute hyperglycaemia induces changes in the transcription levels of 4 major genes in human endothelial cells: macroarrays-based expression analysis.
Thromb Haemost.
87
2002
141
148
271
Furuno
T
Mitsuyama
T
Hidaka
K
Tanaka
T
Hara
N
The role of neutrophil elastase in human pulmonary artery endothelial cell injury.
Int Arch Allergy Immunol.
112
1997
262
269
272
MacGregor
IR
Perrie
AM
Donnelly
SC
Haslett
C
Modulation of human endothelial thrombomodulin by neutrophils and their release products.
Am J Respir Crit Care Med.
155
1997
47
52
273
Szabo
C
Cuzzocrea
S
Zingarelli
B
O'Connor
M
Salzman
AL
Endothelial dysfunction in a rat model of endotoxic shock. Importance of the activation of poly (ADP-ribose) synthetase by peroxynitrite.
J Clin Invest.
100
1997
723
735
274
Hassa
PO
Hottiger
MO
The functional role of poly(ADP-ribose)polymerase 1 as novel coactivator of NF-kappaB in inflammatory disorders.
Cell Mol Life Sci.
59
2002
1534
1553
275
Oliver
FJ
Menissier-de Murcia
J
Nacci
C
et al
Resistance to endotoxic shock as a consequence of defective NF-kappaB activation in poly (ADP-ribose) polymerase-1 deficient mice.
Embo J.
18
1999
4446
4454
276
Rahman
A
Anwar
KN
Uddin
S
et al
Protein kinase C-delta regulates thrombin-induced ICAM-1 gene expression in endothelial cells via activation of p38 mitogen-activated protein kinase.
Mol Cell Biol.
21
2001
5554
5565
277
Michiels
C
Arnould
T
Remacle
J
Endothelial cell responses to hypoxia: initiation of a cascade of cellular interactions.
Biochim Biophys Acta.
1497
2000
1
10
278
Faller
DV
Endothelial cell responses to hypoxic stress.
Clin Exp Pharmacol Physiol.
26
1999
74
84
279
Pinsky
DJ
Liao
H
Lawson
CA
et al
Coordinated induction of plasminogen activator inhibitor-1 (PAI-1) and inhibition of plasminogen activator gene expression by hypoxia promotes pulmonary vascular fibrin deposition.
J Clin Invest.
102
1998
919
928
280
Pinsky
DJ
Yan
SF
Lawson
C
et al
Hypoxia and modification of the endothelium: implications for regulation of vascular homeostatic properties.
Semin Cell Biol.
6
1995
283
294
281
Tsao
PS
Buitrago
R
Chan
JR
Cooke
JP
Fluid flow inhibits endothelial adhesiveness. Nitric oxide and transcriptional regulation of VCAM-1.
Circulation.
94
1996
1682
1689
282
Traub
O
Berk
BC
Laminar shear stress: mechanisms by which endothelial cells transduce an atheroprotective force.
Arterioscler Thromb Vasc Biol.
18
1998
677
685
283
Surapisitchat
J
Hoefen
RJ
Pi
X
et al
Fluid shear stress inhibits TNF-alpha activation of JNK but not ERK1/2 or p38 in human umbilical vein endothelial cells: inhibitory crosstalk among MAPK family members.
Proc Natl Acad Sci U S A.
98
2001
6476
6481
284
Hojo
Y
Saito
Y
Tanimoto
T
et al
Fluid shear stress attenuates hydrogen peroxide-induced c-Jun NH2- terminal kinase activation via a glutathione reductase-mediated mechanism.
Circ Res.
91
2002
712
718
285
Conway
EM
Van de Wouwer
M
Pollefeyt
S
et al
The lectin-like domain of thrombomodulin confers protection from neutrophil-mediated tissue damage by suppressing adhesion molecule expression via nuclear factor kappaB and mitogen-activated protein kinase pathways.
J Exp Med.
196
2002
565
577
286
Karmpaliotis
D
Kosmidou
I
Ingenito
EP
et al
Angiogenic growth factors in the pathophysiology of a murine model of acute lung injury.
Am J Physiol Lung Cell Mol Physiol.
283
2002
L585
L595
287
Schmaier
AH
The plasma kallikrein-kinin system counterbalances the renin-angiotensin system.
J Clin Invest.
109
2002
1007
1009
288
Colman
RW
The contact system: a proinflammatory pathway with antithrombotic activity.
Nat Med.
4
1998
277
278
289
Pixley
RA
De La Cadena
R
Page
JD
et al
The contact system contributes to hypotension but not disseminated intravascular coagulation in lethal bacteremia. In vivo use of a monoclonal anti-factor XII antibody to block contact activation in baboons.
J Clin Invest.
91
1993
61
68
290
Chiron announces results of phase III study of Tifacogin in severe sepsis [press release]. Emeryville, CA: Chiron. November 21, 2001.
291
Riewald
M
Petrovan
RJ
Donner
A
Mueller
BM
Ruf
W
Activation of endothelial cell protease activated receptor 1 by the protein C pathway.
Science.
296
2002
1880
1882
292
Cheng
T
Liu
D
Griffin
JH
et al
Activated protein C blocks p53-mediated apoptosis in ischemic human brain endothelium and is neuroprotective.
Nat Med.
9
2003
338
342

Note added in proof

A recent study demonstrated that activated protein C signals through PAR-1 in cultured endothelial cells, by an EPCR-dependent mechanism.291 Consistent with these results, both EPCR and PAR-1 were shown to be required for mediating the cytoprotective function of activated protein C in hypoxic cultured human brain endothelial cells and in a stroke model of mice.292Collectively, these findings suggest that activated protein C signals through the PAR-1 receptor both in vitro and in vivo. Since PAR-1 is also a receptor for thrombin, these studies raise interesting questions as to how two distinct ligands, namely activated protein C and thrombin, mediate opposing PAR-1 responses (eg, protective and proinflammatory responses, respectively).

Author notes

William C. Aird, Molecular Medicine, Beth Israel Deaconess Medical Center, RW-663, 330 Brookline Ave, Boston, MA 02215; e-mail: waird@caregroup.harvard.edu.