Abstract

Infection with the human immunodeficiency virus (HIV) is associated with a progressive decrease in CD4 T-cell number and a consequent impairment in host immune defenses. Analysis of T cells from patients infected with HIV, or of T cells infected in vitro with HIV, demonstrates a significant fraction of both infected and uninfected cells dying by apoptosis. The many mechanisms that contribute to HIV-associated lymphocyte apoptosis include chronic immunologic activation; gp120/160 ligation of the CD4 receptor; enhanced production of cytotoxic ligands or viral proteins by monocytes, macrophages, B cells, and CD8 T cells from HIV-infected patients that kill uninfected CD4 T cells; and direct infection of target cells by HIV, resulting in apoptosis. Although HIV infection results in T-cell apoptosis, under some circumstances HIV infection of resting T cells or macrophages does not result in apoptosis; this may be a critical step in the development of viral reservoirs. Recent therapies for HIV effectively reduce lymphoid and peripheral T-cell apoptosis, reduce viral replication, and enhance cellular immune competence; however, they do not alter viral reservoirs. Further understanding the regulation of apoptosis in HIV disease is required to develop novel immune-based therapies aimed at modifying HIV-induced apoptosis to the benefit of patients infected with HIV.

Introduction

Patients infected with the human immunodeficiency virus (HIV) experience a progressive decline in CD4 T-cell number, resulting in immunodeficiency and increased susceptibility to opportunistic infections and malignancies. Although CD4 T-cell production is impaired in patients infected with HIV,1there is now overwhelming evidence that the primary basis of T-cell depletion in patients infected with HIV is increased apoptosis of CD4 and CD8 T cells. Since it was first proposed as a potential mechanism of CD4 T-cell depletion in patients infected with HIV,2apoptosis and its dysregulation after HIV infection has become a major focus of research. Although apoptosis may result from the effects of continuous immune activation that occurs in HIV-infected patients, considerable data indicate that there are additional distinct mechanisms by which HIV (and HIV-specific proteins) enhances apoptosis. Importantly, only a minor fraction of apoptotic lymphocytes are physically infected by HIV, indicating that the enhanced apoptosis of lymphocytes seen in infected persons results from mechanism(s) other than direct infection. Thus, understanding of the mechanisms of HIV-associated lymphocyte apoptosis may lead to new and more effective therapies for HIV disease and acquired immunodeficiency syndrome.

Overview of HIV-associated lymphocyte apoptosis

Chronic uncontrolled infections provide continuous antigenic stimulation that causes persistent immune activation and consequent apoptosis. This is the mechanism by which infectious diseases, such as cytomegalovirus, cause enhanced apoptosis and lymphopenia. Chronic HIV infection provides a chronic immunologic stimulus; however, it may be unique in its ability to induce lymphocyte apoptosis through direct or indirect mechanism(s) that are distinct from immune activation alone. Although numerous pathogenic viruses have developed mechanisms to prevent apoptosis of host cells, no such antiapoptotic machinery is present in HIV. Indeed, HIV-encoded proteins may induce apoptosis of infected cells and uninfected cells (ie, paracrine death) through various mechanisms, some of which are defined; others are as yet unidentified (Table 1).

Table 1.

Proposed mechanisms of HIV-associated lymphocyte apoptosis

Effector Proposed mechanism Target cell  
HIV Tat Enhanced Fas sensitivity Infected +  
 Enhanced Fas ligand production  uninfected cells  
HIV Nef Activation Infected +  
 Enhanced FasL production  uninfected cells  
 ? Binding to unidentified receptor  
HIV vpr Cell cycle arrest Infected +  
 Direct effect on mitochondrial permeability  uninfected cells  
HIV protease Cleavage of host structural proteins Infected cells  
Activation-induced cell death HIV-associated activation Uninfected cells 
 Increased TRAIL/APO-2L, FasL, or both  
gp 120/160 Inappropriate activation after CD4 ligation Uninfected cells  
 Enhanced Fas susceptibility/FasL production  
 ? Nonapoptotic death by CXCR4  
Autologous cell–mediated killing Enhanced production of cytotoxic ligands by HIV-infected cells Uninfected cells 
Effector Proposed mechanism Target cell  
HIV Tat Enhanced Fas sensitivity Infected +  
 Enhanced Fas ligand production  uninfected cells  
HIV Nef Activation Infected +  
 Enhanced FasL production  uninfected cells  
 ? Binding to unidentified receptor  
HIV vpr Cell cycle arrest Infected +  
 Direct effect on mitochondrial permeability  uninfected cells  
HIV protease Cleavage of host structural proteins Infected cells  
Activation-induced cell death HIV-associated activation Uninfected cells 
 Increased TRAIL/APO-2L, FasL, or both  
gp 120/160 Inappropriate activation after CD4 ligation Uninfected cells  
 Enhanced Fas susceptibility/FasL production  
 ? Nonapoptotic death by CXCR4  
Autologous cell–mediated killing Enhanced production of cytotoxic ligands by HIV-infected cells Uninfected cells 

Overview of the regulation of apoptosis

Apoptosis is a highly regulated and coordinated cellular death process that is essential for cellular homeostasis. Alterations in the regulation of apoptosis may lead to malignancies,3immunodeficiencies,4 and autoimmune phenomena.5 

Apoptosis regulatory proteins

Many elements influence whether a cell will undergo apoptosis6 (Figure 1). Four cellular receptors induce apoptosis after ligation; they are the Fas receptor,7 p55 tumor necrosis factor (TNF) receptor,8 and TRAIL/APO 2-L (TNF-related apoptosis-inducing ligand) receptors 1 and 2.9 Fas Ligand (FasL), TNF, and TRAIL/APO 2-L, respectively, bind these receptors to initiate apoptosis. In the case of FasL and TNF, membrane-associated proteins may be cleaved by the action of matrix metalloproteases to release soluble ligands that maintain their biologic activity.10-12 It is unknown whether TRAIL/APO 2-L exists as a soluble molecule. Ligation of these death receptors recruits the adaptor proteins FADD (Fas-associated death domain)13-15TRADD (TNF receptor-associated death domain), or both,16,17 which sequentially activate a family of cysteine proteases that cleave at aspartate residues (cysteine-dependent, aspartate-specific protease), or caspases. Caspases are synthesized as inactive zymogens and become activated after proteolytic removal of a terminal prodomain.18-20 Fourteen mammalian caspase family members have been identified, each with varying involvements in the regulation of apoptosis. For example, caspase 8 (FLICE)21-23 and caspase 3 (CPP32)24-26 are involved in apoptosis mediated by Fas, p55 TNF receptor, and TRAIL/APO 2-L receptor ligation. Activated caspases catalyze the cleavage of other caspases, which, in turn, activate various cellular proteases and endonucleases that cleave host cell structural and regulatory proteins and host nuclear DNA,27 ultimately causing the cell to undergo the morphologic and biochemical changes that are characteristic of apoptosis.28 

Fig. 1.

Schematic overview of the regulation of apoptosis.

Fig. 1.

Schematic overview of the regulation of apoptosis.

In addition to receptor-mediated apoptosis, other stimuli (eg, chemotherapy, ultraviolet radiation, and ionizing radiation) induce changes in mitochondria that include opening of the permeability transition pore and loss of mitochondrial inner transmembrane potential, which allows the release of apoptosis regulatory proteins (including cytochrome c, Apaf-1, and caspase 9)29-32 that initiate further caspase activation, ultimately leading to apoptosis. Although classical Fas-induced apoptosis (see above) involves direct caspase activation without mitochondrial involvement (type 1), in certain cell types Fas-induced apoptosis may also require mitochondrial activation (type 2).7 

Antiapoptosis regulatory molecules

In addition to the proteins involved in mediating apoptosis described above, other proteins act to inhibit apoptosis. One such family of regulatory proteins is cellular FLICE-like inhibitory protein (c-FLIP), which inhibits apoptosis by binding to FADD and thus prevents the activation of caspase 8.33,34 The inhibitor of apoptosis proteins (IAP) family, including HIAP, XIAP, and others, acts by inhibiting the activation of caspase 3 and possibly other caspases.35-37 

Bcl2 and related family members,38,39 including BclXS, BclXL, Bad, and Bax, influence apoptosis by regulating the intracellular signals that induce apoptosis. Some family members (Bcl2) are antiapoptotic, whereas others (Bax) are proapoptotic. Cells that contain a predominance of proapoptotic Bcl2 family molecules promote apoptosis, and cells with a predominance of antiapoptotic Bcl2 family proteins are relatively apoptosis resistant. Bcl2 consistently blocks apoptosis induced by anticancer and nitric oxide,41 and these effects may result from the inhibition of calcineurin activation,40,42,43 NFAT activation,40 or transcription of Fas ligand.40 Conversely, reports on the effects of Bcl2 on Fas-induced apoptosis are conflicting: Bcl2 may variably inhibit44 or not inhibit45Fas-induced death. Because members of the Bcl2 family are principally localized within mitochondria, their influence may be greatest in forms of apoptosis that are associated with mitochondrial activation. Thus, Bcl2 overexpression may not inhibit death receptor–initiated apoptosis in cells with a type 1 (mitochondria-independent) Fas pathway, but it may block Fas-initiated death in type 2 (mitochondria-dependent) cells.46 

Physiologic T-cell apoptosis

Healthy subjects orchestrate a physiologic immune response to a foreign antigen by T-cell activation and proliferation. If this T-cell proliferative response were not regulated, each encounter with a foreign antigen would lead to unending T-cell expansion. Down-regulation of T-cell proliferation occurs by an apoptotic program that is initiated after activation47(Figure 2, top). After T-cell activation, c-FLIP expression is reduced, and the cells become susceptible to Fas ligation and to caspase 8–mediated apoptosis.33 Exposure to a second activation stimulus (eg, CD3 stimulation in the absence of CD28 costimulation) promotes de novo production of FasL, leading to both autocrine and paracrine Fas/FasL-mediated T-cell apoptosis.48-52 It is important to note that not all physiologic T-cell apoptosis is regulated solely by Fas/FasL interactions; Fas-deficient cells maintain T-cell receptor (CD3)–induced apoptosis that is inhibited by TNF antagonists.53,54 

Fig. 2.

T-cell apoptosis.

Mechanisms of physiologic T-cell apoptosis (top) and mechanisms of increased T-cell apoptosis associated with HIV infection (bottom). VAD refers to the pan-caspase inhibitor 2-VAD-Fmk.

Fig. 2.

T-cell apoptosis.

Mechanisms of physiologic T-cell apoptosis (top) and mechanisms of increased T-cell apoptosis associated with HIV infection (bottom). VAD refers to the pan-caspase inhibitor 2-VAD-Fmk.

Measurement of apoptosis

As noted, apoptosis is characterized by distinct morphologic and biochemical changes, including chromatin condensation, shrinkage of the cytoplasm, membrane blebbing, and formation of apoptotic bodies. Apoptosis is a complex and sequential process, and, as such, some assays detect changes that occur early, whereas other assays detect later events. The most common assays used in the detection of apoptosis are listed in Table 2 55-99; many have been used to evaluate apoptosis in patients infected with HIV. In a direct comparison of the relative benefits of these assays for use in the evaluation of apoptosis of HIV-infected patients, TUNEL staining was the most specific and therefore may be the most accurate assay to use in this patient population.100 

Table 2.

Assays of apoptosis and their relationship to events of apoptosis

Event Assays Detection  
Changes in nuclear morphology: DNA stains (DAPI) Microscopy  
 Chromatin condensation, segmentation,   
  and formation of apoptotic bodies   
Changes in membrane permeability Vital dyes (PI) Microscopy  
 Permeable DNA stains: (DAPI, Hoechst 33258) Flow cytometry with simultaneous size determination 
Changes in membrane composition: Annexin V binding Flow cytometry  
 Externalization of phosphatidylserine  Confocal and epifluorescence microscopy  
Cleavage of nuclear proteins Poly ADP ribose polymerase Western blot  
Mitochondrial function and integrity   
 Changes in permeability transition (ΔΨm) Vital dyes (DiOC6, JC-1) Flow cytometry 
 Accessibility to mitochondrial antigens Apo 2,7 antibody Flow cytometry  
 Release of cytochrome-c Anti–cytochrome-c antibody Flow cytometry, Western blot  
 Production of free radicals DPPP/dihydroethidium Flow cytometry  
Caspase activation   
 Detection of caspase cleavage product Known caspase substrates; PARP, caspase 3, caspase 8, DNA-PK, PK-C Western blot  
 Detection of active caspase Anti–activated caspase 3 antibody Western blot 
 Detection of caspase activity Cleavage of fluorescent or colorimetric substrate(s) Fluorometer, plate reader  
DNA degradation   
 Large fragments DNA stains (EtBr, SYBR green) Pulse-field gel electrophoresis  
 DNA stains (EtBr) Comet  
 Radioactivity (C14Detection of radio-labeled DNA by filter binding  
 Small fragments DNA stains (EtBr) Agarose gel electrophoresis (DNA ladder) 
 Radioactivity (C14Detection of radio-labeled DNA by filter binding  
 Sub-G1 peak detection DNA stains (PI, Hoechst) Flow cytometry  
Detection of DNA strand breaks Terminal dUTP nick end labeling (TUNEL) In situ hybridization  
  Flow cytometry  
 Ligation-mediated polymerase chain reaction Agarose or polyacrylamide gel electrophoresis 
Event Assays Detection  
Changes in nuclear morphology: DNA stains (DAPI) Microscopy  
 Chromatin condensation, segmentation,   
  and formation of apoptotic bodies   
Changes in membrane permeability Vital dyes (PI) Microscopy  
 Permeable DNA stains: (DAPI, Hoechst 33258) Flow cytometry with simultaneous size determination 
Changes in membrane composition: Annexin V binding Flow cytometry  
 Externalization of phosphatidylserine  Confocal and epifluorescence microscopy  
Cleavage of nuclear proteins Poly ADP ribose polymerase Western blot  
Mitochondrial function and integrity   
 Changes in permeability transition (ΔΨm) Vital dyes (DiOC6, JC-1) Flow cytometry 
 Accessibility to mitochondrial antigens Apo 2,7 antibody Flow cytometry  
 Release of cytochrome-c Anti–cytochrome-c antibody Flow cytometry, Western blot  
 Production of free radicals DPPP/dihydroethidium Flow cytometry  
Caspase activation   
 Detection of caspase cleavage product Known caspase substrates; PARP, caspase 3, caspase 8, DNA-PK, PK-C Western blot  
 Detection of active caspase Anti–activated caspase 3 antibody Western blot 
 Detection of caspase activity Cleavage of fluorescent or colorimetric substrate(s) Fluorometer, plate reader  
DNA degradation   
 Large fragments DNA stains (EtBr, SYBR green) Pulse-field gel electrophoresis  
 DNA stains (EtBr) Comet  
 Radioactivity (C14Detection of radio-labeled DNA by filter binding  
 Small fragments DNA stains (EtBr) Agarose gel electrophoresis (DNA ladder) 
 Radioactivity (C14Detection of radio-labeled DNA by filter binding  
 Sub-G1 peak detection DNA stains (PI, Hoechst) Flow cytometry  
Detection of DNA strand breaks Terminal dUTP nick end labeling (TUNEL) In situ hybridization  
  Flow cytometry  
 Ligation-mediated polymerase chain reaction Agarose or polyacrylamide gel electrophoresis 

HIV-mediated alterations in molecules that regulate the apoptotic process

Cells obtained from HIV-infected patients and cells infected with HIV in vitro show changes in the regulation of Fas and Fas ligand (reviewed in101). Acute HIV infection of the promonocytic cell line U937 is associated with viral replication-dependent apoptosis102 that is characterized by the increased membrane expression of Fas102 and FasL,102 by the down-regulation of antiapoptotic proteins Bcl2 and BclXL,103,104 and by a concomitant increase in proapoptotic BclXS and Bax.103,104 The hypothesis that Fas/FasL interactions may be responsible for HIV-induced apoptosis is supported by the observation that soluble Fas receptor decoys block HIV-associated death in U937 cells.102 This is in marked contrast to the effects of acute HIV infection of T-cell lines, which is Fas independent despite increased Fas expression.105-108 Interestingly, though T cells from HIV-infected patients have altered expression of Bcl2, the expression of Bax, BclXL, and BclXS does not differ from that of uninfected controls.109 

T cells from HIV-infected patients exhibit both increased Fas receptor expression and enhanced susceptibility to Fas-mediated death.110-117 FasL is elevated in peripheral blood mononuclear cells (which contain monocytes)114,118,119from HIV-infected patients, and the plasma level of soluble FasL is increased in HIV-positive patients and correlates with HIV RNA burden.120 The demonstrated increases in Fas expression, Fas susceptibility, and Fas ligand expression suggest that these molecules may be important in some forms (see below) of HIV-induced cell death, though direct T-cell killing is independent of Fas.105-108 

Intracellular levels of c-FLIP in resting cells from HIV-negative patients decrease after activation, resulting in enhanced sensitivity to Fas-mediated apoptosis.33 This observation, coupled with observations that apoptosis in patients infected with HIV occurs in activated CD45 RO+, HLA-DR+, CD28 cells,121-124 suggests that decreased c-FLIP expression may be responsible for the enhanced susceptibility of cells from these patients to apoptosis. However, c-FLIP expression in bulk peripheral blood lymphocytes (PBL) or in purified CD4 or CD8 T cells from HIV-infected patients does not differ from that of HIV-negative patients.125 It remains possible that defined cellular subsets may have reduced levels of c-FLIP that are missed in bulk analysis.

The regulation of TNF, TNF receptors, or both is fundamentally altered in HIV-infected patients.126-129 Both cognate receptors for TNF, p75 TNFR and p55 TNFR,130 are expressed in a variety of cell types. However, only ligation of the p55 TNF receptor leads to apoptosis.16,17,53,131,132 Elevated serum TNF levels are seen in symptomatic HIV-infected patients126-129 but not in asymptomatic patients.133,134 Furthermore, (1) HIV infection of lymphocytes or monocytes results in TNF production,135,136and (2) TNF activates the transcription factor NFkB, which, in turn, activates HIV transcription,137,138 initiating an autocrine loop that results in high levels of TNF production and increased levels of HIV transcription. In addition, elevated serum levels of soluble p75 TNFR are predictive of HIV disease progression, independent of other immunologic or virologic prognostic markers.139 Although little is known of the ability of TNF to induce apoptosis in HIV-infected cells, HIV-infected macrophage-mediated killing of uninfected CD4 T-cell blasts (see below) can be partially reduced by the administration of soluble TNFR decoys,40 and TNF may contribute to apoptosis induced by gp120-mediated cross-linking of CD4138 (see below). The potential role of TNF as a mediator of HIV disease has prompted trials of anti-TNF therapy to retard HIV disease progression. Thalidomide reduces TNF secretion,141 and pentoxifylline reduces TNF mRNA half-life.138 However, clinical trials with each of these agents have consistently failed to show improvement in either immunologic or virologic outcomes.142-144 Other studies using soluble TNF antagonists have had similarly disappointing results.145 

There is also relatively little information concerning the potential role of TRAIL/APO 2-L in apoptosis in HIV-infected patients. Current data suggest that TRAIL/APO 2-L can bind to 1 of 5 receptors, TRAIL/APO 2-L-R1, TRAIL/APO 2-L-R2, TRAIL/APO 2-L-R3, TRAIL/APO 2-L-R4,146 and osteoprotegerin.147Binding of TRAIL/APO 2-L to TRAIL/APO 2-L-R1 or R2 transduces apoptotic signals, whereas binding to TRAIL/APO 2-L-R3 or TRAIL/APO 2-L-R4 does not. The effects of TRAIL/APO 2-L binding to osteoprotegerin are unknown. Although it has been suggested that the relative expression of TRAIL/APO 2-L-R3 and TRAIL/APO 2-L-R4 to TRAIL/APO 2-L-R1 and TRAIL/APO 2-L-R2 influences susceptibility to TRAIL/APO 2-L–mediated killing,148-150 recent studies do not support this hypothesis. Rather, intracellular levels of c-FLIP may correlate with the sensitivity or resistance to TRAIL/APO 2-L–induced apoptosis in target cells.151,152 

Although no studies to date have evaluated the relative expression of TRAIL/APO 2-L receptor(s) or TRAIL/APO 2-L expression in patients infected with HIV, it has been observed that (in contrast to cells from HIV-uninfected patients) cells from HIV-infected patients are susceptible to TRAIL/APO 2-L–mediated killing.153 This finding, together with the fact that activation-induced cell death in patients with HIV infection may be partially inhibited using antagonistic TRAIL/APO 2-L–specific antibodies,154suggests that TRAIL/APO 2-L and TRAIL/APO 2-L receptor dysfunction may contribute to HIV pathogenesis.

Apoptosis of uninfected and infected T cells induced by HIV proteins

HIV infection is associated with enhanced apoptosis in CD4 T cells infected by HIV and in uninfected T cells. In this section we review proposed mechanisms of CD4 T-cell apoptosis, focusing on whether the proposed mechanisms affect infected cells, uninfected cells, or both.

Gp120-induced apoptosis

Gp120 is an HIV viral envelope glycoprotein that can bind to and cross-link the CD4 receptor and the chemokine coreceptors. Cross-linking of CD4 T cells by gp120 causes the induction of enhanced susceptibility to Fas-mediated killing.155 In previously activated cells, gp120 cross-linking results in apoptosis156 (possibly mediated by IFN-γ, TNF, or both157), down-regulation of Bcl-2 expression,158 and activation of caspase 3.159 The apoptotic response to gp 120 is almost completely inhibited by soluble CD4 and by anti-gp120 antibodies.160 Further evidence for the specificity of this interaction is provided by the observation that a point mutation in the V3 loop of gp120 inhibits the induction of apoptosis in CD4 T cells.161 Finally, this interaction must also involve CD4 signaling because deletion or mutation of the intracytoplasmic portion of CD4 also abrogates the apoptotic response.162,163 

Most of the experiments involving gp120-induced apoptosis evaluate apoptosis that occurs after several days. However, recent reports164-166 show that gp120 cross-linking of CD4 and CXCR4 chemokine receptor results in nonapoptotic death within several hours of stimulation by a mechanism that appears to be independent of p56LCK,164 g-protein–coupled signaling,166Fas, or TNF receptors.165 The administration of CXCR4 antagonists blocks this apoptosis response to the HIV envelope.167 

It is appealing to invoke gp120 as a responsible mechanism for CD4 T-cell death in patients infected with HIV because it doesnot depend on the infection of all cells that become apoptotic and it does not require the presence of viable virions. Circulating immune complexes and replication-incompetent viruses that contain gp120 can induce death in a similar manner.168-170 

Apoptosis induced by other HIV proteins

Transfection experiments demonstrate that the ectopic expression of HIV Tat induces apoptosis. Further, gp120/160-deleted HIV maintains its ability to induce infected cell apoptosis, potentially because of the Tat-directed up-regulation of caspase 8171 or because of Fas ligand.172 Importantly, Tat has also been implicated as an inducer of apoptosis in uninfected T cells, potentially by Fas-dependent mechanisms, superoxide dismutase inhibition, or activation of cyclin-dependent kinases.173-175 The ability of Tat to induce uninfected cell death has also been demonstrated in vitro for neurons, lymphocytes, and CD4 T-cell lines. Its clinical relevance is suggested by observations that Tat is readily secreted by infected cells176 and cellular or humoral immunity to Tat may have protective effects against HIV disease progression.177 

Because Nef is essential for viral pathogenicity, HIV-encoded Nef has been suggested as a potential mediator of apoptosis.178This proposal is supported by the following findings: (1) human infection with naturally occurring Nef deletion mutants leads to less rapid CD4 T-cell depletion (compared to strains with Nef),179,180 though the differences may be related to the decreased efficiency of viral replication181-184; (2) Nef synergistically enhances the activating effects of T-cell receptor ligation,185-187 though this enhancement may be stimulus dependent187-191; (3) Nef-expressing T cells coexpress FasL,192 as do infected T cells from SIV-infected macaques but not T cells from macaques infected with similar strains of SIV that contain mutations within the Nef gene193 (the mechanism(s) by which Nef results in activation and FasL production remain unclear, yet mutational analysis indicates that the carboxy terminus of the CD4 receptor associates with both Nef and p56LCK194); lastly, (4) Nef may exert an apoptotic effect on uninfected CD4 T cells by binding to unidentified receptor(s),195 resulting in Fas-independent death.196 In this regard, Nef may induce apoptosis of infected and uninfected cells.

HIV-encoded vpr also has the ability to induce apoptosis through transfection and exogenous treatment. Proposed mechanisms include the induction of G2/M cell cycle arrest197,198 and a direct effect on mitochondrial permeability.199 Vpr also influences viral LTR transcription,200,201 cellular activation, and differentiation,202,203 suggesting a role in the development of HIV reservoirs. The seeming paradox of inducing apoptosis while promoting viral reservoirs is elucidated by data that vpr may, in certain situations, inhibit apoptosis.204-206The observation that virion-associated vpr acts as an immediate early viral protein to induce apoptosis207 is inconsistent with the apparent requirement that viral replication must occur before the onset of infected T-cell apoptosis. In addition, the finding that direct HIV-induced T-cell apoptosis occurs in all phases of the cell cycle107 brings into question the role of vpr in direct infection apoptosis. Vpr is more likely to be involved in regulation of latency, control of replication, and resistance to antiretroviral agents.208 

Knowledge that HIV-encoded protease is a cytotoxic protein that leads to apoptosis in human and bacterial cells after transfection209-212 has been exploited as a method of screening compounds for potential HIV protease inhibitory activity.213 However, the relevance of HIV protease to HIV-infected T-cell death in vitro and in vivo is unknown. HIV protease expression (by Western blotting) correlates with the presence of apoptosis in vitro and in vivo.214 Further studies demonstrate that HIV protease directly cleaves caspase 8214 and modifies cellular susceptibility to apoptosis by virtue of proteolytic degradation of the antiapoptotic protein Bcl2.215 Together these findings indicate that HIV protease may also play a role in the death of HIV-infected T cells. There are no data to suggest that HIV protease may influence the death of uninfected cells.

Indirect mechanisms of HIV-associated apoptosis

In addition to apoptosis induced directly by HIV proteins, HIV infection may induce T-cell apoptosis through indirect mechanisms, including activation-induced cell death and autologous infected cell–mediated killing. The indirect mechanisms of T-cell death mediate the deaths principally of uninfected T cells (Figure 2).

Activation-induced cell death

T cells obtained from HIV-infected patients undergo spontaneous apoptosis at a greater rate than cells from HIV-seronegative subjects.111,216-219 Furthermore, the ex vivo activation of CD4 T cells from HIV-infected patients (using a variety of stimuli) consistently enhances apoptosis compared with cells from uninfected subjects.111,122,124,217-219 This phenomenon, termed activation-induced cell death (AICD), occurs only in cells that have been previously activated,49,52 and it may represent the in vitro model of the effects of repeated antigenic stimulation.49,52,220,221 Naive peripheral blood T cells from HIV-negative patients, when stimulated through the T-cell receptor, undergo proliferation, cytokine secretion,222-226 and the development of susceptibility to apoptosis induced by Fas ligation.48,52,220,221 Subsequent stimulation results in AICD by the de novo production of FasL, which mediates autocrine and paracrine apoptosis.48,52,220,221Both in vivo and in vitro, HIV infection is associated with an activated T-cell phenotype,227-231 increased expression of Fas, enhanced susceptibility to Fas-mediated killing,111,115,116,174,232,233 and increased T-cell–expressed FasL after T-cell receptor stimulation,114,234 suggesting a role for Fas/FasL in HIV-associated AICD. Findings that retinoic acid inhibits FasL expression and resultant apoptosis in vitro221 and that retinoic acid therapy in HIV-infected patients reduces CD4 T-cell depletion235 support a causal role for Fas/FasL interactions in T-cell death induced by HIV.

Elevated levels of apoptosis are seen after mitogenic stimulation or TCR cross-linking of PBL from HIV-seropositive patients.124,154,216-218,227,236,237 The molecular signals responsible for apoptosis in these patients are unclear, but the administration of Fas, TRAIL/APO 2-L, or TNF antagonists reduces AICD in cells from patients infected with HIV,154,237suggesting that all 3 signals—Fas, TNF, and TRAIL/APO 2-L—may be involved.

Autologous infected cell–mediated killing

Macrophages,102,140monocytes,213,238,239 peripheral blood mononuclear cells,240 CD4 T cells,241 and CD8 T cells242 derived from HIV-infected patients may induce the death of uninfected CD4 T lymphocytes. Autologous infected cell–mediated killing may involve gp120 interactions (see below), the Fas/FasL system, or both. Macrophages express basal levels of FasL that are significantly up-regulated after infection with HIV,102 and monocytes from HIV-infected patients have significantly increased FasL expression compared with monocytes from HIV-negative controls.243 HIV-infected macrophages (and, to a lesser extent, uninfected macrophages) have been shown to kill Fas-sensitive T-cell targets102 in a major histocompatibility complex–unrestricted and Fas/TNF-dependent manner.140 Macrophage-mediated killing appears to be selective for uninfected T cells,244 as opposed to the mechanisms involved in infected T-cell death described above. Macrophage-mediated CD4 T-cell apoptosis has implications in vivo because levels of tissue apoptosis directly correlate with levels of macrophage-associated FasL.245 Thus, FasL may be the mediator of uninfected CD4 T-cell death by monocytes, macrophages,213,239 and CD8 T cells.242,246 

CD8 T-cell apoptosis

Although levels of CD8 T-cell apoptosis are consistently elevated in patients infected with HIV (whether this occurs spontaneously, in response to activation stimuli [AICD] or after coincubation with autologous infected cells102,122-124,154,216,227,240,244,247-249), the CD8 T-cell count is not significantly reduced in these patients. This apparent paradox may be resolved by observations in SIV-infected primates receiving total body irradiation, in which it was observed that CD8 T-cell recovery significantly precedes the recovery of CD4 T cells.250 A similar delay in CD4 repopulation is also seen in humans receiving high-dose chemotherapy.251,252 These data have several potential interpretations, yet they demonstrate that CD8 T-cell rebound occurs earlier than CD4 T-cell rebound after PBL depletion. In HIV-infected patients, it may therefore be expected that if rates of CD4 and CD8 T-cell loss were equal, the steady state CD8 number may be greater than the CD4 number because of the quicker recovery times. Further, HIV-associated apoptosis may lead to greater absolute numbers of CD4 T-cell apoptosis than CD8 T-cell apoptosis, because direct infection and gp120-mediated apoptosis selectively target cells that express CD4, whereas gp120 does not bind to (and thus cross-link) CD8. Nonetheless, it has recently been proposed that macrophage-associated gp120 may mediate CD8 T-cell apoptosis through interaction with CXCR4.253 Alternative potential mechanisms may also be involved (see below).

The fact that CD8 T cells from patients with HIV infection are more activated than are similar cells from HIV-uninfected persons227,254-257 suggests that the enhanced state of susceptibility to apoptosis is present in CD8 and in CD4 T cells and that CD8 T cells would be expected to die by apoptosis after exposure to another activation stimulus or with a preformed apoptosis-inducing ligand (eg, macrophage-associated FasL140). Furthermore, CD8 T cells express the CD4 receptor after activation, thereby rendering them susceptible to direct infection by the virus.258,259 In addition, the enhanced expression of CD4 antigen on CD8 T cells would be expected to render these double-positive cells more susceptible to the effects of gp120 cross-linking and subsequent apoptosis. Despite the several possible pathways that may be responsible for CD8 T-cell apoptosis in HIV-infected patients, chronic antigenic stimulation most likely contributes to CD8 T-cell apoptosis. The relative role of direct infection leading to CD8 T cell death remains untested.

Associations of apoptosis with HIV disease progression and response to therapy

Clinical studies in patients infected with HIV measure spontaneous apoptosis, Fas ligation-induced apoptosis, and apoptosis occurring in response to mitogenic activation or TCR cross-linking. In relation to the various mechanisms of apoptosis outlined above, spontaneous apoptosis may reflect infected cell apoptosis or gp120-induced apoptosis; Fas-induced apoptosis may reflect autologous cell-mediated killing of uninfected bystander cells or AICD; apoptosis in response to mitogen or CD3 ligation reflects AICD. In studies in which tissue apoptosis has been measured,260-262 few apoptotic cells are found to be physically infected by virus,260suggesting that tissue apoptosis reflects the killing of uninfected cells by gp120-induced or autologous cell-mediated killing of uninfected cells.

The magnitude of apoptosis observed in HIV-infected patients correlates well with the stage of HIV disease in longitudinal and cross-sectional analyses.263-265 Spontaneous apoptosis is greater in HIV-infected patients with progressive disease than in uninfected patients.266,267 In addition, spontaneous apoptosis in patients with long-term nonprogressive HIV infection are similar to those of HIV-negative patients.268 Thus, the rate of apoptosis correlates inversely with CD4 T-cell depletion. Because recent advances in HIV therapy have resulted in sustained increases in CD4 T cell number, if enhanced apoptosis causes CD4 T-cell depletion then apoptosis must decrease during therapy.

Numerous studies have shown that apoptosis in lymph nodes, rectal mucosa, and PBL subsets from patients infected with HIV decreases dramatically in response to protease inhibitor-based HIV treatment.125,269-274 This effect is seen for spontaneous apoptosis, apoptosis in response to T-cell receptor ligation, apoptosis in response to mitogenic stimulation, and apoptosis in response to Fas receptor ligation.125,271,272,274 The decrease in apoptosis is rapid and is seen as early as 4 days after protease inhibitor therapy is initiated125; it occurs in all patients within 14 days.271-274 Because the decrease precedes significant changes in viral replication, it has been suggested that protease inhibitors may be antiapoptotic,275,276 possibly by virtue of inhibiting the activity of effector proteases involved in apoptosis.

Effects of cytokines on HIV-associated apoptosis

One hallmark of infection with HIV is progressive T-helper-cell dysfunction. As HIV disease progresses, the balance of Th1 cytokines (IL-2 and IFN-γ) that enhance cellular immunity eventually shift to a Th2 cytokine profile (IL-4, IL-5, IL-6, and IL-10) that promotes humoral responses. The suggestion that helper cell dysfunction is central to the pathogenesis of HIV infection277 is supported by observations278-280 that the Th1-promoting cytokine IL-12, or the use of antagonistic antibodies specific for the Th2 cytokines IL-4 and IL-10, restores T-cell proliferative responses to recall antigens in HIV-infected patients. Because of the pervasive effects of cytokines in modulating apoptosis and apoptosis susceptibility, cytokine-based therapy may result in changes in apoptosis. Indeed, it has been reported that resistance to apoptosis in HIV and SIV infection is associated with a predominance of a Th1 phenotype,281 arguing that chronic immune activation and a Th2 shift may promote apoptosis. Consistent with this hypothesis, spontaneous apoptosis in cells from HIV-infected patients is blocked by the administration of IL-12, IFN-γ, anti–IL-4, anti–IL-10, and antilymphotoxin, but not by anti–IL-12 therapy.282Furthermore, IL-12 protects against the enhanced sensitivity to Fas-mediated apoptosis and enhanced sensitivity to AICD seen in HIV-infected patients.219 

Apoptosis in patients with HIV infection is modulated by exogenous cytokines or cytokine antagonists that promote a Th1 helper cell phenotype and by cytokines that promote T-cell proliferation. IL-2 therapy in patients infected with HIV results in increased CD4 T-cell numbers unrelated to decreases in viral replication. Thus, IL-2 may modulate CD4 T-cell survival directly, possibly through an antiapoptotic mechanism, a hypothesis supported by in vitro studies in which clinically relevant concentrations of IL-2 significantly reduce spontaneous apoptosis in CD4+ T cells from HIV-infected patients but not from HIV-uninfected patients.283 

IL-15 is a T-cell growth factor whose effects include T-cell proliferation, enhanced cytotoxicity of T cells and natural killer cells, B-cell proliferation, and immunoglobulin secretion.284 The effects of IL-15 on T cells are related to its ability to bind to a trimeric receptor consisting of the IL-15Rα subunit and the shared IL-2Rβ and IL-2Rγ subunits. Thus, many physiologic effects of IL-15 parallel those of IL-2. In addition, the incubation of peripheral blood mononuclear cells from HIV-infected patients with IL-15 results in enhanced production of the Th1 cytokine IFN-γ,285 CD8 T-cell activation, increased numbers of CD8 T cells,286 enhanced lymphoproliferative responses,287 and decreased spontaneous T-cell apoptosis,288 possibly mediated by increases in Bcl-2 expression.288 It is significant that although IL-2 increases HIV replication, IL-15 does not share this effect.287,288 Finally, IL-16 may have therapeutic implications for HIV-associated apoptosis.

IL-16 is a chemoattractant289 that inhibits lymphocyte activation290 and may also inhibit HIV replication.291 Possibly because of its antiproliferative effects, IL-16 treatment in vitro decreases levels of anti-CD3– or anti-Fas–induced apoptosis in lymphocytes from HIV-infected patients.292 However, the inhibitory effects of IL-16 on apoptosis are not seen in the context of spontaneous apoptosis.292 

T-cell regeneration in response to therapy

The institution of highly active antiretroviral therapy (HAART) has witnessed a major impact on immune reconstitution: sustained increases in numbers of circulating CD4 T cells associated with a rapid drop in plasma viral RNA levels. The mechanisms proposed to explain the increase in numbers of CD4 T cells include cellular redistribution from lymphoid tissue,293 cellular proliferation of the peripheral T-cell pool,294 new T-cell synthesis from a thymic source,295,296 and reduced levels of apoptosis (see above). We have previously demonstrated that HAART therapy rapidly reduces apoptosis in lymphoid tissue273 and significantly decreases apoptosis in PBL.125,273 The decrease in apoptosis occurs before significant changes on plasma viral RNA levels and when patients are receiving only the protease inhibitor component of the HAART regimen.125 This finding has led to the proposal that protease inhibitors have an effect on immune reconstitution that is independent of their ability to suppress HIV replication.275,276,297 In vitro therapy with protease inhibitors has been shown to reduce the expression of selected caspases in treated cells and to reduce the rate of caspase 3 activation.275,297 Additional evidence for an indirect protease inhibitor effect comes from studies that demonstrate sustained CD4 rises in patients who experience virologic failure298-301 and who are receiving protease inhibitor-containing HAART regimens.

The early rise (2 weeks) in CD4 cells attributable to a reduction in apoptosis appears to be followed by a phase of CD4 cell increase due to cellular redistribution and proliferation of predominately memory CD4 T cells302-306 (Figure 3). A possible third phase consists mainly of new T-cell synthesis characterized by cells with a naive phenotype.307 This third phase of T-cell regeneration is characterized by the presence of circular DNA elements formed after the rearrangement of possible T-cell receptor alleles, thereby indicating that these are newly produced T cells that have matured in the thymus.295,296 

Fig. 3.

Kinetics of change in CD4 T-cell number after the initiation of protease inhibitor (PI)–based HAART.

Fig. 3.

Kinetics of change in CD4 T-cell number after the initiation of protease inhibitor (PI)–based HAART.

Alteration of apoptosis as a therapeutic approach in HIV infection

If CD4 T-cell depletion in HIV infection results from enhanced apoptosis, then the prevention of apoptosis might be expected to modify the course of HIV disease. In vitro studies using apoptosis inhibitors (with no intrinsic antiviral properties) on PBL from HIV-infected patients cause increased viral production and increased cell survival.308 These findings suggest that non-apoptotic–infected cells serve as viral reservoirs and that it is unlikely that phenotypic and functional abnormalities of infected cells will be reversed by merely inhibiting apoptosis. Thus, blocking apoptosis alone fails to meet 2 objectives of effective HIV therapy: it does not decrease viral replication or decrease viral reservoirs, and it does not increase cellular immune competence.

The main obstacle to viral eradication in HIV-infected patients (reviewed in Chun and Fauci309) is the presence of chronically infected latent reservoir cells, such as macrophages, and latently infected CD4 T lymphocytes.310-313 In these cellular populations, HIV infection is not associated with apoptosis but with a chronic productively infected phenotype. Indeed, latently infected CD4 T cells have a markedly prolonged half-life (estimated at 6 months), which limits the probability that viral reservoirs can be eliminated by interference in viral replication alone.314In fact, recent estimates based on the half-life of latently infected cells suggest that 60 years of viral suppression would be required to eliminate viral reservoirs.311 A possible way to achieve viral eradication is to target infected macrophages and latently infected CD4 T cells to undergo apoptosis after infection. Along these lines, it has recently been proposed that treatment with a pro–caspase 3 analogue, which contains an HIV protease–specific sequence in its prodomain, may cause apoptosis of all infected cells.315Additional research is required to evaluate the clinical usefulness of this and other approaches designed to enhance the apoptosis of cells that normally function as reservoirs for HIV. The concept of enhancing HIV-associated apoptosis is, however, a potentially significant step forward in attempts to modify apoptosis for the benefit of patients infected with HIV. It further underscores the need for continued efforts to understand the regulation of apoptosis induced by HIV infection.

Acknowledgments

The authors thank Ms A. Carisse for expert secretarial assistance and the entire Badley laboratory staff and Dr B. W. D. Badley for helpful discussions and manuscript review.

Supported by grants from the Medical Research Council of Canada (HOP-36047) and the Doris Duke Foundation (T98026), the AIDS Program Committee of Ontario, and the Ontario HIV Treatment Network.

The publication costs of this article were defrayed in part by page charge payment. Therefore, and solely to indicate this fact, this article is hereby marked “advertisement” in accordance with 18 U.S.C. section 1734.

References

References
1
Hellerstein
MB
Hanley
MB
Cesar
D
et al. 
Directly measured kinetics of circulating T lymphocytes in normal and HIV-1-infected humans.
Nat Med.
5
1999
83
89
2
Ameison
JC
Capron
A
Cell dysfunction and depletion in AIDS: the programmed cell death hypothesis.
Immunol Today.
12
1991
102
105
3
Benitez-Bribiesca
L
Assessment of apoptosis in tumor growth: importance in clinical oncology and cancer therapy.
When Cells Die.
Lockshin
RA
Zakeri
Z
Tilly
JL
1998
453
476
Wiley-Liss
New York, NY
4
Laurence
J
Mitra
D
Steiner
M
Lynch
DH
Siegal
FP
Staiano-Coico
L
Apoptotic depletion of CD4+ T cells in idiopathic CD4+ lymphocytopenia.
J Clin Invest.
97
1996
672
680
5
Budd
RC
Apoptosis in autoimmunity.
When Cells Die.
Lockshin
RA
Zakeri
Z
Tilly
JL
1998
279
304
Wiley-Liss
New York, NY
6
Nagata
S
Apoptosis by death factor.
Cell.
88
1997
355
365
7
Scaffidi
C
Fulda
S
Srinivasan
A
et al. 
Two CD95 (APO-1/Fas) signaling pathways.
EMBO J.
17
1998
1675
1687
8
Darnay
BG
Aggarwal
BB
Early events in TNF signaling: a story of associations and dissociations.
J Leukoc Biol.
61
1997
559
566
9
Wiley
SR
Schooley
K
Smolak
PJ
et al. 
Identification and characterization of a new member of the TNF family that induces apoptosis.
Immunity.
3
1995
673
682
10
Mohler
KM
et al. 
Protection against a lethal dose of endotoxin by an inhibitor of tumor necrosis factor processing.
Nature.
370
1994
218
220
11
McGeehan
GM
et al. 
Regulation of tumor necrosis factor: a processing by a metalloproteinase inhibitor.
Nature.
370
1994
558
561
12
Tanaka
M
Suda
T
Haze
K
et al. 
Fas ligand in human serum.
Nat Med.
2
1996
317
322
13
Chinnaiyan
AM
Tepper
CG
Seldin
MF
et al. 
FADD/MORT1 is a common mediator of CD95 (Fas/APO-1) and tumor necrosis factor receptor-induced apoptosis.
J Biol Chem.
271
1996
4961
4965
14
Memon
SA
Hou
J
Moreno
MB
Zacharchuk
CM
Apoptosis induced by a chimeric Fas/FLICE receptor: lack of requirement for Fas- or FADD-binding proteins.
J Immunol.
160
1998
2046
2049
15
Zhang
J
Cado
D
Chen
A
Kabra
NH
Winoto
A
Fas-mediated apoptosis and activation-induced T-cell proliferation are defective in mice lacking FADD/Mort1.
Nature.
392
1998
296
300
16
Hsu
H
Xiong
J
Goeddel
DV
The TNF receptor 1-associated protein TRADD signals cell death and NF-k B activation.
Cell.
81
1995
495
504
17
Hsu
H
Shu
H-B
Pan
M-G
Goeddel
DV
TRADD-TRAF2 and TRADD-FADD interactions define two distinct TNF receptor 1 signal transduction pathways.
Cell.
84
1996
299
308
18
Miller
DK
The role of the caspase family of cysteine proteases in apoptosis.
Immunology.
95
1997
35
49
19
Stroh
C
Schulze-Osthoff
K
Death by a thousand cuts: an ever increasing list of caspase substrates.
Cell Death Differer.
5
1998
997
1000
20
Cohen
GM
Caspases: the executioners of apoptosis.
J Biochem.
326
1997
1
16
21
Muzio
M
Chinnaiyan
AM
Kischkel
FC
et al. 
FLICE, a novel FADD/homologous ICE/CED-3-like protease, is recruited to the CD95 (Fas/APO-1) death-inducing signaling complex.
Cell.
85
1996
817
827
22
Scaffidi
C
Medema
JP
Krammer
PH
Peter
ME
FLICE is predominantly expressed as two functionally active isoforms, caspase-8A and caspase-8/b.
J Biol Chem.
272
1997
26953
26958
23
Medema
JP
Scaffidi
C
Kischek
FC
et al. 
FLICE is activated by association with the CD95 death-inducing signaling complex (DISC).
EMBO J.
16
1997
2794
2804
24
Fujita
E
Kouroku
Y
Miho
Y
Tsukahara
R
Ishiura
S
Momoi
T
Wortmannin enhances activation of CPP32 (caspase-3) induced by TNF or anti-Fas.
Cell Death Differer.
5
1998
289
297
25
Fernandes-Alnemri
T
Armstrong
RC
Krebs
J
et al. 
In vitro activation of CPP32 and Mch3 by Mch4, a novel human apoptotic cysteine protease containing two FAD-like domains.
Proc Natl Acad Sci U S A.
93
1996
7464
7469
26
Kuman
S
The apoptotic cysteine protease CPP32.
Int J Biochem Cell Biol.
29
1997
393
396
27
Rosen
A
Casciola-Rosen
L
Macromolecular substrates for the ICE-like proteases during apoptosis.
J Cell Biochem.
64
1997
50
54
28
Johnson Webb
S
Harrison
DJ
Wyllie
AH
Apoptosis: an overview of the process and its relevance in disease.
Advances in Pharmacology “Apoptosis.”
Kauffman
S
1997
1
44
29
Saleh
A
Srinivasula
SM
Acharya
S
Fishel
R
Alnemri
ES
Cytochrome c and dATP-mediated oligomerization of Apaf-1 is a prerequisite for procaspase-9 activation.
J Biol Chem.
274
1999
17941
17945
30
Bossy-Wetzel
E
Green
DR
Caspases induce cytochrome c release from mitochondria by activating cystosolic factors.
J Biol Chem.
274
1999
17484
17490
31
Narula
J
Pandey
P
Arbustini
E
et al. 
Apoptosis in heart failure: release of cytochrome c from mitochondria and activation of caspase-3 in human cardiomyopathy.
Proc Natl Acad Sci U S A.
96
1999
8144
8149
32
Soengas
MS
Alarcon
RM
Yoshida
H
et al. 
Apaf-1 and caspase-9 in p53 dependent apoptosis and tumor inhibition.
Science.
284
1999
156
159
33
Irmler
M
Thome
M
Hahne
M
et al. 
Inhibition of death receptor signals by cellular FLIP.
Nature.
388
1997
190
195
34
Tschopp
J
Thome
M
Hofmann
K
Mein
E
The fight of viruses against apoptosis.
Curr Opin Gene Devel.
9
1998
82
87
35
Liston
P
Roy
N
Tamai
K
et al. 
Suppression of apoptosis in mammalian cells by NAIP and a related family of IAP genes.
Nature.
379
1996
349
353
36
Deveraux
QL
Takahaashi
R
Salvesen
GS
Reed
JC
X-linked IAP is a direct inhibitor of cell-death proteases.
Nature.
388
1997
300
304
37
Duckett
CS
Nava
VE
Gedrich
RW
et al. 
A conserved family of cellular genes related to the baculovirus iap gene and encoding apoptosis inhibitors.
EMBO J.
15
1996
2685
2694
38
Shimizu
S
Eguchi
Y
Kamiike
W
Matsuda
H
Tsujimoto
Y
Bcl-2 expression prevents activation of the ICE protease caspase.
Oncogene.
12
1996
2251
2257
39
Reed
JC
Bcl-2 and the regulation of programmed cell death.
J Cell Biol.
124
1994
1
6
40
Srivastava
RK
Sasaki
CY
Hardwick
JM
Longo
DL
Bcl-2-mediated drug resistance: inhibition of apoptosis by blocking nuclear factor of activated T lymphocytes (NFAT)-induced Fas ligand transcription.
J Exp Med.
190
1999
253
265
41
Okuno
S
Shimizu
S
Ito
T
et al. 
Bcl-2 Prevents caspase-independent cell death.
J Biol Chem.
273
1998
34272
34277
42
Shibasaki
F
Kondo
E
Akagi
T
McKeon
F
Suppression of signalling through transcription factor NF-AT by interactions between calcineurin and Bcl-2.
Nature.
386
1997
728
731
43
Wang
HG
Pathan
N
Ethell
IM
et al. 
Ca2+ induced apoptosis through calcineurin dephosphorylation.
Science.
284
1999
339
343
44
Itoh
N
Tsujimoto
Y
Nagata
S
Effect of bcl-2 on Fas antigen-mediated cell death.
J Immunol.
151
1993
621
627
45
Chiu
VK
Walsh
CM
Liu
CC
Reed
JC
Clark
WR
Bcl-2 blocks degranulation but not fas-based cell-mediated cytotoxicity.
J Immunol.
154
1995
2023
2032
46
Scaffidi
C
Schmitz
I
Zha
J
Korsmeyer
SJ
Krammer
PH
Peter
ME
Differential modulation of apoptosis sensitivity in CD95 type I and type II cells.
J Biol Chem.
274
1999
22532
22538
47
Lenardo
M
Chan
FK-M
Hornung
F
et al. 
Mature T lymphocyte apoptosis: immune regulation in a dynamic and unpredictable antigenic environment.
Annu Rev Immunol.
17
1999
221
253
48
Alderson
MR
Tough
TW
Davis-Smith
T
et al. 
Fas ligand mediates activation-induced cell death in human T lymphocytes.
J Exp Med.
181
1995
71
77
49
Brunner
T
Mogil
RJ
LaFace
D
et al. 
Cell-autonomous Fas (CD95)/Fas-ligand interaction mediates activation-induced apoptosis in T-cell hybridomas.
Nature.
373
1995
441
444
50
Dhein
J
Walczak
H
Baumler
C
Debatin
KM
Krammer
PH
Autocrine T-cell suicide mediated by APO-1/(Fas/CD95).
Nature.
373
1995
438
443
51
Ju
ST
Panka
DJ
Cui
H
et al. 
Fas (CD95) FasL interactions required for programmed cell death after T-cell activation.
Nature.
373
1995
444
448
52
Wesselborg
S
Janssen
O
Kabelitz
D
Induction of activation-driven death (apoptosis) in activated but not resting peripheral blood T cells.
J Immunol.
150
1993
4338
4345
53
Zheng
L
Fisher
G
Miller
RE
Peschon
J
Lynch
DH
Lenardo
MJ
Induction of apoptosis in mature T cells by tumour necrosis factor.
Nature.
377
1995
348
351
54
Tucek-Szabo
CL
Andjelic
S
Lacy
E
Elkon
KB
Nikolic-Zugic
J
Surface T cell Fas receptor/CD95 regulation, in vivo activation, and apoptosis: activation-induced death can occur without Fas receptor.
J Immunol.
156
1996
192
200
55
Ormerod
MG
et al. 
Increased membrane permeability of apoptotic thymocytes: a flow cytometric study.
Cytometry.
14
1993
595
602
56
Sun
XM
et al. 
A flow-cytometric method for the separation and quantitation of normal and apoptotic thymocytes.
Anal Biochem.
204
1992
351
356
57
Telford
WG
King
LE
Fraker
PJ
Comparative evaluation of several DNA binding dyes in the detection of apoptosis-associated chromatin degradation by flow cytometry.
Cytometry.
13
1992
137
143
58
Gorczyca
W
Melamed
MR
Darzynkiewicz
Z
Analysis of apoptosis by flow cytometry.
Methods Mol Biol.
91
1998
217
238
59
Darzynkiewicz
Z
et al. 
Cytometry in cell necrobiology: analysis of apoptosis and accidental cell death (necrosis).
Cytometry.
27
1997
1
20
60
Gong
J
Traganos
F
Darzynkiewicz
Z
A selective procedure for DNA extraction from apoptotic cells applicable for gel electrophoresis and flow cytometry.
Anal Biochem.
218
1994
314
319
61
Darzynkiewicz
Z
et al. 
Features of apoptotic cells measured by flow cytometry.
Cytometry.
13
1992
795
808
62
Darzynkiewicz
Z
et al. 
Cell cycle-specific effects of tumor necrosis factor.
Cancer Res.
44
1984
83
90
63
Reutelingsperger
CPM
van Heerde
VW
Annexin V, the regulator of phosphatidylserine-catalyzed inflammation and coagulation during apoptosis.
Cell Mol Life Sci.
53
1997
527
532
64
Rimon
G
et al. 
Increased surface phosphatidylserine is an early marker of neuronal apoptosis.
J Neurosci Res.
48
1997
563
570
65
Vermes
I
et al. 
A novel assay for apoptosis flow cytometric detection of phosphatidylserine expression on early apoptotic cells using fluorescein labelled annexin V.
J Immunol Methods.
184
1995
39
51
66
Koopman
G
et al. 
Annexin V for flow cytometric detection of phosphatidylserine expression on B cells undergoing apoptosis.
Blood.
84
1994
1415
1420
67
Garner
DL
Thomas
CA
Organelle-specific probe JC-1 identifies membrane potential differences in the mitochondrial function of bovine sperm.
Mol Reprod Dev.
53
1999
222
229
68
Reers
M
et al. 
Mitochondrial membrane potential monitored by JC-1 dye.
Methods Enzymol.
260
1995
406
417
69
Reers
M
Smith
TW
Chen
LB
J-Aggregate formation of a carbocyanine as a quantitative fluorescent indicator of membrane potential.
Biochemistry.
30
1991
4480
4486
70
Smiley
ST
Reers
M
Mottola-Hartshorn
C
et al. 
Intracellular heterogeneity in mitochondrial membrane potentials revealed by a J-aggregate-forming lipophilic cation JC-1.
Proc Natl Acad Sci U S A.
88
1991
3671
3675
71
Gorman
AM
et al. 
Use of flow cytometry techniques in studying mechanisms of apoptosis in leukemic cells.
Cytometry.
29
1997
97
105
72
Salvioli
S
et al. 
JC-1, but not DiOC6 (3) or Rhodamine 123, is a reliable fluorescent probe to assess psi changes in intact cells: implications for studies on mitochondrial functionality during apoptosis.
FEBS Lett.
411
1997
77
82
73
Kuhnel
JM
et al. 
Functional assay of multidrug resistant cells using JC-1, a carbocyanine fluorescent probe.
Leukemia.
11
1997
1147
1155
74
Quillet-Mary
A
et al. 
Implication of mitochondrial hydrogen peroxide generation in ceramide-induced apoptosis.
J Biol Chem.
272
1997
21388
21395
75
Herrmann
M
et al. 
A rapid and simple method for the isolation of apoptotic DNA fragments.
Nucleic Acids Res.
22
1994
5506
5507
76
Walker
PR
et al. 
Detection of the initial stages of DNA fragmentation in apoptosis.
BioTechniques.
15
1993
1032
1040
77
Olive
PL
Wlodek
D
Banath
JP
DNA double-strand breaks measured in individual cells subjected to gel electrophoresis.
Cancer Res.
51
1991
4671
4676
78
Arends
MJ
Morris
RG
Wyllie
AH
Apoptosis: the role of the endonuclease.
Am J Pathol.
136
1990
593
608
79
Singh
NP
Stephens
RE
Schneider
EL
Modifications of alkaline microgel electrophoresis for sensitive detection of DNA damage.
Int J Radiat Biol.
66
1994
23
28
80
Singh
NP
et al. 
A simple technique for quantitation of low levels of DNA damage in individual cells.
Exp Cell Res.
175
1988
184
191
81
Chapman
RS
et al. 
Further characterization of the in situ terminal deoxynucleotidyl transferase (TdT) assay for the flow cytometric analysis of apoptosis in drug resistant and drug sensitive leukaemic cells.
Cytometry.
20
1995
245
256
82
Bromidge
TJ
et al. 
Adaptation of the TdT assay for semi-quantitative flow cytometric detection of DNA strand breaks.
Cytometry.
20
1995
257
260
83
Wijsman
JH
et al. 
A new method to detect apoptosis in paraffin sections: in situ end-labelling of fragmented DNA.
Cytochemistry.
41
1993
7
12
84
Gavrieli
Y
Sherman
Y
Ben-Sasson
SA
Identification of programmed cell death in situ via specific labeling of nuclear DNA fragmentation.
J Cell Biol.
119
1992
493
501
85
Staley
K
Blaschke
AJ
Chun
J
Apoptotic DNA fragmentation is detected by a semiquantitative ligation-mediated PCR of blunt DNA ends.
Cell Death Differ.
4
1997
66
75
86
Akasaka
K
Ohrui
H
Meguro
H
Simultaneous determination of hydroperoxide of phosphatidylcholine, cholesterol esters and triacylglycerols by column-switching high-performance liquid chromatography with a post-column detection system.
J Chromatogr.
622
1993
153
159
87
Akasaka
K
Ohrui
H
Meguro
H
Normal-phase high-performance liquid chromatography with a fluorimetric postcolumn detection system for lipid hydroperoxides.
J Chromatogr.
617
1993
205
211
88
Akasaka
K
et al. 
High-performance liquid chromatography and post-column derivatization with diphenyl-1-pyrenylphosphine for fluorimetric determination of triacylglycerol hydroperoxides.
J Chromatogr.
596
1992
197
202
89
Akasaka
K
Ohrui
H
Meguro
H
An aromatic phosphine reagent for the HPLC-fluorescence determination of hydroperoxides-determination of phosphatidylcholine hydroperoxides in human plasma.
Anal Lett.
21
1988
965
90
Thornberry
NA
et al. 
Method for use of AFC-120 (Z-TYR-VAL-ALA-ASP-AFC) in determination of ICE and ICE-like enzyme activity.
Nature.
356
1992
768
774
91
Garcia-Calvo
M
Peterson
EP
Rasper
DM
Vaillancourt
JP
Zamboni-Nicholson
DW
Thornberry
NA
Purification and catalytic properties of human caspase family members.
Cell Death Differ.
6
1999
362
369
92
Thornberry
NA
et al. 
Method for assay of caspase-8 with AFC-140 (Ac-Ile-Glu-Thr-Asp-AFC).
J Biol Chem.
272
1997
17907
17911
93
Eriksson
C
Van Dam
AM
Lucassen
PJ
Bol
JG
Winblad
B
Schultzberg
M
Immunohistochemical localization of interleukin-1beta, interleukin-1 receptor antagonist and interleukin-1beta converting enzyme/caspase-1 in the rat brain after peripheral administration of kainic acid.
Neuroscience.
93
1999
915
930
94
Boyer
PD
Chance
B
Ernster
L
Mitchell
P
Racker
E
Slater
EC
Oxidative phosphorylation and photophosphorylation.
Annu Rev Biochem.
46
1977
955
1026
95
Liu
X
Kim
CN
Yang
J
Jemmerson
R
Wang
R
Induction of apoptotic program in cell-free extracts: requirement for dATP and cytochrome c.
Cell.
86
1996
147
157
96
Jemmerson
R
Johnson
JG
Burrell
E
Taylor
PS
Jenkins
MK
A monoclonal antibody specific for a cytochrome c T cell stimulatory peptide inhibits T cell response and affects the way the peptide associates with antigen-presenting cells.
Eur J Immunol.
21
1991
143
151
97
Goshorn
SC
Retzel
E
Jemmerson
R
Common structural features among monoclonal antibodies binding to the same antigenic region of cytochrome c.
J Biol Chem.
266
1991
2134
2142
98
Narayanan
PK
Goodwin
EH
Lehnert
BE
α Particles initiate biological production of superoxide anions and hydrogen peroxide in human cells.
Cancer Res.
7
1997
3963
3971
99
Rothe
G
Valet
G
Flow cytometric analysis of respiratory burst activity in phagocytes with hydroethidine and 2',7'- dichlorofluorescin.
J Leukoc Biol.
47
1990
440
448
100
McCloskey
TW
Chavan
S
Lakshmi-Tamma
SM
Pahwa
S
Comparison of seven quantitative assays to assess lymphocyte cell death during HIV infection: measurement of induced apoptosis in anti-Fas-treated Jurkat cells and spontaneous apoptosis in peripheral blood mononuclear cells from children infected with HIV.
AIDS Res Hum Retroviruses.
14
1998
1413
1422
101
Kaplan
D
Sieg
S
Role of the Fas/Fas Ligand apoptotic pathway in human immunodeficiency virus type 1 disease.
J Virol.
72
1998
6279
6282
102
Badley
AD
McElhinny
JA
Leibson
PJ
Lynch
DH
Alderson
MR
Paya
CV
Upregulation of Fas ligand expression by human immunodeficiency virus in human macrophages mediates apoptosis of uninfected T lymphocytes.
J Virol.
70
1996
199
206
103
Virk A, Badley AD, Frigas EA, Harmsen WS, Wiesner RH, Paya CV. Bcl-2 and related proteins are selectively modified by HIV infection prior to virus induced apoptotic cell death. In: Program and abstracts of the XI International Conference on AIDS; July 7-12, 1996; Vancouver, British Columbia. Abstract 143.
104
Mathew P, Badley AD, McElhinny JA, Leibson PJ, Paya CV. Modulation of bcl-2 expression in HIV infected U937 cells. Presented at: 2nd National Conference on Human Retroviruses and Related Infections; January 1995; Washington, DC. Abstract 437.
105
Noraz
N
Gozlan
J
Corbeil
J
Brunner
T
Spector
SA
HIV-induced apoptosis of activated primary CD4+ T lymphocytes is not mediated by Fas-Fas ligand.
AIDS.
11
1997
1671
1680
106
Gandhi
RT
Chen
BK
Straus
SE
Dale
JK
Lenardo
MJ
Baltimore
D
HIV-1 directly kills CD4+ T cells by a Fas-independent mechanism.
J Exp Med.
187
1998
1113
1122
107
Glynn
JM
McElligott
DL
Mosier
D
Apoptosis induced by HIV infection in H9 T cells is blocked by ICE-family protease inhibition but not by a Fas (CD95) antagonist.
J Immunol.
157
1996
2754
2758
108
Yagi
T
Sugimoto
A
Tanaka
M
et al. 
Fas/FasL interaction is not involved in apoptosis of activated CD4+ cells upon HIV-1 infection in vitro.
AIDS Res Hum Retroviruses.
18
1998
307
315
109
Regamey
N
Harr
T
Battegay
M
Erb
P
Downregulation of Bcl-2, but not of Bax or Bcl-x, is associated with T lymphocyte apoptosis in HIV infection and restored by antiretroviral thearpy or by interleukin 2.
AIDS Res Hum Retroviruses.
15
1999
803
810
110
Debatin
KM
Fahrig-Faissner
A
Enenkel-Stoodt
S
Kreuz
W
Benner
A
Krammer
PH
High expression of APO-1 (CD95) on T lymphocytes from human immunodeficiency virus-1 infected children.
Blood.
83
1994
3101
3103
111
Katsikis
PD
Wunderlich
ES
Smith
CA
Herzenberg
LA
Herzenberg
LA
Fas antigen stimulation induces marked apoptosis of T lymphocytes in human immunodeficiency virus-infected individuals.
J Exp Med.
181
1995
2029
2036
112
McCloskey
TW
Oyaizu
N
Kaplan
M
Pahwa
S
Expression of the Fas antigen in patients infected with human immunodeficiency virus.
Cytometry.
22
1995
111
114
113
Estaquier
J
Tanaka
M
Suda
T
Nagata
S
Golstein
P
Ameisen
JC
Fas-mediated apoptosis of CD4+ and CD8+ T cells from human immunodeficiency virus-infected persons: differential in vitro preventive effect of cytokines and protease antagonists.
Blood.
87
1996
4959
4966
114
Sloand
EM
Young
NS
Kumar
P
Weichold
FF
Sato
T
Maciejewski
JP
Role of Fas ligand and receptor in the mechanism of T-cell depletion in acquired immunodeficiency syndrome: effect on CD4+ lymphocyte depletion and human immunodeficiency virus replication.
Blood.
89
1997
1357
1363
115
Gehri
R
Hahn
S
Rothen
M
Steurerwald
M
Nuesch
R
Erb
P
The Fas receptor in HIV infection: expression on peripheral blood lymphocytes and role in the depletion of T cells.
AIDS.
10
1996
9
16
116
Kobayashi
N
Hamamoto
Y
Yamamoto
N
Ishii
A
Yonehara
M
Yonehara
S
Anti-Fas monoclonal antibody is cytocidal to human immunodeficiency virus-infected cells without augmenting viral replication.
Proc Natl Acad Sci U S A.
87
1990
9620
9624
117
McCloskey
TW
Oyaizu
N
Bakshi
S
Kowalski
R
Kohn
N
Pahwa
S
CD95 expression and apoptosis during pediatric HIV infection: early upregulation of CD95 expression.
Clin Immunol Immunopathol.
87
1998
33
41
118
Mitra
D
Steiner
M
Lynch
DH
Staiano-Coico
L
Laurence
J
HIV-1 upregulates Fas ligand expression in CD4+ T cells in vitro and in vivo: association with Fas-mediated apoptosis and modulation by aurintricarboxylic acid.
Immunology.
87
1996
581
585
119
Silvetris
F
Camarda
G
Cafforio
P
Dammacco
F
Upregulation of Fas ligand secretion in non-lymphopenic stages of HIV-1 infection.
AIDS.
12
1998
1103
1118
120
Hosaka
N
Oyaizu
N
Kaplan
MH
Yagita
H
Pahwa
S
Membrane and soluble forms of Fas (CD95) and Fas ligand in peripheral blood mononuclear cells and in plasma from human immunodeficiency virus-infected persons.
Infect Dis.
178
1998
1030
1039
121
McCloskey
TW
Bakshi
S
Than
S
Arman
P
Pahwa
S
Immunophenotypic analysis of peripheral blood mononuclear cells undergoing in vitro apoptosis after isolation from human immunodeficiency virus-infected children.
Blood.
92
1998
4230
4237
122
Ledru
E
Lecoeur
H
Garcia
S
Debord
T
Gougeon
ML
Differential susceptibility to activation-induced apoptosis among peripheral Th1 subsets: correlation with Bcl-2 expression and consequences for AIDS pathogenesis.
J Immunol.
160
1998
3194
3206
123
Szondy
Z
Lecoeur
H
Fesus
L
Gougeon
ML
All-trans retinoic acid inhibition of anti-CD3-induced T cell apoptosis in human immunodeficiency virus infection mostly concerns CD4 T lymphocytes and is mediated via regulation of CD95 ligand expression.
J Infect Dis.
178
1998
1288
1298
124
Katsikis
PD
Garcia-Ojeda
ME
Wunderlich
ES
et al. 
Activation-induced peripheral blood T cell apoptosis is Fas independent in HIV-infected individuals.
Int Immunol.
8
1996
1311
1317
125
Badley
AD
Parato
K
Cameron
DW
et al. 
Dynamic correlation of apoptosis and immune activation during treatment of HIV infection.
Cell Death Differ.
6
1999
420
432
126
Zangerle
R
Gallati
H
Sarcletti
M
Watchter
H
Fuchs
D
Tumor necrosis factor alpha and soluble tumor necrosis factor receptors in individuals with human immunodeficiency virus infection.
Immunol Lett.
41
1994
229
234
127
Chollet-Martin
S
Simon
F
Matheron
S
Joseph
CA
Elbim
C
Gougerot-Pocidalo
MA
Comparison of plasma cytokine levels in African patients with HIV-1 and HIV-2 infection.
AIDS.
8
1994
879
884
128
Brown
CC
Poli
G
Lubaki
N
et al. 
Elevated levels of tumor necrosis factor-alpha in Zairian neonates plasmas: implications for perinatal infection with the human immunodeficiency virus.
J Infect Dis.
169
1994
975
980
129
Ayehunie
S
Sonnerborg
A
Yemane-Berhan
T
Zewdie
DW
Britton
S
Strannegard
O
Raised levels of tumor necrosis factor-alpha and neopterin, but not interferon-alpha, in serum of HIV-1 infected patients from Ethiopia.
Clin Exp Immunol.
91
1993
37
42
130
Ruddle
NH
Tumor necrosis factor (TNF- α) and lymphotoxin (TNF-β).
Immunology.
4
1992
327
332
131
Schwandner
R
Wiegmann
K
Bernardo
K
Kreder
D
Kronket
M
TNF receptor death domain-associated proteins TRADD and FADD signal activation of acid sphingomyelinase.
J Biol Chem.
273
1998
5916
5922
132
Baxter
GT
Kuo
RC
Jupp
OJ
Vandenabeele
P
MacEwan
DJ
Tumor necrosis factor-α mediates both apoptotic cell death and cell proliferation in a human hematopoietic cell line dependent on mitotic activity and receptor subtype expression.
J Biol Chem.
274
1999
9539
9547
133
Hober
D
Haque
A
Wattre
P
Production of tumour necrosis factor-alpha and interleukin-1 in patients with AIDS: enhanced TNF-alpha is related to a higher cytotoxic activity.
Clin Exp Immunol.
78
1989
329
333
134
Maury
CPJ
Lahdevirta
J
Correlation of serum cytokine levels with haematological abnormalities in human immunodeficiency virus infection.
J Intern Med.
227
1990
253
257
135
Poli
G
Kinter
A
Justement
JS
Tumour necrosis factor α functions in an autocrine manner in the induction of human immunodeficiency virus expression.
Proc Natl Acad Sci U S A.
87
1990
782
785
136
Vyakarnam
A
McKeating
J
Meager
A
Beverley
PC
Tumour necrosis factors (α, β) induced by HIV-1 in peripheral blood mononuclear cells potentiate virus replication.
AIDS.
4
1990
21
27
137
Osborn
L
Kunkel
S
Nabel
GJ
Tumor necrosis factor α and interleukin 1 stimulate the human immunodeficiency virus enhancer by activation of the nuclear factor kB.
Proc Natl Acad Sci U S A.
86
1989
2336
2340
138
Han
X
Becker
K
Degen
HJ
Jablonowski
H
Strohmeyer
G
Synergistic stimulatory effects of tumour necrosis factor α and interferon γ on replication of human immunodeficiency virus type 1 and on apoptosis of HIV-1 infected host cells.
Eur J Clin Invest.
26
1996
286
292
139
Bilello
JA
Stellrecht
K
Drusano
GL
Stein
DS
Soluble tumor necrosis factor-α receptor type II (sTNFαRII) correlates with human immunodeficiency virus (HIV) RNA copy number in HIV-infected patients.
J Infect Dis.
173
1996
464
467
140
Badley
AD
Dockrell
D
Simpson
M
et al. 
Macrophage-dependent apoptosis of CD4+ T lymphocytes from HIV-infected individuals is mediated by FasL and tumor necrosis factor.
J Exp Med.
185
1997
55
64
141
Moreira
AL
Sampaio
EP
Zmuidzinas
A
Thalodomide exerts its inhibitory action on tumor necrosis factor alpha by enhancing mRNA degradation.
J Exp Med.
177
1993
1675
1680
142
Dezube
BJ
Pardee
AB
Chapman
B
Pentoxifylline decreases tumor necrosis factor expression and serum triglycerides in people with AIDS.
J Acquir Immune Defic Syndr.
6
1993
787
794
143
Luke
DR
McCreedy
BJ
Sarnoski
TP
Phase I/II study of pentoxyfylline with zidovudine on HIV-1 growth in AIDS patients.
Int J Clin Pharmacol Ther Toxicol.
31
1993
343
350
144
Youle
M
Clarbour
J
Farthing
C
Treatment of resistant aphthous ulceration with thalidomide in AIDS.
BMJ.
298
1989
432
145
Walker
RE
Spooner
KM
Kelly
G
et al. 
Inhibition of immunoreactive tumor necrosis factor-α by a chimeric antibody in patients infected with human immunodeficiency virus type 1.
J Infect Dis.
174
1996
63
68
146
French
LE
Tschopp
J
The TRAIL to selective tumor death.
Nature.
5
1999
146
147
147
Emery
JG
McDonnell
P
Burke
MB
et al. 
Osteoprotegerin is a receptor for the cytotoxic ligand TRAIL.
J Biol Chem.
273
1998
14363
14367
148
Degli-Esposti
MA
Dougall
WC
Smolak
PJ
Waugh
JY
Smith
CA
Goodwin
RG
The novel receptor TRAIL-R4 induces NF-kB and protects against TRAIL-mediated apoptosis, yet retains an incomplete death domain.
Immunity.
7
1997
813
820
149
Sheridan
JP
Marsters
SA
Pitti
RM
et al. 
Control of TRAIL-induced apoptosis by a family of signaling and decoy receptors.
Science.
277
1997
818
821
150
Sedger
LM
Shows
DM
Blanton
RA
et al. 
IFN-gamma mediates a novel antiviral activity through dynamic modulation of TRAIL and TRAIL receptor expression.
Immunology.
163
1999
920
926
151
Bretz
JD
Rymaszewski
M
Arscott
PL
et al. 
TRAIL death pathway expression and induction in thyroid follicular cells.
J Biol Chem.
274
1999
23627
23632
152
Griffith
TS
Chin
WA
Jackson
GC
Lynch
DH
Kubin
MZ
Intracellular regulation of TRAIL-induced apoptosis in human melanoma cells.
J Immunol.
161
1998
2833
2840
153
Jeremias
I
Herr
I
Boehler
T
Debatin
KM
TRAIL/Apo-2-ligand-induced apoptosis in human T cells.
Eur J Immunol.
28
1998
143
152
154
Katsikis
PD
Garcia-Ojeda
ME
Torres-Roca
JF
et al. 
Interleukin-1# converting enzyme-like protease involvement in Fas-induced and activation-induced peripheral blood T cell apoptosis in HIV infection: TNF-related apoptosis-inducing ligand can mediate activation-induced T cell death in HIV infection.
J Exp Med.
186
1997
1365
1372
155
Banda
NK
Bernier
J
Kurahara
DK
et al. 
Crosslinking CD4 by human immunodeficiency virus gp120 primes T cells for activation-induced apoptosis.
J Exp Med.
76
1992
1099
1106
156
Accornero
P
Radrizzani
M
Delia
D
Gerosa
F
Kurrle
R
Colombo
MP
Differential susceptibility to HIV-GP120-sensitized apoptosis in CD4+ T-cell clones with different T-helper phenotypes: role of CD95/CD95L interactions.
Blood.
89
1997
558
569
157
Oyaizu
N
McCloskey
TW
Than
S
Hu
R
Kalyanaraman
VS
Pahwa
S
Cross linking of CD4 molecules upregulates Fas antigen expression in lymphocytes by inducing interferon-γ and tumor necrosis factor-α secretion.
Blood.
84
1994
2622
2631
158
Hashimoto
F
Oyaizu
N
Kalyanaraman
VS
Pahwa
S
Modulation of Bcl-2 protein by CD4 cross-linking: a possible mechanism for lymphocyte apoptosis in human immunodeficiency virus infection and for rescue of apoptosis by interleukin-2.
Blood.
90
1997
745
753
159
Cicala
C
Arthos
J
Rubbert
A
et al. 
HIV-1 envelope induces activation of caspase-3 and cleavage of focal adhesion kinase in primary human CD4 (+) T cells.
Proc Natl Acad Sci U S A.
97
2000
1178
1183
160
Laurent-Crawford
AG
Krust
B
Riviere
Y
et al. 
Membrane expression of HIV envelope glycoproteins triggers apoptosis in CD4 cells.
AIDS Res Hum Retroviruses.
9
1993
761
773
161
Laurent-Crawford
AG
Coccia
E
Krust
B
Hovanessian
AG
Membrane-expressed HIV envelope glycoprotein heterodimer is a powerful inducer of cell death in uninfected CD4+ target cells.
Res Virol.
146
1995
5
17
162
Moutouh
L
Estaquier
J
Richman
DD
Corbeil
J
Molecular and cellular analysis of human immunodeficiency virus-induced apoptosis in lymphoblastoid T-cell-line-expressing wild-type and mutated CD4 receptors.
J Virol.
72
1998
8061
8072
163
Guillerm
C
Coudronniere
N
Robert-Hebmann
V
Devaux
C
Delayed human immunodeficiency virus type 1-induced apoptosis in cells expressing truncated forms of CD4.
J Virol.
72
1997
1754
1761
164
Berndt
C
Mopps
B
Angermuller
S
Gierschik
P
Krammer
PH
CXCR4 and CD4 mediate a rapid CD95-independent cell death in CD4+ T cells.
Proc Natl Acad Sci U S A.
95
1998
12556
12561
165
Ohnimus
H
Heinkelein
M
Jassoy
C
Apoptotic cell death upon contact of CD4 T lymphocytes with HIV glycoprotein expressing cells is mediated by caspases but bypasses CD95 (Fas/APO-1) and TNF receptor 1.
J Immunol.
159
1997
5246
5252
166
Blanco
J
Jacotot
E
Cabrera
C
et al. 
The implication of the chemokine receptor CXCR4 in HIV-1 envelope protein-induced apoptosis is independent of the G protein-mediated signalling.
AIDS.
13
1999
909
917
167
Blanco
J
Barretina
J
Henson
G
et al. 
The CXCR4 antagonist AMD3100 efficiently inhibits cell-surface-expressed human immunodeficiency virus type 1 envelope-induced apoptosis.
Antimicrob Agents Chemother.
44
2000
51
56
168
Kameoka
M
Kimura
T
Zheng
YH
et al. 
Protease-defective, gp120-containing human immunodeficiency virus type 1 particles induce apoptosis more efficiently than does wild-type virus or recombinant gp120 protein in healthy donor-derived peripheral blood T cells.
J Clin Microbiol.
35
1997
41
47
169
Ellaurie
M
Calvelli
TA
Rubinstein
A
Human Immunodeficiency virus (HIV) circulating immune complexes in infected children.
AIDS Res Hum Retroviruses.
6
1990
1437
1441
170
Aceituno
E
Castanon
S
Jimenez
C
et al. 
Circulating immune complexes from HIV-1 patients induces apoptosis on normal lymphocytes.
Immunology.
92
1997
317
320
171
Bartz
SR
Emerman
M
Human immunodeficiency virus type 1 Tat induces apoptosis and increases sensitivity to apoptotic signals by up-regulating FLICE/Caspase-8.
J Virol.
73
1999
1956
1963
172
Li-Weber
M
Laur
O
Dern
K
Krammer
PH
T cell activation-induced and HIV tat-enhanced CD95 (APO-1/Fas) ligand transcription involves NF-kB.
Eur J Immunol.
30
2000
661
670
173
Li
CJ
Friedman
DJ
Wang
C
Metelev
V
Pardee
AB
Induction of apoptosis in uninfected lymphocytes by HIV-1 Tat protein.
Science.
268
1995
429
431
174
Westendorp
MO
Frank
R
Ochsenbauer
K
et al. 
Sensitization of T cells to CD95-mediated apoptosis by HIV-1 Tat and gp120.
Nature.
375
1995
497
500
175
Westendorp
MO
Shatrov
VA
Schulze-Osthoff
K
et al. 
HIV-1 Tat potentiates TNF-induced NF-kB activation and cytotoxicity by altering the cellular redox state.
EMBO J.
14
1995
546
554
176
Chang
HC
Samaniego
F
Nair
BC
Buonaguro
L
Ensoli
B
HIV-1 Tat protein exits from cells via a leaderless secretory pathway and binds to extracellular matrix-associated heparan sulfate proteoglycans through its basic region.
AIDS.
11
1997
1421
1431
177
Gallo
RC
Tat as one key to HIV-induced immune pathogenesis and Pat toxoid as an important component of a vaccine.
Proc Natl Acad Sci U S A.
96
1999
8324
8326
178
Azad
AA
Could Nef and Vpr proteins contribute to disease progression by promoting depletion of bystander cells and prolonged survival of HIV-infected cells?
Biochem Biophys Res Comm.
267
2000
677
685
179
Kirchhoff
F
Greenough
TC
Brettler
DB
Sullivan
JL
Desrosiers
RC
Brief report: absence of intact nef sequences in a long-term survivor with nonprogressive HIV-1 infection.
N Engl J Med.
332
1995
228
232
180
Huang
Y
Zhang
L
Ho
DD
Characterization of nef sequences in long-term survivors of human immunodeficiency virus type 1 infection.
J Virol.
69
1995
93
100
181
Cheng-Mayer
C
Iannello
P
Shaw
K
Luciw
PA
Levy
JA
Differential effects of nef on HIV replication: implications for viral pathogenesis in the host.
Science.
246
1989
1629
1632
182
Niederman
TMJ
Thielan
BJ
Ratner
L
Human immunodeficiency virus type 1 negative factor is a transcriptional silencer.
Proc Natl Acad Sci U S A.
86
1989
1128
1132
183
Terwilliger
EF
Sodroski
JG
Rosen
CA
Haseltine
WA
Effects of mutations with the 3'-open reading frame region of human T-cell lymphotropic virus type III (HTLV-III/LAV) on replication and cytopathogenicity.
J Virol.
60
1986
754
760
184
Kestler
HW
III
Ringler
DJ
Mori
K
et al. 
Importance of the nef gene for maintenance of high virus loads and for development of AIDS.
Cell.
65
1991
651
662
185
Rhee
SS
Marsh
JW
HIV-1 Nef Activity in murine T cells: CD4 modulation and positive enhancement.
J Immunol.
152
1994
5128
5134
186
Alexander
L
Du
Z
Rosenzweig
M
Jung
JU
Desrosiers
RC
A role for natural simian immunodeficiency virus and human immunodeficiency virus type 1 nef alleles in lymphocyte activation.
J Virol.
71
1997
6094
6099
187
Schrager
JA
Marsh
JW
HIV-1 Nef increases T cell activation in a stimulus-dependent manner.
Proc Natl Acad Sci U S A.
96
1999
8167
8172
188
Luria
S
Chambers
I
Berg
P
Expression of the type 1 human immunodeficiency virus Nef protein in T cells prevents antigen receptor-mediated induction of interleukin 2 mRNA.
Proc Natl Acad Sci U S A.
88
1991
5326
5330
189
Niederman
TM
Garcia
JV
Hastings
WR
Luria
S
Ratner
L
Human immunodeficiency virus type 1 Nef protein inhibits NF-k B induction in human T cells.
J Virol.
66
1992
6213
6219
190
Iafrate
AJ
Bronson
S
Skowronski
J
Separable functions of Nef disrupt two aspects of T cells receptor machinery: CD4 expression and CD3 signaling.
EMBO J.
16
1997
673
684
191
Greenway
A
Azad
A
McPhee
D
Human immunodeficiency virus type 1 Nef protein inhibits activation pathways in peripheral blood mononuclear cells and T-cell lines.
J Virol.
69
1995
1842
1850
192
Zauli
G
Gibellini
D
Secchiero
P
et al. 
Human immunodeficiency virus type 1 Nef protein sensitizes CD4+ T lymphoid cells to apoptosis via functional upregulation of the CD95/CD95 ligand pathway.
Blood.
93
1999
1000
1010
193
Xu
XN
Screaton
GR
Gotch
FM
et al. 
Evasion of cytotoxic T lymphocyte (CTL) responses by Nef-dependent induction of Fas ligand (CD95L) expression on simian immunodeficiency virus-infected cells.
J Exp Med.
186
1997
7
16
194
Salghetti
S
Mariani
R
Skowronski
J
Human immunodeficiency virus type 1 Nef and p56lck protein-tyrosine kinase interact with a common element in CD4 cytoplasmic tail.
Proc Natl Acad Sci U S A.
92
1995
349
353
195
Otake
K
Fujii
Y
Nakaya
T
et al. 
The carboxyl-terminal region of HIV-1 Nef protein is a cell surface domain that can interact with CD4+ T cells.
J Immunol.
153
1994
5826
5837
196
Okada
H
Takei
R
Tashiro
M
HIV-1 Nef protein-induced apoptotic cytolysis of a broad spectrum of uninfected human blood cells independently of CD95(Fas).
FEBS Lett.
414
1997
603
606
197
Stewart
SA
Poon
B
Jowett
JBM
Chen
ISY
Human immunodeficiency virus type 1 vpr induces apoptosis following cell cycle arrest.
J Virol.
71
1997
5579
5592
198
Yao
X-J
Mouland
AJ
Subbramanian
RA
et al. 
Vpr stimulates viral expression and induces cell killing in human immunodeficiency virus type 1-infected dividing Jurkat T cells.
J Virol.
72
1998
4686
4693
199
Jacotot
E
Ravagnan
L
Loeffler
M
et al. 
The HIV-1 viral protein R induces apoptosis via a direct effect on the mitochondrial permeability transition pore.
J Exp Med.
191
2000
33
45
200
Connor
RI
Chen
BK
Choe
S
Landau
NR
Vpr is required for efficient replication of human immunodeficiency virus type 1 in mononuclear phagocytes.
Virology.
206
1995
936
944
201
Levy
DN
Rafaeli
Y
Weiner
DB
Extracellular vpr protein increases cellular permissiveness to human immunodeficiency virus replication and reactivates virus from latency.
J Virol.
69
1995
1243
1252
202
Levy
DN
Induction of cell differentiation by human immunodeficiency virus 1 vpr.
Cell.
72
1993
541
550
203
He
J
Human immunodeficiency virus type 1 protein R (vpr) arrests cells in the G2 phase of the cell cycle by inhibiting p34cdc2 activity.
J Virol.
69
1995
6705
6711
204
Conti
L
Rainaldi
G
Matarrese
P
et al. 
The HIV-1 vpr protein acts as a negative regulator of apoptosis in a human lymphoblastoid T cell line: possible implications for the pathogenesis of AIDS.
J Exp Med.
187
1998
403
413
205
Fukumori
T
Akari
H
Iida
S
et al. 
The HIV-1 vpr displays strong anti-apoptotic activity.
FEBS Lett.
432
1998
17
20
206
Ayyavoo
V
Mahboubi
A
Mahalingam
S
et al. 
HIV-1 vpr suppresses immune activation and apoptosis through regulation of nuclear factor kB.
Nat Med.
3
1997
1117
1123
207
Hrimech
M
Yao
X-J
Bachand
F
Rougeau
N
Cohen
EA
Human immunodeficiency virus type 1 (HIV-1) vpr functions as an immediate-early protein during HIV-1 infection.
J Virol.
73
1999
4101
4109
208
Poon
B
Grovit-Ferbas
K
Stewart
SA
Chen
ISY
Cell cycle arrest by vpr in HIV-1 virions and insensitivity to antiretroviral agents.
Science.
281
1998
266
269
209
Adams
LD
Tomasselli
AG
Robbins
P
Moss
P
Heinrikson
RL
HIV-1 protease cleaves actin during acute infection human T-lymphocytes.
AIDS Res Hum Retroviruses.
8
1992
291
295
210
Buttner
J
Dornmair
K
Schramm
HJ
Screening of inhibitors of HIV-1 protease using an Escherichia coli cell assay.
Biochem Biophy Res Comm.
233
1997
36
38
211
Konvalinka
J
Litterst
MA
Welker
R
et al. 
An active site mutation in the HIV type 1 proteinase (PR) causes reduced PR activity and loss of PR mediated cytotoxicity without apparent effect on virus maturation and infectivity.
J Virol.
69
1995
7180
7186
212
Rivière
Y
Blank
V
Kourilsky
P
Israel
A
Processing of the precursor of NF-k B by the HIV-1 protease during acute infection.
Nature.
350
1991
625
626
213
Oyaizu
N
Adachi
Y
Hashimoto
F
et al. 
Monocytes express Fas ligand upon CD4 cross-linking and induce CD4+ T cells apoptosis.
J Immunol.
158
1997
2456
2463
214
Phenix BN, Beckett B, Alam A, et al. HIV protease induces apoptosis of HIV infected T cells through activation of caspase 8. Eighth Annual Canadian Conference of HIV/AIDS Research; 1999; Victoria, British Columbia. Abstract 409.
215
Strack
PR
West Frey
M
Rizzo
CJ
et al. 
Apoptosis mediated by HIV protease is preceded by cleavage of Bcl-2.
Proc Natl Acad Sci U S A.
93
1996
9571
9576
216
Meyaard
L
Otto
SA
Jonker
RR
Mijnster
MJ
Keet
RP
Miedema
F
Programmed death of T cells in HIV-1 infection.
Science.
257
1992
217
219
217
Oyaizu
N
McCloskey
TW
Coronesi
M
Chirmule
N
Kalyanaraman
VS
Pahwa
S
Accelerated apoptosis in peripheral blood mononuclear cells (PBMCs) from human immunodeficiency virus type-1 infected patients and in CD4 cross-linked PBMCs from normal individuals.
Blood.
82
1993
3392
3400
218
Groux
H
Torpier
G
Monte
D
Mouton
Y
Capron
A
Ameisen
JC
Activation-induced death by apoptosis in CD4+ T cells from human immunodeficiency virus-infected asymptomatic individuals.
J Exp Med.
175
1992
331
340
219
Estaquier
J
Idziorek
T
Zou
W
et al. 
T helper type 1/T helper type 2 cytokines and T cell death; preventive effect of interleukin 12 on activation-induced and CD95 (Fas/APO-1)-mediated apoptosis of CD4+ T cells from human immunodeficiency virus-infected persons.
J Exp Med.
182
1995
1759
1767
220
Alderson
MR
Armitage
RJ
Maraskovsky
E
et al. 
Fas transduces activation signals in normal human T lymphocytes.
J Exp Med.
178
1993
2231
2235
221
Yang
Y
Mercep
M
Ware
CF
Ashwell
JD
Fas and activation-induced Fas ligand mediate apoptosis of T cell hybridomas: inhibition of Fas ligand expression by retinoic acid and glucocorticoids.
J Exp Med.
181
1995
1673
1682
222
Meuer
SC
Hodgdon
JC
Hussey
RE
Protentis
JP
Schlossman
SF
Reinherz
EL
Antigen-like effects of monoclonal antibodies directed at receptors on human T cell clones.
J Exp Med.
158
1983
988
993
223
Nau
GJ
Kim
D-K
Fitch
FW
Agents that mimic antigen receptor signalling inhibit proliferation of cloned murine T lymphocytes induced by Il-2.
J Immunol.
141
1988
3557
3563
224
Breitmeyer
JB
Oppenheim
SO
Delay
JF
Levine
HB
Schlossman
SF
Growth inhibition of human T cells by antibodies recognizing the T cell antigen receptor complex.
J Immunol.
138
1987
726
731
225
Webb
S
Sprent
J
Downregulation of T cell responses by antibodies to the T cell receptor.
J Exp Med.
165
1987
584
589
226
Mercep
M
Bluestone
JA
Noguchi
PD
Ashwell
JD
Inhibition of transformed T cell growth by monoclonal antibodies directed against distinct activating molecules.
J Immunol.
140
1988
324
335
227
Gougeon
ML
Lecoeur
H
Dulioust
A
et al. 
Programmed cell death in peripheral lymphocytes from HIV-infected persons: increased susceptibility to apoptosis of CD4 and CD8 T cells correlates with lymphocyte activation and with disease progression.
J Immunol.
156
1996
3509
3520
228
Miedema
F
Petit
AJ
Terpstra
FG
et al. 
Immunological abnormalities in human immunodeficiency virus (HIV)-infected asymptomatic homosexual men: HIV affects the immune system before CD4+ T cell deletion occurs.
J Clin Invest.
82
1988
1908
1914
229
Clerici
M
Hakim
F
Venzon
D
et al. 
Changes in interleukin-2 and interleukin-4 production in asymptomatic human immunodeficiency virus-seropositive individuals.
J Clin Invest.
91
1993
759
765
230
Graziosi
C
Pantaleo
G
Fauci
AS
Comparative analysis of constitutive cytokine expression in peripheral blood and lymph nodes of HIV-infected individuals.
Res Immunol.
145
1994
602
605
231
Meyaard
L
Otto
SA
Keet
IP
van Lier
RA
Miedema
F
Changes in cytokine secretion patterns of CD4+ T-cell clones in human immunodeficiency virus infection.
Blood.
84
1994
4262
4268
232
Aries
SP
Schaaf
B
Muller
C
Dennin
RH
Dalhoff
K
Fas (CD95) expression on CD4+ T cells from HIV-infected patients increases with disease progression.
J Mol Med.
73
1995
591
593
233
Silvestris
F
Cafforio
P
Frassanito
MA
et al. 
Overexpression of Fas antigen on T cells in advanced HIV-1 infection: differential ligation constantly induces apoptosis.
AIDS.
10
1996
131
141
234
Baumler
CB
Bohler
T
Herr
R
Benner
A
Krammer
PH
Debatin
KM
Activation of the CD95 (APO-1/Fas) system in T cells from human immunodeficiency virus type-1 infected children.
Blood.
88
1996
1741
1746
235
Yang
Y
Bailey
J
Vacchio
MS
Yarchoan
R
Ashwell
JD
Retinoic acid inhibition of ex vivo human immunodeficiency virus-associated apoptosis of peripheral blood cells.
Proc Natl Acad Sci U S A.
92
1995
3051
3055
236
Sarin
A
Clerici
M
Blatt
SP
Hendrix
CW
Shearer
GM
Henkart
PA
Inhibition of activation-induced programmed cell death and restoration of defective immune responses of HIV+ donors by cysteine protease inhibitors.
J Immunol.
153
1994
862
872
237
Dockrell
DH
Badley
AD
Algeciras-Schimnich
A
et al. 
Activation-induced CD4 T cell death in HIV positive individuals correlates with Fas-susceptibility, CD4 T cell count and HIV plasma viral copy number.
AIDS Res Hum Retro.
15
1999
1509
1518
238
Cottrez
F
Manca
F
Dalgleish
AG
Arenzana-Seisdedos
F
Capron
A
Groux
H
Priming of human CD4+ antigen-specific T cells to undergo apoptosis by HIV-infected monocytes.
J Clin Invest.
99
1997
257
266
239
Orlikowsky
T
Wang
Z-Q
Dudhane
A
Horowitz
H
Riethmuller
G
Hoffman
MK
Cytotoxic monocytes in the blood of HIV type-1 infected subjects destroy targeted T cells in a CD-95-dependent fashion.
AIDS Res Hum Retroviruses.
13
1997
953
960
240
Nardelli
B
Gonzalez
CJ
Schechter
M
Valentine
FT
CD4+ blood lymphocytes are rapidly killed in vitro by contact with autologous human immunodeficiency virus-infected cells.
Proc Natl Acad Sci U S A.
92
1995
7312
7316
241
Kameoka
M
Suzuki
S
Kimura
T
et al. 
Exposure of resting peripheral blood T cells to HIV-1 particles generates CD25+ killer cells in a small subset, leading to induction of apoptosis in bystander cells.
Int Immunol.
9
1997
1453
1462
242
Kojima
H
Eshima
K
Takayama
H
Sitkovsky
MV
Leukocyte function-associated antigen-1 dependent lysis of Fas+ (CD95+/Apo-1+) innocent bystanders by antigen-specific CD8+ CTL.
J Immunol.
158
1997
2728
2734
243
Lewis
DE
Ng Tang
DS
Wang
X
Kozinetz
C
Costimulatory pathways mediate monocyte-dependent lymphocyte apoptosis in HIV.
Clin Immunol.
90
1999
302
312
244
Herbein
G
Van Lint
C
Lovett
JL
Verdin
E
Distinct mechanisms trigger apoptosis in human immunodeficiency virus type-1 infected and in uninfected bystander T lymphocytes.
J Virol.
72
1998
660
670
245
Dockrell
DH
Badley
AD
Villacian
JS
et al. 
The expression of Fas ligand by macrophages and its upregulation by human immunodeficiency virus infection.
J Clin Invest.
101
1998
2394
2405
246
Hadida
F
Vieillard
V
Mollet
L
Clark-Lewis
I
Baggiolini
M
Debre
P
Cutting edge: RANTES regulates Fas ligand expression and killing by HIV-specific CD8 cytotoxic T cells.
J Immunol.
163
1999
1105
1109
247
Lauener
RP
Hüttner
S
Buisson
M
et al. 
T-cell death by apoptosis in vertically human immunodeficiency virus-infected children coincides with expansion of CD8+/interleukin-2 receptor-/HLA-DR+ T cells: sign of a possible role for herpes viruses as cofactors?
Blood.
86
1995
1400
1407
248
Lewis
DE
Ng Tang
DS
Adu-Oppong
A
Schober
W
Rodgers
JR
Anergy and apoptosis in CD8+ T cells from HIV-infected persons.
J Immunol.
153
1994
412
420
249
Meyaard
L
Otto
SA
Keet
IPM
Roos
MTL
Miedema
F
Programmed death of T cells in human immunodeficiency virus infection: no correlation with progression to disease.
J Clin Invest.
93
1994
982
988
250
Fultz
PN
Schwiebert
RS
Su
L
Salter
MM
Effects of total lymphoid irradiation on SIV-infected macaques.
AIDS Res Hum Retroviruses.
11
1995
1517
1527
251
Hakim
FT
Cepeda
R
Kaimei
S
et al. 
Constraints on CD4 recovery postchemotherapy in adults: thymic insufficiency and apoptotic decline of expanded peripheral CD4 cells.
Blood.
90
1997
3789
3798
252
Gratama
JW
Lipovich-Oosterveer
MA
Willemze
R
et al. 
Reduction and repopulation of T-lymphocytes after cytoreductive therapy with or without autologous bone marrow rescue.
Exp Hematol.
14
1986
173
177
253
Herbein
G
Mahlknecht
U
Batliwalla
F
et al. 
Apoptosis of CD8+ T cells is mediated by macrophages through interaction of HIV gp120 with chemokine receptor CXCR4.
Nature.
395
1998
189
194
254
Giorgi
JV
Detels
R
T-cell subset alterations in HIV-infected homosexual men: NIAID multicenter AIDs cohort study.
Clin Immunol Immunopathol.
52
1989
10
18
255
Giorgi
JV
Liu
Z
Hultin
LE
Cumberland
WG
Hennessen
K
Detels
R
Elevated levels of CD38+CD8+ cells in HIV infection add to the prognostic value of low CD4+ T cell levels: results of 6 years follow-up.
J Acquir Immune Defic Syndr.
6
1993
904
912
256
Giorgi
JV
Ho
HN
Hirji
K
et al. 
CD8+ lymphocyte activation at human immunodeficiency virus type 1 seroconversion: development of HLA-DR+CD38-CD8+ cells is associated with subsequent stable CD4+ cell levels.
J Infect Dis.
170
1994
775
781
257
Levacher
M
Hulstaert
F
Tallet
S
Ullery
S
Pocidalo
JJ
Bach
BA
The significance of activation markers on CD8 lymphocytes in human immunodeficiency syndrome: staging and prognostic value.
Clin Exp Immunol.
90
1992
376
382
258
Flamand
L
Crowley
RW
Lusso
P
Colombini-Hatch
S
Margolis
DM
Gallo
RC
Activation of CD8+ T lymphocytes through the T cell receptor turns on CD4 gene expression: implications for HIV pathogenesis.
Proc Natl Acad Sci U S A.
95
1998
3111
3116
259
Yang
LP
Riley
JL
Carroll
RG
et al. 
Productive infection of neonatal CD8+ T lymphocytes by HIV-1.
J Exp Med.
187
1998
1139
1144
260
Finkel
TH
Tudor-Williams
G
Banda
NK
et al. 
Apoptosis occurs predominantly in bystander cells and not in productively infected cells of HIV- and SIV- infected lymph nodes.
Nat Med.
1
1995
129
134
261
Røsok
B
Brinchmann
JE
Stent
G
et al. 
Correlates of apoptosis of CD4+ and CD8+ T cells in tonsillar tissue in HIV Type 1 infection.
AIDS Res Hum Retroviruses.
14
1998
1635
1643
262
Muro-Cacho
CA
Pantaleo
G
Fauci
A
Analysis of apoptosis in lymph nodes of HIV-1 infected persons: intensity of apoptosis correlates with the general state of activatin of the lymphoid tissue and not with stage of disease or viral burden.
J Immunol.
154
1995
5555
5566
263
Patki
AH
Georges
DL
Lederman
MM
CD4+-T-cell counts, spontaneous apoptosis, and Fas expression in peripheral blood mononuclear cells obtained from human immunodeficiency virus type 1-infected subjects.
Clin Diagn Lab Immunol.
4
1997
736
741
264
Prati
E
Gorla
R
Malacarne
F
et al. 
Study of spontaneous apoptosis in HIV + patients: correlation with clinical progression and T cell loss.
AIDS Res Hum Retroviruses.
13
1997
1501
1508
265
Samuelsson
A
Broström
C
Van Dijk
N
Sönnerborg
A
Chiodi
F
Apoptosis of CD4+ and CD19+ cells during human immunodeficiency virus type 1 infection: correlation with clinical progression, viral load, and loss of humoral immunity.
Virology.
238
1997
180
188
266
Liegler
TJ
Yonemoto
W
Elbeik
T
Wittinghoff
E
Buchbinder
SP
Greene
WC
Diminished spontaneous apoptosis in lymphocytes from human immunodeficiency virus-infected long-term nonprogressors.
J Infect Dis.
178
1998
669
679
267
Wasmuth
JC
Klein
KH
Hackbarth
F
Rockstroh
JK
Sauerbruch
T
Spengler
U
Prediction of imminent complications in HIV-1-infected patients by markers of lymphocyte apoptosis.
J Acquir Immune Defic Syndr.
23
2000
44
51
268
Franceschi
C
Franceschini
MG
Boschini
A
et al. 
Phenotypic characteristics and tendency to apoptosis of peripheral blood mononuclear cells from HIV+ long term non progressors.
Cell Death Differ.
4
1997
815
823
269
Chavan
SJ
Tamma
SL
Kaplan
M
Gerstein
M
Pahwa
SG
Reduction in T cell apoptosis in patients with HIV disease following antiretroviral therapy.
Clin Immunol.
93
1999
24
33
270
Kotler
DP
Shimada
T
Snow
G
et al. 
Effect of combination antiretroviral therapy upon rectal mucosal HIV RNA burden and mononuclear cell apoptosis.
AIDS.
12
1998
597
604
271
Johnson
N
Parkin
JM
Anti-retroviral therapy reverses HIV-associated abnormalities in lymphocyte apoptosis.
Clin Exp Immunol.
113
1998
229
234
272
Aries
SP
Weyrich
K
Schaaf
B
Hansen
F
Dennin
RH
Dalhoff
K
Early T-cell apoptosis and Fas expression during antiretroviral therapy in individuals infected with human immunodeficiency virus-1.
Scand J Immunol.
48
1998
86
91
273
Badley
AD
Dockrell
DH
Algeciras
A
et al. 
In vivo analysis of Fas/FasL interactions in HIV-infected patients.
J Clin Invest.
102
1998
79
87
274
Böhler
T
Walcher
J
Hölzl-Wenig
G
et al. 
Early effects of antiretroviral combination therapy on activation, apoptosis and regeneration of T cells in HIV-1 infected children and adolescents.
AIDS.
13
1999
779
789
275
Sloand
EM
Kumar
PN
Kim
S
Chaudhuri
A
Weichold
FF
Young
NS
Human immunodeficiency virus type 1 protease inhibitor modulates activation of peripheral blood CD4+ T cells and decreases their susceptibility to apoptosis in vitro and in vivo.
Blood.
94
1999
1021
1027
276
Phenix
BN
Angel
JB
Mandy
F
et al. 
Decreased HIV-associated T cell apoptosis by HIV protease inhibitors.
AIDS Res Hum Retroviruses.
16
2000
559
567
277
Clerici
M
Shearer
GM
A TH1 to TH2 switch is a critical step in the etiology of HIV infection.
Immunol Today.
14
1993
107
111
278
Clerici
M
Lucey
DR
Berzofsky
JA
et al. 
Restoration of HIV-specific cell-mediated immune response by IL-12 in vitro.
Science.
262
1993
1721
1724
279
Clerici
M
Lucey
DR
Berzofsky
JA
et al. 
Role of IL-10 in T helper cell dysfunction in asymptomatic individuals infected with HIV.
J Clin Invest.
93
1994
768
775
280
Clerici
M
Sarin
A
Coffman
RL
et al. 
Type 1/type 2 cytokine modulation of T cell programmed cell death as a model for HIV pathogenesis.
Proc Natl Acad Sci U S A.
91
1994
11811
11815
281
Gougeon
ML
Garcia
S
Heeney
J
et al. 
Programmed cell death in AIDS-related HIV and SIV infections.
AIDS Res Hum Retroviruses.
9
1993
553
563
282
Clerici
M
Sarin
A
Berzofsky
JA
et al. 
Antigen-stimulated apoptotic T-cell death in HIV infection is selective for CD4+ T cells, modulated by cytokines and effected by lymphotoxin.
AIDS.
10
1996
603
611
283
Adachi
Y
Oyaizu
N
Than
S
McCloskey
TW
Pahwa
S
IL-2 Rescues in vitro lymphocyte apoptosis in patients with HIV infection.
J Immunol.
157
1996
4184
4193
284
Cosman
D
Kumaki
S
Anderson
D
Kennedy
M
Eisenman
J
Park
L
Interleukin 15.
Biochem Soc Trans.
25
1997
371
374
285
Lucey
DR
Pinto
LA
Bethke
FR
et al. 
In vitro immunologic and virologic effects of interleukin 15 on peripheral blood mononuclear cells from normal donors and human immunodeficiency virus type-1 infected patients.
Clin Diagn Lab Immunol.
4
1997
43
48
286
Agostini
C
Trentin
L
Sancetta
R
et al. 
Interleukin-15 triggers activation and growth of the CD8 T-cell pool in extravascular tissues of patients with acquired immunodeficiency syndrome.
Blood.
90
1997
1115
1123
287
Patki
AH
QuiOones-Mateu
ME
Dorazio
D
et al. 
Activation of antigen-induced lymphocyte proliferation by Interleukin-15 without the mitogenic effect of Interleukin-2 that may induce human immunodeficiency virus-1 expression.
J Clin Invest.
98
1996
616
621
288
Chehimi
J
Marshall
JD
Salvucci
O
et al. 
IL-15 enhances immune functions during HIV infection.
J Immunol.
158
1997
5978
5987
289
Center
DM
Kornfeld
H
Cruikshank
WW
Interleukin-16 and function as a CD4 ligand.
Immunol Today.
17
1996
476
481
290
Cruikshank
WW
Lim
K
Theodore
AC
et al. 
IL-16 inhibition of CD3-dependent lymphocyte activation and proliferation.
J Immunol.
157
1996
5240
5248
291
Baier
M
Werner
A
Bannert
N
Metzner
K
Kurth
R
HIV suppression by interleukin-16.
Nature.
378
1995
563
292
Idziorek
T
Khalife
J
Billaut-Mulot
O
et al. 
Recombinant human IL-16 inhibits HIV-1 replication and protects against activation-induced cell death (AICD).
Clin Exp Immunol.
112
1998
84
91
293
Bucy
RP
Hockett
RD
Derdeyn
CA
et al. 
Initial increase in blood CD4+ lymphocytes after HIV antiretroviral therapy reflects redistribution from lymphoid tissues.
J Clin Invest.
103
1999
1391
1398
294
Fleury
S
De Boer
RJ
Rizzardi
GP
et al. 
Limited CD4+ T-cell renewal in early HIV-1 infection: effect of highly active antiretroviral therapy.
Nat Med.
4
1998
794
801
295
Zhang
L
Lewin
SR
Markowitz
M
et al. 
Measuring recent thymic emigrants in blood of normal and HIV-1 infected individuals before and after effective therapy.
J Exp Med.
190
1999
725
732
296
Poulin
JF
Viswanathan
MN
Harris
JM
et al. 
Direct evidence for thymic function in adult humans.
J Exp Med.
190
1999
479
486
297
Weichold
FF
Bryant
JL
Pati
S
Barabitskaya
O
Gallo
RC
Reitz
MSJ
HIV-1 protease inhibitor ritonavir modulates susceptibility to apoptosis of uninfected T cells.
J Hum Virol.
2
1999
261
269
298
Collier
AC
Coombs
RW
Schoenfeld
DA
Treatment of human immunodeficiency virus infection with saquinavir, zidovudine, and zalcitabine.
N Engl J Med.
334
1996
1011
1017
299
Kaufmann
D
Pantaleo
G
Sudre
P
Telenti
A
CD4-cell count in HIV-1 infected individuals remaining viraemic with highly active antiretroviral therapy (HAART).
Lancet.
351
1998
723
724
300
Levitz
SM
Improvement in CD4+ cell counts despite persistently detectable HIV load.
N Engl J Med.
338
1998
1074
1075
301
Piketty
C
Castiel
P
Belec
L
Discrepant responses to triple combination antiretroviral therapy in advanced HIV disease.
AIDS.
12
1998
745
750
302
Mezzaroma
I
Carlesimo
M
Pinter
E
et al. 
Long-term evaluation of T-cell subsets and T-cell function after HAART in advanced stage HIV-1 disease.
AIDS.
13
1999
1187
1193
303
Li
TS
Tubiana
R
Katlama
C
Calvez
V
Ait Mohand
H
Autran
B
Long-lasting recovery in CD4 T-cell function and viral-load reduction after highly active antiretroviral therapy in advanced HIV-1 disease.
Lancet.
351
1998
1682
1686
304
Lederman
MM
Connick
E
Landay
A
et al. 
Immunologic responses associated with 12 weeks of combination antiretroviral therapy consisting of zidovudine, lamivudine, and ritonavir: results of AIDS Clinical Trials Group Protocol 315.
J Infect Dis.
178
1998
70
79
305
Pakker
NG
Roos
MT
van Leeuwen
R
et al. 
Patterns of T-cell repopulation, virus load reduction, and restoration of T-cell function in HIV-infected persons during therapy with different antiretroviral agents.
J Acquir Immune Defic Syndr.
16
1997
318
326
306
Angel
JB
Kumar
A
Parato
K
et al. 
Improvement in cell-mediated immune function during potent anti-human immunodeficiency virus therapy with ritonavir plus saquinavir.
J Infect Dis.
177
1998
898
904
307
Sousa
AE
Chaves
AF
Doroana
M
Antunes
F
Victorino
RMM
Kinetics of the changes of lymphocyte subsets defined by cytokine production at single cell level during highly active antiretroviral therapy for HIV-1 infection.
J Immunol.
162
1999
3718
3726
308
Chinnaiyan
AM
Woffendin
C
Dixit
VM
Nabel
GJ
The inhibition of pro-apoptotic ICE-like proteases enhances HIV replication.
Nat Med.
3
1997
333
337
309
Chun
TW
Fauci
AS
Latent reservoirs of HIV: obstacles to the eradication of virus.
Proc Natl Acad Sci U S A.
96
1999
10958
10961
310
Schrager
LK
D'Souza
MP
Cellular and anatomical reservoirs of HIV-1 in patients receiving potent antiretroviral combination therapy.
JAMA.
280
1998
67
71
311
Finzi
D
Blankson
J
Siliciano
JD
et al. 
Latent infection of CD4+ T cells provides a mechanism for lifelong persistence of HIV-1, even in patients on effective combination therapy.
Nat Med.
5
1999
512
517
312
Ho
DD
Toward HIV eradication or remission: the tasks ahead.
Science.
280
1998
1866
1867
313
Wein
LM
D'Amato
RM
Perelson
AS
Mathematical analysis of antiretroviral therapy aimed at HIV-1 eradication or maintenance of low viral loads.
J Theor Biol.
192
1998
81
98
314
Zhang
L
Ramratnam
B
Tenner-Racz
K
Quantifying residual HIV-1 replication in patients receiving combination antiretroviral therapy.
N Engl J Med.
340
1999
1605
1613
315
Vocero-Akbani
AM
Heyden
NV
Lissy
NA
Ratner
L
Dowdy
SF
Killing HIV-infected cells by transduction with an HIV protease-activated caspase-3 protein.
Nat Med.
5
1999
29
33

Author notes

Andrew D. Badley, Division of Infectious Diseases, Ottawa Hospital Research Institute, 501 Smyth Rd, Ottawa, Ontario K1H 8L6, Canada; e-mail: abadley@ottawahospital.on.ca.