Tlr4-/- mice were mated with C57BL/6 mice, and the fertilized eggs were micro-injected with an endothelial-cell-specific TLR-4 transgene to produce mice called EndotheliumTLR-4 that exclusively express TRL-4 on the endothelium but not on granulocytes or macrophages. Local injection of LPS induced neutrophil recruitment in both EndotheliumTLR-4 and wild-type mice. When IV LPS or intraperitoneal E. coli was infused, neutrophils accumulated in the lungs of wild-type animals but did not accumulate in the peritoneum. However, conversely, the EndotheliumTLR-4 mice showed reduced pulmonary neutrophils and increased peritoneal neutrophils. Furthermore, EndotheliumTLR-4 mice injected with E. coli cleared the bacteria more effectively and did not die, while 50 percent of wildtype mice died within 24 hours. When LPS was administered intratracheally, EndotheliumTLR-4 mice failed to accumulate neutrophils in the lung, but when these mice were transplanted with bone marrow from TLR-4 replete wild-type mice, similar neutrophil accumulation occurred as in wild-type mice. Thus, TLR-4 on neutrophils and macrophages was necessary to respond to an intratracheal LPS challenge.
This study confirms that the 4,000 square meters of 60 trillion endothelial cells play an essential role in host defense. Remarkably, TLR-4 has been linked to multiple diseases ranging from atherosclerosis to prostate cancer and even Alzheimer disease. Endothelial cell TLR-4 polymorphisms may explain an individual’s susceptibility to multiple processes. What is remarkable in this paper is how activation of macrophage/neutrophil TLR-4 may induce a cytokine storm ultimately to the detriment of the host, while exclusive expression of endothelial cell TLR-4 cleared an IV injection of bacteria and permitted survival of the host. Neutrophil/macrophage cytokines were critical to the mortality, as tlr4-/- mice cleared bacteria very slowly (~50 percent at 168 hours), but did not die.
This paper provokes an observation and some questions from this hematologist. Most of our chemotherapy and hematopoietic stem transplant patients survive prolonged neutropenia with bacteremia. Are the 60 trillion endothelial cells clearing all those bacteremias? Does chemotherapy or immunotherapy alter endothelial cell TLR-4 function? Are some individuals with certain polymorphisms of TLR-4 more susceptible to sepsis? The signal transduction cascade downstream from TLR-4 includes two discrete pathways, Mal/MyD88 and TRAM/TRIF, leading to activation of NF-κB, p38 MAP kinase, JNK kinase, and others. Will pharmacologic targeting of these signaling molecules modulate TLR-4 and alter sepsis morbidities?3,4 Clearly, two decades later Janeway’s concepts of innate immunity suggest new ways to improve our patients’ outcomes with hematologic diseases.
Dr. Vercellotti indicated no relevant conflicts of interest.