By Gregory M. Vercellotti, MD

2009-09-01

Dr. Vercellotti indicated no relevant conflicts of interest.

Andonegui G, Zhou H, Bullard D, et al. Mice that exclusively express TLR4 on endothelial cells can efficiently clear a lethal systemic Gram-negative bacterial infection. J Clin Invest. 2009;119:1921-30.

Twenty years ago Charles Janeway proposed the concepts of innate immunity that transformed immunology.^1,2 These ideas suggested that innate immune recognition of microbes depended upon receptors that detected conserved microbial products using pathogen-associated molecular patterns. This evolutionarily conserved mechanism led to the discovery in drosophila of Toll-like receptors (TLR) that could bind ligands of bacteria, viruses, and fungi that were non-self. Today, TLRs are targets for novel therapies to treat sepsis, skin diseases, and cancer. In vitro culture techniques of the vascular endothelium developed some 40 years ago revolutionized our concepts of its function from a bland inert barrier to an active cell-regulating inflammation, thrombosis, and immunity. The vasculature and innate immunity have become intimately intertwined. Andonegui et al., from the laboratory of Paul Kubes, demonstrated that endothelial cells play a critical sentinel role for binding Lipopolysaccharide (LPS) via a TLR-4 receptor and preventing E. coli lethality.

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 Endothelium^TLR-4 that exclusively express TRL-4 on the endothelium but not on granulocytes or macrophages. Local injection of LPS induced neutrophil recruitment in both Endothelium^TLR-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 Endothelium^TLR-4 mice showed reduced pulmonary neutrophils and increased peritoneal neutrophils. Furthermore, Endothelium^TLR-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, Endothelium^TLR-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.

1. Janeway, CA Jr. Approaching the asymptote? Evolution and revolution in immunology. Cold Spring Harb Symp Quant Biol. 1989;54:1-13.
2. Medzhitov R. Approaching the asymptote: 20 years later. Immunity. 2009;30:766-75.
3. O’Neill LA, Bryant CE, Doyle SL. Therapeutic targeting of toll-like receptors for infectious and inflammatory diseases and cancer. Pharmacol Rev. 2009;61:177-97.
4. Castellheim A, Brekke OL, Espevik T, et al.  Innate immune responses to danger signals in systemic inflammatory response syndrome and sepsis. Scand J Immunol. 2009;69:479-91.

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