Endotoxin tolerance: new mechanisms, molecules and clinical significance

Prior exposure of innate immune cells like monocytes/macrophages to minute amounts of endotoxin cause them to become refractory to subsequent endotoxin challenge, a phenomenon called "endotoxin tolerance". Clinically, this state is associated with monocytes/macrophages in sepsis patients where they contribute to "immunosuppression" and mortality. The molecular mechanisms underlying endotoxin tolerance remain elusive. The recent appreciation of inflammation as a self-regulating process, the relative contribution of MyD88 versus TRIF signaling pathways in inducing activation or tolerance, plasticity of NF-kappaB function and the role of chromatin modification and microRNAs in LPS-induced gene reprogramming urges a re-evaluation of endotoxin tolerance. This review integrates these new findings into an up-to-date account of endotoxin tolerance, its molecular basis and clinical implications in different pathologies.