Molecular mechanisms and pathological consequences of endotoxin tolerance and priming

Lipopolysaccharide (LPS), a component of Gram-negative bacteria, is a potent inflammatory stimulant, with high doses due to disseminated bacterial infection resulting in systemic inflammatory response syndrome and death. Lower doses can induce a state of tolerance to subsequent toxic doses of LPS, but extremely low doses have an opposite effect, priming the immune system for an even more violent response to subsequent challenge. A substantial body of research exists on the phenomenon of endotoxin tolerance, which appears to be a state of generalized dampening of inflammatory pathways. Comparatively little is known about the mechanisms or indeed the phenomenon of priming, particularly regarding the shift from a priming to a tolerizing response. Our aim is to review recent findings in the field of the inflammatory response to endotoxin, with a focus on highlighting the gaps in current understanding and attempting to reconcile the competing tolerance and priming phenomena.