Endotoxin tolerance is the situation in which previous exposure to a low level of lipopolysaccharide (LPS) induces a transient period of hyporesponsiveness after subsequent challenge with LPS. LPS-induced inflammatory responses are important for fighting Gram-negative bacterial infections. But if the response gets out of control, endotoxic shock can develop, which can be fatal. So, understanding the ways in which responses to LPS can be regulated could have clinical applications. In a study published in Immunity, Gerry Krystal and colleagues describe a regulatory role for the cytosolic phosphatase SHIP (SRC-homology-2-domain-containing inositol-5-phosphatase) in endotoxin tolerance.

LPS binds to Toll-like receptor 4 (TLR4) and stimulates two signalling pathways — a myeloid differentiation primary-response gene 88 (MyD88)-dependent pathway and a MyD88-independent pathway — both of which lead to activation of nuclear factor-κB (NF-κB). Based on studies showing that NF-κB activation is reduced in endotoxin tolerance and that SHIP inhibits the NF-κB pathway in bone-marrow-derived mast cells stimulated with IgE plus antigen, the authors decided to investigate the role of SHIP in endotoxin tolerance.

SHIP-deficient mice were shown to be more susceptible to LPS-induced toxicity than wild-type mice. The SHIP-deficient mice produced considerably increased amounts of pro-inflammatory cytokines and nitric oxide, indicating that SHIP is a negative regulator of LPS-induced production of inflammatory mediators. In contrast to wild-type cells, bone-marrow-derived macrophages and mast cells from SHIP-deficient mice did not show endotoxin tolerance.

So, how does SHIP mediate endotoxin tolerance? Because cell-surface levels of TLR4 were similar for SHIP-deficient and SHIP-sufficient cells stimulated with a tolerizing dose of LPS, TLR4 expression cannot account for the effect. By contrast, SHIP protein expression was markedly upregulated by SHIP-sufficient cells after low-level stimulation with LPS. SHIP-directed antisense oligonucleotides were then used to decrease SHIP expression in SHIP-sufficient cells, confirming that it is the lack of induction of SHIP expression, and not any secondary effects, that accounts for the inability to generate endotoxin tolerance. In vivo studies using SHIP-deficient mice also supported a role for SHIP in endotoxin tolerance.

Next, because transforming growth factor-β (TGF-β) has been reported to be an inducer of SHIP expression, the authors asked whether autocrine production of TGF-β has a role in the LPS-induced increase in SHIP expression. This was found to be the case, because TGF-β increased SHIP expression by SHIP-sufficient cells that were stimulated with LPS. In addition, neutralizing antibodies specific for TGF-β inhibited the increase in SHIP expression, thereby preventing the induction of endotoxin tolerance.

This study considerably advances our understanding of endotoxin tolerance.