Lipopolysaccharide sensing an important factor in the innate immune response to Gram-negative bacterial infections: benefits and hazards of LPS hypersensitivity

Circulating lipoproteins are a crucial component of host defense against invasive Salmonella typhimurium infection

Background

Circulating lipoproteins improve the outcome of severe Gram-negative infections through neutralizing lipopolysaccharides (LPS), thus inhibiting the release of proinflammatory cytokines.

Methods/Principal Findings

Low density lipoprotein receptor deficient (LDLR−/−) mice, with a 7-fold increase in LDL, are resistant against infection with Salmonella typhimurium (survival 100% vs 5%, p<0.001), and 100 to 1000-fold lower bacterial burden in the organs, compared with LDLR+/+ mice. Protection was not due to differences in cytokine production, phagocytosis, and killing of Salmonella organisms. The differences were caused by the excess of lipoproteins, as hyperlipoproteinemic ApoE−/− mice were also highly resistant to Salmonella infection. Lipoproteins protect against infection by interfering with the binding of Salmonella to host cells, and preventing organ invasion. This leads to an altered biodistribution of the microorganisms during the first hours of infection: after intravenous injection of Salmonella into LDLR+/+ mice, the bacteria invaded the liver and spleen within 30 minutes of infection. In contrast, in LDLR−/− mice, Salmonella remained constrained to the circulation from where they were efficiently cleared, with decreased organ invasion.

Conclusions

plasma lipoproteins are a potent host defense mechanism against invasive Salmonella infection, by blocking adhesion of Salmonella to the host cells and subsequent tissue invasion.

Selective contribution of Tyk2 to cell activation by lipopolysaccharide

Tyk2 deficient mice show a markedly reduced susceptibility to lipopolysaccharide (LPS) induced shock and a partial impairment of IL-12 and interferon (IFN) signals. To examine the underlying mechanisms, we analysed the activation of peritoneal macrophages (PMPhis) and spleen cells after LPS challenge. In PMPhis and spleen cells the contribution of Tyk2 to the induction of the T-cell co-stimulatory molecules CD86, CD40 and MHC II was small or insignificant. By contrast, induced expression of the early activation marker CD69 on PMPhis and splenic cell populations required type I interferons (IFN-I) and Tyk2. The data suggest a selective contribution of Tyk2 to the activation of inflammation-relevant cell types by LPS.