An excessive immune response to bacterial infection can lead to death due to massive tissue damage by the immune system. Kondo et al. found that A4galt-deficient mice that cannot synthesize the globo-series glycosphingolipids, including Gb3 and Gb4, were more likely than wild-type mice to die in an assay of lipopolysaccharide (LPS)–induced lethal shock. Analysis of primary mouse vascular endothelial cell cultures from the A4galt-deficient mice showed that the cells exhibited an enhanced inflammatory response to LPS. Exposure of wild-type endothelial cells to LPS stimulated expression of A4galt and increased the cell surface abundance of Gb4. In vitro studies with recombinant tagged MD-2, a subunit of the Toll-like receptor 4 (TLR4) complex that binds and responds to LPS, showed that Gb4 bound directly to MD-2, and endogenous Gb4 was present with tagged MD-2 immunoprecipitated from an endothelial cell line. Binding studies with Gb4, LPS, and TLR4–MD-2 indicated that Gb4 functioned as a noncompetitive inhibitor of LPS by binding to the receptor, and molecular simulations suggested that Gb4 could make multiple interactions with amino acids in the receptor complex. Administration of Gb4 to mice reduced the lethality of injected LPS, and histological examination showed reduced lung and liver pathology compared with wild-type mice subjected to the same LPS dosing regimen. Thus, Gb4 appears to function as an endogenous inhibitor of the innate immune response by reducing activation of the TLR4–MD-2 complex by bacterial lipids.
Y. Kondo, K. Ikeda, N. Tokuda, C. Nishitani, U. Ohto, S. Akashi-Takamura, Y. Ito, M. Uchikawa, Y. Kuroki, R. Taguchi, K. Miyake, Q. Zhang, K. Furukawa, K. Furukawa, TLF4–MD-2 complex is negatively regulated by an endogenous ligand, globotetraosylceramide. Proc. Natl. Acad. Sci. U.S.A. 110, 4714–4719 (2013). [Abstract] [Full Text]