The Total Synthesis of Proposed Immunoactive Glycolipids from S. Pneumoniae and a Re-evaluation of Their Immunological Activity
Invariant natural killer T cells (iNKT), a subclass of white blood cells, are responsible for the production of pro-inflammatory cytokines which induce a systemic immune response. They are distinctive in having an invariant T-cell receptor that recognizes glycolipid antigens presented by the class I major histocompatibility complex-related protein CD1d, which is conserved across multiple mammalian species in a class of proteins well-renowned for their high degree of polymorphism. This receptor’s first identified antigen is the potent KRN7000, a glycosphingolipid isolated from bacteria that were found on a Japanese marine sponge. The corresponding terrestrial antigen remained unidentified until quite recently, when diacylglycerol-containing glycolipids, reported to activate iNKT cells, were isolated from Streptococcus pneumoniae. We report the total synthesis and immunological re-evaluation of these two glycolipids. The compounds are unable to activate iNKT cells. Computational modelling shows that these ligands, while being capable of interacting with the CD1d receptor, create a different surface for the binary complex that makes formation of the ternary complex with the iNKT T-cell receptor difficult. Together these results suggest that the reported activity might have been due to an impurity in the original isolated sample, and highlights the importance of taking care when reporting biological activity from isolated natural products.