The Identification of the Endogenous Ligands of Natural Killer T Cells Reveals the Presence of Mammalian α-Linked Glycosylceramides

Glycosylceramides in mammalian species are thought to be present in the form of beta-anomers. This conclusion was reinforced by the identification of only one glucosylceramide and one galactosylceramide synthase, both beta-transferases, in mammalian genomes. Thus, the possibility that small amounts of alpha-anomers could be produced by an alternative enzymatic pathway, by an unfaithful enzyme, or spontaneously in unusual cellular compartments has not been examined in detail. We approached the question by taking advantage of the exquisite specificity of T and B lymphocytes and combined it with the specificity of catabolic enzymes of the sphingolipid pathway. Here, we demonstrate that mammalian immune cells produce constitutively very small quantities of alpha-glycosylceramides, which are the major endogenous ligands of natural killer T cells. Catabolic enzymes of the ceramide and glycolipid pathway tightly control the amount of these alpha-glycosylceramides. The exploitation of this pathway to manipulate the immune response will create new therapeutic opportunities.