T-cell recognition of glycolipids presented by CD1 proteins.

The most well-known molecular paradigm of antigen recognition by T cells involves partial digestion of proteins to generate small peptides, which bind to major histocompatibility complex (MHC) proteins. Recent studies of CD1, an MHC class I homolog encoded outside the MHC, have revealed that it presents diverse glycolipids to T cells. The molecular mechanism for lipid antigen recognition involves insertion of the lipid portion of antigens into a hydrophobic groove to form CD1-lipid complexes, which contact T-cell receptors (TCRs). Here, we examine the known antigen structures presented by CD1, the majority of which have sugar moieties that are capable of interacting with TCRs. Recognition of carbohydrate epitopes is precise, and lipid-reactive T cells alter systemic immune responses in models of infectious and autoimmune disease. These findings provide a previously unrecognized mechanism by which the cellular immune system can recognize alterations in many types of carbohydrate structures.