CD1 proteins: targets of T cell recognition in innate and adaptive immunity.

The CD1 family consists of antigen presenting molecules encoded by genes located outside of the major histocompatibility complex. CD1 proteins are conserved among mammalian species and are expressed on the surface of cells involved in antigen presentation. The CD1 system has been shown to be involved in activation of cell-mediated responses, and T cells specific for either CD1 molecules or antigens presented by CD1 have been isolated. Structural and biochemical analyses demonstrate that antigens presented by CD1 are nonpeptide lipid or glycolipid structures, including examples found in the cell walls of pathogenic mycobacteria. The hydrophobic part of these antigens most likely binds in the CD1 ligand-binding groove, whereas the polar headgroup of these antigens appears to make direct contact with the T cell receptor and determines specific recognition. Presentation of antigens by CD1 molecules requires uptake and intracellular processing by antigen presenting cells and can be achieved for both exogenous and endogenous antigens. T cells recognizing CD1 restricted antigens have a broad range of functional activities that suggest that the CD1 system is involved in both innate and adaptive immune responses against microbial infections.