T lymphocytes express either alpha beta or gamma delta T-cell receptor heterodimers. Most alpha beta T cells recognize antigenic peptides bound to major histocompatibility complex molecules but the antigen recognition and biological function of gamma delta T cells is unknown. A major human gamma delta T-cell subset expressing V gamma 2 and V delta 2 germline genes, but having diverse junctional sequences, is found in human mycobacterial lesions and responds in vitro to antigens of bacteria and parasites. In addition, certain haematopoietic tumour cells are specifically recognized and lysed by these T cells. V gamma 2V delta 2-bearing T cells were shown to recognize mycobacterial antigens that are protease resistant and phosphatase sensitive. Because of the difficulty in isolating natural antigens from mycobacterial culture filtrates or extracts, we synthesized a series of monoalkyl phosphates, and found that some, particularly monoethyl phosphate, could mimic the activity of mycobacterial antigens in stimulating these gamma delta T cells. Here we report the identification of natural antigens produced by mycobacteria recognized by human V gamma 2V delta 2-bearing T cells as isopentenyl pyrophosphate and related prenyl pyrophosphate derivatives, compounds involved in the synthesis of complex polyisoprenoid compounds in microbial and mammalian cells. Substitution of phosphate for the pyrophosphate moiety, or elimination of the double bond, greatly reduced antigenic activity of these compounds. These results provide formal evidence that, in contrast to recognition of major histocompatibility complex-bound peptide antigens by alpha beta T cells, human gamma delta T cells can recognize naturally occurring small non-peptidic antigens.

Major histocompatibility complex (MHC) class I and class II molecules bind immunogenic peptides and present them to lymphocytes bearing the alpha beta T-cell antigen receptor (TCR). An analogous antigen-presenting function also has been proposed for the non-MHC-encoded CD1 molecules, a family of non-polymorphic, beta 2-microglobulin-associated glycoproteins expressed on most professional antigen-presenting cells. In support of this hypothesis, CD1 molecules are recognized by selected CD4-CD8- alpha beta or gamma delta TCR+ T-cell clones, and we have recently shown that CD1 molecules restrict the recognition of foreign microbial antigens by alpha beta TCR+ T cells. But the substantial structural divergence of CD1 from MHC class I and class II molecules, raises the possibility that the antigens presented by the CD1 system may differ fundamentally from those presented by MHC-encoded molecules. Here we report that a purified CD1b-restricted antigen of Mycobacterium tuberculosis presented to alpha beta TCR+ T cells is mycolic acid, a family of alpha-branched, beta-hydroxy, long-chain fatty acids found in mycobacteria. This example of non-protein microbial antigen recognition suggests that alpha beta TCR+ T cells recognize a broader range of antigens than previously appreciated and that at least one member of the CD1 family has evolved the ability to present lipid antigens.

Human Vgamma2Vdelta2 T cells play important roles in mediating immunity against microbial pathogens and have potent anti-tumor activity. Vgamma2Vdelta2 T cells recognize the pyrophosphorylated isoprenoid intermediates (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMBPP), an intermediate in the foreign 2-C-methyl-d-erythritol 4-phosphate (MEP) pathway, and isopentenyl pyrophosphate (IPP), an intermediate in the self-mevalonate pathway. Infection with bacteria and protozoa using the MEP pathway leads to the rapid expansion of Vgamma2Vdelta2 T cells to very high numbers through preferential recognition of HMBPP. Activated Vgamma2Vdelta2 T cells produce proinflammatory cytokines and chemokines, kill infected cells, secrete growth factors for epithelial cells, and present antigens to alphabeta T cells. Vgamma2Vdelta2 T cells can also recognize high levels of IPP in certain tumors and in cells treated with pharmacological agents, such as bisphosphonates and alkylamines, that block farnesyl pyrophosphate synthase. Activated Vgamma2Vdelta2 T cells are able to kill most tumor cells because of recognition by T-cell receptor and natural killer receptors. The ubiquitous nature of the antigens converts essentially all Vgamma2Vdelta2 T cells to memory cells at an early age. Thus, primary infections with HMBPP-producing bacteria are perceived by Vgamma2Vdelta2 T cells as a repeat infection. Extensive efforts are underway to harness these cells to treat a variety of cancers and to provide microbial immunity.