T cells play essential effector and regulatory roles in adaptive immune responses to viruses, bacteria, parasites, tumors, transplanted tissues, allergens and even to self antigens. Antigen-specific T cell responses can be detected by functional assays—e.g. lymphoproliferation assays, cytotoxic T cell assays using chromium release, and cytokine production assays such as the ELISpot assay and intracellular cytokine staining—or by antigen-binding methods. T cells use the T cell antigen-receptor (TCR) to recognize their antigens, which are nearly always in the form of peptides bound to major histocompatibility complex molecules (MHC), also called HLA (human leukocyte antigens) in humans.
The affinity of isolated, soluble monomeric MHC/peptide complexes for their specific TCR partners is weak, and the complex has a half-life that is on the order of 10 seconds. The short half-life of this interaction long stymied efforts to identify antigen-specific T cells by ligand binding methods using labeled, monomeric MHC/peptide complexes. In 1994-1995, Michael McHeyzer-Williams, Mark Davis, and I solved this problem by engineering defined, soluble MHC/peptide multimers capable of engaging more than one copy of the TCR on the surface of a T cell, thereby increasing the avidity of the interaction. We accomplished this by enzymatic biotinylation of a soluble MHC molecule fused to a short peptide sequence that is a substrate for the E. coli enzyme BirA, followed by mixing of the biotinylated MHC/peptide complexes with fluorescently labeled streptavidin. These reagents, which we referred to as “MHC Tetramers”, proved to be very effective in identifying antigen-specific T cells by flow cytometry, even those present at low frequencies in fresh populations of lymphocytes sampled directly ex vivo. (See Altman et al., 1996. Science, 274: 94-6.)