Construction of MHC Class I-Peptide Tetrameric Complexes for Analysis of T-Cell-Mediated Immune Resp

Tetrameric major histocompatibility complex (MHC) class I-peptide complexes (tetramers) are a powerful tool in the study of antigen-specific CD8+ T cell responses (1 ,2 ). Although the interaction of a monomeric MHC-peptide complex with the T-cell receptor is of a low affinity and fast off-rate, the avidity of multimeric complexes provides a relatively stable T-cell binding, which allows T-cell enumeration and phenotypic analysis. The tetramer approach is highly sensitive, with a detection limit of approx 0.02% of CD8+ T cells, and the frequencies obtained correlate well with other T-cell assays. Tetramer analysis gives up to five- and 10-fold higher frequencies than the respective interferon-γ-based enzyme-linked immunospot (ELISpot) and limiting dilution assays, although these ratios may depend on the antigenic load. The use of tetrameric MHC-peptide complexes revealed, for the first time, massive clonal expansions of T-cells, which in the case of primary Epstein-Bar virus (EBV) infection reached 45% of circulating CD8+ peripheral blood mononuclear cells (PBMC) (see ref. 3 ). The ease with which tetramer complexes are constructed depends on the particular MHC (called HLA for humans) molecule and the peptide ligand. For example, the HLA-A✻0201 molecule folds relatively easily into a conformationally correct complex compared to the HLA-B57 molecule. A growing list of these reagents is available commercially and also from the National Institutes of Health, which provide tetramers free of charge, but for research purpose only (Table 1 ). The construction of MHC class II-peptide tetrameric complexes has proven to be more problematic than class I, with a fewer publications mainly involving mouse tetramers and no standard protocol. Table 1  MHC Class I Alleles Available at the NIH Tetramer Facility (2001)