Mapping of allospecific T-cell recognition sites encoded by the HLA-DR4 β₁-chain

Genes of the major histocompatibility complex (MHC) influence the immune system by their central role in the activation of T lymphocytes, which have to corecognize antigen in the association with MHC-encoded cell surface molecules. The location and number of sites on HLA class II molecules that interact with T-cell receptors remains unknown. Using a set of ten alloreactive human T-cell clones we have defined the molecular basis of T-cell interaction sites on the HLA-DR4 molecule. At least seven unique determinants are recognized that are confined to an immunodominant region encoded by the third hypervariable region (hvr) of the HLA-DR ß1-chain. Substitutions at amino acid positions 71, 74 and 86 contribute similarly to determinants recognized by alloreactive T cells. A cluster of tightly overlapping sites stimulatory for distinct T cells is contained within the segment of amino acid residues 71 to 86, which is expressed within the HLA-DR4 as well as HLA-DR1 haplotype. Five of the ten T-cell clones are stimulated by HLA-DR1⁺ cells, suggesting that allospecific T-cell receptors directly interact with protein structures determined by the third hvr. These data provide evidence that the third hvr of the HLA-DR ß1-chain encodes for a functional domain on the surface of the molecule that is recognized by a polyclonal T-cell response.