Delineation of functional sites in HLA-B27 antigens. Molecular analysis of HLA-B27 variant Wewak I defined by cytolytic T lymphocytes.

An HLA-B27 positive, Epstein Barr virus-transformed cell line Wewak I was not lysed by Epstein Barr virus-specific cytolytic T lymphocytes restricted by HLA-B27. This line is weakly reactive with a B27-specific monoclonal antibody, M2, which recognizes a majority, but not all, of the HLA-B27-positive cells. To establish the molecular basis for this lack of recognition, the structure of the variant HLA-B27 antigen was compared with the known structure of HLA-B27 from the LG-2 cell line, which is representative of a major subtype of this antigen. Both molecules were almost indistinguishable by isoelectric focusing. However, comparative peptide mapping and sequencing of the difference peptides revealed two amino acid changes: At position 77, Asp in donor LG-2 had changed to Ser in the variant, and at position 152, Val in LG-2 had changed to Glu in the variant. The nature of these substitutions was consistent with the extreme similarity of the isoelectric focusing pattern. An evaluation of these findings in the context of studies on other HLA variants and H-2Kb mutants suggests that both positions 77 and 152 contribute to the determinants recognized by B27-specific cytolytic T lymphocytes. The change at position 152 adds to previous evidence suggesting that the segment 149-156 is critical for cellular recognition. In addition, it is proposed on the basis of structural comparisons that residue 77 may also participate in the epitope recognized by the B27M2 antibody.