A mutational analysis of the Aβ z /Aα d major histocompatibility complex class II molecule that restricts autoreactive T cells in (NZB×NZW)F1 mice. The critical influence of alanine at position 69 in the Aα d chain

Autoimmune symptoms of (NZB×NZW)F1 (H‐2 d/z ) mice are reported to be critically related to the heterozygosity at the H‐2 complex of the murine major histocompatibility complex (MHC). We previously showed that several Aβ z /Aα d MHC class II molecule‐restricted autoreactive T‐cell clones from B/WF1 mice were pathogenic upon transfer to preautoimmune B/WF1 mice. In this study, to identify the crucial amino acid residues in Aβ z /Aα d molecules for T‐cell activation, we generated a panel of transfectant cell lines. These transfectant cell lines express the Aβ z /Aα d MHC molecules with a mutation at each residue α11, α28, α57, α69, α70, α76 of Aα d chain and β86 of Aβ z chain. Replacing α69 alanine with threonine, valine or serine completely eliminated the ability to stimulate autoreactive T‐cell clones without affecting the ability to present foreign antigen keyhole limpet haemocyanin (KLH) or l‐plastin peptide to specific T‐cell clones. Replacing β86 valine with aspartic acid resulted in a decrease in the stimulation for antigen‐reactive as well as autoreactive T‐cell clones. Substitutions at other residues had minimal or no effect on the stimulation of either auto‐ or antigen‐reactive T‐cell clones. These results suggest that alanine at residue 69 of the Aα d chain is critical for the activation of autoreactive Aβ z /Aα d ‐restricted T‐cell clones. Possible explanations for this are discussed.