CTL derived from (C3H x B6.K1)F1 animals were sensitized against L cells that express the transfected gene product Q10d/Ld. These CTL were highly crossreactive against three other class I molecules, H-2Kbm1, H-2Ld, and H-2Kd. In an attempt to define this crossreactive epitope it was noted that between 25 and 39% of amino acids in the alpha helices and central beta strands of these three molecules vary from Q10d. These amino acids represent residues that have been proposed to potentially interact with a peptide antigen or TCR (21). However, all four molecules share the amino acid tyrosine at positions 155 and 156. Additionally, Q10d, H-2Kbm1, and H-2Ld share alanine at position 152, while H-2Kd has an aspartic acid. We showed that these residues were important in controlling this epitope by the finding that anti-Q10d CTL did not recognize H-2Kbm1 revertant molecules that had either the position 152 alanine changed back to the wild-type H-2Kb residue (glutamic acid) or position 155 and 156 tyrosines changed back to wild-type residues arginine and leucine. Further evidence that these molecules share a crossreactive epitope was noted by the failure of (C3H x H-2Kbm1)F1 animals to generate CTL that recognized H-2Ld or H-2Kd, and the inability of (C3H x BALB/c)F1 animals to generate CTL reactive against H-2Kbm1. CTL from these mice were still able to recognize Q10d/Ld indicating that other epitopes could be detected if natural tolerance prevented recognition of the crossreactive epitope. To further define the epitope, CTL clones were generated against Q10d/Ld and maintained on either H-2Kbm1 or BALB/c feeder cells. In addition to testing these clones on the target cells described above, mutant molecules derived from H-2Ld, which have amino acid substitutions in their alpha 1 domain, were analyzed. It was noted that some anti-Q10 clones that did not crossreact on H-2Ld did react against H-2Ld mutant antigens that had H-2Dd amino acid substitutions in the alpha 1 domain at positions 63, 65, 66, and 70. Other clones had differential reactivities on these H-2Ld mutants further substantiating that alpha 1 domain amino acids play a role in controlling the expression of the crossreactive epitope. Thus, four class I molecules with multiple amino acid differences in their alpha 1 and alpha 2 domains share a crossreactive epitope readily recognized by alloreactive CTL.(ABSTRACT TRUNCATED AT 250 WORDS)

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