Analysis of the requirements for class II-restricted T cell recognition of a single determinant reveals considerable diversity in the T cell response and degeneracy of peptide binding to I-Ed.

The amino acid sequences recognized by five I-E(d)-restricted and one E alpha A beta d-restricted murine T cell clones were determined. The clones had been raised to a synthetic peptide representing amino acids 305-328 of influenza virus hemagglutinin. It was found that although all of the T cell clones recognized a single 10-residue region of the peptide, 307KYVKQNTLKL316, different clones could recognize minimal ("core") determinants spanning 8, 9, or 10 of these amino acids. To see whether particular amino acids within the sequence 307-316 were universally important for T cell recognition, the six clones were assayed for their ability to tolerate single amino acid substitutions of the 10 residue peptide. In all, 190 analogues of the peptide in which each amino acid in the sequence was replaced, in turn, by each of the other 19 naturally occurring amino acids were tested. It was shown that 1) the six T cell clones had very different requirements for recognition of the peptide, 2) substitutions at every single position within the peptide could be shown to affect recognition in a T cell-specific manner, and 3) every single position within the peptide could be replaced by a large number of amino acids and still be recognized by at least one T cell clone. These results demonstrate the great diversity exhibited by the T cell repertoire in recognizing a 10-amino acid determinant, as well as the degeneracy of peptide binding to I-E(d).