MOLECULAR ANALYSIS OF A HETEROCLITIC T-CELL RESPONSE TO THE IMMUNODOMINANT EPITOPE OF SPERM WHALE MYOGLOBIN - IMPLICATIONS FOR PEPTIDE PARTIAL AGONISTS

We have investigated the molecular basis for binding and Ag presentati on of an immunodominant Th cell determinant of sperm whale myoglobin, a prototype amphipathic helical structure in the native protein. A ser ies of peptides with three different substitutions at each position we re evaluated for binding to the class II MHC molecule I-A(d) and for a ctivation of two T cell clones with distinct fine specificity, to dete rmine the role of each residue. The assignment of MHC binding and TCR binding residues is consistent with a peptide bound as a twisted beta- strand, with 130 degrees twist similar to that of the influenza hemagg lutinin peptide crystallized in the groove of HLA-DR1. This twist give s the peptide amphipathicity, with a periodicity similar to an alpha-h elix without its being a helix. Two substituted peptides were discover ed to be heteroclitic, but by different molecular mechanisms, one invo lving gain of a favorable residue and one involving loss of an unfavor able one. Complexes of both peptides with I-A(d) had enormously higher affinity for the TCR, but peptide affinity for the MHC molecule was n ot increased, such that the wild-type peptide acted as a partial agoni st and inhibited the response to the heteroclitic ones. Moreover, the magnitude of response was elevated in a way that could not be mimicked by the wild-type peptide even at higher concentration. These results suggest a TCR dwell time requirement for optimal signal transduction t hat may help explain the mechanism of partial agonism.