Functional consequences of the binding of MHC class II-derived peptides to MHC class II.

Three MHC class II-derived synthetic peptides (I-A beta (g7)1-16, I-A beta (g7)52-77 and I-A alpha (g7)63-82YC) were analyzed for their ability to bind to syngeneic and allogeneic MHC class II molecules using a whole cell, competitive peptide binding assay. These studies demonstrated that the A beta (g7)1-16 peptide was able to specifically bind to syngeneic as well as to four allogeneic MHC class II molecules. The A alpha (g7)63-82YC peptide bound to self MHC class II molecules with a lower relative affinity and was able to bind to three out of the four allogeneic cells tested. The binding of the three I-A(g7)-derived peptides to the self MHC class II was functionally significant. The A beta (g7)1-16 and A beta (g7)52-77 peptides inhibited the proliferation of a heat shock protein 60 peptide-specific Th1 clone by MHC blockade. Interestingly, the A alpha (g7)63-82YC peptide appeared to interact directly with T cells as pretreatment of the Th1 clone with this peptide resulted in inhibition of antigen-induced proliferation. This phenomenon was analyzed in more detail and it was found that this peptide could behave as a partial agonist. Incubation of T cells with the A alpha (g7)63-82YC peptide resulted in up-regulation of IL-2R alpha chain expression and induction of IFN-gamma secretion. In addition T cells pretreated with this peptide were rendered hyporesponsive to further antigenic stimulation. Thus, a peptide derived from MHC class II may be used in an immunoregulatory capacity.