EFFECTS OF PH AND POLYSACCHARIDES ON PEPTIDE BINDING TO CLASS-II MAJOR HISTOCOMPATIBILITY COMPLEX-MOLECULES

The binding of immunogenic peptides to class II major histocompatibility molecules was examined at various pH values. We studied binding of peptides containing residues 52-61 from hen egg lysozyme (HEL) to I-A(k) on fixed peritoneal macrophages or to solubilized affinity-purified I-A(k). Optimum binding occurred at pH 5.5-6.0 with accelerated kinetics relative to pH 7.4; equilibrium binding was also higher at pH 5.5-6.0 than at 7.4. Similar enhancement at pH 5-6 was observed for the binding of hemoglobin-(64-76) to I-E(k) and of ribonuclease-(41-61) to I-A(k). In contrast, the binding of HEL-(34-45) to I-A(k) was minimally enhanced at acid pH. Dissociation of cell-associated or purified peptide-I-A(k) complexes was minimal between pH 5.5 and 7.4, with increased dissociation only at or below pH 4.0 [HEL-(46-61)] or pH 5.0 [HEL-(34-45)]. Thus, optimum peptide binding occurs at pH values similar to the endosomal environment, where the complexes appear to be formed during antigen processing. In addition, we examined the effect of a number of polysaccharides on the binding of peptide to I-A(k). None of these competed with the HEL peptide I-125-labeled YE52-61 for binding to I-A(k). [H-3]Dextran also failed to bind purified I-A(k). Polysaccharides do not appear to bind to class II major histocompatibility complex molecules, which explains the T-cell independence of polysaccharide antigens.