INVARIANT-COGNATE PEPTIDE EXCHANGE RESTORES CLASS-II DIMER STABILITY IN HLA-DM MUTANTS

Class II presentation mutants have mutations in the HLA-DMA or B genes and are defective in the presentation of whole exogenous Ags restrict ed by HLA-DR, -DQ, and -DP. The functional defect in Ag presentation i s accompanied by an altered conformation of cell surface class II mole cules and instability of extracted class II dimers in SDS-PAGE; the la tter can be corrected by incubation of mutant cells in an acidic pH in the presence of cognate peptide. Here we investigated the basis for c orrection of class II dimer instability by acid/cognate peptide treatm ent and the extent to which this treatment corrects the class II confo rmational defect in DMB mutants. We found that an acidic pH generates peptide binding sites in class II molecules of DMB mutants by eluting invariant chain (li)-derived peptides from them. Cognate peptides can then bind to the empty binding sites of class II molecules in a pH-ind ependent manner, which results in stabilization of class II dimers. Ac id/peptide treatment also restores the DR polymorphic epitope recogniz ed by mAb 7.3.19.1 but not the DR polymorphic epitope recognized by mA b 16.23; low pH gradually destroys the 16.23 epitope in nonmutant cell s. Mutant 10.24.6, which has a mutation in the DRA coding region creat ing an extra glycosylation site, also has unstable DR dimers whose sta bility is restored by acid/peptide treatment. These results suggest th at the primary phenotypic defect in both the DMB and 10.24.6 mutants i s the abundance of li peptides and lack of cognate peptides bound to c lass II molecules.