A MUTANT HUMAN HISTOCOMPATIBILITY LEUKOCYTE ANTIGEN DR MOLECULE ASSOCIATED WITH INVARIANT CHAIN PEPTIDES

From a human histocompatibility leukocyte antigen (HLA)-DR/DQ hemizygo us, B lymphoblastoid blastoid progenitor, we isolated a cell line, 10. 24.6, with a DR alpha missense mutation (96P --> 96S), which results i n an N-linked carbohydrate addition at position 94 in the DR alpha 2 d omain. Several features of 10.24.6 cells suggest that the mutation dis rupts normal intracellular formation of peptide/DR complexes. The muta nt HLA-DR dimers, though expressed at the cell surface, lack the confo rmation of the mature, peptide-loaded class II molecules of the progen itor cell, as assessed by their loss of binding of certain antibodies and by the lack of stability in detergent (sodium dodecyl sulfate) sol ution. In addition, presentation of endocytosed antigen to HLA-DR-rest ricted T cells is defective in the mutant, but can be restored by tran sfection of a wild type DRA gene. Assays with synthetic peptides indic ate that the 10.24.6 phenotype is not due to an intrinsic inability of the mutant DR molecules to bind peptides. Therefore, to directly eval uate peptide occupancy of the mutant molecules, we analyzed acid-elute d, HLA-DR-associated peptides. The predominant species from the 10.24. 6 mutant is a nested set of invariant chain (Ii)-derived peptides that are undetectable in the DR eluate from progenitor cells. The region o f DR alpha altered in the mutant molecules is thus implicated in norma l formation of peptide/DR complexes. Further, the same set of Ii pepti des associated with the DR molecules is present in the eluate from an antigen presentation mutant with a defect in an major histocompatibili ty complex (MHC)-linked gene. These results suggest that DR molecules in 10.24.6 and in certain presentation mutants are affected at the sam e or related steps in class II molecule biosynthesis, raising the poss ibility that class II molecules interact with an MHC-encoded accessory molecule during antigen presentation.