TRANSPORT AND INTRACELLULAR-DISTRIBUTION OF MHC CLASS-II MOLECULES AND ASSOCIATED INVARIANT CHAIN IN NORMAL AND ANTIGEN-PROCESSING MUTANT-CELL LINES

We have compared the intracellular transport and subcellular distribut ion of MHC class II-invariant chain complexes in a wild-type HLA-DR3 h omozygous cell line and a mutant cell line, T2.DR3. The latter has a d efect in antigen processing and accumulates HLA-DR3 molecules associat ed with an invariant chain-derived peptide (CLIP) rather than the norm al complement of peptides derived from endocytosed proteins. We find t hat in the wild-type cells, CLIP is transiently associated with HLA-DR 3 molecules, suggesting that the peptide is a normal class II-associat ed intermediate generated during proteolysis of the invariant chain. I n the mutant cell line proteolysis of the invariant chain is less effi cient, and HLA-DR3/CLIP complexes are generated much more slowly. Exam ination of the mutant cell line by immunoelectronmicroscopy shows that class II-invariant chain complexes accumulate intracellularly in larg e acidic vesicles which contain lysosomal markers, including beta-hexo saminidase, cathepsin D, and the lysosomal membrane protein CD63. The markers in these vesicles are identical to those seen in the class II- containing vesicles (MIICs) seen in the wild-type cells but the morpho logy is drastically different. The vesicles in the mutant cells are en docytic, as measured by the internalization of BSA-gold conjugates. Th e implication of these findings for antigen processing in general and the nature of the mutation in particular are discussed.