Proteolytic processing of specific antigen was studied using Epstein Barr virus transformed B-lymphoblastoid cells expressing membrane IgG against tetanus toxin. As previously reported (Watts, C., and H.W. Davidson. 1988. EMBO (Eur. Mol. Biol. Organ.) J. 7:1937-1945), receptor-mediated endocytosis of monovalent antigen bound at 0 degrees C began immediately upon shifting the cells to 37 degrees C. In contrast, degradation of antigen, assessed either by the release of acid-soluble radiolabel into the incubation medium, or by SDS-PAGE analysis of total cell-associated antigen, proceeded after a lag of 10-20 min. Degradation was abolished by exposure of the cells to metabolic inhibitors, or by incubation at 20 degrees C, and inhibited in a dose-dependent fashion by chloroquine and by the lysosomal protease inhibitors leupeptin, E-64, and pepstatin A. Analysis of the cell-associated radiolabel by SDS-PAGE and autoradiography after incubations at 37 degrees C revealed the time-dependent generation of distinct antigen fragments. Virtually quantitative immunoprecipitation of these fragments was obtained using a monoclonal anti-human IgG antibody, indicating that the antigen/mIg complex is the initial substrate for processing. We show that the pattern of fragmentation observed varies from one B cell line to another (a) depending on the epitope through which the antigen is bound and endocytosed and (b) depending on whether additional epitopes in the antigen are complexed with anti-tetanus Fabs. The implications of these results for the presentation of major histocompatibility complex restricted antigen fragments, and for intracellular trafficking of ligand/receptor complexes are discussed.

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