MAJOR INVOLVEMENT OF CATHEPSIN-B IN THE INTRACELLULAR PROTEOLYTIC PROCESSING OF EXOGENOUS IGGS IN U937 CELLS

Monoclonal antibodies used for diagnostic and therapeutic purposes beh ave as antigens when injected into patients. They are recognized by T cells in a processed form and in a major histocompatibility complex cl ass II restricted fashion. Monoclonal murine IgG2a were used as a mode l to analyse the early phase of antigen processing in U937 cells. IgG2 a prebound to cell surface Fc receptors were rapidly internalized in t he cells. During internalization, they were proteolysed with a time-de pendent intracellular accumulation of 26, 25, 24, 22 and 14 kDa fragme nts. Comparison of in vitro IgG2a proteolysis by U937 subcellular frac tions or by purified cathepsin B and their intracellular processing in dicated that a major cathepsin B like protease is responsible for IgG2 a intracellular processing in endo-lysosomal compartments of U937 cell s.