Inhibition by leupeptin and antipain of the intracellular proteolysis of Ii.

Intracellular cleavage of Ii was evaluated in immunoprecipitates of radiolabeled Raji cells treated with protease inhibitors (leupeptin, antipain, chymostatin, and pepstatin) or blockers of endosomal function (chloroquine and monensin). Immunoprecipitates with anti-class II and anti-Ii(12-28) sera and VIC-Y1 MoAb revealed Ii cleavage products of 21,000 and 10,000 daltons (p21 and p10) only in leupeptin- and antipain-treated cells. Both p21 and p10 were judged to be N-terminal products because they were recognized with anti-Ii(12-28) and not with anti-Ii(183-193) or anti-Ii(192-211) sera. p10 might be derived from p21 because its intensity was increased in inverse proportion to p21 as a function of leupeptin or antipain concentration. p21, but not p10, was recognized by anti-class II antibody and thus might originate from class II-associated Ii. In pulse-chase studies, p21 and p10 appeared at 2 hr and later after Ii synthesis. p25, an Ii C-terminal fragment, was about 60% reduced by leupeptin or antipain. Intracellular proteolytic cleavage of class II-associated Ii appeared to follow two pathways leading either to N-terminal p21 and p10 or to C-terminal p25. Such cleavages might regulate or catalyze foreign antigen binding to class II.