WORTMANNIN, A PHOSPHATIDYLINOSITOL 3-KINASE INHIBITOR, BLOCKS THE ASSEMBLY OF PEPTIDE-MHC CLASS-II COMPLEXES

Peptide-class II complexes are assembled in endocytic, lysosome-like c ompartments where newly synthesized class II molecules are targeted fr om the trans-Golgi network (TGN), Recent studies have implicated phosp hatidylinositol 3-kinase (PI3-kinase) as an essential component in mem brane trafficking from the TGN to lysosomes, Here, using subcellular f ractionation, we show PI3-kinase activity associated with subcellular fractions which contain the class II peptide-loading compartment (IIPL C) in B cells, At concentrations required for inhibition of PI3-kinase activity in vivo wortmannin blocked the processing and presentation o f antigen by a cells to T cells, Treatment of B cells with wortmannin significantly limited the proteolytic degradation of invariant chain a nd the formation of peptide-class II complexes. Subcellular fractionat ion coupled with pulse-chase analyses showed that invariant chain and class II molecules trafficked to the IIPLC in wortmannin-treated cells , However, wortmannin prevented the maturation and correct targeting t o the IIPLC of cathepsin D, a protease necessary for the degradation o f invariant chain and assembly of processed antigen-class II complexes , These results suggest that II-class II complexes traffic to the IIPL C via a pathway that is relatively insensitive to wortmannin, but sugg est a role for PI3-kinases in the trafficking of other components nece ssary for the assembly of processed antigen class II complexes to the IIPLC.