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 compartments where newly synthesized class II molecules are targeted from the trans-Golgi network (TGN). Recent studies have implicated phosphatidylinositol 3-kinase (PI3-kinase) as an essential component in membrane trafficking from the TGN to lysosomes. Here, using subcellular fractionation, we show PI3-kinase activity associated with subcellular fractions which contain the class II peptide-loading compartment (IIPLC) in B cells. At concentrations required for inhibition of PI3-kinase activity in vivo, wortmannin blocked the processing and presentation of antigen by B cells to T cells. Treatment of B cells with wortmannin significantly limited the proteolytic degradation of invariant chain and the formation of peptide‐class II complexes. Subcellular fractionation 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 to the IIPLC of cathepsin D, a protease necessary for the degradation of invariant chain and assembly of processed antigen‐class II complexes. These results suggest that Ii‐class II complexes traffic to the IIPLC via a pathway that is relatively insensitive to wortmannin, but suggest a role for PI3kinases in the trafficking of other components necessary for the assembly of processed antigen class II complexes to the IIPLC.