How MHC class II molecules reach the endocytic pathway

We have examined trafficking of major histocompatibility complex (MHC) class II molecules in human B cells exposed to concanamycin B, a highly specific inhibitor of the vacuolar H(+)-ATPases required for acidification of the vacuolar system and for early to late endosomal transport. Neutralization of vacuolar compartments prevents breakdown of the invariant chain (Ii) and blocks conversion of MHC class II molecules to peptide-loaded, SDS-stable alpha beta dimers. Ii remains associated with alpha beta and this complex accumulates internally, as ascertained biochemically and by morphological methods. In concanamycin B-treated cells, a slow increase (> 20-fold) in surface expression of Ii, mostly complexed with alpha beta, is detected. This surface-disposed fraction of alpha beta Ii is nevertheless a minor population that reaches the cell surface directly, or is routed via early endosomes as intermediary stations. In inhibitor-treated cells, the bulk of newly synthesized alpha beta Ii is no longer accessible to fluid phase endocytic markers. It is concluded that the majority of alpha beta Ii is targeted directly from the trans-Golgi network to the compartment for peptide loading, bypassing the cell surface and early endosomes en route to the endocytic pathway.