NEW EVIDENCE FOR 2 MHC CLASS II-RESTRICTED ANTIGEN PRESENTATION PATHWAYS BY OVEREXPRESSION OF A SMALL G-PROTEIN

Exogenous Ags may be presented by MHC class II molecules through two d istinct pathways distinguished by their sensitivity to drugs that inhi bit the protein synthesis. Using this approach, we previously showed t hat the subunits Ig-alpha and Ig-beta, associated to B cell Ag recepto r, targeted Ags either to newly synthesized or to preexisting pools of MHC class II molecules, respectively. To further characterize these t wo Ag presentation pathways, we altered the intra-Golgi transport of n ewly synthesized MHC class II by stably overexpressing, in B cells, mu tants of a small G protein involved in the intra-Golgi transport, Rab6 . Overexpression of GTP-bound rab6 (Q72L) mutant proteins reduced the cell surface arrival of MHC class II molecules and consequently slowed down Ag presentation dependent upon newly synthesized class II molecu les. In contrast, this mutant had no effect on Ag presentation depende nt upon preexisting pools of class II molecules, and the overexpressio n of an inactive GDP-bound form of rab6 (T27N) did not affect any Ag p resentation pathway. MHC class II-restricted Ag presentation pathways can therefore be distinguished by their sensitivity to the overexpress ion of proteins modifying the intracellular transport of newly synthes ized class II molecules.