HIV-1 Nef expression induces intracellular accumulation of multivesicular bodies and MHC II complexes: potential role of PI 3- kinase

Running title: Nef induces MHC II and MVB accumulations ABSTRACT Nef alters the cell surface expression of several immunoreceptors, which may contribute to viral escape. We show that Nef modifies MHC II intracellular trafficking and thereby its function. In the presence of Nef, mature, peptide loaded MHC II were down-modulated at the cell surface and accumulated intracellularly, while immature (invariant (Ii) chain associated) MHC II expression at the plasma membrane was increased. Antibody internalization experiments and subcellular fractionation analyses showed that immature MHC II were internalized from the plasma membrane, but had limited access to lysosomes, explaining the reduced Ii chain degradation. Immunoelectron microscopy revealed that Nef expression induced a marked accumulation of MVB containing Nef, MHC II and high amounts of Ii chain. The Nef-induced up-regulation of surface Ii chain was inhibited by LY294002 exposure, indicating the involvement of a PI 3-kinase, whose products play a key role in multivesicular bodies (MVB) biogenesis. Taken together our results indicate that Nef induces an increase of the number of MVB where MHC II complexes accumulate. Given that HIV recruits the MVB machinery for its assembly process, our data raise the possibility that Nef is involved in viral assembly through its effect on MVB.