HIV-1 ENVELOPE PROTEIN IS EXPRESSED ON THE SURFACE OF INFECTED-CELLS BEFORE ITS PROCESSING AND PRESENTATION TO CLASS-II-RESTRICTED T-LYMPHOCYTES

T lymphocytes are activated upon binding of their Ag receptors to a co mplex of Ag-derived peptides and MHC class I or class II molecules exp ressed on the surface of APC. It is now well established that APC degr ade exogenous Ag in acidic endosomal compartments, and that Ag fragmen ts bind to class II molecules moving through these compartments on the ir way to the surface of the APC. Although peptides derived from some endogenous Ag can also bind to class II molecules and subsequently be recognized by class II-restricted T cells, the intracellular trafficki ng pathways that enable endogenous proteins to be processed for associ ation with class II molecules remain controversial. We have analyzed t he mechanism by which the envelope (env) protein of the HIV-1 is proce ssed in infected cells for recognition by class II-restricted T cells. A large number of env-specific class II-restricted human CTL clones w ere shown to lyse B-lymphoblastoid cell lines expressing the env. A no vel dilutional assay proved that recognition of endogenous env protein was not a consequence of release and re-uptake of the env protein and subsequent processing by the standard class II-restricted pathway. Pr ocessing of endogenous env protein required that the protein be co-tra nslationally translocated into the endoplasmic reticulum (ER) and then exit the ER, since the class II-restricted CTL did not recognize env protein localized to the cytosol or retained in the ER of target cells . Under these conditions, however, class I-restricted recognition was readily demonstrated. Finally, class II-restricted recognition was str ikingly dependent upon the steady state level of surface env protein, since extracellular reagents that removed intact env protein from the surface of target cells inhibited recognition. This inhibition operate d at the Ag-processing level rather than at the level of subsequent Ag recognition. These results provide the first direct evidence that end ogenously synthesized membrane proteins enter the class II-restricted Ag-processing pathway after expression on the cell surface in an intac t form.