Association of major histocompatibility complex II with cholesterol- and sphingolipid-rich membranes precedes peptide loading

Major histocompatibility complex class II protein (MHC II) molecules present antigenic peptides to CD4-positive T-cells. Efficient T cell stimulation requires association of MHC II with membrane microdomains organized by cholesterol and glycosphingolipids or by tetraspanins. Using detergent extraction at 37 degrees C combined with a modified flotation assay, we investigated the sequence of events leading to the association of MHC II with cholesterol- and glycosphingolipid-rich membranes (DRMs) that are distinct from tetraspanins. We find two stages of association of MHC II with DRMs. In stage one, complexes of MHC II and invariant chain, a chaperone involved in MHC II transport, enter DRMs in the Golgi stack. In early endosomes, these complexes are almost quantitatively associated with DRMs. Upon transport to late endocytic compartments, MHC II-bound invariant chain is stepwise proteolyzed to the MHC class II-associated invariant chain peptide (CLIP) that remains MHC II-bound and retains a preference for DRMs. At the transition between the two stages, CLIP is exchanged against processed antigens, and the resulting MHC II-peptide complexes are transported to the cell surface. In the second stage, MHC II shows a lower overall association with DRMs. However, surface MHC II molecules occupied with peptides that induce resistance to denaturation by SDS are enriched in DRMs relative to SDS-sensitive MHC II-peptide complexes. Likewise, MHC II molecules loaded with long-lived processing products of hen-egg lysozyme containing the immunodominant epitope 48-61 show a very high preference for DRMs. Thus after an initial mainly intracellular stage of high DRM association, MHC II moves to a second stage in which its preference for DRMs is modulated by bound peptides.