Early endosomes and a late endocytic compartment generate different peptide-class II MHC complexes via distinct processing mechanisms.

Class II MHC Ag-processing compartments and mechanisms were compared for four antigenic epitopes from hen egg lysozyme (HEL) and RNase. T cell assays on subcellular fractions of Ag-pulsed macrophages detected the initial appearance of HEL-(48-61):I-Ak, HEL-(34-45):I-Ak, and RNase-(90-105):I-Ek complexes in a high density late endocytic compartment. In contrast, RNase-(42-56):I-Ak complexes never appeared in high density compartments, but were rapidly generated in low density endosomes. This early endosomal processing mechanism was 1) chloroquine inhibitable; 2) less sensitive than the late endocytic mechanism to 20 degrees C inhibition; 3) partially resistant to depletion of nascent class II MHC molecules with brefeldin A, suggesting some use of recycled class II MHC molecules, whereas the late endocytic processing mechanism was blocked by brefeldin A; and 4) involved in the processing of a DM-independent complex (RNase-(42-56):I-Ak). Thus, distinct processing compartments and mechanisms are identified for different epitopes even within a single Ag.