Peptide loading onto recycling HLA-DR molecules occurs in early endosomes.

Presentation of exogenous antigens to MHC class II-restricted T cells can follow two different processing pathways. The classical pathway requires newly synthesized MHC class II molecules, invariant chain and HLA-DM expression, whereas the alternative pathway is independent of protein synthesis, invariant chain and HLA-DM. In both cases, MHC class II molecules associate with peptides derived from exogenous antigens that have been processed in endocytic compartments. Different endosomal/prelysosomal compartments where peptide/MHC class II complexes and HLA-DM molecules accumulate have been described. We show here that the alternative pathway uses an earlier compartment than the classical pathway. Experiments with chemically cross-linked antigen suggest that recycling MHC class II molecules present rapidly degraded antigens, leading to a rapid immune response to exogenously added influenza virus proteins.