Résumé
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‐liniked antigen suggest that recycling MHC class II molecules present rapidly degraded antigens, leading to a rapid immune response to exogenously added influenza virus proteins.