Proteasomal degradation within endocytic organelles mediates antigen cross‐presentation

Abstract During MHC ‐I‐restricted antigen processing, peptides generated by cytosolic proteasomes are translocated by the transporter associated with antigen processing ( TAP ) into the endoplasmic reticulum, where they bind to newly synthesized MHC ‐I molecules. Dendritic cells and other cell types can also generate MHC ‐I complexes with peptides derived from internalized proteins, a process called cross‐presentation. Here, we show that active proteasomes within cross‐presenting cell phagosomes can generate these peptides. Active proteasomes are detectable within endocytic compartments in mouse bone marrow‐derived dendritic cells. In TAP ‐deficient mouse dendritic cells, cross‐presentation is enhanced by the introduction of human β 2 ‐microglobulin, which increases surface expression of MHC ‐I and suggests a role for recycling MHC ‐I molecules. In addition, surface MHC ‐I can be reduced by proteasome inhibition and stabilized by MHC ‐I‐restricted peptides. This is consistent with constitutive proteasome‐dependent but TAP ‐independent peptide loading in the endocytic pathway. Rab‐ GTP ase mutants that restrain phagosome maturation increase proteasome recruitment and enhance TAP ‐independent cross‐presentation. Thus, phagosomal/endosomal binding of peptides locally generated by proteasomes allows cross‐presentation to generate MHC ‐I‐peptide complexes identical to those produced by conventional antigen processing.