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 introd... More
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 -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-GTPase 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.
