Proteasomal degradation within endocytic organelles can mediate 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 b 2 m, increasing surface expression of MHC-I and suggesting a role for recycling MHC-I molecules. In addition, surface MHC-I is reduced by proteasome inhibition but can be stabilized by the addition of a specific peptide, consistent with constitutive proteasomedependent but TAP-independent peptide loading in the endocytic pathway. Introduction of Rab-GTPase mutants that restrain phagosome maturation increases proteasome recruitment and enhances TAP-independent cross-presentation. 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.