Cross-Presentation IRAP-Rab14 Endosomes for Efficient Conventional Dendritic Cells Require

Dendritic cells (DCs) use cellular pathways collectively referred to as cross-presentation to stimulate CD8 + T cells with peptide Ags derived from internalized, exogenous Ags. We have recently reported that DCs rely on aminoterminal trimming of cross-presented peptides by insulin-responsive aminopeptidase (IRAP), an enzyme localized in a regulated endosomal storage compartment. Considering a report contending that this role is limited to inflammatory DCs (Segura et al. in this study, we examined the role of IRAP in steady-state DC subpopulations. Steady-state conventional DCs (cDCs) and plasmacytoid DCs expressed similar amounts of IRAP. IRAP colocalized with the endosomal markers Rab14 and syntaxin 6, both known to be associated with regulated endosomal storage compartments, in CD8 + and CD8 2 cDCs—however, to a greater extent in the former population. Likewise, IRAP recruitment to phagosomes was significantly stronger in CD8 + DCs. IRAP deficiency compromised cross-presentation of soluble and particulate Ag by both CD8 + and CD8 2 cDCs, again with a stronger effect in the former population. Thus, the requirement of IRAP in cross-presentation extends to steady-state cDCs. Moreover, these data suggest that increased recruitment of an IRAP + /Rab14 + compartment to Ag-containing vesicles contributes to the superior cross-presentation efficacy of CD8 + cDCs. D endritic cells (DCs) have a pivotal role in the orchestration of adaptive immune responses. All aspects of the adaptive immune response—T cell priming and activation , immune tolerance, and immune memory—critically involve the presentation of cognate Ag (1). Like most cells of the body, DCs can present endogenously synthesized Ags, but unlike other cell types, DCs can cross-present exogenous Ags on MHC class I (MHC-I) molecules. Exogenous Ags are also cross-presented by macrophages, monocytes, and B cells, but the only APCs capable of activating or cross-priming naive T CD8 + cells are DCs (2). Immature DCs capture Ags by phagocytosis, pinocytosis, and receptor-mediated endocytosis. Engulfed exogenous Ag is either processed inside lysosomes or shuttled into the cytosol, where it is exposed to the proteasome. Efficient cross-presentation of most studied exogenous Ags depends on the proteasome (3). Amino-terminal trimming and loading on MHC-I of antigenic peptide precursors produced by the proteasome is believed to occur in several cellular compartments. Historically, it was first proposed that loading of MHC-I with cross-presented epitopes occurs in the perinuclear endoplasmic reticulum (4). Consistent with this, knockout (ko) of the murine ERAP-trimming peptidase has been shown to compromise cross-presentation (5, 6). Reimport of …