Presentation of exogenous antigens on MHC-I molecules, termed cross-presentation, is essential for cytotoxic CD8 + T cell responses. In mice, dendritic cells (DCs) that arise from monocytes (mo-DCs) during inflammation have a key function in these responses by cross-presenting antigens locally in peripheral tissues. Whether human naturally-occurring mo-DCs can cross-present is unknown. Here, we use human mo-DCs and macrophages directly purified from ascites to address this question. Single-cell RNA-seq data show that ascites CD1c + DCs contain exclusively monocyte-derived cells. Both ascites mo-DCs and monocyte-derived macrophages cross-present efficiently, but are inefficient for transferring exogenous proteins into their cytosol. Inhibition of cysteine proteases, but not of proteasome, abolishes cross-presentation in these cells. We conclude that human monocyte-derived cells cross-present exclusively using a vacuolar pathway. Finally, only ascites mo-DCs provide co-stimulatory signals to induce effector cytotoxic CD8 + T cells. Our findings thus provide important insights on how to harness cross-presentation for therapeutic purposes.
Cross-presentation, or the presentation of exogenous antigens on MHC-I, is thought to be restricted to dendritic cells (DCs). Here the authors show that human DCs and macrophages developed in vivo from monocytes can both perform cross-presentation using a non-conventional pathway, but only DCs are capable of inducing cytotoxic CD8 + T cells.