Ly6C(low) Monocytes Differentiate into Dendritic Cells and Cross-tolerize T Cells Through PDL-1

Monocyte-derived dendritic cells are active participants during the immune response against infection, but whether they play a role in maintaining self-tolerance under steady-state conditions is not known. Here we investigated the differentiation of monocytes, their ability to ingest apoptotic cells, and their potential functionality in vivo. We observed that Ly6C (Gr-1)(low) mature monocytes up-regulate their MHC II level in the spleen, express high levels of PDL-1 (programmed death ligand 1), and are more efficient than Ly6C(high) immature monocytes in the ingestion of apoptotic cells in vivo. Sorted circulating Ly6C(low) monocytes were able to cross-present both apoptotic cell-associated OVA and soluble OVA protein. Monocytes containing apoptotic cells can further differentiate into CD11c(+)CD8alpha(-)MHC II(+) splenic dendritic cells that maintained high expression of PDL-1. Since wild-type but not PDL-1-deficient peripheral blood monocytes containing apoptotic cell-associated OVA suppressed the response to OVA immunization, PDL-1 expression was required for monocyte-mediated T cell tolerance. These observations demonstrate that Ly6C(low) mature monocytes can promote tolerance to self Ag contained in apoptotic cells through a PDL-1-dependent mechanism.