Identification of a novel dendritic cell-like subset of CD64+ / CD16+ blood monocytes

Human monocytes (Mo) consist of a major subset of Fcγ-receptor I (CD64)-positive typical low accessory phagocytes, and a minor CD64– DC-like subset with high T cell-accessory and IFN-α-releasing activity. Both populations also differentially express CD16 (Fcγ-receptor III). Double labeling with anti-CD64 and anti-CD16 mAb, as performed here, identified four different subsets. The CD64– subset consists of CD64– / 16+ cells with high antigen-presenting cell (APC) function and macrophage-like phenotype, and a CD64– / 16– subset of less active APC but which exhibits a higher mixed lymphocyte reaction (MLR) stimulating and IFN-α-producing capacity, possibly resembling plasmacytoid dendritic cell type II (DC2) blood precursors. As well as the majority of CD64+ cells that appeared CD64+ / 16– and represent typical low-accessory, CD14high Mo, we could identify and describe a novel minor subset of CD64+ / 16+ cells which is unique in combining typical DC and Mo characteristics in the same cell. These are high IL-12 production, high accessory capacity for antigen- or allogen-activated lymphocytes, and high expression of HLA-DR, CD86, and CD11c.