Monocytes and cells of the dendritic cell lineage circulate in blood and eventually migrate into tissue where they further mature and serve various functions, most notably in immune defense. Over recent years these cells have been characterized in detail with the use of cell surface markers and flow cytometry, and subpopulations have been described. The present document proposes a nomenclature for these cells and defines 3 types of monocytes (classical, intermediate, and nonclassical monocytes) and 3 types of dendritic cells (plasmacytoid and 2 types of myeloid dendritic cells) in human and in mouse blood. This classification has been approved by the Nomenclature Committee of the International Union of Immunological Societies, and we are convinced that it will facilitate communication among experts and in the wider scientific community.

In the skin, the lack of markers permitting the unambiguous identification of macrophages and of conventional and monocyte-derived dendritic cells (DCs) complicates understanding of their contribution to skin integrity and to immune responses. By combining CD64 and CCR2 staining, we successfully identified each of these cell types and studied their origin, transcriptomic signatures, and migratory and T cell stimulatory properties. We also analyzed the impact of microbiota on their development and their contribution to skin inflammation during contact hypersensitivity. Dermal macrophages had a unique scavenging role and were unable to migrate and activate T cells. Conventional dermal DCs excelled both at migrating and activating T cells. In the steady-state dermis, monocyte-derived DCs are continuously generated by extravasated Ly-6C(hi) monocytes. Their T cell stimulatory capacity combined with their poor migratory ability made them particularly suited to activate skin-tropic T cells. Therefore, a high degree of functional specialization occurs among the mononuclear phagocytes of the skin.