The monocyte family tree

Monocytes and dendritic cells (DCs) originate from a common macrophage and DC progenitor (MDP) and classical and plasmacytoid DCs develop from an intermediate stage, the common DC progenitor (CDP). Now, Hettinger et al. identify a progenitor for monocytes and monocyte-derived macrophages downstream of the MDP, which they termed the common monocyte progenitor. Common monocyte progenitors had a lineage-negative CD117⁺CD115⁺CD135⁻LY6C⁺CD11b⁻ phenotype and showed high proliferative and clonogenic activity. In mice, common monocyte progenitors gave rise to LY6C^hi and LY6C^low monocytes, and to macrophages following macrophage deletion or inflammation, but they did not give rise to DCs. These observations, together with the results of proteomic analyses, indicate that common monocyte progenitors are committed to monocyte differentiation but that they lack any monocyte functionality.