A close encounter of the third kind: monocyte-derived cells

Recent insights into discrete myeloid developmental pathways have provided critical information about the organization of the murine mononuclear phagocyte compartment. Short-lived dendritic cells (DCs) have been shown to continuously arise from dedicated bone marrow-derived precursors. In contrast, it is now appreciated that most tissue macrophage populations are established before birth and subsequently maintain themselves throughout adulthood by longevity and limited self-renewal. Both of these classical tissue-resident mononuclear phagocyte compartments can be complemented on demand by monocyte infiltrates giving rise to macrophage or DC-like cells, depending on the tissue context they encounter upon extravasation. Monocytes hence have emerged as a versatile emergency squad that can be rapidly recruited to sites of injury to provide a transient supplement with proinflammatory or resolving activities for local mononuclear phagocytes.