Monocytes

Monocytes are circulating leukocytes that are derived from macrophage-dendritic cell precursors (MDPs) in the bone marrow. MDPs develop into common DC progenitors (CDPs) and common monocyte progenitors (cMoPs), which give rise to blood monocytes. Functionally, monocytes are characterized as professional phagocytes that ingest and process foreign materials and cell debris. Additionally, they are capable of antigen presentation and cytokine production, allowing them to influence the adaptive immune response. Moreover, monocytes have been shown to migrate to sites of inflammation where they can differentiate into either inflammatory macrophages or monocyte-derived dendritic cells. In both mice and humans, there are at least two subsets of monocytes known as inflammatory and patrolling monocytes. In mice, inflammatory monocytes are characterized as Ly-6C⁺CD62L/L-Selectin⁺CCR2^highCX3CR1^low cells, while patrolling monocytes are Ly-6C^-CD62L/L-Selectin-CCR2^lowCX3CR1^high cells. In humans, CD14⁺⁺CD16^- classical monocytes have functional properties similar to mouse Ly-6C⁺ inflammatory monocytes. In contrast, human non-classical monocytes, which are CD14⁺CD16⁺⁺, are functionally more similar to the mouse Ly-6C^- monocyte subset. An additional intermediate monocyte subset has also been described in humans that is CD14⁺⁺CD16⁺.