Figure 1.

Murine monocyte and macrophage subsets. Murine monocytes develop from a common myeloid progenitor and leave the bone marrow through the chemokine receptor CCR2. In bloodstream, circulating monocytes are phenotypically and functionally heterogeneous. Non-classical monocytes (Ly6C^low CCR2^low CX3CR1^high CD62L⁻) patrol the vasculature and accumulate at low numbers in the steady state. Inflammatory or classical (Ly6C^high CCR2^high CX3CR1^low CD62L⁺) monocytes have a relatively short-circulating lifespan and preferentially accumulate in inflammatory sites where they give rise to inflammatory M1 macrophages (F4/80⁺ CD11b⁺ CD86⁺ CD206⁻). M1 macrophage subset has high microbicidal capacity due to their ability to produce inflammatory cytokines [TNF, IL-1β], reactive oxygen species (ROS) secretion and the expression of iNOS enzyme that metabolizes arginine to arginine-derived killer molecule NO. Non-classical monocytes can be recruited to tissue and differentiate to M2 macrophages (F4/80⁺ CD11b⁺ CD86⁻ CD206⁺), which secrete anti-inflammatory cytokines (IL-10, IL-4) and contribute to tissue repair mechanisms.