Regulation and consequences of monocytosis

Abstract

Monocytes are part of the vertebrate innate immune system. Blood monocytes are produced by bone marrow and splenic progenitors that derive from hematopoietic stem cells (HSCs). In cardiovascular disease, such as atherosclerosis and myocardial infarction, HSCs proliferate at higher levels that in turn increase production of hematopoietic cells, including monocytes. Once produced in hematopoietic niches, monocytes intravasate blood vessels, circulate, and migrate to sites of inflammation. Monocyte recruitment to atherosclerotic plaque and the ischemic heart depends on various chemokines, such as CCL2, CX3 CL1, and CCL5. Once in tissue, monocytes can differentiate into macrophages and dendritic cells. Macrophages are end effector cells that regulate the steady state and tissue healing, but they can also promote disease. At sites of inflammation, monocytes and macrophages produce inflammatory cytokines, which can exacerbate disease progression. Macrophages can also phagocytose tissue debris and produce pro-healing cytokines. Additionally, macrophages are antigen-presenting cells and can prime T cells. The tissue environment, including cytokines and types of inflammation, instructs macrophage specialization. Understanding monocytosis and its consequences in disease will reveal new therapeutic opportunities without compromising steady state functions.

Keywords: monocytes/macrophages; spleen; stem cells.

Fig. 1. Monocytosis after acute and chronic inflammation

Infection and sterile inflammation, such as myocardial infarction, decrease production of hematopoietic stem cell (HSC) retention factors by hematopoietic niche cells. This leads to increased HSC cycling resulting in elevated hematopoiesis. HSC-derived monocytes in the bone marrow can intravasate into blood circulation in an MCP-1-dependent manner. HSCs can also circulate in the blood and lodge in the spleen, where they produce monocytes in presence of SCF, G-MCSF, IL-3, and IL1β. Release of splenic HSC into the blood depends on AT-II-AT1r signaling. Finally, blood monocytes are recruited to sites of inflammation, where they differentiate into macrophages with several functions, including inflammation, healing, and resolution of inflammation.