Monocyte

Monocytes are a type of leukocyte or white blood cell. They are the largest type of leukocyte in the blood and can differentiate into macrophages and monocyte-derived dendritic cells. As a part of the vertebrate innate immune system monocytes also influence adaptive immune responses and exert tissue repair functions. There are at least three subclasses of monocytes in human blood based on their phenotypic receptors.

Monocytes are amoeboid in appearance, and have nongranulated cytoplasm. Thus they are classified as agranulocytes, although they might occasionally display some azurophil granules and/or vacuoles. With a diameter of 15–22 μm, monocytes are the largest cell type in peripheral blood. Monocytes are mononuclear cells and the ellipsoidal nucleus is often lobulated/indented, causing a bean-shaped or kidney-shaped appearance. Monocytes compose 2% to 10% of all leukocytes in the human body.

Monocytes are produced by the bone marrow from precursors called monoblasts, bipotent cells that differentiated from hematopoietic stem cells. Monocytes circulate in the bloodstream for about one to three days and then typically migrate into organs throughout the body where they differentiate into macrophages and dendritic cells.

The first clear description of monocyte subsets by flow cytometry dates back to the late 1980s, when a population of CD16-positive monocytes was described. Today, three types of monocytes are recognized in human blood:

While in humans the level of CD14 expression can be used to differentiate non-classical and intermediate monocytes, the slan (6-Sulfo LacNAc) cell surface marker was shown to give an unequivocal separation of the two cell types.

Ghattas et al. state that the "intermediate" monocyte population is likely to be a unique subpopulation of monocytes, as opposed to a developmental step, due to their comparatively high expression of surface receptors involved in reparative processes (including vascular endothelial growth factor receptors type 1 and 2, CXCR4, and Tie-2) as well as evidence that the "intermediate" subset is specifically enriched in the bone marrow.

In mice, monocytes can be divided in two subpopulations. Inflammatory monocytes (CX3CR1^low, CCR2^pos, Ly6C^high, PD-L1^neg), which are equivalent to human classical CD14⁺⁺ CD16⁻ monocytes and resident monocytes (CX3CR1^high, CCR2^neg, Ly6C^low, PD-L1^pos), which are equivalent to human non-classical CD14⁺ CD16⁺ monocytes. Resident monocytes have the ability to patrol along the endothelium wall in the steady state and under inflammatory conditions.

Monocytes are mechanically active cells and migrate from blood to an inflammatory site to perform their functions. As explained before, they can differentiate into macrophages and dendritic cells, but the different monocyte subpopulations can also exert specific functions on their own. In general, monocytes and their macrophage and dendritic cell progeny serve three main functions in the immune system. These are phagocytosis, antigen presentation, and cytokine production. Phagocytosis is the process of uptake of microbes and particles followed by digestion and destruction of this material. Monocytes can perform phagocytosis using intermediary (opsonising) proteins such as antibodies or complement that coat the pathogen, as well as by binding to the microbe directly via pattern recognition receptors that recognize pathogens. Monocytes are also capable of killing infected host cells via antibody-dependent cell-mediated cytotoxicity. Vacuolization may be present in a cell that has recently phagocytized foreign matter.