Blood Cells - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 163

Monocytes originate from a myeloid progenitor shared with neutrophils in the
bone marrow and the common monocyte, macrophage, and dendritic cell precursor
(MDP). The latter generates monocytes as well as preclassical and plasmacytoid
dendritic cells via a common dendritic cell precursor. Monocytes exit into the blood
stream by diapedesis, where they circulate for several days.
Monocytes circulate in the blood or reside in the bone marrow and spleen.
They do not proliferate in a steady state. These immunocytes are equipped with
chemokine and adhesion receptors that are used during their migration from blood
to tissues triggered by infection. They produce inflammatory cytokines and take up
toxins and cells.
Circulating monocytes give rise to mature tissue-resident macrophages .Both
monocyte and macrophage lineages are characterized by heterogeneity. Phago-
cytic monocytes defend the body against viruses and bacteria. Monocytes also
differentiate into dendritic cells, mainly during inflammation. Migration to tissues
and differentiation to inflammatory dendritic cells and macrophages are likely
determined by the inflammatory milieu and pathogen-associated pattern-recognition
receptors [ 296 ].
Monocytes are effectors of the innate immunity and inflammation. They kill
pathogens using phagocytosis and production of reactive oxygen species, nitric
oxide, myeloperoxidase, and inflammatory cytokines. They can trigger or suppress
T-cell responses. CD11b naive monocytes can inhibit T-cell proliferation,

at least in vitro, upon cell contact, partly via NO and possibly independently
of FoxP3

+ , Ly6G −

regulatory T cells [ 295 ]. They may contribute to tissue repair and
angiogenesis [ 294 ].
In human monocytes, Wnt3a activates the canonical Wnt axis and decreases
monocyte adhesion to endothelial cells (as well as to fibronectin, laminin, and
collagen) and subsequent transendothelial migration [ 298 ]. Monocytes synthesize
different Frizzled receptor types.

+

3.13.1.1 Monocyte Types

Circulating monocytes are variable in size, granularity, and nuclear morphology.
Monocytes can be identified by expression of large amounts of monocyte differen-
tiation antigen CD14, a lipopolysaccharide receptor.
In

human blood, several monocyte populations exist [ 299 ]: (1) classical,

CD14 high ,CD16 ,CD16 high ;and(3)CD14 − ( ∼ 90%); (2) non-classical, CD14 + + ,

CD16 + (Fc γ R3, a low-affinity receptor for IgG), and CD64 + (Fc γ receptor-1A, a

high-affinity receptor for IgG) monocytes.
The low-affinity IgG Fc receptor Fc R3 on the surface of monocytes and

macrophages, as well as natural killer cells and neutrophils, possesses 2 subtypes
Fc

γ

receptor-3A and -3B (CD16a and CD16b) encoded by the FCGR3A and
FCGR3B genes. In humans, CD14

γ

−
cells specialize in phagocytosis, ROS production, and recognition of TLR ligands.

+ monocytes consist of CD16 + and CD16