defense action of monocyte
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Answer:
Circulating blood monocytes supply peripheral tissues with macrophage and dendritic cell (DC) precursors and, in the setting of infection, also contribute directly to immune defense against microbial pathogens.
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Circulating blood monocytes supply peripheral tissues with macrophage and dendritic cell (DC) precursors and, in the setting of infection, also contribute directly to immune defense against microbial pathogens. In humans and mice, monocytes are divided into two major subsets that either specifically traffic into inflamed tissues or, in the absence of overt inflammation, constitutively maintain tissue macrophage/DC populations. Inflammatory monocytes respond rapidly to microbial stimuli by secreting cytokines and antimicrobial factors, express the CCR2 chemokine receptor, and traffic to sites of microbial infection in response to monocyte chemoattractant protein (MCP)-1 (CCL2) secretion. In murine models, CCR2-mediated monocyte recruitment is essential for defense against Listeria monocytogenes, Mycobacterium tuberculosis, Toxoplasma gondii, and Cryptococcus neoformans infection, implicating inflammatory monocytes in defense against bacterial, protozoal, and fungal pathogens. Recent studies indicate that inflammatory monocyte recruitment to sites of infection is complex, involving CCR2-mediated emigration of monocytes from the bone marrow into the bloodstream, followed by trafficking into infected tissues. The in vivo mechanisms that promote chemokine secretion, monocyte differentiation and trafficking, and finally monocyte-mediated microbial killing remain active and important areas of investigation.
Keywords: inflammation, monocyte differentiation, chemokines, microbial pathogens
Defense Actions.
The major splenic conventional DC (cDC) subsets (CD8+CD4−, CD8−CD4−, and CD8−CD4+) turn over rapidly, with half-lives that range from one and a half to seven days (41, 42). To maintain steady-state splenic cDC populations in mice, a daily influx of ~105 circulating progenitor cells is required (42). Previous studies have identified several candidate circulating DC precursors that appear to be distinct from monocytes (43, 44) or that are monocytic in origin (45). In addition, non-monocytic intrasplenic DC precursors with limited potential for cell division, termed pre-DCs, contribute to the maintenance of all cDC subsets (46, 47).
The major splenic conventional DC (cDC) subsets (CD8+CD4−, CD8−CD4−, and CD8−CD4+) turn over rapidly, with half-lives that range from one and a half to seven days (41, 42). To maintain steady-state splenic cDC populations in mice, a daily influx of ~105 circulating progenitor cells is required (42). Previous studies have identified several candidate circulating DC precursors that appear to be distinct from monocytes (43, 44) or that are monocytic in origin (45). In addition, non-monocytic intrasplenic DC precursors with limited potential for cell division, termed pre-DCs, contribute to the maintenance of all cDC subsets (46, 47).Under homeostatic conditions, MDPs contribute to the steady-state splenic mononuclear phagocyte pool because adoptively transferred cells give rise to CD8+ and CD8− cDCs as well as to splenic marginal zone and marginal sinus macrophages in nonirradiated recipient mice (28). In contrast, more differentiated bone marrow cell populations were much less efficient than MDPs in generating DCs (28) (20). In a separate study, splenic-resident pre-DCs gave rise to all CD8+ and CD8− DC subsets and were 50-fold more efficient in generating CD8−cDCs than were purified blood Ly6Clow monocytes (46). Thus, in the steady state, MDPs and splenic pre-DCs, rather than bone marrow and circulating monocytes, appear to reconstitute cDCs most effectively (Figure 1).