Editing Monocyte (section)

===Specific functions of monocyte subpopulations===
[[File:Monocyte.svg|thumb|Artist's impression of monocyte]]
Aside from their differentiation capacity, monocytes can also directly regulate immune responses. As explained before, they are able to perform phagocytosis. Cells of the classical subpopulation are the most efficient phagocytes and can additionally secrete inflammation-stimulating factors. The intermediate subpopulation is important for [[antigen presentation]] and [[T lymphocyte]] stimulation.<ref name="d595">{{cite journal | last1=Wong | first1=Kok Loon | last2=Tai | first2=June Jing-Yi | last3=Wong | first3=Wing-Cheong | last4=Han | first4=Hao | last5=Sem | first5=Xiaohui | last6=Yeap | first6=Wei-Hseun | last7=Kourilsky | first7=Philippe | last8=Wong | first8=Siew-Cheng | title=Gene expression profiling reveals the defining features of the classical, intermediate, and nonclassical human monocyte subsets | journal=Blood | volume=118 | issue=5 | date=2011-08-04 | issn=0006-4971 | doi=10.1182/blood-2010-12-326355 | pages=e16–e31| pmid=21653326 }}</ref> Briefly, antigen presentation describes a process during which microbial fragments that are present in the monocytes after phagocytosis are incorporated into MHC molecules. They are then trafficked to the cell surface of the monocytes (or macrophages or dendritic cells) and presented as antigens to activate T lymphocytes, which then mount a specific immune response against the antigen. Non-classical monocytes produce high amounts of pro-inflammatory [[cytokine]]s like [[tumor necrosis factor]] and [[interleukin-12]] after stimulation with microbial products. Furthermore, a monocyte patrolling behavior has been demonstrated in humans both for the classical and the non-classical monocytes, meaning that they slowly move along the [[endothelium]] to examine it for pathogens.<ref>{{cite journal |last1=Collison |first1=Joanna L. |last2=Carlin |first2=Leo M. |last3=Eichmann |first3=Martin |last4=Geissmann |first4=Frederic |last5=Peakman |first5=Mark |date=1 August 2015 |title=Heterogeneity in the Locomotory Behavior of Human Monocyte Subsets over Human Vascular Endothelium In Vitro |journal=[[The Journal of Immunology]] |volume=195 |issue=3 |pages=1162–1170 |doi=10.4049/jimmunol.1401806 |pmid=26085686 |doi-access=free}}</ref> Said et al. showed that activated monocytes express high levels of [[PD 1|PD-1]] which might explain the higher expression of PD-1 in CD14<sup>+</sup>CD16<sup>++</sup> monocytes as compared to CD14<sup>++</sup>CD16<sup>−</sup> monocytes. Triggering monocytes-expressed PD-1 by its [[ligand]] PD-L1 induces IL-10 production, which activates [[CD4]] [[Th2 cells]] and inhibits [[CD4]] [[Th1 cell]] function.<ref>{{cite journal |last1=Said |first1=Elias A |last2=Dupuy |first2=Franck P |last3=Trautmann |first3=Lydie |last4=Zhang |first4=Yuwei |last5=Shi |first5=Yu |last6=El-Far |first6=Mohamed |last7=Hill |first7=Brenna J |last8=Noto |first8=Alessandra |last9=Ancuta |first9=Petronela |last10=Peretz |first10=Yoav |last11=Fonseca |first11=Simone G |date=April 2010 |title=Programmed death-1–induced interleukin-10 production by monocytes impairs CD4+ T cell activation during HIV infection |journal=[[Nature Medicine]] |volume=16 |issue=4 |pages=452–459 |doi=10.1038/nm.2106 |pmc=4229134 |pmid=20208540 |last12=Van Grevenynghe |first12=Julien |last13=Boulassel |first13=Mohamed R |last14=Bruneau |first14=Julie |last15=Shoukry |first15=Naglaa H |last16=Routy |first16=Jean-Pierre |last17=Douek |first17=Daniel C |last18=Haddad |first18=Elias K |last19=Sekaly |first19=Rafick-Pierre}}</ref>
Many factors produced by other cells can regulate the [[chemotaxis]] and other functions of monocytes.  These factors include most particularly [[chemokines]] such as [[monocyte chemotactic protein-1]] (CCL2) and [[CCL7|monocyte chemotactic protein-3 (CCL7)]]; certain [[arachidonic acid]] metabolites such as [[leukotriene B4]] and members of the [[5-Hydroxyicosatetraenoic acid and 5-oxo-eicosatetraenoic acid|5-hydroxyicosatetraenoic acid and 5-oxo-eicosatetraenoic acid]] family of OXE1 receptor agonists (e.g., [[5-HETE]] and 5-oxo-ETE); and [[N-Formylmethionine leucyl-phenylalanine]] and other N-formylated [[oligopeptide]]s which are made by bacteria and activate the [[formyl peptide receptor 1]].<ref>{{cite journal |last1=Sozzani |first1=S. |last2=Zhou |first2=D. |last3=Locati |first3=M. |last4=Bernasconi |first4=S. |last5=Luini |first5=W. |last6=Mantovani |first6=A. |last7=O'Flaherty |first7=J. T. |title=Stimulating properties of 5-oxo-eicosanoids for human monocytes: synergism with monocyte chemotactic protein-1 and -3. |journal=The Journal of Immunology |date=15 November 1996 |volume=157 |issue=10 |pages=4664–4671 |doi=10.4049/jimmunol.157.10.4664 |pmid=8906847 |s2cid=23499393 |url=https://www.jimmunol.org/lookup/pmidlookup?view=long&pmid=8906847 |doi-access=free }}</ref>
Other microbial products can directly activate monocytes and this leads to production of pro-inflammatory and, with some delay, of anti-inflammatory [[cytokine]]s. Typical cytokines produced by monocytes are [[tumor necrosis factor|TNF]], [[interleukin-1|IL-1]], and [[interleukin-12|IL-12]].