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Dendritic cell
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===Formation of immature cells and their maturation=== [[File:Immune_Cells_Development.pdf|thumb|400px|Diagram of hematopoiesis from HSC, showing a separate dendritic cell lineage via CDP (Common Dendritic-cell Progenitor).]] Dendritic cells are derived from [[hematopoietic stem cells|hematopoietic bone marrow progenitor cells]] (HSC). These progenitor cells initially transform into immature dendritic cells. These cells are characterized by high endocytic activity and low T-cell activation potential. Immature dendritic cells constantly sample the surrounding environment for pathogens such as [[virus (biology)|viruses]] and [[bacteria]]. This is done through [[pattern recognition receptors]] (PRRs) such as the [[toll-like receptor]]s (TLRs). TLRs recognize specific chemical signatures found on subsets of pathogens. Immature dendritic cells may also [[phagocytosis|phagocytose]] small quantities of membrane from live own cells, in a process called nibbling. Once they have come into contact with a presentable antigen, they become activated into mature dendritic cells and begin to migrate to a [[lymph node]]. Immature dendritic cells phagocytose pathogens and degrade their [[protein]]s into small pieces and upon maturation present those fragments at their cell surface using [[major histocompatibility complex|MHC]] molecules. Simultaneously, they upregulate cell-surface receptors that act as [[co-stimulation|co-receptors]] in T-cell activation such as [[CD80]] (B7.1), [[CD86]] (B7.2), and [[CD40]] greatly enhancing their ability to activate T-cells. They also upregulate [[CC chemokine receptors#CCR7|CCR7]], a chemotactic receptor that induces the dendritic cell to travel through the [[blood]] stream to the [[spleen]] or through the [[lymphatic system]] to a [[lymph node]]. Here they act as [[antigen-presenting cell]]s: they activate [[helper T-cell]]s and [[killer T-cell]]s as well as [[B-cell]]s by presenting them with antigens derived from the pathogen, alongside non-antigen specific costimulatory signals. Dendritic cells can also induce T-cell tolerance (unresponsiveness). Certain C-type lectin receptors (CLRs) on the surface of dendritic cells, some functioning as PRRs, help instruct dendritic cells as to when it is appropriate to induce immune tolerance rather than lymphocyte activation.<ref>{{cite journal |vauthors = Maverakis E, Kim K, Shimoda M, Gershwin M, Patel F, Wilken R, Raychaudhuri S, Ruhaak LR, Lebrilla CB | title = Glycans in the immune system and The Altered Glycan Theory of Autoimmunity | journal = J Autoimmun | volume = 57 | issue = 6 | pages = 1β13 | year = 2015 | pmid = 25578468 | doi = 10.1016/j.jaut.2014.12.002 | pmc=4340844}}</ref> Every helper T-cell is specific to one particular antigen. Only professional [[antigen-presenting cells]] (APCs: macrophages, B lymphocytes, and dendritic cells) are able to activate a resting helper T-cell when the matching antigen is presented. However, in non-lymphoid organs, macrophages and B cells can only activate [[memory T cells]]{{Citation needed|date=December 2008}} whereas dendritic cells can activate both memory and [[naive T cells]], and are the most potent of all the antigen-presenting cells. In the lymph node and secondary lymphoid organs, all three APCs can activate naive T cells. Whereas mature dendritic cells are able to activate antigen-specific naive CD8<sup>+</sup> T cells, the formation of CD8<sup>+</sup> memory T cells requires the interaction of dendritic cells with CD4<sup>+</sup> [[helper T cells]].<ref name="ReferenceA">{{cite journal | pmid= 15475958| year= 2004| last1= Smith| first1= C. M.| title= Cognate CD4(+) T cell licensing of dendritic cells in CD8(+) T cell immunity| journal= Nature Immunology| volume= 5| issue= 11| pages= 1143β8| last2= Wilson| first2= N. S.| last3= Waithman| first3= J| last4= Villadangos| first4= J. A.| last5= Carbone| first5= F. R.| last6= Heath| first6= W. R.| last7= Belz| first7= G. T.| doi= 10.1038/ni1129| s2cid= 27757632}}</ref> This help from CD4<sup>+</sup> T cells additionally activates the matured dendritic cells and licenses (empowers) them to efficiently induce CD8<sup>+</sup> memory T cells, which are also able to be expanded a second time.<ref name="ReferenceA"/><ref name="ReferenceB">{{cite journal | doi = 10.1002/eji.201444477| pmid = 25211552| title = Concurrent interaction of DCs with CD4+and CD8+T cells improves secondary CTL expansion: It takes three to tango| journal = European Journal of Immunology| volume = 44| issue = 12| pages = 3543β59| year = 2014| last1 = Hoyer| first1 = Stefanie| last2 = Prommersberger| first2 = Sabrina| last3 = Pfeiffer| first3 = Isabell A.| last4 = Schuler-Thurner| first4 = Beatrice| last5 = Schuler| first5 = Gerold| last6 = DΓΆrrie| first6 = Jan| last7 = Schaft| first7 = Niels| s2cid = 5655814| doi-access = free}}</ref> For this activation of CD8+, concurrent interaction of all three cell types, namely CD4<sup>+</sup> T helper cells, CD8<sup>+</sup> T cells and dendritic cells, seems to be required.<ref name="ReferenceB"/> As mentioned above, mDC probably arise from [[monocyte]]s, white blood cells which circulate in the body and, depending on the right signal, can turn into either dendritic cells or [[macrophage]]s. The monocytes in turn are formed from stem cells in the [[bone marrow]]. Monocyte-derived dendritic cells can be generated in vitro from [[peripheral blood mononuclear cell]] (PBMCs). Plating of PBMCs in a tissue culture flask permits adherence of monocytes. Treatment of these monocytes with interleukin 4 (IL-4) and granulocyte-macrophage colony stimulating factor (GM-CSF) leads to differentiation to immature dendritic cells (iDCs) in about a week. Subsequent treatment with tumor necrosis factor (TNF) further differentiates the iDCs into mature dendritic cells. Monocytes can be induced to differentiate into dendritic cells by a self-peptide Ep1.B derived from [[apolipoprotein E]].<ref>{{cite journal | vauthors = Stephens TA, Nikoopour E, Rider BJ, Leon-Ponte M, Chau TA, Mikolajczak S, Chaturvedi P, Lee-Chan E, Flavell RA, Haeryfar SM, Madrenas J, Singh B | pmid = 18981105 | volume=181 | issue=10 | title=Dendritic cell differentiation induced by a self-peptide derived from apolipoprotein E. | date=Nov 2008 | journal=J Immunol | pages=6859β71 | doi=10.4049/jimmunol.181.10.6859| s2cid = 23966566 | url=http://www.jimmunol.org/content/jimmunol/181/10/6859.full.pdf | doi-access=free }}</ref> These are primarily [[Tolerogenic dendritic cell|tolerogenic plasmacytoid dendritic cells]].<ref>{{cite journal | vauthors = Bellemore SM, Nikoopour E, Au BC, Krougly O, Lee-Chan E, Haeryfar SM, Singh B | year = 2014 | title = Anti-atherogenic peptide Ep1.B derived from Apolipoprotein E induces tolerogenic plasmacytoid dendritic cells | journal = Clin Exp Immunol | volume = 177 | issue = 3| pages = 732β42 | doi = 10.1111/cei.12370 | pmid = 24784480 | pmc = 4137858 }}</ref>
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