Abstract
NK cells have usually been defined as cells of the innate immune system, although they are also involved in adaptative responses. These cells belong to the innate lymphocyte cells (ILC) family. They remove unwanted cells, tumoral cells and pathogens. NK cells are essential for viral infection clearance and are involved in tolerogenic responses depending on the dynamic balance of the repertoire of activating and inhibitory receptors. NK plasticity is crucial for tissue function and vigilant immune responses. They directly eliminate virus-infected cells by recognising viral protein antigens using a non-MHC dependent mechanism, recognising viral glycan structures and antigens by NCR family receptors, inducing apoptosis by Fas-Fas ligand interaction, and killing cells by antibody-dependent cell cytotoxicity via the FcγIII receptor. Activating receptors are responsible for the clearance of virally infected cells, while inhibitory KIR receptor activation impairs NK responses and facilitates virus escape. Effective NK memory cells have been described and characterised by a low NKG2A and high NKG2C or NKG2D expression. NK cells have also been used in cell therapy. In SARS-CoV-2 infection, several contradicting reports about the role of NK cells have been published. A careful analysis of the current data and possible implications will be discussed.
Highlights
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There are several populations of tissue-resident NK cells: uterine, decidual, adipose, lung, liver, spleen, tonsil, gut [1,2,3,4,5,6]. The antigens of these cells depend on tissue; for example, CD56dim/CD16bright NK cells are more prevalent in the lungs than the CD56bright/CD16dim positive cells and the CD56bright, CD16low, NKG2A/CD94, CD49a+, NCR1+, integrin β7+, CD117+, DX5−, which are located in the uterus [1,2,3,4,5,6,7,8,9]
Several reports have been on NK cell numbers and peripheral blood subpopulations in SARS-CoV-2 infection
Summary
NK cells are usually defined as immune cells that belong to the innate immune response [1,2,3,4]. It was shown that NK cells lack the TCR and BCR receptors capable of binding specific antigens, and it was assumed that these cells lack antigen recognition [1,2,3,4] This definition has changed in the latest years due to the increasing evidence of memory responses of NK cells in animal models and humans [5]. There are several populations of tissue-resident NK cells: uterine, decidual, adipose, lung, liver, spleen, tonsil, gut [1,2,3,4,5,6] The antigens of these cells depend on tissue; for example, CD56dim/CD16bright NK cells are more prevalent in the lungs than the CD56bright/CD16dim positive cells and the CD56bright, CD16low, NKG2A/CD94, CD49a+, NCR1+, integrin β7+, CD117+, DX5−, which are located in the uterus [1,2,3,4,5,6,7,8,9]. CARct.ivAatcintigvcaytitnokginceystroekceinpetosrsremcaeyptors ma play apnlaiymanpoimrtpaonrttarnotlreolieninprpirmiminingg oorr eennhhaanncicnigntghethreespreosnpseonofsoethoefrortehceeprtroersceapndtotrhse adnecdretahsee idnecrease i inhibiitnohriybitroercyerpectoeprsto.rIsn. tIengtergirninssaanndd sseelleecctitninggacaticvtaivteaPteKCPKanCd aennhdanecnehNanKcmeoNbiKlitymaonbdilsietcyreatinodn. secretion
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