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Natural killer cells, also known as NK cells or large granular lymphocytes (LGL), are a type of cytotoxic lymphocyte critical to the innate immune system that belong to the rapidly expanding family of innate lymphoid cells (ILC) and represent 5–20% of all circulating lymphocytes in humans. The role of NK cells is analogous to that of cytotoxic T cells in the vertebrate adaptive immune response. NK cells provide rapid responses to virus-infected cell and other intracellular pathogens acting at around 3 days after infection, and respond to tumor formation. Typically, immune cells detect the major histocompatibility complex (MHC) presented on infected cell surfaces, triggering cytokine release, causing the death of the infected cell by lysis or apoptosis. NK cells are unique, however, as they have the ability to recognize and kill stressed cells in the absence of antibodies and MHC, allowing for a much faster immune reaction. They were named "natural killers" because of the notion that they do not require activation to kill cells that are missing "self" markers of MHC class 1. This role is especially important because harmful cells that are missing MHC I markers cannot be detected and destroyed by other immune cells, such as T lymphocyte cells.
Pattern recognition receptors (PRRs) play a crucial role in the proper function of the innate immune system. PRRs are germline-encoded host sensors, which detect molecules typical for the pathogens. They are proteins expressed, mainly, by cells of the innate immune system, such as dendritic cells, macrophages, monocytes, neutrophils and epithelial cells, to identify two classes of molecules: pathogen-associated molecular patterns (PAMPs), which are associated with microbial pathogens, and damage-associated molecular patterns (DAMPs), which are associated with components of host's cells that are released during cell damage or death. They are also called primitive pattern recognition receptors because they evolved before other parts of the immune system, particularly before adaptive immunity. PRRs also mediate the initiation of antigen-specific adaptive immune response and release of inflammatory cytokines.
DC-SIGN also known as CD209 is a protein which in humans is encoded by the CD209 gene.
Protein tyrosine phosphatase, receptor type, C also known as PTPRC is an enzyme that, in humans, is encoded by the PTPRC gene. PTPRC is also known as CD45 antigen, which was originally called leukocyte common antigen (LCA).
CD94, also known as killer cell lectin-like receptor subfamily D, member 1 (KLRD1) is a human gene.
Ly49 is a family of membrane C-type lectin-like receptors expressed mainly on NK cells but also on other immune cells. Their primary role is to bind MHC-I molecules to distinguish between self healthy cells and infected or altered cells. Ly49 family is coded by Klra gene cluster and include genes for both inhibitory and activating paired receptors, but most of them are inhibitory. Inhibitory Ly49 receptors play a role in the recognition of self cells and thus maintain self-tolerance and prevent autoimmunity by suppressing NK cell activation. On the other hand, activating receptors recognise ligands from cancer or viral infected cells and are used when cells lack or have abnormal expression of MHC-I molecules, which activate cytokine production and cytotoxic activity of NK and immune cells.
TLR 1 is a member of the toll-like receptor family (TLR) of pattern recognition receptors of the innate immune system. TLR1 recognizes pathogen-associated molecular pattern with a specificity for gram-positive bacteria. TLR1 has also been designated as CD281.
Macrophage migration inhibitory factor (MIF), also known as glycosylation-inhibiting factor (GIF), L-dopachrome isomerase, or phenylpyruvate tautomerase is a protein that in humans is encoded by the MIF gene. MIF is an important regulator of innate immunity. The MIF protein superfamily also includes a second member with functionally related properties, the D-dopachrome tautomerase (D-DT). CD74 is a surface receptor for MIF.
CD69 is a human transmembrane C-Type lectin protein encoded by the CD69 gene. It is an early activation marker that is expressed in hematopoietic stem cells, T cells, and many other cell types in the immune system. It is also implicated in T cell differentiation as well as lymphocyte retention in lymphoid organs.
RIG-I is a cytosolic pattern recognition receptor (PRR) responsible for the type-1 interferon (IFN1) response. RIG-I is an essential molecule in the innate immune system for recognizing cells that have been infected with a virus. These viruses can include West Nile virus, Japanese Encephalitis virus, influenza A, Sendai virus, flavivirus, and coronaviruses. RIG-I is structurally considered a helical ATP-dependent DExD/H box RNA helicase, that recognizes short viral double-stranded RNA (dsRNA) in the cytosol during a viral infection or other irregular RNAs. Once activated by the dsRNA, the N-terminus caspase activation and recruitment domains (CARDs) migrate and bind with CARDs attached to mitochondrial antiviral signaling protein (MAVS) to activate the signaling pathway for IFN1. IFN1s have three main functions: to limit the virus from spreading to nearby cells, promote an innate immune response, including inflammatory responses, and help activate the adaptive immune system. Other studies have shown that in different microenvironments, such as in cancerous cells, RIG-I has more functions other than viral recognition. RIG-I orthologs are found in mammals, geese, ducks, some fish, and some reptiles. RIG-I is in most cells, including various innate immune system cells, and is usually in an inactive state. Knockout mice that have been designed to have a deleted or non-functioning RIG-I gene are not healthy and typically die embryonically. If they survive, the mice have serious developmental dysfunction. The stimulator of interferon genes STING antagonizes RIG-1 by binding its N-terminus, probably as to avoid overactivation of RIG-1 signaling and the associated autoimmunity.
C-type lectin domain family 7 member A or Dectin-1 is a protein that in humans is encoded by the CLEC7A gene. CLEC7A is a member of the C-type lectin/C-type lectin-like domain (CTL/CTLD) superfamily. The encoded glycoprotein is a small type II membrane receptor with an extracellular C-type lectin-like domain fold and a cytoplasmic domain with a partial immunoreceptor tyrosine-based activation motif. It functions as a pattern-recognition receptor for a variety of β-1,3-linked and β-1,6-linked glucans from fungi and plants, and in this way plays a role in innate immune response. Expression is found on myeloid dendritic cells, monocytes, macrophages and B cells. Alternate transcriptional splice variants, encoding different isoforms, have been characterized. This gene is closely linked to other CTL/CTLD superfamily members on chromosome 12p13 in the natural killer gene complex region.
Natural cytotoxicity triggering receptor 3 is a protein that in humans is encoded by the NCR3 gene. NCR3 has also been designated as CD337 and as NKp30. NCR3 belongs to the family of NCR membrane receptors together with NCR1 (NKp46) and NCR2 (NKp44).
Leukocyte immunoglobulin-like receptor subfamily B member 4 is a protein that in humans is encoded by the LILRB4 gene.
Killer cell lectin-like receptor subfamily B, member 1, also known as KLRB1, NKR-P1A or CD161, is a human gene.
NKG2-C type II integral membrane protein or NKG2C is a protein that in humans is encoded by the KLRC2 gene. It is also known as or cluster of differentiation 159c (CD159c).
Killer cell lectin-like receptor subfamily G member 1 is a protein that in humans is encoded by the KLRG1 gene.
NKG2D is an activating receptor (transmembrane protein) belonging to the NKG2 family of C-type lectin-like receptors. NKG2D is encoded by KLRK1 (killer cell lectin like receptor K1) gene which is located in the NK-gene complex (NKC) situated on chromosome 6 in mice and chromosome 12 in humans. In mice, it is expressed by NK cells, NK1.1+ T cells, γδ T cells, activated CD8+ αβ T cells and activated macrophages. In humans, it is expressed by NK cells, γδ T cells and CD8+ αβ T cells. NKG2D recognizes induced-self proteins from MIC and RAET1/ULBP families which appear on the surface of stressed, malignant transformed, and infected cells.
Ly6 also known as lymphocyte antigen 6 or urokinase-type plasminogen activator receptor (uPAR) is family of proteins that share a common structure but differ in their tissue expression patterns and function. Ly6 are cysteine-rich proteins that form disulfide bridges and contain a LU domain. These proteins are GPI-anchored to the cell membrane or are secreted. A total of 35 human and 61 mouse Ly6 family members have been identified. Depending on which tissues they are expressed in, LY6 family members have different roles. They are expressed in various types of tissues and their expression dependent on the stage of cell differentiation. For example, they are involved in cell proliferation, cell migration, cell–cell interactions, immune cell maturation, macrophage activation, and cytokine production. Their overexpression or dysregulation, for example due to point mutations, is associated with tumorogenesis and autoimmune diseases. This family was discovered in the 1970s, and these proteins are still used as markers of distinct stage of leukocyte differentiation.
Dectin-2 or C-type lectin domain containing 6A is a protein that in humans is encoded by the CLEC6A gene. Dectin-2 is a member of the C-type lectin/C-type lectin-like domain (CTL/CTLD) superfamily. The encoded protein is a type II transmembrane protein with an extracellular carbohydrate recognition domain. It functions as a pattern recognition receptor recognizing α-mannans and as such plays an important role in innate immune response to fungi. Expression is found on macrophages and dendritic cells. It can also be found at low levels in Langerhans cells and peripheral blood monocytes, where expression levels could be increased upon induction of inflammation.
Paired receptors are pairs or clusters of receptor proteins that bind to extracellular ligands but have opposing activating and inhibitory signaling effects. Traditionally, paired receptors are defined as homologous pairs with similar extracellular domains and different cytoplasmic regions, whose genes are located together in the genome as part of the same gene cluster and which evolved through gene duplication. Homologous paired receptors often, but not always, have a shared ligand in common. More broadly, pairs of receptors have been identified that exhibit paired functional behavior - responding to a shared ligand with opposing intracellular signals - but are not closely homologous or co-located in the genome. Paired receptors are highly expressed in the cells of the immune system, especially natural killer (NK) and myeloid cells, and are involved in immune regulation.
References
- ↑ Rahim MM, Tu MM, Mahmoud AB, Wight A, Abou-Samra E, Lima PD, Makrigiannis AP (2014-04-02). "Ly49 receptors: innate and adaptive immune paradigms". Frontiers in Immunology. 5: 145. doi: 10.3389/fimmu.2014.00145 . PMC 3980100 . PMID 24765094.
- 1 2 3 Schenkel AR, Kingry LC, Slayden RA (2013). "The ly49 gene family. A brief guide to the nomenclature, genetics, and role in intracellular infection". Frontiers in Immunology. 4: 90. doi: 10.3389/fimmu.2013.00090 . PMC 3627126 . PMID 23596445.
- ↑ "Klra MGI Mouse Complex/Cluster/Region Detail - MGI:96876 - killer cell lectin-like receptor, subfamily A". www.informatics.jax.org. Retrieved 2019-06-30.
- ↑ Rahim MM, Makrigiannis AP (September 2015). "Ly49 receptors: evolution, genetic diversity, and impact on immunity". Immunological Reviews. 267 (1): 137–47. doi:10.1111/imr.12318. PMID 26284475. S2CID 13458485.
- ↑ Barten R, Trowsdale J (July 1999). "The human Ly-49L gene". Immunogenetics. 49 (7–8): 731–4. doi:10.1007/s002510050675. PMID 10369937. S2CID 708709.
- ↑ Hao L, Klein J, Nei M (February 2006). "Heterogeneous but conserved natural killer receptor gene complexes in four major orders of mammals". Proceedings of the National Academy of Sciences of the United States of America. 103 (9): 3192–7. Bibcode:2006PNAS..103.3192H. doi: 10.1073/pnas.0511280103 . PMC 1413923 . PMID 16492762.
