Рецепторы распознавания паттернов: семейство и функции.

Family of cell surface receptors
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, as well as by 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. The microbe specific molecules that are recognized by a given PRR are called pathogen associated molecular patterns (PAMPs) and include bacterial carbohydrates (such as lipopolysaccharide or LPS, mannose), nucleic acids (such as bacterial or viral DNA or RNA), bacterial peptides (flagellin, microtubule elongation factors), peptidoglycans and lipoteichoic acids (from Gram positive bacteria), N formylmethionine, lipoproteins and fungal glucans and chitin. Endogenous stress signals are called damage associated molecular patterns (DAMPs) and include uric acid and extracellular ATP, among many other compounds. Since that time many plant PRRs have been predicted by genomic analysis (370 in rice; 47 in Arabidopsis). Unlike animal PRRs, which are associated with intracellular kinases via adaptor proteins (see non RD kinases below), plant PRRs are composed of an extracellular domain, transmembrane domain, juxtamembrane domain and intracellular kinase domain as part of a single protein.

Recognition of extracellular or endosomal pathogen associated molecular patterns is mediated by transmembrane proteins known as toll like receptors (TLRs). TLRs share a typical structural motif, the Leucine rich repeats (LRR), which give them their specific appearance and are also responsible for TLR functionality. Toll like receptors were first discovered in Drosophila and trigger the synthesis and secretion of cytokines and activation of other host defense programs that are necessary for both innate or adaptive immune responses. 10 functional members of the TLR family have been described in humans so far. Nonetheless, TLR11 is only a pseudogene in humans without direct function or functional protein expression. Each of the TLR has been shown to interact with a specific PAMP.

TLRs tend to dimerize, TLR4 forms homodimers, and TLR6 can dimerize with either TLR1 or TLR2.

Many different cells of the innate immune system express a myriad of CLRs which shape innate immunity by virtue of their pattern recognition ability. nonetheless, other PAMPs have been identified in studies as targets of CLRs as well e. g. mannose is the recognition motif for many viruses, fungi and mycobacteria; similarly fucose presents the same for certain bacteria and helminths; and glucans are present on mycobacteria and fungi. In addition, many of acquired nonself surfaces e. g. carcinoembryonic/oncofetal type neoantigens carrying "internal danger source"/"self turned nonself" type pathogen pattern are also identified and destroyed (e. g. by complement fixation or other cytotoxic attacks) or sequestered (phagocytosed or ensheathed) by the immune system by virtue of the CLRs. The name lectin is a bit misleading because the family includes proteins with at least one C type lectin domain (CTLD) which is a specific type of carbohydrate recognition domain. CTLD is a ligand binding motif found in more than 1000 known proteins (more than 100 in humans) and the ligands are often not sugars. If and when the ligand is sugar they need Ca2+ – hence the name "C type", but many of them don't even have a known sugar ligand thus despite carrying a lectin type fold structure, some of them are technically not "lectin" in function.

There are several types of signaling involved in CLRs induced immune response, major connection has been identified between TLR and CLR signaling, therefore we differentiate between TLR dependent and TLR independent signaling. DC SIGN leading to RAF1 MEK ERK cascade, BDCA2 signaling via ITAM and signaling through ITIM belong among the TLR dependent signaling. TLR independent signaling such as Dectin 1, and Dectin 2 – mincle signaling lead to MAP kinase and NFkB activation. Generally there is a large group, which recognizes and binds carbohydrates, so called carbohydrate recognition domains (CRDs) and the previously mentioned CTLDs. Another potential characterization of the CLRs can be into mannose receptors and asialoglycoprotein receptors. is a PRR primarily present on the surface of macrophages and dendritic cells. It belongs into the calcium dependent multiple CRD group. It recognizes and binds to repeated mannose units on the surfaces of infectious agents and its activation triggers endocytosis and phagocytosis of the microbe via the complement system. Specifically, mannose binding triggers recruitment of MBL associated serine proteases (MASPs). The serine proteases activate themselves in a cascade, amplifying the immune response: MBL interacts with C4, binding the C4b subunit and releasing C4a into the bloodstream; similarly, binding of C2 causes release of C2b. Together, MBL, C4b and C2a are known as the C3 convertase. C3 is cleaved into its a and b subunits, and C3b binds the convertase. These together are called the C5 convertase. Similarly again, C5b is bound and C5a is released. C5b recruits C6, C7, C8 and multiple C9s. C5, C6, C7, C8 and C9 form the membrane attack complex (MAC).

This is another large superfamily of CLRs that includes the classic asialoglycoprotein receptor macrophage galactose type lectin (MGL), DC SIGN (CLEC4L), Langerin (CLEC4K), Myeloid DAP12‑associating lectin (MDL)‑1 (CLEC5A), DC‑associated C‑type lectin 1 (Dectin1) subfamily, and DC immunoreceptor (DCIR) subfamily. Furthermore, Dectin subfamily and DCIR subfamily consist of some members as follow. DC‑associated C‑type lectin 1 (Dectin1) subfamily includes dectin 1/CLEC7A, DNGR1/CLEC9A, Myeloid C‑type lectin‑like receptor (MICL) (CLEC12A), CLEC2 (also called CLEC1B) the platelet activation receptor for podoplanin on lymphatic endothelial cells and invading front of some carcinomas, and CLEC12B; while DC immunoreceptor (DCIR) subfamily includes DCIR/CLEC4A, Dectin 2/CLEC6A, Blood DC antigen 2 (BDCA2) ( CLEC4C), and Mincle i. e. macrophage‑inducible C‑type lectin (CLEC4E). The nomenclature (mannose versus asialoglycoprotein) is a bit misleading as these the asialoglycoprotein receptors are not necessarily galactose (one of the commonest outer residues of asialo glycoprotein) specific receptors and even many of this family members can also bind to mannose after which the other group is named.

Some of these proteins recognize endogenous or microbial molecules or stress responses and form oligomers that, in animals, activate inflammatory caspases (e. g. caspase 1) causing cleavage and activation of important inflammatory cytokines such as IL 1, and/or activate the NF κB signaling pathway to induce production of inflammatory molecules. The NLR family is known under several different names, including the CATERPILLER (or CLR) or NOD LRR family. The most significant members of the NLRs are NOD1 and NOD2. They sense the conserved microbial peptidoglycans in the cytoplasm of the cell and therefore represent another level of immune response after membrane bound receptors such as TLRs and CLRs.

Three RLR helicases have so far been described: RIG I and MDA5 (recognizing 5'triphosphate RNA and dsRNA, respectively), which activate antiviral signaling, and LGP2, which appears to act as a dominant negative inhibitor. RLRs initiate the release of inflammatory cytokines and type I interferon (IFN I). It has been suggested that the main antiviral program induced by RLR is based on ATPase activity. RLRs often interact and create cross talk with the TLRs in the innate immune response and in regulation of adaptive immune response.

A number of PRRs do not remain associated with the cell that produces them. Complement receptors, collectins, ficolins, pentraxins such as serum amyloid and C reactive protein, lipid transferases, peptidoglycan recognition proteins (PGRPs) and the LRR, XA21D are all secreted proteins. One very important collectin is mannan binding lectin (MBL), a major PRR of the innate immune system that binds to a wide range of bacteria, viruses, fungi and protozoa. MBL predominantly recognizes certain sugar groups on the surface of microorganisms but also binds phospholipids, nucleic acids and non glycosylated proteins. Once bound to the ligands MBL and Ficolin oligomers recruit MASP1 and MASP2 and initiate the lectin pathway of complement activation which is somewhat similar to the classical complement pathway.

Plants contain a significant number of PRRs that share remarkable structural and functional similarity with drosophila TOLL and mammalian TLRs. The first PRR identified in plants or animals was the Xa21 protein, conferring resistance to the Gram negative bacterial pathogen Xanthomonas oryzae pv. oryzae. Since that time two other plants PRRs, Arabidopsis FLS2 (flagellin) and EFR (elongation factor Tu receptor)have been isolated. Plant PRRs either exist as surface localized receptor kinases (RKs) or receptor like proteins (RLPs) that contain multiple ligand binding ectodomains that perceive PAMPs or DAMPs. The corresponding PAMPs for FLS2 and EFR have been identified. Upon ligand recognition, the plant PRRs transduce "PAMP triggered immunity" (PTI). Plant immune systems also encode resistance proteins that resemble NOD like receptors (see above), that feature NBS and LRR domains and can also carry other conserved interaction domains such as the TIR cytoplasmic domain found in Toll and Interleukin Receptors. The nucleotide binding and leucine rich repeat (NBS LRR) proteins are required for detecting nonindigenous molecular signatures from pathogens.

Research groups have recently conducted extensive research into the involvement and potential use of patient's immune system in the therapy of various diseases, the so called immunotherapy, including monoclonal antibodies, non specific immunotherapies, oncolytic virus therapy, T cell therapy and cancer vaccines. NOD2 has been associated through a loss and gain of function with development of Crohn's disease and early onset sarcoidosis. Mutations in NOD2 in cooperation with environmental factors lead to development of chronic inflammation in the intestine. Therefore, it has been suggested to treat the disease by inhibiting the small molecules, which are able to modulate the NOD2 signaling, particularly RIP2. Two therapeutics have been approved by FDA so far inhibiting the phosphorylation on RIP2, which is necessary for proper NOD2 functioning, gefitinib and erlotinib. Additionally, research has been conducted on GSK583, a highly specific RIP2 inhibitor, which seems highly promising in inhibiting NOD1 and NOD2 signaling and therefore, limiting inflammation caused by NOD1, NOD2 signaling pathways. Another possibility is to remove the sensor for NOD2, which has been proved efficient in murine models in the effort to suppress the symptoms of Crohn's disease. Type II kinase inhibitors, which are highly specific, have shown promising results in blocking the TNF arising from NOD dependent pathways, which shows a high potential in treatment of inflammation associated tumors.

Another possible exploitation of PRRs in human medicine is also related to tumor malignancies of the intestines. Helicobacter pylori has been shown by studies to significantly correlate with the development of a gastrointestinal tumors. In a healthy individual Helicobacter pylori infection is targeted by the combination of PRRs, namely TLRs, NLRs, RLRs and CLR DC SIGN. In case of their malfunction, these receptors have also been connected to carcinogenesis. When the Helicobacter pylori infection is left to progress in the intestine it develops into chronic inflammation, atrophy and eventually dysplasia leading to development of cancer. Since all types of PRRs play a role in the identification and eradication of the infection, their specific agonists mount a strong immune response to cancers and other PRR related diseases. The inhibition of TLR2 has been shown to significantly correlate with improved state of the patient and suppression of the gastric adenocarcinoma.

The PRRs are also tightly connected to the proper function of neuronal networks and tissues, especially because of their involvement in the processes of inflammation, which are essential for proper function but may cause irreparable damage if not under control. The TLRs are expressed on most cells of the central nervous system (CNS) and they play a crucial role in sterile inflammation. After an injury, they lead to impairment of axonal growth and slow down or even halt the recovery altogether. Another important structure involved in and potentially exploitable in therapy after injury is the inflammasome. Through its induction of proinflammatory cytokines, IL 1β and IL 18 it has been proposed, that inhibition of inflammasome may also serve as an efficient therapeutic method. The involvement of inflammasome has also been researched in several other diseases including experimental autoimmune encephalomyelitis (EAE), Alzheimer's and Parkinson's diseases and in atherosclerosis connected with type II diabetes in patients. The suggested therapies include degradation of NLRP3 or inhibit the proinflammatory cytokines.