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cells (306), the (302), and (238), cell (196), are (87), pmid (80), doi (79), pmc (51), immune (50), that (50), cd8 (48), memory (46), cd4 (45), with (40), for (38), also (38), exhaustion (37), can (37), edit (35), which (34), #antigen (33), mhc (32), these (32), #selection (29), this (28), immunology (27), their (27), from (26), associated (26), during (26), activation (26), they (25), molecules (24), cancer (23), positive (23), tcr (23), thymus (21), response (20), invariant (20), infection (20), expression (20), helper (19), system (18), killer (18), regulatory (18), cytotoxic (18), self (18), have (18), journal (17), negative (17), receptor (17), surface (17), then (17), thymocytes (17), other (16), mucosal (16), s2cid (16), role (16), class (15), 1038 (15), 1016 (15), not (15), development (15), immunity (14), innate (14), after (14), thymic (14), main (14), into (14), may (13), cytokines (13), 2015 (13), transplantation (13), known (13), proteins (13), process (13), activated 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aturation of b cells into plasma cells and memory b cells and activation of cytotoxic t cells and macrophages these cells are also known as cd4 t cells as they express the cd4 glycoprotein on their surfaces helper t cells become activated when they are presented with peptide antigens by mhc class ii molecules which are expressed on the surface of antigen presenting cells apcs once activated they divide rapidly and secrete cytokines that regulate or assist the immune response these cells can differentiate into one of several subtypes which have different roles cytokines direct t cells into particular subtypes 16 cd4 helper t cell subsets cell type cytokines produced key transcription factor role in immune defense related diseases th1 ifnγ il 2 tbet produce an inflammatory response key for defense against intracellular bacteria viruses and cancer ms type 1 diabetes th2 il 4 il 5 il 13 gata 3 immunologically important against extracellular pathogens such as worm infections asthma and other allergic diseases th17 il 17f il 17a il 22 rorγt defense against gut pathogens and at mucosal barriers ms rheumatoid arthritis psoriasis th9 17 18 il 9 irf4 pu 1 defense against helminths parasitic worms and cell dependent allergic inflammation multiple sclerosis tfh il 21 il 4 bcl 6 help b cells produce antibodies asthma and other allergic diseases th22 19 18 il 22 ahr pathogenesis of allergic airway diseases and predominantly anti inflammatory crohn s disease rheumatoid arthritis tumors cytotoxic cd8 t cells edit main article cytotoxic t cell superresolution image of a group of cytotoxic t cells surrounding a cancer cell cytotoxic t cells t c cells ctls t killer cells killer t cells destroy virus infected cells and tumor cells and are also implicated in transplant rejection these cells are defined by the expression of the cd8 protein on their cell surface cytotoxic t cells recognize their targets by binding to short peptides 8 11 amino acids in length associated with mhc class i molecules present on the surface of all nucleated cells cytotoxic t cells also produce the key cytokines il 2 and ifnγ these cytokines influence the effector functions of other cells in particular macrophages and nk cells memory t cells edit main article memory t cell antigen naive t cells expand and differentiate into memory and effector t cells after they encounter their cognate antigen within the context of an mhc molecule on the surface of a professional antigen presenting cell e g a dendritic cell appropriate co stimulation must be present at the time of antigen encounter for this process to occur historically memory t cells were thought to belong to either the effector or central memory subtypes each with their own distinguishing set of cell surface markers see below 20 subsequently numerous new populations of memory t cells were discovered including tissue resident memory t trm cells stem memory tscm cells and virtual memory t cells the single unifying theme for all memory t cell subtypes is that they are long lived and can quickly expand to large numbers of effector t cells upon re exposure to their cognate antigen by this mechanism they provide the immune system with memory against previously encountered pathogens memory t cells may be either cd4 or cd8 and usually express cd45ro 21 memory t cell subtypes central memory t cells t cm cells express cd45ro c c chemokine receptor type 7 ccr7 and l selectin cd62l central memory t cells also have intermediate to high expression of cd44 this memory subpopulation is commonly found in the lymph nodes and in the peripheral circulation note cd44 expression is usually used to distinguish murine naive from memory t cells effector memory t cells t em cells and t emra cells express cd45ro but lack expression of ccr7 and l selectin they also have intermediate to high expression of cd44 these memory t cells lack lymph node homing receptors and are thus found in the peripheral circulation and tissues 22 t emra stands for terminally differentiated effector memory cells re expressing cd45ra which is a marker usually found on naive t cells 23 tissue resident memory t cells t rm occupy tissues skin lung etc without recirculating one cell surface marker that has been associated with t rm is the intern αeβ7 also known as cd103 24 virtual memory t cells t vm differ from the other memory subsets in that they do not originate following a strong clonal expansion event thus although this population as a whole is abundant within the peripheral circulation individual virtual memory t cell clones reside at relatively low frequencies one theory is that homeostatic proliferation gives rise to this t cell population although cd8 virtual memory t cells were the first to be described 25 it is now known that cd4 virtual memory cells also exist 26 regulatory cd4 t cells edit main article regulatory t cell regulatory t cells are crucial for the maintenance of immune tolerance their major role is to shut down t cell mediated immunity toward the end of an immune reaction and to suppress autoreactive t cells that escaped the process of negative selection in the thymus two major classes of cd4 t reg cells have been described foxp3 t reg cells and foxp3 t reg cells regulatory t cells can develop either during normal development in the thymus and are then known as thymic t reg cells or can be induced peripherally and are called peripherally derived t reg cells these two subsets were previously called naturally occurring and adaptive or induced respectively 27 both subsets require the expression of the transcription factor foxp3 which can be used to identify the cells mutations of the foxp3 gene can prevent regulatory t cell development causing the fatal autoimmune disease ipex several other types of t cells have suppressive activity but do not express foxp3 constitutively these include tr1 and th3 cells which are thought to originate during an immune response and act by producing suppressive molecules tr1 cells are associated with il 10 and th3 cells are associated with tgf beta recently th17 cells have been added to this list 28 innate like t cells edit innate like t cells or unconventional t cells represent some subsets of t cells that behave differently in immunity they trigger rapid immune responses regardless of the major histocompatibility complex mhc expression unlike their conventional counterparts cd4 t helper cells and cd8 cytotoxic t cells which are dependent on the recognition of peptide antigens in the context of the mhc molecule overall there are three large populations of unconventional t cells nkt cells mait cells and gammadelta t cells now their functional roles are already being well established in the context of infections and cancer 29 furthermore these t cell subsets are being translated into many therapies against malignancies such as leukemia for example 30 natural killer t cell edit main article natural killer t cell natural killer t cells nkt cells not to be confused with natural killer cells of the innate immune system bridge the adaptive immune system with the innate immune system unlike conventional t cells that recognize protein peptide antigens presented by major histocompatibility complex mhc molecules nkt cells recognize glycolipid antigens presented by cd1d once activated these cells can perform functions ascribed to both helper and cytotoxic t cells cytokine production and release of cytolytic cell killing molecules they are also able to recognize and eliminate some tumor cells and cells infected with herpes viruses 31 mucosal associated invariant t cells edit main article mucosal associated invariant t cell mucosal associated invariant t mait cells display innate effector like qualities 32 33 in humans mait cells are found in the blood liver lungs and mucosa defending against microbial activity and infection 32 the mhc class i like protein mr1 is responsible for presenting bacterially produced vitamin b metabolites to mait cells 34 35 36 after the presentation of foreign antigen by mr1 mait cells secrete pro inflammatory cytokines and are capable of lysing bacterially infected cells 32 36 mait cells can also be activated through mr1 independent signaling 36 in addition to possessing innate like functions this t cell subset supports the adaptive immune response and has a memory like phenotype 32 furthermore mait cells are thought to play a role in autoimmune diseases such as multiple sclerosis arthritis and inflammatory bowel disease 37 38 although definitive evidence is yet to be published 39 40 41 42 gamma delta t cells edit main article gamma delta t cell gamma delta t cells γδ t cells represent a small subset of t cells which possess a γδ tcr rather than the αβ tcr on the cell surface the majority of t cells express αβ tcr chains this group of t cells is much less common in humans and mice about 2 of total t cells and are found mostly in the gut mucosa within a population of intraepithelial lymphocytes in rabbits sheep and chickens the number of γδ t cells can be as high as 60 of total t cells the antigenic molecules that activate γδ t cells are still mostly unknown however γδ t cells are not mhc restricted and seem to be able to recognize whole proteins rather than requiring peptides to be presented by mhc molecules on apcs some murine γδ t cells recognize mhc class ib molecules human γδ t cells that use the vγ9 and vδ2 gene fragments constitute the major γδ t cell population in peripheral blood these cells are unique in that they specifically and rapidly respond to a set of nonpeptidic phosphorylated isoprenoid precursors collectively named phosphoantigens which are produced by virtually all living cells the most common phosphoantigens from animal and human cells including cancer cells are isopentenyl pyrophosphate ipp and its isomer dimethylallyl pyrophosphate dmpp many microbes produce the active compound hydroxy dmapp hmb pp and corresponding mononucleotide conjugates in addition to ipp and dmapp plant cells produce both types of phosphoantigens drugs activating human vγ9 vδ2 t cells comprise synthetic phosphoantigens and aminobisphosphonates which upregulate endogenous ipp dmapp activation edit see also t cell receptor signaling pathway the t lymphocyte activation pathway t cells contribute to immune defenses in two major ways some direct and regulate immune responses others directly attack infected or cancerous cells 43 activation of cd4 t cells occurs through the simultaneous engagement of the t cell receptor and a co stimulatory molecule like cd28 or icos on the t cell by the major histocompatibility complex mhcii peptide and co stimulatory molecules on the apc both are required for production of an effective immune response in the absence of co stimulation t cell receptor signalling alone results in anergy the signalling pathways downstream from co stimulatory molecules usually engages the pi3k pathway generating pip3 at the plasma membrane and recruiting ph domain containing signaling molecules like pdk1 that are essential for the activation of pkc θ and eventual il 2 production optimal cd8 t cell response relies on cd4 signalling 44 cd4 cells are useful in the initial antigenic activation of naive cd8 t cells and sustaining memory cd8 t cells in the aftermath of an acute infection therefore activation of cd4 t cells can be beneficial to the action of cd8 t cells 45 46 47 the first signal is provided by binding of the t cell receptor to its cognate peptide presented on mhcii on an apc mhcii is restricted to so called professional antigen presenting cells like dendritic cells b cells and macrophages to name a few the peptides presented to cd8 t cells by mhc class i molecules are 8 13 amino acids in length the peptides presented to cd4 cells by mhc class ii molecules are longer usually 12 25 amino acids in length 48 as the ends of the binding cleft of the mhc class ii molecule are open the second signal comes from co stimulation in which surface receptors on the apc are induced by a relatively small number of stimuli usually products of pathogens but sometimes breakdown products of cells such as necrotic bodies or heat shock proteins the only co stimulatory receptor expressed constitutively by naive t cells is cd28 so co stimulation for these cells comes from the cd80 and cd86 proteins which together constitute the b7 protein b7 1 and b7 2 respectively on the apc other receptors are expressed upon activation of the t cell such as ox40 and icos but these largely depend upon cd28 for their expression the second signal licenses the t cell to respond to an antigen without it the t cell becomes anergic and it becomes more difficult for it to activate in future this mechanism prevents inappropriate responses to self as self peptides will not usually be presented with suitable co stimulation once a t cell has been appropriately activated i e has received signal one and signal two it alters its cell surface expression of a variety of proteins markers of t cell activation include cd69 cd71 and cd25 also a marker for t reg cells and hla dr a marker of human t cell activation ctla 4 expression is also up regulated on activated t cells which in turn outcompetes cd28 for binding to the b7 proteins this is a checkpoint mechanism to prevent over activation of the t cell activated t cells also change their cell surface glycosylation profile 49 the t cell receptor exists as a complex of several proteins the actual t cell receptor is composed of two separate peptide chains which are produced from the independent t cell receptor alpha and beta tcrα and tcrβ genes the other proteins in the complex are the cd3 proteins cd3εγ and cd3εδ heterodimers and most important a cd3ζ homodimer which has a total of six itam motifs the itam motifs on the cd3ζ can be phosphorylated by lck and in turn recruit zap 70 lck and or zap 70 can also phosphorylate the tyrosines on many other molecules not least cd28 lat and slp 76 which allows the aggregation of signalling complexes around these proteins phosphorylated lat recruits slp 76 to the membrane where it can then bring in plc γ vav1 itk and potentially pi3k plc γ cleaves pi 4 5 p2 on the inner leaflet of the membrane to create the active intermediaries diacylglycerol dag inositol 1 4 5 trisphosphate ip3 pi3k also acts on pip2 phosphorylating it to produce phosphatidlyinositol 3 4 5 trisphosphate pip3 dag binds and activates some pkcs most important in t cells is pkc θ critical for activating the transcription factors nf κb and ap 1 ip3 is released from the membrane by plc γ and diffuses rapidly to activate calcium channel receptors on the er which induces the release of calcium into the cytosol low calcium in the endoplasmic reticulum causes stim1 clustering on the er membrane and leads to activation of cell membrane crac channels that allows additional calcium to flow into the cytosol from the extracellular space this aggregated cytosolic calcium binds calmodulin which can th...
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