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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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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 then activate calcineurin calcineurin in turn activates nfat which then translocates to the nucleus nfat is a transcription factor that activates the transcription of a pleiotropic set of genes most notable il 2 a cytokine that promotes long term proliferation of activated t cells plc γ can also initiate the nf κb pathway dag activates pkc θ which then phosphorylates carma1 causing it to unfold and function as a scaffold the cytosolic domains bind an adapter bcl10 via card caspase activation and recruitment domains domains that then binds traf6 which is ubiquitinated at k63 513 523 50 this form of ubiquitination does not lead to degradation of target proteins rather it serves to recruit nemo ikkα and β and tab1 2 tak1 51 tak 1 phosphorylates ikk β which then phosphorylates iκb allowing for k48 ubiquitination leads to proteasomal degradation rel a and p50 can then enter the nucleus and bind the nf κb response element this coupled with nfat signaling allows for complete activation of the il 2 gene 50 while in most cases activation is dependent on tcr recognition of antigen alternative pathways for activation have been described for example cytotoxic t cells have been shown to become activated when targeted by other cd8 t cells leading to tolerization of the latter 52 in spring 2014 the t cell activation in space tcas experiment was launched to the international space station on the spacex crs 3 mission to study how deficiencies in the human immune system are affected by a microgravity environment 53 t cell activation is modulated by reactive oxygen species 54 antigen discrimination edit a unique feature of t cells is their ability to discriminate between healthy and abnormal e g infected or cancerous cells in the body 55 healthy cells typically express a large number of self derived pmhc on their cell surface and although the t cell antigen receptor can interact with at least a subset of these self pmhc the t cell generally ignores these healthy cells however when these very same cells contain even minute quantities of pathogen derived pmhc t cells are able to become activated and initiate immune responses the ability of t cells to ignore healthy cells but respond when these same cells contain pathogen or cancer derived pmhc is known as antigen discrimination the molecular mechanisms that underlie this process are controversial 55 56 clinical significance edit deficiency edit main article t cell deficiency hiv infected t cell causes of t cell deficiency include lymphocytopenia of t cells and or defects on function of individual t cells complete insufficiency of t cell function can result from hereditary conditions such as severe combined immunodeficiency scid omenn syndrome and cartilage hair hypoplasia 57 causes of partial insufficiencies of t cell function include acquired immune deficiency syndrome aids and hereditary conditions such as digeorge syndrome dgs chromosomal breakage syndromes cbss and b cell and t cell combined disorders such as ataxia telangiectasia at and wiskott aldrich syndrome was 57 the main pathogens of concern in t cell deficiencies are intracellular pathogens including herpes simplex virus mycobacterium and listeria 58 also fungal infections are also more common and severe in t cell deficiencies 58 cancer edit further information t cell lymphoma cancer of t cells is termed t cell lymphoma and accounts for perhaps one in ten cases of non hodgkin lymphoma 59 the main forms of t cell lymphoma are extranodal t cell lymphoma cutaneous t cell lymphomas sézary syndrome and mycosis fungoides anaplastic large cell lymphoma angioimmunoblastic t cell lymphoma exhaustion edit t cell exhaustion is a poorly defined or ambiguous term 60 61 there are three approaches to its definition 60 the first approach primarily defines as exhausted the cells that present the same cellular dysfunction typically the absence of an expected effector response the second approach primarily defines as exhausted the cells that are produced by a given cause typically but not necessarily chronic exposure to an antigen finally the third approach primarily defines as exhausted the cells that present the same molecular markers typically programmed cell death protein 1 pd 1 60 indeed it is now starting to emerge that exhaustion might not be the only t cell dysfunctional state 62 in fact tolerization anergy cell death ignorance senesence and exclusion have recently emerged as additional sources and or states of t cell dysfunction in cancer and chronic viral infection 63 dysfunctional t cells are characterized by progressive loss of function changes in transcriptional profiles and sustained expression of inhibitory receptors at first cells lose their ability to produce il 2 and tnfα which is followed by the loss of high proliferative capacity and cytotoxic potential and eventually leads to their deletion exhausted t cells typically indicate higher levels of cd43 cd69 and inhibitory receptors combined with lower expression of cd62l and cd127 exhaustion can develop during chronic infections sepsis and cancer 64 exhausted t cells preserve their functional exhaustion even after repeated antigen exposure 65 during chronic infection and sepsis edit t cell exhaustion can be triggered by several factors like persistent antigen exposure and lack of cd4 t cell help 66 antigen exposure also has effect on the course of exhaustion because longer exposure time and higher viral load increases the severity of t cell exhaustion at least 2 4 weeks exposure is needed to establish exhaustion 67 another factor able to induce exhaustion are inhibitory receptors including programmed cell death protein 1 pd1 ctla 4 t cell membrane protein 3 tim3 and lymphocyte activation gene 3 protein lag3 68 69 soluble molecules such as cytokines il 10 or tgf β are also able to trigger exhaustion 70 71 last known factors that can play a role in t cell exhaustion are regulatory cells t reg cells can be a source of il 10 and tgf β and therefore they can play a role in t cell exhaustion 72 furthermore t cell exhaustion is reverted after depletion of t reg cells and blockade of pd1 73 t cell exhaustion can also occur during sepsis as a result of cytoki...