A T cell, or T lymphocyte, is a type of lymphocyte (a subtype of white blood cell) that plays a central role in cell-mediated immunity. T cells can be distinguished from other lymphocytes, such as B cells and natural killer cells, by the presence of a T-cell receptor on the cell surface. They are called T cells because they mature in the thymus from thymocytes (although some also mature in the tonsils). The several subsets of T cells each have a distinct function. The majority of human T cells, termed alpha beta T cells (αβ T cells), rearrange their alpha and beta chains on the cell receptor and are part of the adaptive immune system. Specialized gamma delta T cells, (a small minority of T cells in the human body, more frequent in ruminants), have invariant T-cell receptors with limited diversity, that can effectively present antigens to other T cells and are considered to be part of the innate immune system.
Effector cells are the superset of all the various T cell types that actively respond immediately to a stimulus, such as co-stimulation. This includes helper, killer, regulatory, and potentially other T cell types. Memory cells are their opposite counterpart that are longer lived to target future infections as necessary.
T helper cells (TH cells) assist other white blood cells in immunologic processes, including maturation of B cells into plasma cellsand memory B cells, and activation of cytotoxic T cells and macrophages. These cells are also known as CD4+ T cells because they express the CD4 glycoprotein on their surfaces. Helper T cells become activated when they are presented with peptideantigens by MHC class II molecules, which are expressed on the surface of antigen-presenting cells (APCs). Once activated, they divide rapidly and secrete small proteins called cytokines that regulate or assist in the active immune response. These cells can differentiate into one of several subtypes, including TH1, TH2, TH3, TH17, TH9, or TFH, which secrete different cytokines to facilitate different types of immune responses. Signalling from the APC directs T cells into particular subtypes.
Cytotoxic T cells (TC 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 also known as CD8+ T cells since they express the CD8 glycoprotein at their surfaces. These cells recognize their targets by binding to antigen associated with MHC class I molecules, which are present on the surface of all nucleated cells. Through IL-10, adenosine, and other molecules secreted by regulatory T cells, the CD8+ cells can be inactivated to an anergic state, which prevents autoimmune diseases.
Antigen-naïve 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). 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.
Memory T cell subtypes
Central memory T cells (TCM 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 (TEM cells and TEMRA 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. TEMRA stands for terminally differentiated effector memory cells re-expressing CD45RA, which is a marker usually found on naive T cells.
Tissue resident memory T cells (TRM) occupy tissues (skin, lung, etc..) without recirculating. One cell surface marker that has been associated with TRM is the integrin αeβ7.
Virtual memory T cells 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, it is now known that CD4 virtual memory cells also exist.
Regulatory T cells (suppressor T cells) are crucial for the maintenance of immunological 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. Suppressor T cells along with Helper T cells can collectively be called Regulatory T cells due to their regulatory functions. Two major classes of CD4+ Treg cells have been described — FOXP3+ Treg cells and FOXP3− Treg cells.
Regulatory T cells can develop either during normal development in the thymus, and are then known as thymic Treg cells, or can be induced peripherally and are called peripherally derived Treg cells. These two subsets were previously called “naturally occurring”, and “adaptive” or “induced”, respectively. 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 cell have suppressive activity, but do not express FOXP3. These include Tr1 cells 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, Treg17 cells have been added to this list.
Natural killer T cells
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 peptide antigens presented by major histocompatibility complex (MHC) molecules, NKT cells recognize glycolipid antigen presented by a molecule called CD1d. Once activated, these cells can perform functions ascribed to both Th and Tc cells (i.e., 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.
Mucosal associated invariant T cell
MAIT cells display innate, effector-like qualities. In humans, MAIT cells are found in the blood, liver, lungs, and mucosa, defending against microbial activity and infection. The MHC class I-like protein, MR1, is responsible for presenting bacterially-produced vitamin B metabolites to MAIT cells. After the presentation of foreign antigen by MR1, MAIT cells secretes pro-inflammatory cytokines and are capable of lysing bacterially-infected cells. MAIT cells can also be activated through MR1-independent signaling. In addition to possessing innate-like functions, this T cell subset supports the adaptive immune response and has a memory-like phenotype. Furthermore, MAIT cells are thought to play a role in autoimmune diseases, such as multiple sclerosis, arthritis and inflammatory bowel disease,although definitive evidence is yet to be published.
Gamma delta T cells (γδ T cells) represent a small subset of T cells that possess a distinct T cell receptor (TCR) on their surfaces. A majority of T cells have a TCRcomposed of two glycoprotein chains called α- and β- TCR chains. However, in γδ T cells, the TCR is made up of one γ-chain and one δ-chain. 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 lymphocytes known as 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 widely 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, though. Human Vγ9/Vδ2 T cells, which constitute the major γδ T cell population in peripheral blood, 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 highly 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.
All T cells originate from haematopoietic stem cells in the bone marrow. Haematopoietic progenitors (lymphoid progenitor cells) from haematopoietic stem cells populate the thymus and expand by cell division to generate a large population of immature thymocytes. The earliest thymocytes express neither CD4 nor CD8, and are therefore classed as double-negative (CD4−CD8−) cells. As they progress through their development, they become double-positive thymocytes (CD4+CD8+), and finally mature to single-positive (CD4+CD8− or CD4−CD8+) thymocytes that are then released from the thymus to peripheral tissues. There is some evidence of double-positive T-cells in the periphery, though their prevalence and function is uncertain. In laboratory, T-cells can be converted into functional neurons within three weeks.
About 98% of thymocytes die during the development processes in the thymus by failing either positive selection or negative selection, whereas the other 2% survive and leave the thymus to become mature immunocompetent T cells. Increasing evidence indicates microRNAs, which are small noncoding regulatory RNAs, could impact the clonal selection process during thymic development. For example, miR-181a was found to play a role in the positive selection of T lymphocytes.
The thymus contributes fewer cells as a person ages. As the thymus shrinks by about 3% a year throughout middle age, a corresponding fall in the thymic production of naïve T cells occurs, leaving peripheral T cell expansion to play a greater role in protecting older subjects.
Common lymphoid precursor cells that migrate to the thymus become known as T-cell precursors (or thymocytes) and do not express a T cell receptor. Broadly speaking, the double negative (DN) stage is focused on producing a functional β-chain whereas the double positive (DP) stage is focused on producing a functional α-chain, ultimately producing a functional αβ T cell receptor. As the developing thymocyte progresses through the four DN stages (DN1, DN2, DN3, and DN4), the T cell expresses an invariant α-chain but rearranges the β-chain locus. If the rearranged β-chain successfully pairs with the invariant α-chain, signals are produced which cease rearrangement of the β-chain (and silence the alternate allele) and result in proliferation of the cell. Although these signals require this pre-TCR at the cell surface, they are independent of ligand binding to the pre-TCR. These thymocytes will then express both CD4 and CD8 and progress to the double positive (DP) stage where selection of the α-chain takes place. If a rearranged β-chain does not lead to any signalling (e.g. as a result of an inability to pair with the invariant α-chain), the cell may die by neglect (lack of signalling).
Positive selection “selects for” T cells capable of interacting with MHC. Positive selection involves the production of a signal by double-positive precursors that express either MHC Class I or II restricted receptors. The signal produced by these thymocytes result in RAG gene repression, long-term survival and migration into the medulla, as well as differentiation into mature T cells. The process of positive selection takes a number of days.
Double-positive thymocytes (CD4+/CD8+) move deep into the thymic cortex, where they are presented with self-antigens. These self-antigens are expressed by thymic cortical epithelial cells on MHC molecules on the surface of cortical epithelial cells. Only those thymocytes that interact with MHC-I or MHC-II appropriately (i.e., not too strongly or too weakly) will receive a vital “survival signal”. All that cannot (i.e., if they do not interact strongly enough, or if they bind too strongly) will die by “death by neglect” (no survival signal). This process ensures that the selected T-cells will have an MHC affinity that can serve useful functions in the body (i.e., the cells must be able to interact with MHC and peptide complexes to effect immune responses). The vast majority of developing thymocytes will die during this process.
A thymocyte’s fate is determined during positive selection. Double-positive cells (CD4+/CD8+) that interact well with MHC class II molecules will eventually become CD4+cells, whereas thymocytes that interact well with MHC class I molecules mature into CD8+ cells. A T cell becomes a CD4+ cell by down-regulating expression of its CD8 cell surface receptors. If the cell does not lose its signal, it will continue downregulating CD8 and become a CD4+, single positive cell. But, if there is a signal interruption, the cell stops downregulating CD8 and switches over to downregulating CD4 molecules, instead, eventually becoming a CD8+, single positive cell.
This process does not remove thymocytes that may cause autoimmunity. The potentially autoimmune cells are removed by the process of negative selection, which occurs in the thymic medulla (discussed below).
Negative selection removes thymocytes that are capable of strongly binding with “self” MHC peptides. Thymocytes that survive positive selection migrate towards the boundary of the cortex and medulla in the thymus. While in the medulla, they are again presented with a self-antigen presented on the MHC complex of medullary thymic epithelial cells (mTECs). mTECs must be AIRE+ to properly express self-antigens from all tissues of the body on their MHC class I peptides. Some mTECs are phagocytosed by thymic dendritic cells; this allows for presentation of self-antigens on MHC class II molecules (positively selected CD4+ cells must interact with MHC class II molecules, thus APCs, which possess MHC class II, must be present for CD4+ T-cell negative selection). Thymocytes that interact too strongly with the self-antigen receive an apoptotic signal that leads to cell death. However, some of these cells are selected to become Treg cells. The remaining cells exit the thymus as immature naïve T cells (also known as recent thymic emigrants ). This process is an important component of central tolerance and serves to prevent the formation of self-reactive T cells that are capable of inducing autoimmune diseases in the host.
In summary, β-selection is the first checkpoint, where the T cells that are able to form a functional pre-TCR with an invariant alpha chain and a functional beta chain are allowed to continue development in the thymus. Next, positive selection checks that T cells have successfully rearranged their TCRα locus and are capable of recognizing peptide-MHC complexes with appropriate affinity. Negative selection in the medulla then obliterates T cells that bind too strongly to self-antigens expressed on MHC molecules. These selection processes allow for tolerance of self by the immune system. Typical T cells that leave the thymus (via the corticomedullarly junction) are self-restricted, self-tolerant, and singly positive.
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 PKCθ, and eventual IL-2production. Optimal CD8+ T cell response relies on CD4+ signalling. CD4+ cells are useful in the initial antigenic activation of naïve 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.
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,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 naïve T cells is CD28, so co-stimulation for these cells comes from the CD80 and CD86 proteins, which together constitute the B7protein, (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 Treg 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.
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 PKCθ, 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.
PLCγ can also initiate the NF-κB pathway. DAG activates PKCθ, 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 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. 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.
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.
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”. T cell activation is modulated by reactive oxygen species.
T cells can “think” and distinguish
A unique feature of T cells is their ability to discriminate between healthy and abnormal (e.g. infected or cancerous) cells in the body. 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.
T cell deficiency
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. 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).
The main pathogens of concern in T cell deficiencies are intracellular pathogens, including Herpes simplex virus, Mycobacterium and Listeria. Also, fungal infectionsare also more common and severe in T cell deficiencies.
Cancer of T cells is termed T-cell lymphoma, and accounts for perhaps one in ten cases of non-Hodgkin lymphoma. 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
T cell exhaustion
T cell exhaustion is a state of dysfunctional T cells. It is 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 followed by the loss of high proliferative capacity and cytotoxic potential, eventually leading 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. Exhausted T cells preserve their functional exhaustion even after repeated antigen exposure.
T cell exhaustion can be triggered by several factors like persistent antigen exposure and lack of CD4 T cell help. 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. Another factors 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). Soluble molecules such as cytokines IL-10 or TGF-β are also able to trigger exhaustion. Last known factors that can play a role in T cell exhaustion are regulatory cells. Treg cells can be a source of IL-10 and TGF-β and therefore they can play a role in T cell exhaustion. Furthermore T cell exhaustion is reverted after depletion of Treg cells and blockade of PD1. T cell exhaustion can also occur during sepsis as a result of cytokine storm. Later after the initial septic encounter anti-inflammatory cytokines and pro-apoptotic proteins take over to protect the body from damage. Sepsis also carries high antigen load and inflammation. In this stage of sepsis T cell exhaustion increases. Currently there are studies aiming to utilize inhibitory receptor blockades in treatment of sepsis.
While during infection T cell exhaustion can develop following persistent antigen exposure after graft transplant similar situation arises with alloantigen presence. It was shown that T cell response diminishes over time after kidney transplant. These data suggest T cell exhaustion plays an important role in tolerance of a graft mainly by depletion of alloreactive CD8 T cells. Several studies showed positive effect of chronic infection on graft acceptance and its long-term survival mediated partly by T cell exhaustion. It was also shown that recipient T cell exhaustion provides sufficient conditions for NK cell transfer. While there are data showing that induction of T cell exhaustion can be beneficial for transplantation it also carries disadvantages among which can be counted increased number of infections and the risk of tumor development.
During cancer T cell exhaustion plays a role in tumor protection. According to research some cancer-associated cells as well as tumor cells themselves can actively induce T cell exhaustion at the site of tumor.
T cell exhaustion can also play a role in cancer relapses as was shown on leukemia.
Some studies even suggested that it is possible to predict relapse of leukemia based on expression of inhibitory receptors PD-1 and TIM-3 by T cells. In recent years there are a lot of experiments and clinical trials with immune checkpoint blockers in cancer therapy. Some of them were approved as valid therapies and are now used in clinics. Inhibitory receptors targeted by those medical procedures are vital in T cell exhaustion and blocking them could reverse these changes.[8 In 2015, a team of researchers led by Dr. Alexander Marson at the University of California, San Francisco successfully edited the genome of human T cells using a Cas9 ribonucleoprotein delivery method. This advancement has potential for applications in treating “cancer immunotherapies and cell-based therapies for HIV, primary immune deficiencies, and autoimmune diseases.”
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