Regulatory T cell

The regulatory T cells (Tregs

Mouse models have suggested that modulation of T_reg cells can treat autoimmune disease and cancer and can facilitate organ transplantation^[4] and wound healing.^[5] Their implications for cancer are complicated. T_reg cells tend to be upregulated in individuals with cancer, and they seem to be recruited to the site of many tumors. Studies in both humans and animal models have implicated that high numbers of T_reg cells in the tumor microenvironment is indicative of a poor prognosis, and T_reg cells are thought to suppress tumor immunity, thus hindering the body's innate ability to control the growth of cancerous cells.^[6] Immunotherapy research is studying how regulation of T cells could possibly be utilized in the treatment of cancer.^[7]

Populations

T regulatory cells are a component of the immune system that suppress immune responses of other cells. This is an important "self-check" built into the immune system to prevent excessive reactions. Regulatory T cells come in many forms with the most well-understood being those that express CD4, CD25, and FOXP3 (CD4⁺CD25⁺ regulatory T cells). These T_reg cells are different from helper T cells.^[8] Another regulatory T cell subset is T_reg17 cells.^[9] Regulatory T cells are involved in shutting down immune responses after they have successfully eliminated invading organisms, and also in preventing autoimmunity.^[10]

CD4⁺ FOXP3⁺ CD25(high) regulatory T cells have been called "naturally occurring" regulatory T cells^[11] to distinguish them from "suppressor" T cell populations that are generated in vitro. Additional regulatory T cell populations include Tr1, T_h3, CD8⁺CD28⁻, and Qa-1 restricted T cells. The contribution of these populations to self-tolerance and immune homeostasis is less well defined. FOXP3 can be used as a good marker for mouse CD4⁺CD25⁺ T cells, although recent studies have also shown evidence for FOXP3 expression in CD4⁺CD25⁻ T cells. In humans, FOXP3 is also expressed by recently activated conventional T cells and thus does not specifically identify human T_regs.^[12]

Development

All T cells derive from progenitor cells in the

The process of T_reg selection is determined by the affinity of interaction with the self-peptide MHC complex. Selection to become a T_reg is a "

After interaction with self-peptide MHC complex, T cell has to upregulate IL-2R, CD25 and TNFR superfamily members GITR, OX40 and TNFR2 to become CD25⁺FOXP3^- T_reg cell progenitor. To become mature T_reg, FOXP3 transcription factor has to be upregulated which is driven by γ-chain (CD132) dependent cytokines, in particular IL-2 and/or IL-15.^[14][15] Only IL-2 alone is not sufficient to stimulate Foxp3 expression, other cytokines are needed. Whereas IL-2 is produced by self-reactive thymocytes, IL-15 is produced by stromal cells of the thymus, mainly mTECs and cTECs.^[14]

Recently, other subset of T_reg precursors was identified. This subset lacks CD25 and has low expression of Foxp3. Its development is mainly dependent on IL-15. This subset has lower affinity for self antigens than CD25⁺Foxp3^high subset.  Both subsets generate mature T_reg cells after stimulation with IL-2 with comparable efficiency both in vitro and in vivo. CD25⁺Foxp3^high progenitors exhibit increased apoptosis and develop into mature T_reg cells with faster kinetics than Foxp3^low progenitors.^[16] T_regs derived from CD25⁺Foxp3^high progenitors protect from experimental auto-immune encephalomyelitis, whereas those derived from CD25⁺Foxp3^low progenitors protect from T-cell induced colitis.^[14]

Mature CD25+Foxp3+ Tregs can be also divided into two different subsets based on the expression level of CD25, GITR, and PD-1. Tregs expressing low amounts of CD25, GITR and PD-1 limit the development of colitis by promoting the conversion of conventional CD4⁺ T cells into pTreg. Tregs highly expressing CD25, GITR and PD-1 are more self-reactive and control lymphoproliferation in peripheral lymph nodes - they may have the ability to protect against autoimmune disorders.^[14]

Foxp3⁺ T_reg generation in the thymus is delayed by several days compared to T_eff cells and does not reach adult levels in either the thymus or periphery until around three weeks post-partum. T_reg cells require

Thymic recirculation

There was observed, that some FOXP3⁺ T_reg cells are recirculating back to thymus, where they have developed. This T_reg were mainly present in thymic medulla, which is the main site of T_reg cells differentiation.^[17] The presence of this cells in thymus or addition into fetal thymic tissue culture suppress development of new T_reg cells by 34–60%,^[17] but Tconv cells are not affected. That means, that recirculating T_reg to thymus inhibited just de novo development of T_reg cells. Molecular mechanism of this process works due to the ability of T_reg to adsorb IL-2 from the microenvironments, thus being able to induce apoptosis of other T cells which need IL-2 as main growth factor.

The molecular mechanism by which regulatory T cells exert their suppressor/regulatory activity has not been definitively characterized and is the subject of intense research. In vitro experiments have given mixed results regarding the requirement of cell-to-cell contact with the cell being suppressed. The following represent some of the proposed mechanisms of immune suppression:

• Regulatory T cells produce a number of inhibitory cytokines. These include TGF-β,^[24] Interleukin 35,^[25] and Interleukin 10.^[26] It also appears that regulatory T cells can induce other cell types to express interleukin-10.^[27]
• Regulatory T cells can produce Granzyme B, which in turn can induce apoptosis of effector cells. Regulatory T cells from Granzyme B deficient mice are reported to be less effective suppressors of the activation of effector T cells.^[28]
• Reverse signalling through direct interaction with dendritic cells and the induction of immunosuppressive indoleamine 2,3-dioxygenase.^[29]
• Signalling through the ectoenzymes
  CD73 with the production of immunosuppressive adenosine.^[30][31]
• Through direct interactions with dendritic cells by
  dendritic cells provides distinction between mechanisms described for human cells versus mouse cells.^[34]
• Another control mechanism is through the IL-2 feedback loop. Antigen-activated T cells produce IL-2 which then acts on IL-2 receptors on regulatory T cells alerting them to the fact that high T cell activity is occurring in the region, and they mount a suppressory response against them. This is a negative feedback loop to ensure that overreaction is not occurring. If an actual infection is present other inflammatory factors downregulate the suppression. Disruption of the loop leads to hyperreactivity, regulation can modify the strength of the immune response.[35] A related suggestion with regard to interleukin 2 is that activated regulatory T cells take up interleukin 2 so avidly that they deprive effector T cells of sufficient to avoid apoptosis.^[18]
• A major mechanism of suppression by regulatory T cells is through the prevention of
  CTLA-4.^[36]

T regulatory lymphocytes develop during ontogeny either in the thymus or in the periphery. Accordingly, they are divided into natural and induced T regulatory cells.^[37]

Natural T regulatory lymphocytes (tTregs, nTregs) are characterized by continuous expression of FoxP3 and T cell receptor (TCR) with relatively high autoaffinity. These cells are predominantly found in the body in the bloodstream or lymph nodes and serve mainly to confer tolerance to autoantigens.^[37]

Induced (peripheral) T regulatory cells (iTregs, pTregs) arise under certain situations in the presence of IL-2 and TGF-b in the periphery and begin to express FoxP3 inducibly, thus becoming the functional equivalent of tTreg cells. iTregs, however, are found primarily in peripheral barrier tissues, where they are primarily involved in preventing inflammation in the presence of external antigens.^[37]

The main features that differentiate tTreg and iTreg cells include Helios and Neuropilin-1, the presence of which suggests origin in the thymus. Another feature distinguishing these two Treg cell populations is the stability of FoxP3 expression in different settings.^[37]

Induced T regulatory cells

Induced regulatory T (iT_reg) cells (CD4⁺ CD25⁺ FOXP3⁺) are suppressive cells involved in tolerance. iT_reg cells have been shown to suppress T cell proliferation and experimental autoimmune diseases. These cells include T_reg17 cells. iT_reg cells develop from mature CD4⁺ conventional T cells outside of the thymus: a defining distinction between natural regulatory T (nT_reg) cells and iT_reg cells. Though iT_reg and nT_reg cells share a similar function iT_reg cells have recently been shown to be "an essential non-redundant regulatory subset that supplements nT_reg cells, in part by expanding TCR diversity within regulatory responses".^[38] Acute depletion of the iT_reg cell pool in mouse models has resulted in inflammation and weight loss. The contribution of nT_reg cells versus iT_reg cells in maintaining tolerance is unknown, but both are important. Epigenetic differences have been observed between nT_reg and iT_reg cells, with the former having more stable FOXP3 expression and wider demethylation.

The small intestinal environment is high in vitamin A and is a location where retinoic acid is produced.

Approximately 30%–40% of colonic FoxP3+ Treg cells express the transcription factor RORγt.[49] The iTregs are able to differentiate into RORγt-expressing cells and thus acquire the phenotype of Th17 cells. These cells are associated with the functions of mucosal lymphoid tissues such as the intestinal barrier. In the intestinal lamina propria, 20-30% of Foxp3+ T regulatory cells expressing RORyt are found and this high proportion is strongly dependent on the presence of a complex gut microbiome. In germ-free (GF) mice, the population of RORγt+ T regulatory cells is strongly reduced, whereas recolonization by the specific pathogen-free (SPF) microbiota restores normal numbers of these lymphocytes in the gut. The mechanism by which the gut microbiota induces the formation of RORγt+ Treg cells involves the production of short-chain fatty acids (SCFAs), on which this induction is dependent. SCFAs are a by-product of fermentation and digestion of dietary fiber, therefore, microbial-free mice have very low concentrations of both SCFAs and RORγt Treg cells. Induction of RORγt Treg cells is also dependent on the presence of dendritic cells in adults, Thetis cells in neonatal and antigen presentation by MHC II.^[50][51]

RORγt+ Treg cells are not present in the thymus and do not express Helios or

Induction of RORγt+ Treg cells in lymph nodes of the small intestine is crucial for the establishment of intestinal luminal antigen tolerance. These cells are particularly important in the prevention of food allergies. One mechanism is the production of suppressive molecules such as the cytokine IL-10. These cells also suppress the Th17 cell population and inhibit the production of IL-17, thus suppressing the pro-inflammatory response.^[50]

In mice, colonic RORγt+ Tregs are absent during the first two weeks after birth. Generation of RORγt+ Treg early after birth is essential to prevent the development of various intestinal immunopathologies later in life. Particularly crucial is a time period of gradual transition from relying solely on maternal milk to incorporating solid food, between 15 and 20 days of age, when a large number of microbial antigens is introduced and commensal microbiota are settling in the intestine. During this time, protective RORγt+ Treg cells are induced by the microbial antigens and normal intestinal homeostasis is sustained by induction of tolerance to commensal microbiota. Lack of RORγt+ Treg cell induction led in mice to the development of severe colitis.^[52] The quantity of early-life-induced RORγt+ Tregs is influenced by maternal milk, particularly by the amount of IgA antibodies present in the maternal milk. In adult mice, RORγt+ Tregs and IgA exhibit mutual inhibition. Similarly, mice nursed by foster mothers with higher IgA titers in their milk will develop fewer RORγt+ Tregs compared to those fed with milk containing lower IgA titers.^[53]

RORγt+ Tregs were also shown for their importance in oral tolerance and prevention of food allergies. Infants with developed food allergies have different composition of fecal microbiota in comparison to healthy infants and have increased

Deficiency of tryptophan, an essential amino acid, alters commensal microbiota metabolism which results in expansion of RORγt+ Treg cells and reduction of Gata3+ Treg cells. This induction is possibly regulated by stimulation of Aryl hydrocarbon receptor by metabolites produced by commensal bacteria using tryptophan as an energy source.^[55]

Lower number of RORγt+ Treg cells is present in germ free mice colonized with microbiota associated with

Pathological may be involvement of RORγt+ regulatory T cells in colorectal cancer. It was found, that RORγt+ Tregs which are able to express IL-17 are expanded in colorectal cancer and as cancer develops, they lose the ability to express anti-inflammatory IL-10. Similarly such RORγt+ Tregs expressing IL-17 are expanded in mucosa of patients with Crohn´s disease.[57]^[58] Depletion of RORγt+ Tregs in mice with colorectal cancer caused enhancement of reactivity of tumor-specific T cells and improved cancer immune surveillance. This improvement is not caused by the loss of IL-17 as that was proved to promote cancer progression.^[58] In tumors of mice with conditional knockout of RORγt+ Tregs was confirmed downregulation of IL-6, reduction of IL-6 expressing CD11c+ dendritic cells and overexpression of CTLA-4. IL-6 mediates activation of STAT3 transcription factor which is critical for proliferation of cancer cells.^[59]

Gata3+ regulatory T lymphocytes

Another important subset of Treg cells are Gata3+ Treg cells, which respond to IL-33 in the gut and influence the regulation of effector T cells during inflammation. Unlike RORγt+ Treg cells, these cells express Helios and are not dependent on the microbiome.^[51][60]

Gata3+ T regs are major

T_reg cells play major roles during HIV infection. They suppress the immune system, thus limiting target cells and reducing inflammation, but this simultaneously disrupts the clearance of virus by the cell-mediated immune response and enhances the reservoir by pushing CD4⁺ T cells to a resting state, including infected cells. Additionally, T_reg cells can be infected by HIV, increasing the size of the HIV reservoir directly. Thus, T_reg cells are being investigated as targets for HIV cure research.^[63] Some T_reg cell depletion strategies have been tested in SIV infected nonhuman primates, and shown to cause viral reactivation and enhanced SIV specific CD8⁺ T cell responses.^[64]

Regulatory T cells have a large role in the pathology of visceral leishmaniasis and in preventing excess inflammation in patients cured of visceral leishmaniasis.

CD4⁺ regulatory T cells are often associated with solid tumours in both humans and murine models. Increased numbers of regulatory T cells in breast, colorectal and ovarian cancers is associated with a poorer prognosis.^[65]

CD70⁺ non-Hodgkin lymphoma B cells induce FOXP3 expression and regulatory function in intratumoral CD4⁺CD25⁻ T cells.^[66]

There is some evidence that T_reg cells may be dysfunctional and driving neuroinflammation in

Additionally, while regulatory T cells have been shown to increase via polyclonal expansion both systemically and locally during healthy pregnancies to protect the fetus from the maternal immune response (a process called maternal immune tolerance), there is evidence that this polyclonal expansion is impaired in preeclamptic mothers and their offspring.[69] Research suggests reduced production and development of regulatory T cells during preeclampsia may degrade maternal immune tolerance, leading to the hyperactive immune response characteristic of preeclampsia.^[70]

Cancer

Most tumors elicit an immune response in the host that is mediated by tumor antigens, thus distinguishing the tumor from other non-cancerous cells. This causes large numbers of

Although high levels of TILs were initially thought to be important in determining an immune response against cancer, it is now widely recognized that the ratio of T_reg to effector T cells in the tumor microenvironment is a determining factor in the success of the immune response against the cancer. High levels of T_reg cells in the tumor microenvironment are associated with poor prognosis in many cancers,[73] such as ovarian, breast, renal, and pancreatic cancer.^[72] This indicates that T_reg cells suppress effector T cells and hinder the body's immune response against the cancer. However, in some types of cancer the opposite is true, and high levels of T_reg cells are associated with a positive prognosis. This trend is seen in cancers such as colorectal carcinoma and follicular lymphoma. This could be due to T_reg cells' ability to suppress general inflammation which is known to trigger cell proliferation and metastasis .^[72] These opposite effects indicate that T_reg cells' role in the development of cancer is highly dependent on both type and location of the tumor.

Although it is still not entirely understood how T_reg cells are preferentially trafficked to the tumor microenvironment, the chemotaxis is probably driven by the production of chemokines by the tumor. T_reg infiltration into the tumor microenvironment is facilitated by the binding of the chemokine receptor CCR4, which is expressed on T_reg cells, to its ligand CCL22, which is secreted by many types of tumor cells.^[74] T_reg cell expansion at the site of the tumor could also explain the increased levels of T_reg cells. The cytokine, TGF-β, which is commonly produced by tumor cells, is known to induce the differentiation and expansion of T_reg cells.^[74]

Forkhead box protein 3 (

T_reg cells present in the tumor microenvironment (TME)  can be either induced T_regs or natural (thymic) T_regs which develop from naive precursors. However, tumor-associated T_regs may also originate from IL-17A⁺Foxp3⁺ T_regs which develop from Th17 cells.[76]^[77]

In general, the immunosuppression of the tumor microenvironment has largely contributed to the unsuccessful outcomes of many cancer immunotherapy treatments. Depletion of T_reg cells in animal models has shown an increased efficacy of immunotherapy treatments, and therefore, many immunotherapy treatments are now incorporating T_reg depletion.^[2]

Cancer therapies targeting regulatory T lymphocytes

T_regs in the TME are abundantly effector T_regs which over-express immunosuppressive molecules such as CTLA-4. Anti-CTLA-4 antibodies cause depletion of T_regs and thus increase CD8⁺ T cells effective against the tumor. Anti-CTLA-4 antibody ipilimumab was approved for patients with advanced melanoma. Immune-checkpoint molecule PD-1 inhibits activation of both conventional T cells and T_regs and use of anti-PD-1 antibodies may lead to activation and immunosuppressive function of T_regs. Resistance to anti-PD-1-mAb treatment is probably caused by enhanced T_reg cell activity. Rapid cancer progression upon PD-1 blockade is called hyperprogressive disease. Therapies targeting T_reg suppression include anti-CD25 mAbs and anti-CCR4 mAbs. OX40 agonist and GITR agonists are currently being investigated.^[76][78] Therapy targeting TCR signaling is also possible by blocking tyrosine kinases. For example, tyrosine-kinase inhibitor dasatinib is used for treatment of chronic myeloid leukemia and is associated with T_reg inhibition.^[79]

Molecular characterization

Similar to other T cells, regulatory T cells develop in the

In addition to the search for novel protein markers, a different method to analyze and monitor T_reg cells more accurately has been described in the literature. This method is based on DNA methylation analysis. Only in T_reg cells, but not in any other cell type, including activated effector T cells, a certain region within the FOXP3 gene (TSDR, T_reg-specific-demethylated region) is found demethylated, which allows to monitor T_reg cells through a PCR reaction or other DNA-based analysis methods.^[85] Interplay between the Th17 cells and regulatory T cells are important in many diseases like respiratory diseases.^[86]

Recent evidence suggests that

Regulatory T cell epitopes ('Tregitopes') were discovered in 2008 and consist of linear sequences of amino acids contained within monoclonal antibodies and immunoglobulin G (IgG). Since their discovery, evidence has indicated Tregitopes may be crucial to the activation of natural regulatory T cells.[88]^[89][90]

Potential applications of regulatory T cell epitopes have been hypothesised: tolerisation to transplants, protein drugs, blood transfer therapies, and

Genetic mutations in the gene encoding FOXP3 have been identified in both humans and mice based on the heritable disease caused by these mutations. This disease provides the most striking evidence that regulatory T cells play a critical role in maintaining normal immune system function. Humans with mutations in FOXP3 develop a severe and rapidly fatal autoimmune disorder known as Immune dysregulation, Polyendocrinopathy, Enteropathy X-linked (IPEX) syndrome.[97]^[98]

The

See also

• List of distinct cell types in the adult human body

References