FOXP3 (forkhead box P3), also known as scurfin, is a protein involved in immune system responses. [5] A member of the FOX protein family, FOXP3 appears to function as a master regulator of the regulatory pathway in the development and function of regulatory T cells. [6] [7] [8] Regulatory T cells generally turn the immune response down. In cancer, an excess of regulatory T cell activity can prevent the immune system from destroying cancer cells. In autoimmune disease, a deficiency of regulatory T cell activity can allow other autoimmune cells to attack the body's own tissues. [9] [10]
While the precise control mechanism has not yet been established, FOX proteins belong to the forkhead/winged-helix family of transcriptional regulators and are presumed to exert control via similar DNA binding interactions during transcription. In regulatory T cell model systems, the FOXP3 transcription factor occupies the promoters for genes involved in regulatory T-cell function, and may inhibit transcription of key genes following stimulation of T cell receptors. [11]
The human FOXP3 genes contain 11 coding exons. Exon-intron boundaries are identical across the coding regions of the mouse and human genes. By genomic sequence analysis, the FOXP3 gene maps to the p arm of the X chromosome (specifically, Xp11.23). [5] [12]
Foxp3 is a specific marker of natural T regulatory cells (nTregs, a lineage of T cells) and adaptive/induced T regulatory cells (a/iTregs), also identified by other less specific markers such as CD25 or CD45RB. [6] [7] [8] In animal studies, Tregs that express Foxp3 are critical in the transfer of immune tolerance, especially self-tolerance. [13]
The induction or administration of Foxp3 positive T cells has, in animal studies, led to marked reductions in (autoimmune) disease severity in models of diabetes, multiple sclerosis, asthma, inflammatory bowel disease, thyroiditis and renal disease. [14] Human trials using regulatory T cells to treat graft-versus-host disease have shown efficacy. [15] [16]
Further work has shown that T cells are more plastic in nature than originally thought. [17] [18] [19] This means that the use of regulatory T cells in therapy may be risky, as the T regulatory cell transferred to the patient may change into T helper 17 (Th17) cells, which are pro-inflammatory rather than regulatory cells. [17] Th17 cells are proinflammatory and are produced under similar environments as a/iTregs. [17] Th17 cells are produced under the influence of TGF-β and IL-6 (or IL-21), whereas a/iTregs are produced under the influence of solely TGF-β, so the difference between a proinflammatory and a pro-regulatory scenario is the presence of a single interleukin. IL-6 or IL-21 is being debated by immunology laboratories as the definitive signaling molecule. Murine studies point to IL-6 whereas human studies have shown IL-21.[ citation needed ] Foxp3 is the major transcription factor controlling T-regulatory cells (Treg or CD4+ cells). [20] CD4+ cells are leukocytes responsible for protecting animals from foreign invaders such as bacteria and viruses. [20] Defects in this gene's ability to function can cause IPEX syndrome (IPEX), also known as X-linked autoimmunity-immunodeficiency syndrome as well as numerous cancers. [21] While CD4+ cells are heavily regulated and require multiple transcription factors such as STAT-5 and AhR in order to become active and function properly, Foxp3 has been identified as the master regulator for Treg lineage. [20] Foxp3 can either act as a transcriptional activator or suppressor depending on what regulators such as deacetylases and histone acetylases are acting on it. [20] The Foxp3 gene is also known to convert naïve T-cells to Treg cells, which are capable of in vivo and in vitro suppressive capabilities suggesting that Foxp3 is capable of regulating the expression of suppression-mediating molecules. [20] Clarifying the gene targets of Foxp3 could be crucial to the comprehension of the suppressive abilities of Treg cells.
In human disease, alterations in numbers of regulatory T cells – and in particular those that express Foxp3 – are found in a number of disease states. For example, patients with tumors have a local relative excess of Foxp3 positive T cells which inhibits the body's ability to suppress the formation of cancerous cells. [22] Conversely, patients with an autoimmune disease such as systemic lupus erythematosus (SLE) have a relative dysfunction of Foxp3 positive cells. [23] The Foxp3 gene is also mutated in IPEX syndrome (Immunodysregulation, Polyendocrinopathy, and Enteropathy, X-linked). [24] [25] Many patients with IPEX have mutations in the DNA-binding forkhead domain of FOXP3. [26]
In mice, a Foxp3 mutation (a frameshift mutation that result in protein lacking the forkhead domain) is responsible for 'Scurfy', an X-linked recessive mouse mutant that results in lethality in hemizygous males 16 to 25 days after birth. [5] These mice have overproliferation of CD4+ T-lymphocytes, extensive multiorgan infiltration, and elevation of numerous cytokines. This phenotype is similar to those that lack expression of CTLA-4, TGF-β, human disease IPEX, or deletion of the Foxp3 gene in mice ("scurfy mice"). The pathology observed in scurfy mice seems to result from an inability to properly regulate CD4+ T-cell activity. In mice overexpressing the Foxp3 gene, fewer T cells are observed. The remaining T cells have poor proliferative and cytolytic responses and poor interleukin-2 production, although thymic development appears normal. Histologic analysis indicates that peripheral lymphoid organs, particularly lymph nodes, lack the proper number of cells.[ citation needed ]
In addition to Foxp3's role in regulatory T cell differentiation, multiple lines of evidence have indicated that Foxp3 play important roles in cancer development.
Down-regulation of Foxp3 expression has been reported in tumour specimens derived from breast, prostate, and ovarian cancer patients, indicating that Foxp3 is a potential tumour suppressor gene. Expression of Foxp3 was also detected in tumour specimens derived from additional cancer types, including pancreatic, melanoma, liver, bladder, thyroid, cervical cancers. However, in these reports, no corresponding normal tissues were analyzed, therefore it remained unclear whether Foxp3 is a pro- or anti-tumourigeneic molecule in these tumours.[ citation needed ]
Two lines of functional evidence strongly supported that Foxp3 serves as a tumour suppressive transcription factor in cancer development. First, Foxp3 represses expression of HER2, Skp2, SATB1 and MYC oncogenes and induces expression of tumour suppressor genes P21 and LATS2 in breast and prostate cancer cells. Second, over-expression of Foxp3 in melanoma,[ citation needed ] glioma, breast, prostate and ovarian cancer cell lines induces profound growth inhibitory effects in vitro and in vivo. However, this hypothesis need to be further investigated in future studies.[ citation needed ]
Foxp3 is a recruiter of other anti-tumor enzymes such as CD39 and CD8. [21] The overexpression of CD39 is found in patients with multiple cancer types such as melanoma, leukemia, pancreatic cancer, colon cancer, and ovarian cancer. [21] This overexpression may be protecting tumorous cells, allowing them to create their “escape phase”. [21] A cancerous tumor's “escape phase” is where the tumor grows quickly and it becomes clinically invisible by becoming independent of the extracellular matrix and creating its own immunosuppressive tumor microenvironment. [21] The consequences of a cancer cell reaching the “escape phase” is that it allows it to completely evade the immune system, which reduces the immunogenicity and ability to become clinically detected, allowing it to progress and spread throughout the body. Some cancer patients have also been known to display higher numbers of mutated CD4+ cells. These mutated cells will then produce large quantities of TGF-β and IL-10, (a Transforming Growth Factor β and an inhibitory cytokine respectively,) which will suppress signals to the immune system and allow for tumor escape. [21] So, Foxp3 polymorphism (rs3761548) might contribute to cancer development like gastric cancer through influencing Treg cell activity and secretion of immunomodulatory cytokines such as IL-10, IL-35, and TGF-β. [27] In one experiment a 15-mer synthetic peptide, P60, was able to inhibit Foxp3's ability to function. P60 did this by entering the cells and then binding to Foxp3, where it hinders Foxp3's ability to translocate to the nucleus. [28] Due to this, Foxp3 could no longer properly suppress the transcription factors NF-kB and NFAT; both of which are protein complexes that regulate transcription of DNA, cytokine production and cell survival. [28] This would inhibit a cell's ability to perform apoptosis and stop its own cell cycle, which could potentially allow an affected cancerous cell to survive and reproduce.
Mutations or disruptions of the Foxp3 regulatory pathway can lead to organ-specific autoimmune diseases such as autoimmune thyroiditis and type 1 diabetes mellitus. [29] These mutations affect thymocytes developing within the thymus. Regulated by Foxp3, it's these thymocytes that during thymopoiesis, are transformed into mature Treg cells by the thymus. [29] It was found that patients who have the autoimmune disease systemic lupus erythematosus (SLE) possess Foxp3 mutations that affect the thymopoiesis process, preventing the proper development of Treg cells within the thymus. [29] These malfunctioning Treg cells aren’t efficiently being regulated by its transcription factors, which cause them to attack cells that are healthy, leading to these organ-specific autoimmune diseases. Another way that Foxp3 helps keep the autoimmune system at homeostasis is through its regulation of the expression of suppression-mediating molecules. For instance, Foxp3 is able to facilitate the translocation of extracellular adenosine into the cytoplasm. [30] It does this by recruiting CD39, a rate-limiting enzyme that's vital in tumor suppression to hydrolyze ATP to ADP in order to regulate immunosuppression on different cell populations. [30]
T cells are one of the important types of white blood cells of the immune system and play a central role in the adaptive immune response. T cells can be distinguished from other lymphocytes by the presence of a T-cell receptor (TCR) on their cell surface.
Autoimmune polyendocrine syndromes (APSs), also called polyglandular autoimmune syndromes (PGASs) or polyendocrine autoimmune syndromes (PASs), are a heterogeneous group of rare diseases characterized by autoimmune activity against more than one endocrine organ, although non-endocrine organs can be affected. There are three types of APS, and there are a number of other diseases which involve endocrine autoimmunity.
The regulatory T cells (Tregs or Treg cells), formerly known as suppressor T cells, are a subpopulation of T cells that modulate the immune system, maintain tolerance to self-antigens, and prevent autoimmune disease. Treg cells are immunosuppressive and generally suppress or downregulate induction and proliferation of effector T cells. Treg cells express the biomarkers CD4, FOXP3, and CD25 and are thought to be derived from the same lineage as naïve CD4+ cells. Because effector T cells also express CD4 and CD25, Treg cells are very difficult to effectively discern from effector CD4+, making them difficult to study. Research has found that the cytokine transforming growth factor beta (TGF-β) is essential for Treg cells to differentiate from naïve CD4+ cells and is important in maintaining Treg cell homeostasis.
Cytotoxic T-lymphocyte associated protein 4, (CTLA-4) also known as CD152, is a protein receptor that functions as an immune checkpoint and downregulates immune responses. CTLA-4 is constitutively expressed in regulatory T cells but only upregulated in conventional T cells after activation – a phenomenon which is particularly notable in cancers. It acts as an "off" switch when bound to CD80 or CD86 on the surface of antigen-presenting cells. It is encoded by the gene CTLA4 in humans.
Immunodysregulation polyendocrinopathy enteropathy X-linked syndrome is a rare autoimmune disease. It is one of the autoimmune polyendocrine syndromes. Most often, IPEX presents with autoimmune enteropathy, dermatitis (eczema), and autoimmune endocrinopathy, but other presentations exist.
Immune tolerance, also known as immunological tolerance or immunotolerance, refers to the immune system's state of unresponsiveness to substances or tissues that would otherwise trigger an immune response. It arises from prior exposure to a specific antigen and contrasts the immune system's conventional role in eliminating foreign antigens. Depending on the site of induction, tolerance is categorized as either central tolerance, occurring in the thymus and bone marrow, or peripheral tolerance, taking place in other tissues and lymph nodes. Although the mechanisms establishing central and peripheral tolerance differ, their outcomes are analogous, ensuring immune system modulation.
Immune dysregulation is any proposed or confirmed breakdown or maladaptive change in molecular control of immune system processes. For example, dysregulation is a component in the pathogenesis of autoimmune diseases and some cancers. Immune system dysfunction, as seen in IPEX syndrome leads to immune dysfunction, polyendocrinopathy, enteropathy, X-linked (IPEX). IPEX typically presents during the first few months of life with diabetes mellitus, intractable diarrhea, failure to thrive, eczema, and hemolytic anemia. unrestrained or unregulated immune response.
The autoimmune regulator (AIRE) is a protein that in humans is encoded by the AIRE gene. It is a 13kbp gene on chromosome 21q22.3 that encodes 545 amino acids. AIRE is a transcription factor expressed in the medulla of the thymus. It is part of the mechanism which eliminates self-reactive T cells that would cause autoimmune disease. It exposes T cells to normal, healthy proteins from all parts of the body, and T cells that react to those proteins are destroyed.
In immunology, peripheral tolerance is the second branch of immunological tolerance, after central tolerance. It takes place in the immune periphery. Its main purpose is to ensure that self-reactive T and B cells which escaped central tolerance do not cause autoimmune disease. Peripheral tolerance can also serve a purpose in preventing an immune response to harmless food antigens and allergens.
Interleukin 35 (IL-35) is a recently discovered anti-inflammatory cytokine from the IL-12 family. Member of IL-12 family - IL-35 is produced by wide range of regulatory lymphocytes and plays a role in immune suppression. IL-35 can block the development of Th1 and Th17 cells by limiting early T cell proliferation.
Zinc finger protein Helios is a protein that in humans is encoded by the IKZF2 gene. This protein is a member of Ikaros family of transcription factors.
Tumor necrosis factor receptor superfamily member 18 (TNFRSF18), also known as glucocorticoid-induced TNFR-related protein (GITR) or CD357. GITR is encoded and tnfrsf18 gene at chromosome 4 in mice. GITR is type I transmembrane protein and is described in 4 different isoforms. GITR human orthologue, also called activation-inducible TNFR family receptor (AITR), is encoded by the TNFRSF18 gene at chromosome 1.
The interleukin-2 receptor alpha chain is a protein involved in the assembly of the high-affinity interleukin-2 receptor, consisting of alpha (IL2RA), beta (IL2RB) and the common gamma chain (IL2RG). As the name indicates, this receptor interacts with interleukin-2, a pleiotropic cytokine which plays an important role in immune homeostasis.
T helper 3 cells (Th3) are a subset of T lymphocytes with immunoregulary and immunosuppressive functions, that can be induced by administration of foreign oral antigen. Th3 cells act mainly through the secretion of anti-inflammatory cytokine transforming growth factor beta (TGF-β). Th3 have been described both in mice and human as CD4+FOXP3− regulatory T cells. Th3 cells were first described in research focusing on oral tolerance in the experimental autoimmune encephalitis (EAE) mouse model and later described as CD4+CD25−FOXP3−LAP+ cells, that can be induced in the gut by oral antigen through T cell receptor (TCR) signalling.
Autoimmune polyendocrine syndrome type 1 (APS-1), is a subtype of autoimmune polyendocrine syndrome. It causes the dysfunction of multiple endocrine glands due to autoimmunity. It is a genetic disorder, inherited in autosomal recessive fashion due to a defect in the AIRE gene , which is located on chromosome 21 and normally confers immune tolerance.
CD25 deficiency or interleukin 2 receptor alpha deficiency is an immunodeficiency disorder associated with mutations in the interleukin 2 receptor alpha (CD25) (IL2RA) gene. The mutations cause expression of a defective α chain or complete absence thereof, an essential part of high-affinity interleukin-2 (IL-2) receptors. The result is a syndrome described as IPEX-like or a SCID.
Autoimmune enteropathy is a rare autoimmune disorder characterized by weight loss from malabsorption, severe and protracted diarrhea, and autoimmune damage to the intestinal mucosa. Autoimmune enteropathy typically occurs in infants and younger children however, adult cases have been reported in literature. Autoimmune enteropathy was first described by Walker-Smith et al. in 1982.
STAT3 gain-of function (GOF) is a rare genetic disorder of the immune system, leading to early-onset autoimmunity and a variety of multi-organ disorders. The condition is progressive and manifests through a broad spectrum of clinical symptoms, including lymphadenopathy, autoimmune cytopenias, growth delays, enteropathy, lung disease, endocrine disorders, arthritis, autoimmune hepatitis, neurological diseases, vasculopathy, eczema, infections, and multiorgan autoimmunity. Patients experience recurring infections.
Alexander Rudensky is an immunologist at Memorial Sloan Kettering Cancer Center known for his research on regulatory T cells and the transcription factor Foxp3.
Frederick J. "Fred" Ramsdell is an American immunologist. Ramsdell graduated from the University of California, San Diego in 1983 with a bachelor's degree in biology and from the University of California, Los Angeles in 1987 with a Ph.D. in immunology. As a postdoc he worked at the National Institutes of Health and subsequently in biotech companies in the Seattle area. He has served as a senior executive at several biotech companies Darwin Molecular/Celltech, ZymoGenetics, Novo Nordisk, and aTyr Pharma. Since the beginning of 2016, he has been Research Director at the Parker Institute for Cancer Immunotherapy in San Francisco.