Regulatory T cells sometimes known as
suppressor T cells, are a specialized subpopulation of T cells which suppresses activation of the
immune systemAn immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...
and thereby maintains
tolerance to self-antigensImmune tolerance or immunological tolerance is the process by which the immune system does not attack an antigen. It can be either 'natural' or 'self tolerance', in which the body does not mount an immune response to self antigens, or 'induced tolerance', in which tolerance to external antigens can...
. The existence of regulatory T cells was the subject of significant controversy among immunologists for many years. However, advances in the molecular characterization of this cell population have firmly established their existence and their critical role in the vertebrate immune system. Interest in regulatory T cells has been heightened by evidence from experimental mouse models suggesting that the immunosuppressive potential of these cells can be harnessed to treat
autoimmune diseaseAutoimmune diseases arise from an overactive immune response of the body against substances and tissues normally present in the body. In other words, the body actually attacks its own cells. The immune system mistakes some part of the body as a pathogen and attacks it. This may be restricted to...
s and manipulated to facilitate organ transplantation and
cancer immunotherapyCancer immunotherapy is the use of the immune system to reject cancer. The main premise is stimulating the patient's immune system to attack the malignant tumor cells that are responsible for the disease...
.
T regulatory cell 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
CD4CD4 is a glycoprotein expressed on the surface of T helper cells, monocytes, macrophages, and dendritic cells. It was discovered in the late 1970s and was originally known as leu-3 and T4 before being named CD4 in 1984...
,
CD25CD25 is the alpha chain of the IL-2 receptor. It is a type I transmembrane protein present on activated T cells, activated B cells, some thymocytes, myeloid precursors, and oligodendrocytes that associates with CD122 to form a heterodimer that can act as a high-affinity receptor for IL-2.CD25 is...
, and
Foxp3FOXP3 is a protein involved in immune system responses. A member of the FOX protein family, FOXP3 appears to function as a master regulator in the development and function of regulatory T cells....
(CD4+CD25+ regulatory T cells, or "T
regs"). These cells are involved in shutting down immune responses after they have successfully tackled invading organisms, and also in regulating immune responses that may potentially attack one's own tissues (
autoimmunityAutoimmunity is the failure of an organism to recognize its own constituent parts as self, which allows an immune response against its own cells and tissues. Any disease that results from such an aberrant immune response is termed an autoimmune disease...
).
CD4
+Foxp3
+ regulatory T cells have been referred to as "naturally-occurring" regulatory T cells to distinguish them from "suppressor" T cell populations that are generated
in vitro. The regulatory T cell field is further complicated by reports of additional suppressive T cell populations, including Tr1, Th3, CD8
+CD28
-, and Qa-1 restricted T cells. The contribution, however, of these populations to self-tolerance and immune homeostasis is less well defined. The lack of a clear defining marker for regulatory T cells presents a serious challenge to researchers.
Development
All T cells come from progenitor cells from the
bone marrowBone marrow is the flexible tissue found in the interior of bones. In humans, bone marrow in large bones produces new blood cells. On average, bone marrow constitutes 4% of the total body mass of humans; in adults weighing 65 kg , bone marrow accounts for approximately 2.6 kg...
, which become committed to their lineage in the
thymusThe thymus is a specialized organ of the immune system. The thymus produces and "educates" T-lymphocytes , which are critical cells of the adaptive immune system....
. All T cells begin as
CD4CD4 is a glycoprotein expressed on the surface of T helper cells, monocytes, macrophages, and dendritic cells. It was discovered in the late 1970s and was originally known as leu-3 and T4 before being named CD4 in 1984...
-
CD8CD8 is a transmembrane glycoprotein that serves as a co-receptor for the T cell receptor . Like the TCR, CD8 binds to a major histocompatibility complex molecule, but is specific for the class I MHC protein. There are two isoforms of the protein, alpha and beta, each encoded by a different gene...
-
TCRThe T cell receptor or TCR is a molecule found on the surface of T lymphocytes that is responsible for recognizing antigens bound to major histocompatibility complex molecules...
- cells at the DN (double-negative) stage, where an individual cell will rearrange its T cell receptor genes to form a unique, functional molecule, which they, in turn, test against cells in the thymic cortex for a minimal level of interaction with self-
MHCMajor histocompatibility complex is a cell surface molecule encoded by a large gene family in all vertebrates. MHC molecules mediate interactions of leukocytes, also called white blood cells , which are immune cells, with other leukocytes or body cells...
. If they receive these signals, they proliferate and express both CD4 and CD8, becoming double-positive cells. The selection of T
regs occurs on radio-resistant haemopoietically-derived MHC class II-expressing cells in the medulla or Hassal’s corpuscles in the thymus. It seems that, at the DP (double-positive) stage, they are selected by their interaction with the cells within the thymus, begin the transcription of Foxp3, and become T
reg cells, although they may not begin to express Foxp3 until the single-positive stage, at which point they are functional T
regs. T
reg do not have the limited
TCRThe T cell receptor or TCR is a molecule found on the surface of T lymphocytes that is responsible for recognizing antigens bound to major histocompatibility complex molecules...
expression of NKT or γδ T cells; T
reg have a larger TCR diversity than effector T cells, biased towards self-peptides.
The exact process of T
reg selection is still unknown, but appears to be a process determined by the affinity of interaction with the self-peptide MHC complex. Selection to become a T
reg is a “Goldilocks” process; T cell that receives very strong signals will undergo apoptotic death; a cell that receives a weak signal will survive and be selected to become an effector cell. If a T cell receives an intermediate signal, then it will become a regulatory cell. Due to the
stochasticStochastic refers to systems whose behaviour is intrinsically non-deterministic. A stochastic process is one whose behavior is non-deterministic, in that a system's subsequent state is determined both by the process's predictable actions and by a random element. However, according to M. Kac and E...
nature of the process of T cell activation, all T cell populations with a given TCR will end up with a mixture of T
eff and T
reg – the relative proportions determined by the affinities of the T cell for the self-peptide-MHC. Even in mouse models with TCR-transgenic cells selected on specific-antigen-secreting stroma, deletion or conversion is not complete.
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
CD28CD28 is one of the molecules expressed on T cells that provide co-stimulatory signals, which are required for T cell activation. CD28 is the receptor for CD80 and CD86 . When activated by Toll-like receptor ligands, the CD80 expression is upregulated in antigen presenting cells...
co-stimulationDuring the activation of lymphocytes, co-stimulation is often crucial to the development of an effective immune response. Co-stimulation is required in addition to the antigen-specific signal from their antigen receptors.- Co-stimulation T cells require :...
and
B7.2Cluster of Differentiation 86 is a protein expressed on antigen-presenting cells that provides costimulatory signals necessary for T cell activation and survival...
expression is largely restricted to the medulla, the development of which seems to parallel the development of Foxp3+ cells. It has been suggested that the two are linked, but no definitive link between the processes has yet been shown. TGF-β is not required for T
reg functionality, in the thymus, as thymic T
reg from TGF-β insensitive TGFβRII-DN mice are functional.
Function
To function properly, the
immune systemAn immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...
must discriminate between self and non-self. When self/non-self discrimination fails, the immune system destroys cells and tissues of the body and as a result causes autoimmune diseases. Regulatory T cells actively suppress activation of the immune system and prevent pathological self-reactivity, i.e. autoimmune disease. The critical role regulatory T cells play within the immune system is evidenced by the severe autoimmune syndrome that results from a genetic deficiency in regulatory T cells.
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 vitroIn vitro refers to studies in experimental biology that are conducted using components of an organism that have been isolated from their usual biological context in order to permit a more detailed or more convenient analysis than can be done with whole organisms. Colloquially, these experiments...
experiments have given mixed results regarding the requirement of cell-to-cell contact with the cell being suppressed. The immunosuppressive cytokines TGF-beta and
Interleukin 10Interleukin-10 , also known as human cytokine synthesis inhibitory factor , is an anti-inflammatory cytokine. In humans IL-10 is encoded by the IL10 gene....
(IL-10) have also been implicated in regulatory T cell function.
An important question in the field of immunology is how the immunosuppressive activity of regulatory T cells is modulated during the course of an ongoing immune response. While the immunosuppressive function of regulatory T cells prevents the development of autoimmune disease, it is not desirable during immune responses to infectious microorganisms. Current hypotheses suggest that, upon encounter with infectious microorganisms, the activity of regulatory T cells may be downregulated, either directly or indirectly, by other cells to facilitate elimination of the infection. Experimental evidence from mouse models suggests that some pathogens may have evolved to manipulate regulatory T cells to immunosuppress the host and so potentiate their own survival. For example, regulatory T cell activity has been reported to increase in several infectious contexts, such as retroviral infections (the most well-known of which is
HIVHuman immunodeficiency virus is a lentivirus that causes acquired immunodeficiency syndrome , a condition in humans in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive...
), mycobacterial infections (like
tuberculosisTuberculosis, MTB, or TB is a common, and in many cases lethal, infectious disease caused by various strains of mycobacteria, usually Mycobacterium tuberculosis. Tuberculosis usually attacks the lungs but can also affect other parts of the body...
), and various parasitic infections including
LeishmaniaLeishmania is a genus of Trypanosomatid protozoa, and is the parasite responsible for the disease leishmaniasis. It is spread through sandflies of the genus Phlebotomus in the Old World, and of the genus Lutzomyia in the New World. Their primary hosts are vertebrates; Leishmania commonly infects...
and
malariaMalaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...
.
Studies of human subjects with a history of leishmania infection suggest that modulation of CD8+ suppressor T cells is, at least partly, mediated by cytokines. Leishmania specific CD4+ helper T cells predominate in adults with strong protective immunity (skin-test positive with no history of clinical infection). When added to autologous leishmania infected macrophages these T cells cause parasite death and secretion of large amounts of interferon-gamma and lymphotoxin. CD8+ T suppressor cells predominate in patients with no protective immunity (visceral leishmaniasis patients). When added to autologous peripheral blood mononuclear cells isolated after successful treatment, these T cells inhibit interferon-gamma secretion and proliferation and increase interleukin-6 and interleukin-10 secretion. A soluble factor(s) generated by antigen or phytohemagglutinin stimulation of leishmania-specific CD4+ helper T cells from skin-test positive adults killed CD8+ T cells but not CD4+ helper T cells when added to culture media. Soluble factors generated by antigen stimulation of peripheral blood mononuclear cells from skin-test positive adults prevented CD8+ suppressor T cell mediated increases in interleukin-10 secretion. These findings suggest that antigen stimulation of CD4+ helper T cells results in production of cytokines that kill or down regulate CD8+ T suppressor cells. Once the leishmania infection has been eliminated and leishmania antigens are gone, CD8+ T suppressor cells down-regulate CD4+ T helper cells. Isolation of cytokines that inhibit and kill CD8+ T suppressor cells might be useful in treating diseases that involve immune suppression such as leishmaniasis, AIDS, and certain cancers.
References
Molecular characterization
Similar to other T cells, regulatory T cells develop in the
thymusThe thymus is a specialized organ of the immune system. The thymus produces and "educates" T-lymphocytes , which are critical cells of the adaptive immune system....
. The latest research suggests that regulatory T cells are defined by expression of the forkhead family
transcription factorIn molecular biology and genetics, a transcription factor is a protein that binds to specific DNA sequences, thereby controlling the flow of genetic information from DNA to mRNA...
FOXP3FOXP3 is a protein involved in immune system responses. A member of the FOX protein family, FOXP3 appears to function as a master regulator in the development and function of regulatory T cells....
(forkhead box p3). Expression of
FOXP3FOXP3 is a protein involved in immune system responses. A member of the FOX protein family, FOXP3 appears to function as a master regulator in the development and function of regulatory T cells....
is required for regulatory T cell development and appears to control a genetic program specifying this cell fate. The large majority of Foxp3-expressing regulatory T cells are found within the
major histocompatibility complexMajor histocompatibility complex is a cell surface molecule encoded by a large gene family in all vertebrates. MHC molecules mediate interactions of leukocytes, also called white blood cells , which are immune cells, with other leukocytes or body cells...
(MHC) class II restricted
CDThe cluster of differentiation is a protocol used for the identification and investigation of cell surface molecules present on white blood cells, providing targets for immunophenotyping of cells...
4-expressing (CD4
+) helper T cell population and express high levels of the interleukin-2 receptor alpha chain (CD25). In addition to the Foxp3-expressing CD4
+CD25
+, there also appears to be a minor population of MHC class I restricted CD8
+ Foxp3-expressing regulatory T cells. Unlike conventional T cells, regulatory T cells do not produce IL-2 and are therefore anergic at baseline.
A number of different methods are employed in research to identify and monitor T
reg cells. Originally, high expression of CD25 and CD4 surface markers was used (CD4+CD25+ cells). This is problematic as CD25 is also expressed on non-regulatory T cells in the setting of immune activation such as during an immune response to a pathogen. As defined by CD4 and CD25 expression, regulatory T cells comprise about 5-10% of the mature CD4
+ helper T cell subpopulation in mice and humans, while about 1-2% of T
reg can be measured in whole blood. The additional measurement of cellular expression of Foxp3 protein allowed a more specific analysis of T
reg cells (CD4+CD25+Foxp3+ cells). However, Foxp3 is also transiently expressed in activated human effector T cells, thus complicating a correct T
reg analysis using CD4, CD25 and Foxp3 as markers in humans. Therefore, some research groups use another marker, the absence or low-level expression of the surface protein CD127 in combination with the presence of CD4 and CD25. Several additional markers have been described, e.g., high levels of CTLA-4 (cytotoxic T-lymphocyte associated molecule-4) and GITR (glucocorticoid-induced TNF receptor) are also expressed on regulatory T cells, however the functional significance of this expression remains to be defined. There is a great interest in identifying cell surface markers that are uniquely and specifically expressed on all Foxp3-expressing regulatory T cells. However, to date no such molecule has been identified.
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 methylationDNA methylation is a biochemical process that is important for normal development in higher organisms. It involves the addition of a methyl group to the 5 position of the cytosine pyrimidine ring or the number 6 nitrogen of the adenine purine ring...
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-demthylated region) is found demethylated, which allows to monitor T
reg cells through a PCR reaction or other DNA-based analysis methods.
See reference list for Treg epigenetic analysis
Recent evidence suggests that mast cells may be important mediators of T
reg-dependent peripheral tolerance.
Genetic deficiency
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 suffer from a severe and rapidly fatal autoimmune disorder known as
Immune dysregulation,
Polyendocrinopathy,
Enteropathy
X-linked (
IPEX) syndrome.
The IPEX syndrome is characterized by the development of overwhelming systemic autoimmunity in the first year of life, resulting in the commonly observed triad of watery diarrhea, eczematous dermatitis, and endocrinopathy seen most commonly as insulin-dependent
diabetes mellitusDiabetes mellitus, often simply referred to as diabetes, is a group of metabolic diseases in which a person has high blood sugar, either because the body does not produce enough insulin, or because cells do not respond to the insulin that is produced...
. Most individuals have other autoimmune phenomena including Coombs-positive hemolytic anemia, autoimmune thrombocytopenia, autoimmune neutropenia, and tubular nephropathy. The majority of affected males die within the first year of life of either metabolic derangements or sepsis. An analogous disease is also observed in a spontaneous Foxp3-mutant mouse known as “scurfy”.