File Name: t cell receptor signalling in the control of regulatory t cell differentiation and function .zip
- T-Cell Tolerance: Central and Peripheral
- Nutrient mTORC1 signaling underpins regulatory T cell control of immune tolerance
- Treg Heterogeneity, Function, and Homeostasis
- T cell receptor signaling for γδT cell development
It was created in December with the purpose of disseminating scientific information derived from primary and secondary research and presenting cases coming from the practice of Rheumatology in Latin America. Since its foundation, the Journal has been characterized by its plurality with subjects of all rheumatic and osteomuscular pathologies, in the form of original articles, historical articles, economic evaluations, and articles of reflection and education in Medicine. It covers an extensive area of topics ranging from the broad spectrum of the clinical aspects of rheumatology and related areas in autoimmunity both in pediatric and adult pathologies , to aspects of basic sciences.
T-Cell Tolerance: Central and Peripheral
It was created in December with the purpose of disseminating scientific information derived from primary and secondary research and presenting cases coming from the practice of Rheumatology in Latin America. Since its foundation, the Journal has been characterized by its plurality with subjects of all rheumatic and osteomuscular pathologies, in the form of original articles, historical articles, economic evaluations, and articles of reflection and education in Medicine.
It covers an extensive area of topics ranging from the broad spectrum of the clinical aspects of rheumatology and related areas in autoimmunity both in pediatric and adult pathologies , to aspects of basic sciences. It is an academic tool for the different members of the academic and scientific community at their different levels of training, from undergraduate to post-doctoral degrees, managing to integrate all actors inter-and transdisciplinarily.
It is intended for rheumatologists, general internists, specialists in related areas, and general practitioners in the country and abroad. It has become an important space in the work of all rheumatologists from Central and South America. The activation of T cells is initiated by the presentation of exogenous or endogenous antigens, by antigen presenting cells through the major histocompatibility complex, which binds to a special receptor on T cells.
This recognition triggers a cascade of intracellular signaling that leads to an increase in integrin expression, cytoskeletal modifications, and transcription factors production involved in the liberation of cytokines and inflammatory mediators.
One of the most important inducers in cell activation is the enzymatic complex with tyrosine kinase action. Regulating the kinases signaling, as well as the adapter proteins involved in T cell activation, is essential for maintaining an activation threshold, as well as the modulation of cell response.
The phosphorylation of the positive regulation sites of these proteins is important to allow an active configuration of the protein and thereby its maximum capacity as kinase. The phosphorylation of negative regulation sites leads to a closed configuration of the protein that reduces its kinase function, and thereby inhibits its own function.
The alteration in signaling by the modification of certain cytoplasmic proteins in some cases is associated with the development of autoimmune diseases, such as systemic lupus erythematosus.
Under physiological conditions the T cell receptor complex regroups with protein complexes that interact harmonically to generate an internal signal. The altered signaling events are partly responsible for an anomalous expression of cytokines, including the interleukin-6 IL-6 , IL, IL-2, IFN, and CD40 linking, these modifications affect the cells ability to over-stimulate T and B cells, resulting in an increased production of autoantibodies and the triggering of the autoimmune disease..
The mechanisms of activation and regulation of T lymphocytes involve a cascade of internal signaling events in which a large number of proteins play a relevant role, inducing in some cases phosphorylation and activation of tyrosine kinases, which leads to cellular activation with release of cytokines and other soluble factors.
An alteration in the proteins involved in the signaling events may result in the loss of their effector mechanism, which would mean a change in the activation of the cell. The functional changes induced as a consequence of these alterations generate different behaviors in the T and B lymphocytes, producing, in some cases, overexpression of inducing proteins and an increase in the synthesis of antibodies. This review aims to include the most important proteins and markers involved in internal signaling events in T lymphocytes, as well as modifications in the expression of some of them induced by mutations or by external factors that trigger autoimmune processes such as systemic lupus erythematosus SLE Table 1.
Relationship between the normal lymphocyte activation and systemic lupus erythematosus. The T-cell receptor TCR is a heterodimer composed of an alpha chain and a beta chain that shares structural similarity with immunoglobulins, having a variable domain and a constant domain. Both chains are linked through disulfide bridges at an end close to the cell membrane. The variable domain consists of amino acid sequences encoded by variable V , diverse D and junction J gene segments.
The D segment is found only in the segments that encode for the receptor beta chain. The TCR of T lymphocytes has the ability to recognize peptides that are bound to the major histocompatibility complex, expressed on the surface of antigen-presenting cells.
The binding of the TCR and the MHC-associated peptide generates the first cytoplasmic signal for the lymphocyte activation through the CD3 co-receptor.
LCK activates ZAP by the phosphorylation of its regulatory domain, allowing its complete function as a kinase. The binding of the peptide major histocompatibility complex pMHC and the TCR originates a primary signal that by itself is not able to generate the activation of the lymphocytes, since it needs other secondary signals. The works of Tsokos et al. Experimental designs in which anti-CD3 monoclonal antibodies are used to stimulate T lymphocytes have demonstrated an increase in the activity of protein tyrosine kinases and in the intracellular calcium levels in autoreactive T lymphocytes of patients with SLE, but not in autoreactive T lymphocytes of patients with mixed connective tissue disease MCTD or in control T lymphocytes.
Other groups have reported other types of alterations in the intracellular signaling pathways in T cells of patients with SLE, in the internal signaling based on the antigenic recognition by TCR and in the signal transduction in the T lymphocytes. Domain 1 has enzymatic activity of phosphatase and domain 2, although its function is not completely known, may be involved in the regulation of the substrates with which D1 interacts.
The recruiting of the CD4 co-receptor allows the proximity of LCK kinase to its substrates in the chains of the CD3 co-receptor; and in this way, the phosphorylation of the ITAMs in their tyrosine residues. Although the structure and biochemical function of SYK and ZAP are similar, there is a great difference in their expression; this is due to the expression pattern of ZAP, which is restricted to T and natural killer lymphocytes, whereas SYK can be found in B lymphocytes, macrophages, monocytes, mast cells and platelets.
The signalosome is a protein complex that is composed of several signaling elements that are associated and regulated by the activity of scaffold proteins. It is composed of a short extracellular domain, a transmembrane domain and a long cytoplasmic domain that contains tyrosine residues.
These adaptors share a similar structure consisting of two SH3 domains and one SH2 domain. Through their SH2 domains they bind the phosphorylated residues of LAT and allow the binding of other proteins by their exposed SH3 domains.
Each adaptor protein associated with LAT is involved in different signaling pathways, allowing the interaction with different effector proteins. This indicates an upregulation at the transcriptional level of the protein or a decrease in the proteolitic degradation of the protein. The activation of the T lymphocyte requires the regulation of the organization of the actin cytoskeleton to facilitate the immune synapse, allowing the communication with the antigen presenting cells and facilitating different events that involve: differentiation into different cell lineages, migration through the tissues, cellular adherence, cellular reorientation and secretion of cell mediators such as cytokines.
The reorganization of the cytoskeleton is accompanied by an important number of internal signaling events. WASP is recognized for the Wiskott-Aldrich syndrome, an X chromosome linked inherited immunodeficiency which results from the mutation of the WASP gene, specifically a mutation in the WH1 domain, a region which is required for the protein stabilization through the association with WIP; in the absence of this interaction, WASP is degraded.
R, which contributes to signaling abnormalities. The binding of the TCR with the peptide bound with the MHC initiates a series of signaling events that prepare the lymphocyte for the cell differentiation, proliferation and effector function. The mitogen activated protein kinase MAPK cascade is a signaling way for cellular processes such as differentiation, cell growth, proliferation, mobility and response to stress. The JNK cascade, also called stress-activated protein kinase cascade SAPKs , 44 involves different signaling pathways responsible for the cell response to stress stimuli.
The function of the three isoforms of JNK is the phosphorylation of c-Jun inhibiting its degradation and allowing the interaction with other transcription factors.
The defective MAP kinase activity can alter the coordination of the signals that are necessary for the normal production of IL-2 and the maintenance of the tolerance in the T lymphocytes of patients with autoimmune disease. The regulation of calcium is mediated by channels located both in the endoplasmic reticulum and in the cell membrane.
Ins 1,4,5 P3 receptor is a glycoprotein bound to the membrane of the endoplasmic reticulum which acts as a calcium channel that releases the calcium stored in the reticulum to the cytoplasm, increasing intracellular concentrations. The STIM proteins are transmembrane proteins located in the endoplasmic reticulum which act as sensors of the calcium levels maintaining the homeostasis within the cell. Once the low calcium concentrations are detected, is generated a change from a basal dimeric state into an oligomeric state, which migrates to the plasma membrane and is able to trigger the entry of calcium into the cell through transmembrane calcium channels ORAI calcium release-activated calcium modulator 1 by domains located in the C-terminal cytoplasmic region.
Calcium within the cell acts as a second messenger for the activation of NFAT. High concentrations of calcium within the cell are crucial for the activation of the intracellular calcium sensor, calmodulin. The calcium as a second messenger of multiple signaling pathways is closely regulated to ensure an optimal cell response to a stimulus; alterations in its concentration involve abnormal signals that have implications in the cellular response.
The signal generated by the activation of the TCR by itself fails to trigger a complete activation of the lymphocyte; the presence of co-receptors that provide additional signals by means of signaling pathways in common with the main signal is necessary to guarantee a signal sufficiently strong for the activation of the T lymphocyte.
The CD28 co-receptor has great importance in the cell activation for its participation in the generation of co-stimulator signals. It is a transmembrane glycoprotein that is mainly expressed both in activated and resting T lymphocytes.
CD28 co-receptor: the CD28 co-receptor has great importance in cellular activation because of its participation in the generation of a costimulatory signal. The amino acids that comprise the cytoplasmic tail of the CD28 co-receptor possess intrinsic enzymatic activity.
This cytoplasmic tail has 4 tyrosines that are key for intracellular signaling and are phosphorylated by FYN or by LCK. PI3K is part of the family of enzymes that regulate the biological function through the generation of lipids as second messengers.
It is divided into different classes according to their function; the PI3K class I predominates mainly in the lymphocytes. In turn, the PI3K are divided into 2 groups, according to their activation, being the group IA activated by receptors associated with tyrosine kinases, cytokine receptors and coreceptors.
It has been demonstrated that without the second costimulatory signal, the T lymphocytes fail in the production of IL-2 and a state of anergy is established. Lipid rafts are microdomains in the cell membrane composed of lipids different than those that are normally part of the cell membrane, among these lipids there are the glycosphingolipids, sphingomyelin, cholesterol, among others. These lipid rafts create an environment that allows to accumulate or secrete different proteins in a specific region.
Sphingolipids have a high boiling point, which allows them to form arrays separated from the layer of phospholipids which have a lower boiling point; the cholesterol is preferably mixed within sphingolipids to stabilize the structure and fluidity of the membrane. Therefore, the lipid rafts can act as platforms that move freely in the cell membrane. The complexity of the environment of the lipid rafts allows a temporospatial regulation of the T lymphocyte signal.
The addition of saturated groups to the proteins with intracellular, extracellular or transmembrane domains increases the affinity of these proteins to the organized environment of the lipid rafts. LCK is modified with the presence of groups of saturated lipids and its presence in the lipid rafts is required for the transduction of the signal through the activation of the TCR. Cytotoxic T-lymphocyte antigen 4 CTLA-4 is a co-receptor which plays a fundamental role in the negative regulation of the activation signal of the T lymphocyte; it is a glycoprotein that shares great similarity with CD28 and, like this one, possesses extracellular domains similar to those of immunoglobulins.
CTLA-4 inhibits the lymphocyte activation through the reduction in the production of IL-2 and the decrease in the expression of the IL-2 receptor, stopping the cell cycle in G1 phase. CTLA-4 has a cytoplasmic tail without intrinsic enzyme activity composed by characteristic pattern sequences YVKM and 2 tyrosine residues involved in the activity and regulation of this protein Y, Y In its non-phosphorylated form, the YVKM pattern of CTLA-4 is associated with the AP50 subunit of the clathrin-associated protein complex, which determines the cytoplasmic state of the protein.
CTLA-4 acts through phosphatases that act directly on specific substrates, however, how CTL-4 signaling exerts its inhibitory effect has been quite controversial and even contradictory. Ubiquitination is the process by which the cells can discriminate proteins which will be degraded; this takes place thanks to the labeling of the protein with ubiquitin, which allows its degradation. Ubiquitin is a peptide of 76 amino acids; it binds to proteins through 3 enzymes, E1, E2 and E3 by the process of ubiquitination.
The first step of ubiquitination consists in the formation of a thioester bond with the glycine residue of the C-terminal of ubiquitin and the sulfhydryl group or thiol of the cysteine of E1 in its active center. The second step consists in the transfer of ubiquitin from an E1 enzyme to an E2 conjugating enzyme Ubc.
The final step consists in the binding of E2-ubiquitin to an E3 ligase; which catalyzes the formation of an isopeptide bond between the glycine of the C-terminal of the ubiquitin and the lisine of the specific substrate.
Polyubiquitinated proteins labeled in their lysine 48 are substrates for protein degradation by the 26S proteasome. The proteins of the family Casitas B-lineage lymphoma Cbl are molecular adaptor proteins; they bind to proteins for their ubiquitination and degradation.
In mammals there are 3 genes that encode for the proteins of the Cbl family: c-cbl, cbl-b and cbl-3 also known as cbl-c. Negative regulation of the activation signal of the TCR complex. The negative regulation of the activation signal of the TCR is important to avoid anergy processes.
In the T lymphocytes, the c-Cbl and Cbl-b proteins are responsible for the signaling control through the ubiquitination of active receptors and tyrosine kinase associated receptors. When the CTLA-4 co-receptor is activated, there is activation of the Cbl-b protein for the regulation of the lymphocyte signal; it interacts with the p85 subunit of the enzyme PI3K, thus regulating the signal coming from CD28 coreceptor. The regulation of the activation of the kinases involved in the activation of the T lymphocyte is essential to maintain an activation threshold.
The proximity of CSK to its substrate is important and it is achieved by its interaction with the phosphoprotein associated with glycosphingolipid-enriched microdomain PAG.
PAG, also known as Csk-binding protein CBP is an ubiquitous transmembrane protein with multiple tyrosine residues which when they are phosphorylated, they act as binding pockets for proteins containing SH2 domains; in addition, it has 2 proline-rich sequences that allow the interaction with proteins containing SH3 domains. When the activation of the T lymphocyte occurs, PAG is rapidly dephosphorylated, resulting in the release of CSK from the membrane, and it is sequestered in the cytoplasm by the Ras GTPase-activating protein-binding protein 1 G3BP ; this allows it to be far from the immune synapse.
Tyrosine kinase phosphatases PTP play an essential role, both in maintaining the activated phenotype of T lymphocytes, as well as in the reversal of the activated state to a resting state when the immune response ends. This negative regulation is possible thanks to its interaction with the SH3 domain of the CSK proteins by proline-rich pattern sequences, while PEP dephosphorylates the tyrosines of the positive regulatory regions of the proteins of the SRC family, CSK phosphorylates the negative regulatory regions thereof.
This polymorphism is characterized by the substitution of the amino acid arginine by a tryptophane in the position of the amino acid sequence. This proline-rich portion is an important binding site for the C-terminal domain of CSK, whereby this polymorphism interrupts the interaction between PTPN22 and CSK; in this way, the suppression of T lymphocyte activation does not take place.
The net effect of this polymorphism on the interaction between PTPN22 and CSK for the generation of autoimmune disease is a subject of controversy and the proposed mechanisms of action are based on animal models.
Nutrient mTORC1 signaling underpins regulatory T cell control of immune tolerance
A T cell is a type of lymphocyte. T cells are one of the important white blood cells of the immune system, and play a central role in the adaptive immune response. T cells can be easily distinguished from other lymphocytes by the presence of a T-cell receptor TCR on their cell surface. T cells are born from hematopoietic stem cells ,  found in the bone marrow. Then, developing T cells migrate to the thymus gland to mature. T cells derive their name from this organ where they develop or mature. T cell differentiation also continues after they have left the thymus.
Treg Heterogeneity, Function, and Homeostasis
Mytrang H. Nixon, Kristelle J. Sabatini, Ming O. J Exp Med 6 January ; 1 : e T reg cell—specific ablation of the Rag family small GTPases RagA and RagB impairs amino acid—induced mTORC1 signaling, causing defective amino acid anabolism, reduced T reg cell proliferation, and a rampant autoimmune disorder similar in severity to that triggered by T reg cell—specific TCR deficiency.
T-regulatory cells Tregs represent a unique subpopulation of helper T-cells by maintaining immune equilibrium using various mechanisms. The role of T-cell receptors TCR in providing homeostasis and activation of conventional T-cells is well-known; however, for Tregs, this area is understudied. In the last two decades, evidence has accumulated to confirm the importance of the TCR in Treg homeostasis and antigen-specific immune response regulation. In this review, we describe the current view of Treg subset heterogeneity, homeostasis and function in the context of TCR involvement. Recent studies of the TCR repertoire of Tregs, combined with single-cell gene expression analysis, revealed the importance of TCR specificity in shaping Treg phenotype diversity, their functions and homeostatic maintenance in various tissues.
T cell receptor signaling for γδT cell development
Besides the classical T-helper 1 and T-helper 2, other subsets have been identified, including T-helper 17, regulatory T cell, follicular helper T cell, and T-helper 9, each with a characteristic cytokine profile. For a particular phenotype to be differentiated, a set of cytokine signaling pathways coupled with activation of lineage-specific transcription factors and epigenetic modifications at appropriate genes are required. The effector functions of these cells are mediated by the cytokines secreted by the differentiated cells.
Somatic recombination of TCR genes in immature thymocytes results in some cells with useful TCR specificities, but also many with useless or potentially self-reactive specificities. Thus thymic selection mechanisms operate to shape the T-cell repertoire. Thymocytes that have a TCR with low affinity for self-peptide—MHC complexes are positively selected to further differentiate and function in adaptive immunity, whereas useless ones die by neglect.
Tregs are immunosuppressive and generally suppress or downregulate induction and proliferation of effector T cells. Mouse models have suggested that modulation of Tregs can treat autoimmune disease and cancer and can facilitate organ transplantation  and wound healing. Tregs 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 Tregs in the tumor microenvironment is indicative of a poor prognosis , and Tregs are thought to suppress tumor immunity, thus hindering the body's innate ability to control the growth of cancerous cells. T regulatory cells are a component of the immune system that suppress immune responses of other cells.