Different types of T lymphocytes exist and each type performs a specialized function.  Helper T cells, also known as TH cells, serve as messengers of the immune system.  When helper T cells are activated, these cells proliferate and produce cytokines, which are small proteins that control the extent of immune response that a biological organism will express.  Several types of cytokines exist and each cytokine triggers the maturation of helper T cells into specific subtypes such as helper T cell subtype 1, 2 and 17 (Balandina et al., 2005).

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Another type of T lymphocyte is the cytotoxic T cell (CTL), which is also known as the TC cell.  Cytotoxic T cells are responsible for the destruction of cells that were infected by viruses, as well as for the annihilation of tumor cells.  These cells also play a major role in determining whether a cell is normally part of the system or a foreign cell.  Cytotoxic T cells are known to be the first in line to react and reject if an organ transplanted into a recipient is not compatible in terms of tissue compatibility.

The cytotoxic T cells that are associated with tissue rejection are designated at CD8+ T cells, based on the idea that these cytotoxic T cells produce a specific CD8 glycoprotein that is displayed on its cell membrane.  These CD8 glycoproteins interact with helper T lymphocytes, of which may trigger its transformation into regulatory T cells.  The major role of regulatory T cells is to inhibit the onset of an autoimmune disorder within the system of the organism.

Another type of T lymphocyte is the memory T cell which is produced as soon as an infection occurs in an individual.  These specific T cells continue to circulate in the blood system for several months even when the infection has been stopped.  Memory T cells further differentiate into effector T cells when they find the same antigen that was associated with the previous infection, hence the term memory is appropriate to apply to this type of function.  Two general types of memory Tcells include the central and effector cells.   Memory T cells are known to carry either the CD4 or the CD8 glycoprotein on its plasma membrane (Wildin et al., 2001).

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Regulatory T cells are another type of T lymphocytes that play a crucial role in sustaining the tolerance of theimmune system (Khattri et al., 2003).  These T cells were earlier called suppressor T cells and their primary role is to prevent immunity that is caused by other T cells during the later stages of an immune response (Fontenot et al., 2003).  Regulatory T cells are also responsible for inhibiting the immune reaction towards other cells of the body or self cells because this is important in maintaining the stability of the entire immunological system.

Specific conditions result in the immune response towards self cells and this is usually observed when the thymus experiences difficulty in distinguishing self cells from non-self or foreign cells.  There are two major types of CD4+ regulatory T cells that have been characterized.  Naturally occurring regulatory T cells originate from the thymus and are sometimes referred to as CD4/CD25/ForP3 T regulatory cells.

The other major type of CD4+ regulatory T cell is the adaptive T regulatory cell which is produced when an innate immune response occurs.  This type of cell is also called Tr1 or Th3 cell (Hori et al., 2003).  A distinguishing feature that facilitates is the differentiation of naturally occurring T regulatory cells from the rest of types of T lymphocytes is the occurrence of the FoxP3 molecule that is situated within the cytoplasm.  Research has shown that mutations incurred in the FOXP3 gene results in the inability of regulatory T cells to differentiate, which in turn influences the onset of the lethal condition of autoimmunity.

Another type of T lymphocyte is the natural killer T cell which is serves as the communicator between the adaptive and the innate immune systems.  Different from the standard T lymphocyte that identifies protein antigens that are harbored by the major histocompatibility complex (MHC), the natural killer T lymphocytes distinguish glycolipid antigens that are shown by the protein molecule CD1d.  The natural killer T lymphocyte is then activated and subsequently acquires the ability to execute specific functions that are related to Th and Tc cells, including the secretion of cytokines and the expression of cytolytic molecules that are responsible for the destruction of foreign cells that have invaded the body.

Gamma-delta T lymphocytes represent a small subtype of T cells that carry a unique receptor on their plasma membrane.  Most of the T lymphocytes carry a receptor that is comprised by two strings of alpha and beta glycoproteins.  In gamma-delta T lymphocytes, the receptor is composed of a gamma and a delta glycoprotein string.  Gamma-delta T lymphocytes are quite rare, comprising only approximately 5% of the entire T lymphocyte population but are found in high concentrations in the epithelial lining of the stomach, as part of population of intraepithelial lymphocytes.

Protein antigens that trigger a response from gamma-delta T cells have still yet to be determined.  It is interesting to know that gamma-delta T lymphocytes are not exclusively assigned to carry major histocompatibility complex molecules and are actually capable of identifying entire proteins than simply perceiving short protein chains carried by the major histocompatibility complex molecules on lymphocytes that harbor antigens.

There are particular gamma-delta T cells that have the capacity to identify a specific type of major histocompatibility complex molecules, specifically class IB.  In the human immune system, the Vgamma9/Vdelta2 T lymphocyte makes up the majority of the gamma-delta T lymphocyte population that is circulating in the bloodstream.  These specific T lymphocytes have the function of quickly reacting to a tiny metabolite that originates from microorganisms, known as the isopentenyl pyrophosphate precursor.

 Another type of T lymphocyte is the autoaggressive T cell which is highly specialized based on its ability to secrete the CD40 protein molecule (Miura et al., 2004).  CD40 is generally linked to T lymphocytes that present fragments of an antigen and this protein molecule is commonly produced by a subtype of T helper lymphocytes.

Th40 cells are present in all human beings but the levels of this specific type of cells quickly increase during conditions of autoimmunity.  In patients diagnosed with type 1 diabetes mellitus, the Th40 cells usually react to self-antigens.  In the case of patients diagnosed with non-autoimmune individuals, the Th40 lymphocytes do not react with these antigens.  An important function of the CD40 protein molecule on T lymphocytes is to trigger the recombinase proteins RAG1 and RAG2 in directing the receptor of a T lymphocyte.

The receptor of a T lymphocyte serves as the route through which a T lymphocyte is able to identify a specific antigen.  It has been determined that the two recombinase proteins be produced exclusively in the thymus during the development of T lymphocytes.  It was, however, observed that the RAG proteins were secreted again by the T lymphocytes that were circulating in the bloodstream.  In addition, it was also observed that the CD40 protein associated with Th40 lymphocytes, resulting in the production of RAG proteins.  Subsequent to the expression of the RAG proteins, modifications in the receptor of the T cells take place, suggesting that the Th40 lymphocytes carry the ability to adapt for the entire duration of an individual’s life.

The procedure of modifying the expression of the receptors of T cells that are circulating in the bloodstream has been names as TDR revision.  Research has reported that TCR revision influences the expansion of the repertoire of T lymphocytes, as well as generates T lymphocytes that are autoaggressive.  This observation therefore shows that the revision of the receptors of T cells is another method of T lymphocyte tolerance.

THE DEVELOPMENT OF T LYMPHOCYTES IN THE THYMUS

T lymphocytes are developed from the stem cells of the hematopoietic system which is located in the bone marrow.  These stem cells migrate from the bone marrow to the thymus, where they are expected to proliferate through the process of cell division and generate immature thymocytes.  Young thymocytes generally do not secrete any CD4 or CD8 proteins hence these cells have been described as CD4-CD8- cells.

During their course of development, the young thymocytes acquire the capability of secreting CD4 and CD8 molecules and these thymocytes are now classified as CD4+CD8+ (Sakaguchi, 2000).  Further maturation of these cells involves the specific secretion of only one protein, either the CD4 or the CD8 molecule.  Once this has been determined in each thymocytes, these cells are then expelled by the thymus in order for them to circulate to the rest of the body.  Approximately 98% of the thymocytes produced by the thymus are expected to disintegrate during its maturation, resulting in only 2% of the thymocytes reaching maturity and categorically expelled by the thymus as immunocompetent T lymphocytes.

Immature thymocytes that harbor the two types of antigens, CD4 and CD8, migrate into the core of the thymus in order to associate with antigens that are generated by the T lymphocyte.  These thymocytes interact with the major histocompatibility complex molecules that are present of the plasma membrane of the cells of the epithelium.  Only a specific fraction of thymocytes will be able to completely bind with the major histocompatibility complex molecules thus resulting in a signal that indicates the survival of the thymocyte.

The rest of the thymocytes that were not able to achieve a complete association with the major histocompatibility complex molecules are thus classified as incapable of performing immune functions or even illicit an immune response.  This specific group of thymocytes thus undergoes the process of apoptosis, which involved the disintegration of the nucleus which eventually leads to cell death.  The cellular debris that results from the apoptotic pathway are enveloped and eaten by macrophages.  The entire process of differentiating immuno-competent from immuno-incompetent thymocytes based on the ability to illicit an immune reaction is known as positive selection.

Thymocytes that endure positive selection travel towards the perimeter of the cortical and middle regions of the thymus. While situated in the middle region of the thymus, the thymocytes are presented with an antigen of its self in association with the major histocompatibility complex molecules on antigen-presenting cells (APCs), including the dendritic cells and macrophages.  Thymocytes that intensely associate with the antigen are sent a signal that induces their programmed death and the rest of the thymocytes that were first generated are trigger to disintegrate during this selection process in the thymus.

A small fraction of the cells that survived is induced to develop regulatory T cells.  The other cells subsequently depart the thymus as differentiation naive T lymphocytes.  This mechanism is designated as negative selection, an essential process of immunological tolerance that controls the development of T lymphocytes that react to self cells and are able of to cause an autoimmune disease in the body of an individual (Shevach, 2000).

Although the specific mechanisms of activation vary slightly between different types of T cells, the “two-signal model” in CD4+ T cells holds true for most. Activation of CD4+ T cells occurs through the engagement of both the T cell receptor and CD28 on the T cell by the Major histocompatibility complex peptide and B7 family members on the APC respectively. Both are required for production of an effective immune response; in the absence of CD28 co-stimulation, T cell receptor signalling alone results in anergy. The signalling pathways downstream from both CD28 and the T cell receptor involve many proteins.

The first signal is provided by binding of the T cell receptor to a short peptide presented by the major histocompatibility complex (MHC) on another cell. This ensures that only a T cell with a TCR specific to that peptide is activated. The partner cell is usually a professional antigen presenting cell (APC), usually a dendritic cell in the case of naïve responses, although B cells and macrophages can be important APCs. The peptides presented to CD8+ T cells by MHC class I molecules are 8-9 amino acids in length; the peptides presented to CD4+ cells by MHC class II molecules are longer, 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 (Brunkow et al., 2001). 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 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.

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 importantly 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, Trim, 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 PLCγ, VAV1, Itk and potentially PI3K. Both PLCγ and PI3K act on PI(4,5)P2 on the inner leaflet of the membrane to create the active intermediaries di-acyl glycerol (DAG), inositol-1,4,5-trisphosphate (IP3) and phosphatidlyinositol-3,4,5-trisphosphate (PIP3). DAG binds and activates some PKCs, most importantly in T cells PKCθ, which is important for activating the transcription factors NF-κB and AP-1. IP3 is released from the membrane by PLCγ and diffuses rapidly to activate receptors on the ER which induce the release of calcium. The released calcium then activates calcineurin, and calcineurin activates NFAT, which then translocates to the nucleus. NFAT is a transcription factor which activates the transcription of a pleiotropic set of genes, most notably IL-2, a cytokine which promotes long term proliferation of activated T cells.

Overview of interactions between T cells and APCs T cells are a subset of lymphocytes that play a large role in the cell-mediated immune response of adaptive immunity. T lymphocytes are divided into 2 major classes distinguished by the expression of the cell surface molecules CD4 or CD8. CD8 cytotoxic T lymphocytes (CTLs) primarily destroy virus-infected cells whereasCD4 helper T lymphocytes are involved in activating B cells and macrophages.

The response of both classes of T lymphocytes is dependent on interactions through the T cell receptor (TCR) and coreceptors (CD4 or CD8) with target APCs that present peptides in the context of MHC class I or class II molecules. MHC class I and class II molecules are structurally similar but differ in their source of antigenic peptide and in the transport mechanisms for their peptides. Class I MHC molecules present self peptides or viral products from the cytosol

to CD8 T lymphocytes, whereas MHC class II molecules present peptides from pathogens that either reside or were endocytosed into intracellular compartments (Fontenot and Rudensky, 2005). T cells are activated on encountering antigen, which results in either lysis of target cells by CD8

T lymphocytes or recruitment of other effector cells by CD4 cells.  Modulation of the plasma membrane alters the T cell Ca2+ response.  A series of studies from showedthat unsaturated free fatty acids (FFAs) could inhibit specific aspects of cytotoxic T cell function by perturbing membranes.  Initially, it was shown that short-term exposure of murine allogeneic effector T cells to low levels of unsaturated FFAs (10 mol/L), including PUFAs, inhibited lysis of target APCs. The change in lysis of target cells was a direct consequence of the FFA added to the CTLs, because lysis could be inhibited by

extracting the unsaturated FFA with bovine serum albumin before CTL-target conjugation. Specific aspects of T cell function inhibited by unsaturated lipids included the initial rise in intracellular [Ca2+] on conjugate formation, protein phosphorylation

events and subsequent CTL esterase release.  On the other hand, release of inositol phosphates and binding to target cells were unaffected. Because the inhibition in CTL calcium release linearly correlated with the decrease in membrane acyl chain order induced by the presence of increasing unsaturation in the plasma membrane , it was hypothesized that modulation of membrane structure affected T cell Ca2+ signaling.

References

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Brunkow ME, Jeffery EW, Hjerrild KA, Paeper B, Clark LB, Yasayko SA, Wilkinson JE, Galas D, Ziegler SF and Ramsdell F (2001):  Disruption of a new forkhead/winged-helix protein, scurfin, results in the fatal lymphoproliferative disorder of the scurfy mouse. Nat. Genet.  27:68–73.

Fontenot JD, Gavin MA and Rudensky AY (2003):  Foxp3 programs the development and function of CD4-CD25 regulatory T cells. Nat. Immunol.  4:330–336.

Fontenot JD and Rudensky AY (2005):  A well adapted regulatory contrivance:  Regulatory T cell development and the forkhead family transcription factor Foxp3. Nat. Immunol. 6:331–337.

Hori S, Nomura T and Sakaguchi S (2003):  Control of regulatory T cell development by the transcription factor Foxp3. Science 299:1057–1061

Khattri R, Cox T, Yasayko SA and Ramsdell F (2003):  An essential role for Scurfin in CD4-CD25 T regulatory cells. Nat. Immunol.  4:337–342.

Miura Y, Thoburn CJ, Bright EC, Phelps ML, Shin T, Matsui EC, Matsui WH, Arai S, Fuchs EJ and Vogelsang GB (2004):  Association of Foxp3 regulatory gene expression with graft-versus-host disease. Blood  104:2187–2193.

Sakaguchi S (2000):  Regulatory T cells: key controllers of immunologic self-tolerance.  Cell 101:455–458.

Shevach EM (2000): Regulatory T cells in autoimmmunity. Annu. Rev. Immunol.  18:423–449.

Wildin RS, Ramsdell F, Peake J, Faravelli F, Casanova JL, Buist N, Levy-Lahad E, Mazzella M, Goulet O and Perroni L (2001): X-linked neonatal diabetes mellitus, enteropathy and endocrinopathy syndrome is the human equivalent of mouse scurfy. Nat. Genet.  27:18–20.

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