T cells constitute 70–80% of blood lymphocytes. Unlike B cells, they do not have microvilli on their surface. They bear specialized surface receptors called T cell receptors (TCR).

T Cell Receptor

 The T cell receptors (TCR) of T cells are equivalent to the surface immunoglobulins (B cell receptors) of the B cells. Their main function is antigen recognition. Unlike B cell receptor which binds to antigen directly, TCR does not recognize antigen by itself. It can only respond to an antigen which is processed and presented by the antigen presenting cells, such as macrophages.

TCR–CD3 Complex

Most T cell receptors (95%) comprise of two chains (α and β) which in turn have three regions—(1) extracellular domain, (2) transmembrane domain, and (3) cytoplasmic tail. The extracellular domain of each polypeptide chain has 2 regions (variable and constant region). About 5% of TCRs do not have α/β chains, instead they bear γ/δ chains. TCR is active only when both the chains (α and β) complex with CD3 molecule (Fig. 1).

Fig1. Structure of T cell receptor - CD3 complex.

- The variable region of α and β chains of TCR bind to the presented antigens. They are polymorphic in nature. Rearrangement of α and β genes during T cell development can produce large number of different combinations of TCRs. Each TCR is capable of recognizing a particular epitope of an antigen

- T he CD3 complex consists of three pairs of polypeptide chains—ξξ (zeta-homodimer), δε (delta–epsilon heterodimer) and γε (gamma–epsilon heterodimer)

- Following binding of antigen to α and β chains of TCR, a signal is generated that is transmitted through the CD3 complex leading to activation of T cells.

T Cell Development

The major events of T cell maturation take place in thymus, in contrast to bone marrow for B cells.

- The progenitor T cells are originated from the bone marrow (or liver in fetal life) and then migrate to thymus through bloodstream

- Developing T cells in the thymus (collectively called as thymocytes) pass through series of stages that are marked by characteristic changes in their cell surface markers

- Most of the development events take place in the cortex of thymus, under the influence of thymic stromal cells which secrete thymic hormones and lymphopoietic growth factor IL-7.

 The sequence of events of T cell development is as follows (Fig. 2):

- Double negative (DN) T cells: T cell precursors after entering into the thymus transform into double negative T cells (CD4– CD8–). These cells are so called because, they do not express the surface markers of mature T cells, i.e. CD4 and CD8 molecules. DN T cells first express CD3 molecule and then undergo further development

* Five percent of T cell precursors carrying TCR γδ develop into mature γδ T cells

* T he remaining (95%) of the cells express TCR αβ and subsequently express both CD4 and CD8 molecules to become double positive (DP) T cells.

- Double positive (DP) T cells (CD4+ CD8+): They are immature T cells, carrying both CD4 and CD8 molecules on their surface. They further undergo one of the following fate:

* Positive selection: The 5% of DP T cells, whose αβ receptors are capable of recognizing their MHC molecules are positively selected. This results in MHC restriction

* Death by neglect: Majority of DP cells (95%) fail positive selection because they do not specifically recognize their MHC molecules

* Negative selection: The survived cells that undergo positive selection (5%) are MHC restricted. However, some of these surviving cells (2–5% ) react to the self- antigens and therefore, they are selected to be killed by apoptosis and removed (negatively selection)

* T he remaining double positive T cells (2–5%) having αβ type TCR selectively shut off the expression of either CD4 or CD8 molecules and eventually become single positive mature T cells (CD4+/CD8– or CD4–/CD8+).

- Mature T cells (e.g. CD4+ helper T cells and CD8+ cytotoxic T cells) acquire thymus specific antigens, then are released into the circulation and migrate to the peripheral lymphoid organs where they respond to the antigenic stimulus.

Fig2. T cell development.

Types of T Cells

Effector T Cells

There are two types of effector T cells—(1) CD4+ helper T cells and (2) CD8+ cytotoxic T cell.

- Helper T cells: Helper T cells (TH ) possess CD4 molecules as surface receptors. They recognize the antigenic peptides that are processed by antigen presenting cells and presented along with MHC-II molecules (major histocompatibility complex)

* Following antigenic stimulus, the helper T cells differentiate into either of the two types of cells— (1) T_H 1 and (2) T_H 2 subset; each secrete specific cytokines which modulate the cellular and humoral immune responses respectively

* T_H 17 cells: Recently a third subset of T helper cells called T_H 17 cell has been discovered. It produces IL 17 and IL-22, and is primarily involved in recruiting neutrophils. They contribute to the pathogenesis of many autoimmune inflammatory diseases such as rheumatoid arthritis and others.

- Cytotoxic T cells: In contrast to TH cells, cytotoxic T cells (TC ) possess CD8 molecules and recognize the intracellular antigens (e.g. viral antigens or tumor antigens) that are processed by any nucleated cells and presented along with MHC-I. In general, TC cells are involved in destruction of virus infected cells and tumor cells.

Rare Subtypes of T Cells

- Regulatory T cells (T_REG cells): The T_REG cells (formerly known as suppressor T cells) are a subpopulation of T cells which regulate the immune system

* They provide tolerance to self-antigens (known as peripheral tolerance), and thus prevent the development of autoimmune disease

* Surface markers: TREG cells possess surface markers such as CD4, CD25 and Foxp3 (a forkhead family transcription factor)

* Deficiency of Foxp 3 receptors leads to a severe form of autoimmune disease known as Immune dysregulation, Polyendocrinopathy, Enteropathy X-linked (IPEX) syndrome.

* γδ T cells: γδ T cells represent a small subset of T cells (5%) that possess a distinct TCR composed of γ and δ chains; instead of α/β chains. They lack both CD4 and CD8 molecules

* T hey differ from the conventional αβ T cells by the fact that they do not require antigen processing and MHC presentation of peptides

* T hey are part of innate immunity as the γδ receptors exhibit limited diversity for the antigen

* T hey are usually found in the gut mucosa, within a population of lymphocytes known as IELs

* T he function of γδ T cells is not known, they may encounter the lipid antigens that enter through the intestinal mucosa.

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المزيد

مواضيع ذات صلة

Macrophage

B Lymphocytes

Immunologic Manifestations of Infectious Mononucleosis

Peripheral Lymphoid Organs

Central Lymphoid Organs

Effector Functions of Complement

Complement Pathways

Mechanisms of innate immunity

Immunologic Manifestations of Viral, Rickettsial, and Mycoplasmal Diseases

Immunologic Manifestations of Fungal Diseases

The Structure of Antibodies

Myelomas and Monoclonal Antibodies