T cells are clonally restricted, so that each T cell expresses a receptor that can interact with a given peptide. Each lymphocyte makes only one type of antigen receptor and can recognize only a very limited number of antigens. Because receptors differ on each clone of cells, the entire lymphocyte population has an enormous number of different, specific antigen receptors.

The TCR of most T lymphocytes is composed of an alpha and beta polypeptide chain, with constant regions located close to the cell surface and the part that binds to the antigenic pep tide of appropriate fit located away from the cell surface. The difference in structure of the distal regions of the alpha and beta chains allows the development of different clones of T cells. The TCR reacts with antigen in the context of MHC class I or II molecules on an APC (Fig. 1).

Fig1. Recognition of peptide-MHC by TCR. This ribbon dia gram is drawn from the crystal structure of the extracellular portion of  a peptide-MHC bound to a T cell antigen receptor (TCR) that is spe cific for the peptide displayed by the MHC molecule. The peptide can  be seen attached to the cleft at the top of the MHC molecule, and one  residue of the peptide contacts the variable (V) region of a TCR.  (Adapted from Bjorkman PJ: MHC restriction in three dimensions: a view of T cell receptor/ligand interactions, Cell 89:167-170, 1997.)

T cells recognize protein antigens in the form of peptide fragments presented at the cell surface by MHC I or II molecules. When the antigen-specific TCR on the T cell surface (specifically the zeta-beta chains) of the CD3 complex inter acts with the appropriate peptide-MHC complex, it triggers phosphorylation of the intracellular domains of the CD3 zeta chains. Subsequently, the zeta-associated protein 70 (ZAP-70) binds to the phosphorylated zeta chains and is activated.

Simultaneous colligation of the cell marker CD4 (or CD8) with the MHC class II (or I) molecule results in the phosphorylation of particular kinases. These events stimulate the activation of at least three intracellular signaling cascades. T cell activation also requires a second costimulatory signal (e.g., interaction between marker CD28 on T cells and marker CD80 on APCs). This interaction also triggers several intracellular signaling pathways.

 T Cell Activation

 T cell activation requires a minimum of two signals:

 • Signal 1 is delivered by the TCR-CD3 complex through interaction of the TCR α and β chains as they recognize peptide presented by a class I CD8+ T cell or a class II CD4+ T cell MHC molecule.

• Signal 2 is usually provided by the engagement of CD28 on the T cell with the costimulatory molecule CD80 or CD86 on the APC. The surface markers CD 137 and CD134 also provide costimulation to T cells.

The optimal combination of effector function, proliferation, and survival requires both signals. Delivery of signal 1 without costimulation, which often occurs in tumor-infiltrating lymphocytes, leads to anergy and apoptosis, which limits the anti-tumor response of the cells.

Activation of T cells can lead to the following:

 • Cell division • Cytokine secretion by T cells

 • Expression by T cells of antigens associated with activated state

Activated T cells frequently express activation antigens (Box 1). Expression of CD69 occurs within 12 hours of activation, followed by CD25 (IL-2 receptor) and CD71 (transferrin receptor) in 1 to 3 days. Alternatively, in the case of Tc cells, interaction with antigen through the specific TCR leads to destruction of target cells.

Box1. Screening for Congenital Immunodeficiencies

If a cell does not receive a full set of signals, it will not divide and may even become anergic. Peripheral T cells generally exist in a resting state (G0 or G1). T cell activation is a complex reaction involving transmembrane signaling and intracellular enzyme activation steps. It is through soluble cytokines that T cell regulation influences the action of other T cells, accessory cells, and nonimmune constituents. When activated by the proper signals, T cells may carry out one or more of the following functions:

 1. Proliferation

 2. Differentiation

3. Production of cytokines

 4. Development of effector function

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مواضيع ذات صلة

Antigen Processing and Antigen Presentation to T Cells

Cytotoxic T Lymphocytes

T Regulatory Lymphocytes

Helper T Lymphocytes

Sites of Lymphocyte Development

MONOCYTE-MACROPHAGE DISORDERS

Clinical Features of HIV Infection and AIDS

Pathogenesis of AIDS

Congenital Immunodeficiencies : Defects in Lymphocyte Activation and Function

Congenital Immunodeficiencies: Defects in Lymphocyte Maturation

Congenital Immunodeficiencies: Defects in Innate Immunity

Congenital (Primary) Immunodeficiencies