Dental Microbiology #211IMMUNOLOGY

Lecture 5

Cellular Immunity: The functions of T cells

Topics

• The Ag-specific T cell receptor

• Distinct populations of T cells

•The T effector cells- what do they do and how

Cell-mediated immunity is operated by T lymphocytes, and by antigen-presenting cells (APC)

The T lymphocyte

The T lymphocytes undergo maturation in the thymus from undifferentiated lymphoid progenitors that arrive from the bone marrow. T cells are characterized by the expression of antigen-specific T cell receptors (TCR).

T cells produce and express only the cell-bound molecules.

Structure of the T cell Receptor Fig. 1

The TCR is composed of two polypeptide chains: the (the L chain counterpart) and the (the H chain counterpart of Ig). The extracellular portion of each chain has a variable (V) and constant (C) domain. Both chains have carbohydrate moieties (blue hexagons)

Each TCR molecule has only one binding site.

Two classes of T lymphocytes

Following a complex process of maturation in the thymus, T cells separate into two distinct populations:They are called CD4+ and CD8+ T cells.

Mature T cells express either the CD4 or the CD8 molecule on their surface but not both, and perform distinct biologic functions.

CD4 and CD8 act as co-receptors Fig 2

The CD4 and CD8 molecules are called co-receptors because, when the T cell binds the Ag-MHC complex, they interact both with the TCR on the T cell surface and with the MHC on the APC

Effector T cells

The binding of Ag peptide-MHC complex present on an APC to the TCR starts a chain of intracellular signaling events that results in the activation, proliferation and conversion of a resting T cell into an effector T cell and a memory T cell

There are 3 types of effector T cells:CD4+ T helper (Th)-1CD4+ T helper (Th)-2

CD8+ Cytotoxic T lymphocytes (CTL)

The helper cells help to organize the type and direction of the immune response through the production of small molecules with powerful and diverse biologic functions, called cytokines.

CTL act by direct killing of the cells on the surface of which they encounter foreign Ag-MHC class-I

Th-1, Th-2 and CTL differ markedly in biologic function and in the type of cytokines they produce

Properties of Th-1 cells

CD4+ Th-1 cells produce the cytokines Interleukin-2 (IL-2), Interferon (IFN) and Tumor Necrosis Factor and (TNF and )"at the site of infection. A typical effect of Th-1 cell activation is the defensive mechanism against infection with tubercle bacilli. Th-1 cells are also involved in the reaction to foreign bodies called granulomas in which concentric layers of T cells, macrophages and fibroblasts surround and isolate the foreign pathogen

Effector function of T lymphocytes Fig. 3

Granuloma Fig 4

A core composed of infected macrophages and concentric layers of healthy macrophages, T cells and fibroblasts

Properties of Th-2 cells

In order to be able to produce Ab, most B cells require T cell help.

CD4+Th-2 cells produce the cytokines interleukin-4, 5, 6, 10 and 13 (IL-4, IL-5, etc). IL-4, 5 and 6 provide the Th-2 cells with the ability to help B cells to produce antibody.

Effector function of T lymphocytes Fig. 5

Plasma cell

Th2 cell interacts with Ag MHC-class-II on B cell surface and releases IL-4, IL-5, IL-6.

B cell divides then differentiates to become a plasma cell.

T cell help for Ab formation by B cells Fig 6

B cells are also APC.They pick up Ag via the BCR, internalize,process and load the fragments onto MHC II.Ag-MHC at the surface presents Ag to a Th cellThe Th2 cells secreteIL4, IL5, IL6 etc which induces the B cell to produce Ab.

Properties of CTL

CD8+CTL kill virus-infected cells and also some tumour cells, by direct binding to the MHC class-I-Ag peptide complex expressed on the surface of the target cells.

The killing occurs through the focused release of toxic molecules called perforins and granzymes which penetrate into the target cell and destroy it.

Effector function of T lymphocytes Fig. 7

Cytotoxic Molecules of CD8+ T cells Fig 8

Perforin as well as several other cytotoxic molecules are stored in intracellular granules. When a CD8+ T cell recognizes its target, the perforin molecules released from the granules polymerize on the surface of the target cell and form pores

One CTL kills multiple targets Fig 9

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