Biology of Disease

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Memb

rane

Memb

rane

TCR

CD8

MHC class I

Virus infected cell

Tc (CD8+)

Viral peptide

B)

Figure 4.24 (A) Molecular model showing the recognition of a MHC Class I-peptide complex by a TC cell receptor. PDB file 1NAM. (B) A

schematic showing the interactions between a virus-infected cell and a TC,as described in the text. TCR, T cell receptor.

A)

The TC cell is activated when the T cell receptor recognizes the complex of MHC

Class I protein with its bound virus derived peptide and binds to it (Figure 4.24

(A)and(B)). An accessory protein, CD8, is also involved in this interaction as

it also binds to the MHC protein. Once bound, the TC cell is stimulated into

a series of mitotic divisions to form a clone of cells, which then differentiate

into cytotoxic T lymphocytes (CTL). These cells resemble TC cells but their

cytoplasm is more granular, owing to the presence of vesicles containing

cytotoxic proteins called granzymes and perforins. The CTL binds to a virus

infected cell using the same mechanism as the Tc cell, and then releases these

cytotoxic proteins which destroy the target cell. Cytotoxic T lymphocytes also

release IL-2 and IFN F which stimulate NK cells and macrophages to kill virus

infected cells.

Activation of TH cells

When stimulated by an immunogen, TH cells secrete cytokines required for

specific and nonspecific immune responses. Thus they are required for an

effective immune response to the whole range of infectious agents, from

viruses to multicellular parasites. In order to activate a TH cell, an immunogen

has to be taken up by an antigen presenting cell (APC). Here, its proteins are

hydrolyzed within endocytic vesicles and the resulting peptides of 12–19 amino

acid residues are attached to MHC encoded Class II proteins and transported