Cytotoxic T Lymphocytes (CTL)

Cytotoxic T lymphocytes are lymphocytes that kill other ("target") cells.

Targets may include:

There is also evidence that CTL are active in some autoimmune disorders, e.g. help destroy the beta cells of the islets of Langerhans, leading to Type I diabetes mellitus.

Properties of CTLs

Most of them

An example will show the beauty and biological efficiency of CTLs.

Every time you get a virus infection, say influenza (flu), the virus invades certain cells of your body (in this case cells of the respiratory passages). Once inside, the virus subverts the metabolism of the cell to make more virus. This involves synthesizing a number of different macromolecules encoded by the viral genome.

In due course, these are assembled into a fresh crop of virus particles that leave the cell (often killing it in the process) and spread to new target cells.

Except while in transit from their old homes to their new, the viruses work inside of your cells safe from any antibodies that might be present in blood, lymph, and secretions.

But early in the process, infected cells display fragments of the viral proteins in their surface class I molecules. CTLs specific for that antigen will be able to bind to the infected cell and often will be able to destroy it before it can release a fresh crop of viruses.

In general, the role of the CD8+ T cells is to monitor all the cells of the body, ready to destroy any that express foreign antigen fragments in their class I molecules.

Some CD4+ T cells can develop into CTLs, but they can attack only those cell types (e.g. B cells, macrophages, dendritic cells) that express class II MHC molecules. Virtually every cell in the body expresses class I MHC molecules, so CD8+ CTLs are not limited in the targets they can attack.

Mechanisms of Killing

There are two.

Perforin/Granzyme Killing

CTLs have cytoplasmic granules that contain the proteins perforin and granzymes. When the CTL binds to its target, the contents of the granules are discharged by exocytosis.

Both in structure and function, the interaction of CTL and its target resembles the synapses of the nervous system.
Link to more details about the "immunological synapse"

FasL/Fas Killing

CTLs express on their surface molecules of a transmembrane protein, the death activator designated Fas ligand (FasL).

Most potential CTL targets express a receptor for FasL designated Fas.

When cytotoxic T cells recognize (bind to) their target,

Other Cytotoxic "Killer" Cells

There is another population of lymphocytes that function as "killer" cells but are not T cells. These so-called Natural Killer (NK) cells are discussed on a separate page. [Link to it.]

Unlike CTL, which are mediators of adaptive immunity; that is, learn to respond to a particular antigen, NK cells are part of the innate immune system.

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5 April 2011