ABSTRACT
Cytotoxic T cells kill their infected targets with great precision and neatness, by inducing apoptosis in the infected cell while sparing adjacent normal cells; this strategy minimizes tissue damage (Fig. 27.2). CD8 cytotoxic T cells release two types of preformed cytotoxin-the fragmentins or granzmes, which seem able to induce apoptosis in any type of target cell, and the protein perforin, which is thought to act as a translocator protein to enable granzymes to cross the membrane of the target cell (Fig. 27.3). A membrane-bound molecule, the Fas ligand, which is expressed on CD8 T cells as well as on some CD4 T cells can also induce apoptosis by binding to Fas on a limited range of target cells. Together, these properties allow the cytotoxic T cell to attack and destroy virtually any infected cell. Cytotoxic CD8 T cells also produce the cytokine interferon (IFN)-g; this cytokine inhibits viral replication, induces MHC class I expression, and also activates macrophages. As well as combating infection by viruses and intracytosolic bacteria, CD8 T cells are important in controlling some protozoal infections; they are crucial, for example, in host defense against Toxoplasma gondii, an intracellular protozoan.
