Cytotoxic granules house potent proapoptotic toxins critical for antiviral responses and immune homeostasis

General Introduction Cytotoxic lymphocytes and apoptosis The immune system of high-order organisms is a highly specialized compartment that eliminates transformed cells and cells infected with viruses or bacteria through a controlled process of cell-mediated cytotoxicity. The immune cells responsible for mediating cell death are collectively called cytotoxic lymphocytes (CLs) and are made up of natural killer (NK) cells and cytotoxic T lymphocytes (CTL). CLs are distinguished primarily by their respective mechanism of antigen recognition. NK cells form part of the innate immune response, a generalized first line of defense. NK cells are generally CD3aeCD56+lymphocytes that recognize and respond to abnormal cells through an imbalance of facilitatory and inhibitory receptors (Bottino et al., 2004; Moretta et al., 2004). CTLs formpart of the adaptive immune response, a more specific response that is generated subsequent to and as a consequence of the innate response. These cells use their clonotypic T-cell receptors (TcRs) to recognize a peptide antigen presented on the major histocompatability complex (MHC) proteins on the surface of the target cell. CTLs can be identified on the basis of expression of CD3 and CD8 (CD3+CD8+) on their cell surface. In addition, some CD4+T cells (typically T-helper cells) can have limited cytotoxic capacity