What are killer cells and what do they do?

Abstract

Cytotoxic effector function of the immune system is mediated predominantly by killer cells and with the exception of complement-mediated lysis all forms of immune-directed cell death are attributable to these cells. The heterogeneous collection of cells with cytotoxic properties ranges from the phylogenetically primitive granulocyte and natural killer (NK) cell lineages to the more complex and versatile macrophage and antigen-specific cytotoxic T lymphocyte. Most killer cells are capable of the dual functions of cytotoxicity and immunoregulation. Granulocytes, macrophages and NK cells constitute and effective ‘first line’ cellular defence against invasive microbial pathogens and emerging malignancies. Through direct cytotoxicity or by the secretion of cytokines, NK cells also prevent overproliferation of precursor cell populations, thereby exerting a more discriminating control over antigen-specific T and B cell responses. The ultimate refinement of the killer cell is the cytotoxic T cell which is directed by an antigen receptor comparable in specificity and diversity to the immunoglobulin molecule. The mechanisms underlying the initial interaction between killer cell and target and the subsequent lytic event are imperfectly understood. Although many cytokines with lytic properties have been identified and in some cases cloned, their relative importance and intricate interactions with other components of the immune system are still largely unknown. In addition to the prevention of infection and malignancy, killer cells of all lineages are important in the pathogenesis of human disease. Of particular interest is the role of macrophages, NK cells and cytotoxic T cells in autoimmunity. Although there is little evidence that these cells account for defective immunoregulation, there is a strong suspicion that killer cells are important mediators of tissue injury and organ dysfunction in these diseases.