Fighting Virus and Parasites with Fish Cytotoxic Cells

Cell-mediated cytotoxic (CMC) activity is the main cellular immunological response to kill tumor cells, virus-infected cells and parasites (Groscurth, 1989). In mammalian species this is carried out by several leucocyte populations depending on the non-specific/innate and specific/adaptive immune response. Among the last ones, the CMC activity is carried out by cytotoxic T lymphocytes (CTLs), expressing the co-receptor CD8, after repeated antigen contact and restricted to major histocompatibility complex (MHC) I. Among the innate cytotoxic cells, acting without previous neither sensitization nor MHC I restriction, the most important are the natural killer (NK) cells, which consist on large granular lymphocytes (markers: CD16/56+CD8-). However, other cell types such as the lymphokine-activated killer cells (LAK), adherent lymphokine-activated killer cells (ALAK), antibody-dependent cytotoxic cells (ADCC), macrophages, neutrophils and acidophils are also responsible for innate CMC activity (Groscurth, 1989). This CMC activity has been described in all the vertebrate animals with substantial differences. In the case of fish, the first vertebrate group showing both innate and adaptive immune system, they are not an exception. However, deeper studies are needed to clearly understand the appearance and evolution of the fish cytotoxic cells and their activity from an evolutionary point of view. Furthermore, the great potential of aquaculture industry and lack of commercial antiviral and antiparasitic vaccines for fish make necessary to increase the knowledge on the CMC activity of fish.