CLONING AND REGULATION OF THE MAJOR HISTOCOMPATIBILITY CLASS I ALPHA GENE IN THE TELEOST FISH GILTHEAD SEABREAM

Major histocompatibility complex (MHC) genes encode key molecules in the immune response. Antigens properly processed are expressed in the membrane bound to either class I or class II MHC proteins and then recognized by lymphocytes through the cell-surface T-lymphocyte receptors (TCR). However, contact with the CD8 (in cytotoxic T lymphocytes) or CD4 (in helper T lymphocytes) co-receptors are necessary for triggering the adaptive immune response leading to pathogen elimination. MHC genes have been cloned in several species, representing all the vertebrate groups. From an evolutionary point of view, the adaptive immune response appears in fish and the presence of MHC and MHC-related genes has been evidenced in all species studied. In teleost or bony fish MHC genes are not clustered, unlike in cartilaginous fish and tetrapods, indicating differences in evolution. Therefore, a study of the teleost fish MHC genes is essential for understanding the importance and significance of this genetic divergence; at the same time, the knowledge will add to the recent advances made in fish immunology. Since the early nineties when fish MHC genes were first cloned, numerous sequences have been published in many bony fish species but mainly for evolutionary purposes. Interestingly, little information exists regarding their regulation and function. Thus, further characterization of the fish MHC genes at different levels will be a great help for understanding the innate-adaptive immune system interactions.