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Two immunological reactions against foreign antigens can be distinguished: (1) the humoral immune response, involving the production of immunoglobulins by plasma cells; (2) the cell-mediated immune responses, mediated by thymus-derived cells. All jawed vertebrates show these two immunological pathways. In lower vertebrates, the existence of cell-mediated immunity has been shown by transplantation experiments resulting in allograft rejection. The basic phenomena observed are similar to those in mammals. Lymphocyte heterogeneity, particularly the participation of T-like lymphocytes in fish immunity, is now well established. T-like cells were defined as sIg − cells which proliferate in the presence of specific mitogens for mammalian T lymphocytes (Con A and PHA). The mixed leucocyte reaction (MLR) has been demonstrated in all vertebrates, from shark to mammals. The in vitro anti-hapten antibody response to a T-dependent antigen requires the participation of sIg + and sIg − cells and monocytes. In addition, fish sIg − lymphocytes specifically proliferate in the presence of processed and presented antigen, reminiscent of the mammalian classical T-cell/B-cell/APC collaboration. Hitherto, only a few attempts to generate antibodies, reacting exlusively with fish T-cell markers, have been successful. The use of molecular tehniques such as PCR and library screening has allowed the identification of cDNA clones encoding TCR-α and -β chains in several fish species. TCRδ-like chains not yet detected in teleosts have been isolated in shark, suggesting that TCRγδ-like receptors should exist in all gnathostomes. Whereas the teleost TCRβ locus shows a translocon organisation comparable to that of its mammalian counterpart, elasmobranch TCRβ genes are arranged in multiple clusters. As in mammals, the TCR repertoire in fish species appears to be considerable. |
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