Functional and molecular characterization of teleost leukocytes.

The coupling of immunologically relevant in vitro assay systems, cell separation techniques, and the development of distinct clonal leukocyte lines has established the existence of T, B, natural killer, and accessory cell equivalents in teleosts. B cells are directly defined by monoclonal antibodies to teleost immunoglobulin (Ig) and identification of Ig H and L chain genes. As in mammals, fish B cells show Ig H-chain gene rearrangements, allelic exclusion, produce both membrane-bound and secreted forms of Ig, and transduce intracellular proliferative signals upon anti-Ig cross-linking. It has also been found that some fish B cells express a unique chimeric Ig chain with sequence homology to mammalian Ig delta. Teleost T cells are still indirectly defined as sIg- lymphocytes due to a lack of definitive surface markers. These mIg- lymphocytes are the responding cells in mixed leukocyte cultures, proliferate specifically to autologously processed and presented antigen, provide helper function for in vitro antibody responses, and produce interleukin-like factors upon activation. Recent identification of teleost T-cell receptor alpha and beta genes has now permitted the unequivocal genetic demonstration that some of these mIg- cells are bona fide T cells. It is anticipated that such long-term clonal cell lines will be indispensable tools for dissecting the physiology, biochemistry and molecular biology of teleost immune responses.