IMMUNOCYTOCHEMICAL DETECTION AND CYTOMORPHOLOGY OF LYMPHOCYTE SUBPOPULATIONS IN A TELEOST FISH DICENTRARCHUS-LABRAX

The monoclonal antibodies DLT15 and DLIg3 directed against thymocytes and serum immunoglobulins of the sea bass (Dicentrarchus labrax, L.) w ere used to study cells from the thymus, head kidney, spleen, gut-asso ciated lymphoid tissue and peripheral blood leukocytes of this fish by immunofluorescence and pre-embedding immunoelectron microscopy. Immun ofluorescence and flow cytometry of leukocyte fractions revealed a lar ge number of DLT15-positive cells in the thymus (similar to 80%) and i ntestine (similar to 55%) and fewer cells in the spleen (similar to 7% ), head kidney (similar to 6%) and peripheral blood (similar to 3%). D LT15-positive cells had two main morphologies, both detectable among t hymocytes: a large round heterochromatic nucleus with light sparse cyt oplasm (type a) and an irregular and heterocromatic nucleus with cytop lasm rich in polysomes and mitochondria (type b). Type b was most repr esented in spleen, head kidney, intestine and blood. We suggest that t he type b morphology represents more differentiated lymphocytes. Flow cytometry revealed numerous DLIg3 positive cells in the head kidney (s imilar to 33%), spleen (similar to 30%) and peripheral blood leukocyte s (similar to 21%) and fewer positive cells in the intestine (similar to 3%) and thymus (similar to 2%). DLIg3-positive cells had the morpho logy of lympho cytes (with a large round nucleus) or macrophages in al l tissues. Plasma cells lacked membrane immunoreactivity. This is the first ultrastructural characterisation of putative T- and B-lymphocyte subpopulations in a fish species; these subpopulations are differenti ally distributed in teleost lymphoid organs.