Ontogeny of IgM-bearing cells and changes in the immunoglobulin M-like protein level (IgM) during larval stages in sea bass (Dicentrarchus labrax)

The ontogenetic development of IgM positive cells was described as demonstrated by immunofluorescent staining (FITC) and flow cytometric analysis (FACS) of cells stained for IgM expression with a mouse anti-sea bass IgM monoclonal antibody. In addition, the immunoglobulin (IgM) level in larvae was measured using a sensitive ELISA and the development of the lymphoid organs was studied histologically. A small amount of IgM was detected by ELISA in the eggs, decreasing regularly until day 5 post-hatching, with no IgM being detected from day 5 to day 15. IgM was again detectable from day 15 to day 30 together with the detection of IgM positives cells by FACS analysis. During this period the thymus and the head kidney became lymphoid. From day 30 to day 95, the IgM level increased and correlated with the body weight of the larvae. These results show that the IgM detected from day 0 to day 5 derive from maternal fish while the IgM detected from day 15 are neo-synthesised IgM. Moreover, mature B cells and strong IgM synthesis are present in 50-day-old sea bass larvae when the lymphoid organs are fully developed.