IgM expression and B lymphocyte antibacterial activity of grass carp (Ctenopharyngodon idella)

Grass carp (Ctenopharyngodon idella) is a unique aquaculture species in China, which is mainly distributed in the Yangtze River, Pearl River and Heilongjiang River, and its aquaculture output has long been the highest in freshwater aquaculture in China. With the increase of culture scale and density, the environment continues to deteriorate, resulting in frequent outbreaks of a variety of diseases. Therefore, it is necessary to study its disease resistance mechanism. B lymphocytes are the key cells of adaptive immune response, and can secrete a variety of immunoglobulins which play an important role in the immune response. Respiratory burst is an oxygen-dependent killing mechanism of immune cells when pathogenic microorganisms invade the body. Previous studies suggested that fish B lymphocytes can participate in non-specific immune process through phagocytosis. However, the study of the phagocytic activity of B lymphocytes in C. idella is still not well known. To elucidate the immune function of IgM, real-time fluorescence quantitative PCR (qRT-PCR) was used to detect the expression of IgM in embryonic development, the distribution of IgM in different tissues and the transcription of IgM stimulated by bacteria. The results showed that during embryonic development, the expression of IgM was not significantly changed from cleavage stage to pre-hatching stage, and it significantly increased after hatching stage; IgM was distributed in all tested tissues, with the highest expression in the head kidney, and its expression was significantly up-regulated after the stimulation of different bacteria. B lymphocytes were isolated and purified from the C. idella blood and verified by Giemsa staining, RT-PCR and indirect immunofluorescence. The results of bacterial phagocytosis test showed that B lymphocytes had phagocytic ability to Aeromonas hydrophila. Furthermore, we also found that the production of reactive oxygen species (ROS) and nitric oxide (NO) in grass carp B cells could be significantly up-regulated after being infected by microbes. Finally, the serum conditioning assay showed that the release of ROS from B lymphocytes could be enhanced through serum conditioning, however, there was no influence on NO production. Taken together, the results of this study showed that B lymphocytes of C. idella have the potential of phagocytic ability to bacteria and can directly participate in anti-bacterial immunity by producing non-specific immunity such as respiratory burst.