B cells in fish were recently proven to have potent innate immune activities like macrophages. This inspired us to further explore the innate nature of B cells in fish. Moreover, antimicrobial peptides (AMPs) are representative molecules of innate immunity, and they can modulate the functions of macrophages. These make fish an appropriate model to study the interactions between B cells and AMPs. Interestingly, the results in this study revealed that the IgM+ and IgT+ B cells of rainbow trout could express multiple AMP genes, including four cathelicidin genes and one β-defensin gene. The expression levels of the cathelicidin genes were obviously higher than that of the β-defensin gene. Further studies revealed that intracellular, extracellular, in vitro, and in vivo stimulations could significantly increase the expression of the cathelicidin genes in trout IgM+ and IgT+ B cells but not the expression of the β-defensin gene, indicating that cathelicidin peptides are the main innate immune effectors of trout B cells. More interestingly, we found that cathelicidin peptides could significantly enhance the phagocytic, intracellular bactericidal, and reactive oxygen species activities of trout IgM+ and IgT+ B cells, a phenomenon previously reported only in macrophages, and these activities might also be mediated by the P2X7 receptor. These results collectively suggest that B cells play multiple roles in the innate immunity of fish, and they provide new evidence for understanding the close relationship between B cells and macrophages in vertebrates.

Based on microbiological and histopathological examinations and DNA sequencing, several outbreaks of mycobacteriosis in the reared sturgeons, including Chinese sturgeon (Acipenser sinensis Gray) and Amur sturgeon (Acipenser schrencki), were identified during 2009 to 2010. Forty-nine isolates of non-tuberculous mycobacteria(NTM) were isolated from 19 diseased sturgeons. In total, seven species of Mycobacterium were identified, namely, Mycobacterium chelonae, Mycobacterium marinum, Mycobacterium gordonae, Mycobacterium fortuitum, Mycobacterium szulgai, Mycobacterium arupense and Mycobacterium porcinum. Among them, M. marinum was found to be more prevalent (89.5%) compared with the other mycobacterial species. When two molecular biological methods, PCR-DGGE (denaturing gradient gel electrophoresis) analysis and rpoB gene library sequencing, were used to analyse the mycobacterial DNAs extracted from the diseased fish tissues, mixed infections of two or three mycobacterial species were found being the predominant infection form (94.7%) in sturgeon mycobacteriosis. M. marinum was the only one species that caused sturgeon mycobacteriosis alone. Virulence assay showed that M. marinum possessed stronger pathogenicity to zebrafish killing 100% of fish in 28 days at 10 3 cfu/fish than the other species. These results suggested that M. marinum is the major pathogenic bacteria in sturgeon mycobacteriosis. To the best of our knowledge, this study is the first report on mycobacteriosis in farmed Chinese and Amur sturgeons as well as the first isolation of M. porcinum and M. arupense from fish.