Differential gene expression and immune response of Nile tilapia (Oreochromis niloticus) challenged intraperitoneally with Photobacterium damselae and Aeromonas hydrophila demonstrating immunosuppression

Abstract Nile tilapia, Oreochromis niloticus, one of the most important aquatic species in Egypt, has suffered intensive losses from bacterial pathogens; Aeromonas hydrophila and Photobacterium damselae during the onset of the summer season. The immune mechanisms involved in the infection of P. damselae in fish are still poorly recognized. In this study, O. niloticus intraperitoneally injected by two bacterial inocula (1.7 × 105 CFU ml−1 for A. hydrophila and 2 × 106 CFU ml−1 for P. damselae). Blood was sampled after 1, 3 and 7 days of each bacterial injection to evaluate total, differential leukocytic counts, and alterations in cellular innate immunity parameters; phagocytic activity, respiratory burst, serum lysozyme, bactericidal activity, complement system, and pro-inflammatory cytokines. Simultaneously, the mRNA expression levels of three immune-relevant genes (TLR-7, NFκB, MHC-IІα) and four pro-inflammatory cytokines related genes (IL-6, IL-8, CC-chemokine, IL-1β, and TNF-α) were evaluated in the spleen and head-kidney using q-PCR. Our data implied that tested immunological parameters and its relevant genes expression showed distinct patterns. In general, a notable increase in the leukocyte counts and innate immune responses, mainly in the respiratory burst, lysozyme, and complement (C4), but with a marked decrease in the phagocytic activity, principally on days 3 and 7 of challenge experiment. Moreover, transcript levels of immune-related genes (IL-6, IL-8, CC-chemokine, IL-1β, TLR-7, and NF-κB) were significantly down-regulated among different time points of the challenge experiment. A positive correlation was noted between gene expression obtained and tested phagocytic activity, implying that these pathogens can subvert the immune defense of the host. Together, these results symbolize the disparity in the kinetics of inherent immunity responses in peripheral blood and the molecular mechanism in immune organs used for combating these bacterial pathogens.