Abstract
Though some freshwater fish have been successfully cultivated in saline-alkali water, the survival rates of freshwater fish are greatly affected by different saline-alkali conditions. The mechanisms of immune adaptation or immunosuppression of freshwater fish under different saline-alkali stress remain unclear. Here, grass carp were exposed to 3‰ and 6‰ salinity for 30 days. It was observed that salinity treatments had no obvious effects on survival rates, but significantly increased the percent of unhealthy fish. Salinity treatments also increased the susceptibility of grass carp against Flavobacterium columnare infection. The fatality rate (16.67%) of grass carp treated with 6‰ salinity was much lower than that treated with 3‰ salinity (40%). In the absence of infection, higher numbers of immune-related DEGs and signaling pathways were enriched in 6‰ salinity-treated asymptomatic fish than in 3‰ salinity-treated asymptomatic fish. Furthermore different from salinity-treated symptomatic fish, more DEGs involved in the upstream sensors of NOD-like receptor signaling pathway, such as NLRs, were induced in the gills of 6‰ salinity-treated asymptomatic fish. However in the case of F. columnare infection, more immune-related signaling pathways were impaired by salinity treatments. Among them, only NOD-like receptor signaling pathway was significantly enriched at early (1 and/or 2 dpi) and late (7 dpi) time points of infection both for 3‰ salinity-treated and 6‰ salinity-treated fish. Besides the innate immune responses, the adaptive immune responses such as the production of Ig levels were impaired by salinity treatments in the grass carp infected with F. columnare. The present study also characterized two novel NLRs regulated by salinity stress could inhibit bacterial proliferation and improve the survival rate of infected cells. Collectively, the present study provides the insights into the possible mechanisms why the percent of unhealthy fish in the absence of infection and mortality of grass carp in the case of F. columnare infection were much lower in the 6‰ salinity-treated grass carp than in 3‰ salinity-treated grass carp, and also offers a number of potential markers for sensing both environmental salinity stress and pathogen.