Antimicrobial responses of teleost phagocytes and innate immune evasion strategies of intracellular bacteria

•The reactive oxygen response and immune evasion of intracellular bacteria.•The nitric oxide response and immune evasion of intracellular bacteria.•NRAMP/Scl11a natural resistance to infection with intracellular bacteria.•Iron deprivation and tryptophan degradation and intracellular bacteria.•Mechanisms of innate immune evasion by M. marinum.

During infection, macrophage lineage cells eliminate infiltrating pathogens through a battery of antimicrobial responses, where the efficacy of these innate immune responses is pivotal to immunological outcomes. Not surprisingly, many intracellular pathogens have evolved mechanisms to overcome macrophage defenses, using these immune cells as residences and dissemination strategies. With pathogenic infections causing increasing detriments to both aquacultural and wild fish populations, it is imperative to garner greater understanding of fish phagocyte antimicrobial responses and the mechanisms by which aquatic pathogens are able to overcome these teleost macrophage barriers. Insights into the regulation of macrophage immunity of bony fish species will lend to the development of more effective aquacultural prophylaxis as well as broadening our understanding of the evolution of these immune processes. Accordingly, this review focuses on recent advances in the understanding of teleost macrophage antimicrobial responses and the strategies by which intracellular fish pathogens are able to avoid being killed by phagocytes, with a focus on Mycobacterium marinum.