Antimicrobial mechanisms of fish leukocytes

Early activation and coordination of innate defenses are critical for effective responses against infiltrating pathogens. Rapid engagement of immune cells provides a critical first line of defense soon after pathogen infiltration. Activation leads to a well-orchestrated set of events that sees the induction and regulation of intracellular and extracellular antimicrobial defenses. An array of regulatory mediators, highly toxic soluble molecules, degradative enzymes and antimicrobial peptides provides maximal protection against a wide range of pathogens while limiting endogenous damage to host tissues. In this review we highlight recent advances in our understanding of innate cellular antimicrobial responses of teleost fish and discuss their implications to cell survival, immunomodulation and death. The evolutionary conservation of these responses is a testament to their effectiveness against pathogen infiltration and their commitment to effective maintenance of host homeostasis. Importantly, recent developments in teleost fish systems have identified novel host defense strategies that may be unique to this lower vertebrate group or may point to previously unknown innate mechanisms that also play a significant role in higher vertebrate host immunity.