Immunomodulatory and cellular effects of Aeromonas salmonicida melanin in Atlantic salmon (Salmo salar).

Aeromonas salmonicida subspecies salmonicida is a Gram-negative fish pathogen that synthesizes and secretes melanin, a dark pigment with a complex molecular structure. Melanin is formed by polymerized phenolic and/or indole monomers, which possess antioxidant, radical scavenging, photo-protective, and even semiconducting properties. The role of melanin in A. salmonicida pathogenesis is unknown. In this study, we evaluated the effect of A. salmonicida melanin on Atlantic salmon (Salmo salar) head kidney and spleen. Atlantic salmon were intraperitoneally injected with A. salmonicida melanin (100 μg/100 g of fish). Fish injected with Tris-OH 20 mM pH 8.0 buffer were used as a control. Head kidney tissue samples were taken at innate immune response (6, 24, 48 h post-injection (hpi)), adaptive immune activation (168 hpi), and metabolic adjustment phase (336 hpi) for transcriptomics and histological analysis. Transcriptomic analysis followed an innate immune response kinetics with a peak of dysregulated genes at 24 hpi, followed by a decrease at 48 hpi. At 7 days post-injection (168 hpi), a second peak of dysregulated genes was observed, and at 14 day post-infection (336 hpi) the gene expression decreased. These results suggests that A. salmonicida melanin is not toxic to Atlantic salmon, remains in the animal, and induction of innate and adaptive immunity. Gene ontology analysis, indicate that A. salmonicida melanin induced an anti-viral innate and T-cytotoxic cell adaptive immune response. Immunohistochemistry showed that melanin promote the synthesis of CD10 and IgM in spleen and head kidney, suggesting that A. salmonicida melanin induce lymphocyte division. In summary, melanin produced by A. salmonicida appears to promote the division of antigen-presenting cells, potentially supporting the formation of cellular niches for intracellular infection and growth.