Immune response of yellowtail kingfish, Seriola lalandi, to infection with Photobacterium damselae subsp. damselae

Yellowtail kingfish (Seriola lalandi, YTK) has many characteristics suitable for aquaculture, but infectious diseases have the ability to hinder the development of the industry. Photobacterium damselae subsp. damselae (Pdd) is an emerging pathogen for S. lalandi and other cultured fish species, causing severe economic losses. A better understanding of the immune response of YTK to Pdd infection is crucial for the development of effective approaches to disease management. In the present work, we evaluated the immunopathological response of YTK to Pdd infection and examined differences between those fish that survived or succumbed to infection. Mortality of juvenile fish (36-38 g) increased with increasing infectious dose and the LD50 was estimated to be 8.0×104 CFU/fish. Infection with Pdd induced both innate and adaptive immune responses in YTK. Compared to control (non-challenged) fish, the tissues of infected fish showed major histological changes, including infiltration of inflammatory cells, macrophage aggregations, and cell necrosis. Challenge infection induced significant increases in the expression of both pro-inflammatory and anti-inflammatory cytokine genes in the blood after three days. Fish which did not survive infection had a higher expression of tissue cytokine genes than those which survived up to eight days.