Modulation of rainbow trout (Oncorhynchus mykiss) intestinal immune gene expression following bacterial challenge

The mucosal immune systems of fishes are still poorly understood, and defined models for studying natural host-pathogen interactions are lacking. The objective of this study was to evaluate different challenge models and pathogens to examine the magnitude of change in intestinal immune gene expression. Rainbow trout were exposed by immersion to Yersinia ruckeri or by intraperitoneal injection with Flavobacterium psychrophilum. At 3, 9, or 10 days post-challenge, pathogen load was quantified by plate count and intestinal tissue was removed and immune gene expression measured by real-time PCR. In general, the magnitude of infection was correlated with change in immune gene transcript abundance. We found that messages for the innate immune molecules, SAA, IL-8, INF-γ and TNF-α, as well as the message for IgM, were up-regulated in intestinal tissue in both challenge paradigms. A >250-fold increase was observed in SAA and 20-fold increase of IL-8 gene transcript abundance occurred on day 10 following challenge with F. psychrophilum. Within individual fish, there was a positive correlation between bacteria load in the spleen and the increase of immune gene message between 3 and 10 days post-infection. These findings demonstrate that measurable changes in immune gene expression occur in the intestine of rainbow trout following bath challenge with Y. ruckeri or injection challenge with F. psychrophilum.