Abstract
Rainbow trout Oncorhynchus mykiss surviving proliferative kidney disease ( PKD ) are reported not to develop the disease upon re‐exposure. However, the mechanisms involved in the immune response to re‐exposure are unknown. We examined disease susceptibility and the immune response of naive 1+ rainbow trout when first exposed to Tetracapsuloides bryosalmonae in comparison with that of 1+ rainbow trout re‐exposed to T . bryosalmonae . PKD pathogenesis, parasite burden and transcriptional signatures of the host immune response were assessed at 10, 25 and 40 d.p.e (days post‐exposure). In addition, we evaluated the presence of IgM+ B cells in the blood and the posterior kidney. The exposure of 1+ rainbow trout to T. bryosalmonae for the first time resulted in 100% infection prevalence, high parasite burdens and severe clinical PKD , while re‐exposed fish were either able to avoid reinfection completely or mount an earlier and more efficient adaptive‐type immune response. This response was characterized by a greater amount of IgM+ B cells in the blood and elevated mRNA levels of secretory IgM in the posterior kidney which minimized pathogen burden and kidney inflammation. Our findings suggest that rainbow trout is able to develop immune protection against T. bryosalmonae .