Abstract

Renibacterium salmoninarum is a facultative intracellular pathogen often found in host phagocytes where it appears to successfully avoid the fish host's immunological defenses. The objective of this investigation was to determine whether the soluble extracellular protein produced by R. salmoninarum may contribute to the immunomodulation in bacterial kidney disease via inhibition of host phagocyte respiratory burst and/or phagocytosis mechanisms. Splenic cells from healthy adult brook trout (Salvelinus fontinalis) were incubated with two different concentrations of extracellular protein or viable R. salmoninarum. Splenic cell cultures were then evaluated for respiratory burst activity via flow cytometry with the dichlorofluorescin diacatate assay and for phagocytosis via light microscopic assessment of microsphere engulfment. Respiratory burst activity was inhibited in all experimental groups as compared to controls, while no differences were noted in phagocytic abilities in cells among the treatment groups.