The effect of early exposure of Coho salmon (Oncorhynchus kisutch) eggs to the p57 protein of Renibacterium salmoninarum on the development of immunity to the pathogen

Bacterial kidney disease (BKD) in salmonids is caused by Renibacterium salmoninarum (Rs) which is transmitted vertically through salmonid eggs. This study investigated the effect of vertical transmission of one protein of Rs, p57, on the development of the immune system of coho salmon (Oncorhynchus kisutch) and on the susceptibility of the fish to subsequent challenge with Rs. It was shown that exposure of coho salmon at the egg stage to the p57 protein of Rs induces at least partial immunosuppression. Coho salmon eggs were injected with varying amounts of p57. The eggs were fertilized, the progeny were vaccinated against Rs, and then challenged with live, virulent Rs. Fish that had been injected as eggs with 100 ng of p57 demonstrated a significantly higher cumulative percent mortality (50%) than those from the saline-injected eggs (controls) (14%). The same fish also produced significantly lower levels of antibodies against p57, although not against whole Rs cells, than those derived from saline-injected eggs. It appeared, therefore, that there was some specificity to the immune suppression. A decreased proportion of phagocytic cells isolated from fish exposed as eggs to 100 ng p57, showed respiratory burst activity (0.4%) compared with the proportion of cells isolated from the saline-injected (control) group (16.5%). These data suggest that early (egg stage) exposure of coho salmon to p57 results in long-term immunosuppression and a decreased ability in the animal to resist subsequent challenges with Rs. (C) 1996 Academic Press Limited