Enhanced immunogenicity of Renibacterium salmoninarum in chinook salmon after removal of the bacterial cell surface-associated 57 kDa protein

A study was conducted to determine the effect of endogenous serine protease activity on the immunogenicity of Renibacterium salmoninarum cells in chinook salmon Oncorhynchus tshawytscha. Salmon were immunized with either R. salmoninarum cells possessing p57 (p57 super(+)) or substantially depleted of p57 (p57 super(+)). The resultant antisera examined by whole cell ELISA and immunoblot procedures using p57 super(+), p57 super(-), proteinase-K-treated, and periodate-treated whole cells. These analyses revealed that the removal of p57 by the endogenous serine protease significantly enhanced the immunogenicity of the cell, resulting in a 20-fold increase in detectable antibody titers. The bulk of this antibody activity was directed at sites blocked by the presence of the p57 molecule. Furthermore, proteinase-K and periodate treatment of R. salmoninarum cells revealed that the increased antibody activity almost exclusively reacted with carbohydrate moieties on the p57 super(-) cell.