Immunological responses in Atlantic salmon, Salmo salar L., against purified serine protease and haemolysins from Aeromonas salmonicida

Abstract. A method for purification of the 70‐KDa extracellular serine protease of Aeromonas salmonicida by hydrophobic chromatography and ion exchange is described. The purified protease, adsorbed onto mineral particles, was used for immunization of salmon. Other groups of salmon were immunized with particles coated with purified glycerophospholipid:cholesterol acyltransferase (GCAT) or with the acyltransferase complexed with LPS (GCAT‐LPS). Humoral immune responses assayed after 6 weeks by ELISA, showed relatively good responses against GCAT‐LPS, while titres of antisera against the purified protease and GCAT were scarcely above those of control sera. However, the antibody responses to each toxin were shown by Western blotting to be specific and qualitatively similar to responses seen in rabbits. The toxin preparations were also used (in combination with whole bacteria) for vaccination of salmon. On challenge 3 months later, only GCAT‐LPS elicited significant immunological protection. However, a more convincing protection was seen when total extracellular product was present in the vaccine.