Antigenic characterization of the salmonid pathogen Piscirickettsia salmonis

Piscirickettsia salmonis, the etiological agent of salmonid rickettsial septicemia, was purified from infected immortal chinook salmon (Oncorhynchus tshawytscha) embryo cells by a combination of differential and Percoll density gradient centrifugation. Immune sera from rabbits immunized with purified whole cells of P. salmonis reacted with four protein antigens and two carbohydrate antigens with relative molecular sizes of 65, 60, 54, 51, 16, and approximately 11 kDa, respectively. The carbohydrate antigens appear to be mainly core region lipo-oligosaccharide with lesser amounts of lipopolysaccharide. Serum from convalescent rainbow trout (Oncorhynchus mykiss) and coho salmon (Oncorhynchus kisutch) reacted with several minor immunoreactive protein antigens between 10 and 70 kDa in size and a carbohydrate antigen with a relative molecular size of approximately 11 kDa. The salmonid immune system did not appear to elicit a strong humoral response against this intracellular pathogen. Indirect immunofluorescence microscopy, immunogold transmission electron microscopy, and biotin labeling of intact P. salmonis cells suggest that the immunoreactive antigens identified with rabbit antisera are surface exposed and differ significantly from those identified with salmonid antisera.