Streptococcus iniaeの免疫原性における莢膜の役割

Streptococcus iniae causes an acute systemic disease in Japanese flounder Paralichthys olivaceus, one of the important cultured fish in Japan. Although commercial S. iniae vaccines have been developed and used in farms since 2005, the immunoprotection mechanism in vaccinated fish has not been elucidated yet. To verify the involvement of capsular polysaccharides of S. iniae in immunoprotection, we compared the protective efficacy of formalin-killed cells (FKC) of S. iniae NUF631 (capsulated) with those of its isogenic capsular-deleted mutants in S. iniae infection of Japanese flounder. As a result, high protection was achieved by immunization with NUF631 FKC but not with mutant FKC. Viable count of intravenously inoculated NUF631 decreased in the kidney of flounder immunized with NUF631 FKC, and NUF631 cells opsonized with anti-NUF631 flounder serum elevated the phagocytic activity and reactive oxygen species production of flounder peritoneal macrophages. In electron microscopic examination, electronically dense materials were observed on the capsule of NUF631 cells pretreated with the antiserum, and in western blot analysis flounder antibody was detected in the antiserum components bound to NUF631 cells. These findings indicate that capsular polysaccharides are important protective antigens and anti-capsule antibody plays a protective role as an opsonin in flounder S. iniae infection