Immunoproteomic analysis of the antibody response obtained in Nile tilapia following vaccination with a Streptococcus iniae vaccine

Streptococcus iniae is one of the most economically important Gram-positive pathogens in cultured fish species worldwide. The USDA-ARS Aquatic Animal Health Research Unit developed a modified (contains concentrated culture supernatant) S. iniae bacterin that has been demonstrated to be efficacious, and protection is mediated by specific anti-S. iniae antibodies. Although effective, the specific vaccine components important for efficacy are not known. In the present study, an immunoproteomic approach was utilized to identify whole-cell lysate proteins of S. iniae that stimulated specific antibody production in Nile tilapia (Oreochromis niloticus) following vaccination. Groups of tilapia were vaccinated by intraperitoneal injection with the modified S. iniae bacterin or were mock-vaccinated, and at 30 d post-vaccination sera samples were obtained from individual fish. Vaccination of tilapia with the S. iniae vaccine stimulated significantly elevated specific antibody responses against proteins of the bacterium and passive immunization of tilapia with this serum demonstrated the antibodies were highly protective. Whole-cell lysate proteins of S. iniae were separated by 2D-PAGE and were probed with a pooled serum sample from vaccinated tilapia. A total of eleven unique immunogenic proteins were positively identified by mass spectrometry. Based on research conducted on homologous proteins in other Streptococcus spp., antibodies specific for three of the identified proteins, enolase, glyceraldehyde-3-phosphate dehydrogenase, and fructose-bisphosphate aldolase, are likely involved in protection from streptococcosis caused by S. iniae.