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Background   Streptococcus iniae  is a Gram-positive bacterium and a severe pathogen of a wide range of farmed fish.  S. iniae  possesses a virulence-associated streptolysin S cluster composed of several components, one of which is SagE. SagE a transmembrane protein with one major extracellular region named ECR. This study aimed to develop a SagE-based DNA candidate vaccine against streptococcosis and examine the immunoprotective mechanism of the vaccine.    Results  We constructed a DNA vaccine, pSagE, based on the  sagE  gene and examined its immunological property in a Japanese flounder ( Paralichthys olivaceus ) model. The results showed that at 7 days post-vaccination, expression of SagE at transcription and translation levels was detected in the tissues of the vaccinated fish. After challenge with  S. iniae  at one and two months post-vaccination, pSagE-vaccinated fish exhibited relative percent survival (RPS) of 95% and 88% respectively. Immunological analysis showed that (i) pSagE significantly upregulated the expression of a wide range of immune genes, (ii) pSagE induced the production of specific serum antibodies that bound whole-cell  S. iniae , and (iii) treatment of  S. iniae  with pSagE-induced antibodies blocked bacterial invasion of host cells. To localize the immunoprotective domain of SagE, the ECR-expressing DNA vaccine pSagEECR was constructed. Immunization analysis showed that flounder vaccinated with pSagEECR exhibited a RPS of 68%, and that pSagEECR induced serum antibody production and immune gene expression in a manner similar to, though to lower magnitudes than, those induced by pSagE.    Conclusions  We in this study developed a DNA vaccine, pSagE, which induces highly protective immunity against  S. iniae . The protective effect of pSagE is probably due to its ability to elicit systemic immune response, in particular that of the humoral branch, which leads to production of specific serum antibodies that impair bacterial infection. These results add insights to the immunoprotective mechanism of fish DNA vaccine.

SagE induces highly effective protective immunity against Streptococcus iniae mainly through an immunogenic domain in the extracellular region