Alpha-galactosylceramide enhances protective immunity induced by DNA vaccine of the SAG5D gene of Toxoplasma gondii

BACKGROUND Toxoplasmosis caused by the intracellular parasite Toxoplasma gondii (T. gondii) is a global epidemic parasitic disease. DNA vaccines play an important role in preventing the spread of toxoplasmosis. SAG family genes encoding particular surface proteins of T. gondii are the best candidates of DNA vaccine. As a member of SAG family genes, SAG5 gene has been proved to have better antigenic than SAG1. In addition, alpha-Galactosylceramide (α-GalCer) was used to be an adjuvant in malaria vaccine and received positive results. In this study, the effect of the DNA vaccine enhanced by α-GalCer was evaluated by immunizing BALB/c mice. METHODS In the present study, SAG5D gene of T. gondii was cloned, sequenced, and biologically characterized. BALB/c mice were randomly divided into five groups, including three experimental groups (pEGFP-C1-SAG5D, α-GalCer and α-GalCer/pEGFP-C1-SAG5D) and two control groups (PBS and pEGFP-C1), and were immunized intramuscularly three times. The levels of IgG antibodies and cytokine productions in mouse sera were determined by enzyme-linked immunosorbent assays (ELISA). Two weeks after the last immunization, all mice were challenged intraperitoneally with 1 × 10(4) tachyzoites of T. gondii and the survival time of mice was recorded. RESULTS A significant level of increase of IgG response against the soluble tachyzoite antigens (STAg) was detected by ELISA in experimental group. It revealed relatively high level of IFN-γ production by the spleen cells. There were higher productions of interleukin-4 (IL-4) in α-GalCer treated groups compared to control groups. Challenge experiment showed a longer survival period (11 days compared with 5 days in control) in SAG5D DNA vaccinated mice was found after a lethal challenge with T. gondii RH strain. CONCLUSIONS The present study suggested that T. gondii SAG5D was a novel and positive DNA vaccine candidate against toxoplasmosis. In addition, the adjuvant (α-GalCer) enhanced the body's cellular immune response and prolonged the survival time of mice after challenge.