Immunogenic and functional characterization of Toxoplasma gondii surface antigens (SAG1 and SAG2)

Toxoplasmosis is a zoonotic infection caused by the protozoan parasite Toxoplasma gondii. Among the different forms involved in the parasite’s life cycle, tachyzoites are the most infectious and known to infect a wide variety of tissues to produce inflammatory responses. Two of the most abundant cell surface proteins on the tachyzoite surface, SAG1 and SAG2, are known to play important role during the invasion of host cells. In order to explore the potential of using SAG1 and SAG2 antigens in diagnosis and vaccine development, a recombinant DNA for a chimeric SAG1/2 gene was constructed and the fusion protein was produced in the yeast Pichia pastoris. The sensitivity and specificity of the fusion protein as an antigen were confirmed through western blotting using serum from toxoplasmosis positive individuals. The immunogenicity of the recombinant protein was seen in immunized mice, where specific antibodies were detected. Splenocytes from immunized mice readily proliferate and secrete large amount of IFN-γ when stimulated with the total lysate antigen of tachyzoite, suggesting that a T helper 1 type immune response was induced. Immunized mice have a higher survival rate when challenged with virulent tachyzoites (RH strain) compared to control (non-protein immunized) mice. This suggests that the chimeric protein is potentially a good candidate for diagnostic and vaccine development against T. gondii. Since SAG1 and SAG2 are known to play a role in tachyzoite invasion, the effect of silencing these two genes using RNA interference on host tissue invasion was examined. The siRNA molecules were able to downregulate the expression of both SAG1 and SAG2 in tachyzoites when compared to controls. However, the silencing of SAG1 and SAG2 appears to have little effect on the invasion of host cells by tachyzoites. This suggests that host cell invasion by tachyzoites may not depend on SAG1 and SAG2 alone but other surface antigens and/or secreted proteins might be involved.