Comparative evaluation of live attenuated and killed tachyzoites as vaccine candidates for toxoplasmosis.

Toxoplasma gondii (T.gondii) an obligate intracellular protozoan, causes toxoplasmosis, leading to significant economic losses and posing serious public health challenges worldwide. Developing an effective vaccine for toxoplasmosis in humans remains difficult. In this study, we evaluated the protective immunity of tachyzoites local virulent strain exposed to 0.25 KGy as a live attenuated vaccine and 1.5 KGy as a killed vaccine. Swiss albino mice were immunized with three doses of each vaccine at 2-week intervals. Four weeks after the final immunization, mice were challenged with 2500 tachyzoites of RH HXGPRT (–) strain. Mice immunized with live attenuated vaccine showed a significant p < 0.05 increase in survival time (67.778 ± 26.4 days) compared to a control group, mice group immunized with killed tachyzoites, and the group injected with adjuvant. In the live attenuated vaccinated group, the mice percentage that still survived along six months of follow-up was 57.1% without any signs of acute toxoplasmosis. The infection reduction percentage was significantly higher (99.8%) in the live attenuated vaccine group, with the complete absence of tachyzoites in liver impression smears. Our results demonstrated that live attenuated vaccine triggered a strong immune response, marked by significantly elevated levels of IFN-γ, IL-12, and IL-17 cytokines, and increased percentages of CD4 (+) and CD8(+) T-lymphocytes. High levels of Toxoplasma-specific IgG were maintained. Histopathological analysis showed a complete absence of the parasite in the liver and spleen of mice vaccinated with live attenuated vaccine, with hepatic cells appearing normal and moderate aggregations of inflammatory cells observed in the hepatic portal area. In conclusion, the comparison between the use of killed and live attenuated tachyzoites proved that the use of live gamma-irradiated attenuated tachyzoites is effective in eliciting cellular and humoral immune responses against acute toxoplasmosis in mice, presenting potential candidate prepared by conventional approach for the development of a vaccine against toxoplasmosis. The sheep isolate strain used in our study represents a virulent strain with characteristics similar to those that circulate in natural populations, which makes it an appropriate candidate for vaccine development in livestock and then for humans.