Immunogenicity of an in-silico designed multi-epitope DNA vaccine encoding ROP21 and ROP29 of Toxoplasma gondii against both acute and chronic toxoplasmosis in BALB/c mice

Background: Toxoplasma infections are highly prevalent worldwide and can cause serious complications in immunocompromised individuals and lead to congenital infections in neonates. Despite ongoing efforts to develop T. gondii vaccines, none have been developed. A potential target, the ROP21 protein catalyzes the phosphorylation of additional protein substrates and ROP29 is critical for chronic Toxoplasma gondii infection. In this study, recombinant plasmid with epitopes of ROP21 and ROP29 were used as DNA vaccine. Methods: An immunoinformatics approach was employed to design a multi-epitope DNA vaccine encoding T. gondii ROP21 and ROP29. The bioinformatic outputs supported the immunogenic and non-allergic nature of multi-epitope vaccine. The recombinant plasmid was transfected in HEK cells. Thereafter, the protective effect of the DNA vaccine was evaluated in BALB/c mice by way of vaccination and challenge of these mice with acute RH and chronic PRU strains of T. gondii, respectively. The immunological responses of the control and vaccinated groups were assessed using survival time, lymphocyte proliferation assays, cytokine and antibody measurements, and determination of the parasite load in the spleen with real-time PCR. Findings: Multiple epitope (ROP21 and ROP29) DNA immunization stimulated cellular and humoral immune reactions in BALB/c mice and lengthened their life following challenge. Multiple epitope proteins increased significantly the total IgG antibody concentrations, triggered increased IFN-gamma cytokine production, increased the mixed IgG1/IgG2a response with a predominance of IgG2a, prolonged the survival duration and reduced the number of brain cysts. The in vivo findings indicated that the multi-epitope DNA vaccine elicited significant production of IgG antibodies (122.16 ng/ml) as well as IFN-gamma (12.37 pg/ml) (p < 0.05). Moreover, a significantly reduced parasite-burden (CT: 35.62) and prolonged survival time (14 days) were observed in the immunized groups compared to the controls (p < 0.05). Low IL-4 (5.63 pg/ml) values were detected in vaccinated mice compared to the PBS control (p > 0.05). Conclusions: We found that multiepitope protein vaccination could provide more protective immunity against chronic and acute toxoplasmosis infection compared to control.