Toxoplasma gondii is a threat for immunocompromized individuals, and no treatment is available for enhancing immunity against infection. Molecular adjuvants may improve the efficacy of DNA vaccine-induced T cell immunity. Here, we report that cocktailed DNA immunization with ROP5 and ROP18 boosted immune responses induced by a single DNA immunization with ROP5 or ROP18, but also that co-administration of molecular adjuvant IL-33 enhanced immune efficacy induced by this cocktailed DNA vaccination. These improved immune responses were characterized by higher Toxoplasma-specific IgG2a titers, Th1 responses associated with the production of IFN-γ, IL-2, IL-12, as well as cell-mediated activity with higher frequencies of CD8+ and CD4+ T cells. More importantly, this enhanced immunity has the ability to confer remarkable protection against a high dose lethal challenge of the T. gondii RH strain and thus against chronic infection with the T. gondii PRU strain. These data show that IL-33 is a promising immunoadjuvant to facilitate humoral as well as cellular immunity in a vaccine setting against T. gondii, and suggest that it should be evaluated in strategies against other apicomplexan parasites.

Toxoplasma gondii is an obligate intracellular parasite, which is responsible for a widely distributed zoonosis. Effective vaccines against toxoplasmosis are necessary to protect the public health. The aim of this study is to evaluate the immune efficacy of DNA vaccines encoding TgMIC5 and TgMIC16 genes against T. gondii infection. The recombinant plasmid pVAX-MIC5 and pVAX-MIC16 were constructed and injected intramuscularly in mice. The specific immune responses and protection against challenge with T. gondii RH tachyzoites were evaluated by measuring the cytokine levels, serum antibody concentrations, lymphocyte proliferation, lymphocyte populations, and the survival time. The protection against challenge with the T. gondii RH tchyzoites and PRU cysts was examined by evaluation of the reduction in the brain cyst burden. The results indicated that immunized mice showed significantly increased levels of IgG, IFN-γ, IL-2, IL-12p70, and IL-12p40 and percentages of CD4+ and CD8+ T cells. Additionally, vaccination prolonged the mouse survival time and reduced brain cysts compared with controls. Mouse groups immunized with a two-gene cocktail of pVAX-MIC5 + pVAX-MIC16 were more protected than mouse groups immunized with a single gene of pVAX-MIC5 or pVAX-MIC16. These results demonstrate that TgMIC5 and TgMIC16 induce effective immunity against toxoplasmosis and may serve as a good vaccine candidate against T. gondii infection.