Immunization with Toxoplasma gondii GRA17 Deletion Mutant Induces Partial Protection and Survival in Challenged Mice

Abstract

Toxoplasmosis remains a world-threatening disease largely because of the lack of a fully effective vaccine. Here, we created a Δ<i>GRA17</i> mutant by disrupting the virulence factor <i>GRA17</i> using CRISPR-Cas9 method. Then, we tested whether Δ<i>GRA17</i> tachyzoites can be used as a live-attenuated vaccine against acute, chronic, and congenital <i>Toxoplasma gondii</i> infection in mice. Immune response evoked by Δ<i>GRA17</i> immunization suggested a sequential Th1 and Th2 T cell response, indicated by high levels of Th1 and a mixed Th1/Th2 cytokines at 28 and 70 days after immunization, respectively. Δ<i>GRA17</i>-mediated immunity fully protected mice against lethal infection with wild-type (wt) RH strain, heterologous challenge with PYS, and TgC7 strains of the Chinese ToxoDB#9 genotype, and <i>T. gondii</i> Pru strain. Although parasite cysts were detected in 8 out of 10 immunized mice, cyst burden in the brain was significantly reduced (<i>P</i> < 0.05) in immunized mice (53 ± 15 cysts/brain) compared to non-immunized mice (4,296 ± 687 cysts/brain). In respect to congenital infection, the litter size, survival rate, and body weight (BW) of pups born to Δ<i>GRA17</i>-immunized dams were not different compared to pups born to naïve control dams (<i>P</i> = 0.24). However, a marked reduction in the litter size (<i>P</i> < 0.001), survival rate, and BW (<i>P</i> < 0.01) of pups born to non-immunized and infected dams was detected. Also, immunized dams infected with type II Pru strain had significantly (<i>P</i> < 0.001) less cyst burden in the brain compared with non-immunized and infected dams. These findings show that immunization with Δ<i>GRA17</i> strain evokes cell-mediated and neutralizing antibody responses and confers some degree of protection against challenge with homologous and heterologous virulent <i>T. gondii</i> strains.