Immunization With a Live-Attenuated RH:Δ Strain of Induces Strong Protective Immunity Against Toxoplasmosis in Mice.

Toxoplasmosis, one of the most important health-threatening diseases worldwide, is caused by , which infects a wide range of warm-blooded animals and humans, leading to enormous health and socioeconomic concerns. can establish chronic infection to evade the immune response in hosts. Once a chronic infection has been established, the available treatments cannot efficiently control this stage of efficiently. Moreover, the available treatments rely only on a few drugs, such as sulfapyridine and pyrimethamine, that tend to have severe side effects. Given these factors, vaccination has been considered to be the most efficient method to prevent and control this disease. However, there is currently lack of effective vaccine available for use to prevent toxoplasmosis apart form Toxovax, the only available vaccine, which is used in sheep to prevent abortion. To address this problem, we knocked out the gene of the type I strain using the CRISPR-Cas9 system, constructed a live-attenuated vaccine and evaluated its protective efficacy in a mouse model. Immunization of mice with RH:Δ induced a high level of -specific IgG1, IgG2a and total IgG 42 days after immunization. There was a significant increase in the levels of cytokines in the splenocyte suspensions of RH:Δ-infected mice, and a mixed Th1/Th2 response was induced in the mice. Remarkably, after heterologous challenges with tachyzoites of the RH, PYS and Pru strains and cysts of the Pru strain by different infection routes, the immunized animals were protected from toxoplasmosis with a 100% survival rate, in both acute and chronic infection. In addition, compared with control mice, the Pru cyst load was clearly reduced in the brains of RH:Δ-infected immunization-mice. Our study demonstrated that the RH:Δ strain was able to evoke strong anti- immune responses and provide effective protection against parasite strains with different levels of virulence, suggesting that the RH:Δ strain may represent a promising live-attenuated vaccine against toxoplasmosis, which is worthy of further evaluation in food-producing animals and in definitive feline host.