Long-Term Immunity to Lethal Acute or Chronic Type II Toxoplasma gondii Infection Is Effectively Induced in Genetically Susceptible C57BL/6 Mice by Immunization with an Attenuated Type I Vaccine Strain

C57BL/6 (B6) mice are genetically highly susceptible to chronic type II Toxoplasma gondii infections that invariably cause lethal toxoplasmic encephalitis. We examined the ability of an attenuated type I vaccine strain to elicit long-term immunity to lethal acute or chronic type II infections in susceptible B6 mice. Mice immunized with the type I cps1-1 vaccine strain were not susceptible to a lethal (100-cyst) challenge with the type II strain ME49. Immunized mice challenged with 10 ME49 cysts exhibited significant reductions in brain cyst and parasite burdens compared to naive mice, regardless of the route of challenge infection. Remarkably, cps1-1 strain-immunized B6 mice chronically infected with ME49 survived for at least 12 months without succumbing to the chronic infection. Potent immunity to type II challenge infections persisted for at least 10 months after vaccination. While the cps1-1 strain-elicited immunity did not prevent the establishment of a chronic infection or clear established brain cysts, cps1-1 strain-elicited CD8⁺ immune T cells significantly inhibited recrudescence of brain cysts during chronic ME49 infection. In addition, we show that uracil starvation of the cps1-1 strain induces early markers of bradyzoite differentiation. Collectively, these results suggest that more effective immune control of chronic type II infection in the genetically susceptible B6 background is established by vaccination with the nonreplicating type I uracil auxotroph cps1-1 strain.Toxoplasma gondii is a common and significant obligate intracellular pathogen of humans and animals. There are three clonal types that exist which are also thought to be derived when T. gondii acquired oral infectivity (50). Virulence in mice is strain specific where type I clones are universally virulent, type II clones are of intermediate virulence, and type III clones are avirulent. Ingestion of contaminated food sources is the most common route of human infection, resulting in systemic disease that can be divided into two stages: the acute disseminating tachyzoite stage and the chronic encysted bradyzoite stage (12). Recrudescent infections arising from reactivation of preexisting chronic latent cyst stages are particularly severe in the context of immune deficiency such as AIDS (38), and improved treatments and the development of vaccines to reduce disease burden are important therapeutic objectives. Strategies with the potential to eradicate the latent cyst stages present in already-infected individuals could be helpful but, unfortunately, the biology of cyst development, as well as the immune control mechanisms of latent stages, are relatively poorly understood at this time. Clearly, CD8⁺ T cells and gamma interferon (IFN-γ) are significant effectors in mediating resistance to acute and chronic T. gondii infection (17, 19).Numerous studies have evaluated responses to vaccines based on protein or DNA components of T. gondii with various degrees of success (1, 2, 4, 5, 9, 11, 22, 24, 25, 28, 36, 37, 40, 44, 46). Virulent parasite strains, as well as attenuated T. gondii strains, have been paramount in dissecting the immunobiology of host response in regard to understanding adaptive immune responses that may be helpful in vaccine design. Dense granule protein 6 (GRA6), GRA4, and rhoptry bulb protein 7 (ROP7) were recently identified as parasite antigens possessing a H-2L^d-restricted major histocompatibility complex class I (MHC-I) epitope that correlates with stage-specific expression and resistance to lethal chronic type II infections in the H-2L^d background (BALB/c). These data further define a potential molecular basis for genetic susceptibility to lethal type II chronic infections in the C57BL/6 H-2^b MHC-I-restricted background (6, 16). Vaccine models using either live attenuated parasites, such as type I strain ts-4, or irradiated tachyzoites, have had the greatest success in providing complete protection against lethal type I challenges. These studies also report more significant reductions in type II cyst burdens than component vaccines or whole-dead parasite vaccines (42, 48, 53, 54). However, live parasite-based vaccines such as strain ts-4 are still slowly replicating and retain a significant potential for virulence in the immunocompromised host. Furthermore, immune protection elicited by strain ts-4 is not long-lasting and significantly decreases within months after immunization (27).From the same parental RH strain that strain ts-4 was developed (45), our laboratory developed a fully attenuated nonreplicating type I cps1-1 strain that exhibits a severe uracil auxotrophy. The cps1-1 strain in a single immunization elicits complete immune protection and is able to clear high lethal dose virulent type I infection (14). Significantly, this highly attenuated strain is completely avirulent at extreme doses in immunocompromised hosts, such as in Tyk2^−/− mice (49), which cannot control inflammation, and also in IFN-γ^−/− mice (14, 20). The cps1-1 strain elicits potent Th1 immunity to lethal type I challenge infection after immunization of BALB/c, C57BL/6, Tyk2^−/− (C57BL/6), or MyD88^−/− (C57BL/6) mice (13, 14, 20, 49, 51, 56). Immunity to lethal type I challenge infection induced by the cps1-1 strain is dependent on CD8⁺ T cells (20), local production of IFN-γ (20), and interleukin-12 (IL-12) p70 (20, 51, 56). Remarkably, the potent immunity elicited by vaccination with the cps1-1 strain does not require systemic IFN-γ (20).We show here that the immunity induced in C57BL/6 mice after vaccination with the cps1-1 strain provides a surprisingly effective and complete protection from a lethal oral or intraperitoneal (i.p.) challenge infection of type II cysts from the ME49 strain. We address the durability of cps1-1 strain-induced immunity to lethal type II cyst challenge infection by different routes and find that immunization with the cps1-1 strain provides long-term protective immunity to lethal type II challenge. Vaccination with the cps1-1 strain also markedly reduces the cyst burden and protects susceptible C57BL/6 mice from succumbing to chronic infection. CD8⁺ immune T cells elicited by vaccination with the cps1-1 strain prevent cyst recrudescence during chronic infection.