Immunity conferred by drug-cured experimental Trypanosoma cruzi infections is long-lasting and cross-strain protective

Abstract

Background The long term and complex nature of Chagas disease in humans has restricted studies on vaccine feasibility. Animal models also have limitations due to technical difficulties in monitoring the extremely low parasite burden that is characteristic of chronic stage infections. Advances in imaging technology offer alternative approaches that circumvent these problems. Here, we describe the use of highly sensitive whole body in vivo imaging to assess the efficacy of recombinant viral vector vaccines and benznidazole-cured infections to protect mice from challenge with Trypanosoma cruzi. Methodology/Principal Findings Mice were infected with T. cruzi strains modified to express a red-shifted luciferase reporter. Using bioluminescence imaging, we assessed the degree of immunity to re-infection conferred after benznidazole-cure. Mice infected for 14 days or more, prior to the initiation of treatment, were highly protected from challenge with both homologous and heterologous strains (>99% reduction in parasite burden). Sterile protection against homologous challenge was frequently observed. This level of protection was considerably greater than that achieved with recombinant vaccines. It was also independent of the route of infection or size of the challenge inoculum, and was long-lasting, with no significant diminution in immunity after almost a year. When the primary infection was benznidazole-treated after 4 days (before completion of the first cycle of intracellular infection), the degree of protection was much reduced, an outcome associated with a minimal T. cruzi-specific IFN-γ+ T cell response. Conclusions/Significance Our findings suggest that a protective Chagas disease vaccine must have the ability to eliminate parasites before they reach organs/tissues, such as the GI tract, where once established, they become largely refractory to the induced immune response. AUTHOR SUMMARY Chagas disease, which is caused by the protozoan parasite Trypanosoma cruzi, is a major public health problem throughout Latin America. Attempts to develop a vaccine have been hampered by technical difficulties in monitoring the extremely low parasite burden during the life-long chronic stage of infection. To circumvent these issues, we used highly sensitive bioluminescence imaging to assess the ability of recombinant viral vector vaccines and drug-cured infections to confer protection against experimental challenge in mice. We observed that drug-cured infections were much more effective than subunit vaccines, with many instances of sterile protection. Efficacy was independent of the route of infection or size of the challenge inoculum, and was undiminished after almost a year. In addition, drug-cured infections conferred a high level of cross-strain protection. The highly sensitive imaging procedures enabled us to visualise parasite distribution in mice where sterile protection was not achieved. This suggested that to confer sterile protection, vaccines must prevent the infection of organs/tissues that act as parasite reservoirs during the chronic stage. Once established at these sites, parasites become largely refractory to vaccine-induced elimination.