Vaccination with Trypomastigote Surface Antigen 1-Encoding Plasmid DNA Confers Protection against Lethal Trypanosoma cruzi Infection

DNA vaccination was evaluated with the experimental murine model of Trypanosoma cruzi infection as a means to induce antiparasite protective immunity, and the trypomastigote surface antigen 1 (TSA-1), a target of anti-T. cruzi antibody and major histocompatibility complex (MHC) class I-restricted CD8⁺ cytotoxic T-lymphocyte (CTL) responses, was used as the model antigen. Following the intramuscular immunization of H-2^b and H-2^d mice with a plasmid DNA encoding an N-terminally truncated TSA-1 lacking or containing the C-terminal nonapeptide tandem repeats, the antibody level, CTL response, and protection against challenge with T. cruzi were assessed. In H-2^b mice, antiparasite antibodies were induced only by immunization with the DNA construct encoding TSA-1 containing the C-terminal repeats. However, both DNA constructs were efficient in eliciting long-lasting CTL responses against the protective H-2K^b-restricted TSA-1_515–522 epitope. In H-2^d mice, inoculation with either of the two TSA-1-expressing vectors effectively generated antiparasite antibodies and primed CTLs that lysed T. cruzi-infected cells in an antigen-specific, MHC class I-restricted, and CD8⁺-T-cell-dependent manner. When TSA-1 DNA-vaccinated animals were challenged with T. cruzi, 14 of 22 (64%) H-2^b and 16 of 18 (89%) H-2^d mice survived the infection. The ability to induce significant murine anti-T. cruzi protective immunity by immunization with plasmid DNA expressing TSA-1 provides the basis for the application of this technology in the design of optimal DNA multicomponent anti-T. cruzi vaccines which may ultimately be used for the prevention or treatment of Chagas’ disease.

Chagas’ disease, caused by the intracellular protozoan parasite Trypanosoma cruzi, is a lifelong health problem in Central and South America, where an estimated 18 million people are infected with this parasite and 90 million are at risk of infection (35, 65). Following a short-lived acute-phase illness characterized by fever and a patent parasitemia, infected individuals enter a nearly aparasitemic asymptomatic chronic phase, where most remain for the remainder of their lifetime. However, at 10 to 20 years postinfection nearly 30% of infected individuals develop severe cardiomyopathy, which is responsible for most of the 50,000 deaths caused by Chagas’ disease each year (45). Although reduviid vector control and blood bank screening measures have had a major impact in reducing transmission of T. cruzi (65), the operational costs to maintain such control programs, behavioral differences among vector species, existence of animal reservoirs, persistence of parasites in chronically infected patients, and lack of adequate chemotherapies to treat the infection will likely prevent these control measures alone from completely eradicating T. cruzi. An additional approach that could contribute significantly to control the transmission of Chagas’ disease is the development of anti-T. cruzi vaccines. To date, however, vaccine production for T. cruzi has been a low priority despite the current knowledge about the protective roles that antibodies, type 1 cytokines, and CD8⁺ T cells play in resistance to experimental T. cruzi infections (53).During T. cruzi infection, both chagasic patients and experimental animals produce strong immune responses to molecules from the infective nonreplicative trypomastigote stage and the replicative amastigote forms (3, 4, 14, 29). Among these, trypomastigote surface antigen 1 (TSA-1) (15, 38), a major trypomastigote surface antigen and the first identified member of the trans-sialidase gene superfamily (48), is a target of protective immune responses in mice (61, 66). Immunization with an amino-proximal fragment of TSA-1 induces a strong antibody response and protects mice against an otherwise lethal challenge with T. cruzi (66). Our studies have recently identified TSA-1 as the first bona fide target of CD8⁺ cytotoxic T lymphocytes (CTL) in T. cruzi-infected mice and demonstrated that the adoptive transfer of TSA-1-specific gamma interferon (IFN-γ)- and tumor necrosis factor alpha-producing CTL lines protects naive animals against lethal T. cruzi infection (61). Moreover, we have recently determined that TSA-1 and amastigote surface protein-1 and -2 (33, 44), which are also recognized by murine CTL (32), represent three target molecules of T. cruzi-specific human CD8⁺ CTL (62). These studies demonstrated the validity of the mouse model to identify target antigens of protective anti-T. cruzi immune responses and provide a strong incentive for the development of vaccines as a potential control measure against Chagas’ disease. For this purpose, and given the success of plasmid DNA vaccination in specifically stimulating a broad spectrum of immune responses to the vector-encoded target antigen (12), we have chosen to investigate DNA-based immunization as a system to generate vaccine-induced resistance against T. cruzi and have used TSA-1 as a model antigen for its initial evaluation. In this report we document that intramuscular injection of BALB/c and C57BL/6J mice with TSA-1-encoding plasmid DNA induces antibodies, CTL, and significant protection against lethal challenge with T. cruzi.