Protection of mice against Trypanosoma cruzi by immunization with paraflagellar rod proteins requires T cell, but not B cell, function

Previous studies have shown that immunization of mice with the paraflagellar rod proteins (PAR) of Trypanosoma cruzi induces an immune response capable of protecting mice against an otherwise lethal challenge with this parasite. Herein, we define immunologic responses that do or do not play a critical role in PAR-mediated protection. Firstly, PAR-immunized Ab-deficient (μMT) strain mice survived an otherwise lethal T. cruzi challenge, indicating that a B cell response is not required for PAR-induced immunity. However, β₂m -/- mice, which are severely deficient in MHC class I and TCRαβ⁺CD8⁺CD4^- T cells, did not survive challenge infection following PAR immunization, indicating that MHC class I/CD8⁺ T cell function is necessary for protection induced by PAR immunization. Surprisingly, PAR-immunized mice depleted of CD4⁺ T cells survived a T. cruzi challenge for >84 days postinfection while maintaining a parasitemia that is generally thought to be lethal (i.e., >10⁶ trypomastigotes/ml), thus associating CD4⁺ T cell function with the process of parasite clearance. Consistent with this association, CD4⁺ T cells from PAR-immunized mice released INF-γ and stimulated T. cruzi-infected macrophages to release nitric oxide. The importance of IFN-γ in PAR-induced protective immunity is further indicated by the observation that PAR-immunized INF-γ knockout mice developed an extremely high parasitemia and did not survive a challenge infection. Thus, while Ab-mediated immune mechanisms are not required for protection induced by PAR immunization, T cell responses are necessary for both elimination of bloodstream parasites and survival.