CD4⁺ T Cell Responses to Toxoplasma gondii Are a Double-Edged Sword

CD4⁺ T cells have been found to play critical roles in the control of both acute and chronic Toxoplasma infection. Previous studies identified a protective role for the Toxoplasma CD4⁺ T cell-eliciting peptide AS15 (AVEIHRPVPGTAPPS) in C57BL/6J mice. Herein, we found that immunizing mice with AS15 combined with GLA-SE, a TLR-4 agonist in emulsion adjuvant, can be either helpful in protecting male and female mice at early stages against Type I and Type II Toxoplasma parasites or harmful (lethal with intestinal, hepatic, and spleen pathology associated with a storm of IL6). Introducing the universal CD4⁺ T cell epitope PADRE abrogates the harmful phenotype of AS15. Our findings demonstrate quantitative and qualitative features of an effective Toxoplasma-specific CD4⁺ T cell response that should be considered in testing next-generation vaccines against toxoplasmosis. Our results also are cautionary that individual vaccine constituents can cause severe harm depending on the company they keep.