The live attenuated DGAT1-knockout whole-cell Toxoplasma vaccine confers protective immunity against acute and chronic toxoplasmosis.

The intravacuolar parasite Toxoplasma gondii scavenges fatty acids from host mammalian cells and stores excess in lipid droplets. To investigate the physiological relevance of neutral lipid storage in Toxoplasma, we generated a mutant lacking DGAT1, an ER-localized enzyme that synthesizes triacylglycerols, from the virulent type I RH strain of T. gondii. Compared to WT, RH ∆DGAT1 parasites grow poorly in mammalian cells, form few LD, suffer from lipotoxicity, and do not cause disease or lethality in immunocompetent and immunodeficient mice. Importantly, mice immunized with RH ∆DGAT1 parasites mount strong, long-term immune responses involving both cellular and humoral components, with higher levels of T. gondii-specific IgG antibodies, effector memory T cells, and both pro-inflammatory and anti-inflammatory cytokines, indicating a mixed Th1/Th2 response with Th1 predominance. This immunity provides complete, long-lasting protection (up to 6 months) against rechallenge from homologous type I (acute infection) and heterologous cyst-forming type II (chronic infection) T. gondii strains. Additional analyses reveal that IFN-γ, CD8+ T cells, as well as B cells are crucial for defending against type I T. gondii in immunized mice. Overall, our live-attenuated RH ∆DGAT1 strain is a promising vaccine candidate and a model for studying immune responses that control T. gondii infections.