A novel Toxoplasma gondii thioredoxin (TgTrx1) is important for parasite fitness and virulence.

The protozoan parasite Toxoplasma gondii relies on antioxidant proteins and systems to protect against the host's immune responses and to neutralize free radicals produced by its own metabolism. In this study, we identified and characterized a new thioredoxin protein, TgTrx1, which is mainly found in the cytoplasm of T. gondii tachyzoites and contains a conserved -CXXC- catalytic motif. Using CRISPR-Cas9 gene editing, we disrupted the TgTrx1 gene to generate a knockout strain (RHΔtrx1) and studied the effect of gene loss on various aspects of the infection process. RHΔtrx1 parasites showed a marked reduction in their ability to invade host cells, secrete microneme proteins, replicate intracellularly, egress from host cells, and tolerate oxidative stress. They also displayed abnormal mitochondrial morphology and asynchronous cell division. Transcriptomic analysis revealed significant changes in the expression of genes involved in oxidative stress response and bradyzoite differentiation. Mice injected intraperitoneally with 10⁶ RHΔtrx1 tachyzoites showed no clinical symptoms. However, the immunity induced by these attenuated tachyzoites conferred only partial protection against subsequent acute and chronic T. gondii infections. This limited protective effect is likely related to the parasite's impaired replication, which may lead to rapid clearance by the host immune system and insufficient antigenic stimulation to elicit a fully protective immune response. These findings establish TgTrx1 as a multifunctional redox protein important for T. gondii survival, redox balance, synchronous cell division, and virulence.