Toxoplasma gondii GRA3 activates interferon-stimulated genes and STAT6 by the cGAS-STING pathway to promote parasite proliferation.

Toxoplasma gondii is an opportunistic protozoan parasite that can establish latent infections in humans, causing toxoplasmosis in immunocompromised individuals. Type I interferons (IFN-I), particularly IFN-β, are critical for controlling Toxoplasma gondii infection, but the parasite has evolved various strategies to manipulate the host immune response. Interferon regulatory factor 3 (IRF3) is a key transcription factor that regulates the expression of antiviral genes, including IFN-I and ISGs. Unlike IFN-β, IRF3-activated ISG56 can enhance T. gondii proliferation. Furthermore, STAT6 activation has also been reported to promote the proliferation of Toxoplasma gondii. In this study, we found that GRA3 is highly expressed in the less virulent ME49 strain. Furthermore, we discovered that GRA3 interacted with STING to activate the cGAS/STING pathway. This interaction promotes STING oligomerization and the nuclear translocation of phosphorylated-IRF3, which in turn enhances IFN-β production. GRA3 in ME49 tachyzoites promoted both IRF3-mediated ISG56 expression and STAT6 phosphorylation, thereby enhancing the proliferation of these less virulent parasites. Interestingly, GRA3 enhances parasite proliferation via a mechanism mediated by ISG56 and STAT6, rather than by IFN-β. This study highlights how less virulent strains modulate host immunity to promote T. gondii survival and replication, establish latent infections, and ultimately achieve widespread dissemination in humans.