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

## TL;DR

This study shows how the Toxoplasma gondii parasite uses a protein called GRA3 to manipulate the host's immune system, promoting its own survival and replication.

## Contribution

The study reveals a novel mechanism by which GRA3 activates the cGAS-STING pathway to enhance parasite proliferation via ISG56 and STAT6.

## Key findings

- GRA3 interacts with STING to activate the cGAS/STING pathway, increasing IFN-β production.
- GRA3 promotes ISG56 and p-STAT6, which enhance T. gondii proliferation in less virulent strains.
- The parasite uses ISG56 and p-STAT6, not IFN-β, to manipulate host immunity for its benefit.

## Abstract

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.

Toxoplasma gondii is a common parasite that can infect nearly all warm-blooded animals, including humans. While healthy individuals usually do not develop symptoms, those with weakened immune systems may suffer from severe disease. The host defends against T. gondii infection by producing key immune molecules such as IFN-β, but the parasite has evolved various strategies to manipulate host immune defenses to support its survival and replication. In this study, we focused on GRA3, a protein highly expressed in the less virulent ME49 strain. We found that GRA3 interacts with the host protein STING to activate the cGAS/STING signaling pathway, leading to increased production of ISG56, p-STAT6, and IFN-β. Interestingly, unlike IFN-β which helps suppress the parasite, ISG56 and p-STAT6 actually enhances T. gondii proliferation. Our findings reveal how T. gondii can finetune host immune responses to strike a balance between activation and evasion, facilitating the establishment of chronic and latent infection. Understanding this mechanism may help develop better treatments for infections caused by T. gondii.