ROP39 is an Irgb10-specific parasite effector that modulates acute Toxoplasma gondii virulence

Toxoplasma gondii (T. gondii) is a zoonotic apicomplexan parasite that is an important cause of clinical disability in humans. On a global scale, one third of the human population is infected with T. gondii. Mice and other small rodents are believed to be responsible for transmission of T. gondii to the domestic cat, its definitive host. Interferon-inducible Immunity-Related GTPases (IRG proteins) are important for control of murine T. gondii infections. Virulence differences between T. gondii strains are linked to polymorphic rhoptry proteins (ROPs) that cooperate to inactivate individual IRG family members. In particular, the pseudokinase ROP5 isoform B is critically important in laboratory strains of mice. We identified T. gondii ROP39 in complex with ROP5B and demonstrate its contribution to acute T. gondii virulence. ROP39 directly targets Irgb10 and inhibits homodimer formation of the GTPase leading to an overall reduction of IRG protein loading onto the parasitophorous vacuolar membrane (PVM). Maintenance of PVM integrity rescues the parasite from IRG protein-mediated clearance in vitro and in vivo. This study identifies a novel T. gondii effector that is important for specific inactivation of the IRG resistance system. Our data reveal that yet unknown T. gondii effectors can emerge from identification of direct interaction partners of ROP5B.Author summary: Toxoplasmosis is one of the more common parasitic zoonoses world-wide. Its causative agent, Toxoplasma gondii (T. gondii), is an obligate intracellular food-borne parasite. It has a very broad host range establishing a productive infection in virtually all warm-blooded animals including humans. A mouse model of toxoplasmosis is of medical importance because the relationship with one of its most important natural hosts, the mouse, determines the impact of T. gondii infections in humans. In the acute phase of infection, the parasite seeks to reach an intermediate level of virulence to increase chances for transmission. Virulence is a multifactorial phenotype; we and others have described several effector proteins of the parasite that specifically inhibit certain host cell GTPases (IRG proteins). Only the combination of all parasite effectors determines the full virulence potential of the parasite. Here, we demonstrate that T. gondii has evolved a virulence effector, ROP39, that specifically targets Irgb10 to escape IRG-mediated control. This is a novel aspect of the infection biology of this world-wide zoonotic pathogen. It advances our understanding of the coevolutionary relationship of T. gondii and the mouse, an important intermediate host for transmission.