ROP16-mediated activation of STAT6 enhances cyst development of type III Toxoplasma gondii in neurons

Author summaryToxoplasma gondii is a parasite that establishes a latent infection in the brain of its host. In humans who become immunocompromised, this latent infection can reactivate to cause serious neurologic complications. The parasite establishes a latent infection by transitioning from a fast-growing state to a slow growing one. This slow growing form resides within cysts, which are commonly found in neurons. While several parasite factors that drive cyst formation have been identified, only a few host cell genes that promote encystment have been found. Using different models of encystment, including human and mouse primary neuronal cultures, we show that phosphorylation and, thus, activation of the host cell transcription factor STAT6 by the parasite kinase ROP16 is required for efficient encystment of certain T. gondii strains but not others. Collectively, this work suggests that T. gondii strain-specific pathways to encystment exist and identifies a direct link between parasite manipulation of the host cell and encystment. Toxoplasma gondii establishes a long-lived latent infection in the central nervous system (CNS) of its hosts. Reactivation in immunocompromised individuals can lead to life threatening disease. Latent infection is driven by the ability of the parasite to convert from the acute-stage tachyzoite to the latent-stage bradyzoite which resides in long-lived intracellular cysts. While much work has focused on the parasitic factors that drive cyst development, the host factors that influence encystment are not well defined. Here we show that a polymorphic secreted parasite kinase (ROP16), that phosphorylates host cell proteins, mediates efficient encystment of T. gondii in a stress-induced model of encystment and primary neuronal cell cultures (PNCs) in a strain-specific manner. Using short-hairpin RNA (shRNA) knockdowns in human foreskin fibroblasts (HFFs) and PNCs from transgenic mice, we determined that ROP16's cyst enhancing abilities are mediated, in part, by phosphorylation-and therefore activation-of the host cell transcription factor STAT6. To test the role of STAT6 in vivo, we infected wild-type (WT) and STAT6KO mice, finding that, compared to WT mice, STAT6KO mice have a decrease in CNS cyst burden but not overall parasite burden or dissemination to the CNS. Finally, we found a similar ROP16-dependent encystment defect in human pluripotent stem cell-derived neurons. Together, these findings identify a host cell factor (STAT6) that T. gondii manipulates in a strain-specific manner to generate a favorable encystment environment.

Automated summary

Toxoplasma gondii establishes a long-term latent infection in the host's central nervous system. It has been discovered that the parasite kinase ROP16 phosphorylates and activates the host cell transcription factor STAT6, which plays a critical role in promoting the encystment of the parasite.