The Role of ROP16 in Type III Toxoplasma gondii Tachyzoite and Bradyzoite Stages

Abstract

In humans, clinically relevant disease involving Toxoplasma gondii, an obligate intracellular parasite, results from T. gondii’s life-long persistence in the central nervous system (CNS). Successful T. gondii persistence is driven by the ability of the parasite to evade the immune response during acute infection and disseminate throughout the body. Once the parasite has disseminated to the CNS it switches from a fast-replicating form, the tachyzoite, to a slow-growing encysted form, the bradyzoite. Additionally, an emerging body of studies indicates that the genotype of the infecting T. gondii strain can influence CNS disease outcomes. These differences are thought to be driven by polymorphic effector proteins that can modulate different immune responses in a strain-specific manner. The work in this dissertation will address the role of one such polymorphic effector protein, rhoptry protein 16 (ROP16). Most work on ROP16 has been done in a hypervirulent strain of T. gondii where it has little effect on acute virulence in mice. In addition, the hypervirulent nature of the strain means studies on chronic infection cannot be done. Recent work revealed a role for ROP16 in a less virulent strain during in vivo infection. To further address the role of ROP16 in this less virulent strain in both tachyzoites and bradyzoites, we used a panel of ROP16 mutants and infected cells cultured in both tachyzoite and bradyzoite growth conditions. We have found that during tachyzoite growth ROP16 facilitates parasite growth and survival in a strain-specific manner by reducing host cell reactive oxygen species (ROS) through the activation of a host cell transcription factor Signal Transducer and Activator of Transcription 6 (STAT6). We also observed that under bradyzoite-inducing conditions ROP16 facilitates encystment through the activation of STAT6 in vitro and that, compared to wild-type mice, STAT6 knockout (STAT6KO) mice have fewer CNS cysts despite having similar parasite burdens.