The apicoplast is important for the viability and persistence ofToxoplasma gondiibradyzoites

Toxoplasma gondii is responsible for toxoplasmosis, a disease that can be serious when contracted during pregnancy, but can also be a threat for immunocompromised individuals. Acute infection is associated with the tachyzoite form that spreads rapidly within the host. However, under stress conditions, some parasites can differentiate into cyst-forming bradyzoites, residing mainly in the central nervous system, retina and muscle. Because this latent form of the parasite is resistant to all currently available treatments, and is central to persistence and transmission of the parasite, new specific therapeutic strategies targeting this developmental stage need to be discovered. T. gondii contains a plastid of endosymbiotic origin called the apicoplast, which is an appealing drug target because it is essential for tachyzoite viability and contains several key metabolic pathways that are largely absent from the mammalian host. Its function in bradyzoites, however, is unknown. Our objective was thus to study the contribution of the apicoplast to the viability and persistence of bradyzoites during chronic toxoplasmosis. We have used complementary strategies based on stage-specific promoters to generate conditional bradyzoite mutants of essential apicoplast genes. Our results show that specifically targeting the apicoplast in both in vitro or in vivo -differentiated bradyzoites leads to a loss of long-term bradyzoite viability, highlighting the importance of this organelle for this developmental stage. This validates the apicoplast as a potential area to look for new therapeutic targets in bradyzoites, with the aim to interfere with this currently incurable parasite stage. Significance Statement In its intermediate hosts, the parasite Toxoplasma gondii can persist as a cyst-contained developmental form that might reactivate and cause severe pathologies. Importantly, this form is resistant to current anti-parasitic drugs. T. gondii harbors a plastid of endosymbiotic origin called the apicoplast, containing important and potentially druggable metabolic pathways, but whose contribution to the fitness and viability of persistent parasites has never been assessed. We have generated conditional mutants specifically affected for the homeostasis of the apicoplast in cyst-contained parasites and showed that this organelle is important for persistence of these particular developmental forms. Our work thus validates the apicoplast as a relevant drug target in the context of chronic T. gondii infection.