Single-cell analysis of brain-derived Toxoplasma bradyzoites reveals a novel cell cycle regulated by AP2XI-6.

Toxoplasma gondii prevalence is due, in part, to its ability to persist in hosts while retaining the capacity to transmit and recrudesce. A process that is poorly understood. Through single-cell RNA profiling of in vivo-derived bradyzoites, we discovered that they are heterogeneous and not G1-arrested, as expected from in vitro studies. Instead, they progress through two cell cycles that branch from a single G1. While in G1b, in vivo-derived bradyzoites express cyst wall proteins predicted to replenish the wall that shields cysts. One cell cycle is common amongst tachyzoites and in vitro- and in vivo-derived bradyzoites; the other is unique to in vivo-derived bradyzoites (BCC). We demonstrated that AP2XI-6, expressed in G1 in vivo, is dispensable for tachyzoite growth and cyst formation in vitro but necessary for encystation in vivo. We propose that AP2XI-6 is a driver of replication through the BCC that is required for cyst maturation and persistence.