Toxoplasma gondii restricts the developmental trajectory of placental trophoblasts from an infection-resistant cell fate

Pregnancy is one of the most critical points of vulnerability to Toxoplasma infection. Despite this, no specific mechanisms used by the parasite to cross the placenta have been discovered. Here, we investigate an entirely new phenomenon where Toxoplasma is able to coopt canonical host cell development to propagate its own transmission through the placenta. Using a bipotent stem cell system, we can grow cells that recapitulate placental trophoblast progenitors, called cytotrophoblasts (CYT), in addition to both terminally differentiated lineages, extravillous trophoblasts (EVT) and syncytiotrophoblasts (SYN). These cells vary in their susceptibility to T. gondii infection, with SYN being unusually resistant and EVT being comparably permissive. Importantly, the high proportion of infection-resistant SYN at the surface of the maternofetal interface prevents the vast majority of parasite invasion to deeper cell layers. Using scRNA and scATAC-seq datasets of 50,000 cells, we have convincingly determined that T. gondii-infected progenitor CYT undergo global transcriptional changes consistent with a departure from their stem state and activation of host cell differentiation. Moreover, these data indicate that this parasite-induced host cell differentiation is specifically antagonistic to the infection-resistant SYN cell lineage. We see simultaneous reduction of transcripts and proteins governing SYN development and induction of transcripts governing EVT development. We also demonstrate that parasite infection of 3D primary trophoblast organoids (a system that best recapitulates inter-cell type signaling) for just 24 hours is sufficient to elicit CYT differentiation to EVT-like cells that, ordinarily, must be chemically induced for multiple days in culture. Finally, we use a novel functional assay to show that Toxoplasma infection results in such severe lineage restriction that host cells almost completely lose their capacity to fuse with other SYN. Taken together, these results indicate that Toxoplasma infection dysregulates progenitor trophoblast cells at both the levels of cell fate and cell function, meaning, infection of progenitor cells specifically promotes one cell lineage at the cost of another. This significant shift of CYT away from SYN and towards EVT could alter the balance of resistant and susceptible cells at the maternofetal interface, potentiating robust T. gondii dissemination through placental tissue and, ultimately, into the fetal compartment.