Tissue barriers of the human placenta to infection with Toxoplasma gondii

Toxoplasma gondii is a ubiquitous, obligate intracellular parasite capable of crossing the placenta to cause spontaneous abortion, preterm labor, or significant disease in the surviving neonate. Exploration of the cellular and histological components of the placental barrier is in its infancy, and both how and where T. gondii breaches it are unknown. The human placenta presents two anatomical interfaces between maternal cells and fetal cells (trophoblasts): (i) the villous region where maternal blood bathes syncytialized trophoblasts for nutrient exchange and (ii) the maternal decidua, where mononuclear, extravillous trophoblasts anchor the villous region to the uterus. Using first-trimester human placental explants, we demonstrate that the latter site is significantly more vulnerable to infection, despite presenting a vastly smaller surface. This is consistent with past findings concerning two vertically transmitted viruses and one bacterium. We further explore whether three genetically distinct T. gondii types (I, II, and III) are capable of preferential placental infection and survival in this model. We find no difference in these strains' ability to infect placental explants; however, slightly slower growth is evident in type II (Prugniaud [Pru]) parasites relative to other cell types, although this did not quite achieve statistical significance.