Intracellular bacteria in placental basal plate localize to extravillous trophoblasts.

We previously showed that intracellular bacteria are present in the human placental maternal-fetal interface (basal plate). To determine the bacterial niche, basal plate biopsies were 1) examined histologically, and 2) cultured ex vivo, infected with either gram negative (Escherichia coli) or positive (Listeria monocytogenes) bacteria, and examined by histological staining, immunofluorescence, and transmission electron microscopy. We found bacteria in fetal extravillous trophoblasts (EVTs) in basal plate biopsies. Both E. coli and L. monocytogenes also predominantly invaded EVTs in basal plate explants where they replicated and formed clusters or existed as single organisms. EVTs are the cell type most susceptible to bacterial colonization, likely due to their expression of major histocompatibility antigen and immune-privileged status. Pathogens persisting and replicating in the EVTs may constitute a source of intrauterine colonization that leads to adverse outcomes such as preterm birth.