SARS-CoV-2 delta and omicron variants alter trophoblast cell fusion and syncytiotrophoblast dynamics: new insights into placental vulnerability

Pregnancy is associated with an increased risk of severe COVID-19. In addition, SARS-CoV-2 infection during gestation has been linked to adverse obstetrical outcomes and placental abnormalities. Nevertheless, the susceptibility of early trophoblast cells to SARS-CoV-2 and the potential consequences of infection on trophoblast function remain unclear. In this study, we assessed the permissiveness of first trimester trophoblast cells to SARS-CoV-2 infection and its impact on trophoblast cells fusion. To address this, we isolated primary cytotrophoblast (CTB) cells from first trimester human placentas and allow their differentiation into STB in vitro. These cells were infected with SARS-CoV-2 variants of concern, including Delta and Omicron (BA.1, BA.2, BA.5). Viral replication was assessed by RT-qPCR and immunofluorescence, while host cell responses, including expression of viral entry receptors and innate immunity genes, were measured by RT-qPCR. Trophoblast fusion was evaluated by staining and calculating the fusion index. In parallel, placental tissues from SARS-CoV-2-infected pregnancies were analyzed by immunohistochemistry to quantify syncytial knots (SK) formation in vivo. Our results demonstrate that both first trimester CTB and STB are permissive to SARS-CoV-2 infection in a variant- and donor-dependent manners, with Delta exhibiting higher replication efficiency compared to Omicron variants. In STB, viral replication did not correlate with the induction of entry receptors or type III interferon responses. However, in CTB, viral replication was significantly associated with enhanced cell fusion. In parallel, an increased number of SK was observed in infected placental areas in vivo compared to non-infected regions from the same placenta and to gestational age-matched controls. Altogether, these in vitro and in vivo results suggest that SARS-CoV-2 infection in early pregnancy may alter STB turnover, potentially contributing to placental dysfunction and adverse pregnancy outcomes.