Dissecting placental host-pathogen interactions: Rift Valley fever virus infection in early human trophoblast stem cells.

Rift Valley fever virus (RVFV) is a mosquito-borne Phlebovirus and zoonotic pathogen affecting maternal-fetal health. Vertical transmission is linked to miscarriage and severe fetal outcomes, but mechanisms of placental pathogenesis remain unclear. We used first-trimester human trophoblast stem cells (hTSCs) to model infection at the maternal-fetal interface. Immunofluorescence, qRT-PCR, western blotting, and single-cell transcriptomics showed that hTSCs are highly susceptible to RVFV. Strand-specific viral transcriptomics confirmed the ambisense S segment and revealed preferential transcription of the M and S segments over L. RVFV induced G1 arrest, impairing trophoblast proliferation and differentiation, and drove widespread transcriptional reprogramming, including strong interferon lambda 1 (IFNL1) but modest type I interferon responses, and dysregulation of inflammatory and preeclampsia-associated genes such as RUNX1 and TGFBRAP1. Recombinant IFN-λ pretreatment reduced RVFV protein expression, highlighting hTSCs as a robust model and IFN-λ as a promising antiviral strategy.