Spatiotemporal immunofluorescent evaluation of porcine reproductive and respiratory syndrome virus transmission across the maternal-fetal interface

ABSTRACT Porcine reproductive and respiratory syndrome virus (PRRSV) infection causes severe reproductive failure characterized by high fetal morbidity and mortality leading to substantial economic losses to the swine industry. Evaluation of spatiotemporal transmission of PRRSV at the maternal-fetal interface (MFI) is critical for understanding fetal infection. Localization of PRRSV-2 strain NVSL 97-7895 at different regions of the MFI in 20 pregnant gilts at 2, 5, 8, 12 and 14 days post-inoculation (dpi) were analyzed by immunofluorescence (IF). Samples of MFI were collected from 15 inoculated and 5 control gilts and transplacental PRRSV transmission assessed in randomly selected fetuses from each litter. Localization of NVSL 97-7895 antigen immunoreactivity in the MFI was focused in three major areas: endometrial connective tissues (ENDO), the feto-maternal junction (FMJ) and fetal placenta (PLC). NVSL 97-7895 was detected at the FMJ by 2 dpi. At 2, 5 and 8 dpi, NVSL 97-7895 was localized within the ENDO and FMJ, whereas at 12 and 14 dpi, it was mainly localized in the PLC. Using a novel IF strategy for counting and size sorting NVSL 97-7895 viral antigen in situ, results of this study indicate that non-cell-associated mechanisms are involved in PRRSV transmission across the MFI. Porcine reproductive and respiratory syndrome virus crosses the maternal-fetal interface by 2 days post-inoculation and may breach the interface via non-cell-associated mechanisms such as cell-free virions or extracellular vesicles.