Background: Environmental exposure to respiratory syncytial virus (RSV) is a leading cause of respiratory infections in infants, but it remains unknown whether this infection is transmitted transplacentally from the lungs of infected mothers to the offspring. We sought to test the hypothesis that RSV travels from the respiratory tract during pregnancy, crosses the placenta to the fetus, persists in the lung tissues of the offspring, and modulates pre- and postnatal expression of growth factors, thereby predisposing to airway hyperreactivity.
Methodology: Pregnant rats were inoculated intratracheally at midterm using recombinant RSV expressing red fluorescent protein (RFP). Viral RNA was amplified by RT-PCR and confirmed by sequencing. RFP expression was analyzed by flow cytometry and viral culture. Developmental and pathophysiologic implications of prenatal infection were determined by analyzing the expression of genes encoding critical growth factors, particularly neurotrophic factors and receptors. We also measured the expression of key neurotransmitters and postnatal bronchial reactivity in vertically infected lungs, and assessed their dependence on neurotrophic signaling using selective biological or chemical inhibition.
Principal findings: RSV genome was found in 30% of fetuses, as well as in the lungs of 40% of newborns and 25% of adults. RFP expression was also shown by flow cytometry and replicating virus was cultured from exposed fetuses. Nerve growth factor and its TrkA receptor were upregulated in RSV- infected fetal lungs and co-localized with increased cholinergic innervation. Acetylcholine expression and smooth muscle response to cholinergic stimulation increased in lungs exposed to RSV in utero and reinfected after birth, and blocking TrkA signaling inhibited both effects.
Conclusions/significance: Our data show transplacental transmission of RSV from mother to offspring and persistence of vertically transmitted virus in lungs after birth. Exposure to RSV in utero is followed by dysregulation of neurotrophic pathways predisposing to postnatal airway hyperreactivity upon reinfection with the virus.