Nanoparticles increase human bronchial epithelial cell susceptibility to respiratory syncytial virus infection via nerve growth factor-induced autophagy.

Cytotoxic and neuroinflammatory effects of TiO2 nanoparticles (TiO2 -NP) in human airways are mediated by nerve growth factor (NGF), which is also implicated in the pathophysiology of respiratory syncytial virus (RSV) infection. We tested the hypothesis that exposure to TiO2 -NP results in increased susceptibility to RSV infection and exacerbation of airway inflammation via NGF-mediated induction of autophagy in lower respiratory tract cells. Human primary bronchial epithelial cells were exposed to TiO2 -NP for 24 h prior to infection with recombinant red RSV (rrRSV). Expression of NGF and its TrkA and p75NTR receptors was measured by real-time PCR and fluorescence-activated cell sorting (FACS). Autophagy was assessed by beclin-1 expression analysis. Cell death was studied by FACS after annexin V/propidium iodide staining. rrRSV infection efficiency more than doubled in human bronchial cells pre-exposed to TiO2 -NP compared to controls. NGF and its TrkA receptor were upregulated in RSV-infected bronchial cells pre-exposed to TiO2 -NP compared to controls exposed to either rrRSV or TiO2 -NP alone. Silencing NGF gene expression with siRNA significantly inhibited rrRSV infection. rrRSV-infected cells pre-exposed to TiO2 -NP also showed increase in necrotic cell death and reduction in apoptosis, together with 4.3-fold increase in expression of the early autophagosomal gene beclin-1. Pharmacological inhibition of beclin-1 by wortmannin resulted in increased apoptotic rate along with lower viral load. This study shows that TiO2 -NP exposure enhances the infectivity of RSV in human bronchial epithelial cells by upregulating the NGF/TrkA axis. The mechanism of this interaction involves induction of autophagy promoting viral replication and necrotic cell death.