Analysis of Antiviral Response in Human Epithelial Cells Infected with Hepatitis E Virus

Hepatitis E virus (HEV) is a major cause of enterically transmitted acute hepatitis in developing nations and occurs in sporadic and epidemic forms. The disease may become severe with high mortality (20%) among pregnant women. Due to lack of efficient cell culture system and small animal model, early molecular events of HEV infection are not yet known. In the present study, human lung epithelial cells, A549, were infected with HEV to monitor expression levels of genes/proteins in antiviral pathways. Both live and UV inactivated virus elicited robust induction of inflammatory cytokines/chemokines such as IL-6, IL-8, TNF-alpha, and RANTES within 12 h of infection. Cells exposed to soluble capsid protein showed no induction suggesting the capsid structure and not the protein being detected as the pathogen pattern by cells. A delayed upregulation of type I interferon genes only by the live virus at 48 h post HEV infection indicated the need of virus replication. However, absence of secreted interferons till 96 h suggested possible involvement of post-transcriptional regulation of type I IFN expression. HEV infected cells showed activation of both NF-kappa B and IRF3 transcription factors when seen at protein levels; however, reporter gene assays showed predominant expression via NF-kB promoter as compared to IRF3 promoter. Knockdown experiments done using siRNAs showed involvement of MyD88 and TRIF adaptors in generating antiviral response thus indicating role of TLR2, TLR4 and TLR3 in sensing viral molecules. MAVS knockdown surprisingly enhanced only proinflammatory cytokines and not type I IFNs. This suggested that HEV not only down-regulates RIG-I helicase like receptor mediated IFN induction but also employs MAVS in curtailing host inflammatory response. Our findings uncover an early cellular response in HEV infection and associated molecular mechanisms suggesting the potential role of inflammatory response triggered by HEV infection in host immune response and pathogenesis.