Critical Role of Interferon-a Constitutively Produced in Human Hepatocytes in Response to RNA Virus Infection

Several viruses are known to infect human liver and cause the hepatitis, but the interferon (IFN) response, a first-line defense against viral infection, of virus-infected hepatocytes is not clearly defined yet. We investigated innate immune system against RNA viral infection in immortalized human hepatocytes (HuS-E/2 cells), as the cells showed similar early innate immune responses to primary human hepatocytes (PHH). The low-level constitutive expression of IFN-a1 gene, but not IFN-b and IFN-l, was observed in both PHH and HuS-E/2 cells in the absence of viral infection, suggesting a particular subtype(s) of IFN-a is constitutively produced in human hepatocytes. To examine the functional role of such IFN-a in the antiviral response, the expression profiles of innate immune-related genes were studied in the cells with the treatment of neutralization against type I IFN receptor 2 (IFNAR2) or IFN-a itself to inhibit the constitutive IFN-a signaling before and after virus infection. As the results, a clear reduction of basal level expression of IFN-inducible genes was observed in uninfected cells. When the effect of the inhibition on the cells infected with hepatitis C virus (HCV) was examined, the significant decrease of IFN stimulated gene expression and the enhancement of initial HCV replication were observed, suggesting that the steady-state production of IFN-a plays a role in amplification of antiviral responses to control the spread of RNA vira