RIG-I and MDA5 are the major intracellular sensors for the induction of IFN-β, by dsRNA, in Huh7 cells. It was found that the expression of RIG-I, MDA5 and the signalling mediators TRIF, MAVS and MyD88 were upregulated in Huh7 HCV RNA replicon cells. Similar findings were obtained in Huh7 cells infected with JFH-1 HCV. When replicon and HCV infected cells were further stimulated by dsRNA the induction of these genes were reduced compare to control cells. The expression of NS3 and NS3/4A proteins, using adenovirus vectors, reduced RIG-I, MDA5, MAVS and MyD88 expression levels in response to dsRNA stimulation. Ablation of MDA5, MAVS, MyD88 and TRIF in HCV infected Huh7.5 cells, using siRNAs, increased virus replication demonstrating their importance in inhibiting HCV infection. These results show that HCV inhibits the induction of interferon in response to dsRNA and that NS3 plays an important role in this inhibition.
Analysis of gene expression during HCV infection showed significant changes in the expression of genes involved in the innate immune response, interferon, apoptosis and cell cycle regulation pathways. These include SOCS1, 2 and 3, OAS3, MXA1, IRF9, IRF1, IFNAR2 and STAT1. SOCS3 knockdown by siRNAs reduced HCV replication indicating an additional mechanism of IFN inhibition by HCV.
Infection with HCV also caused cell cycle arrest leading to DNA fragmentation and apoptosis. A decrease in cyclin B1 and an increase in GADD45-β expression during HCV infection indicates that they may play an important role in HCV induced alterations to the cell cycle.