Dicer Deletion Leads to Different Antiviral Responses in Mouse Embryonic Stem Cells

Abstract

Interferon (IFN) response is the primary antiviral mechanism presumably developed in most, if not all, types of mammalian cells. However, recent studies have demonstrated that embryonic stem cells (ESCs) from both human and mouse are deficient in expressing type I IFN. It is proposed that ESCs may use RNA interference (RNAi) as an alternative antiviral system, which is the major antiviral mechanism in invertebrates but not normally operational in mammalian cells. RNAi uses short interfering RNA (siRNA) derived from viral RNA to limit viral replication. Since Dicer is a key enzyme for siRNA biogenesis, Dicer knockout ESCs (D-/-ESCs) could be used as a loss of function model to determine antiviral RNAi activity in ESCs. In this study, we compared the infectivity of Chikungunya virus (ChikV) and La Crosse virus (LaCV) in normal mouse ESCs and D-/-ESCs. It is expected that ESCs would be more susceptible to viral infection than D-/-ESCs if RNAi is functional ESCs. This is true in the case of LaCV infection, but surprisingly, D-/-ESCs is more resistant to ChikV infection than ESCs, indicating that other antiviral pathways may also be involved. These results could be explained by our recent findings that D-/-ESCs have gained the ability to express IFNβ, which is otherwise deficient in wild-type ESCs. Furthermore, D-/-ESCs have constitutively active double-stranded RNA (dsRNA)-activated protein kinase (PKR), which also plays an important role in antiviral response. Our results demonstrated that inhibiting PKR increased the infectivity of both viruses in D-/-ESCs and ESCs. Incubation of ESCs with neutralizing antibodies against IFNβ did not have an effect on viral infectivity in ESCs as expected. However, neutralizing antibody treatment increased the infectivity of ChikV, but not LaCV, in D-/-ESCs. Based on these findings, we can conclude that PKR can provide antiviral activity to both ESCs and D-/-ESCs, but IFNβ is only produced in D-/-ESCs and shows its effect in a virus-dependent manner. We are currently investigating the molecular mechanisms underlying this interesting finding.