Inhibition of the Alpha/Beta Interferon Response by Mouse Hepatitis Virus at Multiple Levels

Mouse hepatitis virus (MHV) was used as a model to study the interaction of coronaviruses with the alpha/beta interferon (IFN-α/β) response. While MHV strain A59 appeared to induce IFN-β gene transcription and low levels of nuclear translocation of the IFN-β transcription factor interferon regulatory factor 3 (IRF-3), MHV did not induce IFN-β protein production during the course of infection in L2 mouse fibroblast cells. In addition, MHV was able to significantly decrease the level of IFN-β protein induced by both Newcastle disease virus (NDV) and Sendai virus infections, without targeting it for proteasomal degradation and without altering the nuclear translocation of IRF-3 or IFN-β mRNA production or stability. These results indicate that MHV infection causes an inhibition of IFN-β production at a posttranscriptional level, without altering RNA or protein stability. In contrast, MHV induced IFN-β mRNA and protein production in the brains of infected animals, suggesting that the inhibitory mechanisms observed in vitro are not enough to prevent IFN-α/β production in vivo. Furthermore, MHV replication is highly resistant to IFN-α/β action, as indicated by unimpaired MHV replication in L2 cells pretreated with IFN-β. However, when L2 cells were coinfected with MHV and NDV in the presence of IFN-β, NDV, but not MHV, replication was inhibited. Thus, rather than disarming the antiviral activity induced by IFN-β pretreatment completely, MHV may be inherently resistant to some aspects of the antiviral state induced by IFN-β. These findings show that MHV employs unique strategies to circumvent the IFN-α/β response at multiple steps.