Mx is dispensable for interferon-mediated resistance of chicken cells against influenza A virus

The type I interferon (IFN) system plays an important role in antiviral defence against influenza A viruses (FLUAV) which are natural chicken pathogens. Studies in mice identified the Mx1 protein as key effector molecule of the IFN-induced antiviral state against FLUAV. Chicken Mx genes are highly polymorphic, and recent studies suggested that an Asn/Ser polymorphism at amino acid position 631 might determine the antiviral activity of the chicken Mx protein. By employing chicken embryo fibroblasts with defined Mx-631 polymorphisms and retroviral vectors for the expression of Mx isoforms in chicken cells and embryonated eggs, we show here that neither the 631Asn nor the 631Ser variant of chicken Mx was able to confer antiviral protection against several low and high pathogenic FLUAV strains. Using a siRNA-mediated knockdown approach we noted that the antiviral effect of type I IFN in chicken cells was not dependent on Mx, suggesting that some other IFN-induced factors must contribute to inhibition of FLUAV in chicken cells. Finally, we found that both isoforms of chicken Mx protein appear to lack GTPase activity which might explain the observed lack of antiviral activity.