The Ebola virus VP35 protein functions as a type IIFN antagonist

An assay has been developed that allows the identification of molecules tha t function as type I IFN antagonists. Using this assay, we have identified an Ebola virus-encoded inhibitor of the type I IFN response, the Ebola viru s VP35 protein. The assay relies on the properties of an influenza virus mu tant, influenza delNS1 virus, which lacks the NS1 ORF and, therefore, does not produce the NS1 protein. When cells are infected with influenza delNS1 virus, large amounts of type I IFN are produced. As a consequence, influenz a delNS1 virus replicates poorly. However, high-efficiency transient transf ection of a plasmid encoding a protein that interferes with type I IFN-indu ced antiviral functions, such as the influenza A virus NS1 protein or the h erpes simplex virus protein ICP34.5, rescues growth of influenza delNS1 vir us. When plasmids expressing individual Ebola virus proteins were transfect ed into Madin Darby canine kidney cells, the Ebola virus VP35 protein enhan ced influenza delNS1 virus growth more than 100-fold. VP35 subsequently was shown to block double-stranded RNA- and virus-mediated induction of an IFN -stimulated response element reporter gene and to block double-stranded RNA - and virus-mediated induction of the IFN-P promoter. The Ebola virus VP35 therefore is likely to inhibit induction of type I IFN in Ebola virus-infec ted cells and may be an important determinant of Ebola virus virulence in v ivo.