In 2016, the order Mononegavirales was emended through the addition of two new families (Mymonaviridae and Sunviridae), the elevation of the paramyxoviral subfamily Pneumovirinae to family status (Pneumoviridae), the addition of five free-floating genera (Anphevirus, Arlivirus, Chengtivirus, Crustavirus, and Wastrivirus), and several other changes at the genus and species levels. This article presents the updated taxonomy of the order Mononegavirales as now accepted by the International Committee on Taxonomy of Viruses (ICTV).

An assay has been developed that allows the identification of molecules that 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 virus VP35 protein. The assay relies on the properties of an influenza virus mutant, 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, influenza delNS1 virus replicates poorly. However, high-efficiency transient transfection of a plasmid encoding a protein that interferes with type I IFN-induced antiviral functions, such as the influenza A virus NS1 protein or the herpes simplex virus protein ICP34.5, rescues growth of influenza delNS1 virus. When plasmids expressing individual Ebola virus proteins were transfected into Madin Darby canine kidney cells, the Ebola virus VP35 protein enhanced 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-β promoter. The Ebola virus VP35 therefore is likely to inhibit induction of type I IFN in Ebola virus-infected cells and may be an important determinant of Ebola virus virulence in vivo .

The Ebola virus VP35 protein was previously found to act as an interferon (IFN) antagonist which could complement growth of influenza delNS1 virus, a mutant influenza virus lacking the influenza virus IFN antagonist protein, NS1. The Ebola virus VP35 could also prevent the virus- or double-stranded RNA-mediated transcriptional activation of both the beta IFN (IFN-β) promoter and the IFN-stimulated ISG54 promoter (C. Basler et al., Proc. Natl. Acad. Sci. USA 97:12289-12294, 2000). We now show that VP35 inhibits virus infection-induced transcriptional activation of IFN regulatory factor 3 (IRF-3)-responsive mammalian promoters and that VP35 does not block signaling from the IFN-α/β receptor. The ability of VP35 to inhibit this virus-induced transcription correlates with its ability to block activation of IRF-3, a cellular transcription factor of central importance in initiating the host cell IFN response. We demonstrate that VP35 blocks the Sendai virus-induced activation of two promoters which can be directly activated by IRF-3, namely, the ISG54 promoter and the ISG56 promoter. Further, expression of VP35 prevents the IRF-3-dependent activation of the IFN-α4 promoter in response to viral infection. The inhibition of IRF-3 appears to occur through an inhibition of IRF-3 phosphorylation. VP35 blocks virus-induced IRF-3 phosphorylation and subsequent IRF-3 dimerization and nuclear translocation. Consistent with these observations, Ebola virus infection of Vero cells activated neither transcription from the ISG54 promoter nor nuclear accumulation of IRF-3. These data suggest that in Ebola virus-infected cells, VP35 inhibits the induction of antiviral genes, including the IFN-β gene, by blocking IRF-3 activation.

The members of the family Filoviridae , Marburg virus (MBGV) and Ebola virus (EBOV), are very similar in terms of morphology, genome organization, and protein composition. To compare the replication and transcription strategies of both viruses, an artificial replication system based on the vaccinia virus T7 expression system was established for EBOV. Specific transcription and replication of an artificial monocistronic minireplicon was demonstrated by reporter gene expression and detection of the transcribed and replicated RNA species. As it was shown previously for MBGV, three of the four EBOV nucleocapsid proteins, NP, VP35, and L, were essential and sufficient for replication. In contrast to MBGV, EBOV-specific transcription was dependent on the presence of the fourth nucleocapsid protein, VP30. When EBOV VP30 was replaced by MBGV VP30, EBOV-specific transcription was observed but with lower efficiency. Exchange of NP, VP35, and L between the two replication systems did not lead to detectable reporter gene expression. It was further observed that neither MBGV nor EBOV were able to replicate the heterologous minigenomes. A chimeric minigenome, however, containing the EBOV leader and the MBGV trailer was encapsidated, replicated, transcribed, and packaged by both viruses.