Human cell line tropism, interferon system interactions and NSs mechanisms by the Ntepes and Gabek Forest phleboviruses

Phleboviruses are a large genus within the family Phenuiviridae, class Bunyaviricetes. These arboviruses are present all over the world, and several members can cause severe disease. New viruses are continuously discovered, but rarely isolated and analysed in cell culture to assess their risk to humans. Here, we describe the in cellulo characteristics for two closely related African phleboviruses, Ntepes virus (NTPV) and Gabek Forest virus (GFV), for which human seroprevalence has been reported but pathogenicity remains unknown. For both viruses, human cell lines from liver, lung, and kidney were permissive, and their capacity to suppress and cope with the antiviral type I interferon (IFN) system was comparable to Rift Valley fever phlebovirus MP-12. Consequently, their non-structural protein NSs, a well-known virulence factor of Phenuiviridae, was able to interfere with both the induction and signaling of IFN. Blockade of IFN induction is a conserved NSs activity, and we found that in the case of NTPV and GFV, the NSs targets MAVS (Mitochondrial antiviral signaling protein), a cellular signaling adapter which plays a key role in the induction of IFN gene expression. NTPV and GFV NSs thereby strongly and specifically bind the N-terminal CARD (Caspase recruitment domain) of human MAVS, thus occupying the very domain that is required for MAVS to relay the signal coming from the virus sensor RIG-I. Thus, the cell line tropism, IFN system interactions and NSs mechanisms suggest that NTPV and GFV fulfill the in cellulo criteria for successful infection of humans.