The Crimean-Congo Hemorrhagic Fever Virus NSm Protein is Dispensable for Growth In Vitro and Disease in Ifnar^-/- Mice

Crimean-Congo hemorrhagic fever virus (CCHFV) is a tri-segmented, tick-borne nairovirus that causes disease of ranging severity in humans. The CCHFV M segment encodes a complex glycoprotein precursor (GPC) that undergoes extensive endoproteolytic cleavage, giving rise to two structural proteins (Gn and Gc) required for virus attachment and entry, and to multiple non-structural proteins (NSm, GP160, GP85, and GP38). The functions of these non-structural proteins remain largely unclear. Here, we investigate the role of NSm during infection by generating a recombinant CCHFV lacking the complete NSm domain (10200∆NSm) and observing CCHFV ∆NSm replication in cell lines and pathogenicity in Ifnar^-/- mice. Our data demonstrate that the NSm domain is dispensable for viral replication in vitro, and, despite the delayed onset of clinical signs, CCHFV lacking this domain caused severe or lethal disease in infected mice.