Murine norovirus virulence factor 1 (VF1) protein contributes to viral fitness during persistent infection

Abstract

Background: Murine norovirus (MNV) is widely used as a model for studying norovirus biology. While MNV isolates vary in their pathogenesis, infection of immunocompetent mice mostly results in persistent infection. The ability of a virus to establish a persistent infection is dependent on its ability to subvert or avoid the host immune response. Previously, we described the identification and characterization of virulence factor 1 (VF1) in MNV, and established that it acts as an innate immune antagonist. Here, we explore the role of VF1 during persistent MNV infection in an immunocompetent host. Methods:Using reverse genetics, we generated VF1-knockout MNV-3 that contained a single or a triple termination codon inserted in the VF1 open reading frame.Results: VF1-knockout MNV-3 replicated to comparable levels to the wildtype virus in tissue culture. Comparative studies between MNV-3 and an acute MNV-1 strain show that MNV-3 VF1 exerts the same functions as MNV-1 VF1, but with reduced potency. Mice infected with VF1-knockout MNV-3 showed significantly reduced replication kinetics during the acute phase of the infection, but viral loads rapidly reached the levels seen in mice infected with wildtype virus after phenotypic restoration of VF1 expression. Infection with an MNV-3 mutant that had three termination codons inserted into VF1, in which reversion was suppressed, resulted in consistently lower replication throughout a three-month persistent infection in mice, suggesting a role for VF1 in viral fitness in vivo. Conclusion:Our results indicate that VF1 expressed by a persistent strain of MNV also functions to antagonise the innate response to infection. We found that VF1 is not essential for viral persistence, but instead contributes to viral fitness in mice. These data fit with the hypothesis that noroviruses utilise multiple mechanisms to avoid and/or control the host response to infection and that VF1 is just one component of this.

Interferon responses to norovirus infections: current and future perspectives

“…3, Table 2). The VF1 protein is the first norovirus protein shown to antagonize the IFN response [11,14,88]. It is a small 213-amino acid protein encoded by an alternate open reading frame overlapping the VP1 sequence.…”

“…As our understanding of innate sensing and restriction of norovirus infections has grown, so too has our knowledge of the wide range of strategies the viruses use to evade immune responses (, ). The VF1 protein is the first norovirus protein shown to antagonize the IFN response [11, 14, 88]. It is a small 213-amino acid protein encoded by an alternate open reading frame overlapping the VP1 sequence.…”

“…It is a small 213-amino acid protein encoded by an alternate open reading frame overlapping the VP1 sequence. It is a mitochondrial protein, present only in MNV, and is not encoded by HuNoV [11, 88]. When RAW264.7 cells are infected with MNV1 M1, a VF1-deleted mutant, they show an increased induction of IFN-β and an impaired ability to activate apoptotic pathways compared to those infected with the wild-type virus [11].…”

“…There also appears to be a strain-specific difference in IFN-β induction following MNV infection in vitro , even in the absence of differences in growth kinetics. The persistent S99 strain, for example, elicits a significantly attenuated induction compared to the acute MNV1.CW3 virus [ 87 ], and the persistent MNV3 induces considerably higher levels compared to the acute MNV1.CW1 [ 88 ]. A variant of MNV3 that does not persist also showed higher IFN-β secretion compared to MNV1.CW3 [ 14 ].…”