For CPXV, viral titers in supernatants increased dramatically 48 hpi at the same time point as clear cytopathic effects were observed, indicating an inefficient control of the infection in these cells. Although displaying a lytic infection, production of viral LV progeny did not increase with increased amounts of lyzed cells, indicating a less efficient replication than for CPXV. Natural infections of bank voles with CPXV and LV are believed to occur without symptoms. If these infections also cause lytic infections of cells in the natural host in vivo remains to be shown. Interestingly, PUUV replicated efficiently, showing increased production of progeny virus over time, without clearly inducing IFN-b or Mx2. This suggests a strong PUUV-mediated repression of viral recognition and/or inhibition of IFN-activation in these cells. In contrast infection of human fibroblasts with the same PUUV-stock resulted in induction of IFN-b and MxA, and inhibition of progeny virus production over time, suggesting that PUUV regulate bank vole cells in a different manner than human cells. In contrast to what was AMN107 observed for PUUV, both IFN-b and Mx2 were clearly induced in TBEV-infected VEFs, and virus production did not increase over time. Hence, these results indicate an innate immune reaction that keeps TBEV in check, although not strong enough to completely inhibit progeny virus production. This in vitro model together with our initial characterizations will simplify future studies addressing virus-host interactions between bank vole borne viruses and natural host-derived cells, including possible regulatory effects on innate immune activation as well as on antiviral responses. The model presented here could easily be extended to other natural host species for hantaviruses, and hence facilitate future investigations aiming at understanding how hantavirus regulation of natural host cells differs from that of human cells, and might also provide a reliable method for isolation of wild-type viruses. The genetic diversity within bank voles might lead to differences in outcome of infection. We believe that isolations of VEFs from genetically different bank voles isolated in different geographical localities, can serve as a useful tool for studies addressing the effect of genetic differences on innate immune responses and other markers of infection, which might lead to a better understanding of bank vole-borne virus pathogenesis. Further, information on the role of infected fibroblasts during viral infections is not available and hopefully the VEF system presented here provide new knowledge about infections of bank vole cells in general. Experimental infection of natural hosts with hantaviruses have earlier been established for Sin Nombre virus, Seoul virus and PUUV. In addition, genes encoding deer mouse chemokines and cytokines have been sequenced, thereby enabling studies of immune responses during SNV-infection of the natural host in vivo. However, to our knowledge this is the first report of an in vitro-model based on cells from a natural hantavirus host. It is not known why hantavirus infection in humans causes disease, while the infections in natural hosts are asymptomatic.