Characterization of West Nile virus strains isolated in Italy

West Nile virus (WNV) is a neurotropic flavivirus that is maintained in an enzootic cycle between mosquitoes and birds, but can also infect and cause disease in humans and other vertebrate species. Most of WNV infections in humans are asymptomatic, but approximately 20% of infected people develop clinical symptoms, although severe neurological diseases are observed in less than 1% of them. WNV is the most widely distributed arbovirus in the world and has been recently associated with outbreaks of meningo-encephalitis in Europe, including Italy, caused by different viral strains belonging to distinct lineages 1 and 2. The hypothesis is that genetic divergence among viral strains currently circulating in Italy might reflect on their pathogenic potential and that the rapid spread of WNV with increased pathogenicity within naïve population suggest that epidemic forms of the virus may encode mechanisms to evade host immunity. Infection with WNV triggers a delayed host response that includes a delay in the production of interferon-α (IFN-α). IFNs are a family of immuno-modulatory cytokines that are produced in response to virus infection and serve as integral signal initiators of host intracellular defenses. The increased number of human cases and the lack of data about virulence of European WNV isolates highlight the importance to achieve a better knowledge on this emerging viral infection. In the present study, we investigate the phenotypic and IFN-α-regulatory properties of different WNV lineage 1 and 2 strains that are circulating in Europe/Italy in two cell lines: Vero and 1321N1. We demonstrate that: Vero and 1321N1 cells are capable of supporting WNV replication where different WNV strains show similar growth kinetics; WNV lineage 2 strain replicated in Vero and 1321N1 cells as efficiently as WNV lineage 1 strains; and both lineages 1 and 2 were highly susceptible to the antiviral actions of IFN-α.