Structural gene (prME) chimeras of St. Louis encephalitis virus and West Nile virus exhibit altered in vitro cytopathic and growth phenotypes.

Maharaj PD, Anishchenko M, Langevin SA, Fang Y, Reisen WK, Brault AC.

Source

University of California, Davis, and CDC;

Abstract

Despite utilizing the same avian hosts and mosquito vectors, St. Louis encephalitis virus (SLEV) and West Nile virus (WNV) display dissimilar vector infectivity and vertebrate pathogenic phenotypes. SLEV exhibits a low oral infection threshold for Culex mosquito vectors and is avirulent in avian hosts, producing low-magnitude viremias. In contrast, WNV is less orally infective to mosquitoes and elicits high-magnitude viremias in a wide range of avian species. In order to identify the genetic determinants of these different phenotypes and to assess the utility of mosquito and vertebrate cell lines for recapitulating in vivo differences observed between these viruses, reciprocal WNV and SLEV premembrane and envelope protein (prME) chimeric viruses were generated and growth of these mutant viruses characterized in mammalian (Vero), avian (duck) and mosquito cells [Aedes (C6/36) and Culex (CT)]. In both vertebrate lines, WNV grew to 100-fold higher titer than SLEV and growth and cytopathogenicity phenotypes, determined by chimeric phenotypes, were modulated by genetic elements outside of the prME gene region. Both chimeras exhibited distinctive growth patterns from SLEV in C6/36 cells, indicating the role of both structural and nonstructural gene regions for growth in this cell line. In contrast, growth of chimeric viruses was indistinguishable from the virus containing homologous prME genes in CT cells, indicating that structural genetic elements could specifically dictate growth differences of these viruses in relevant vectors. These data provide genetic insight into divergent enzootic maintenance strategies that could also be useful for the assessment of emergence mechanisms of closely related flaviviruses.