North American Powassan virus encompasses diverse in vitro phenotypes

Powassan virus (POWV) is a tick-borne orthoflavivirus which has resulted in increasing human cases over the past two decades. Despite high prevalence in ticks and evidence of broad distribution in North America, fewer than 50 human cases are detected annually with evidence of undetected asymptomatic infections. Experimental studies of the relationships between POWV genetic diversity and disease potential are currently lacking. In the present study, 17 isolates originating from 14 locations in the United States and Canada were used to assess virus biology, in vitro phenotypic diversity, and replication in multiple mammalian cell lines. Differences were observed in plaque size, with lineage II Northeast viruses generating significantly smaller plaques than both lineage I and lineage II Midwest viruses. Another significant phenotype was broad differences in replication in all cell lines, with lineage I viruses often generating significantly higher titers. Following serial passage of all viruses on deer mouse cells, we identified two residues in the envelope protein (164 and 308) that changed in two viruses, however the role of these mutations is unknown. These results support the observation that the North American POWV population may be highly genetically and phenotypically diverse. The degree to which in vitro phenotype reflects differences in transmission and pathogenesis remains to be determined.