Ashley Sobel Leonard, Micah T. McClain, Gavin J. D. Smith, David Wentworthd, Rebecca A. Halpin, Xudong Lin, Amy Ransier, Timothy B. Stockwell, Suman Das, Anthony S. Gilbert, Robert Lambkin-Williams, Geoffrey S. Ginsburg, Christopher W. Woods, Katia Koelle
Journal of Virology
October 5, 2016
Knowledge of influenza evolution at the point of transmission and at the intra-host level remains limited, particularly for human hosts. Here, we analyze a unique viral dataset of next-generation sequencing (NGS) samples generated from a human influenza challenge study wherein 17 healthy subjects were inoculated with egg-passaged virus. Nasal wash samples collected from 7 of these subjects were successfully deep sequenced. From these, we characterized changes in the subjects’ viral populations during infection and identified differences between the virus in these samples and the viral stock used to inoculate the subjects. We first calculated pairwise genetic distances between the subjects’ nasal wash samples, the viral stock, and the A/Wisconsin/67/2005 (H3N2) reference strain used to generate the stock virus. These distances revealed that considerable viral evolution occurred at various points in the human challenge study. Further quantitative analyses indicated that: (1) the viral stock contained genetic variants that originated and likely were selected for during the passaging process; (2) direct intranasal inoculation with the viral stock resulted in a selective bottleneck that reduced nonsynonymous genetic diversity in the viral hemagglutinin and nucleoprotein; and (3) intrahost viral evolution continued over the course of infection. These intrahost evolutionary dynamics were dominated by purifying selection. Our findings indicate that rapid viral evolution can occur during acute influenza infection in otherwise healthy human hosts when the founding population size of the virus is large, as is the case with direct intranasal inoculation.