Neuraminidase Activity Modulates Cellular Coinfection during Influenza A Virus Multicycle Growth.

Infection of individual cells by multiple virions plays critical roles in the replication and spread of many viruses, but mechanisms that control cellular coinfection during multicycle viral growth remain unclear. Here, we investigate virus-intrinsic factors that control cellular coinfection by influenza A virus (IAV). Using quantitative fluorescence to track the spread of virions from single infected cells, we identify the IAV surface protein neuraminidase (NA) as a key determinant of cellular coinfection. We map this effect to NA's ability to deplete viral receptors from both infected and neighboring uninfected cells. In cases in which viral infectious potential is low, genetic or pharmacological inhibition of NA increases the local spread of infection by increasing the viral load received by neighboring cells. These results identify virus-intrinsic factors that contribute to cellular multiplicity of infection and suggest that optimal levels of NA activity depend on the infectious potential of the virus in question. IMPORTANCE Influenza virus populations are comprised of particles that are mostly noninfectious or only partly infectious. As a result, multiple virions are frequently needed for influenza to infect a new cell. Despite its importance in viral spread, mechanisms that control cellular coinfection are not well established. By tracking the local spread of virions from single infected cells, we identify an important role for the viral receptor-destroying enzyme neuraminidase in modulating the degree of coinfection that occurs during multicycle virus growth. We find that decreasing neuraminidase activity facilitates viral attachment to neighboring cells and increases the infectious load that these cells receive. These results identify a genetic mechanism through which the frequency of coinfection may be tuned, with implications for virus evolution.