(217d) Spatial Patterns of Growth and Inhibition in Spreading Infections of Vesicular Stomatitis Virus

Viruses infect their multi-cellular hosts through processes that play out over multiple scales: a virus particle infects a susceptible host cell, producing virus progeny that then spread to other cells within the host and initiate new rounds of infection.  Infected cells make not only virus progeny, but also virus-like defective interfering particles (DIPs), which lack essential genes needed for replication.  DIPs are found in natural isolates of influenza A, dengue and West Nile viruses, but their roles in infection spread and disease pathogenesis are not known.  DIPs alone are non-infectious.  However, in co-infections with intact virus, DIP genomes can interfere with virus growth by competing for viral polymerase, replicating at the expense of viral genomes, or by binding packaging proteins, assembling DIP progeny at the expense of intact virus particles.  To elucidate how DIP-virus interactions play out in spreading infections we engineered wild-type and DIP strains of vesicular stomatitis virus to encode and express red and green fluorescent proteins, respectively.  Moreover, the reporter-DIP was further designed to carry a deletion of the gene for the viral surface glycoprotein G (VSV-ΔG-eGFP).  With this system we were able to monitor the dynamics of wild-type and defective virus gene expression through their respective reporter proteins in single co-infected cells.  In co-infections of wild-type (RFP) the reporter-DIP inhibited production of infectious virus to the same extent as standard non-reporter DIPs, but reporter-DIPs were less potent in their ability to inhibit wild-type (RFP) reporter protein production.  Heterogeneity in protein expression patterns was apparent among cells co-infected with the same dose of wild-type and reporter-DIPs, but wild-type protein expression (RFP) overall decreased as DIP reporter protein (GFP) expression increased.  When infections were permitted to spread spatially across a population of susceptible cells, the heterogeneity in reporter protein increased.  Unlike spreading infections in the presence of natural DIPs, which can completely arrest infection spread, co-infection with reporter-DIP produced many local areas of wild-type virus escape, and complete arrest of spread was not observed.  These results suggest that competition for packaging protein G can reduce wild-type virus production at the cell level to the same extent as natural DIPs, but this reduction is not sufficient to fully inhibit wild-type virus spread in host cell populations.  More broadly, these results highlight how similar patterns of virus growth and inhibition at the level of individual cells can exhibit distinctly different properties of infection spread or containment when virus and DIPs propagating spatially across a population of susceptible cells.