| Influenza is a negative sense, single-stranded RNA virus that is associated with 114,000 hospitalizations and 36,000 deaths annually in the United States. Many of these deaths are due to complications from secondary bacterial infections, and it is well-appreciated that the virus, host, and bacterial species all make contributions to these deadly outcomes. This thesis describes my evaluation of all three of these components as distinct contributors to deadly influenza virus:Streptococcus pyogenes super-infections. Using an influenza virus:S. pyogenes super-infection model, I compared an influenza virus isolate that causes death (A/Puerto Rico/8/34-H1N1, PR8) with one that allows for survival after inoculation with bacteria (A/swine/Texas/4199-2/98-H3N2, TX98). To specifically evaluate the virus gene contributions to these distinct phenotypes, I used reverse genetics to create a panel of influenza virus reassortants. This approach allowed me to identify the influenza virus hemagglutinin (HA) and nonstructural (NS) genes as key regulators of survival during an influenza virus:S. pyogenes super-infection. To define the host immune responses that dictate susceptibility to a super-infection, I compared host immune responses induced during the course of PR8 and TX98 infections. This comparison showed that induction of the cytokine IL-27 within 24 hours after TX98 infection is associated with survival, while increased levels of the pro-inflammatory cytokines IFN-gamma, IL-6, and TNF-alpha were detected after infection with the deadly PR8 virus. Finally, using a vaccine against the M protein of S. pyogenes that is currently being evaluated in clinical trials, I present evidence that vaccination against this virulence factor can prevent death within our influenza virus:S. pyogenes super-infection model. The protection observed provides evidence that vaccine-induced immunity against bacteria can protect against a secondary bacterial infection, even in a host whose immune system is weakened by a primary virus infection. Overall, this work provides us with viral, host, and bacterial targets that can be further evaluated for their ability to prevent death after super-infection. The impact of these findings will be discussed in the context of historical knowledge of these viral, host, and bacterial factors, and future work will be toward a full appreciation of these complex interactions during polymicrobial infections. |
