A VLP approach to combat postinfluenza bacterial infections

Clinical and historical data underscore the ability of influenza viruses to ally with certain bacterial species and predispose the host for bacterial infections after an influenza infection. Bacterial pneumonia postinfluenza was identified as the major cause of mortality during the most devastating influenza pandemics in 1918/19 and 1957/58 with S.aureus and S.pneumoniae being the most commonly associated etiological agents. Vaccination is the best method to prevent infectious disease. There is, however, no S.aureus prophylaxis available, S.pneumoniae vaccines were shown to be largely ineffective and the prevalence of multi-drug-resistant bacterial strains is alarming. Yet, it was shown that by limiting primary viral infection, influenza vaccines are able to decrease subsequent secondary bacterial infections, leaving influenza vaccines as only promising measure to prevent secondary bacterial complications. Current influenza vaccines, however, are only effective in preventing an infection with a highly similar influenza strain. Thus, mutated or novel influenza strains are still able to cause disease and make patients susceptible to potential difficult-to-treat bacterial superinfections. Three components of the influenza virus have been identified as key players involved in increasing the host susceptibility to postinfluenza bacterial infections. They include the influenza surface protein neuraminidase and non-structural viral proteins (termed NS1 and PB1-F2). These components were either shown to modify the lung environment of infected hosts, thereby facilitating colonization of bacteria or suppress the host innate defense by modulating key regulatory factors of innate immunity. These viral components, however, are not standardized or even absent from current vaccine preparations. In the present study we aim to produce virus-like particles, which resemble the authentic virus, but are non-infectious. The main constituent is the influenza hemagglutinin, which is the only standardized component of current vaccines. We will supplement viral components critical for the viral/bacterial synergism at different concentrations to this virus-like particle preparation. We hypothesize that immunity provided by the supplemented antigens plays a critical role in limiting bacterial superinfections. Above that, our test setup will mimic a situation of influenza vaccine mismatch, as vaccinated mice are infected with a heterologous influenza virus before bacterial superinfection with S.aureus and S.pneumoniae. Our hypothesis will be evaluated in established postinfluenza superinfection mouse models. Above that, we will investigate whether immunity provided by supplemented antigens interferes with suggested mechanisms of viral/bacterial synergism, such as modifications of the lung environment or the modulation of host factors of innate immunity. Our approach is highly innovative as no recombinant influenza vaccines have been tested in superinfection models so far and results of our study may shed light on viral proteins that in the past have been left unattended in the field of influenza immunology and vaccine development.

Clinical and historical data underscore the ability of influenza viruses to make an individual more susceptible to secondary bacterial infections, which frequently results in severe illness and mortality. In fact, pneumonia linked to infections with the bacterial strains Staphylococcus aureus or Streptococcus. pneumoniae (Pneumococcus) were identified as major causes of mortality during the most devastating influenza pandemics in 1918/19 (the "Spanish Flu") and 1957/58 (the "Asian Flu"). Vaccination is the best method to prevent an infectious disease. There is currently no S. aureus prophylaxis available and pneumococus vaccines are conditionally effective. On top, the rise of multi-drug-resistant bacterial strains is alarming. It has already been established that a previous influenza virus infection favors a bacterial secondary infection. Furthermore, there is evidence that influenza immunity conferred by a previous influenza infection may not only mitigate a later influenza infection, it also may mitigate bacterial secondary infections occurring shortly after an influenza virus infection. Yet, the efficacy of influenza vaccine-conferred immunity in this respect remains a little investigated field. Within this project, we evaluated the efficacy of immunity conferred by a recombinant protein-based vaccine preparation on bacterial infections occurring within a narrow time window after an influenza infection. The main component of conventional influenza vaccines, the protein influenza hemagglutinin (HA), changes rapidly, occasionally rendering influenza vaccines ineffective (vaccine mismatch). Our main focus, therefore, was to investigate the efficacy of matched as well as mismatched influenza HA-specific immunity in the context of bacterial infections in temporal proximity to an influenza infection. In both cases, our results in mouse models suggest that influenza hemagglutinin-targeted immunity results in a milder course of disease and reduces mortality. However, only when the influenza vaccine is adjusted to the circulating virus strain we could benefit from a high protection also from bacterial secondary infection after an influenza infection. As a next logical step, we tested whether the supplementation of additional influenza antigens (influenza neuraminidase (NA), nonstructural protein (NS1), PB1-F2) to the vaccine preparation has potential to increase the protective efficacy of an influenza vaccine against postinfluenza S.aureus and S.pneumoniae infections in a scenario of an influenza vaccine mismatch. In our simple experimental set-up (single immunization), we observed a trend towards increased protection, but this was not significant. Yet, further studies investigating higher antigen doses, the addition of adjuvants or multiple vaccine doses may succeed in achieving the desired results.