Inactivated and live, attenuated influenza vaccines protect mice against influenza:Streptococcus pyogenes super-infections☆
Introduction
Influenza A virus infections that are complicated by secondary bacterial pneumonia make a significant contribution to deaths during both influenza virus epidemics [1] and pandemics [2]. During typical influenza seasons, Streptococcus pneumoniae, Staphylococcus aureus, Haemophilus influenzae, and Streptococcus pyogenes [3], [4], [5] are all known contributors to the “excess mortality” that results from influenza virus super-infections. In fact, in the 1918–1919 influenza pandemic, S. pneumoniae and S. pyogenes (Group A Streptococcus) were the most frequently observed bacterial species in the lungs of infected soldiers [6], and together they likely contributed to as many as 90% of deaths attributed to this pandemic [2]. More recently, findings from the H1N1 swine-origin influenza virus pandemic demonstrate that 29% of deaths were due to secondary bacterial pneumonia in an autopsy series, with 27% of these fatalities being associated with S. pyogenes super-infection [7]. Furthermore, S. pneumoniae and S. pyogenes were the most frequent species associated with increased parapneumonic empyema in a study conducted during the 2009 H1N1 pandemic in Utah [8]. The incidence of invasive diseases caused by S. pyogenes in England increased significantly (26%) in December 2010 and January 2011 in all age groups, due, in part, to widespread influenza infections. Interestingly, the greatest percentage of invasive disease episodes associated with laboratory confirmed influenza infection during this period were caused by S. pyogenes [9].
Several studies have assessed the efficacy and effectiveness of influenza vaccines to prevent influenza-like illness [10], [11], [12], [13], but less information is available regarding the ability of influenza vaccines to limit secondary bacterial complications [14], [15], [16], [17]. Since secondary bacterial infections are the primary cause of mortality associated with influenza virus, methods to limit these complications are currently being sought [18]. The purpose of this study was to directly compare the contributions of IIV and LAIV toward protection in a murine model of influenza virus:S. pyogenes super-infection. We report that both IIV and LAIV vaccines induced systemic (serum) antibody responses, with LAIV also eliciting local (mucosal) IgA antibodies. Subsequently, mice vaccinated against influenza virus demonstrated reduced inflammatory cytokines within bronchoalveolar lavage fluid (BALF), decreased recruitment of inflammatory cells to the lungs, and increased survival, compared to unvaccinated control mice. Despite limiting mortality associated with these super-infections, similar levels of viable bacteria were detected within the lungs of both vaccinated and unvaccinated mice, an outcome that was not observed after sub-lethal inoculation with S. pyogenes alone. Thus, immunity induced after vaccination against influenza virus (either IIV or LAIV) prevented super-infections within mice, albeit incompletely. Overall, protection against super-infection was similar for recipients of either IIV or LAIV.
Section snippets
Mice
Adult (6–8-week-old) female BALB/cJ mice were obtained from Harlan Laboratories (Indianapolis, IN) and housed in groups of four, with 24-h access to food and water. All animal experiments were performed following the guidelines established and approved by the Animal Care and Use committee at the University of South Dakota (Vermillion, SD).
Super-infection model
Viruses expressing the hemagglutinin (HA) and neuraminidase (NA) from A/Hong Kong/1/68-H3N2 were created as described previously [19], [20], and this
Influenza virus:S. pyogenes super-infections can be modeled using an H3N2 HA-expressing virus and MGAS315 bacteria
We first established a murine model of influenza virus:S. pyogenes super-infection using a previously characterized strain of HK68 virus [19] and MGAS315 bacteria [23] for super-infection. Mice were divided into groups that received either HK68 virus alone, MGAS315 bacteria alone, or HK68 virus + MGAS315 bacteria. A control group that received neither virus nor bacteria was also included (Fig. 1). The 3 groups inoculated with either HK68 virus alone, MGAS315 bacteria alone, or HK68 virus + MGAS315
Discussion
The mortality associated with influenza is often due to secondary bacterial complications, including those caused by S. pyogenes [2], [7], [8]. In this study, we established a model of influenza:S. pyogenes super-infection and compared the independent contributions of IIV and LAIV vaccines toward preventing morbidity and mortality associated with this super-infection. Our results demonstrate that despite differences in vaccine-induced immunity, both vaccines provided significant, albeit
Acknowledgments
The authors recognize the technical contributions of Kara Pitchford and Kevin Cwach. We also thank Melanee Clark and the Animal Resource Center at USD and acknowledge the contributions of the Flow Cytometry Core Facility at Sanford Research, in particular Satoshi Nagata and John Lee. Finally, we thank Jonathan A. McCullers for providing HK68 viruses for use in these studies and for critically reading the manuscript.
Conflict of interest: The authors have no conflicts of interest to disclose.
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Portions of this manuscript were presented at the Options for the Control of Influenza VII (September, 2010) in Hong Kong SAR (Abstract Number P-569) and the International Conference on Gram-Positive Pathogens (October, 2010) in Omaha, NE (Poster Number 40).