The Role of IL-27 in Susceptibility to Post-Influenza Staphylococcus Aureus Pneumonia

Influenza is a common respiratory virus and Staphylococcus aureus frequently causes secondary pneumonia during influenza infection, leading to increased morbidity and mortality. Influenza has been found to attenuate subsequent Type 17 immunity, enhancing susceptibility to secondary bacterial infections. IL-27 is known to inhibit Type 17 immunity, suggesting a potential critical role for IL-27 in viral and bacterial co-infection

STAT2 Signaling Regulates Macrophage Phenotype During Influenza and Bacterial Super-Infection

Influenza is a common respiratory virus that infects between 5 and 20% of the US population and results in 30,000 deaths annually. A primary cause of influenza-associated death is secondary bacterial pneumonia. We have previously shown that influenza induces type I interferon (IFN)-mediated inhibition of Type 17 immune responses, resulting in exacerbation of bacterial burden during influenza and Staphylococcus aureus super-infection. In this study, we investigated the role of STAT2 signaling during influenza and influenza-bacterial super-infection in mice. Influenza-infected STAT2−/− mice had increased morbidity, viral burden, and inflammation when compared to wild-type mice. Despite an exaggerated inflammatory response to influenza infection, we found increased bacterial control and survival in STAT2 deficient mice during influenza-MRSA super-infection compared to controls. Further, we found that increased bacterial clearance during influenza-MRSA super-infection is not due to rescue of Type 17 immunity. Absence of STAT2 was associated with increased accumulation of M1, M2 and M1/M2 co-expressing macrophages during influenza-bacterial super-infection. Neutralization of IFNγ (M1) and/or Arginase 1 (M2) impaired bacterial clearance in Stat2−/− mice during super-infection, demonstrating that pulmonary macrophages expressing a mixed M1/M2 phenotype promote bacterial control during influenza-bacterial super-infection. Together, these results suggest that the STAT2 signaling is involved in suppressing macrophage activation and bacterial control during influenza-bacterial super-infection. Further, these studies reveal novel mechanistic insight into the roles of macrophage subpopulations in pulmonary host defense

A novel outbred mouse model of 2009 pandemic influenza and bacterial co-infection severity

Influenza viruses pose a significant health risk and annually impose a great cost to patients and the health care system. The molecular determinants of influenza severity, often exacerbated by secondary bacterial infection, are largely unclear. We generated a novel outbred mouse model of influenza virus, Staphylococcus aureus, and coinfection utilizing influenza A/CA/07/2009 virus and S. aureus (USA300). Outbred mice displayed a wide range of pathologic phenotypes following influenza virus or co-infection ranging broadly in severity. Influenza viral burden positively correlated with weight loss although lung histopathology did not. Inflammatory cytokines including IL-6, TNF-α, G-CSF, and CXCL10 positively correlated with both weight loss and viral burden. In S. aureus infection, IL-1β, G-CSF, TNF-α, and IL-6 positively correlated with weight loss and bacterial burden. In co-infection, IL-1β production correlated with decreased weight loss suggesting a protective role. The data demonstrate an approach to identify biomarkers of severe disease and to understand pathogenic mechanisms in pneumonia. © 2013 McHugh et al

Influenza virus induces variable weight loss and inflammation in outbred mice.

<p>Diversity outbred mice were infected with 1x10⁶ pfu of influenza A/California/07/2009 virus and were harvested five days later. A, weight loss over time as percentage of pre-challenge starting weight (N=30). B, correlation between starting weight and weight loss percentage (N=30). C, bronchoalveolar lavage cell counts correlated with weight loss percentage (N=14). D, bronchoalveolar lavage protein concentration correlated with weight loss percentage (N=14). R, Pearson’s Rho; P indicates significance as listed. Solid bar indicates group mean.</p

Inflammatory cytokine levels correlated with weight loss or viral burden in influenza virus, <i>S. aureus</i> co-infected mice.

<p>Diversity outbred mice were infected with 1x10⁶ pfu of influenza A/California/07/2009 virus followed by infection with <i>S. aureus</i> USA300 on day 6 and then harvested 2 days later. A, weight loss percentage correlated with cytokine expression in lung homogenate (by Lincoplex) (N=12). B-D, influenza virus M protein gene expression correlated with cytokine expression in lung homogenate (N=12). E, F variable pathology in co-infected individual mice. R, Pearson’s Rho; P indicates significance as listed.</p

Viral burden correlates with weight loss in outbred mice.

<p>Diversity outbred mice were infected with 1x10⁶ pfu of influenza A/California/07/2009 virus and were harvested five or eight days later. A, day five influenza virus M protein gene expression (determined by qRT-PCR) correlated with weight loss percentage (N=13). B, day eight influenza virus M protein gene expression correlated with weight loss percentage (N=13). R, Pearson’s Rho; P indicates significance as listed.</p

Variable lung pathology in influenza virus infected outbred mice.

<p>Diversity outbred mice were infected with 1x10⁶ pfu of influenza A/California/07/2009 virus and were harvested five days later. A, B, individual mice with similar influenza viral burden (relative M protein gene expression, lower number) but disparate weight loss (weight loss percentage, upper number). C, D, individual mice with similar weight loss but different viral burdens. E, individual mouse with extreme weight loss and high viral burden, but with minimal lung inflammation pathology. F, individual mouse with no weight loss and low viral burden, but with significant lung injury. G, histologic scoring of lung parenchymal inflammation. </p

Inflammatory cytokine levels correlated with weight loss or bacterial burden in <i>S. aureus</i> infected mice.

<p>Diversity outbred mice were infected <i>S. aureus</i> USA300 and then harvested 2 days later. A, C, E, F, weight loss percentage correlated with cytokine expression in lung homogenate (by Lincoplex) (N=15). B, D, bacterial burden correlated with cytokine expression in lung homogenate (N=15). R, Pearson’s Rho; P indicates significance as listed.</p

Inflammatory cytokine levels correlated with influenza viral burden or weight loss.

<p>Diversity outbred mice were infected with 1x10⁶ pfu of influenza A/California/07/2009 virus and were harvested five days later. A, cytokine levels in lung homogenate (by Lincoplex) correlated with influenza virus M protein gene expression (by qRT-PCR) (N=13). B, cytokine levels in serum correlated with influenza virus M protein gene expression (N=14). C, D, cytokine levels in serum correlated with weight loss percentage (N=14). R, Pearson’s Rho; P indicates significance as listed.</p

Inflammatory cytokine levels correlated with influenza virus induced weight loss at day eight.

<p>Diversity outbred mice were infected with 1x10⁶ pfu of influenza A/California/07/2009 virus and were harvested eight days later. A-C, weight loss percentage correlated with lung homogenate cytokine concentration (by Lincoplex) (N=12). R, Pearson’s Rho; P indicates significance as listed.</p