Bacterial secondary infections played significant roles in the increased morbidity and mortality during past influenza virus pandemics (3
). Although antibiotics have decreased to some extent the impact of secondary infections, bacterial pneumonia following influenza virus infection has continued to pose a significant threat to human health. Several mechanisms have been proposed to explain the association between primary influenza viral and secondary bacterial infections, including increased colonization, changes in airway function, increased respiratory compromise, increased inflammatory responses, and synergy between influenza viral and bacterial replication (7
). This study reveals that death of progenitor basal epithelial cells and loss of airway epithelial cell reproliferation and lung repair responses may also play a role in disease severity.
The upper respiratory tract contains numerous physical barriers that impede the introduction of foreign materials, including bacteria, into the lung. Damage and necrosis of epithelial cells lining the respiratory tree lead to loss of mucociliary clearance and can facilitate colonization of the lungs with bacteria present in the oronasopharynx (11
). It has also been proposed that bacterial colonization of the upper respiratory tract is accelerated due to epithelial cell damage and exposure of attachment sites by viral neuraminidase (NA) activity (6
). In addition, inflammatory responses, including fibrin deposition, tissue repair, and regenerative processes, can result in the exposure of bacterial attachment sites. Several studies have suggested that enhanced influenza viral replication could also occur due to increased proteolytic processing of viral HA0
protein by bacterial proteases (27
). Although increased viral replication was not observed in our study, we did observe an increase in pulmonary bacterial 16S rRNA expression during Mex09-S. pneumoniae
Once bacterial coinfection is established, severity of respiratory compromise can occur in response to blockage and loss of small airway function and development of exudative transmural pleuritis (11
). Additionally, dysregulation of immune responses can contribute to disease severity and compromise of lung function during influenza virus and bacterial coinfection (29
). Our results, however, showed that activation of inflammatory responses prior to death during bacterial coinfection, as measured by gene expression, appeared to be independent of lung pathology and survival. Primary viral infection with Mex09 resulted in increased inflammatory and cell death responses that were accompanied by strong lung repair and reproliferation responses compared to NIH50 primary viral infection. In contrast, animals infected with Mex09 plus S. pneumoniae
showed slightly decreased activation of inflammatory response-related gene expression relative to Mex09-only infection but with a significant loss of lung repair responses.
Repair of lung epithelium involves several steps: proliferation and differentiation of progenitor basal epithelial cells, migration/spreading of these cells to cover the denuded areas, and resolution (22
). Our study suggests that a more virulent primary influenza viral infection, with increased loss of the more superficial airway epithelial cells, may lead to increased bacterial replication and attachment to the exposed basal epithelial cells and the associated loss of these critical respiratory epithelial progenitor cells (29
). The lack of expression of genes encoding key growth factors normally produced by basal cells and the marked loss of basal epithelial cells observed histologically in the lungs of Mex09-S. pneumoniae
-infected animals support this hypothesis. Loss of basal epithelial cells would preclude the reestablishment of the normal airway epithelium, resulting in increased and sustained respiratory compromise and death (30
). Increased lung damage and lack of lung repair processes could also lead to loss of tissue integrity and spread of bacteria outside the lung. This is supported by the bacteremia observed in the Mex09-S. pneumoniae
The association between failure to restore lung epithelium and lethality of coinfections also raises the possibility of therapeutically targeting these repair pathways, such as treatment with exogenous KGF, which has been shown to have a protective effect in a variety of lung injury models (31
). In combination with antibiotic and antiviral therapies, treatments that stimulate lung repair responses could be beneficial in improving survival.
The present study has demonstrated that outcome of influenza virus and bacterial coinfection correlated with the pathogenicity of the primary viral infection, enhanced bacterial replication, and the extent of lung repair required for survival. Increased loss of superficial respiratory epithelial cells during a more pathogenic viral infection led to exposure and subsequent death of progenitor basal epithelial cells by bacterial infection, resulting in a loss of critical respiratory epithelial cell reproliferation and lung repair processes that also likely contributed to the development of late-stage bacteremia. Thus, the severity of bacterial pneumonia resulting from enhanced lung damage from the primary viral infection, and the amount of repair necessary for recovery, may be an intrinsic measure of influenza virus pathogenicity and may help explain differences in the numbers of bacterial pneumonia-associated deaths during influenza pandemics.