Discussion
In this study, we comparatively evaluated AHR in a mouse model of bronchial asthma with either A(H1N1)pdm09 infection or seasonal H1N1 infection. Enhanced AHR was observed in asthmatic mice with A(H1N1)pdm09 infection, which peaked at 7 days post-infection and subsequently diminished at 10 days post-infection. Histopathological analysis showed that the onset of lung inflammation in asthmatic mice with A(H1N1)pdm09 infection occurred earlier and was more prominent compared to that in mice with seasonal H1N1 infection; these effects peaked 7 days post-infection and diminished by 10 days post-infection, which was consistent with the observed changes in AHR. These data suggest that A(H1N1)pdm09 induces enhanced AHR complication as a severe phenotype of pneumonia in mice with asthma.
The severity of AHR reflects the inflammatory state of the airways18. Several studies have reported that AHR can be enhanced by inflammatory and Th2 cytokines, such as tumour necrosis factor (TNF)-α, interleukin (IL)-6, and IL-1319-21. TNF-α secreted from airway macrophages or airway epithelial cells after respiratory virus infection increases levels of adhesion molecules, such as intercellular adhesion molecule-1 on epithelial cells, thereby inducing the recruitment of eosinophils and contributing to epithelial damage and AHR22-26. TNF-α is associated with wheezing in human infants19. IL-6 is secreted from epithelial cells in respiratory virus infection and induces airway inflammation and bronchospasms in patients with asthma and upper respiratory tract infections20, 23. Our previous study showed that IL-6 and TNF-α levels in the bronchoalveolar lavage fluid of A(H1N1)pdm09-infected mice were significantly higher than those in seasonal H1N1-infected mice within 3 days after infection13. Therefore, A(H1N1)pdm09 infection may enhance AHR by inducing the production of high levels of inflammatory cytokines during lung inflammation in asthmatic mice, which may also occur in human cases.
Notably, the enhanced AHR in A(H1N1)pdm09-infected mice at 3 and 7 days post-infection decreased to the same level as in control mice at 10 days post-infection, demonstrating that AHR and airway constriction induced by A(H1N1)pdm09 infection is temporary and limited to the acute phase of infection. This finding is supported by our data in A(H1N1)pdm09-infected paediatric participants showing that AHR was alleviated by 3 months after discharge compared to findings at 1 month after discharge. Bozanich et al34 reported that increased AHR observed in seasonal H3N1 influenza-infected non-asthmatic wild-type mice at 4 days post-infection returned to control levels at 20 days post-infection, which is consistent with our findings. Together, these data indicate that it is pivotal to treat patients with severe asthma exacerbation in the acute phase of post-A(H1N1)pdm09 infection. Established treatments for rescuing acute severe asthma exacerbation complicated with severe pneumonia resulting from A(H1N1)pdm09 infection have not yet been developed. We are currently investigating approaches for treating acute severe asthma exacerbation occurring with A(H1N1)pdm09 infection.
There were some limitations to this study. First, we did not evaluate AHR, inflammatory or Th2 cytokines, or virus titres in the bronchoalveolar lavage fluid at the same time. Lung tissues were collected and used for pathological analysis after AHR evaluation, as we previously reported the cytokine profiles in the bronchoalveolar lavage fluid of A(H1N1)pdm09 mice12. Second, we could not evaluate AHRs of the paediatric participants before or during A(H1N1)pdm09 infection for ethical reasons to verify whether or not enhanced AHR during the infection were alleviated in the post-infection phase. Instead, we measured AHR at 1 and 3 months after discharge.
In conclusion, AHR was significantly enhanced in asthmatic mice with A(H1N1)pdm09 infection compared to that occurring in asthmatic mice with seasonal influenza infection. Furthermore, A(H1N1)pdm09-infected asthma model mice showed more severe pulmonary inflammation in the acute phase post-infection. Enhanced AHR subsequently returned to normal levels with the amelioration of lung inflammation, suggesting that appropriate treatment during the acute phase after A(H1N1)pdm09 infection is essential for avoiding severe respiratory conditions.