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.