Reduced Lung Function in a Chronic Asthma Model Is Associated with Prolonged Inflammation, but Independent of Peribronchial Fibrosis

In asthma, mechanisms contributing to chronicity remain to be determined. Recent models of sensitisation with prolonged airway allergen challenges reproduce typical features of chronic asthma. However, the interplay between inflammation, structural changes and lung function is poorly understood. This study was performed to delineate functional, structural and immunological airway changes after cessation of long term challenges to elucidate factors contributing to the development of prolonged lung function changes.Mice sensitised systemically were consecutively challenged intranasally with ovalbumin for two or eight weeks. After the end of challenges, lung function, airway inflammation, features of airway remodelling, local T-cell cytokines and systemic ovalbumin-specific antibodies were monitored. Long term challenges resulted in airway hyperresponsiveness lasting 2 weeks and reduced baseline lung function for 6 weeks after their cessation. In contrast, these changes resolved within one week after short term challenges. Prolonged transforming growth factor beta (TGF-beta)1 production and marked peribronchial fibrosis were only induced by long term challenges. Importantly, fibrosis became apparent only after the onset of lung function changes and outlasted them. Further, long term challenges led to prolonged and intense airway inflammation with marked lymphocytosis, but moderate eosinophilia, sustained IL-5 production and ovalbumin-specific IgG2a antibodies, the latter suggesting a Th1 component to the immune response. In contrast, following short term challenges airway inflammation was dominated by eosinophils and associated with a strong, but transient IL-13 response.Prolonged lung function changes after long term allergen challenges seem to develop and resolve independently of the persistent peribronchial fibrosis. They are more closely associated with intense airway inflammation, marked lymphocytosis, prolonged IL-5 and TGF-beta1 production in the airways and a Th1 immune response

Airway inflammation, goblet cell hyperplasia and airway fibrosis following long and short term OVA challenges.

<p>Representative photomicrographs of paraffin-embedded lung sections stained with H&E (left column), Alcian-PAS (middle column) and Masson's trichrome (right column) from short term OVA challenged animals (d–l), long term OVA challenged animals (m–u) and after PBS treatment (a–c) 3 days (row 2 and 5), 4 weeks (row 3 and 6) and 8 weeks (row 4 and 7) after challenges. Magnification: 20-fold for H&E, 20-fold for Masson's trichrome, and 40-fold for Alcian-PAS stained sections.</p

Lung function following long and short term airway challenges.

<p>(a) Following OVA sensitisation and short or long term OVA challenges or after sham sensitisation and long or short term PBS challenges, airway responsiveness to MCh was assessed 48 hours after final airway challenge and then weekly for 8 weeks. Mean±SEM of Penh values at 50 mg/ml MCh are shown for the first 4 weeks only from 3 independent experiments (n≥12). No differences between groups were detected after week 4. (b) Baseline Penh values were assessed during the same measurements. Mean±SEM from 5 independent experiments are illustrated (n≥15). Significant differences: * long term OVA challenges versus PBS control, +short term OVA challenges versus PBS control, levels of significance: */+ p<0,01, **/++ p<0,001.</p

Cytokines in BAL fluid after long and short term airway challenges.

<p>Following OVA sensitisation and short or long term OVA challenges or after sham sensitisation and long or short term PBS challenges, concentrations of (a) IL-5, (b) IL-13 and (c) activated TGF-β1were measured in BAL fluid by ELISA 3 days to 8 weeks after final airway challenge, n≥15 per group from 5 independent experiments. Significant differences: * OVA challenges versus PBS controls, +long term versus short term OVA challenges, */+ p<0.05, **/++ p<0.01, ***/+++ p<0.001.</p

Eosinophils and lymphocytes in BAL fluid after long and short term airway challenges.

<p>Following OVA sensitisation and short or long term OVA challenges or after sham sensitisation and long or short term PBS challenges, numbers of (a) eosinophils, and (b) lymphocytes were determined in BAL fluid from 3 days to 8 weeks after final airway challenge, n≥12 per group from 5 independent experiments. Significant differences: * OVA challenges versus PBS controls, +long term versus short term OVA challenges, */+ p<0,05, **/++ p<0,01, ***/+++ p<0,001.</p

OVA specific serum antibodies following sensitisation and short or long term challenges

<p>Serum concentrations of OVA-specific antibodies post challenges were assessed by ELISA. In PBS controls no OVA-specific IgG1 or IgG2a was detected and there was a background of OVA-specific IgE of maximally 1.0 IU/ml. Significant differences: *versus respective PBS control, +versus respective time point after short term OVA challenges, */+ p<0.05, **/++ p<0.01, ***/+++ p<0,001. Means±SEM are shown, n≥9 from 3 independent experiments.</p

Reduced lung function in a chronic asthma model is associated with prolonged inflammation, but independent of peribronchial fibrosis

$\textit {Background:}$ In asthma, mechanisms contributing to chronicity remain to be determined. Recent models of sensitisation with prolonged airway allergen challenges reproduce typical features of chronic asthma. However, the interplay between inflammation, structural changes and lung function is poorly understood. This study was performed to delineate functional, structural and immunological airway changes after cessation of long term challenges to elucidate factors contributing to the development of prolonged lung function changes. $\textit {Methodology/Principal Findings:}$ Mice sensitised systemically were consecutively challenged intranasally with ovalbumin for two or eight weeks. After the end of challenges, lung function, airway inflammation, features of airway remodelling, local T-cell cytokines and systemic ovalbumin-specific antibodies were monitored. Long term challenges resulted in airway hyperresponsiveness lasting 2 weeks and reduced baseline lung function for 6 weeks after their cessation. In contrast, these changes resolved within one week after short term challenges. Prolonged transforming growth factor beta (TGF-$\beta$)1 production and marked peribronchial fibrosis were only induced by long term challenges. Importantly, fibrosis became apparent only after the onset of lung function changes and outlasted them. Further, long term challenges led to prolonged and intense airway inflammation with marked lymphocytosis, but moderate eosinophilia, sustained IL-5 production and ovalbumin-specific IgG2a antibodies, the latter suggesting a Th1 component to the immune response. In contrast, following short term challenges airway inflammation was dominated by eosinophils and associated with a strong, but transient IL-13 response. $\textit {Conclusions:}$ Prolonged lung function changes after long term allergen challenges seem to develop and resolve independently of the persistent peribronchial fibrosis. They are more closely associated with intense airway inflammation, marked lymphocytosis, prolonged IL-5 and TGF-$\beta$1 production in the airways and a Th1 immune response