Progressive increase in allergen concentration abrogates immune tolerance in ovalbumin-induced murine model of chronic asthma

Persistent inflammation and remodeling of airways are the major hallmarks of asthma. Though airway inflammation diminishes in ovalbumin (OVA)-based mouse model of chronic asthma owing to immune-tolerance linked with repeated allergen exposure, which limits the application of the disease model. Accordingly, the present study was designed to develop a murine model of chronic asthma which presents persistent airway inflammation coupled with remodeling traits. Herein, OVA-sensitized BALB/c mice were challenged with increasing (modified protocol) or constant concentration (conventional protocol) of the allergen for 6 weeks; 3 times/week. The results, indeed, revealed that mice subjected to modified protocol demonstrate an improved response to the allergen as reflected by the significant increase in inflammatory cells particularly, eosinophils in bronchoalveolar lavage fluid compared to conventional protocol. Moreover, the expression of Th2 cytokines and their responsible transcription factors (GATA-3 and STAT-6) was markedly enhanced in lungs. The increase in inflammation was further accompanied by a marked increase in mucus production, collagen deposition, and the expression of allied factors (Muc5ac, Col1 alpha 1, and alpha-SMA). Interestingly, pre-treatment of dexamethasone, a corticosteroid (0.5 mg/kg b.wt., i.p.), suppressed the allergen-induced airway inflammation and mucus production without altering collagen deposition. Failure of dexamethasone seems to be related to their ineffectiveness to modulate the expression of TGF-beta, MMP-9, COL1 alpha 1, and alpha-SMA. Overall, our results strongly suggest that mice underwent modified chronic protocol bears more resemblance with asthmatics as it imitates persistent airway inflammation allied with steroid-refractory remodeling traits; hence, may be useful for the evaluation of new/alternative drugs in steroid-refractory asthmatic conditions.