Dissociation between airway responsiveness to methacholine and responsiveness to antigen

Repeated aerosolized antigen challenges to brown Norway (BN) rats generate nonspecific airway hyperresponsiveness (AHR). On the other hand, some studies have demonstrated that repeated antigen challenge could attenuate antigen-specific AHR in BN rats. The authors questioned whether such dissociation in airway responses actually occurs when assessed in a single study in the same animals. The authors simultaneously measured AHR to methacholine and antigen-specific AHR in rats that were repeatedly exposed to aerosolized ovalbumin (OA) for 1 or 3 months after sensitization. Four days after the last challenge, airway responses to methacholine and OA, morphometry of the airways, the cell profile in bronchoalveolar lavage fluid, and cytokine messenger ribonucleic acid (mRNA) expression in the lungs were evaluated. The two types of AHR were modulated in opposite directions by repeated antigen challenges. The AHR to methacholine was significantly increased in the rats receiving antigen challenges compared with the control rats receiving saline challenges after sensitization; whereas, the antigen-specific AHR was significantly decreased. The number of alveolar macrophages in lavaged fluid and the expression of transforming growth factor-beta1 mRNA in lung tissue was significantly different between the antigen-challenged rats and the control rats. In conclusion, dissociation between nonspecific airway hyperresponsiveness and antigen-specific airway hyperresponsiveness in brown Norway rats after repeated antigen challenges was demonstrated. Sustained airway inflammation with macrophages and/or upregulation of transforming growth factor-beta1 messenger ribonucleic acid in the lung tissue may be responsible for this dissociation.