Eosinophil traffic in the circulation following allergen challenge

Background: Eosinophils contribute to the pathogenesis of asthma and localize to the lung after allergen exposure by uncertain mechanisms. Methods: We used intrabronchial instillation of allergen to model the interaction between inhaled allergen and the lung. We measured the number of peripheral blood leukocytes and the expression of VLA‐4 (CD49d), Mac‐1 (CD11b) and PSGL‐1 (CD162) up to 4 h after instillation of allergen into a bronchus of eight atopic asthmatics. For controls, we instilled normal saline into a subset of the asthmatic subjects, and allergen into nonatopic, nonasthmatic subjects. Results: There were changes of total leukocyte number, number of polymorphonuclear leukocytes, lymphocytes, monocytes and eosinophils in all three groups (atopic asthmatics instilled with allergen, atopic asthmatics instilled with saline, nonatopic nonasthmatic subjects instilled with allergen), which were likely related to bronchoscopy. However, the decrease of eosinophils was significant only in the atopic asthmatics instilled with allergen. The remaining eosinophils in the allergen challenged asthmatics were not activated as defined by cell density or change of expression of VLA‐4, Mac‐1 and PSGL‐1. Conclusions: While eosinophils rapidly and specifically leave the circulation after allergen challenge of atopic asthmatics, the remaining circulating eosinophils are not activated.