Degranulation status of airway tissue eosinophils in mouse models of allergic airway inflammation

Eosinophil degranulation is a characteristic feature of asthma and allergic rhinitis. However, degranulated eosinophils have not been convincingly dem onstrated in the common mouse models of these airway diseases. This study u ses eosinophil peroxidase (EPO) histochemistry and transmission electron mi croscopy (TEM) analysis to assess eosinophil degranulation in the airways o f ovalbumin (OVA)-sensitized and challenged BALB/c and C57BL/6 mice. Using TEM we also examined mouse and human blood eosinophils after in vitro incub ation with formyl-Met-Leu-Phe (fMLP) or phorbol myristate acetate (PMA). Al though OVA exposure induced significant nasal and lung eosinophilia, we did not observe any of the known cellular processes by which eosinophils relea se their granule products, i.e., eosinophil cytolysis, piecemeal degranulat ion, and exocytosis. The occurrence of other allergen-induced degranulation events was ruled out because no difference in granule morphology was obser ved between lung-tissue eosinophils and blood or bone-marrow eosinophils fr om control animals. Accordingly, there was no detectable extracellular EPO in lung tissues of allergic mice. Similarly, mouse blood eosinophils remain ed nondegranulated in vitro in the presence of fMLP and PMA, whereas the sa me treatment of human eosinophils resulted in extensive degranulation. This investigation indicates that OVA-induced airway inflammation in the presen t mouse strains does not involve significant eosinophil degranulation, It i s speculated that this dissimilarity from the human disease may be due to a fundamental difference in the regulation of mouse and human eosinophils.