Airway inflammation and altered alveolar macrophage phenotype pattern after repeated low-dose allergen exposure of atopic asthmatic subjects

BACKGROUND: The alveolar macrophage (AM) constitutes an important link between pulmonary innate and adaptive immunity due to its antigen-presenting capacity and ability to express different immunomodulating mediators. The role of AMs in the pathogenesis of allergic inflammation has yet to be fully determined. OBJECTIVE: To investigate clinical effects and any change in the AM phenotype pattern after inhalation of sub-clinical doses of allergen by asthmatic patients. METHODS: Eight subjects with allergic asthma underwent repeated low-dose allergen provocations equivalent to 10% of PD20. AMs recovered with bronchoalveolar lavage (BAL) were characterized by flow cytometric analysis of adhesion molecules, co-stimulatory molecules and markers for AM population activation and heterogeneity. RESULTS: An allergic airway inflammation, sub-clinical in six out of eight subjects, was obtained after low-dose allergen provocations, as determined by increased airway methacholine reactivity, increased BAL fluid total cell and eosinophil counts and increased serum ECP levels. The AMs showed a post-challenge altered phenotype pattern with a decreased expression of CD11a, CD16, CD71 and HLA class I and an increased expression of CD11b and CD14. The AMs were positive for CD83 and a weak post-challenge increase in the CD83 expression was found. CONCLUSION: Repeated low-dose allergen exposure induces an allergic airway inflammation in asthmatic subjects. The inflammation is associated with an altered AM phenotype pattern, consistent with an influx of monocytes and a hypothetical increased accessory cell function in the airways, possibly contributing to the development and sustenance of airway inflammation in asthma.