Fibrocytes contribute to the fibrotic changes most frequently observed in forms of asthma where inflammation is driven by T helper type 2 (Th2) cells. The mechanisms that regulate the profibrotic function of asthmatic fibrocytes are largely unknown. We isolated circulating fibrocytes from patients with allergen-exacerbated asthma, who showed the presence of fibrocytes, together with elevated concentrations of interleukin (IL)-4 and IL-13 and slightly increased concentrations of the Th17 cell-derived IL-17A, in induced sputum. Fibrocytes stimulated with IL-4 and IL-13 produced high levels of collagenous and non-collagenous matrix components and low levels of proinflammatory cytokines. Conversely, fibrocytes stimulated with IL-17A proliferated and released proinflammatory factors that may promote neutrophil recruitment and airway hyperresponsiveness. IL-17A also indirectly increased α-smooth muscle actin but not collagen expression in fibrocytes. Thus, fibrocytes may proliferate and express a predominant profibrotic or proinflammatory phenotype in asthmatic airways depending on the local concentrations of Th2- and Th17-derived cytokines.
We investigated the kinetics of allergen-induced eotaxin expression and its relationship to eosinophil accumulation and activation in the airways of patients with allergic asthma. Twenty-four patients with allergic asthma and late asthmatic responses to allergen inhalation were randomly allocated into three groups of eight patients each, who received bronchoscopy with bronchial biopsies and BAL at 2, 4 and 24 h, respectively, after the inhalation of the diluent and the allergen. The expression of eotaxin mRNA and protein and eotaxin release were evaluated by in situ hybridization, immunohistochemistry, immunocytochemistry, and radioimmunoassay. Increased transcription from the eotaxin gene preceded the appearance of the late asthmatic response and the influx of activated eosinophils in bronchial tissue and BAL fluid (BALF). This was followed by increased cell expression of eotaxin protein (P<0.001) and increased eotaxin release (P<0.001), which correlated with the numbers of total and activated eosinophils and the level of airflow obstruction at 4 h after allergen exposure (P<0.05 for all correlations). At 24 h after allergen inhalation, enhanced eotaxin expression declined without a similar reduction in the numbers of eosinophils in bronchial biopsies and when there was a further increase in the number of these cells in BALF (P<0.05). These results indicate that eotaxin contributes to the early phase of allergen-induced recruitment of activated eosinophils into the airways of patients with allergic asthma and that other factors are implicated in the persistence of eosinophil infiltration.
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