Rationale: Cystic fibrosis (CF) is characterized by bronchoalveolar neutrophilia and submucosal lymphocytosis. We hypothesized that Th17 lymphocytes are part of this submucosal infiltrate.

Objectives: Quantification and phenotyping of the lymphocytic infiltrate in the bronchial submucosa of patients with CF (n = 53, of which 20 were newly diagnosed), non-CF bronchiectasis (n = 17), and healthy control subjects (n = 13).

Methods: We measured IL-17 levels in bronchoalveolar lavage and CD4⁺, CD8⁺, and IL-17⁺ cell counts in endobronchial biopsies. Correlations were made with infection status and other inflammatory markers. Potential cellular sources of IL-17 were determined by double staining.

Measurements and Main Results: IL-17⁺ cell counts (median [interquartile range] cells/mm²) were significantly higher in patients with established CF (205 [115–551]) and non-CF bronchiectasis (245 [183–436]) than in control subjects (53 [12–82]) (P < 0.01 for both). Patients with newly diagnosed CF had intermediate counts (171 [91–252]). IL-17–positive CD4⁺ T cells, γδT cells, natural killer T cells, and neutrophils were identified. Bronchoalveolar lavage IL-17 levels (pg/ml) were highest in established CF (14.6 [2.2–38.4]), low in newly diagnosed CF and control subjects (1.7 [1.7–1.74]; 1.7 [1.7–3]), and intermediate in non-CF bronchiectasis (9.1 [1.7–34] pg/ml) (Kruskal-Wallis P = 0.001). There was a significant correlation between IL-17 and neutrophil counts (P < 0.001, R = 0.6) as well as IL-4 (P < 0.001, R = 0.84).

Conclusions: Th17 lymphocytes are present in the airway submucosa in CF, even in a young, newly diagnosed group. Other IL-17⁺ cells include neutrophils, γδ T cells, and natural killer T cells.