Airway, but not serum or urinary, levels of YKL-40 reflect inflammation in early cystic fibrosis lung disease

Background: Cystic fibrosis (CF) lung disease begins in early life and is progressive with the major risk factor being an exaggerated inflammatory response. Currently, assessment of neutrophilic inflammation in early cystic fibrosis (CF) lung disease relies on bronchoalveolar lavage (BAL). The chitinase-like protein YKL-40 is raised in sputum and serum of adults with CF. We investigated YKL-40 in BAL, serum and urine to determine whether this reflected inflammation and infection in young children with CF. Methods: YKL-40 was measured in matched samples of BAL, serum and urine obtained from 36 infants and young children with CF participating in an early surveillance program. Levels were compared to clinical data and markers of inflammation detected in the lung. Results: YKL-40 in BAL correlated with pulmonary infection [beta=1.30 (SE 0.34), p < 0.001] and BAL markers of inflammation [macrophage number: r(2) = 0.34, p < 0.001; neutrophil number: r(2) = 0.74, p < 0.001; neutrophil elastase: r(2) = 0.47, p < 0.001; CXCL8: r(2) = 0.45, p < 0.001; IL-beta: r(2) = 0.62, p < 0.001]. YKL-40 was detectable in serum but levels did not correlate with BAL levels in the same individuals (r(2) = 0.04, p = 0.14) or with inflammatory markers. YKL-40 was below the limit of detection in urine (30 pg/ml). Conclusions: This study demonstrates that levels of the chitinase-like protein YKL-40 reflect airway inflammation and infection in early CF lung disease. The lack of increased YKL-40 in serum in the absence of systemic inflammation limits the benefit of this potential biomarker in early disease.