Upregulation of S100 Calcium Binding Protein A9 Levels in the Lungs of Patients with Idiopathic Pulmonary Fibrosis

Background: Neutrophilic inflammation is a predominant characteristic of idiopathic pulmonary fibrosis (IPF). S100 Calcium-Binding Protein A9 (S100A9) is a neutrophil-derived protein and is involved in the development of neutrophil-related chronic inflammatory disorders. However, the role of S100A9 in IPF has not been evaluated.Methods : S100A9 concentrations were measured by ELISA in the BAL fluid obtained from NCs (n = 33) and patients with IPF (n = 87), NSIP (n = 22), HP (n = 19), or sarcoidosis (n = 10).Results: The S100A9 levels in BALF were significantly higher in patients with IPF than in those with NC (0.4 [0.18–0.9] vs. 0 [0–0.5] ng/mL, p < 0.001), HP (0.19 [0.07–0.33] ng/mL, p = 0.043), or sarcoidosis (0.06 [0–0.11] ng/mL, p < 0.001) patients. A S100A9 level of 0.093 ng/mL had discriminating powers of 78.79% for specificity and 81.61% for sensitivity between IPF patients and NCs. S100A9 levels were also correlated with neutrophil numbers (r = 0.356, p = 0.0007) and S100A9 was expressed on neutrophils and macrophages in the BALF of IPF patients. Patients with S100A9 levels above 0.5535 ng/mL or a neutrophil percentage above 49.09% (n = 43) had significantly lower survival rates than those with S100A9 levels at or below 0.5535 ng/mL and a neutrophil percentage at or below 49.09% (n = 41) (HR, 9.28; p = 0.0004).Conclusion: S100A9 may participate in the development and progression of IPF. The levels of S100A9 in BALF may be a surrogate marker for diagnosing IPF and predicting its prognosis.