S100A9 in BALF is a candidate biomarker of idiopathic pulmonary fibrosis.

BACKGROUND Idiopathic pulmonary fibrosis (IPF) is a specific form of chronic, progressive fibrosing interstitial pneumonia of unknown cause, and the prognosis remains poor. On the other hand, other fibrotic interstitial pneumonias such as idiopathic nonspecific interstitial pneumonia (I-NSIP) and collagen vascular disease-associated interstitial pneumonia (CVD-IP) resemble IPF, but they respond to therapy and the prognosis is better. We searched for biomarkers to distinguish IPF from other fibrotic interstitial pneumonias and investigated whether S100A9 could be useful for discriminating types of fibrotic interstitial pneumonia based on our preliminary proteomic findings. METHODS We measured S100A9 levels in serum and bronchoalveolar lavage fluid (BALF) from 28 patients with IPF, 15 with I-NSIP, 20 with cryptogenic organizing pneumonia (COP), 35 with CVD-IP and 23 healthy individuals (controls) using enzyme-linked immunosorbent assays. S100A9 in the lung was also immunohistochemically localized. RESULTS S100A9 levels in BALF, but not in serum, were significantly elevated in patients with IPF compared with I-NSIP, COP, CVD-IP and healthy individuals. S100A9 immunoreactivity was localized mainly in macrophages and neutrophils in lung specimens from patients with IPF. The results of receiver operating characteristic (ROC) curve analysis showed that BALF S100A9 levels had sufficient specificity and sensitivity to distinguish IPF from I-NSIP and CVD-IP. CONCLUSION S100A9 in BALF might serve as a candidate biomarker to discriminate between IPF and other fibrotic interstitial pneumonias.