Smoking is a major risk factor for IPF, and studies have shown that patients with smoking-related lung diseases such as pulmonary Langerhans cell histiocytosis (PLCH) and desquamative interstitial pneumonitis (DIP) have much higher levels of SPP1 expression in BAL cells compared to those from IPF subjects [29]. Here, SPP1 is linked to idiopathic pulmonary fibrosis.