CHI3L1 and pulmonary fibrosis: In the process of pulmonary fibrosis, monocytes are recruited into the lung in response to tissue injury and differentiate into long-lived macrophages producing TGF-β, CCL18, CHI3L1, MMPs, eventually, leading to fibroblast activation, myofibroblast differentiation, and extracellular matrix (ECM) remodeling (7).