Krebs von den Lungen 6 (KL-6) is a mucin-like, high molecular weight glycoprotein expressed on the surface membrane of alveolar epithelial cells (AEC-II) and bronchiolar epithelial cells. It exists as a soluble component in the pulmonary epithelial lining fluid, resulting from proteolytic cleavage. The elevated KL-6 serum concentration is mainly the result of two mechanisms: The elevated pulmonary production is due to diffuse hyperplasia of alveolar epithelial cells (AEC), also known as pneumocytes. The increase in spillover to the systemic circulation is caused by leakage of the alveolo-capillary membrane. Over 350 papers have been published on the clinical significance of KL-6 in various types of interstitial lung disease (ILDs), indicating that KL-6 serum levels are a valuable tool for detecting the presence of AEC injury, evaluating disease activity, and predicting clinical outcomes in various types of ILDs.
