SERPINE1 and idiopathic pulmonary fibrosis: Other proposed IPF biomarkers include Krebs von den Lungen-6 antigen (KL-6) [17], CCL-18 [18], monocyte chemotactic protein-1 (MCP-1; also known as CC chemokine ligand 2) [19], tissue inhibitor of metalloproteinase-1 (TIMP-1) [20], and plasminogen activator inhibitor-1 (PAI-1) [21].