Transforming growth factor-β (TGF-β) activation is involved in various pathogeneses, such as fibrosis and malignancy. We previously showed that TGF-β was activated by serine protease plasma kallikrein-dependent digestion of latency-associated peptides (LAPs) and developed a method to detect LAP degradation products (LAP-DPs) in the liver and blood using specific monoclonal antibodies. Clinical studies have revealed that blood LAP-DPs are formed in the early stages of liver fibrosis. This study aimed to identify the cell source of LAP-DP formation during liver fibrosis. The N-terminals of LAP-DPs ending at residue Arg58 (R58) were stained in liver sections of a bile duct-ligated liver fibrosis model at 3 and 13 days. R58 LAP-DPs were detected in quiescent hepatic stellate cells at day 3 and in macrophages on day 13 after ligation of the bile duct. We then performed a detailed analysis of the axial localization of R58 signals in a single macrophage, visualized the cell membrane with the anti-CLEC4F antibody, and found R58 LAP-DPs surrounded by the membrane in phagocytosed debris that appeared to be dead cells. These findings suggest that in the early stages of liver fibrosis, TGF-β is activated on the membrane of stellate cells, and then the cells are phagocytosed after cell death.
Keywords: LAP; TGF-β; liver fibrosis; plasma kallikrein.