Macrophage-mediated phagocytosis of apoptotic cholangiocytes contributes to reversal of experimental biliary fibrosis

Abstract

Studies have suggested the reversibility of liver fibrosis, but the mechanisms of fibrosis reversal are poorly understood. We investigated the possible functional link between apoptosis, macrophages, and matrix turnover in rat liver during reversal of fibrosis secondary to bile duct ligation (BDL). Biliary fibrosis was induced by BDL for 4 wk. After Roux-en-Y (RY)-bilio-jejunal-anastomosis, resolution of fibrosis was monitored for up to 12 wk by hepatic collagen content, matrix metalloproteinase (MMP) expression and activities, and fibrosis-related gene expression. MMP expression and activities were studied in macrophages after engulfment of apoptotic cholangiocytes in vitro. Hepatic collagen decreased to near normal at 12 wk after RY-anastomosis. During reversal, profibrogenic mRNA declined, whereas expression of several profibrolytic MMPs increased. Fibrotic septa showed fragmentation at week 4 and disappeared at week 12. Peak histological remodeling at week 4 was characterized by massive apoptosis of cytokeratin 19+ cholangiocytes, >90% in colocalization with CD68+ macrophages, and a 2- to 7.5-fold increase in matrix-degrading activities. In vitro, phagocytosis of apoptotic cholangiocytes induced matrix-degrading activities and MMP-3, -8, and -9 in rat peritoneal macrophages. We concluded that reconstruction of bile flow after BDL leads to an orchestrated fibrolytic program that results in near complete reversal of advanced fibrosis. The peak of connective tissue remodeling and fibrolytic activity is associated with massive apoptosis of cholangiocytes and their phagocytic clearance by macrophages in vivo. Macrophages upregulate MMPs and become fibrolytic effector cells upon apoptotic cholangiocyte engulfment in vitro, suggesting that phagocytosis-associated MMP induction in macrophages significantly contributes to biliary fibrosis reversal.

Metalloproteinases in liver fibrosis: current insights

“…This model is irreversible in mice; however, it is possible to reconnect the bile duct in rat models. 31 Due to wound healing after surgery, this model is usually performed in studies focused on chronic liver injury. Probably, the most important modification of the BDL model is partial BDL (pBDL).…”

Cannabinoid Receptor Type I Modulates Alcohol-Induced Liver Fibrosis

“…We found a strong upregulation of MMP-2, -3 and -13 in WT mice compared with CB1 -/-animals. This seemingly contradictory observation with regards to the balance between fibrogenesis and fibrolysis can be explained by the very complex and different regulation of MMP expression and enzymatic activation during the peak of fibrosis formation and ECM degradation (34,37). Certain MMPs, such as profibrogenic MMP-2, facilitate and promote degradation of basement membrane with subsequent activation, migration and proliferation of HSC/myofibroblasts, resulting in progression of liver injury and fibrosis (38), whereas others, such as MMP-3, -9 and -13 participate in the degradation of extracellular matrix and fibrosis resolution (35,39).…”

Involvement of cholangiocyte proliferation in biliary fibrosis

“…The reduction of fibrosis within cirrhotic liver tissue would lead to a reduction of portal hypertension and consequent clinical complications, thus improving overall liver function, which would extend the complication-free patient survival time and reduce the need for liver transplants [26] . Studies have suggested the reversibility of liver fibrosis, but the mechanisms for such a reversal are poorly understood [79] . In BDL rats, Popov and colleagues introduced macrophages to damaged biliary epithelium via Roux-en-Y bilio-jejunal anastomosis.…”

“…In BDL rats, Popov and colleagues introduced macrophages to damaged biliary epithelium via Roux-en-Y bilio-jejunal anastomosis. After engulfing apoptotic cholangiocytes, macrophages upregulate matrix metalloproteinases and become fibrolytic effector cells [79] . This suggests a link between apoptotic epithelial cells, macrophages, and the reversal of fibrosis.…”