Clinical meaning
Primary sclerosing cholangitis (PSC) is a chronic, progressive cholestatic liver disease characterized by inflammation and fibrosis of the intrahepatic and extrahepatic bile ducts, leading to multifocal biliary strictures, cholestasis, and ultimately biliary cirrhosis. The pathogenesis involves a complex interplay of autoimmune dysregulation, genetic susceptibility (HLA-B8, HLA-DR3 haplotypes), and the gut-liver axis. PSC is strongly associated with inflammatory bowel disease — 70-80% of PSC patients have ulcerative colitis (UC) — supporting the 'aberrant gut homing' hypothesis: activated lymphocytes primed in the inflamed intestinal mucosa express adhesion molecules (alpha4-beta7 integrin, CCR9) that normally direct them back to the gut, but aberrant expression of the corresponding ligands (MAdCAM-1) on hepatic endothelium allows these gut-activated T-cells to be recruited to the liver, where they initiate periductal inflammation. The histological hallmark is 'onion-skin' periductal fibrosis — concentric rings of collagen deposited around medium and large bile ducts by activated portal myofibroblasts, progressively narrowing the ductal lumen. As bile ducts stricture, bile flow is impeded (cholestasis), causing upstream bile acid accumulation. Retained bile acids are directly cytotoxic to hepatocytes and cholangiocytes: hydrophobic bile acids (deoxycholic acid,...