Objectives: Autoimmune pancreatitis (AIP) is a unique clinical entity that has been recently proposed, and it is frequently associated with bile duct stricture. The aim of this study was to investigate the pathophysiology of the biliary tract involvement in patients with AIP.
Methods: We evaluated the clinicopathologic findings in 16 patients with AIP. Surgical resection was performed in 7 of the patients because of suspicion of a pancreatic tumor; 8 of the other patients were treated with oral prednisolone (PSL) therapy, and the remaining patient was observed clinically and not treated. The pancreas, bile duct, and gallbladder in the surgical cases were examined histologically and immunohistochemically. We also assessed the clinical manifestations and diagnostic imaging findings before and after oral PSL therapy in the 8 patients treated with PSL.
Results: Stricture of the extrahepatic bile duct was detected in 88% (14/16) of the patients. Thickening of the bile duct wall was detected in 94% (15/16), and thickening of the gallbladder wall was observed in 56% (9/16). Histologically, the bile duct and gallbladder wall were characterized by diffuse lymphoplasmacytic infiltration and marked interstitial fibrosis. Immunohistochemically, the diffusely infiltrating cells consisted of predominantly CD8- or CD4-positive T lymphocytes and IgG4-positive plasma cells. These findings were the same as in the inflammatory process that was observed in the pancreas. After oral PSL therapy, the pancreatic enlargement and irregular narrowing of the main pancreatic duct improved to almost their normal size in all 8 patients; however, stricture of the extrahepatic bile duct persisted in 4 of the patients (57%, 4/7) in whom it was detected before PSL therapy.
Conclusions: Based on the pathophysiologic and histologic findings and the response to PSL therapy, the biliary involvement in AIP developed by the same mechanism as the pancreatitis. CD8- and CD4-positive lymphocytes and IgG4-positive plasma cells may play an important role in the pathogenesis of AIP.