[Type 2 autoimmune pancreatitis: a clinicopathological analysis]

Objective: To investigate the clinicopathological characteristics of type 2 autoimmune pancreatitis (AIP) and to explore its relationship with inflammatory bowel disease (IBD). Methods: AIP cases confirmed by pathology in the First Affiliated Hospital of Zhejiang University School of Medicine from 2009 to 2024 were collected. According to the International Consensus Diagnostic Criteria (ICDC) for AIP, 11 patients were identified as histological level 1 (definite) or level 2 (probable) type 2 AIP. Their clinical manifestations, laboratory test results, imaging features, and histopathological characteristics were analyzed, and a follow-up was conducted. Meanwhile, 130 patients with type 1 AIP diagnosed in our hospital during the same period were selected as control group. Results: Among 141 AIP patients, 11 cases (7.8%, 11/141) were diagnosed with type 2 AIP, including 7 cases of histologically level 1 and 4 cases of level 2. There were 10 male patients and 1 female patient, with a median age of 37(31,46) years (range: 25-47 years). Three patients were complicated with ulcerative colitis (UC). Compared with type 1 AIP patients, type 2 AIP patients were younger, often presented with acute pancreatitis or abdominal pain as the initial symptom, and had a close association with IBD (P<0.05). Laboratory tests showed that only 1 patient had slightly elevated serum IgG4, while the other 10 patients had normal serum IgG4 levels. Serum CA19-9 was elevated in 8 patients, and the percentage of peripheral blood neutrophils was increased in 9 patients. Imaging findings revealed diffuse pancreatic enlargement in 8 patients and localized enlargement in 3 patients (2 cases in the pancreatic head and 1 case in the pancreatic body-tail). Magnetic resonance cholangiopancreatography (MRCP) showed main pancreatic duct stenosis in 5 cases (5/7). Histopathological features included 7 cases of level 1 type 2 AIP that showed neutrophilic infiltration in the pancreatic duct epithelium and massive neutrophilic infiltration between the acini. Immunohistochemistry showed that only 1 case had <5 IgG4-positive plasma cells per high-power field (HPF), while the other 10 cases were negative. All 11 patients with type 2 AIP received steroid therapy, and no recurrence was observed during the follow-up period of 5 to 174 months. Conclusions: Type 2 AIP has unique clinicopathological characteristics. It is more commonly found in young patients and often presents with manifestations similar to acute pancreatitis. Histologically, neutrophilic infiltration in the ductal epithelium is the common feature. Type 2 AIP is closely associated with IBD, especially UC.