Autoimmune pancreatitis with normal serum igG4 concentrations: What is the correct classification for such patients?

Abstract

The autoimmune pancreatitis (AIP) seen in Japan occurs preferentially in older men, presents with jaundice in many cases, and is seldom associated with symptoms and signs of acute pancreatitis. Elevated serum IgG4 concentrations are noted in a high proportion of cases, and the pancreas characteristically shows a marked lymphoplasmacytic infiltrate with IgG4-positive plasma cells and fibrosis. This entity has been termed lymphoplasmacytic sclerosing pancreatitis (LPSP) and is now known also as type 1 (IgG4-related) AIP. In contrast, data to date suggest that a different pattern (i.e., a different type) of AIP is more common in Western countries. In the West, AIP often presents at a younger age and has a balanced sex distribution. Moreover, AIP in the West is frequently complicated by inflammatory bowel disease. Most importantly, the pathological features of this other form of AIP, now termed type 2 AIP, consist of neutrophilic infiltration of the pancreatic duct epithelium: The granulocytic epithelial lesion (GEL). IgG4-positive plasma cells are generally absent from the tissue lesions of type 2 AIP, and serum IgG4 concentrations are usually normal. In our experience in considering all cases of AIP, among patients with normal serum IgG4 concentrations, the percentage of women is high and symptoms of acute pancreatitis such as abdominal pain are common. Segmental as opposed to diffuse pancreatic swelling is common in this AIP subtype, but extrapancreatic lesions were few. In particular, salivary and lacrimal gland enlargement and dysfunction tend not to occur in this AIP subtype, and IgG4-positive plasma cells are not found on tissue specimens. Patients with normal serum IgG4 concentrations should be classified as type 1 (IgG4-related AIP) if histopathologic examination and immunohistochemical studies of pancreatic specimens demonstrate the typical hematoxylin and eosin features of IgG4-related disease as well as increased numbers of IgG4-positive plasma cells. Type 2 AIP, which is usually associated with normal serum IgG4 concentrations and is linked to pathological findings that contrast sharply with those of type 1 (IgG4-related) AIP, is not a form of IgG4-related disease and should be regarded as a separate disease entity.