Clinical and morphological characteristics of type 1 and 2 autoimmune pancreatitis

Abstract

Rationale: In the recent years, an increased interest to autoimmune pancreatitis (AIP) has been seen, related to growing diagnostic potential. In its turn, this leads to an increase in numbers of diagnosed AIP cases. At present, two types of AIP have been described with diverse clinical manifestation and morphology of the pancreas. However, the reproducibility of the differential diagnosis between AIP type 1 and 2 is low even among pancreatic pathologists.Aim: To identify criteria for the morphologic diagnosis of AIP type 1 and 2.Materials and methods: A morphological study of biopsy and surgical specimens from 26 patients with AIP was performed. There were 22 cases of AIP type 1 and 4 cases of AIP type 2. In addition to hematoxylin eosin staining of the specimens, immunohistochemistry was used with counting of CD138+ absolute numbers, determination of IgG+ and IgG4+ cells in the inflammatory infiltrates, as well as the ratios of IgG4+/IgG+ and IgG4+/CD138+ cells.Results: AIP type 1 was characterized by storiform fibrosis of the pancreatic tissue (81.8% cases), involving the parapancreatic fat tissue, by moderateto-severe lymphoplasmocytic infiltration and signs of obliterative/non-obliterative phlebitis. Type 2 AIP was characterized by severe fibrosis with predominantly periductal (centrilobular) fibrosis and mild chronic inflammatory infiltration of the pancreas, while there was no extension of fibrosis and inflammatory infiltration to the parapancreatic tissues in any case. The mean number of CD138+ cell in AIP type 1 was 101.2 ± 27.9 per 1 high-power field (HPF), and in AIP type 2, it was 42.8 ± 20.9 per 1 HPF. The mean absolute number of IgG+ cells in AIP type 1 was 99.6 ± 25.7 per 1 HPF, whereas in AIP type 2, 42.1 ± 20.8 per 1 HPF. In AIP type 1, the mean number of IgG4+ plasmatic cells in the infiltrates was 74.5 ± 27.2 per 1 HPF, whereas in AIP type 2, it was 3.4 ± 2.7 per 1 HPF. The IgG4+/IgG+ ratio was 75 ± 12.6% vs. 8.4 ± 6.2%, and the IgG4+/CD138+ ratio was 72.4 ± 12.3% vs. 8.3 ± 5.9% in AIP type 1 and type 2, respectively.Conclusion: For the differential diagnosis of type 1 and 2 AIP, it is necessary to take into consideration not only typical histological abnormalities, but also the numbers of CD138+, IgG+ and IgG4+ cells within the inflammatory infiltrate, as well as the IgG4+/IgG+ and IgG4+/CD138+ ratios.

Keywords

• autoimmune pancreatitis
• igg
• igg4
• morphological criteria