Aims/hypothesis: According to the consensus criteria developed for type 1 diabetes, an individual can be diagnosed with insulitis when ≥ 15 CD45+ cells are found within the parenchyma or in the islet-exocrine interface in ≥ 3 islets. The aim of this study was to determine the frequency of individuals with type 2 diabetes fulfilling these criteria with reference to non-diabetic and type 1 diabetic individuals.
Methods: Insulitis was determined by examining CD45+ cells in the pancreases of 50, 13 and 44 organ donors with type 2 diabetes, type 1 diabetes and no diabetes, respectively. CD3+ cells (T cells) infiltrating the islets were evaluated in insulitic donors. In insulitic donors with type 2 diabetes, the pancreases were characterised according to the presence of CD68 (macrophages), myeloperoxidase (MPO; neutrophils), CD3, CD20 (B cells) and HLA class I hyperstained islets. In all type 2 diabetic donors, potential correlations of insulitis with dynamic glucose-stimulated insulin secretion in vitro or age, BMI, HbA1c or autoantibody positivity were examined.
Results: Overall, 28% of the type 2 diabetic donors fulfilled the consensus criteria for insulitis developed for type 1 diabetes. Of the type 1 diabetic donors, 31% fulfilled the criteria. None of the non-diabetic donors met the criteria. Only type 1 diabetic donors had ≥ 15 CD3+ cells in ≥ 3 islets. Type 2 diabetic donors with insulitis also had a substantial number of CD45+ cells in the exocrine parenchyma. Macrophages constituted the largest fraction of CD45+ cells, followed by neutrophils and T cells. Of type 2 diabetic pancreases with insulitis, 36% contained islets that hyperstained for HLA class I. Isolated islets from type 2 diabetic donors secreted less insulin than controls, although with preserved dynamics. Insulitis in the type 2 diabetic donors did not correlate with glucose-stimulated insulin secretion, the presence of autoantibodies, BMI or HbA1c.
Conclusions/interpretation: The current definition of insulitis cannot be used to distinguish pancreases retrieved from individuals with type 1 diabetes from those with type 2 diabetes. On the basis of our findings, we propose a revised definition of insulitis, with a positive diagnosis when ≥ 15 CD3+ cells, not CD45+ cells, are found in ≥ 3 islets.
Keywords: HLA; Inflammation; Insulin secretion; Insulitis; Islets; Macrophages; T cells; Type 1 diabetes; Type 2 diabetes.