Exclusive Expression of MHC Class II Proteins on CD45⁺ Cells in Pancreatic Islets of NOD Mice

The expression of MHC class II molecules on β-cells of the pancreatic islet has been proposed to play a role in the genesis of insulin-dependent diabetes mellitus in the NOD mouse. We investigated this by immunofluorescent double labeling of islet cells with anti-MHC and anti-CD45 to identify cells of hematopoietic origin. MHC class I expression increased with age on CD45⁻ islet cells. MHC class II expression was not observed on CD45⁻ islet cells at any age; the only cells in the islet that were MHC class II positive were also CD45+. This indicates that all MHC class II–positive cells in the islet are lymphoid cells that infiltrate the islet, whereas the islet endocrine cells express no MHC class II molecules. However, an increase in MHC class I expression occurred on β-cells, and this may play a role in immunopathogenesis.