Figure 1.

Multitasking by insulin granules. Insulin secretory granules (right) are unique features of β-cells (green; left) in pancreatic islets. Light organelles represent immature granules, whereas mature granules are dark and dense spheres containing insulin crystals, which are distinct from somatostatin granules in delta cells (blue). Native proteins as well as posttranscriptionally or posttranslationally modified proteins and sphingolipids have now been identified in these characteristic organelles that act as autoantigenic targets or immune modulators of islet-specific autoreactive CD4 and CD8 T cells. (Electron microscopy graph kindly shared by Dr. Ben Giepmans [2].)