Macrophages and pancreatic islet amyloidosis

Islet amyloid formed from islet amyloid polypeptide (IAPP, amylin) is found in spontaneously diabetic monkeys and cats. Islet amyloidosis is progressive, apparently irreversible and is associated with destruction of insulin-secreting cells. The role of macrophages in the destruction and removal of islet amyloid is unknown. Therefore, the presence and morphology of macrophages were determined by electron and quantitative light microscopy in islets of diabetic and nondiabetic man and monkeys and in transgenic mice expressing the gene for human IAPP. Tissue macrophages were present in all pancreatic sections and tissue distribution was similar in exocrine and endocrine areas. There was no difference in macrophage density in amyloidotic and amyloid-free islets in monkeys and man. Macrophage density was similar in islets of transgenic mice expressing human IAPP which do not contain amyloid in vivo but in which fibrils are formed in vitro following islet isolation compared to islets from mice expressing rat IAPP which is not amyloidogenic. IAPP amyloid fibrils were visible by electron microscopy in lysosomes of pancreatic macrophages in man, monkeys and human IAPP transgenic mice. Thus, human IAPP is internalised but inefficiently degraded by tissue macrophages. Diabetes-associated amyloidosis is not associated with visible recruitment of macrophages for removal of amyloid or islet debris.