Metabolically inactive insulin: friend or foe in the prevention of autoimmune diabetes?

About 20 years ago an American study suggested that daily subcutaneous injections of a metabolically inactive insulin analogue with a single amino acid substitution (aspartic acid instead of phenylalanine) at position 25 of the B chain was as effective as intact insulin in preventing autoimmune diabetes in NOD mice. In this issue of Diabetologia Grönholm et al (DOI: 10.​1007/​s00125-017-4276-5) report that parenteral administration of the same insulin analogue has no preventive effect whatsoever on the development of diabetes in NOD mice; in fact, high doses of the metabolically inactive insulin accelerated disease development. The authors were also unable to show any tolerogenic effect of an insulin peptide mimetope given via a subcutaneous osmotic pump. These data do not support the use of metabolically inactive insulin for the prevention of autoimmune diabetes and question whether insulin alone, intact or inactivated has any role in preventing progression to symptomatic diabetes. Future and ongoing intervention trials in humans with preclinical type 1 diabetes should indicate whether the administration of oral insulin has any protective, neutral or even predisposing effects on the development of symptomatic diabetes.