Radioimmunoassay of the A-chain of insulin

Recognition of human insulin in vitro by T cells from subjects treated with animal insulins

Structurally defined proteins and peptides have provided considerable information about the specificity and regulation of immune responses in inbred animals. Many diabetics require therapy with insulin; therefore, we used this defined protein as a model antigen to investigate immune responses in the outbred human population. In this report, we examine human T cell recognition of antigenic determinants on various insulins. A group of 25 subjects was selected from over 200 diabetics because of the magnitude of their in vitro responses. 13 of the 25 had significant T cell responses to human insulin despite treatment with only beef/pork insulin mixtures. This autoimmunity may be attributed to crossreactivity of lymphocytes highly reactive to "foreign" epitopes on therapeutic insulins. Alternatively, identical determinants shared by human and animal insulins may be recognized. By employing additional insulins not used therapeutically and isolated A and B chains, several potential mechanisms for lymphocyte autoreactivity to human insulin were demonstrated. Some epitopes are conformational and require recognition of an intact molecule, whereas other epitopes may arise from antigen processing at the cellular level. Studies using zinc-free insulins suggest that zinc-induced alterations of the molecular surface may result in some shared reactivities between animal and human insulin. Furthermore, T cell reactivity against "foreign" epitopes is more complex than anticipated from differences in amino acid sequence. The response patterns of many subjects indicate that the A-chain loop associates with the N-terminal B chain to form a complex determinant. This determinant is recognized more often than individual amino acids. We conclude that insulin therapy generates polyclonal T cell responses directed at multiple epitopes on the molecule. Many of these epitopes are not identified by amino acid exchanges and their presence on human insulin leads to apparent autoimmunity.