Inhibition of insulin-specific autoreactive T-cells by sulphatide which is variably expressed in beta cells

Sulphatide and insulin are present in the secretory granules and at the surface of beta cells in islets of Langerhans. Insulin autoantibodies and T-cell reactivity against insulin exist during the development of Type I (insulin-dependent) diabetes during which active beta cells may be more vulnerable than passive. Our aims were to examine the presence of sulphatide in active and passive beta cells and to clarify whether sulphatide influences the direct autoimmunity against insulin.

We incubated rat islets in 2.8, 11.0 or 20.0 mmol/l glucose for 24 h and did an electron microscopic evaluation after labelling with a specific anti-sulphatide monoclonal antibody. The reactivity of an insulin-specific T-cell clone isolated from a patient with Type I diabetes, was examined using insulin or insulin B-chain (B11-27) peptide incubated together with sulphatide.

We detected lower amounts of sulphatide per insulin secretory granule in active compared with passive beta cells (p = 0.003). The presence of sulphatide in vitro at doses of 43-8.3 micromol/l resulted in greatly reduced proliferation (median 3.4 % of control value, p = 0. 0004) of the insulin-specific T-cell clone. No inhibition was found using the precursor of sulphatide, galactosylceramide, or GM1. Sulphatide did not reduce non-aspecific proliferation (induced by phorbol myristate acetate plus anti-CD3) or specific proliferation induced by insulin peptide.

These results imply that sulphatide possibly affect processing of the insulin molecule. Sulphatide which has been reported to interfere with phagosome-lysosome fusion, conceivably interacts with insulin. We hypothesize a (patho)physiological role of sulphatide, variably expressed in beta cells, by reducing the antigenicity of insulin. [Diabetologia (1999) 42: 1212-1218]