In this paper we report the concentration of terminal complement complexes (TCCs, SC5b-9, an index of complement activation) in newly diagnosed insulin-dependent diabetes mellitus (IDDM) patient serum and normal human serum. In the nine patients studied, levels of serum soluble TCCs were approximately 1.6-fold higher than in sera obtained from normal control individuals. On incubation of rat islet cells with diluted serum (10%, v/v, concentration), complement activation was increased at a significantly faster rate and the total TCC concentration was significantly higher in culture medium containing IDDM patient serum than in medium containing control serum. The concentration of anti-(glutamic acid decarboxylase) autoantibodies in newly diagnosed IDDM patient serum was on average 60-fold higher than in normal human control serum. IDDM patient serum (10%, v/v) induced apoptosis in islet cells, as determined by islet cell density changes and DNA fragmentation patterns. However, serum from IDDM patients was not able to induce apoptosis of the cells when complement components (C1q and C3) or antibodies were depleted. In addition, glutamine and the potent antioxidant 1-pyrrolidinecarbodithioic acid partially reversed cell death induced by IDDM patient serum in a concentration-dependent manner. The ATP concentration in islet cells incubated for 24 h in the presence of diluted IDDM patient serum was reduced to 4.4% of that observed in islet cells incubated in fetal calf serum or 7.3% of that observed in islet cells incubated in normal human serum. On the basis of these observations, we suggest that the pathway of IDDM patient serum-induced islet cell apoptosis may involve antibody-dependent complement activation, free radical generation and a precipitous fall in ATP levels.

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