Complement-dependent antibody-mediated cytotoxicity (C'AMC) to pancreatic islet cells in the spontaneously diabetic BB/OK rat: interference from cell-bound and soluble inhibitors.

Sera from 30 newly diagnosed diabetic BB/OK rats were analyzed cross-sectionally for complement-dependent antibody-mediated cytotoxicity (C'AMC) to pancreatic islet cells using different 51Cr release test systems. The sera contained enough active complement to lyse either sheep erythrocytes or 51Cr-labelled rat islet cells that had been sensitized with specific rabbit antibodies, but, in the presence of BB/OK rat islet cell antibody they released significant amounts of islet cell-bound 51Cr only after addition of rabbit complement. In a one-step assay, 19 out of the 30 sera produced lysis significantly above that by sera from the non-diabetes-prone WOK strain. This was increased 2.5-fold (p less than 0.01) by briefly washing the serum-treated cells before adding complement (two-step assay), indicating that C'AMC inhibitory activity was present in the diabetic sera. Some inhibitory activity could still be detected in heated sera but only when they were added to the cells immediately before the rabbit complement. Islet cell lysis was still substantial after preferential inactivation of factor B of the alternative complement activation pathway (by heating at 50 degrees C), and thus mainly depended on the classical pathway. From these findings it is concluded that (a) islet cell surface antibodies in diabetic rats activate heterologous complement via the classical pathway, (b) anti-islet C'AMC is sensitive both to species-restricted interference in interactions between homologous antibody and homologous complement in the target cell membrane, and to a distinct serum inhibitor that impairs the ability of membrane-fixed BB/OK rat antibody to interact with rabbit complement.