Quantitative determination of islet cell surface antibodies using ¹²⁵I-protein A

A quantitative method to measure islet cell surface antibodies in human patients has been developed using ¹²⁵I-protein A. Isolated, dispersed, viable rat islet cells prepared by collagenase digestion were fixed in 4% paraformaldehyde to allow storage for up to 7 wk at 4°C. Human sera, heat inactivated and adsorbed with rat liver and kidney powder (100 mg/ml), were incubated with the fixed cells (50 x 10³) for 60 min at 37°C. Thereafter the cells were washed and exposed to 5 x 10⁵ cpm ¹²⁵I-protein A, which binds to IgG attached to the cell surface. Assay precision (14%) and reproducibility (16%) were established by repeated analysis of pooled sera from healthy individuals and IDDM patients using pooled batches of islet cells. Using this method, islet cell surface antibodies were detected in 35% of insulin-dependent diabetic patients.