A growing body of experimental evidence suggests that immune mechanisms in conjunction with genetic susceptibility and environmental factors, contribute to the pathogenesis of Type I diabetes mellitus. The hallmark of Type I diabetes is the selective loss of Beta cells from the pancreatic islets. Using an indirect immunofluorescence assay, we have detected in the serum of patients with Type I diabetes antibodies binding to the surface membrane of viable and dispersed rat or mouse islet cells. Whether these islet cell surface antibodies (ICSA) contribute to the Beta cell loss of Type I diabetes has not been demonstrated. The overall objective of this research proposal is to establish methodologies that will measure the cytotoxicity of ICSA in vitro. A specific aim of this proposal is to establish sensitive assays that will measure both Beta cell dysfunction and death after dispersed viable islet cells coated with ICSA are exposed to complement. The role of an antibody-dependent cell-mediated cytotoxic reaction (ADCC) will be investigated by using viable dispersed suspensions of rat islet cells as target cells, ICSA from diabetic serum, and isogeneic rat or normal lymphocytes as effector cells. The sensitivity of the ADCC reaction to detect circulating ICSA will be compared to the detection of ICSA by indirect immunofluorescence. The ability of diabetic lymphocytes to mediate ADCC as well as possible serum inhibitory factors to ADCC will be evaluated. Furthermore, the effector cell population responsible for ADCC, when rat islet cells are used as target cells, will be investigated. After the successful development of methodologies to evaluate the cytotoxicity of ICSA in vitro, a prospective family study will be initiated to evaluate the functional and prognostic significance of ICSA. After identification of new-onset Type I diabetics, all siblings and parents will be assessed for ICSA, and prospectively followed. The correlation over time of ICSA with glucose tolerance in vivo and complement dependent cytotoxicity and ADCC in vitro will be assessed.