Induction in C57BL/KsJ Mice of Complement-dependent Antibody Cytotoxic to Cultured Beta Cells

Despite widespread evidence that autoimmune mechanisms may contribute to the beta cell necrosis associated with type I insulin-dependent diabetes mellitus (IDDM), it has not heretofore been demonstrated that islet cell antibodies (ICAs), directed primarily against cytoplasmic antigens, are capable of specific lysis of beta cells. We utilized a readily accessible source of mouse pancreatic islets [CBA/J mice lacking exocrine pancreas (exocrine pancreatic insufficiency syndrome)] to experimentally induce ICAs in inbred mice. Homogenates of these islets were injected weekly for four weeks into syngeneic (CBA/J) and allogeneic (A/J, C57BL/6J, C57BL/KSJ) recipients. Only C57BL/KsJ inbred mice showed the induction of a high titer (≥160) antiserum cytotoxic to ⁵¹Cr-labeled CBA/J lymph node target cells. Neither the immunized C57BL/KsJ mice with circulating ICAs nor any of the other immunized strains showed any decrease in glucose tolerance as compared with vehicle controls. Moreover, no morphologic evidence of islet necrosis or atrophy was apparent. Thus the ICAs induced were reactive with alloantigenic determinants of the donor and unreactive with antigenic determinants of the recipient strain. The C57BL/KsJ antiserum was further screened for anti-islet cell cytotoxic activity using both a ⁵¹Cr release assay from CBA/J islet cell monolayer cultures, and immunocytochemical staining of sections of Bouin's fixed, paraffin-embedded pancreas. This antiserum was cytotoxic to CBA/J beta cells in monolayer culture, but not the other non-beta islet cell types. Immune lysis of the beta cells required rabbit complement. At a concentration of 1% antiserum and 4% complement, beta cell lysis was evident between 3 and 4 h at 37°C. Ultrastructural examination of beta cells exhibiting cytopathic changes revealed cytoplasmic disarray rather than any obvious lytic events at the plasma membrane. Grossly distended, rough, endoplasmic reticulum containing intra-cisternal type A retrovirus was the most prominent feature distinguishing antiserum and control serum-treated beta cells. This model system suggests that ICAs which recognize beta cell cytoplasmic antigens are capable of specifically lysing beta cells via a complement-dependent mechanism. Immunocytochemical staining revealed that, in addition to islet beta cells, the antiserum (1/500 dilution) stained a macrophage-like cell in spleen and lymph nodes, as well as an epithelial-like cell in the thymus. The possibility is discussed that this multiple specificity may have been due to passenger leukocytes present in the islet homogenates used to immunize.