A murine mixed islet-lymphocyte coculture system (MILC) was used to quantitate the immunogenicity of a pure population of pancreatic β-cells to more clearly define whether stimulator major histocompatibility complex (MHC) class ll-positive dendritic cells are a major component leading to islet immunogenicity. Pancreatic β-cells express MHC class I antigen but not class II antigen. These experiments compared the in vitro immunogenicity of fluorescence-activated cell sorted (FACS-IV) pure β-cells (MHC class I-positive cells only) relative to unpurified dispersed islet cells (MHC class I-positive cells and class II-positive cells). The results demonstrated the surprising finding that pure DBA/2J (H-2b) pancreatic β-cells stimulated a strong cytotoxic T-lymphocyte (CTL) response when exposed to C57BL/6 (H-2b) allosplenocytes in the MILC, similar to DBA/2J nonpurified dispersed islet cells. Furthermore, the stimulation of CTL by both purified β-cells and nonpurified dispersed islet cells was blocked by addition of MHC-specific anti-class I monoclonal antibody directed against stimulator MHC antigen. The data imply that the highly immunogenic MHC class II-positive passenger leukocytes present in the islets were not necessary for the generation of the immune response in the presence of MHC class l-positive β-cells. Although most of the pretreatment regimens attempting to decrease islet immunogenicity have been directed at eliminating the MHC class II-positive passenger leukocytes from the islets, this work suggests that modulation of MHC class I antigen may be an important approach.

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