Structure, Function, and Immunogenicity of Human Insulinoma Cells

Dissociated human insulinoma cells were plated onto plastic multiwell dishes. Cells were maintained for 1 mo on plastic with three passages. Cultures consisted of small colonies with some areas of stratification and few intercellular spaces. Ultrastructural studies indicated that cultured cells had epithelial features with desmosomes at cell-to-cell contacts and intermediate filaments in addition to secretory granules in the cytoplasm. Insulin and C-peptide were released in equimolar amounts in culture media. When challenged for 30 min with 16.7 mM glucose, 1 mM 3-isobutyl-1-methylxanthine, 4 mM tolbutamide, or 10⁻⁶ M glucagon, insulinoma cells responded by a 1.5-, 1.5-, 2-, or 3-fold increase, respectively, in insulin release above baseline levels. A 15-min challenge with 10⁻⁵ M isoproterenol increased insulin secretion by 1.85-fold. By indirect immunofiuorescence, an anti-insulin antibody reacted positively with cell cytoplasm, whereas anti-somatostatin and anti-glucagon antibodies did not. Insulinoma cell surface expressed class I MHC molecules but not class II molecules. Immediately after isolation, crude insulinoma cells were contaminated by 2% of DR⁺ cells from nonislet components that disappeared after several weeks in culture. The ability of insulinoma cells to stimulate allogenic T-lymphocyte proliferation was assessed by [³H]thymidine incorporation in mixed culture combinations. Crude insulinoma cells elicited a strong lymphoproliterative response with a stimulation index ranging between 3.5 and 7, whereas no stimulation was found after 1 mo in culture. It is postulated that absence of class II-positive cells in the stimulatory cell preparation conditioned this immune tolerance across the major histocompatibility barrier. These results obtained with human insulin-secreting cells underline the need of in vitro manipulation for the success of human islet allografts in insulin-dependent diabetes mellitus.