Suppression of insulitis in non-obese diabetic (NOD) mice by oral insulin administration is associated with selective expression of interleukin-4 and -10, transforming growth factor-beta, and prostaglandin-E.

Oral administration of autoantigens suppresses development of autoimmunity in several animal models, and is being tested in clinical trials in patients with autoimmune diseases such as multiple sclerosis and rheumatoid arthritis. Non-obese diabetic (NOD) mice spontaneously develop insulin-dependent diabetes mellitus at 15 to 20 weeks of age, after mononuclear cell (MNC) infiltration of the pancreatic islets of Langerhans and destruction of insulin-producing beta cells. We have previously shown that oral administration of insulin suppresses insulitis and development of diabetes in the NOD mouse. Oral insulin has no metabolic effect on blood glucose. Oral insulin mediates its effect through a T cell-dependent mechanism as shown by adoptive transfer and T cell depletion experiments, but the mechanisms responsible have not been fully explored. We now report a serial analysis of the cells and cytokines associated with development of diabetes in NOD mice, and contrast this with the findings in animals fed equine insulin or a control protein (ovalbumin). Animals were fed 1 mg twice a week for 5 weeks, beginning at 5 weeks of age. Marked insulitis in naive or ovalbumin-fed NOD mice occurred at 10 weeks, at which time a dense peri-islet and intra-islet MNC infiltration was observed. Immunohistological studies using monoclonal antibodies showed that infiltrating MNC consisted mainly of CD4+ T cells ( > 75% of leukocytes) plus smaller numbers of macrophages and CD8+ T cells. These cells displayed evidence of immune activation with expression of receptors for interleukin-2 (IL-2R) plus Th1 cytokines; dense labeling for IFN-gamma and tumor necrosis factor-alpha, plus lesser amounts of IL-2, was observed. MNC lacked labeling for IL-4, IL-10, prostaglandin-E, or transforming growth factor-beta. By contrast, at 10 weeks, pancreatic tissues from NOD mice fed insulin showed considerably less insulitis, and the residual MNC, although still largely CD4+ T cells plus macrophages, showed dense labeling for IL-4, IL-10, prostaglandin-E, and transforming growth factor-beta and an absence of IL-2, IFN-gamma or tumor necrosis factor-alpha Taken together with our previous findings, these data indicate that oral administration of insulin affects the development of diabetes in NOD mice through the generation of cells that elaborate immunoregulatory cytokines within the target organ and shift the balance from a Th1 to a Th2 pattern of cytokine expression.