ORAL INSULIN FOR DIABETES PREVENTION IN NOD MICE - POTENTIATION BY ENHANCING TH2 CYTOKINE EXPRESSION IN THE GUT THROUGH BACTERIAL ADJUVANT

Oral administration of insulin suppresses the development of diabetes in nonobese diabetic (NOD) mice and deviates the cytokine balance in t he islets of Langerhans from a Th1 to a Th2 type cytokine pattern. How ever, the effect of oral insulin is limited and disease suppression is limited to a narrow dose range. Therefore we tried to improve the out come of suboptimal insulin dosing by bacterial adjuvant. Mice treated with a suboptimal dose of oral insulin showed no change in diabetes in cidence although a shift from Th1 towards Th2 cytokine expression occu rred in inflamed islets. Significant suppression of diabetes developme nt was only seen in NOD mice receiving both, insulin and the bacterial preparation OM-89 as adjuvant. OM-89 is a protein extract of Escheric hia coli, with nonspecific immunostimulatory properties. Potentiation of the effect of oral insulin by the adjuvant was associated with upre gulation of interleukin (IL)-4 Th2 cells in infiltrated islets and sus tained local IL-2 gene expression. RT PCR analyses of cytokine express ion in the gut showed a clear deviation to Th2 type reactivity and dow nregulation of inducible nitric oxide (NO) synthase (iNOS) expression by the bacterial adjuvant but not by oral insulin alone. Since macroph ages are the primary target cells of adjuvant action we tested its eff ect on mouse macrophages in vitro. Treatment with OM-89 induced transi ent release of tumour necrosis factor alpha and nitrite but rendered m acrophages refractory to restimulation by the potent macrophage activa tor lipopolysaccharide. In conclusion, the protective effect of oral i nsulin can be potentiated by pretreatment with the bacterial adjuvant OM-89. This effect correlates with enhanced Th2 cytokine and decreased iNOS gene expression in the gut, probably due to the downregulation o f proinflammatory mediators by exposure to the adjuvant.