Molecular analysis of innate immunity in type 1 diabetes

Introduction: Type I diabetes (T1D) is an autoimmune disease of children and young adults that selectively destroys the insulin-producing islet cells in the pancreas. NOD mice, the best validated animal model of T1D, manifests islet infiltration as early as 4 weeks, leading to β-cell destruction and eventually overt diabetes. This study was designed to determine the role of TRAIL, FASL and IFN-γ, components of innate immune system, in the biology of β-cell destruction in NOD mice. Experiments: To test the hypothesis that differential expression of TNF superfamily ligands (TRAIL and FASL) affects tolerance mechanisms, I assessed the diabetes susceptibility of TRAIL-deficient NOD.Tnfsf10⁻/⁻ mice and of mice which exhibited allelic Fasl expression (NOD.D1Bax208ᵐ¹ and NOD.D1Bax208ᵐ²). Immune cells from these three mouse lines were systematically characterised by flowcytometry, and their diabetogenic potential determined by adoptive transfer experiments. β-cell response in a TRAIL deficient environment was assayed in islet cultures. Micro array analysis was performed to compare the β-cell response in mice with allelic Fasl expression. To investigate the role of IFN-γ in NOD diabetes, I assessed the diabetes susceptibility of targeted gene deficient mutants of Ifng (NOD.Ifng⁻/⁻) and the β chain of Ifng receptor (NOD.Ifngr2⁻/⁻). I examined the role of IFN-γ in BCG-induced diabetes protection by assessing diabetes susceptibility of BCG treated NOD.Ifng⁻/⁻ and NOD.Ifngr2⁻/⁻ mice, and by characterising their immune cell compartments. Further, the contribution of IFN-γ in accelerated precipitation of lupus in NOD mice was studied by administering 1mg BCG to NOD.Ifng⁻/⁻ and NOD.Ifngr2⁻/⁻ mice. Results: Targeted deletion of TRAIL resulted in increased severity of spontaneous insulitis and an increased incidence of diabetes. This was the first ever demonstration of an increased incidence of spontaneous diabetes in NOD mice bearing a targeted deletion of TRAIL. Similarly, florid lymphoid infiltrates and high diabetes incidence in NOD. D1Bax208ᵐ² mice, which exhibited low FASL-expression, provided direct evidence for a protective role of this death ligand in autoimmune diabetes. Despite increased susceptibility to autoimmunity, tolerance mechanisms in these mouse strains were unaffected. In adoptive transfer experiments, numbers of diabetogenic T cells were found to be unchanged in NOD.Tnfsf10⁻/⁻ and NOD. D1Bax208ᵐ² donor mice, whereas NOD.Tnfsf10⁻/⁻ and NOD. D1Bax208ᵐ² recipient mice showed increased susceptibility to diabetes. These findings raised the possibility of TRAIL and FASL expression in the nonhematopoietic compartment conferring protection against T1D. LPS activation of NOD.Tnfsf10⁻/⁻ islets and comparison of gene expression profiles between NOD. D1Bax208ᵐ¹ and NOD. D1Bax208ᵐ² mice indicated that TRAIL and FASL regulated the islet-expression of the potent chemokine, Interferon-γ-induced protein-10 (IP-10) and the suppressor of cytokine signalling (SOCS1), a strong inhibitor of IFN-γ signalling. TRAIL and FASL-mediated suppression of IP-10, and upregulation of Socs1 transcription, rendered the NOD-islets less susceptible to immune destruction in T1D. Proinflammatory cytokine IFN-γ was not essential for diabetes development in NOD mice, but was vital for BCG-mediated diabetes protection. Diabetes incidence in NOD.Ifng⁻/⁻ mice was similar to that of WT NOD/Lt mice, however, BCG failed to inhibit the disease in mice lacking IFN-γ-signalling. IFN-γ-induced elimination of effector T cells played a role in BCG-mediated diabetes protection in NOD mice. At a high-dose of 1mg, BCG precipitated a systemic form of autoimmunity that had characteristics similar to human SLE, in these mice. However, I found that deficiency in IFN-γ-signalling abrogated the influence of BCG in mediating the expression of NODs tendency towards autoimmunity. Conclusion: This study assessed the mechanism of protection mediated by TRAIL, FASL and IFN-γ, components of the innate immune system, in NOD diabetes. I observed a remarkable parallel between the β-cell protective effects of TRAIL and FASL, the two homologous TNF superfamily ligands. An exciting role for the proinflammatory cytokine IFN-γ, in modulating NOD diabetes in a BCG-mediated T1D-protection model, was also identified. Results discussed here provide insight into the islet immune specific regulatory role of TRAIL, FASL and IFN-γ in the pathogenesis of diabetes and suggest therapeutically beneficial strategies for islet preservation.