The role of MDA5 in virus induced innate immune responses and type 1 diabetes

Type 1 Diabetes (T1D), also known as Juvenile Diabetes, is characterized by the autoimmune destruction of insulin-producing β-cells in the pancreas. Autoimmunity has been reported to result from both genetic and epigenetic factors, with viral infection proposed as a plausible environmental factor. A number of viral infections, including rotavirus (RV), coxsackievirus, mumps virus and cytomegalovirus, have been associated with the development of T1D. Of these viruses, arguably the strongest evidence for causation in T1D exists for RV. RV infection correlates with T1D in epidemiology, T-cell epitope assessments in T1D patients share homology with RVencoded peptides and RV infection in murine models provides some support for a role in the development of autoimmunity. Therefore, I have focussed on RV in this study. It was recently demonstrated that polymorphisms in the interferon-induced helicase-C domain-containing protein 1 (ifih1) gene have been correlated with the risk of developing T1D. Interestingly, the product of the ifih1 gene, melanoma differentiation associated protein 5 (MDA5), plays an important role in viral infection. Understanding the role of MDA5, and the influence of single nucleotide polymorphisms (SNPs), in virus infection is important to understand whether MDA5 affects the risk of T1D, as well as how viral infection might trigger autoimmune destruction of pancreatic β-cells. To test a putative role for MDA5 in the development of T1D, MDA5-ablated cells and mice were investigated for their response to RV infection. Additionally, a series of mutations that included the T1D-correlated SNPs were engineered into MDA5 expression constructs to investigate the role of ifih1 SNPs in cell signalling in response to rotavirus. In this investigation, MDA5 was found to play an important role in the response to RV. MDA5 functioned not only via its established effect in inducing the potent antiviral, type I interferon (IFN) response, but also via the induction of apoptosis. Moreover, the apoptosis induced by MDA5 plays a major role in protecting uninfected cells. In vivo, increased RV titre was found together with impaired IFN and nuclear-factor-kappa B (NFκB) signalling in the ifih1-/- cells. The effects of MDA5 in X the response to RV varied between different organs in the animals. Importantly, it was demonstrated that MDA5 strongly induced the immune response to RV in the pancreas. In addition, the T1D-correlated ifih1 SNPs was discovered to affect the function of MDA5 in cell lines. Furthermore, the different SNPs had different consequences for the MDA5-dependent induction of IFNs and apoptosis. The ifih1 SNPs, A946T and E627X, but not I923V, was found to impair NFκB activation and apoptosis in response to RV infection. Alternatively, I923V, but not A946T and E627X, retained the capacity to induce IFNβ upon RV infection. In aggregate, the data suggest that the minor alleles of T1D-related ifih1 SNPs diminish the innate immune response to RV infection. Individuals with A946T and E627X will have reduced apoptosis, while those with I923V and E627X will have a blunted IFN response to RV infection, both resulting in an impaired ability to limit RV production compared to individuals with the major alleles of the SNPs. These suggest that higher RV titres lead to severe infection that could be fatal, which likely explains the rarity of these polymorphisms in the population, despite the predicted protective effect of these SNPs in T1D. However, individuals with the major alleles of these SNPs, although better able to limit the virus, may be more inclined to produce an excessive innate immune response that predisposes them to progress to an inappropriate adaptive response, with ensuing autoimmune destruction of their β-cells.