Th17 immunity has been shown to regulate autoimmune diabetes in mice. IL-17 neutralization prevented development of diabetes when given postinitiation of insulitis but not earlier, suggesting interference with the effector phase of the disease. Islet-cell Ag-specific Th17 cells converted into IFN-γ–secreting Th1-like cells and caused diabetes in mice recipients. The role of IL-17 in human type 1 diabetes (T1D) is, however, not established. In this study, we show upregulation of Th17 immunity in peripheral blood T cells from children with T1D. This was characterized by increased IL-17 secretion and expression of IL-17, IL-22, and retinoic acid-related orphan receptor C isoform 2, but also FOXP3 transcripts upon T cell activation in vitro. Also, circulating memory CD4 cells from children with T1D showed the same pattern of IL-17, IL-22 and FOXP3 mRNA upregulation, indicating IL-17 pathway activation in vivo. IL-17–positive T cells appeared to be CD4+ cells expressing TCR-αβ and CCR6, and a subpopulation showed coproduction of IFN-γ. Given the Th17 immunity in T1D, we demonstrated that IL-17 had detrimental effects on human islet cells in vitro; it potentiated both inflammatory and proapoptotic responses. Our findings highlight the role of IL-17 immunity in the pathogenesis of human T1D and point to a potential therapeutic strategy.

The steep increase in the incidence of type 1 diabetes (T1D), in the Western world after World War II, cannot be explained solely by genetic factors but implies that this rise must be due to crucial interactions between predisposing genes and environmental changes. Three parallel phenomena in early childhood – the dynamic development of the immune system, maturation of the gut microbiome, and the appearance of the first T1D-associated autoantibodies – raise the question whether these phenomena might reflect causative relationships. Plenty of novel data on the role of the microbiome in the development of T1D has been published over recent years and this review summarizes recent findings regarding the associations between islet autoimmunity, T1D, and the intestinal microbiota.

Upregulation of IL-17 immunity and detrimental effects of IL-17 on human islets have been implicated in human type 1 diabetes. In animal models, the plasticity of Th1/Th17 cells contributes to the development of autoimmune diabetes. In this study, we demonstrate that the upregulation of the IL-17 pathway and Th1/Th17 plasticity in peripheral blood are markers of advanced β cell autoimmunity and impaired β cell function in human type 1 diabetes. Activated Th17 immunity was observed in the late stage of preclinical diabetes in children with β cell autoimmunity and impaired glucose tolerance, but not in children with early β cell autoimmunity. We found an increased ratio of IFN-γ/IL-17 expression in Th17 cells in children with advanced β cell autoimmunity, which correlated with HbA1c and plasma glucose concentrations in an oral glucose tolerance test, and thus impaired β cell function. Low expression of Helios was seen in Th17 cells, suggesting that Th1/Th17 cells are not converted thymus-derived regulatory T cells. Our results suggest that the development of Th1/Th17 plasticity may serve as a biomarker of disease progression from β cell autoantibody positivity to type 1 diabetes. These data in human type 1 diabetes emphasize the role of Th1/Th17 plasticity as a potential contributor to tissue destruction in autoimmune conditions.

Background and Aims: Several factors support the view of inflammatory bowel disease [IBD] origin in the host responsiveness to intestinal bacteria, although no single bacterial species has been shown as a causative agent in the pathogenesis. Our aim was to analyse the fecal microbiota of paediatric IBD patients at different stages of the disease. In addition, the characteristics of immune response to the bacterial isolates showing very low abundance in IBD were studied. Methods: Fecal samples [1-3 samples/child] were collected from 10 paediatric patients with crohn's disease [CD],] and 12 with ulcerative colitis [UC] and from 8 healthy children, for polyphasic microbiological analysis (culture, real-time polymerase chain reaction [PCR], and denaturing gradient gel electrophoresis). In addition, in vitro cytokine responses of peripheral blood mononuclear cells to the bacterial isolates, which showed very low abundance in IBD, were studied. Results: Although predominant bacterial diversity was higher in IBD, the numbers of Lachnospiraceae and Coriobacteriaceae bacteria were lower in IBD patients as compared with control children [p < 0.05]. In addition, Ruminococcaceae population diversity was lower in IBD [p < 0.05] and correlated negatively with fecal calprotectin levels. Both abundance and diversity of bifidobacterial populations were lower in children with IBD [p < 0.05], and particularly low numbers of certain bifidobacterial isolates were detected. In CD, we found enhanced up-regulation of interleukin-6 transcripts and impaired RAR-related orphan receptor C response to bifidobacteria, whereas decreased interferon-gamma response was observed in both CD and UC. Conclusion: We demonstrate altered fecal microbiota in paediatric IBD, particularly low numbers and diversity of bifidobacterial populations. Interestingly, immunological response to bifidobacteria differed between paediatric CD patients and control children.

Accumulated data suggest that the gut microbiota is involved at two different steps in the evolution of type 1 diabetes. At the first step, the intestinal tract is colonized by a microbial community unable to provide an adequate education of the immune system. As a consequence, the infant acquires susceptibility to immune-mediated diseases, type 1 diabetes included. At the other step, the young child seroconverts to positivity for diabetes-associated autoantibodies. This is preceded or accompanied by a decrease in the diversity of the intestinal microbiota and an increased abundance of Bacteroides species. These changes will affect the disease process promoting progression toward overt type 1 diabetes. By providing specific probiotics, one can affect the colonization of the intestinal tract in the newborn infant or strengthen the immune education in early life. Human milk oligosaccharides function as nutrients for "healthy" bacteria. Dietary interventions applying modified starches can influence the numbers and activities of both autoreactive and regulatory T cells and provide protection against autoimmune diabetes in non-obese diabetic mice. Modulation of the intestinal microbiome holds the promise of effective protection against human type 1 diabetes.