Abstract:

Type 1 diabetes (T1D) is characterized by a specific autoimmune destruction of insulin-producing pancreatic β cells, leading to chronic hyperglycemia. The T1D pathology is associated with alterations of gut microbiota, even before the clinical diagnostic. Mucosal-Associated Invariant T (MAIT) cells are innate-like T lymphocytes that express a semi-invariant T cell receptor, which is restricted to a non-classical MHC class I-related protein 1 (MR1) and altered in patients with T1D. These cells can be activated by bacterial metabolites derived from the riboflavin biosynthesis pathway, or by pro‑inflammatory cytokines such as IL-18. MAIT cells are abundant at mucosal sites and harbor a protective Th17 transcriptomic profile in the intestine. This feature is lost in non-obese diabetic (NOD) mice, a model that closely mimics human pathology.

From a cohort of children with recent-onset T1D and associated controls, including very young children with the most severe disease forms, we extensively characterized blood MAIT cells, using flow cytometry and transcriptomic analyses. In addition to MAIT cell frequency reduction in T1D patients, we highlight an extensive activation of these cells marked by a polyfunctional production of IFNg, granzyme B and TNFa. The frequency of these activated populations is linked with the disease severity, especially in patients aged below 7 years. Single-cell analyses allowed to decipher six terminally differentiated MAIT cell clusters, including distinct MAIT1 and MAIT17 clusters. This highlights the extensive diversification of blood MAIT cells in early life. Furthermore, fecal microbiota from T1D children induces a lower maturation of MAIT cells in vitro, linked to blood MAIT cell alterations. To investigate the impact of the human gut microbiota on MAIT cells in physiopathological T1D development, we performed fecal microbiota transplantation from patients to four week-old NOD mice. We observed reduced intestinal immune cell activation associated with a defective epithelial cell homeostasis in mice transferred with T1D microbiota. This affects the epithelial renewal process, leading to an accumulation of terminally differentiated goblet cells. Ileal MAIT cells are less abundant and mature, with a defective Th17 profile. The intestinal integrity declined and bacterial 16S DNA translocation increased toward pancreatic lymph nodes. In addition, the lack of MAIT cell maturity is highly correlated with the bacterial compounds’ translocation. Moreover, high level of T-cell activation (CD69 and PD-1) is observed in the liver of NOD mice. Those results suggest that blood MAIT cell activation in patients might also reflect an increased translocation upon human intestinal dysbiosis. Single-cell transcriptomic data from the ileum of transferred NOD mice suggests a loss of MAIT cells supportive CD80/86 and IL-23 signals issued from myeloid cells, the latter interactions shifting towards a harmful state marked by TNFaproduction, notably from macrophages and neutrophils. Altogether, those alterations are associated to increased and accelerated diabetes onset upon T1D microbiota transfer. Transfer of T1D microbiota in MR1-/- NOD mice, which lack MAIT cells, revealed an unchanged incidence, emphasizing the role of MAIT cells in the transmission of T1D physiopathology through the intestinal microbiota.