The intestinal microbiome plays an essential role in host health, where microbial communities have immune-modulating activity and impact on regulatory immune cell compartmentalisation in the gut associated lymphoid tissues. Increasing evidence points to a dysfunctional relationship between the microbiome and host tissues in individuals who develop type 1 diabetes (T1D). Alterations in gut microbial composition and diversity have been observed in cohorts of individuals who go on to develop diabetes and the amount of antibody binding to common microbes is altered in diabetics, controlled in part by T1D risk HLA haplotypes. Antibodies play a key role in the composition and healthy function of the gut microbiota.  Development of the most aggressive form of T1D, with the near complete destruction of insulin producing beta cells in children diagnosed under the age of 7, is associated with an increased number and proportion of B lymphocytes infiltrating the pancreas.  Using samples from tissues across affected and systemic tissues in T1D (duodenum, spleen, pancreatic lymph node and pancreas tissues) from nPOD, we will test the hypothesis that perturbations in microbial populations and intestinal barrier function promote cross-reactivity to pancreatic beta-cell antigens thus driving development of T1D in children. We will apply a sequencing approach to deeply characterise the mucosal-associated microbial community in intestinal tissues derived from control, autoantibody positive and T1D donors. In parallel, in our approved study, we will test whether T and B cells present in the gut are related to those found in the spleen, pancreatic lymph nodes and the pancreas using a sequencing-based approach to identify cells with a shared ancestry. We will trace related clones across sites through the B/T cell receptor sequences and determine their binding characteristics using cloning and expression approaches to test for microbial and pancreatic beta-cell specificity.