Cellular changes in the gut of Rheumatoid Arthritis and the role of endothelial cells

Rheumatoid arthritis (RA) is a systemic autoimmune disorder leading to joint inflammation and bone destruction influenced by genetic and environmental factors. There is emerging evidence suggesting a fundamental role for the intestinal microbiota, intestinal barrier and the intestinal immune system in disease onset and progression. One mechanism by which the intestine might influence arthritis is the generation of systemic antibodies of the type immunoglobulin G (IgG) targeting translocating intestinal bacteria, which might contribute to inflammation and bone destruction in RA. The vasculature, whose inner lining is formed by endothelial cells, connects distant body sites, including the gut and the joints, and orchestrates immune cell trafficking and the systemic dissemination of antibodies. This work investigated the interplay between the intestinal immune system and the pathogenesis of RA with a particular focus on endothelial cells and generation of antibodies targeting intestinal bacteria. To study that, we performed time-course experiments in a mouse model of RA, the collagen induced arthritis (CIA), which resembles important features of human RA including autoimmunity preceding clinical onset joint inflammation and bone destruction. In this model we observed a tendency of increased colonic endothelial leakiness in the pre-disease phase, which might be a predictor of clinical disease progression. Our messenger ribonucleic acid (mRNA) sequencing data of ileal sorted endothelial cells showed increased signs of endothelial activation and immune cell attraction, especially in the pre-disease state. Although this increased activation did not lead to major changes in the cellular composition of the ileum, as our imaging mass cytometry (IMC) data reveals, we saw changes in the estimated cellular interaction between different immune cell types. In the bone, endothelial cells showed most regulation in the active disease phase, and a strong interferon I gene signature. This emphasises the role of the environment in which endothelial cells reside and how they react to the same trigger. Endothelial cells in the intestine are more likely to encounter bacterial compounds such as lipopolysaccharide (LPS), which might act as a second trigger to induce endothelial activation. Along the CIA time-course, we performed 16S ribosomal RNA (16S rRNA) sequencing of stool bacteria, showing a change in the microbial composition in the gut. By enzyme linked immune immunosorbent assay (ELISA) we observed increased levels of IgGs reactive to gut bacterial components. To see which bacteria are bound by serum IgG we performed 16S rRNA sequencing of stool bacteria bound to IgG, which showed preferential IgG binding of bacteria associated with colonizing the respiratory tract. For rare taxa, the IgGhigh and complete bacterial fractions converged in bacterial composition through disease progression which indicates that the immune system starts to recognize and react to almost all rare taxa as disease advances, indicating enhanced immune cell activation. In a human cohort, involving healthy subjects, early RA patients (disease duration below 1 year), RA patients (disease duration below 1 year), and patients with inflammatory bowel disease (IBD), we performed IMC and mRNA sequencing of ileal biopsies to identify changes in the different conditions. Although the IMC data as well as the sequencing did not reveal major differences between the different conditions, we saw a tendency towards more immune cell activation and the generation of immune cell patches in the intestinal tract in IBD as well as early RA, whereas the RA patients were more similar to the healthy controls. Altogether our data highlights a role for the intestinal tract especially in the pre- and early disease setting of human RA and CIA, which indicates a role in disease initiation and progression.