P173 Characterization of the gut microbiota and the colonic immune response in acute severe ulcerative colitis : the multi-omics ITAC project

Background Limited understanding exists regarding the pathogenesis of acute severe ulcerative colitis (UC). Microorganisms are proposed as potential triggers due to the resemblances to infectious colitis and the essential role played by gut microbiota in UC-related inflammation. Our aim was to identify microbiome elements and host factors associated with acute severe UC. Methods This was a prospective study across three referral centers comparing two UC patient groups: acute severe hospitalized UC (as per Truelove and Witts criteria) and non-severe UC. We analyzed gut microbiota using 16S rRNA gene sequencing and conducted single-cell RNA-Seq on rectal biopsies in a subgroup of patients to uncover cellular subtypes and pathways involved in mucosal inflammation. We utilized whole blood RNA-Seq to investigate the host pathways involved in mediating systemic inflammation. Results Forty-one patients (23 (56%) female, median (interquartile range IQR) age 42 (34 – 57) years were included: 19 with acute severe UC and 22 with non-severe UC. Compared to patients with non-severe UC, those with acute severe UC displayed distinct gut microbiota with reduced diversity, increased Proteobacteria (Escherichia/Shigella genus), and decreased Lachnospiraceae and Ruminococcaceae. In severe cases (n=4 - 13,047 cells), single-cell RNA-Seq analysis of rectal biopsies revealed distinctive patterns compared to non-severe cases (n=5 - 18,433 cells): plasmablasts showed an altered transcriptomic profile with heightened IgG expression, and there was an expanded cluster of interleukin (IL) 26 expressing T cell population. Innate immune cells exhibited a pro-inflammatory profile marked by increased expression of the IL1B gene. In blood samples, we observed no distinct differentiation of transcriptomic profiles between severe and non-severe cases. Conclusion Our multi-omics study reveals key cellular and bacterial components in acute severe UC pathogenesis. We identified Proteobacteria, especially Escherichia coli as a potential pathobiont in acute severe UC. At the colonic level, we observed an increased IgG/IgA ratio, while both T cells and innate immune cells indicated a pro-Th17 mucosal environment. Enhanced systemic inflammation in acute severe UC was not reflected by specific changes in immune circulating cells. These insights may pave the way for future research focusing on microbiome modulation, interventions targeting plasmablasts, or nuanced inhibition of the Th17/IL-23 axis in acute severe UC.