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Background Biologic therapy has significantly improved treatment for UC, but nearly two-thirds of patients attenuate the response. Additional therapeutic modalities are therefore needed to address the underlying pathophysiology of UC. Faecal microbiota transplant (FMT) is an emerging therapy for the treatment of UC, but several randomised controlled trials have shown variable efficacy of FMT, and the microbial mechanisms responsible for clinical response are not well understood. Therefore, using samples from our pilot FMT study (Jacob, V, et al. Inflamm. Bowel Dis. 2017), we aim to identify the core transferable microbiota (CTM) in UC patients responsive to FMT therapy and to define the therapeutic mechanism of these strains in pre-clinical models. Methods IBD disease activity scores were used to define a clinical response. Metagenomic sequencing of a donor, recipient, and 4-week post-FMT faecal samples was performed to define the CTM. Strain-level transferability was defined using the StrainFinder algorithm. To define the transferable immune-reactive microbiota (TIM), IgA-seq was performed on a donor, recipient, and 4-week post-FMT faecal samples. TIM strains were isolated from faecal samples and gnotobiotic mouse models were used to evaluate their impact on mucosal immunity and mouse models of colitis. Results Here, we defined a CTM associated with clinical response to FMT for UC. Strain-level tracking of the CTM confirmed that clinical response correlated with strain transferability. In addition, we defined a core TIM by IgA-seq that correlated with clinical response. In humanised mouse models, these TIM were found to induce IgA in a T-cell independent manner. Colonisation of germ-free mice with a core TIM strain of Odoribacter induced IL-10-dependent, RORgt+/Foxp3+ iTreg cells and reduced the severity of transfer T-cell colitis in mono-colonised RAG−/− mice. Conclusion Our data highlight an immune-reactive, core transferable microbiota in responders to FMT for UC. Using pre-clinical mouse models of colitis, we define the mechanistic impact of these TIM in shaping mucosal immunity and guiding the response to UC. This work provides a framework for the rational selection of TIM for microbial-therapy in IBD.

OP40 A core transferable microbiota in responders to faecal microbiota transplant for ulcerative colitis shape mucosal T-cell immunity