Gut microbial dysbiosis is associated with the development of autoimmune disease, but the mechanisms by which microbial dysbiosis affects the transition from asymptomatic autoimmunity to inflammatory disease are incompletely characterized. Here, we identify intestinal barrier integrity as an important checkpoint in translating autoimmunity to inflammation. Zonulin family peptide (zonulin), a potent regulator for intestinal tight junctions, is highly expressed in autoimmune mice and humans and can be used to predict transition from autoimmunity to inflammatory arthritis. Increased serum zonulin levels are accompanied by a leaky intestinal barrier, dysbiosis and inflammation. Restoration of the intestinal barrier in the pre-phase of arthritis using butyrate or a cannabinoid type 1 receptor agonist inhibits the development of arthritis. Moreover, treatment with the zonulin antagonist larazotide acetate, which specifically increases intestinal barrier integrity, effectively reduces arthritis onset. These data identify a preventive approach for the onset of autoimmune disease by specifically targeting impaired intestinal barrier function.

Background To investigate whether methotrexate treatment may affect the susceptibility to infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Methods Clinical assessment of symptoms, SARS-CoV-2 RNA, and anti-SARS-CoV-2 IgG in an initial case series of four families and confirmatory case series of seven families, within which one family member developed coronavirus disease 19 (COVID-19) and exposed another family member receiving methotrexate treatment; experimental part with methotrexate treatment of mice and organoids followed by the assessment of mRNA and protein expression of the SARS-CoV-2 receptor angiotensin-converting enzyme (ACE)-2. Results In the initial case series, three of four women on a joint ski trip developed COVID-19, while the fourth woman, under treatment with methotrexate, remained virus-free. Two of the three diseased women infected their husbands, while the third husband treated with methotrexate remained virus-free. In addition, 7 other families were identified in a follow-up case series, in which one member developed COVID-19, while the other, receiving methotrexate, remained healthy. Experimentally, when mice were treated with methotrexate, ACE2 expression significantly decreased in the lung, in the intestinal epithelium, and in intestinal organoids. Conclusion These clinical and experimental data indicate that methotrexate has certain protective effects on SARS-CoV-2 infection via downregulating ACE2.

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BackgroundIntestinal dysbiosis has been associated with the development and progression of rheumatoid arthritis (RA) [1].Microbiota-derived metabolites such as short-chain fatty acids (SCFA) gained attention in research of inflammatory diseases as promising targets for new therapeutics. Previous work in our lab showed the potent preventive effect of SCFA on the onset of inflammatory arthritis (2, 3). Although effective potential therapeutic approaches are arising, their underlying mechanisms remain elusive [4-6].Cellular derived histamine is considered as pro-inflammatory mediator to induce acute allergic symptoms and to maintain chronicity. Interestingly, we identified an unexpected pro-resolving pathway of SCFA-induced histamine secretion by the gut microbiota. In contrast to the effective, direct Treg inducing 2-week delayed resolving effects of the SCFA propionate, we report on an indirect but ten times faster histamine-mediated pro-resolving mechanism via cells of the nervous system.ObjectivesTo understand the rapid pro-resolving effects of microbiota-derived histamine on synovial inflammation.MethodsWe used 16s rRNA sequencing, HPLC size chromatography exclusion, FMT and meta-transcriptomics to assess propionate-induced changes in microbiota composition and the secreted metabolite profile. Mice with collagen- or serum-induced arthritis (CIA/SIA) were treated orally, i.p. or intrathecally with histamine or specific receptor agonists. Cellular changes in spleen, lymph nodes and joints were assessed by Cytek spectral flow cytometry, inflammatory infiltration and bone erosion were analyzed by histology and µCT. We analyzed differences in CNS and PNS via RNAseq of the spinal cord and peripheral nerves. Effects on vessel leakiness and cell infiltration in the inflamed joints were assessed via lightsheet microscopy and in vivo PET-CT.ResultsHere, we show that therapeutic treatment of CIA mice with the SCFA propionate starting from peak of disease (30 dpi) strongly induced resolution of synovial inflammation after 14 days of treatment. We demonstrate that oral propionate-treatment causes beneficial changes in the microbiota composition and thereby alters the secreted metabolite profile. These metabolites are able to induce rapid resolution of inflammation already after 2 days. By untargeted metabolomics we were able to identify histamine as highly potent metabolite that is increased upon propionate treatment. Oral treatment of CIA mice with histamine or a histamine 3 receptor (H3R) agonist significantly improved clinical symptoms. H3R is mainly expressed on cells of the nervous system. Upon oral H3R treatment we found alterations in activation marker expression in the spinal cord. We further were able to identify changes in the composition and activation of spinal cord (CNS) and the N. plantaris (PNS) of arthritic mice induced by H3R agonist treatment through RNAseq. Moreover, lightsheet microscopy and in vivo PET-CT scans revealed alterations in vessel leakiness and cell infiltration in the inflamed joints.ConclusionIn summary, these data show that the SCFA propionate effectively regulates ongoing inflammation by promoting histamine secretion of the gut microbiota and subsequent H3R-mediated neuronal effector functions that drive the fast resolution of synovial inflammation.References[1]J. U. Scher et al., eLife 2, e01202 (2013).[2]S. Lucas et al., Nat Commun 9, 55 (2018).[3]N. Tajik et al., Nat Commun 11, 1995 (2020).[4]W. Walrabenstein et al., Rheumatology (2023).[5]J. Häger et al., Nutrients 11, (2019).[6]K. Dürholz et al., Nutrients 12, 3207 (2020).Acknowledgements:NIL.Disclosure of InterestsNone Declared.