Background/Purpose: While alterations in tryptophan (Trp) metabolism have been broadly implicated across autoimmune diseases (including RA, SpA, SLE, and MS), the specific role(s) of Trp metabolites in the development of disease are not well characterized. We previously identified significant alterations in tryptophan metabolism in mice with collagen-induced arthritis (CIA) that occurred in a microbiome-dependent manner, including significantly elevated indole. As indole is exclusively produced via microbial Trp metabolism, we modulated indole production through either antibiotic-mediated microbiome depletion or reduction of dietary Trp to dissect the effects of tryptophan, indole, and the microbiome on the development of CIA.

Methods: To regulate dietary Trp, 6wk-old male DBA/1J mice were maintained on either a Trp-sufficient (TS, 0.18% Trp) or Trp-low (TL, 0.05% Trp) diet. For the microbiome depletion studies, mice were treated with broad-spectrum antibiotics (ampicillin, neomycin, metronidazole, and vancomycin) in the setting of a Trp-sufficient diet. CIA was induced by immunization of bovine type II collagen (CII) emulsified in CFA at days 0 and 21. Indole or vehicle were added back either in drinking water or by oral gavage. CIA severity was assessed clinically and histologically. On day 35, serum and tissues were analyzed by flow cytometry, ELISA, and immunohistochemistry.

Results: Depletion of either the microbiome or dietary Trp protected mice from developing CIA. Adding back indole in either setting rescued CIA severity, suggesting that indole is sufficient to incite disease. These findings were confirmed further by amelioration of disease in mice colonized with tryptophanase-deficient bacteria, which are unable to produce indole. Indole supplementation in the TL setting lead to significant increases in RA-relevant serum cytokines (IL-6, TNF, IL-1β) and IL-17-producing, CII-reactive Th17 cells. Indole supplementation also resulted in increased activation of complement by CII-specific antibodies, C3 deposition in the joints, CII-specific IgG2b, and antibody galactosylation compared to the Trp-sufficient group.

Conclusion: Altogether, our data suggest that the presence of either the microbiota or dietary Trp alone are insufficient to induce disease, and that microbial metabolism of Trp into indole is essential for CIA development. Indole supplementation induces a pro-inflammatory, Th17-skewed immune signature. As IgG2b and galactosylation are thought to promote complement binding and activation, our findings suggest that indole supplementation may enhance pathogenic autoantibody function, which is consistent with the observed increase in complement activation and C3 deposition in the joints. While the cellular target(s) of indole are yet to be identified, this data provides a novel mechanism through which altered tryptophan metabolism promotes disease development in the CIA model. Furthermore, these findings identify microbiome-mediated modulation of Trp metabolism as a potential pathway to block or treat disease.

« Back to ACR Convergence 2023

ACR Meeting Abstracts - https://acrabstracts.org/abstract/microbiota-dependent-indole-production-is-required-for-the-development-of-collagen-induced-arthritis/