Modulation of Inflammatory Arthritis in Mice by Gut Microbiota Through Mucosal Inflammation and Autoantibody Generation

OBJECTIVE

Observations of microbial dysbiosis in patients with rheumatoid arthritis (RA) have raised interest in studying microbial-mucosal interactions as a potential trigger of RA. Using the murine collagen-induced arthritis (CIA) model, we undertook this study to test our hypothesis that microbiota modulate immune responses leading to autoimmune arthritis.

METHODS

CIA was induced by immunization of mice with type II collagen (CII) in adjuvant on days 0 and 21, with arthritis appearing on days 23 and 24. Intestinal microbiota were profiled by 16S ribosomal RNA sequencing every 7 days during the course of CIA, and intestinal mucosal changes were evaluated on days 14 and 35. Then, microbiota were depleted either early (7 days before immunization) or late (day 21 after immunization) by administration of broad-spectrum antibiotics. Disease severity, autoantibody and systemic cytokine production, and intestinal mucosal responses were monitored in the setting of microbial reduction.

RESULTS

Significant dysbiosis and mucosal inflammation occurred early in CIA, prior to visible arthritis, and continued to evolve during the course of disease. Depletion of the microbiota prior to the induction of CIA resulted in an ~40% reduction in disease severity and in significantly reduced levels of serum inflammatory cytokines and anti-CII antibodies. In intestinal tissue, production of interleukin-17A (IL-17A) and IL-22 was delayed. Unexpectedly, microbial depletion during the late phase of CIA resulted in a >50% decrease in disease severity. Anti-CII antibodies were mildly reduced but were significantly impaired in their ability to activate complement, likely due to altered glycosylation profiles.

CONCLUSION

These data support a model in which intestinal dysbiosis triggers mucosal immune responses that stimulate T and B cells that are key for the development of inflammatory arthritis.

Bacteroidales recruit IL-6-producing intraepithelial lymphocytes in the colon to promote barrier integrity

Interactions between the microbiota and distal gut are important for the maintenance of a healthy intestinal barrier; dysbiosis of intestinal microbial communities has emerged as a likely contributor to diseases that arise at the level of the mucosa. Intraepithelial lymphocytes (IELs) are positioned within the epithelial barrier, and in the small intestine they function to maintain epithelial homeostasis. We hypothesized that colon IELs promote epithelial barrier function through the expression of cytokines in response to interactions with commensal bacteria. Profiling of bacterial 16S ribosomal RNA revealed that candidate bacteria in the order Bacteroidales are sufficient to promote IEL presence in the colon that in turn produce interleukin-6 (IL-6) in a MyD88 (myeloid differentiation primary response 88)-dependent manner. IEL-derived IL-6 is functionally important in the maintenance of the epithelial barrier as IL-6-/- mice were noted to have increased paracellular permeability, decreased claudin-1 expression, and a thinner mucus gel layer, all of which were reversed by transfer of IL-6+/+ IELs, leading to protection of mice in response to Citrobacter rodentium infection. Therefore, we conclude that microbiota provide a homeostatic role for epithelial barrier function through regulation of IEL-derived IL-6.

Microbiota modify inflammatory arthritis through mucosal inflammation and autoantibody generation

Mucosal dysbiosis occurs in patients with rheumatoid arthritis (RA), but its role in disease pathophysiology has not been well defined. Using the collagen-induced arthritis (CIA) murine model, we investigated the importance of microbiota in developing inflammatory arthritis. Dysbiosis in CIA occurred in two phases, at 14 days and 35 days after the initial immunization of bovine type II collagen (CII) in complete Freund’s adjuvant. In parallel, mucosal barrier impairment was observed with an initial insult observed at day 14 that stabilized through day 21 but worsened by day 35. Mucsoal inflammation also was observed as determined by colon histology and elevation of tissue cytokines IL-17A and IL-22. Depletion of microbiota through administration of broad-spectrum antibiotics resulted in decreased disease severity and mucosal inflammation. Unexpectedly, we observed that microbial depletion during the late, effector phase of CIA, resulted in more profound suppression of disease. Both disease severity and serum anti-CII autoantibodies correlated with microbial load, further supporting the role of microbiota driving the disease process. Finally, we confirmed that in CIA, the presence of an intact microbiota associated with the presence of RA-relevant anti-citrullinated protein antibodies, possibly through dysbiosis-stimulated activation of the host’s peptidyl arginine deiminase 4 enzyme. Clearly additional studies are needed to solidify the role of the microbiota in driving CIA, but our data together with those of others’ supports a model in which intestinal dysbiosis triggers mucosal immune responses that stimulate T and B cell activities that lead to inflammatory arthritis.

Early microbial changes affect mucosal immunity in autoimmune arthritis

Multiple lines of evidence now suggest that autoimmune disease may result from microbial-mucosal interactions. Microbial dysbiosis occurs in new onset seropositive rheumatoid arthritis (RA); and during the preclinical phase of RA, there is an expansion of mucosal autoantibodies and plasmablasts. These findings strongly suggest that a mucosal site may drive the initial break in tolerance that leads to RA. Using the murine collagen-induced arthritis model of RA, we find that microbiome diversity changes quickly after immunization with bovine type II collagen, at the time when disease-related autoantibodies develop, but before clinical CIA is evident. Significant alterations in taxa clustered in Clostridia families occur in CIA. Interestingly, using IgA-seq in human at-risk for RA study groups we find that those subjects with autoantibodies have similar alterations in Clostridia families. Furthermore, during the development of murine CIA, increased intestinal permeability and altered cytokine profiles paralleled disease development and microbiota alterations. Taken together, these data suggest a model in which early microbial alterations stimulate mucosal immune responses that drive the development of pathogenic autoantibodies. Future studies are aimed at identifying specific bacterial species and elucidating the pathway by which they stimulate autoantibody production.

Colonic intraepithelial lymphocytes produce IL-6 in response to resident bacteria to modulate epithelial barrier function

Interactions between the microbiota and distal gut are important for the maintenance of a healthy immune system. Dysbiosis of colon bacteria has emerged as a likely contributor to diseases that arise at the level of the mucosa. Intraepithelial lymphocytes (IELs) are positioned within the epithelial barrier, and in the small intestine, function to maintain epithelial homeostasis. We hypothesized that IELs of the colon modulate epithelial barrier function through the liberation of cytokines stimulated by interactions with resident bacteria. Our data demonstrate that IL-6 is a major cytokine secreted by colonic IELs in a microbe-dependent fashion. We identify Alistipes species of the phylum Bacteroidetes as candidates to recruit IELs and stimulate their IL-6 secretion. IEL-derived IL-6 is functionally important in the maintenance of the epithelial barrier as IL-6−/− and antibiotic-treated mice were noted to have increased paracellular permeability and closer interaction with luminal bacteria. IL-6 was found to signal in colonic epithelial cells and resulted in increased epithelial barrier integrity and claudin1 expression in model epithelia. Therefore, we conclude that the host microbiota provides a homeostatic role for epithelial barrier function through regulation of IEL derived IL-6.