Here, we review previous findings regarding the role of microbiota in human RA and mouse models of arthritis. We have shown that some Japanese RA patients carry increased numbers ofin the intestine and have used gnotobiotic tools to show that a-dominated microbiota induced Th17 cell-dependent arthritis in mice [ 12 ]. In contrast, another study showed thatsuppresses the development of arthritis [ 13 ]. These results support the idea that differentspecies have different effects on arthritis.

The human intestine contains over 1000 bacterial species and 10bacterial cells [ 1 ], but it is difficult to detect most anaerobic bacteria with culture techniques. However, the new sequencing technology, known as “high-throughput microbial DNA sequencing”, has allowed us to better characterize the human microbiota. Recent advances in our understanding of mucosal immunity have clarified the correlation between the gut microbiota and the host immune system [ 2 3 ]. Most of the interactions between a host and its commensal microbiota are symbiotic. However, microbial abnormalities, called “dysbiosis” are thought to correlate with various kinds of diseases. Unsurprisingly, intestinal dysbiosis was observed in inflammatory bowel disease [ 4 5 ]. Interestingly, dysbiosis is also found in diseases that affect the tissues out of the gut. Recent studies have suggested that the composition of the microbiota is altered in type 1 diabetes, multiple sclerosis, and autism [ 6 8 ]. Moreover, dysbiosis has been reported in patients with rheumatoid arthritis (RA) in the United States, China, and Finland [ 9 11 ].

The gut microbiota contains the largest abundance of microorganisms in our body. The previous experiments in germ-free mice revealed that the gut microbiota shapes the intestinal immune system [ 22 23 ]. Recent studies in several countries have found that the composition of the intestinal microbiota is altered in patients with recent-onset RA. Commensal segmented filamentous bacteria (SFB) induce Th17 cells in the intestine and trigger arthritis in mice [ 24 25 ]. Therefore, the gut microbiota is thought to be an important environmental factor in the development of arthritis.

Rheumatoid arthritis is a chronic autoimmune inflammatory disease characterized by auto-antibody production and destruction of bone in multiple joints ( Figure 1 ) [ 14 ]. Recent studies have demonstrated that over 100 genetic susceptibility loci are involved in RA [ 15 16 ]. However, the environmental factors that affect the development of RA are not fully understood. It was recently shown that an immunoglobulin A (IgA) anti-citrullinated protein antibody (ACPA) is detectable before the onset of arthritis [ 17 18 ], suggesting that RA originates at mucosal sites, such as the oral cavity and the gut., a major pathogenic bacterium of periodontal diseases, may correlate with the development of RA [ 19 20 ], because this bacterium is the only known pathogen that expresses a peptidylarginine deiminase and may be related to ACPA [ 21 ].

K/BxN T cell receptor transgenic mice developed inflammatory arthritis, with increased numbers of Th17 cells in the small intestine and spleen [ 24 ]. The severity of arthritis and the titers of auto-antibodies directed against glucose-6-phospate isomerase were reduced when the mice were reared under GF condition. Mono-colonization with SFB was sufficient to cause the development of Th17 cell-dependent arthritis in this model. Therefore, a particular gut commensal microbiota is sufficient to induce arthritis in mice.

Several animal studies have clearly demonstrated that gut microbiota plays an important role in arthritis development ( Table 1 ). We and others have shown that SKG mice, which spontaneously develop arthritis under conventional conditions, did not develop arthritis in a germ-free (GF) environment [ 12 26 ]. We also showed that the mono-colonization of GF-SKG mice withwas sufficient to induce arthritis with a fungal injection.

Another study based on metagenomic shotgun sequencing showed that RA patients in China had an increased abundance ofin the gut, on the tooth, and in the saliva [ 11 ]. However, the abundance ofin the gut was only elevated in the first year after disease onset. The authors showed that the dysbiosis observed in RA patients partly improved after treatment with disease-modifying drugs.

5. Correlation between Prevotella and Arthritis

Prevotella copri was first isolated from human fecal samples in Japan [ P. copri was elevated in untreated recent-onset RA patients [ P. copri were reduced in patients with chronic RA, patients with psoriatic arthritis, and healthy volunteers. They also found that the relative abundance of P. copri in the intestine correlated with an absence of human leukocyte antigen (HLA)-DRB1. Moreover, P. copri –colonized mice displayed exacerbated colitis when they were treated with dextran sulfate sodium in their drinking water. However, the mechanistic link between the increased number of P. copri in the gut and arthritis is unknown. was first isolated from human fecal samples in Japan [ 28 ]. It is an obligately anaerobic, non-spore-forming Gram-negative bacterium. Interestingly, Scher et al. showed that the abundance ofwas elevated in untreated recent-onset RA patients [ 10 ]. By contrast, the numbers ofwere reduced in patients with chronic RA, patients with psoriatic arthritis, and healthy volunteers. They also found that the relative abundance ofin the intestine correlated with an absence of human leukocyte antigen (HLA)-DRB1. Moreover,–colonized mice displayed exacerbated colitis when they were treated with dextran sulfate sodium in their drinking water. However, the mechanistic link between the increased number ofin the gut and arthritis is unknown.

P. copri –dominated microbiota from RA patients (RA-SKG mice) showed increased numbers of Th17 cells in the large intestine. We also confirmed that the abundance of P. copri increased in the large intestines of RA-SKG mice, but not in the small intestines. Intriguingly, the RA-SKG mice developed severe arthritis when they were injected with a low dose of zymosan (a fungal component). We also showed that lymphocytes from regional lymph nodes and large intestines of the RA-SKG mice produced high levels of IL-17 in response to the arthritis-related auto-antigen RPL23A. Moreover, bone marrow-derived dendritic cells stimulated with P. copri expressed high levels of IL-6 and IL-23 in an in vitro analysis. In addition, P. copri -monocolonized SKG mice showed Th17 cell-dependent arthritis development upon fungal injection, thus indicating that dysbiosis dominated by P. copri contributes to arthritis development. In the future, more intensive studies are needed to investigate whether P. copri elicits severe arthritis compared to other intestinal bacteria in in vivo experiments. Therefore, we produced intestinal microbiota-humanized mice and analyzed the severity of their arthritis [ 12 ]. It has been reported that SKG mice develop Th17 cell-dependent autoimmune arthritis, clinically resembling human RA [ 29 ]. We used GF-SKG mice, which contain no bacterium in their gut. The GF-SKG mice showed no signs of arthritis when they were treated with a fungal component, suggesting that microbial stimuli are important for disease development. We colonized the GF-SKG mice with fecal samples from RA patients or healthy controls. The mice were kept in separate vinyl isolators. The SKG mice colonized with a–dominated microbiota from RA patients (RA-SKG mice) showed increased numbers of Th17 cells in the large intestine. We also confirmed that the abundance ofincreased in the large intestines of RA-SKG mice, but not in the small intestines. Intriguingly, the RA-SKG mice developed severe arthritis when they were injected with a low dose of zymosan (a fungal component). We also showed that lymphocytes from regional lymph nodes and large intestines of the RA-SKG mice produced high levels of IL-17 in response to the arthritis-related auto-antigen RPL23A. Moreover, bone marrow-derived dendritic cells stimulated withexpressed high levels of IL-6 and IL-23 in an in vitro analysis. In addition,-monocolonized SKG mice showed Th17 cell-dependent arthritis development upon fungal injection, thus indicating that dysbiosis dominated bycontributes to arthritis development. In the future, more intensive studies are needed to investigate whetherelicits severe arthritis compared to other intestinal bacteria in in vivo experiments.

P. copri protein (Pc-p27) [ P. copri may contribute to the pathogenesis of RA. It will be interesting to analyze whether RA patients showing IgA response to Pc-p27 are also positive for IgA-ACPA. Very recently, Annalisa et al. used liquid chromatography–tandem mass spectrometry to identify an HLA-DR-presented peptide (T cell epitope) from a 27-kDaprotein (Pc-p27) [ 30 ]. The peptide was detected in the synovial tissue, synovial fluid mononuclear cells, and peripheral blood mononuclear cells (PBMCs) of some RA patients. To investigate the Th1/Th17 responses to this peptide, the authors stimulated PBMCs from 40 RA patients with the peptide and evaluated the cytokines produced, with enzyme-linked immunosorbent assays (ELISA). They found that the levels of interferon γ (IFN-γ) were elevated in 42% of RA patients. However, PBMCs from only one RA patient showed increased IL-17 production. They also evaluated the IgG and IgA antibody responses to Pc-p27. IgG antibody responses were observed in 13% of new-onset RA patients and 20% of chronic RA patients. IgA antibody responses were also detected in approximately 10% of both new-onset and chronic RA patients. From these observations, they concluded thatmay contribute to the pathogenesis of RA. It will be interesting to analyze whether RA patients showing IgA response to Pc-p27 are also positive for IgA-ACPA.