Majority of rheumatic diseases are complex and multifactorial in etiology. Emerging studies has suggested that the change of human microbiota, especially in the gut, play a pivotal role in its pathogenesis. Dysequilibrium of the gut microbiota triggers the imbalance between pro- and anti- inflammatory immune responses and results in different rheumatic manifestations, such as rheumatoid arthritis (RA) and spondyloarthritis (SpA). In this article, current and future role of the human gut microbiota in rheumatic diseases are discussed.
Human microbiota, a complex population of microorganisms, consists of both commensals and pathogenic microbial cells . They reside in the body of every person and interact with us in many different ways . Nowadays, millions of sequences can be generated from a single tissue sample by a sequencing technology, which allow us to understand more about the microorganisms’ world . The microbiome can be defined as a collective genome of the microbes, which include bacteria, fungi, protozoa, and viruses . Gut harbors approximately 1014 microbial cells, which have 100- fold more genes than the human native genome . In the past decade, multiple studies demonstrated that the disruption of microbial ecosystems within human body might trigger medical disorders, particularly rheumatic diseases .
2 The role of gut microbiota in rheumatic diseases
There are three main roles of the gut microbiota in rheumatic diseases. First, the microbes can act on an immune defense mechanism by competing with potential pathogens for space and food . The commensal bacteria exhaust the nutrients and limit the growth of the bacteria newcomers.
Second, they can activate the immune system directly . In the lamina propria, the microbial products cross the epithelial barrier and attach to the specific receptors on the surface of antigen-presenting cells, such as dendritic cells and macrophages [4,5]. They activate the related T cells and initiate the production of proinflammatory cytokines, including interleukin (IL)-1, IL-6, IL-17, tumor necrosis factor a, and various antibodies [4,5]. The relevant cytokines and antibodies are released into the circulation and migrate to the target organs or tissues and cause focal inflammation, which result in various rheumatic manifestations [4,5].
Third, the overgrown gut microbes can produce excess metabolites that can trigger rheumatic-related comorbidities . For example, short-chain fatty acids (SCFA) such as acetate, butyrate, and propionate, can increase adiposity via G- protein-coupled signaling pathways (GPR41, GPR43) . Increasing adiposity or obesity is a well-established associating factor for rheumatoid arthritis (RA) and psoriatic arthritis (PsA). In fact, appropriate level of SCFA, that is, butyrate, is essential for human gut metabolism. Butyrate is a major energy source for the colonocytes. It can also improve the epithelial integrity of the colonic epithelium and suppress colonic inflammation [1,5].
Another excess metabolite, secondary bile acids, can deactivate the farnesoid X receptor and its inhibition results in adipose tissue production too . The gut microbes can also convert choline and L- carnitine to trimethylamine, which is metabolized in the liver and produce a toxic chemical, that is, trimethylamine-N-oxide (TMAO) . Ample evidence demonstrated that TMAO contributed to cardiovascular events in RA .
3 Human gut microbiota and dysbiosis
Human intestinal tract is sterile at the time of the birth, but intestinal colonization starts soon after delivery. It takes about two years for the well-establishment of microbiota colonization . Infants who have contact with maternal vaginal and perineal flora during vaginal delivery and those who have exposed to skin flora during caesarian section will have different composition in their gut flora . Feeding with breast-milk containing different milk microbiota and maternal antibodies is another way that may affect the development of their gut flora .
During growth, the composition of the gut microbiota are affected by individual’s lifestyle, diet, and medication use . Westernized diet with high-saturated fatty acids and sucrose, and low fiber, promote epithelial inflammation and stimulate the pathogenic bacterial growth inside the gut . Frequent use of the antibiotics further affects the healthy commensals growth and enhances the proliferation of the new coming gut pathogens .
More than 1,000 microbial species can be identified in our gastrointestinal tract currently. Majority of the bacteria (90% of the total population) belong to two phyla: (1) Firmicutes and (2) Bacteroidetes. Most bacteria in the Firmicutes group have gram-positive cell wall, while those in Bacteroidetes group have gram-negative cell wall. Firmicutes phylum bacteria, such as Clostridia, Faecalibacteria, and Lachnospiraceae, are butyrate producers, and they possess anti-inflammatory effect in the gut . Remaining 10% are mainly consist of the bacterial phyla Actinobacteria, Proteobacteria, and Verrucomicrobia .
Dysbiosis refers to dysequilibrium or imbalance in the human gut microbiota ecosystem. It is intimately related to both intestinal and extraintestinal disorders, such as inflammatory bowel diseases, metabolic syndrome, cardiovascular disease, and obesity . Their current evidence in rheumatic diseases is reviewed in the following sections.
4 Rheumatoid arthritis (RA)
Periodontitis was observed to be significantly common in patients with RA . DNA of bacteria such as Fusobacterium nucleatum and Porphyromonas gingivalis present in the oral cavity can also be detected in the synovial fluid in patients with RA [9,10]. P. gingivalis is able to produce peptidylarginine deiminase, which induce the protein citrullination process inside the inflamed gingiva [11,12]. This localized process leads to the systemic production of anti-cyclic citrullinated protein (anti-CCP) antibodies and triggers articular inflammation in genetically susceptible persons.
Some animal models demonstrated that introduction of bacteria into the gut of germ-free mice could provoke arthritis through the stimulation of T-helper 17 cells [13,14]. Thus, investigators also tried to look for the effect of gut dysbiosis in RA. Vaahtovuo et al compared the composition of the gut microbiota in patients with RA and fibromyalgia and identified a significant difference between them. Patients with RA had significantly less Bifidobacteria, Bacteroides fragilis, Bacteroides-Porphyromonas-Prevotella, and Eubacterium rectale-Closteridium coccoides group . Other two studies from the United States and Japan showed similar findings of high prevalence of Prevotella copri in patients with early RA, but not in those with chronic RA type [16,17]. It implied that the degree of dysbiosis could be varied with RA disease duration and activity, but further trials are definitely required to establish their correlation.
5 Spondyloarthritis (SpA)
Spondyloarthritis (SpA) is a group of inflammatory arthropathies, including ankylosing spondylitis (AS), PsA, reactive arthritis, and enteropathic arthritis, and they all closely relate to HLA-B27 Multiple evidence supports that the HLA-B27 expression play an important role in SpA pathogenesis, but not all HLA-B27 carriers have this disease. In animal models, HLA-B27-positive transgenic rats did not exhibit any articular inflammation in germfree gut environment, but this inflammation suppression was rapidly reversed once their gut flora was disrupted . Environmental factor (gut dysbiosis) is required to trigger the positive HLA-B27 expression in SpA and initiate further systemic inflammation.
A recent study analyzed the fecal microbiota in the HLA-B27-positive transgenic rats and demonstrated a temporal relationship between the bowel inflammation and the fecal microbiota changes, with a reduction in the Firmicutes population and an expansion in Proteobacteria species . Animal studies highlighted that SpA disease development was somehow microbiota dependent, even in the presence of HLA-B27.
In human subjects with SpA, their gut was also found to have certain extent of gut dysbiosis, with a narrower spectrum of microbiome and reduction in the Firmicutes phylum bacteria [20,21,22]. These findings were similar to those suffering from inflammatory bowel disease (IBD), and it might explain why these diseases frequently co-existed together. In fact, half (50%) of the patients with SpA were estimated to have subclinical or flank inflammation in their gut . Disruption of the gut barrier with the immune system exposed to abnormal intestinal microbes drive immune cascade and promote systemic inflammations in multiple sites.
Currently, investigators are now trying to evaluate the application of fecal transplant to restore the gut microbial ecosystem in treating IBD, hoping to apply the same in managing SpA in future.
6 Systemic lupus erythematosus (SLE)
The underlying etiology of systemic lupus erythematosus (SLE) remains poorly understood; human microbiota has been proposed as a new causative factor in the past few years. A landmark trial conducted in Spain found that patients with SLE possessed a significantly lower Firmicutes–to-Bacteroidetes ratio than the healthy controls; the result was similar to those with Crohn’s disease . Another recent study demonstrated an identical result in Chinese patients with SLE .
On the basis of periodontitis–RA association, researchers suggested that SLE might be related to oral dysbiosis. A Brazilian study found that the bacterial load of the dental plaque in patients with SLE was significantly higher (one-log difference) than the controls, and patients wi t h SLE tended to have a narrower microbial spectrum, but with a greater proportion of pathogenic bacteria, including Fretibacterium, Prevotella nigrescent, and Selenomonas .
7 Systemic sclerosis (SSc)
Many patients with systemic sclerosis (SSc) suffer from gastrointestinal dysfunctions, including heartburn, hypomotility, dysphagia, small bowel bacterial overgrowth, and malabsorption. A Swedish study found that patients with SSc and gut dysbiosis (less Faecalibacterium and Clostridiaceae, more Lactobacillus) had more esophageal dysfunction and malnutrition and had more extraintestinal manifestations, including pulmonary fibrosis and telangiectasia .
A study conducted in the United States showed similar findings. They demonstrated that both the fecal matter and the colonoscopic lavage material from patients with SSc had diminished amounts of commensal genera— Faecalibacterium and Clostridium—but an increase in the genera Fusobacterium, Ruminococcus, and Lactobacilli[29,30].
Lactobacillus is supposed to be a beneficial intestinal genus, but it was noted that this commensal was overexpressed in all above-mentioned studies on SSc [28,29,30]. Probiotic therapy with Lactobacillus may not be benefited to every patient with SSc a n d gastrointestinal dysfunction, further study is definitely needed to explore its safety.
8 Behcet’s disease (BD)
Behcet’s disease (BD) is a systemic vasculitis involving blood vessels of various sizes. The underlying pathogenesis is still not well understood. Some researchers suggested that gut dysbiosis might participate in BD. An Italian study highlighted that there was a significant depletion of Roseburia and Subdoligranulum (both are butyrate producers) in patients with BD and their blood butyrate level was significantly diminished . Another study in Japan also supported that Clostridia was less commonly found in the gut microbiota of patients with BD .
Apart from gut dysbiosis, investigators from the United Kingdom discovered that there was a significant increase in colonization of Rothia dentocariosa and Streptococcus sanguinis in t h e saliva of patients with BD, while a Turkey study found that Haemophilus parainfluenzae was more in their subjects’ specimen . The different outcome in the studies could be due to different analytic methods for microbes and ethnicities, but both of them demonstrated that microbial dysequilibrium was prevalent and widespread in BD [31,34].
9 Osteoarthritis (OA)
Evidence for the direct correlation between intestinal microbiota and osteoarthritis (OA) is absent. However, obesity is a well-known risk factor for OA and there is a strong link between the gut dysbiosis and the development of obesity, as mentioned earlier. Proper manipulation of the intestinal microbiota may aid to halt the development of OA .
10 Juvenile idiopathic arthritis (JIA)
Juvenile idiopathic arthritis (JIA) is the rheumatic arthritis of unknown origin in children; it is thought to occur in genetically susceptible children with possible environmental triggers. An investigation found that the diversity of the phylum Firmicutes was greatly reduced in children with polyarticular JIA . Another study also showed a lower Firmicutes-to-Bacteroidetes ratio in patients with JIA, which was consistent with other rheumatic diseases mentioned earlier . Figure 1 shows the taxonomy of some human gut bacteria.
11 Potential therapeutic role of gut microbiota
Theoretically, restoring the gut commensals and eliminating the pathogenic microbes can reverse the imbalanced immune reactions in rheumatic diseases. Microbiota modulation as a therapeutic or preventive measure for rheumatic diseases is now under investigation.
A recent Japanese study found that the anti-CCP antibody could cross-react with microbial proteins and plants, not only limited to human autoantigens. This observation supports the hypothesis of food or microbes inducing RA via anti-CCP antibody production [1,38,39].
Nevertheless, the evidence of the role of diet and rheumatic diseases remains scant or even conflicting [1,39]. Mediterranean diet, characterized by high amounts of fruit, vegetables, whole grains, fish and olive oil, less red meat, and moderate alcohol intake, was previously thought to have a beneficial effect in reducing disease activity in RA [1,40]. But a recent prospective analysis did not show a n y positive result on it . Fish oil contains a long chain n-3 polyunsaturated fatty acids, is believed to lower the inflammation in RA, but not yet proven . To date, there is still no data about the association between a specific dietary pattern and the gut microbiota.
Probiotics refer to living bacteria and prebiotics refer to non-digestible food ingredients for selected bacteria. They are used to modify the gut microbiota by promoting the growth of commensals bacteria, such as Lactobacilli and Bifidobacteria [2,4]. Oral administration of Lactobacillus casei had been shown in suppressing RA progression in animal model . Two randomized double-blinded control studies that assessed the use of probiotics, L. casei, in RA had demonstrated a promising outcome in reducing DAS28 among them [42,43]. Hopefully probiotics supplement may be used as an adjunctive therapy for RA in future.
Fecal microbiota transplant (FMT) is the transplant of fecal bacteria from a healthy donor to a recipient. FMT restores the recipient’s microbiota colonization and recovers his or her immune defense system. It had been successfully applied in treating recurrent Clostridium difficile infection, inflammatory bowel disease, irritable bowel disease, and pouchitis [44,45,46]. The application of FMT was also expanding to other medical illnesses such as hepatic encephalopathy, chronic hepatitis B, metabolic syndrome, multiple sclerosis, and myoclonus-dystonia, but not for rheumatic diseases yet.
Gut dysbiosis is believed to be one of the important causative factors for rheumatic diseases. The imbalance of the bacterial phylum in the intestine, especially Firmicutes (Faecalibacterium, Clostridium, Ruminococcus, Roseburia, Subdoligranulum, Coprococcus) and Bacteroidetes (Prevotella, Bacteroides) is commonly found among them. Analysis of the intestinal microbiome is considered as a novel diagnostic and prognostic tool for rheumatic diseases nowadays. The modulation of intestinal microbiome by diet, probiotic, and fecal transplantation might be a therapeutic option for rheumatic diseases in the near future.
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