Rheumatic disease and the microbiome

Abstract

Every human is intimately associated with a large and diverse population of microorganisms living on the skin and mucous membranes. These commensal organisms are known as the microbiome, or microbiota, and are acquired in young childhood. The microbiome is critically important in establishing a fully function immune system. For example, Th17 T helper cells are not present in a germ-free environment. The relationship of the microbiome to autoimmune disease is being explored actively. Mechanisms by which the microbiome may influence these diseases include, but are not limited to, molecular mimicry as well as induction and regulation of both Th17 and regulatory T cells. There are ample data that a specific oral microbe, Porphyromonas gingivalis, the only bacteria with the enzyme peptidylarginine deiminase, is involved in the pathogenesis of rheumatoid arthritis. Connection between other rheumatic autoimmune diseases and the microbiome remains to be made.

We are born wet, naked and basically sterile. The first two states are rapidly corrected, usually by our mothers. The last one, sterility, is also usually changed by our mothers, but this takes many months to several years. Over the first several years of life each of us establishes a community of microorganisms that are commensal and inhabit niches on skin and mucous membranes. These microorganisms are collectively known as the microbiome, or microbiota, and are predominately obtained from one’s mother (1). The microbiome is usually a large and diverse community, such that about 90% of the cells associated with any one human are from these commensal organisms while are 10% are of human origin. There is a true commensal relationship as the host uses these organisms for digestion, nutrient production, detoxification, defense against pathogens and development of the immune system. From a genetics standpoint, humans have about 23,000 genes but an individual’s microbiome may consist of many dozen species with as many as 4,000,000 genes. The great majority of the microbiome is found in the gut, from the mouth to the anus, and is predominately either Bacteroidies or Firmicules species. We have evolved over the millennia with the microbiome and its importance in human illness, including autoimmune disease, is just being explored.

A number of factors may affect with acquisition and maintenance of the microbiome. In particular, diet may drastically alter the microbiome. And, since the middle of the 20th century, use of antibiotics affects the organisms that are part of any individual microbiome. Several authors have proposed that the rising incident and prevalence of autoimmune disease as well as the increased incidence and prevalence in the developed world compared to the developing world might be attributable to changes in microbiome. However, data supporting these hypotheses have not been produced. Nonetheless, the role of the microbiome in the immune system of the host organism and in autoimmune disease is under intense investigation, spurned in part by the knowledge that most experimental models of autoimmune disease are affected by a germ-free environment (2). That is, an individual’s microbiome is possibly an environmental factor that influences predilection to autoimmune disease.

If the microbiome is part of the pathogenesis of autoimmune disease, what mechanisms shall we consider? First, commensal organisms could supply antigens that play a role in molecular mimicry. For autoimmune illnesses in which the causative organism has been identified, molecular mimicry is a part of the etiology (3,4). For autoimmune rheumatic illness, molecular mimicry has been proposed as an initiating factor for autoimmunity (5). There are accumulating data that the gut microbiome has a role in induction or activation of Th17 T helper cells and Treg cells, either of which might have a role in autoimmune disease. Segmented, filamentous bacteria have a fundamental role in the development of Th17 cells (6). Meanwhile, gut helminths up-regulate regulatory T cells (7), and instillation of helminths can ameliorate disease in animal models of type 1 diabetes, multiple sclerosis, inflammatory bowel disease, rheumatoid arthritis or systemic lupus erythematosus (7). Early stage human trials of helminthes for inflammatory bowel disease have been undertaken (8). Whether there specific or only non-specific effects of the microbiome on autoimmune disease, or whether any such effects are active or bystander, remains to be determined.

Evidence has accumulated that oral flora may be critical in the pathogenesis of rheumatoid arthritis and that molecular mimicry may be the mechanism (reviewed in 9). Since the initial description of antibodies binding citrillunated peptides in the sera of rheumatoid arthritis patients by Walter van Venrooij and his colleagues (10), the presence of these antibodies (anti-CCP) have become an important part of the diagnostic procedure in the disease, and may well be involved in the pathogenesis of joint destruction (11). On the basis of expression of the enzyme peptidylarginine deiminase, which coverts arginine to citrulline when part of a polypeptide and the epidemiological association of rheumatoid arthritis with periodontal disease, Porphyromonas gingivalis, the only bacteria to possess this enzyme, was proposed as an etiological agent in rheumatoid arthritis almost a decade ago (12). Since then, a large body of data has accumulated suggesting an initial immune response to citrullinated peptides produced by P. gingivalis leads to an autoimmune response to several citrullinated self-proteins, and that such an autoimmune response may underlie the pathogenesis of rheumatoid arthritis (reviewed in 9 and 11, see 13–15 for recent data).

Antibodies to P gingivalis-citrullinated peptides are also found in subjects at risk for rheumatoid arthritis by virtue of HLA genetics or family history (16–18). Among 284 subjects with RA-risk HLA alleles or a family history of the disease, 117 were rheumatoid factor or anti-CCP positive. This positivity was associated with antibodies binding P gingivalis (16). In another study of first degree relatives of rheumatoid arthritis patients, a significant fraction of both those relatives with and without anti-CCP or rheumatoid factor were found to bind a panel of citrullinated polypeptides. Further, these antibodies were associated with at least one tender joint on examination, but HLA typing was not given (18). Other recent data in an American Indian population also show antibodies to citrullinated proteins in the sera of relatives of patients. This positivity was found more often in those relatives with 2 shared epitope alleles (17). Thus, rheumatoid arthritis-associated autoimmunity and rheumatoid arthritis may arise in genetically susceptible individuals as a result of an immune response to citrullinated peptides from P gingivalis.

In this issue of the International Journal of Rheumatic Diseases, Khantisopon and colleagues examined P. gingivalis and periodontal disease in Thai rheumatoid arthritis patients (19). In a cross-sectional study, 196 consecutive patients attending an academic rheumatology clinic had a complete dental examination. Moderate or severe periodontal disease was found in 99% of patients, which is higher than in other studies of rheumatoid arthritis patients. Patients with severe periodontal disease were older, more likely to be men and use tobacco. There were no clinical correlations of periodontal disease, but this lack of association is to be expected when virtually all patients had the condition. Results for anti-CCP are not given (19).

In a second study in this issue, Alipour and colleagues have studied probiotic treatment of rheumatoid arthritis (20). Lactobacillus casei was given to rheumatoid arthritis patients in a randomized, double-blind, placebo design for 8 weeks. Even in this short study with a small number of subjects (30 in each group), the authors found efficacy of probiotic treatment. Tender and swollen joints counts were reduced as were C-reactive protein levels. The DAS28 decreased significantly in the treatment group. However, as well reviewed in the present paper, there have been several others trials of probiotics for rheumatoid arthritis that did not show improvement (20). The differences between these negative studies and the present positive trial may be related to species and dose of the probiotic bacteria. Certainly, further studies of probiotic treatment are warranted. But, probiotic bacteria are not part of the normal human microbiome. In fact, probiotic bacteria do not become part of the microbiome when given orally. That is, shortly after administration is discontinued, probiotic bacteria are completely eliminated from the gut. As knowledge develops concerning the relationship of the microbiome to rheumatoid arthritis, trials altering the microbiome on a long term basis by introduction or elimination of particular bacterial strains may be considered for controlled studies in the disease.

Evidence for the involvement of the microbiome in the etiology and pathogenesis of other rheumatic, inflammatory illnesses is less direct (21). Nonetheless, of particular interest in regards to autoimmune disease, is evidence for the contribution of the microbiome to the development of Th17 T helper cells (22). These cells are shown to be important for the pathogenesis of several autoimmune diseases (23), and the induction of these cells is dependent upon the microbiome. That immune system development depends on the microbiome has been amply demonstrated by germ-free conditions. Germ-free individuals show reduced peripheral CD4+ T cells, reduced immunoglobulin levels, immune deviation towards a Th2 phenotype, among other defects. A paper in Cell by Ivanov and colleagues showed that mono-association of segmented filamentous bacteria with germ-free animals was sufficient to induce fully functional Th17 T helper cells (8). These provocative data suggest that manipulation of the microbiome to alter immune phenotype might be possible. Another recent paper shows that this same bacteria can drive experimental autoimmune disease (24). Under germ-free conditions the K/BxN mouse, which under specific pathogen free conditions develops inflammatory arthritis, has greatly attenuated disease. Mono-association with segmented filamentous bacteria restores gut-associated Th17 cells, autoantibody production and arthritis in this arthritis model (24). Other animal models of autoimmunity also depend on gut-derived Th17 cells (25).

Investigation of the role of a particular member of the mouth microbiome; namely, P gingivalis, in the pathogenesis of rheumatoid arthritis has been ongoing for several years. The accumulated data demonstrate a strong association as well as a plausible biological mechanism. Involvement of the microbiome in other rheumatic disease has not been studied extensively. However, gut-associated organisms are critical to the development and activation of the immune system, especially with regards to cell types intimately associated with autoimmunity. These data indicate that the relationship of the microbiome to autoimmune rheumatic disease is an area of high interest.