Published: Nov 8, 2013 | Updated: Nov 8, 2013
By Nancy Walsh, Staff Writer, MedPage Today

Full Story:  http://www.medpagetoday.com/Rheumatology/Arthritis/42799

Action Points

The inflamed joints and systemic inflammation characteristic of early rheumatoid arthritis (RA) have been linked with an altered pattern of gut microbial colonization, suggesting a new explanation for autoimmunity and having potential implications for treatment, researchers reported.

Using shotgun gene sequencing, a group of researchers led by Dan L. Littman, MD, PhD, of New York University found that 75% of patients with new-onset, untreated RA had a distinct predominance of intestinal microbiota, with an expansion of pro-inflammatory Prevotella copri.

In contrast, that pattern was only seen in 11.5% of patients with longstanding RA (P<0.01), in 37.5% of those with psoriatic arthritis (P<0.05), and in 21.4% of healthy controls (P<0.01), Littman and colleagues reported online in eLife.

Moreover, individuals with high prevalence of P. copri showed marked reductions inBacteroides and other species considered beneficial.

“The clinical implication of this is that if we can find ways to manipulate the intestinal microbiome, we may be able to have an impact on a number of diseases, including rheumatoid arthritis,” said Gail Hecht, MD, the chief of gastroenterology at Loyola University in Chicago, who wasn’t involved in the study.

“That could be through diet, targeted antibiotic therapy, or by the provision of ‘good’ bacteria that might displace the Prevotella, which appears to be associated with the development of rheumatoid arthritis,” she told MedPage Today.

The Second Hit Hypothesis

Rheumatoid arthritis is a complex, multifactorial disorder with genetic and environmental influences that develops over a long period. Initially, often for many years, patients have detectable autoantibodies such as rheumatoid factor and anti-citrullinated peptide antibodies, with no signs or symptoms of clinical disease.

With time, however, inflammatory cytokines begin to appear, autoantibody titers rise further, and genetic epitope spreading commences.

Finally, according to the “second hit” hypothesis, an environmental event propels the underlying autoimmune state into clinical disease.

Hypothesized sources for that second hit have included events in the lung, such as effects from cigarette smoking, and in the periodontal tissue, from oral pathogens.

The gut microbiome has been linked with the development of gastrointestinal autoimmunity, such as in inflammatory bowel disease, and has been implicated in arthritis in mouse models. However, a role in human extra-gastrointestinal autoimmune disease has not previously been described.

To explore this, the researchers sequenced specific regions of the relevant 16S gene in 44 fecal samples from patients newly diagnosed with RA who had not yet received any immunosuppressive therapy, along with 26 samples from patients with established RA, 16 samples from patients with psoriatic arthritis, and 28 samples from healthy controls.

In the new-onset group, the samples showed a positive association with two variants of Prevotella and a negative correlation with Bacteroides, Lachnospiraceae, and Clostridia.

The researchers then introduced P. copri into the gastrointestinal tract of mice, and within 2 weeks this organism dominated their gut microbiota, with a loss of protective Bacteroides andLachnospiraceae.

Those P. copri-colonized mice subsequently were given the colitis-inducing agent dextran sulfate sodium in their drinking water, and developed more severe colitis than mice colonized with Bacteroides, showing greater weight loss and upregulation of interferon gamma.

“These data suggest that a Prevotella-defined microbiome may have the propensity to support inflammation in the context of a genetically susceptible host,” Littman and colleagues observed.

A Role for CRP?

They also noted that they initially found it surprising that the prevalence of P. copri in patients with established RA was similar to that in healthy controls.

Possible explanations for this, they suggested, was that the Prevotella-predominant microbiome needs an inflammatory environment to thrive, and with treatment for chronic disease the inflammation has been diminished.

Another possibility is that the change in the gut colonization is a result of an inflammatory factor or process specific to new-onset disease, such as C-reactive Protein (CRP), which binds to bacterial cell walls and upregulates the complement system and macrophage activity.

CRP typically is high in early-phase RA and diminishes with treatment, and also is less characteristic in other autoimmune diseases.

A striking difference was seen in CRP levels in participants in this study, with mean levels of 20.6 mg/L in the new-onset RA group compared with 8.2 mg/L in the established RA patients, 7.6 mg/L in the psoriatic arthritis group, and 1.1 mg/L in healthy controls.

“Whether or not CRP itself represents a specific response to the presence of P. copri in new onset RA is an area of future investigation,” Littman and colleagues wrote.

It’s also possible that some metabolite of P. copri in the gut is driving the process, as has been shown for atherosclerosis, where the gut metabolism of L-carnitine from red meat produces a pro-atherogenic metabolite known as trimethylamine-N-oxide that has been associated with cardiovascular disease, they explained.

Genes, Environment, and Treatment

Another surprising finding in the study was that there seemed to be an inverse, rather than a direct, correlation between the P. copri-dominant microbiota and RA shared-epitope risk alleles found at the human leukocyte-antigen (HLA) locus.

The HLA DRB1 alleles are known to increase RA disease risk in carriers.

Animal studies have revealed that these HLA genes not only influence RA susceptibility but also help regulate the gut microbiota through effects on mucosal characteristics and permeability. The researchers then considered whether similar processes may occur in humans, and found that the P. copri dominance was stronger in non-allele carriers.

“As in the case of cigarette smoking, this could also represent a gene-environment interaction that contributes to RA pathogenesis. It is conceivable that a certain threshold for P. copriabundance may be necessary to overcome the lack of genetic predisposition in RA subjects, while a lower abundance may be sufficient to trigger disease in those carrying risk alleles,” they explained.

Analysis of the enzymatic activities influenced by the Prevotella genome also provided a novel insight into RA treatment, suggesting an explanation for the high variability in RA patient response to methotrexate, an inhibitor of dihydrofolate (DHF) reductase that is considered the cornerstone of therapy.

In patients whose gut biome is dominated by Prevotella, purine pathways are downregulated, including the pathway that metabolizes tetrahydrofolate (THF).

“The THF biosynthetic pathway encoded by the gut metagenome, which includes a DHF reductase enzyme, may compete with host DHF reductase for [methotrexate] binding and metabolism. If so, an increase in DHF reductase-high microbiota in some RA subjects (i.e.,Bacteroides overabundant) may help explain, at least partially, why only about half of RA patients respond adequately to oral [methotrexate],” they wrote.

Larger studies are needed to provide detailed explorations of potential mechanisms of the risk allele gene-environment interactions and the influence of the microbiome on response to medication, Littman’s group noted.

Cause or Effect?

In an editorial accompanying the study, Diane Mathis, PhD, of Harvard Medical School pointed out a further area of needed research.

“One critical avenue of study will be to determine whether the association between P. copriand rheumatoid arthritis reflects cause, effect, or co-association,” she stated.

The most intriguing possibility is that the association is indeed causative, according to S. Louis Bridges, MD, PhD, of the University of Alabama at Birmingham, who also was not involved in the study.

“We’ve known for a long time that there are genetic predispositions to these diseases but that there are environmental components as well. So one could posit that this particular type of bacteria is one of — if not the — instigating factor that causes the diseases, but there’s a lot of work that still needs to be done to determine if it’s cause, effect, or an incidental relationship,” Bridges told MedPage Today.

Mathis also called for an explanation of how “arthritogenic signals” from the gut reach the joints.

“The mysterious microbial world within us is beginning to reveal its secrets, but many mysteries remain to be solved in the years to come,” she concluded.