A gut feeling about arthritis

The gut microbiota of patients recently diagnosed with rheumatoid arthritis is enriched in microbes belonging to the Prevotella genus.

T he intestinal microbiota-the complex universe of microbes that are normally res ident in the gastrointestinal tract-influences many aspects of life, including glucose metab olism, neural processes, the circadian clock and our immune defenses (Gordon, 2012). It has been known for decades that proper development of the immune system requires the gut microbiota, and that alterations in the repertoire of microbes are often associated with immunological disorders, in particular autoimmune diseases in which the body mistakenly attacks its own tissues (Ivanov and Honda, 2012).
While it is not very surprising that intestinal bacteria affect susceptibility to and/or the severity of autoimmune disorders localized to the gutnotably inflammatory bowel disease-their ability to profoundly impact other immune disorders, including arthritis (Wu et al., 2010), experimental allergic encephalomyelitis (Lee et al., 2011) and type1 diabetes (Kriegel et al., 2011), came as a surprise. Now, in eLife, Dan Littman of New York University School of Medicine and coworkersincluding Jose Scher, Andrew Sczesnak and Randy Longman as joint first authors-have taken an important step in extending these findings to humans: they analyzed the faecal microbiota of healthy individuals and of various cohorts of patients with inflammatory arthritis, and docu mented a striking enrichment of bacteria belong ing to the Prevotella genus in patients recently diagnosed with rheumatoid arthritis (Scher et al., 2013).
Rheumatoid arthritis affects the body as a whole, but particularly the hands, wrists and other synovial joints (Figure 1) (McInnes and Schett, 2011). It is a multifactorial disease that has both autoimmune and inflammatory components, and it is subject to diverse genetic, environmental and epigenetic influences. Scher et al.-who are based at various institutions in the US, Italy and the UK-assembled a cohort of subjects who were being treated for chronic rheumatoid arthritis, and a second cohort who had only recently been diagnosed and were therefore not yet receiving treatment. They also recruited sub jects being treated for another form of inflamma tory arthritis (psoriatic arthritis) and a cohort of healthy controls matched for age and ethnicity.
Scher et al. then cataloged the intestinal microbiota of the four groups by sequencing micro bial genes called 16S ribosomal RNA geneswhich can be used to distinguish between bacterial species-present in their faeces. They found a marked overrepresentation of Prevotella species, in particular P. copri, in most, though not all, members of the recentonset rheumatoid arthritis group, but in few of the individuals in the other Copyright Mathis. This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.

MICROBIOTA
A gut feeling about arthritis The gut microbiota of patients recently diagnosed with rheumatoid arthritis is enriched in microbes belonging to the Prevotella genus. Image Genomes from the P. copri species of gut microbe were abundant in the faeces of patients with rheumatoid arthritis three groups. Conversely, the microbiota of the latter three cohorts was enriched in Bacteroides species.
To obtain a more indepth view, Scher et al. then 'shotgun' sequenced the full faecal micro biomes of a subset of the recentonset rheumatoid arthritis patients and healthy controls. Two impor tant results emerged. First, the gutassociated P. copri entities in the controls were not exactly the same as those present in the recently diag nosed rheumatoid arthritis patients. Second, significant differences were observed between the metagenomes (the inventory of genes) of the two cohorts-with the recentonset rheumatoid arthritis patients showing relatively few genes for enzymes that metabolize vitamins and purines, and relatively many genes for enzymes that metabolize cysteine. While the functional relevance of these differences remains unexplored, they may prove to be useful biomarkers for diagnosis or prognosis.
As a first step in functionally dissecting the association between P. copri and autoinflammatory disease, Scher et al. used oral gavage to introduce the bacterium into the stomachs of mice that had been treated with antibiotics. Sequencing of faecal 16S RNA revealed that P. copri domi nated the gut microbiota within two weeks of introduction. The mice also showed a reduced level of Bacteroides (and a few other species), analogous to the findings in the human cohorts. When the P. copricolonized mice were chal lenged with dextran sulfate sodium, a model for inflammatory bowel disease, they exhibited more severe inflammation of the colon than did control animals.
Scher et al. have taken an important step towards translating to humans recent findings on the gut microbiota-immune system interface obtained from studies on mice. Several lines of investigation now beg pursuit. One critical avenue of study will be to determine whether the associ ation between P. copri and rheumatoid arthritis reflects cause, effect or coassociation. The fact that the recentonset rheumatoid arthritis patients alone exhibited high circulating levels of Creactive protein (a marker of systemic inflammation) raises the possibility that the observed microbiota differ ences reflect a subclinical inflammatory response, perhaps an immediate precursor of clinical rheu matoid arthritis. More generally, any effects of the disease itself-or of the drugs used to treat the disease-on the microbiota need to be assessed. In humans, certain members of the HLA family of immunerelated genes are known to increase the risk of rheumatoid arthritis. The observation that recentonset rheumatoid arthritis patients with an excess of P. copri were less likely to possess some of these risk alleles raises the question of to what extent the association between P. copri and rheumatoid arthritis reflects a shared dependence on the HLA locus. Indeed, when human alleles associated with susceptibility or resistance to arthritis were introduced into mice, they had a profound impact on the faecal micro biota, as well as influencing gut permeability and certain populations of intestinal immune cells (Gomez et al., 2012).
Another important avenue of investigation will be to elucidate the mechanisms by which P. copri predisposes its host to autoimmunity or inflammation, in particular to a gutdistal disorder. How are arthritogenic signals generated in the gut transposed to the joint? The mysterious micro bial world within us is beginning to reveal its secrets, but many mysteries remain to be solved in the years to come.