Age and Fecal Microbial Strain-Specific Differences in Patients With Spondyloarthritis

Matthew L. Stoll; Pamela F. Weiss; Jennifer E. Weiss; Peter A. Nigrovic; Barbara S. Edelheit; S. Lou Bridges Jr; Maria I. Danila; Charles H. Spencer; Marilynn G. Punaro; Kenneth Schikler; Andreas Reiff; Ranjit Kumar; Randy Q. Cron; Casey D. Morrow; Elliot J. Lefkowitz

Disclosures

Arthritis Res Ther. 2018;20(14) 

In This Article

Discussion

Mechanisms by which an altered microbiota might be associated with inflammatory arthritis are as yet unknown. Animal studies clearly demonstrate a causal relationship, as an animal model of SpA is abrogated in the germ-free environment.[47] Herein, we report strain-level differences in fecal abundance of F. prausnitzii between children with ERA and healthy controls. Additionally, at the functional level, decreased genetic abundance of the butanoate pathway was observed in children with ERA. Children with ERA also had increased abundance of B. fragilis. Notably, adults with longstanding SpA had similar trends with respect to F. prausnitzii abundance, but opposite trends with respect to Bacteroides, with decreased abundance of Bacteroides genus seen in adult SpA patients compared to adult healthy controls.

This work supports a potential role of F. prausnitzii in the pathogenesis of SpA. Although previous studies assessing the microbiota in adults with SpA have not reported alterations in the abundance of this organism,[5,6,43] this was reported previously in a pediatric study of SpA patients.[8] This particular species is generally considered to have anti-inflammatory effects through production of SCFAs, such as butyrate,[10] or by direct effects on cytokine production,[39] and our results demonstrated decreased genetic capacity to synthesize butyrate among ERA patients compared to healthy control pediatric subjects. SCFAs serve as major sources of energy for the intestinal enterocytes and also regulate the differentiation of T cells, promoting a regulatory phenotype.[46,48] A meta-analysis of studies of patients with IBD showed decreased intestinal abundance of F. prausnitzii.[9] Our work extends these previous observations, by showing that the specific strain within F. prausnitzii may be of particular relevance. Recent studies indicated that the A2-165 strain, as compared to L2/6 and other strains, may have greater capacity to induce IL-10 production[40] and has increased promoter activity at the butyryl-CoA CoA transferase gene, which codes for the enzyme that catalyzes the final step in the butyrate synthesis pathway.[41] Thus, not only did ERA patients demonstrate less representation of the butanoate pathway, but their promoter activity at a key locus determining butyrate production may also be diminished. Not surprisingly in light of these results, a study of fecal metabolomics demonstrated diminished representation of the butanoate pathway in ERA patients compared to controls.[11]

Another important finding in this work pertained to the abundance of Bacteroides. Consistent with previous observations in children with multiple forms of JIA including ERA,[7,8,13] this organism was relatively overrepresented among pediatric SpA patients. This finding appears to be uniquely associated with pediatric subjects; several studies in adults with both RA[14,15,49] and SpA[5,6] have shown decreased abundance of Bacteroides in patients compared to healthy controls. Similar to these other reports, adult SpA patients in this study showed decreased abundance of Bacteroides.

One potential way to reconcile these findings is that B. fragilis may contribute to JIA not by directly causing an inflammatory process, but rather through its effects on the ontogeny of the immune system. Vatanen et al.[50] compared the fecal microbiota of children up to age 3 years in areas with high (Estonia and Finland) and low (Russia) incidences of pediatric autoimmunity, finding among the differences decreased fecal abundance of Bacteroides, particularly Bacteroides dorei, in the Russian children. That the differences in the incidence of autoimmunity may have been related to the abundance of Bacteroides was suggested by findings that the lipopolysaccharide (LPS) of B. dorei generated a less robust inflammatory reaction than did that of Escherichia coli yet failed to induce tolerance to repeated doses of endotoxin; furthermore, the LPS of B. dorei, unlike that of E. coli, did not prevent development of type 1 diabetes when injected intraperitoneally into young nonobese diabetic mice. Thus, elevated fecal abundance of Bacteroides may represent an illustration of the hygiene hypothesis, which posits that abnormal early life microbial exposures result in altered immune priming and consequent increased risk of autoimmune diseases.[51]

The findings of our study need to be interpreted in view of its limitations. Despite including subjects from around the country, the sample size was small, and we cannot exclude geographic contributions to the findings. HLA-B27 itself may also have contributed to the variance,[52] although among the ERA patients there was no evident clustering based upon the presence versus absence of this marker, and there is to our knowledge no published data indicating that the HLA-B27 allele affects the microbiota in humans. In addition, all adult participants with longstanding SpA had been exposed to a variety of immunosuppressive therapies. Furthermore, three of them had been exposed to sulfasalazine, which is in part an antibiotic; however, excluding them did not alter our findings. We also do not have information on subclinical intestinal inflammation, which is frequently present in patients with SpA.[53] Finally, many of the adult and pediatric controls had been referred to evaluate for arthritis; however, these subjects were evaluated by an attending pediatric or adult rheumatologist, who excluded a diagnosis of arthritis on the basis of the historical and physical examination findings. The strengths of the study are that the pediatric arm was limited to treatment-naïve subjects, we used a novel informatics approach to identify strain-level differences between patients and controls, and we included shotgun sequencing data that corroborated the 16S findings.

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