Your <scp>FLORA</scp> Matters: Microbiome Modulation in Psoriatic Arthritis

The last decade witnessed a paradigm shift in our understanding of immune-mediated inflammatory diseases (IMIDs), with an emerging role of the microbiome in disease pathogenesis and progression. Barrier sites, such as the gut, skin, and lung, are not merely anatomic structures but active interfaces between host and the environment, mediating complex host–microbe interplay.1, 2 Disruption of these interfaces—referred to as “barrier dysfunction”—is now a hallmark feature across multiple IMIDs, including spondyloarthritides, inflammatory bowel disease (IBD), psoriasis, psoriatic arthritis (PsA), and even rheumatoid arthritis.2-4 Among these, PsA stands out as a particularly compelling disease model. PsA is uniquely characterized by its multitissue involvement—skin, joints, entheses, and gut—several of which constitute critical barrier sites. A growing body of literature pinpoints intestinal dysbiosis in PsA pathogenesis, and recent studies suggest dysbiosis of the skin microbiome as well.5, 6 These findings raise a tantalizing question of whether targeted modulation of the microbiome may improve outcomes in PsA. In this issue of Arthritis & Rheumatology, Qin and colleagues report a novel investigation into this question through the FLORA trial, a randomized study evaluating the impact of fecal microbiota transplantation (FMT) in patients with PsA receiving methotrexate.7 The authors applied high-resolution shotgun metagenomic sequencing to samples from both donors and recipients, aiming to identify microbiome profiles associated with treatment response. Although FMT has been trialed in other IMIDs with mixed results, FLORA represents the first and only controlled study of FMT in a spondyloarthritis population, making this work both timely and novel. The rationale for using FMT in PsA stems from compelling, if still evolving, evidence that the gut microbiota modulates systemic immunity. PsA shares genetic and immunopathologic features with IBD, particularly Crohn disease, in which gut dysbiosis is well characterized. Animal models have demonstrated that alterations in gut microbial composition can drive Th17-mediated inflammation at distal sites, including joints and skin. Clinical studies in PsA have identified reduced microbial diversity and altered abundance of key taxa, including Prevotella, Bacteroides, and Faecalibacterium. FMT, the transfer of fecal material from a healthy donor to a recipient, has been most extensively studied in Clostridioides difficile infection, in which it has demonstrated high efficacy. Its application to chronic inflammatory conditions, however, remains experimental. Small trials in ulcerative colitis have shown promise, with steroid-free remission achieved in a subset of patients. In contrast, results in systemic lupus erythematosus and multiple sclerosis have been variable. Thus, the field has been eagerly awaiting controlled trials exploring the role of FMT in arthritides. Qin et al randomized patients with active PsA (on stable methotrexate therapy) to receive FMT from four different healthy donors or a sham procedure consisting of colored saline. Although the original trial's primary endpoint was negative, with FMT achieving lower treatment success (40%) compared to sham (81%),8 the present microbiome-focused analysis provides rich insights into microbial community dynamics and potential predictive biomarkers of response. The most notable finding was the identification of two distinct baseline gut microbiota profiles among patients with PsA: a P-type, dominated by Prevotella species, and a B-type, which is dominated by Bacteroides species and associated with greater microbial richness. Patients with B-type profiles appeared more likely to benefit clinically from FMT. This stratification introduces the possibility that microbial community type could serve as a predictive biomarker for response to microbiome-based interventions—an idea that warrants urgent validation. However, these results should be confirmed as microbiome trajectories varied among clinical responders: three remained B-type, two transitioned from B- to P-type, and one shifted from P- to B-type. These transitions suggest that donor–recipient compatibility may be critical and that pretreatment microbial profiling could inform personalized donor selection strategies in future FMT trials. Although these findings are compelling, several limitations still need further consideration. First, the clinical endpoints employed—treatment failure and the Health Assessment Questionnaire Disability Index—are not standard metrics in PsA trials. More widely accepted and broadly validated composite indices, such as the Disease Activity index for Psoriatic Arthritis or the Minimal Disease Activity criteria, would provide greater granularity and enhance interpretability. Without these, it remains difficult to align microbial changes with meaningful improvements in disease activity. Second, although the microbiome analysis is sophisticated, it excludes data from sham-treated patients, limiting the ability to discern natural variability and placebo-associated shifts. This is particularly important given the unexpectedly high placebo response (81%) in the original trial. Including all arms in future analyses will be essential for robust interpretation. Third, although the study includes multiple timepoints (baseline, week 4, week 12, week 26), there is a missed opportunity for integrated longitudinal analysis. Understanding the temporal dynamics of microbial communities—how they shift, stabilize, or regress—could shed light on whether FMT induces lasting ecological reprogramming or only transient perturbations. Finally, the functional implications of these microbial shifts remain largely unexplored. Although taxonomic changes are informative, they do not necessarily reflect alterations in microbial function or metabolite production. The integration of metabolomics, transcriptomics, and immunophenotyping will be critical in future studies to link microbial composition with downstream effects on host immunity. Qin and colleagues should be commended for pioneering the first FMT trial in PsA and for advancing our understanding of microbiome–host interactions in this complex disease. Despite the negative primary outcome, their post hoc analysis reveals important microbial features that could inform future therapeutic strategies. The identification of P- and B-type microbial profiles, with differing associations to treatment response, provides a foundation for precision microbiome interventions in PsA. To translate these insights into clinical practice, future trials must incorporate validated disease activity metrics, sham-controlled microbiome analyses, and integrated multi-omics profiling. Only through such rigor can we unravel the complex interplay between gut ecology and systemic inflammation and harness the microbiome as a true therapeutic frontier in rheumatology. Both authors contributed to at least one of the following manuscript preparation roles: conceptualization AND/OR methodology, software, investigation, formal analysis, data curation, visualization, and validation AND drafting or reviewing/editing the final draft. As corresponding author, Dr Elewaut confirms that the authors have provided the final approval of the version to be published and takes responsibility for the affirmations regarding article submission (eg, not under consideration by another journal), the integrity of the data presented, and the statements regarding compliance with institutional review board/Declaration of Helsinki requirements. Disclosure Form: Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.