Scientists Map the Hidden Cellular Wiring That Drives Rheumatoid Arthritis Joint Destruction

Rheumatoid arthritis has long been treated as a disease of the immune system, an autoimmune storm that floods the joints with inflammatory cells and destroys cartilage and bone. Yet even as drugs targeting TNF-alpha, IL-6, and the JAK/STAT pathway have transformed care for millions of patients, a substantial fraction of people still experience refractory disease or incomplete remission. A new cross-species study published in Frontiers in Molecular Medicine argues that this treatment gap reflects a blind spot in current therapeutic paradigms: while existing biologics dampen systemic immune activation, they frequently fail to arrest the localized, self-amplifying remodeling of the synovial stroma that physically drives joint destruction. By stitching together single-cell and spatial transcriptomic data from mice and humans, the research maps, in unprecedented detail, the conserved signaling circuitry that turns the joint’s structural scaffolding into an aggressive, destructive machine.

The study, led by Marxa L. Figueiredo of Purdue University, integrated publicly available single-cell RNA sequencing datasets spanning multiple preclinical mouse models of inflammatory arthritis, together with 12 healthy controls and 15 rheumatoid arthritis samples. The models were chosen deliberately to capture complementary facets of the disease: collagen-induced arthritis replicates the chronic, autoimmune, T-cell and B-cell-dependent phases of systemic joint destruction; serum transfer-induced arthritis bypasses adaptive priming to mimic the acute, antibody- and innate immune-mediated effector phase; antigen-induced arthritis models localized, antigen-specific, T-cell-driven joint inflammation; and the glucose-6-phosphate isomerase model represents metabolic-immune crosstalk with rapid systemic pathology. By converging on the programs shared across these diverse etiologies, the team sought a conserved landscape of synovial remodeling rather than a model-specific snapshot.

The bioinformatic pipeline was substantial. Count matrices were processed through Trailmaker and ICARUS v3, with background noise removed using transcripts-per-cell thresholds, dead cells filtered by mitochondrial content, doublets excluded with scDblFinder, and batches corrected using Harmony. Unsupervised Leiden clustering with UMAP visualization resolved the integrated mouse synovium into 25 distinct clusters, which the researchers organized into functional niches: a structural and fibroproliferative compartment, a vascular and endothelial compartment, a myeloid immune compartment, a lymphoid and mast compartment, and even a peripheral glial niche containing Schwann cells. Within the structural niche alone, profiling distinguished universal Pi16-positive sublining progenitors, specialized Calb1-positive stromal cells, mechanosensitive Dkk3-positive fibroblasts, fibrogenic Postn-positive fibroblasts, activated fibroblast-like synoviocytes, myofibroblasts, inflammatory progenitors, and joint-destructive Prg4/Il6-positive lining fibroblasts, alongside osteoblast and chondrocyte lineages.

Differential expression analysis comparing inflamed and healthy joints revealed a pronounced transcriptional imbalance. Upregulated genes in arthritic synovium were strongly enriched for innate immune cascades, including neutrophil degranulation and platelet activation and aggregation, as well as extracellular matrix organization, supramolecular fiber organization, focal adhesion, regulation of proteolysis, IL-17A signaling, and TNF production. Protein-protein interaction network analysis identified interconnected modules centered on collagen formation, ECM organization, and actin filament regulation. Conversely, downregulated genes represented a loss of homeostatic programs: tissue development, blood vessel development, glial cell fate specification, biomineral tissue development, and fatty acid metabolism. The picture that emerges is a joint shifting away from protective, regenerative maintenance toward aggressive immune activation and pathological matrix remodeling, a dual failure that helps explain why suppressing inflammation alone often cannot stop structural damage.

The study’s most striking mechanistic finding came from cell-cell communication modeling with CellChat, which inferred ligand-receptor interactions across the diseased synovium. Vascular mural cells, sublining progenitors, fibrogenic sublining fibroblasts, and lining fibroblasts emerged as strongly upregulated signaling hubs. Within the Notch family, the highest-strength disease-state links were autocrine Jag1-Notch3 signaling within vascular mural cells and endothelial-to-mural Dll4-Notch3 communication from activated endothelial cells. Beyond this structural feedback loop, mural Jag1-Notch2 and endothelial Dll4-Notch2 signaling maintained strong communication with infiltrating inflammatory monocytes and sublining macrophages, with an additional Dll4-Notch1 interaction targeting monocytes. This positions the vascular-mural unit as an actionable upstream trigger that simultaneously drives structural remodeling and macrophage activation in the inflamed joint.

Downstream of this perivascular trigger, regulatory network inference with SCENIC revealed a synchronized, stress-induced transcriptional program. Vascular mural cells, chondrocytes, inflammatory progenitors, specialized sublining subsets, and macrophage populations all displayed coordinated activity of the Atf3 and Fos regulons, transcription factors of the AP-1 family that govern inflammatory stress responses, clustering with the immediate-early gene Egr1 and Krüppel-like factors Klf4 and Klf10. The researchers suggest this shared module may function as a unified regulatory hub coordinating myofibroblast differentiation and the aggressive fibrotic response characteristic of chronic joint disease. Replicate-aware pseudobulk DESeq2 modeling confirmed that these cluster-specific patterns mirror the global trends, supporting the findings under a statistically rigorous framework with a Benjamini-Hochberg adjusted threshold.

Crucially, the mouse findings translated to human disease. The team analyzed a high-resolution human rheumatoid arthritis synovial dataset generated with 10x Genomics Visium HD spatial transcriptomics, processed through the VST-DAVis pipeline. A single sample yielded 474,081 tissue-associated 8-micrometer bins with a mean of 197.2 UMIs per bin, allowing unsupervised clustering of spatially resolved tissue domains. Human vascular mural lineages separated into NOTCH3, ACTA2, and TAGLN-enriched mural cells and MYH11-expressing perivascular fibroblasts, mirroring the mouse compartments. Weighted gene co-expression network analysis returned a single dominant module, SM01, a stress and matrix-remodeling hub containing VIM, FN1, MMP2, CTSB, PLA2G2A, CD74, COL6A2, SELENOP, and COL3A1. This module was selectively and highly enriched in human activated FLS and lining FLS, the functional homologs of the mouse sublining fibroblast and mural hubs, suggesting the Atf3/Fos stress-response pathway and its downstream matrix-remodeling program are conserved drivers of the inflammatory stromal phenotype in human RA.

The spatial data then did what conventional single-cell sequencing cannot: it placed these circuits in physical context. Activated FLS emerged as a central spatial coordinator, acting as the primary source of outgoing Collagen, FN1 (fibronectin), Tenascin, and THBS (thrombospondin) ligands while remaining highly receptive to incoming NOTCH, THBS, and FN1 signaling. Spatial chord diagrams confirmed perivascular fibroblasts presenting JAG1 to NOTCH3 on vascular mural cells, alongside a multicellular structural network mediated by THBS2, THBS4, and COMP linking lining and activated FLS to SDC1 and CD47 receptors on plasma cells and activated FLS. Most tellingly, spatial overlay maps showed that FN1 and THBS signaling networks co-localize at high density within a distinct inflammatory niche restricted to the synovial tissue’s edge, heavily populated by activated FLS, lining FLS, and perivascular mural cells. The disease, in other words, has an address.

The authors are careful to note the limitations. The spatial transcriptomics dataset, while high-resolution and derived from untreated RA tissue, comes from a single human donor and serves as a proof-of-concept bridge that will benefit from broader validation. CellChat infers potential microenvironmental signaling rather than confirming direct physical cell-cell contact in vivo, so functional assays will be needed to validate the predicted pathways. Even so, the model that emerges is coherent and therapeutically provocative: vascular-derived Jag1 and Dll4 activate Notch3 and Notch2 on adjacent stromal and immune cells, initiating an Atf3/Fos transcriptional cascade that drives both myeloid activation and a matrix-remodeling program within a localized, self-amplifying spatial niche at the tissue’s edge.

That model points toward a stroma- and vascular-directed therapeutic paradigm that differs fundamentally from conventional immunomodulatory biologics. Rather than broadly suppressing systemic immunity, selective blockade of Notch3/2 activation or endothelial ligand presentation could specifically uncouple vascular-to-stromal and vascular-to-immune crosstalk, preventing both pathogenic fibroblast activation and immune infiltrate recruitment. Disrupting this structural trigger could also destabilize the self-amplifying niche at the tissue’s edge. The authors propose that combining perivascular Notch pathway inhibition with downstream targeted therapies against matrix-remodeling networks such as THBS or FN1, or against AP-1 transcription factors like Atf3 and Fos, may represent a synergistic strategy to simultaneously halt chronic inflammation and structural joint destruction. For the many patients whose disease resists today’s immune-targeted drugs, that multi-tier approach, aimed at the joint’s own cellular architecture rather than only its inflammatory guests, offers a genuinely new map of where to intervene.

Subject of Research: Conserved stromal remodeling and Notch/AP-1 signaling pathways in inflammatory arthritis identified through cross-species single-cell and spatial transcriptomics

Article Title: Cross-species single-cell and spatial transcriptomics map the conserved stromal remodeling landscape of inflammatory arthritis

Article References: Figueiredo, M. L. (2026). Cross-species single-cell and spatial transcriptomics map the conserved stromal remodeling landscape of inflammatory arthritis. Frontiers in Molecular Medicine, 6, 1951540. https://doi.org/10.3389/fmmed.2026.1951540

Image Credits: AI Generated

DOI: 10.3389/fmmed.2026.1951540

Keywords: rheumatoid arthritis, single-cell RNA sequencing, spatial transcriptomics, synovial fibroblasts, Notch signaling, AP-1 transcription factors, vascular mural cells, extracellular matrix remodeling, fibroblast-like synoviocytes, CellChat, Visium HD, mouse models