Pacucci, V. A. | Search bioRxiv

bioRxiv2026

Synovial lining expressed mechanosensor PIEZO1 drives inflammation-permissive macrophage phenotypes and joint inflammation

Synovial tissue-resident macrophages regulate immune homeostasis within the joint, but can adopt an inflammation-permissive phenotype that promotes immune cell infiltration in rheumatoid arthritis (RA). Understanding the factors that drive this phenotypic switch may help prevent the localisation of inflammation in the joints of individuals at-risk of RA. We identified the mechanosensitive ion channel PIEZO1 as a potential regulator of lining-layer synovial tissue macrophage (STM) function. PIEZO1 was highly expressed in homeostatic, tissue resident TREM2pos lining-layer STMs and in its pathogenic chemokine producing TREM2low phenotype that characterises the hyperplastic lining-layer in active RA. Intra-articular injection of a PIEZO1 agonist in mice induced neutrophil and monocyte infiltration, whereas inhibition of PIEZO1 signalling restored the protective macrophage phenotype. Thus, mechanosensing via PIEZO1 is a defining feature of the joint lining-layer, and its aberrant activation by mechanical stress may lead to the localisation of inflammation within the joint, facilitating a transition from asymptomatic autoimmunity of at-risk RA to clinical disease.

immunology↗

bioRxiv2025

The ontogeny of myeloid-stromal synovial tissue niches in rheumatoid arthritis.

Recent single-cell multi-omic and spatial analyses of synovial biopsies have transformed our understanding of myeloid cell-driven mechanisms underlying human joint pathology and tissue homeostasis in Rheumatoid arthritis (RA). However, the developmental trajectories of synovial tissue macrophage (STM) subsets in humans remain poorly understood, due in part to the lack of models that faithfully replicate synovial tissue niches. This hinders the exploration of the therapeutic potential of targeting specific synovial myeloid cell clusters. Using multi-omics analyses of synovial tissue from an allogeneic bone marrow transplant recipient, we show that joint-specific tissue-resident STM subsets, including both health- and disease-associated clusters, can derive from peripheral blood monocytes. Analysis of embryonic synovial joints revealed that macrophage localization and maturation in the joints are preceded by local stromal niche specialisation, indicating that synovial fibroblasts (FLS) provide tissue-specific instructive cues to STM precursors. To elucidate human STM developmental trajectories, we established a SNP-based fate-tracking human synovial organoid system by embedding distinct blood-derived myeloid precursors, together with FLS clusters from RA synovial biopsies and endothelial cells, into 3D structures. These organoids reproduced key synovial tissue features, including lining and sublining architecture and stromal-myeloid cell cluster composition. Importantly, they supported differentiation of all resident STM subsets: homeostatic lining TREM2pos macrophages, their pathogenic TREM2lowSPP1pos counterparts that characterize the RA hyperplastic lining, and both homeostatic and RA-associated perivascular LYVE1pos STM clusters, all traced to monocytic precursors. In summary, we show that development of STM subsets is driven by fibroblast-conditioned spatial niches. We have established a novel, tractable ex vivo platform to dissect the niche-specific cues driving homeostatic versus pathogenic phenotypic clusters. One Sentence SummaryHuman tissue-resident STMs develop from monocytes under the guidance of cues from FLS within discrete spatial locations.

immunology↗