Deconstruction of tophi and synovium defines SPP1+ macrophages involved in extracellular matrix remodelling in gout.

Objectives: Gout is a prevalent condition characterised by high serum urate levels, leading to crystal deposition in the joints. Although many studies have explored the mechanisms underlying gout flares, the dynamics of tophus formation remain unknown. This study provides the first transcriptomic profile of the tophaceous gout joint and investigates immune-stromal cell interactions in the pathogenesis of tophus formation.

Methods: Single-cell transcriptomic profiling was conducted on 44,221 synovial tissue cells from patients with intercritical gout (without tophi) and tophaceous gout. Spatial transcriptomics showed gene expression patterns in the corona and fibrovascular zones of tophi. Gene expression pattern comparisons, pseudotime, and differential gene enrichment analyses were conducted on stage-specific macrophage subsets. Immunofluorescence and flow cytometry on the tophi samples validated the transcription results and visualised the spatial localisation of the macrophage-fibroblast subpopulation. Differential causal inference combined with Mendelian randomisation elucidated gene regulatory networks and their causal relationships with gout pathology.

Results: We identified SPP1⁺/MMP9^+/CHI3L1⁺ macrophages within the corona zone exclusive to tophaceous gout, exhibiting extracellular matrix regulation genes, enhanced integrin-mediated interactions with stromal cells and transitional potential towards osteoclast differentiation. Notably, coexpression of the fibroblast marker S100A4 and COL6A2 in these macrophages suggested a fibroblast-like phenotype. Distinct CD4⁺ T-cell transcriptional profiles between disease states indicated a phenotypic shift from inflammatory to immune-regulatory subsets during tophus development.

Conclusions: This study reveals a novel macrophage population (SPP1⁺/MMP9^+/CHI3L1⁺) with dual immunomodulatory and matrix-remodelling capabilities, exclusively present in tophus tissues but absent in intercritical gout synovium or flare-associated synovial fluid. It may play a role in the pathogenesis of fibrosis and bone erosion in gout.