Background/Purpose: Synovial tissue macrophages are an exquisitely plastic pool of innate cells that play a key role in RA disease progression. However, the precise nature, diversity, and function of macrophage subsets within the inflamed joint remains unexplored. Therefore, the aims of this study are to phenotypically, transcriptionally and functionally characterise synovial tissue macrophages residing within the inflamed joint.

Methods: Rheumatoid Arthritis (RA), Psoriatic Arthritis (PsA), Osteoarthritis (OA), Arthralgia and healthy control synovial tissue biopsies and synovial fluid mononuclear cells were analysed using the following panel (CD40,-CD45,-CD64,-CD68,-CD163,-CD206,-CD253,-CCR4,-CCR7,-CXCR1,-CXCR3). CD206+CD163+ and CD206-CD163- macrophages were sorted from RA synovial-tissue by FACSAria sorter; RNAseq and FLIM analysis, autologous T-cell co-culture and heathy synovial fibroblast experiments were performed. Cytokine expression was measured by MSD.

Results: A spectrum of macrophage activation states exists within the inflamed synovium. Within this spectrum, significant enrichment of dominant CD206+CD163+ macrophage subtype is present in synovial-tissue versus fluid (p< 0.05). CD206+CD163+ synovial tissue macrophages express significantly more CD40 than synovial fluid (p< 0.0003), positively correlating with disease activity (r=0.6, p< 0.01), with baseline levels predicting response to therapy (p< 0.05). Moreover, CD206+CD163+CD40+ macrophages are enriched in RA synovial tissue compared to PsA and OA pathotypes (p< 0.05). While the CD206+CD163+ subset is present in healthy synovial tissue, expression of CD40 is completely absent in healthy synovium (p< 0.05). Protective barrier-like CX₃CR1-expressing macrophages are depleted in RA synovial tissue and this occurs prior to clinical manifestations. RNA-seq analysis indicates that CD206+CD163+ population is transcriptionally distinct from synovial tissue CD206-CD163-, synovial fluid CD206+CD163+, and RA monocyte-derived M1/M2 macrophages, with unique tissue-resident gene signatures. Moreover, differing metabolic demands between CD206+CD163+ and CD206-CD163- subsets was demonstrated by RNAseq and FLIM analysis. Finally, CD206+CD163+ macrophages spontaneously secrete high levels of key pro-inflammatory mediators (reversed through inhibition of CD40 signalling) which in turn can activate healthy synovial fibroblasts, thus further contributing to the local inflammatory response.

Conclusion: This data identifies for the first-time enrichment of a previously undescribed dysfunctional dominant and transcriptionally distinct macrophage subtype in RA synovial tissue. Taken together, this data provides a greater understanding of the critical role tissue-resident macrophages play in perpetuating inflammation in RA. Further investigation of the molecular patterns and cues that shape specific synovial macrophage subsets may provide opportunities to reinstate RA joint homeostasis.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/significant-enrichment-of-pathogenic-cd206cd163-macrophages-in-rheumatoid-arthritis-synovial-tissue-with-distinct-transcriptional-signatures/