Background/Purpose: Macrophages in the synovial lining of the joint are critical players in the pathogenesis of rheumatoid arthritis (RA). While they are potent producers of inflammatory molecules, there is also evidence that macrophage play a role in the resolution of inflammation. A better understanding of macrophage heterogeneity in humans could help pinpoint pathways and potential therapeutic targets for treating RA that are specific to particular subpopulations.

Methods: We obtained synovial tissue samples from the wrist joints of patients with active RA through minimally invasive ultrasound-guided synovial biopsy. Tissue was processed into single-cell suspension and enriched for CD45+ immune cells by Fluorescence Activated Cell Sorting (FACS).  Next, droplet-based single cell RNA-sequencing (scRNA-seq) was performed using the 10X Genomics instrument. The sequenced reads were aligned and mapped to genes using CellRanger pipeline, followed by R package.

Results: We previously used single-cell RNA-sequencing to report the presence of 4 distinct subpopulations of mouse synovial macrophages. Based on their distinct transcriptional profiles, we characterize these populations as synovial lining, interstitial, antigen presenting, and infiltrating. Here, we examined the single cell expression profiles of RA synovial biopsy samples to investigate whether humans exhibit similar disposition of synovial macrophage heterogeneity as observed in mice. Using integrative gene module scoring of top 10 orthologous markers, the labels of 4 mouse subpopulations were projected onto human macrophages. Large variations in the composition of macrophage subpopulations was observed among patients. Strikingly, patients with higher proportions of synovial lining and interstitial macrophage subpopulations are significantly associated with lower clinical severity of RA, which is consistent with their anti-inflammatory phenotypes observed in mice. We also compared the 4 mouse subpopulations against an independent dataset generated by Accelerating Medicines Partnership (AMP) Consortium, where synovial myeloid cells from patients with RA or osteoarthritis (OA) were profiled. We observed the enrichment of synovial lining expression signatures in OA, which is generally associated with lower levels of inflammation. In contrast, signatures of antigen presenting and infiltrating subsets are enriched in the RA patients with high immune cell infiltration, supporting the expansion of these subsets by recruitment of circulating monocytes.

Conclusion: We were able to characterize human synovial macrophage heterogeneity by translating our findings from subpopulations in mice. Further investigation of the activation of these macrophages in the inflamed joint may lead to the development of therapies to target the pathogenic function of specific subpopulations in RA.

ACR Meeting Abstracts - https://acrabstracts.org/abstract/characterizing-heterogeneity-of-synovial-macrophages-in-rheumatoid-arthritis-patients/