Background/Purpose: Rheumatoid arthritis (RA) manifests in persistent synovial inflammation, cellular infiltration and pro-inflammatory cytokine production, and results in progressive joint destruction. Macrophages have been implicated in RA progression and persistence through production of degradative enzymes, cytokines, and chemokines. However, the mechanisms underlying these activities are not fully elucidated. We previously demonstrated that naïve mouse joints contain MHC II⁺ and MHC II^– macrophages, the majority being MHC II^–tissue-resident macrophages that can limit initiation of an acute model of inflammatory arthritis. However, macrophages are plastic and can alternate their phenotype from an immunosuppressive to a proinflammatory phenotype depending on the microenvironment. Thus, we have optimized a multi-parameter flow cytometry protocol to isolate synovial macrophage subsets to perform subset-specific transcriptomic and metabolic analysis.

Methods: K/BxN serum transfer-induced arthritis was initiated in 10-12 week old female C57BL/6 mice and clinical severity was assessed over a 21-day period. Flow cytometric analysis was employed to delineate macrophage subsets via expression of MHC II and CX3CR1 to obtain distinct macrophage populations. These populations were sorted throughout the course of arthritis and RNAseq was performed to obtain transcriptional profiles. In addition, flow cytometric analysis was employed to determine mitochondrial function of synovial macrophage subsets throughout the course of arthritis through the use of MitoSox Red, mBBr and JC-10 dyes.

Results: We observe that both the genetic and metabolic profiles of synovial macrophage subsets shift throughout the course of arthritis and contract back towards their steady-state phenotype during resolution of disease. PCA and clustering analysis of synovial macrophage subsets show that these populations display distinct genetic signatures and have identified gene clusters and cellular processes that dictate their function during arthritis intiation, progression and resolution. Further, synovial macrophages appear to shift towards a more metabolically active phenotype, with an increase in mitochondrial membrane potential, at the height of inflammation and contract back to normal as disease wanes.

Conclusion: We conclude that inflammation induces genetic and metabolic alterations in synovial macrophage subsets that coincide with initiation, progression and resolution phases of an acute model of inflammatory arthritis. These alterations indicate that specific macrophage subsets possess distinct genetic and metabolic profiles coinciding with designated functionality during the course of disease, thereby providing insight into potentially useful targets for therapy.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/genetic-and-metabolic-signatures-of-purified-synovial-macrophage-subsets-during-an-acute-murine-model-of-inflammatory-arthritis/