Background/Purpose: Macrophages are vital contributors to both pro-inflammatory signaling and tissue repair processes involved in the pathogenesis and remission of Rheumatoid Arthritis (RA). Recent research shows that synovial macrophages display significant heterogeneity in transcriptional profile, function, and origin, which may drive the diverging roles of macrophages in joint disease. However, the dynamics of macrophage subpopulations over the course of disease needs further characterization. 

Methods: Steady-state CD45+CD11b+Ly6G-SiglecF-CD64+ cells were extracted and sorted from the ankle joints of C57Bl/6 mice. Bone marrow chimeras (BMC) were generated by irradiating CD45.2 mice (recipient) with shielded ankle joints and transplanting cells from CD45.1 mice (donor). Serum transfer arthritis (STIA) was induced in BMCs by IV injection of K/BxN serum (85µl). Ankle joints were extracted at 0 (controls), 1, 7, 14, and 56 days after IV injection (n = 4-6/group) and CD45+CD11b+CD4-CD8-Ly6G-SiglecF-NK1.1-CD64+ cells were sorted. CITE-seq (Cellular Indexing of Transcriptomes and Epitopes) using the 10X Chromium instrument and CellRanger v7 pipeline was performed on all samples. Seurat v5 was used to perform clustering and differential gene expression analysis. 

Results: We defined 4 clusters of steady state populations using semi-supervised clustering and manual annotation based on expression of CX3CR1 and MHCII surface proteins: synovial lining (CX3CR1+), antigen-presenting (MHCII+), interstitial (CX3CR1- MHCII-) and recently-infiltrated (CX3CR1+ MHCII+). BMC controls showed that the MHCII+ and CX3CR1+ MHCII+ populations were CD45.1 bone-marrow derived (~13%), whereas the CX3CR1+ and CX3CR1-MHCII- populations were CD45.2 tissue-resident (~86%). During the peak of STIA (day 7 and 14), the proportion of bone marrow-derived macrophages greatly increased and included both subpopulations that resembled steady-state MHCII+ and CX3CR1+ MHCII+ (~85%) and a small novel population of Acp5+ cells (~5%). By day 56, the synovial macrophage composition resembled control conditions but with all populations exhibiting a greater proportion of CD45.1 bone marrow derived cells.  

Conclusion: Our findings suggest that RA involves an influx of circulation-derived cells which are largely transcriptionally distinct from tissue-resident steady-state subpopulations. However, repopulation of the niche after the resolution of inflammation is driven by a combination of tissue-resident and circulation-derived macrophages.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/synovial-macrophage-heterogeneity-and-dynamics-in-steady-stateand-rheumatoid-arthritis-mouse-model-time-course/