Background/Purpose: Monocytes are critical for the pathogenesis of rheumatoid arthritis (RA). However, depletion of peripheral monocytes (PM) is not sufficient to rescue disease in RA mouse models. Recently, we identified a novel extravascular cell type, namely tissue-resident monocyte lineage cells (TR-MC) which are distinct from macrophages and PM. Equivalent TR-MC have been identified in human synovium, therefore, investigating TR-MC will provide insight on developing cell therapy for RA. TR-MC in mice are sensitive to clodronate liposomes (clo-lip) and are required for the development of acute RA-like disease in mouse. Nevertheless, the ontogeny of TR-MC and the dynamics of TR-MC during inflammation remained unclear. 

Methods: Ccr2-/-, Nr4a1-se2-def, Lyve1-GFP/+, Ccr2 GFP/+, CD45.1 and wild-type C57BL/6 mice were used in this study. CD45.1  CD45.2 bone marrow chimera (BMC) mice were generated with ankle shielding. CD45.1 & CD45.2 parabiosis mice were generated. Clo-lip was administered to BMC mice to deplete TR-MC and cell numbers were measured via flow cytometry. Serum transfer-induced arthritis (STIA) was initiated on BMC mice. Cellular indexing of transcriptomes and epitopes by sequencing (CITE-seq) was performed on synovial CD45+CD11b+Ly6G−SiglecF−CD4-CD8-NK1.1-CD19-CD64−MHCII-CD177- cells from steady-state BMC, day 7 clo-lip BMC, and day 7, 14, 56 STIA BMC mice.

Results: To determine the dependence of TR-MC on PM, we compared the number of TR-MC in CM-depleted (Ccr2-/-), NCM-depleted (Nr4a1-se2-def), and control mice. The results showed that TR-MC were significantly reduced in CM-depleted mice but remained comparable in NCM-depleted, suggesting a subpopulation of TR-MCs is dependent on Ccr2. Similarly, we identified a consistent subpopulation of CD45.1 donor-derived TR-MC in steady-state BMC mice demonstrating that a portion of TR-MC are bone marrow derived, while the remainder were CD45.2 long-lived recipient cells. TR-MC subpopulations have also been identified using parabiosis mice. CITE-seq results of steady-state BMC showed that BM-derived and long-lived TR-MC could be distinguished by RNA expression of Ccr2 and Lyve1, respectively. These results were confirmed using Lyve1-GFP/+ and Ccr2 GFP/+ mice. GO results showed that long-lived TR-MC were enriched in homeostasis maintenance pathways, whereas BM-derived TR-MC showed enrichment in adaptive immunity associated processes. When treated with clo-lip, long-lived TR-MC were partially depleted, while the numbers of BM-derived TR-MC were not significantly changed. CITE-seq results during STIA time course showed a gradual increase in BM-derived TR-MC relative to long-lived TR-MC during the development of arthritis. During recovery, the populations returned to relatively equal numbers.

Conclusion: Our study revealed two distinct populations of TR-MC: 1) long-lived TR-MC, which are sensitive to clo-lip and are outnumbered by 2) BM-derived CCR2-dependent TR-MC during STIA development. BM-derived TR-MC serve to connect innate and adaptive immune responses, whereas long-lived TR-MC are involved in maintaining synovial tissue homeostasis. The findings above about the subpopulations of TR-MC will help develop treatments for RA. 

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/tissue-resident-monocyte-lineage-cells-tr-mc-exist-as-two-different-subpopulations/