Background/Purpose:

Synovial tissue macrophages (STMs) are critical in the pathogenesis of rheumatoid arthritis (RA). During homeostasis, the majority of murine synovial tissue resident macrophages (TRMs) are MHCII^–Ly6C^– and TRMs in other tissues are F4/80^hi. In our novel CD11C-Flip-KO (HUPO) mouse model of RA, erosive, progressive arthritis develops spontaneously. This study examined the F4/80^hi subsets of ankle macrophages from HUPO mice with arthritis and littermate controls.

Methods:

Arthritis was evaluated by clinical score. Cell types within the ankle joints were determined by flow cytometry. STMs were defined as CD45⁺CD11b⁺CD64⁺F4/80⁺, which were further grouped into subsets according to the expression of F4/80, MHCII and Ly6C. Subsets of F4/80^hi STMs were isolated for RNAseq.

Results:

All STMs that are F4/80^hi were also Ly6C^– in both HUPO and control mice. In the controls mice, under homeostatic conditions, ~80% of the Ly6C^–F4/80^hi STMs were MHCII^– and 20% MHCII⁺, and those that are MHCII^– were considered TRMs. In contrast, in HUPO mice with arthritis the F4/80^hiMHCII⁺ macrophages are greatly increased representing >90%, while the F4/80^hi MHCII^– macrophages were markedly reduced (<10%), and this reduction was inversely correlated with arthritis severity and duration. In HUPO mice, by flow cytometry, IL-6 expressing cells are increased in both MHCII^{+ and –} subsets, while IL-10 expressing cells were reduced, compared with controls. The F4/80^hi STM populations were further analyzed by RNAseq. From over 5,000 differentially expressed genes, a similarity analysis demonstrated limited correlation between HUPO and control F4/80^hi STMs. Further, functionally distinct clusters were identified between HUPO and controls. For both MHCII^{+ and-} subsets, cell differentiation and cell morphogenesis pathways were reduced, while pathways for innate immunity and inflammation were increased in the HUPO mice. In contrast, differences between the MHCII⁺ and the MHCII^– subsets were observed for antigen processing and presentation and the regulation of hematopoiesis in the control mice, while metabolic processes and for tRNA and ntRNA regulation were differentially expressed in the HUPO mice.

Conclusion:

These observations demonstrate that F4/80^hi STMs are markedly different under homeostatic and inflammatory conditions. The reduction of TRMs and an increase of inflammatory F4/80^hiMHCII⁺ macrophages contribute to the pathogenesis of HUPO arthritis. Further studies on the regulation of macrophage biology under homeostatic and chronic inflammatory conditions will help inform the pathogenesis of RA.

ACR Meeting Abstracts - https://acrabstracts.org/abstract/f480hi-synovial-macrophages-in-the-pathogeneses-of-spontaneous-inflammatory-arthritis-in-cd11c-flip-ko-hupo-mice/