Background/Purpose:

We established a mouse model (HUPO) by deletion of Flip in CD11c+ cells that spontaneously develops erosive inflammatory arthritis resembling rheumatoid arthritis (RA). Since CD11c is expressed in F4/80^hi macrophages (MΦs) and since MΦs and MΦ-derived cytokines are important in RA and are increased in the joints of HUPO mice, we examined the phenotype and function of MΦs in the joints of HUPO mice.

Methods:

HUPO arthritis was evaluated by clinical score. Five subsets of synovial tissue MΦs were defined by flow cytometry: three F4/80^int (FI) subsets included Ly6C+MHCII- (FI1), Ly6C+MHCII+ (FI2) and Ly6C-MHCII+ (FI3); and 2 F4/80^hi (FH) subsets were further defined as MHCII^– (FH1) and MHCII⁺ (FH2). These synovial MΦ populations, as well as classical (CM) and non-classical (NCM) monocytes from HUPO mice with arthritis and littermates controls, were sorted by flow cytometry and processed for RNAseq. The gene expression profiles and pathways were analyzed by Genee software and GOrilla database. The origin of monocytes and macrophages from monocyte bone marrow progenitors was performed by BrdU labeling, bone marrow chimeras and treatment with clodronate liposomes (CL).

Results:

The total number of synovial MΦs was increased in HUPO mice, especially the FI2, FI3 and FH2 subsets, while there was a significant reduction of FH1 MΦs. Further, the % of FH1 MΦs, which are the tissue resident MΦs (TRM) of the synovial tissue in control mice, but not other MΦ subsets, inversely correlated with clinical arthritis and granulocyte accumulation. Bone marrow chimera experiments confirmed that arthritis developed after the FH1 population decreased. Fifty-60% of CMs and FI1 and FI2 MΦs were BrdU+ within 24 hours, similar to the controls. In HUPO mice FI3, FH1 and FH2 MΦs became BrdU+ on days 3 and 5, consistent with their origin from monocytes. Examination of cells 30 minutes after BrdU injection and following treatment with CL, suggested that at least a portion of the FI1 population derived directly from the bone marrow monocytes in HUPO mice, rather than the circulation. Further, the reduction of the FI2 and FI3 populations following CL is consistent with their origin from circulating monocytes. HUPO FI3 MΦs demonstrated the most distinct gene expression pattern from controls, suggested that FI3 subset may represent a turning point in synovial MΦ homeostasis. Comparing FH subsets between HUPO and control mice, the differentially expressed genes (DEGs) down regulated were those regulating intracellular signaling, wound healing and lipid homeostasis. In contrast, the DEGs up regulated related to pro-inflammatory signaling, endogenous TLR ligands, angiogenesis and joint destruction in HUPO mice.

Conclusion:

HUPO arthritis develops following the reduction of FH1 TRMs, which inversely correlate with inflammation. In HUPO mice with arthritis, in contrast to controls, bone marrow monocytes, at least in part, have direct access to the joint. These observations suggest a potential new paradigm in which a reduction of TRM may contribute to the pathogenesis of RA.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/loss-of-synovial-tissue-resident-macrophages-permits-monocyte-to-macrophage-differentiation-and-inflammation-in-hupo-mice/