Background/Purpose:

We have generated a CD11c-Flip-KO mouse line (HUPO) that spontaneously develops erosive arthritis with incidence 70-80% at age ≥ 20 weeks. This study aimed at understanding the role of monocytes in HUPO arthritis and to define the molecular changes during differentiation to macrophages in the inflamed joint compared with controls.

Methods:

Arthritis was evaluated by clinical score. Cell types from blood and ankle joints are determined by flow cytometry. Cell proliferation are determined by BrdU incorporation and populations of cells were isolated for RNAseq.

Results:

Circulating monocytes, defined as CD45⁺CD11b⁺CD115⁺F4/80^lo, were further characterized as classical (CM), Ly6C⁺CD62L⁺, or non-classical (NCM), Ly6C^–CD62L^–. CM were markedly increased in HUPO mice with arthritis and positively correlated with arthritis severity. 70 -90% of CM are also CCR2⁺ in HUPO mice with arthritis, similar to that in control mice. In contrast, in the HUPO mice without arthritis, only 0-7% of these cells are CCR2⁺, and the CM numbers were normal. Monocytes recently recruited into synovium are defined as CD45⁺Ly6C⁺CD64⁺CD11b⁺F4/80^intLy6C⁺MHCII^–. This population demonstrated reduced CD115 and increase F4/80 compared with CMs. Within 24 hours after BrdU injection, in both HUPO and control mice, >60% of these Ly6C⁺MHCII^– macrophages incorporated BrdU, comparable to CM and bone marrow monocytes. In contrast, < 1% in NCM incorporated BrdU. In HUPO mice during further differentiation the macrophages became Ly6C⁺MHCII⁺ and then Ly6C^–MHCII⁺, with less BrdU incorporation in the more differentiated cells. A similar transition was observed in the control mice except the Ly6C⁺MHCII⁺ population was not identified. CMs and F4/80^int macrophages were isolated for RNAseq. In the controls, clusters of differentially expressed genes were identified in CMs, and Ly6C⁺MHCII^– and Ly6C^–MHCII⁺ joint macrophages. For example Immune response and leukocyte activation were increase in CMs while cell cycle and cell division were increased in Ly6C⁺MHCII^– macrophages. There were differences in these transcriptional profiles of the macrophages subset between HUPO and control mice, including those regulating cell cycle and differentiation as well as for monocyte differentiation and antigen processing.

Conclusion:

These observations demonstrate that during chronic inflammation, CMs enter the joints and undergo macrophage differentiation during homeostasis and chronic inflammation. The differences in transcriptional profiles between the control and HUPO mice will provide important insights into the mechanisms contributing to chronic inflammation.

« Back to 2017 ACR/ARHP Annual Meeting

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