Synovial Tissue Resident Macrophages Play the protective role in the Development of Inflammatory Arthritis in CD11c-Flip-KO Mice

Qi-Quan Huang¹, Renee Doyle¹, Robert Birkett¹ and Richard M. Pope¹

Background/Purpose: We have generated a CD11c-Flip-KO (HUPO) mouse line that spontaneously develops erosive arthritis resembling rheumatoid arthritis.  Arthritis spontaneously developed beginning at 5weeks of age in ~10% of HUPO, progressing to ~80% incidence at ≥20 weeks.  HUPO mice crossed with Rag-/- mice developed a mild non-progressive arthritis due to macrophage infiltration, supporting an important role for macrophages in disease pathogenesis.  This study examined the phenotype of macrophages in the joints of littermate control mice and of ≥ 20 week old HUPO mice with and without arthritis. 

Methods: Ankle joint cells were dissected and digested with collagenase.  Cells were analyzed by flow cytometry employing multicolor fluorochrome-antibodies.  Arthritis were evaluated by clinical score for the inflammation, joint deformity and grip strength, ranging from 0-28.

Results:  Under homeostatic conditions in control mice synovial macrophages are defined as CD11b⁺CD64⁺F4/80⁺, about 90% co-express CD11c⁺. Further, macrophages may be defined as F4/80^hi or F4/80^mid (Figure).  The F4/80^mid Ly6C⁺ MHCII^– are macrophages recently differentiated from peripheral blood monocytes.  Increase of MHCII and loss of Ly6C represents progressive stages of macrophage differentiation.  The percent F4/80^mid Ly6C⁺ MHCII^– macrophages were increased (p <0.001) in ankles of HUPO mice with arthritis, compared with littermate controls.  The F4/80 ^hi macrophages are Ly6C^–, and either MHCII⁺ or MHCII^–.  During homeostasis, the F4/80^hi MHCII^– macrophages are considered as tissue resident macrophages.  F4/80^hi MHCII^– tissue resident macrophages are greatly reduced and F4/80^hi MHCII⁺ macrophages increased (p < 0.01-0.001) in HUPO mice with arthritis, compared with littermate controls or HUPO mice without arthritis.  F4/80^hi MHCII^– macrophages were also decreased in the joints of HUPO mice at 4 weeks, before arthritis onset.  In HUPO mice the F4/80^hi MHCII^– macrophages were inversely correlated (p = 0.023) with arthritis severity, while the F4/80^hi MHCII⁺ macrophages were positively correlated (p = 0.024).  Further supporting the role of F4/80^hi MHCII^– macrophages in suppressing inflammation, the percent F4/80^hi MHCII^– macrophages was strongly, inversely correlated (r = -0.874, p<0.001) with the percent granulocytes in the synovial tissue.   

Conclusion:   In HUPO mice the reduction of synovial tissue resident macrophages, which are also CD11c⁺, was highly associated with inflammation and the onset and severity of arthritis.  The influx of peripheral blood monocytes and their differentiation into macrophages was observed in HUPO arthritis.  Approaches to preserve tissue resident macrophages or to decrease influx of peripheral blood monocytes may be effective therapeutically in HUPO, and potentially other forms of inflammatory arthritis.