Nonclassical Ly6C− Monocytes Drive the Development of Inflammatory Arthritis in Mice

• Ly6C−, but not Ly6C+, monocytes are crucial for initiation of arthritis
• Ly6C− monocytes give rise to inflammatory macrophages during arthritis
• Tissue-resident synovial macrophages limit development of arthritis
• Recruited macrophages change phenotype from M1 to M2 in situ

SummaryDifferent subsets and/or polarized phenotypes of monocytes and macrophages may play distinct roles during the development and resolution of inflammation. Here, we demonstrate in a murine model of rheumatoid arthritis that nonclassical Ly6C− monocytes are required for the initiation and progression of sterile joint inflammation. Moreover, nonclassical Ly6C− monocytes differentiate into inflammatory macrophages (M1), which drive disease pathogenesis and display plasticity during the resolution phase. During the development of arthritis, these cells polarize toward an alternatively activated phenotype (M2), promoting the resolution of joint inflammation. The influx of Ly6C− monocytes and their subsequent classical and then alternative activation occurs without changes in synovial tissue-resident macrophages, which express markers of M2 polarization throughout the course of the arthritis and attenuate joint inflammation during the initiation phase. These data suggest that circulating Ly6C− monocytes recruited to the joint upon injury orchestrate the development and resolution of autoimmune joint inflammation.

Graphical AbstractFigure optionsDownload as PowerPoint slide