Locally renewing resident synovial macrophages provide a protective barrier for the joint [RNA-seq]

Macrophages (MFs) are considered to contribute to chronic inflammatory diseases such as rheumatoid arthritis 1. However, both the exact origin and role of MFs during inflammatory joint disease remain unclear. Here, we used multiple fate-mapping approaches in conjunction with 3D-light-sheet fluorescence microscopy and single cell RNA sequencing to perform a comprehensive spatiotemporal analysis of the composition, origin and differentiation of MF subsets within the healthy and inflamed joint and subsequently studied their roles during arthritis. This approach revealed dynamic membrane-like structures consisting of a distinct population of CX3CR1+ tissue-resident MFs that formed an internal immunological barrier at the synovial lining and physically secluded the joint. Barrier-forming MFs displayed features otherwise typical of epithelial cells, and maintained their numbers through a pool of locally proliferating CX3CR1- mononuclear cells embedded into the synovial tissue. Unlike recruited monocyte-derived MFs, which actively contributed to joint inflammation, such epithelial-like CX3CR1+ lining MFs restricted the inflammatory reaction by providing a tight junction-mediated shield for intra-articular structures. Our data thus reveal an unexpected functional diversification among synovial MFs and have important implications for the general role of MFs in health and disease. Overall design: mRNA profiles of sorted GFP+ lining macrophages and GFP- interstitial macrophages of healthy Cx3cr1-GFP mice and in vitro differentiated bone marrow-derived macrophages (BMDM) were generated by bulk RNA sequencing, in triplicate.