In arthritis, activated FLS contribute to pathogenesis by damaging synovial membranes and secreting inflammatory cytokines, which in turn recruit immune cells.32,44,45 In line with these reports, data replotted from a recently published single-cell RNA-seq data set of FLS from 51 OA and RA patients68 confirm detectable levels of proinflammatory genes IL-6, KC, MCP-1, IL-8, IL1-R1, SDF-1, and Fractalkine (Fig. 2A), which matches our mouse data in Figure 1. Here, IL1R1 is linked to rheumatoid arthritis.