New insights into macrophage heterogeneity in rheumatoid arthritis

•Recent single cell RNA sequencing studies of human and mouse joint tissue revealed high heterogeneity of lining and interstitial macrophages in health and disease.•Human and mouse tissue resident macrophages partially resemble one another.•Adult synovial tissue resident macrophages comprise a mixture of cells arising from embryonic and adult precursors.•Embryonic origin of synovial tissue resident macrophages subsets remain to be formally demonstrated.•Arthritis disrupts lining and sub-lining macrophage niches and extends macrophages heterogeneity.