MERTK and rheumatoid arthritis: Three tissue macrophage clusters (M-0, M-1 and M-2) were abundant in both osteoarthritis and rheumatoid arthritis synovium and expressed the phagocytic factors CD206 (also known as macrophage mannose receptor (MMR)) and CD163 and MERTK (Extended Data Fig. 6b–d), suggesting a homeostatic debris-clearing function25,26.