MT2A and rheumatoid arthritis: The two clusters identified as pro-inflammatory macrophage populations (IL-1B+CCL20+ and SPP1+MT2A+) are specifically enriched in RA compared to healthy synovial tissue; in fact, the SPP1+ cluster appears to be absent in healthy synovium, indicating that it is a disease-specific myeloid subpopulation (Fig. 5, G and H).