SPP1 and rheumatoid arthritis: These transcriptomic profiles suggest that BALF FCN+ and FCN+SPP1+ macrophage clusters predominant in severe COVID-19 shared pathogenic molecular pathways, including the expression of their signature mediators S100A12 and SPP1 (Figure 1H) with ST CD48hiS100A12+ and CD48+SPP1+ clusters predominant in active RA.