SPP1 and rheumatoid arthritis: The split-points and distances in the second branch of hierarchical clustering indicate that the proinflammatory clusters (i.e., FCN1+ and FCN1+SPP1+) that dominate in severe COVID-19 share transcriptomic profiles with proinflammatory clusters (i.e., CD48hiS100A12+ and CD48+SPP1+, respectively) in active RA.