SPP1 and COVID-19: SPP1 and CD68 staining of postmortem lung tissue confirmed abundant clusters of SPP1+ macrophages in alveoli of COVID-19 patients (n = 2), while it was rare in normal lung (n = 3; Figure 5, A and B and Supplemental Table 2) and sparse in alveoli of bacterial (n = 3) and H1N1 (n = 3) pneumonia (Supplemental Figure 9, A and B).