KRT18 and COVID-19: In addition to an increased level of PD-L1 in pleural effusion (Figure 2), the expression of PD-L1 on macrophages also shows enhancement in lung tissues of deceased COVID-19 patients (Figure 3), i.e., the distribution of the PD-L1 staining is similar to CD68 (macrophages), but different from CK18 (epithelial) and CD34 (endothelial), as previously reported by [8].