CD4 and COVID-19: Notably, as shown in Figure 7C, Granzyme B was observed to be significantly highly expressed in COVID-19 patients versus healthy control across all identified immune cell types (all p < 0.05), TMEM173 was substantially up-regulated in COVID-19 patients particularly in monocytes, neutrophils, and CD4 T cells; and PD-L1 expression remained low for all cells, indicating the immune system of COVID-19 patients was highly activated without exhaustion regardless of disease severity, which was not explored previously.