PF4 and COVID-19: In the interactions between CD14+ monocytes and HLA_DR+ Tregs, ligand-receptor pairs including TNF_VSIR, TNF_TNFRSF1B, TNF_TNFRSF1A, and TNF_RIPK1 were weakened in the SA, SP, and SR groups (Figure 4E), while the ligand-receptor pair of PF4_CXCR3 was enhanced in the eight COVID-19 patient groups, except for the healthy controls (Figure 4E).