CD4 and COVID-19: Figures S1–S5 visually represent these results of the FACS analyses. As shown in Figure S6 and Table 5, the relative proportion of monocytes, macrophages, and NK cells expressing CD200R was significantly higher in COVID-19 patients than in healthy controls, suggesting the involvement of this immune checkpoint in the immunological response to COVID-19 infection of these immune cells. Moreover, TIGIT+CD16+CD45+ (p < 0.0001), TIGIT+CD56+CD45+ (p = 0009), and TIGIT+CD39+CD4+CD3+CD45+ (p = 0.0017) cells were significantly increased in COVID-19 when compared with healthy controls.