KLRK1 and COVID-19: Thus, the functions of S309-CAR-NK cells in controlling COVID-19 progression can be classified into two aspects: 1) The direct binding of S309-CAR molecules to free SARS-CoV-2 virus particles; 2) The activation of CAR-NK cells via the binding of S309-CAR or NKG2D or other natural receptors exerts natural cytotoxicity to directly kill SARS-CoV-2-infected cells.