HOXD13 and COVID-19: Moreover, in independent studies that included COVID-19 patients with severe to critical disease, increased levels of specific sICPs, such as the non-classic histocompatibility antigen G (HLA-G), sCD279, soluble programmed cell death 1 (sPD-1), soluble programmed cell death-ligand 1 (sPDL1), sTIM-3, and soluble CD137 (sCD137) (12, –, 18) were found in peripheral blood.