TIGIT and COVID-19: Other examples include circulating PD-L1, which induces immune suppression and damage, and associates with COVID-19 pathogenesis and mortality23, soluble LILRB4 (sLILRB4), which can be produced by a splice variant24 and suppresses T cell responses and elicits T cell anergy or activation of Treg or T suppressor cells24–26, and PVR (CD155) which is a ligand for CD226 (DNAM-1) and TIGIT expressed on NK cells and a subset of T cells27, but the soluble form inhibits NK cells CD226-mediated cytokine production28.