TABLE 1.

The expression and mechanism of the immune checkpoints.

Immune checkpoints	Expression	Ligand	Mechanisms	PMID
PD-1	Activated T cells, Tregs, B cells, NK cells, DCs, macrophages, and monocytes	PD-L1 and PD-L2	ITSM recruits SHP-2, which acts as a bridge between two PD-1 molecules and induces inhibitory function of PD-1	30851633, 32184441, and 28443090
CTLA-4	Activated T cells and Tregs	CD80 and CD 86	Conserved YVKM motif in the cytoplasmic tail of CTLA-4 mediates recruitment of SH2-domain-containing proteins to regulate immune response	10411922, 18845758, and 29794465
LAG-3	Activated T cells, Tregs, NK cells, DCs, and B cells	MHC-II, LESCtin, Galectin-3, FGL-1, and α-synuclein	The KIEELE motif is considered to be essential for LAG-3 mediated inhibition	33488626 and 34067904
TIM-3	Activated T cells, TH17 cells, Tregs, DCs, NK cells, and monocytes	Galectin-9, CEACAM-1, HMGB-1, and PtdSer	TIM-3 exerts its function through several tyrosine residues	29069302 and 31676858
VISTA	Myeloid cell, T cells, and Tregs	VSIG-3 and PSGL-1	VISTA has the potential function of both a receptor and a ligand. The precise mechanism of VISTA needs to be explored	29375120 and 31690319
IDO1	A heme-containing enzyme participates in tryptophan metabolism	L-Tryptophan	Accumulation of kynurenine metabolites leads to suppression of T cells and induction of Tregs	20720200 and 33883013