Table 1.

Comparison of CD28 and CD28-like receptors

	Percent Identity	Chromosome		Structure		Expression	Ligand
	(with CD28 extracellular domain)	Human	Mouse	Ligand-binding motif	Cytoplasmic tyrosine signaling motifs	Cell type
CD28	100%	2q33	1C1	MYPPPY	PI3K motif, PP2A	T, plasma cell, NK, NKT	B7-1, B7-2, ICOSL (only human)
CTLA-4 (CD152)	30%	2q33	IC2	MYPPPY	PI3K motif, PP2A, SHP2, PKC-η	T, B, NK, NKT, DC	B7-1, B7-2
ICOS (CD278)	27%	2q33	1C2	FDPPPF	PI3K	T, B (only human)	ICOSL
PD-1 (CD279)	23%	2q37	1D		ITIM, ITSM, SHP1, SHP2	T, B, NK, NKT, DC, myeloid	PD-L1, PD-L2
TMIGD2 (IGPR2)	10%	3q13.2	16A1		Y192 and Y222	only human: T, APC	HHLA2

Abbreviations: act. = activated (cell type); B = B cell; DC = dendritic cell; mφ = macrophage; mo = monocyte; MSC = mesenchymal stem cells; NK = natural killer cell; NKT = natural killer cell T cell; T = T cell.