Table 1.

Characteristics and distribution of T cell co-stimulatory molecules^a

Name	Cell expression	Naïve T-cell expression?	Inducible expression?	Ligand
CD28	T cells	Yes	Yes	B7.1 or B7.2
ICOS	T cells, NK cells		Yes	ICOSL
OX40	T, B cells, DCs, eosinophils		Yes	OX40L
4-1BB	T cells, DCs, macrophages, NK cells, eosinophils		Yes	4-1BBL
CD27	T and B cells	Yes	Yes	CD70
CD30	T and B cells		Yes	CD30L
CD40L	T and B cells, NK cells, DCs basophils, mast cells, eosinophils		Yes	CD40
HVEM	T and B cells, monocytes, DCs	Yes	Nob	LIGHT
CTLA-4	T cells		Yes	B7.1 or B7.2
PD-1	T and B cells, myeloid cells		Yes	PD-L1 or PD-L2

^aAbbreviations: CTLA-4, cytotoxic T lymphocyte antigen-4; DCs, dendritic cells; HVEM, herpesvirus entry mediator; ICOS, inducible co-stimulatory molecule; L, ligand; NK, natural killer; PD-1, program death-1.

^bDownregulated.