GITR/GITRL: More than an effector T cell co-stimulatory system

Glucocorticoid-induced TNFR-related protein (GITR) is a member of the TNFR superfamily, expressed in several cells and tissues including T lymphocytes, NK cells and antigen-presenting cells (APC). GITR activation, upon interaction with its ligand (GITRL), functions as a co-activating signal. GITRL is mainly expressed on APC and GITR/GITRL interaction is important for the development of immune response. This review summarizes recent results about the GITR/GITRL system, focusing on the interplay between APC, effector and regulatory T cells.