T cell costimulatory blockade: new therapies for transplant rejection.

Optimal T cell responses occur when T cells receive both antigen-specific signals through the T cell receptor and non-antigen-specific costimulatory signals through accessory cell surface molecules. The best understood costimulatory receptor is CD28. Signals through the T cell receptor and CD28 cooperatively induce cytokine gene expression and promote T cell proliferation and survival. Negative signals delivered through a related cell surface receptor, cytotoxic T lymphocyte antigen (CTLA-4), act to terminate immune responses and are required for normal immune homeostasis. This article reviews T cell costimulation, including the CD28/CTLA-4 system and other potential costimulatory pathways (such as CD40/CD154), the role of these pathways in normal immune responses, and the potential for the inhibition of these pathways to induce transplantation tolerance.