CO-STIMULATION IN T-CELL RESPONSES

Antigen-specific T cell responses have primarily been considered in te rms of activation signals delivered through the TCR and the co-stimula tory molecule CD28. In the past few years, studies have demonstrated t he critical importance of inhibitory signals for regulating lymphocyte activation. CD28 and its homologue cytotoxic T lymphocyte antigen-4 ( CTLA-4) share the same counter-receptors on antigen-presenting cells, but recent experiments have shown that CD28 and CTLA-4 have opposite e ffects on T cell activation. The mechanisms responsible for integratin g these activation and inhibitory signals at the cellular and molecula r levels are just beginning to be elucidated.