Fig. 3

T cell activation and inactivation mechanism. a upon infection the full activation of specific T cell immune response requires two signals; binding of MHC/Ag complex on the APC (ex: DC) with T cell receptor (TCR) and interaction of B7-molecules (CD80/86) to their ligand (CD28) on the surface of T cell. b Activated T cell expresses a surface immunosuppressive molecule called CTLA-4 which compete with CD28 molecule to bind the B7 molecules. The balance between activation and inactivation signals keeps cytotoxic activity in check, while allowing T-cell function to work in a self-limited manner. c Tumor cells produce a suppressive cytokine that lead to the upregulation of CTLA-4 on the surface of T cells. This mechanism allows tumor cells to evade the cytotoxic effect of T cells