Figure 1: Role of CTLA-4 in T-cell activation. (1) CTLA-4 is a negative regulator of T-cell activation. (2) Conventional T cells are activated by engagement of MHC and B7. (3) Upon activation, T-cells express CTLA-4. Binding of CTLA-4 with B7 inhibit T cell activation. (4) Blockade of CTLA-4 produces the liberation of CD28 that engages with B7 activating T cells. APC: antigen presenting cell; MHC: major histocompatibility complex; and TCR: T-cell receptor.