Figure 1. CTLA-4 is a negative regulator of T cell activation.
Conventional T cells are activated by engagement of MHC (signal 1) and B7 (signal 2). Upon activation, T cells express CTLA-4 on the cell surface. CTLA-4 engagement with B7 inhibits T cell activation. Antibody blockade of CTLA-4 interaction with B7 prevents this inhibitory signal.