Figure 1.
(a) T cell activation requires both signal 1, mediated by antigen presentation on MHC by dendritic cells to the T‐cell receptor (TCR), and costimulatory signals from CD80/86 engagement with CD28 on the surface of T cells. This initial activation sends signals to release cytotoxic T‐lymphocyte‐associated antigen‐4 (CTLA‐4) from intracellular vesicles. (b) Downregulation of T cell activity can occur distinctly through two mechanisms. CTLA‐4 is upregulated on the surface of T cells in response to initial activation, and outcompetes binding to CD80/86. In the periphery, tumor cells can present programmed death ligand‐1 to programmed death‐1 on the surface of T cells and also induce downregulation of T cell activity. APC, antigen‐presenting cell; MHC, major histocompatibility complex.