Figure 2. Costimulatory and coinhibitory molecules and their receptors.

Costimulatory molecules, expressed on APCs, engage receptors on T cells concurrent with antigen recognition and induce signals that are required for naive T cell activation or that enhance effector/memory T cell responses. Coinhibitory receptors (CTLA-4 and PD-1) are expressed on activated T cells and, upon binding ligands on APCs, inhibit T cell responses. The major costimulatory and coinhibitory molecules, which belong to the B7/CD28 families or TNF/TNFR superfamilies, are shown in the table.