Figure 1. PD-1/28, a chimeric co-stimulatory converter. PD-1/28 is composed of the extracellular domain of the co-inhibitory receptor programmed cell death 1 (PDCD1, best known as PD-1) and the intracellular domain of the co-stimulatory molecule CD28. Upon binding to PD-1 ligands expressed on the surface of cancer cells, PD-1/28 results in increased cytokine secretion, upregulation of T-cell activation markers, improved proliferative potential and superior antitumor activity in xenograft models of human melanoma.