CD273, also known as B7-DC or PD-L2, is a 42 kDa type I transmembrane glycoprotein belonging to the B7 family of the immunoglobulin superfamily. It is primarily expressed in dendritic cells, liver, and a few transformed cell lines. CD273 acts as a ligand for the coinhibitory receptor PD-1 (CD279), playing a crucial role in immune regulation by inhibiting T cell activation, proliferation, and cytokine production. Unlike PD-L1, CD273 has a higher affinity for PD-1 and is particularly upregulated in the presence of Th2 cytokines such as IL-4. Additionally, CD273 can also bind to repulsive guidance molecule b (RGMb), which is involved in regulating immune responses through BMP and neogenin signaling pathways.