Besides well-known B7-CD28 binding, it has been shown that the binding between PD-1 and its ligands PD-L1/L2 is also involved in costimulatory signaling. And the existence of non-PD-1 ligand(s) for PD-L1/L2 has been speculated.
So I wish to raise the following three questions:
1. Can we identify such human ligand(s) using e.g. yeast two-hybrid method?