Non-PD-1 ligand(s) for PD-L1/L2?

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?

2. If so, can we determine the stoichiometry of the protein complex composed of PD-L1/L2 and speculated non-PD-1 ligand(s) using AFM in solution?

The question below is slightly related to the blockade of PD-1-PD-L1/L2 binding. (see, e.g., here and here.)

3. Can we measure the binding force between PD-L1/L2 and speculated non-PD-1 ligand(s) using AFM?