ZAP70 and neoplasm: ,10 Also, the interaction between PD-1 or PD-2 on the surface of T cells and its ligand PD-L1 on the surface of tumor cells or antigen-presenting calls (APCs) suppresses ζ-chain-associated protein kinase (ZAP70) by SH2 containing protein tyrosine phosphatase-2 (SHP2).