PDCD1 and neoplasm: PD-L2’s binding affinity to PD-1 is two- to six-fold higher than that of PD-L1 (Ghiotto et al. 2010), and it competes with PD-L1 to bind PD-1 (Ghiotto et al. 2010) and inhibits the anti-tumor immunity through suppressing CD8+ T cells (Latchman et al. 2001).