Fig. 1

The signaling pathway of PD-1/PD-L1. a PD-L1 consists of an extracellular domain, a transmembrane domain, and an intracytoplasmic region but lacks intracellular signaling. The intracytoplasmic region consists of three conserved sequences including RMLDVEKC, DTSSK, and QFEET motifs. The part of the RMLDVEKC motif and the entire DTSSK motif that have been identified by MotifFinder are RNA pol-like motifs. b Antigens are presented by APCs as antigenic peptides, which are recognized by the T-cell receptor (TCR; Signal 1). The second signal (Signal 2) is delivered when B7 (CD80 and CD86) on the APCs engage CD28 on the T cells