Using in vitro reconstitution of vesicles, cellular reconstitution, and cell culture assays, Zhao et al. found that in antigen-presenting cells (APCs) – including tumor cells – co-expressing PD-1 and PD-L1, the two bind in cis with high affinity, preventing PD-L1 from engaging in trans with T cells. Selective anti-PD-1 antibody blockade of cis signaling between PD-1 and PD-L1 on APCs freed up PD-L1 to engage with T cells and triggered exhaustion. This research indicates that PD-1 negatively regulates PD-L1 in cis and that the net outcome of the PD-1/PD-L1 pathway depends on PD-1 expression on both APCs and T cells.
The PD-1 pathway, consisting of the co-inhibitory receptor PD-1 on T cells and its ligand (PD-L1) on antigen-presenting cells (APCs), is a major mechanism of tumor immune evasion. PD-1 and PD-L1 blockade antibodies have produced remarkable clinical activities against a subset of cancers. Binding between T cell-intrinsic PD-1 and APC-intrinsic PD-L1 triggers inhibitory signaling to attenuate the T cell response. Here, we report that PD-1 is co-expressed with PD-L1 on tumor cells and tumor-infiltrating APCs. Using reconstitution and cell culture assays, we demonstrate that the co-expressed PD-1 binds to PD-L1 in cis. Such interaction inhibits the ability of PD-L1 to bind T cell-intrinsic PD-1 in trans and, in turn, represses canonical PD-L1/PD-1 inhibitory signaling. Selective blockade of tumor-intrinsic PD-1 frees up tumor-intrinsic PD-L1 to inhibit T cell signaling and cytotoxicity. Our study uncovers another dimension of PD-1 regulation, with important therapeutic implications.