The PD-L1 interactome demonstrates bidirectional signaling coordinating immune suppression and cancer progression in HNSCC.

The programmed death receptor 1 (PD-1) and ligand (PD-L1) are validated cancer targets; however, emerging mechanisms and impact of PD-L1 intracellular signaling on cancer behavior are poorly understood. PD-L1 intracellular signaling increased clonogenicity, motility, and invasiveness in multiple head and neck squamous cell carcinoma (HNSCC) models, and PD-1 binding enhanced these effects. Protein: protein proximity labeling revealed the PD-L1 interactome, distinct for unbound and bound PD-1, which initiated cancer cell-intrinsic signaling. PD-L1 binding partners interleukin enhancer-binding factors 2 and 3 transduced their effect through STAT3. PD-L1 intracellular domain deletion (Δ260-290) disrupted signaling and reversed pro-growth properties. In humanized HNSCC in vivo models bearing T cells, PD-1 binding triggered PD-L1 signaling, and dual PD-L1 and STAT3 inhibition were required to achieve tumor control. Upon PD-1 binding, the PD-L1 extracellular and intracellular domains exert a synchronized effect to promote immune evasion by inhibiting T cell function while simultaneously enhancing cancer cell invasive properties.