Proximal Proteomics Reveals Potential Regulators of Programmed Death-Ligand 1 in Human Cancer

Programmed death-ligand 1 (PD-L1) is a key mediator of tumor immune evasion, but the protein networks governing its post-translational regulation remain incompletely understood. Here, we used PhastID-based proximity labeling to map the PD-L1 proximal proteome and integrated these data with pan-cancer proteomics and immunohistochemical analysis of clinical tumor samples. We found that C-terminal-proximal proteins were enriched in protein degradation, ER-associated quality control, and immune-related pathways, whereas N-terminal-proximal proteins were mainly associated with protein localization and intracellular transport. Integrated analysis identified CAND1, TXN, and FBXO2 as candidate PD-L1 regulators. Knockdown of CAND1, TXN, or FBXO2 reduced PD-L1 protein expression in cancer cells. Mechanistically, CAND1 interacted with PD-L1 and was detected in a complex containing PD-L1 and FBXO22. CAND1 depletion promoted K48-linked ubiquitination and degradation of PD-L1, while deletion of the PD-L1 intracellular domain or mutation of K271 and K281 attenuated PD-L1 ubiquitination. Functionally, CAND1 knockdown enhanced T cell-mediated tumor cell killing, and elevated CAND1 expression was associated with an immunosuppressive tumor microenvironment. Together, our findings define the PD-L1 proximal proteomic landscape and identify CAND1-mediated regulation of PD-L1 stability as a potential mechanism of tumor immune escape.