Pharmacological tumor PDL1 depletion with chlorambucil treats ovarian cancer and melanoma: improves antitumor immunity and renders anti-PDL1-resistant tumors anti-PDL1-sensitive through NK cell effects

BackgroundTumor intracellular programmed cell death ligand-1 (PDL1) mediates pathologic signals that regulate clinical treatment responses distinctly from surface-expressed PDL1 targeted by alpha PDL1 immune checkpoint blockade antibodies.MethodsWe performed a drug screen for tumor cell PDL1 depleting drugs that identified Food and Drug Administration (FDA)-approved chlorambucil and also 9-[2-(phosphonomethoxy)ethyl] guanine. We used in vitro and in vivo assays to evaluate treatment and signaling effects of pharmacological tumor PDL1 depletion focused on chlorambucil as FDA approved, alone or plus alpha PDL1.ResultsPDL1-expressing mouse and human ovarian cancer lines and mouse melanoma were more sensitive to chlorambucil-mediated proliferation inhibition in vitro versus corresponding genetically PDL1-depleted lines. Orthotopic peritoneal PDL1-expressing ID8agg ovarian cancer and subcutaneous B16 melanoma tumors were more chlorambucil-sensitive in vivo versus corresponding genetically PDL1-depleted tumors. Chlorambucil enhanced alpha PDL1 efficacy in tumors otherwise alpha PDL1-refractory, and improved antitumor immunity and treatment efficacy in a natural killer cell-dependent manner alone and plus alpha PDL1. Chlorambucil-mediated PDL1 depletion was relatively tumor-cell selective in vivo, and treatment efficacy was preserved in PDL1KO hosts, demonstrating tumor PDL1-specific treatment effects. Chlorambucil induced PDL1-dependent immunogenic tumor cell death which could help explain immune contributions. Chlorambucil-mediated PDL1 reduction mechanisms were tumor cell-type-specific and involved transcriptional or post-translational mechanisms, including promoting PDL1 ubiquitination through the GSK3 beta/beta-TRCP pathway. Chlorambucil-mediated tumor cell PDL1 depletion also phenocopied genetic PDL1 depletion in reducing tumor cell mTORC1 activation and tumor initiating cell content, and in augmenting autophagy, suggesting additional treatment potential.ConclusionsPharmacological tumor PDL1 depletion with chlorambucil targets tumor-intrinsic PDL1 signaling that mediates treatment resistance, especially in alpha PDL1-resistant tumors, generates PDL1-dependent tumor immunogenicity and inhibits tumor growth in immune-dependent and independent manners. It could improve treatment efficacy of selected agents in otherwise treatment-refractory, including alpha PDL1-refractory cancers, and is rapidly clinically translatable.