Cyclin D–CDK4 kinase destabilizes PD-L1 via cullin 3–SPOP to control cancer immune surveillance

Abundance of PD-L1, the ligand of the anti-cancer immunotherapy target PD-1, is negatively regulated by poly-ubiquitination via the cyclin D–CDK4/cullin 3–SPOP axis and PD-1 blockade treatment in mice improved survival when combined with CDK4/6 inhibitors. Immune checkpoint blockade therapy with PD-1 or PD-L1 inhibitors has been approved for the treatment of several types of cancer, but only a small percentage of patients respond to this from of treatment. Wenyi Wei and colleagues report that the abundance of the PD-L1 protein is negatively regulated during cell cycle progression, in part through the activity of the upstream cyclin D–CDK4–SPOP–FZR1 signalling pathway. CDK4/6 inhibitor treatment increases PD-L1 levels and promotes immune evasion in vitro and in vivo. On the other hand, combined treatment with a CDK4/6 inhibitor and a PD-1 blocker has greater anti-tumour efficacy than treatment with each drug alone in mouse models. This combined approach has the potential to improve the treatment of patients with cancer. Treatments that target immune checkpoints, such as the one mediated by programmed cell death protein 1 (PD-1) and its ligand PD-L1, have been approved for treating human cancers with durable clinical benefit1,2. However, many patients with cancer fail to respond to compounds that target the PD-1 and PD-L1 interaction, and the underlying mechanism(s) is not well understood3,4,5. Recent studies revealed that response to PD-1–PD-L1 blockade might correlate with PD-L1 expression levels in tumour cells6,7. Hence, it is important to understand the mechanistic pathways that control PD-L1 protein expression and stability, which can offer a molecular basis to improve the clinical response rate and efficacy of PD-1–PD-L1 blockade in patients with cancer. Here we show that PD-L1 protein abundance is regulated by cyclin D–CDK4 and the cullin 3–SPOP E3 ligase via proteasome-mediated degradation. Inhibition of CDK4 and CDK6 (hereafter CDK4/6) in vivo increases PD-L1 protein levels by impeding cyclin D–CDK4-mediated phosphorylation of speckle-type POZ protein (SPOP) and thereby promoting SPOP degradation by the anaphase-promoting complex activator FZR1. Loss-of-function mutations in SPOP compromise ubiquitination-mediated PD-L1 degradation, leading to increased PD-L1 levels and reduced numbers of tumour-infiltrating lymphocytes in mouse tumours and in primary human prostate cancer specimens. Notably, combining CDK4/6 inhibitor treatment with anti-PD-1 immunotherapy enhances tumour regression and markedly improves overall survival rates in mouse tumour models. Our study uncovers a novel molecular mechanism for regulating PD-L1 protein stability by a cell cycle kinase and reveals the potential for using combination treatment with CDK4/6 inhibitors and PD-1–PD-L1 immune checkpoint blockade to enhance therapeutic efficacy for human cancers.