Regulation of PD-L1 expression in the tumor microenvironment

Abstract

Programmed death-ligand 1 (PD-L1) on cancer cells engages with programmed cell death-1 (PD-1) on immune cells, contributing to cancer immune escape. For multiple cancer types, the PD-1/PD-L1 axis is the major speed-limiting step of the anti-cancer immune response. In this context, blocking PD-1/PD-L1 could restore T cells from exhausted status and eradicate cancer cells. However, only a subset of PD-L1 positive patients benefits from α-PD-1/PD-L1 therapies. Actually, PD-L1 expression is regulated by various factors, leading to the diverse significances of PD-L1 positivity. Understanding the mechanisms of PD-L1 regulation is helpful to select patients and enhance the treatment effect. In this review, we focused on PD-L1 regulators at the levels of transcription, post-transcription, post-translation. Besides, we discussed the potential applications of these laboratory findings in the clinic.

Keywords: Cancer immunology; PD-1; PD-L1; Post-transcriptional modification; Transcriptional regulation.

Fig. 1

The regulators of PD-L1 expression. PD-L1 abundance is regulated by genomic alterations (amplification or translocation), epigenetic modifications (methylation of histone or CpG island, and histone acetylation), transcriptional regulation (inflammatory stimuli and oncogenic signals), post-transcriptional regulation (miRNA, the status of 3′- UTR, RAS, and Angiotensin II), and post-translational modification (ubiquitination, phosphorylation, glycosylation, palmitoylation). H3K4me3: tri-methylation of histone H3 on lysine 4; H3K27me3: tri-methylation of histone H3 on lysine 27; EGFR: epidermal growth factor receptor; IRF: interferon-responsive factor; IFN: interferon; DSB: double-strand break; GSK3β: glycogen synthase kinase 3β; PI3K: phosphoinositide 3-kinase; NF-κB: Nuclear factor kappa-B; HIF-1α: hypoxia-inducible factor-1α; ALK: Anaplastic lymphoma kinase; ER: endoplasmic reticulum

Fig. 2

The current immune checkpoint inhibitor therapy could be further enhanced by regulating PD-L1 expression. Agents regulating PD-L1 expression might have a synergistic effect with the current immune checkpoint inhibitors. For example, targeting therapies such as CDK4/6 inhibitor upregulated PD-L1 expression and promoted immune escape. This treatment-induced immune evasion could be overcome by combination therapies containing α-PD-1/PD-L1 (the left panel). Besides, adjuvant treatment regulating PD-L1 expression might elevate the sensitivity to α-PD-1/PD-L1 or other immune checkpoint inhibitors. For instance, metformin downregulated PD-L1 by promoting endoplasmic-reticulum-associated degradation, and the combination therapy of metformin and α-CTLA-4 had a synergistic antitumor activity (the right panel). CDK: cyclin-dependent kinase; CTLA-4: cytotoxic T Lymphocyte antigen 4; ER: endoplasmic reticulum