Regulatory mechanisms of PD-1/PD-L1 in cancers

Abstract

Immune evasion contributes to cancer growth and progression. Cancer cells have the ability to activate different immune checkpoint pathways that harbor immunosuppressive functions. The programmed death protein 1 (PD-1) and programmed cell death ligands (PD-Ls) are considered to be the major immune checkpoint molecules. The interaction of PD-1 and PD-L1 negatively regulates adaptive immune response mainly by inhibiting the activity of effector T cells while enhancing the function of immunosuppressive regulatory T cells (Tregs), largely contributing to the maintenance of immune homeostasis that prevents dysregulated immunity and harmful immune responses. However, cancer cells exploit the PD-1/PD-L1 axis to cause immune escape in cancer development and progression. Blockade of PD-1/PD-L1 by neutralizing antibodies restores T cells activity and enhances anti-tumor immunity, achieving remarkable success in cancer therapy. Therefore, the regulatory mechanisms of PD-1/PD-L1 in cancers have attracted an increasing attention. This article aims to provide a comprehensive review of the roles of the PD-1/PD-L1 signaling in human autoimmune diseases and cancers. We summarize all aspects of regulatory mechanisms underlying the expression and activity of PD-1 and PD-L1 in cancers, including genetic, epigenetic, post-transcriptional and post-translational regulatory mechanisms. In addition, we further summarize the progress in clinical research on the antitumor effects of targeting PD-1/PD-L1 antibodies alone and in combination with other therapeutic approaches, providing new strategies for finding new tumor markers and developing combined therapeutic approaches.

Keywords: Combination therapy; PD-1; PD-L1; Regulatory mechanism; Tumor immunity.

Fig. 1

The immune regulation mechanism of the PD-1/PD-L1 axis. Antigen-presenting cells (APCs) deliver tumor antigens to the T-cell receptor (TCR) via the major histocompatibility complex(MHC), and when the MHC-antigen complex specifically binds to the TCR, it triggers a series of signal transductions, including the phosphatidylinositol signaling and mitogen- activated protein kinases signaling pathways, thus activating the immune responses of effector T cells. Upon PD-L1 binding to PD-1, phosphorylation of tyrosine residues in the ITSM and ITIM domains of the PD-1 cytoplasmic region occurs, recruiting and activating SHP2. Subsequently, recruited SHP-2 mediates dephosphorylation of TCR-associated CD3 and ZAP70 signalosomes, while inhibiting CD28 co-stimulatory signals. This further attenuates downstream TCR signaling strength and cytokine secretion, such as IL-2, ultimately inhibiting the function of T cells. Figure created with BioRender

Fig. 2

Role of PD-1/PD-L1 in transplantation and autoimmune diseases. During organ transplantation, PD-1 is highly expressed on the surface of infiltrating T cells in grafts. The negative regulatory signal mediated by PD-1/PD-L1 can inhibit the excessive activation of T cells, induce immune tolerance, and effectively reduce the immune rejection between the host and the donor after surgery. Blocking PD-1/PD-L1 promotes proliferation of infiltrating T cells in grafts, exacerbating post-transplant immune rejection reactions and inducing severe and persistent tissue damage. Similarly, the disruption of the balance between PD-1 and PD-L1 signals can lead to the occurrence of many autoimmune diseases, such as type 1 diabetes mellitus (T1DM), multiple sclerosis (MS), systemic lupus erythematosus (SLE), and rheumatoid arthritis (RA). Figure created with BioRender

Fig. 3

The regulation of PD-1/PD-L1 signaling on immune cells. a. The PD-1/PD-L1 pathway promotes the exhaustion and apoptosis of effector T cells. Tex cells are characterized by increased expression of highly inhibitory receptors, including PD-1, LAG3, and TIGIT, decreased cytokine production such as TNF, IL-2, and IFN-γ, metabolic alterations, and impaired proliferative capacity and survival. b. PD-1/PD-L1 promotes the generation and development of induced Tregs (iTregs) by reducing the phosphorylation of PI3K/Akt/mTOR and S6, while enhancing PTEN, thus enhancing the immune suppression functions of Treg cells and inducing immune tolerance. c. PD-1/PD-L1 can promote the polarization of tumor-associated macrophages (TAM) toward the M2 phenotype, releasing large amounts of fibroblast growth factor, VEGF, TNF-α, and other cytokines to promote angiogenesis and support immune suppression, invasion, and metastasis of cancer cells, accelerating cancer progression. d. PD-1 on NK cells binds to PD-L1 on cancer cells, inhibiting the degranulation and cytotoxic function of NK cells, decreasing their ability to kill tumor cells, and promoting tumor immune escape. The use of PD-1 and PD-L1 inhibitors may reactivate the anti-tumor immune response of the above immune cells. Figure created with BioRender

Fig. 4

PD-1/PD-L1 promotes tumorigenesis and development. PD-1/PD-L1 signaling can alter cellular energy synthesis and metabolic pathways by disrupting aerobic glycolysis, thereby promoting fatty acid oxidation (FAO) as the main source of energy in T cells. Additionally, NAMPT, a component of NAD⁺ metabolism, can enhance PD-L1 expression in tumor cells through Stat1-dependent IFN-γ signaling, thereby inhibiting T cell function and remodeling the local tumor microenvironment, ultimately exerting significant effects on tumor metastasis, recurrence, and prognosis. Furthermore, the gut microbiota can also improve tumor progression and enhance T cell immune responses, suggesting its potential use in improving the efficacy of PD-1 blockade therapy. Figure created with BioRender

Fig. 5

The effects of nuclear PD-L1 on the tumor itself. PD-L1 not only promotes tumor cells to evade immune surveillance but also facilitates tumor progression in an immune-independent manner. PD-L1, which is expressed at high levels in tumor cells, can enter the nucleus by binding to the nuclear entry protein KPNB1 and exert pro-cancer effects. Nuclear PD-L1 can also trigger the upregulation of immune checkpoint genes, including PD-L2 and VISTA, thereby enhancing the anti-tumor responses of PD-1 inhibition. Under hypoxic conditions, treatment with TNFα and CHX can promote the nuclear translocation of PD-L1, which then interacts with p-Stat3-Y705. Subsequently, p-Stat3-Y705 combines with the GSDMC promoter region, leading to the upregulation of GSDMC gene expression. Furthermore, GSDMC is cleaved and activated by caspase-8, triggering pyroptosis in cells and necrosis in the hypoxic areas of tumors. Figure created with BioRender

Fig. 6

Genetic and transcriptional regulatory mechanisms of PD-L1 expression. At the gene level, tumors with PD-L1 gene amplification in chromosome 9p24.1 have a higher mutation load and a close correlation with increased PD-L1 expression. Epigenetic modifications of the PD-L1 promoter region, including DNA methylation, histone methylation, and acetylation, are also important in the regulation of PD-L1 expression. For example, TNF-α/TGFβ1 induces demethylation of the PD-L1 promoter by decreasing DNMT1 (DNA methyltransferase) levels, resulting in PD-L1 up-regulation and thus exerting immunosuppressive effects. At the transcriptional level, PD-L1 expression is primarily regulated by transcription factors, including STAT, MYC, NF-κB, IRF1, AP-1, and HIF-1α, as well as signaling pathway effector molecules such as MAPK/PI3K/Akt, JAK/STAT3, and EGFR/MAPK. Figure created with BioRender

Fig. 7

Mechanisms of PD-L1 expression regulation at post-transcriptional and post-translational. Non-coding RNAs such as miR-34, miR-200, and miR-197 can suppress PD-L1 mRNA expression by directly binding to PD-L1 3′UTR. The m⁶A modification of PD-L1 mRNA is critical for regulating the expression and stability of PD-L1 and mediating tumor immune escape. For instance, demethylases such as FTO and ALKBH can remove the m⁶A modification on PD-L1 mRNA, increasing its stability and promoting high PD-L1 expression. The METTL3/IGF2BP3 axis also enhances the stability of PD-L1 mRNA by upregulating its m⁶A modification, further promoting tumor immune evasion. Additionally, post-translational modifications, including phosphorylation, ubiquitination, glycosylation, and palmitoylation can regulate PD-L1 protein expression by influencing its activity, stability, and membrane expression in cancer cells. Figure created with BioRender