PD-1/PD-L1: Immune Checkpoint Signaling Balance and Differential Mechanisms of Inhibitors

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This article focuses on the molecular characteristics of the PD-1/PD-L1 immune checkpoint pathway and the mechanisms of action of its inhibitors, systematically elaborating the balance between T cell activation signals and inhibitory signals as well as the molecular pathways by which tumor cells utilize PD-L1 to achieve immune evasion, analyzing the differences between PD-1 and PD-L1 in terms of expressing cell types and expression regulatory conditions, and exploring the systematic differences between PD-1 inhibitors and PD-L1 inhibitors in antibody structure, mechanisms of action, and incidence of adverse reactions.

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PD-1/PD-L1: Immune Checkpoint Signal Balance and Mechanistic Differentiation of Inhibitors

Summary: This article focuses on the molecular characteristics of the PD-1/PD-L1 immune checkpoint pathway and the mechanisms of action of its inhibitors. It systematically elaborates on the balance between T-cell activation signals and inhibitory signals, and the molecular pathways by which tumor cells exploit PD-L1 to achieve immune evasion. It analyzes the differences between PD-1 and PD-L1 in terms of expressing cell types and expression regulatory conditions, and discusses the systematic differences between PD-1 inhibitors and PD-L1 inhibitors in antibody structure, mechanism of action, and incidence of adverse reactions.

I. Signal Balance of the Immune System and the Physiological Significance of Immune Checkpoints.

The human body has a complete self-protection system called the immune system, which can eliminate "foreign" components in the body, such as tumor cells, viruses, and bacteria, while avoiding damage to its own tissues and organs. In the immune system, T cell-mediated cellular immunity is the "main force" for eliminating tumors. T cells need to be activated to exert anti-tumor effects, and the magnitude and quality of T cell activation depend on the balance between activation signals and inhibitory signals——activating antibodies sending activation signals are like the accelerator of a car, while inhibitory antibodies sending inhibitory signals are like stepping on the brake.

Immune checkpoints can be considered the brake system of immune cells, referring to a series of molecules expressed on immune cells that can regulate the degree of immune activation, keeping T cell activation levels within a normal range and preventing damage to healthy tissues of the body. PD-1/PD-L1 and CTLA-4 are currently the most well-known immune checkpoints. This "accelerator-brake" balance framework is the foundation for understanding the physiological function and pathological significance of the PD-1/PD-L1 pathway——under physiological conditions, the brake system prevents autoimmune damage, while in the tumor state, the brake is abnormally exploited.

II. Molecular Characteristics of PD-1 and PD-L1 and Differences in Expression Regulation.

PD-1 (programmed death receptor 1) is an immune checkpoint receptor expressed on activated T cells and is an important immunosuppressive molecule. PD-1 regulates the immune system and promotes self-tolerance by down-regulating the immune system's response to human cells and by inhibiting T cell inflammatory activity. This can prevent autoimmune diseases, but it can also prevent the immune system from killing cancer cells. PD-L1 is the ligand of PD-1; PD-L1 is associated with immune system suppression and can transmit inhibitory signals.

At the level of expression regulation, there are noteworthy temporal and spatial differences between the two. PD-1 can be expressed on the surface of immune cells such as T cells and B cells, but when T cells are not activated, PD-1 is almost not expressed; only after T cell activation is PD-1 expressed on the T cell surface——this "activation-induced expression" feature means that inhibition of the PD-1 pathway mainly acts on immune responses that have already been initiated, rather than on resting T cells. In addition to being expressed on the tumor surface to participate in immune evasion, PD-L1 is also expressed on the surface of antigen-presenting cells (such as DC cells and macrophages) and vascular endothelial cells under IFN-γ stimulation. This "inflammation-induced expression" pattern means that the expression level of PD-L1 is closely related to the inflammatory state of the tumor microenvironment, and also constitutes the biological basis for using PD-L1 expression as a predictive biomarker of efficacy.

III. Molecular Pathways of Tumor Immune Evasion and the Intervention Principles of Inhibitors.

Our body possesses the ability to self-eliminate tumors, but there are always some "cunning" tumor cells that can escape immune surveillance and grow wantonly, harming the health of the body. Tumor cells precisely exploit the characteristics of immune checkpoints: tumor cells express PD-L1 protein on their surface, and once PD-1 and PD-L1 bind, they transmit negative regulatory signals to T cells, causing T cells to be unable to recognize cancer cells, thereby enabling tumor cells to achieve immune evasion.

Therapy that blocks the PD-1/PD-L1 pathway has become a focus of cancer immunotherapy. PD-1 monoclonal antibodies and PD-L1 monoclonal antibodies are drugs used for cancer immunotherapy. PD-1 monoclonal antibodies or PD-L1 monoclonal antibodies block the binding of PD-1 to PD-L1, restore T cell tumor activity, activate T cells, and kill tumor cells. Once the anti-tumor immune cycle is established, a durable anti-tumor effect can be produced. From a molecular perspective, the essence of this intervention is spatial blockade: antibodies occupy the ligand-receptor binding interface, preventing the transmission of negative signals, thereby relieving the inhibition of T cell activation.

IV. Systematic Differences Between PD-1 Inhibitors and PD-L1 Inhibitors.

Although the two classes of drugs act on the same pathway, they exhibit systematic differences across multiple dimensions.

In terms of drug structure, most PD-1 inhibitors are IgG4 antibodies, with T cells as the target cells, affecting the function of immune T cells; the IgG4 antibody structure has poor stability, so almost all PD-1 inhibitors have been modified to increase stability, and imperfect modifications may lead to adverse reactions. Most PD-L1 inhibitors are IgG1 antibodies, with tumor cells as the target cells, affecting tumor cells; IgG1 monoclonal antibodies are the most commonly used monoclonal antibodies, with a long half-life and good stability. This structural difference determines the pharmacokinetic characteristics and effector function profiles of the two classes of drugs.

In terms of mechanism of action, PD-1 inhibitors bind to PD-1 on the surface of T cells, blocking the binding of PD-1 to PD-L1/PD-L2, relieving the inhibition of T cell activation and proliferation, keeping tumor-specific T cells in an activated state, and restoring the killing function of T cells; however, PD-1 inhibitors simultaneously block the binding of PD-1 on the surface of T cells to PD-L1 and PD-L2 on the surface of immune cells, increasing the risk of potential autoimmune reactions. Compared with PD-1 inhibitors, PD-L1 inhibitors simultaneously bind to PD-L1 on the surface of tumor cells and antigen-presenting cells, resulting in stronger overall immune efficacy, restoring T cell-mediated anti-tumor immune effects, and not affecting the physiological function of PD-L2; PD-L1 inhibitors have the ability to block the binding of PD-L1 on the surface of tumor cells to B7-1 on the surface of T cells, which is conducive to comprehensively activating T cells; in addition, the B7-1 molecule on the surface of dendritic cells binds to the highly expressed PD-L1 on themselves, making it difficult to bind CD28 to initiate immune activation. PD-L1 inhibitors relieve the self-inhibition of DC cells, further strengthening the anti-tumor immune response; PD-L1 monoclonal antibodies directionally block the binding of PD-L1 to PD-1, preserve the function of macrophage PD-L2, and avoid the occurrence of side effects such as interstitial lung disease.

In terms of the incidence of adverse reactions, while immune checkpoint inhibitors improve the killing effect on tumor cells, activated T cells attacking normal tissues, increased autoantibodies, and increased cytokines induce autoimmune inflammation, producing a series of immune-related adverse reactions. Compared with PD-1 inhibitors, PD-L1 inhibitors have a lower incidence of grade 3 or higher adverse events, which may be related to the fact that PD-L1 inhibitors also preserve the PD-L2 pathway, maintaining the local homeostasis of macrophage PD-L2 and reducing the occurrence of related adverse reactions. A meta-analysis including 125 clinical studies of PD-1/PD-L1 monotherapy showed that patients receiving PD-1 inhibitor treatment had a higher incidence of grade 3 or higher adverse events than patients receiving PD-L1 inhibitors. Another meta-analysis systematically analyzing 23 clinical studies of immunotherapy for non-small cell lung cancer showed that PD-1 monoclonal antibodies had a higher incidence of immune-related adverse events, and the incidence of interstitial pneumonia was twice that of PD-L1 inhibitors, with statistical significance.

V. Conclusion.

As the most representative pathway in the immune checkpoint family, PD-1/PD-L1 constitutes the core target of tumor immunotherapy by virtue of the "accelerator-brake" balance between T cell activation signals and inhibitory signals, the differentiated regulatory patterns of activation-induced PD-1 expression and inflammation-induced PD-L1 expression, and the molecular pathway by which tumor cells achieve immune evasion through the binding of PD-L1 to PD-1.

In basic research on the PD-1/PD-L1 pathway and the development of immunotherapeutic drugs, efficient and reliable binding activity detection tools are the core means for evaluating blocking antibodies and candidate molecules. TR-FRET technology, with its homogeneous operation, high signal-to-noise ratio, and direct reflection of antibody competitive binding, has become an ideal platform for evaluating PD-1/PD-L1 interactions. To meet the needs of PD-1/PD-L1 pathway research and immune checkpoint inhibitor development, UniOne® provides the UniOne® TR-FRET Human PD1/PD-L1 Binding Kit. This product is suitable for applications such as quantitative analysis of PD-1 and PD-L1 binding activity, in vitro screening and activity evaluation of PD-1/PD-L1 blocking antibodies, determination of the half-maximal inhibitory concentration of candidate molecules, and validation of the mechanism of action of immune checkpoint inhibitors.

This article is reviewed and published by the technical expert team of UA

Disclaimer: This article partially utilizes artificial intelligence assistance in its creation. If any content involves copyright or intellectual property issues, please let us know and we promise to verify and remove it as soon as possible.

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