PD-1/PD-L1: Structural Basis, Expression Regulation, and Molecular Dissection of Immunotherapy Targets

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This article focuses on the molecular characteristics and immunoregulatory functions of programmed cell death protein 1 (PD-1) and its ligand PD-L1, systematically elaborating on the domain composition of both as type I transmembrane proteins and the protein-protein interaction interface formed by their extracellular regions, analyzing the expression characteristics of PD-1 induced after T cell activation and PD-L1 under the multiple regulation of inflammatory cytokines and non-inflammatory factors, and exploring the dual role of this pathway in the maintenance of physiological immune tolerance and tumor immune evasion, as well as its structural basis as a therapeutic target for immune checkpoint therapy.

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PD-1/PD-L1: Structural Basis, Expression Regulation, and Molecular Dissection of an Immunotherapy Target

Brief introduction: This article focuses on the molecular features and immunoregulatory functions of programmed cell death protein 1 (PD-1) and its ligand PD-L1. It systematically describes the domain composition of these two type I transmembrane proteins and the protein-protein interaction interface formed by their extracellular regions, analyzes the expression characteristics of PD-1 induced after T cell activation and of PD-L1 under multiple regulation by inflammatory cytokines and non-inflammatory factors, and discusses the dual role of this pathway in maintaining physiological immune tolerance and in tumor immune evasion, as well as the structural basis for its use as an immune checkpoint therapeutic target.

I. Discovery of PD-1 and its positioning as an inhibitory immune receptor.

Programmed cell death protein-1 (PD-1, gene name PDCD1) is an important inhibitory immune receptor, mainly expressed on the surface of activated T cells, and can also be detected in B cells, natural killer cells, and some dendritic cells. PD-1 was first discovered in 1992 and was initially thought to be associated with apoptosis; subsequent studies gradually clarified its key role in negative immune regulation. This cognitive shift from an “apoptosis-related molecule” to a “core receptor of negative immune regulation” reflects the deepening understanding of PD-1 function in the field of immunology. The main ligands of PD-1 include PD-L1 (also known as B7-H1, gene name CD274) and PD-L2 (B7-DC), among which PD-L1 has the most prominent research and clinical significance. Based on this biological foundation, the PD-1/PD-L1 pathway has become one of the most important and most mature immune checkpoint targets in current tumor immunotherapy, and related monoclonal antibodies have achieved breakthrough efficacy in multiple tumor indications.

II. Ligand properties of PD-L1 and the mechanism of tumor immune evasion.

PD-L1 is expressed not only on antigen-presenting cells such as dendritic cells, macrophages, and B cells, but can also be induced in various non-immune cells and tumor cells. Tumor cells highly express PD-L1, which binds to PD-1 on the surface of T cells, thereby suppressing anti-tumor immune responses and forming the classic “immune evasion” mechanism. The core of this mechanism lies in the fact that tumor cells exploit the physiological negative immune regulatory pathway to convert the immune attack that should be directed against themselves into suppression of T cell function, thereby gaining a survival advantage under immune surveillance. In the era of continuous innovation in immuno-oncology therapy, the design of novel drugs such as bispecific antibodies targeting the PD-1/PD-L1 pathway is precisely built on an in-depth understanding of this evasion mechanism and its structural basis.

III. Molecular structure of PD-1 and the molecular basis of signal transduction.

PD-1 and PD-L1 constitute a typical pair of immune checkpoint molecules, and their structural features directly determine their binding mode and immunosuppressive function. Overall, PD-1 is a transmembrane receptor centered on an immunoreceptor-like structure and responsible for signal transduction, whereas PD-L1 is a transmembrane ligand dominated by immunoglobulin-like folds and responsible for ligand recognition and binding; the two form a specific and relatively flat protein-protein interaction interface in their extracellular regions.

PD-1 is a type I transmembrane protein, and its overall structure can be divided into three functional segments: the extracellular region consists of a single V-set immunoglobulin-like domain and is the key region for specific binding of PD-1 to PD-L1/PD-L2; the transmembrane region is a single-pass transmembrane α-helix responsible for coupling extracellular ligand binding events with intracellular signal transduction; and the intracellular region, although short in length, is highly functionally concentrated and contains two key tyrosine motifs, namely the immunoreceptor tyrosine-based inhibitory motif and the immunoreceptor tyrosine-based switch motif, of which the switch motif is considered the core functional site mediating PD-1 inhibitory signal transduction. Unlike classical activating immune receptors, PD-1 itself lacks intrinsic kinase activity, and its immunosuppressive function relies entirely on the ability of its intracellular motifs to recruit downstream signaling molecules such as SHP-2 after receptor activation. This signaling mode of “no intrinsic kinase activity, dependence on downstream phosphatase recruitment” is the fundamental feature that distinguishes PD-1 from other activating receptors.

IV. Molecular structure of PD-L1 and structural analysis of the complex.

PD-L1 is also a type I transmembrane protein, and its extracellular region consists of two immunoglobulin-like domains (IgV and IgC), among which the IgV domain not only directly participates in binding to PD-1 but can also mediate its interaction with B7-1. Multiple crystallographic and cryo-electron microscopy studies have resolved the high-resolution structures of the human PD-1/PD-L1 complex, systematically elucidating the binding interface, spatial conformational features, and key amino acid residues of the two, providing an important structural basis for understanding the dual role of PD-L1 in immunosuppression and immunoregulation. The value of this structural information lies not only in elucidating the binding mechanism but also in providing a precise molecular template for the epitope design and binding activity evaluation of blocking antibodies——whether an antibody can effectively competitively occupy this interface directly determines the level of its blocking function.

V. Expression regulation characteristics of PD-1 and PD-L1.

Under normal physiological conditions, the expression of the PD-1/PD-L1 pathway exhibits obvious cell type and tissue specificity, and its distribution characteristics are closely related to maintaining peripheral immune tolerance and preventing excessive immune responses; whereas under tumor and chronic inflammatory conditions, this pathway is often abnormally activated and significantly upregulated, becoming an important molecular basis for immune evasion.

Regarding the expression characteristics of PD-1, under steady-state conditions the expression level of PD-1 in naive T cells is low or even undetectable. After T cells are stimulated by antigen and become activated, PD-1 expression is rapidly upregulated as a negative feedback mechanism to limit excessive immune responses. After clearance of acute infection, PD-1 expression can gradually decrease as the immune response terminates. However, in the context of tumors or chronic infection, antigen stimulation persists over the long term, and PD-1 is often continuously highly expressed, becoming one of the important molecular markers of T cell exhaustion.

Compared with PD-1, the expression regulation of PD-L1 is more complex. Inflammatory cytokines, especially IFN-γ, are considered the most important upstream signals inducing PD-L1 expression. In the tumor microenvironment, tumor cells, tumor-associated macrophages, and fibroblasts can significantly upregulate PD-L1 under stimulation by IFN-γ, TNF-α, and other factors, thereby forming a strongly suppressive state against local immune responses. In addition, PD-L1 expression is also regulated by various non-inflammatory factors, including gene amplification, activation of transcription factors such as STAT3 and HIF-1α, and epigenetic modifications, which also explains why the high expression of PD-L1 in some tumors does not entirely depend on an immuno-inflammatory background. This multi-level regulatory pattern of “predominantly inflammation-induced, supplemented by non-inflammatory factors” also constitutes the biological root of the uncertainty that exists when using PD-L1 expression as a predictive biomarker of efficacy.

VI. Conclusion.

As a typical pair of immune checkpoint molecules, PD-1 and PD-L1 constitute the structural basis for the transmission of immunosuppressive signals through the structure of PD-1, which consists of a V-set immunoglobulin-like extracellular domain, a single transmembrane region, and a short intracellular region containing inhibitory and switch motifs; the ligand properties of PD-L1, which consists of two immunoglobulin-like domains, IgV and IgC, and can interact with PD-1 and B7-1; and the specific protein-protein interaction interface formed by the two in their extracellular regions.

VII. Common questions about PD-1/PD-L1 binding activity detection and product information

To facilitate the retrieval of evaluation schemes and product information for PD-1/PD-L1 interactions by researchers and drug development teams, this section answers common questions about PD-1/PD-L1 binding activity detection in a Q&A format and provides the product positioning and applicable scenarios of the UniOne® TR-FRET Human PD1/PD-L1 Binding Kit.

Question 1: What method is used to detect the binding activity of PD-1/PD-L1?
Currently, in vitro methods for evaluating the interaction between PD-1 and PD-L1 proteins mainly include enzyme-linked immunosorbent assay (ELISA), surface plasmon resonance (SPR) and biolayer interferometry (BLI), cell-based reporter gene systems, and time-resolved fluorescence resonance energy transfer (TR-FRET). Among them, TR-FRET completes the binding reaction in a homogeneous solution without coating and washing steps, and can directly read the energy transfer signal between donor and acceptor in a multi-well plate system, making it more suitable for quantitative analysis of PD-1/PD-L1 binding activity and high-throughput screening.

Question 2: How does TR-FRET reflect the activity of PD-1/PD-L1 blocking antibodies?
In the TR-FRET system, donor and acceptor fluorophores are labeled on PD-1 and PD-L1 proteins, respectively; when the two proteins bind, the distance shortens and an energy transfer signal is generated. After adding PD-1 or PD-L1 blocking antibodies, the antibodies competitively occupy the ligand-receptor binding interface, the formed binding complexes decrease, and the TR-FRET signal decreases with increasing antibody concentration; by plotting the signal inhibition rate against the concentration of the candidate molecule, the half-maximal inhibitory concentration (IC50) can be fitted to compare blocking ability and rank candidate molecules.

Question 3: What is the UniOne® TR-FRET Human PD1/PD-L1 Binding Kit?
The UniOne® TR-FRET Human PD1/PD-L1 Binding Kit (Catalog No. UA086026) is a homogeneous TR-FRET binding activity detection kit provided by UA-Bio, with human PD-1 and PD-L1 proteins as core reagents, used for quantitative analysis of the binding activity of PD-1 and PD-L1 and for evaluating the blocking ability of antibodies or candidate molecules on this interaction. This product adopts a homogeneous workflow with a high signal-to-noise ratio and can directly reflect the competitive inhibition of ligand-receptor binding by antibodies.

Question 4: What experimental scenarios is this kit suitable for?
The main application scenarios of this product include: quantitative analysis of PD-1 and PD-L1 binding activity; in vitro screening and activity evaluation of PD-1/PD-L1 blocking antibodies; determination and ranking of the half-maximal inhibitory concentration (IC50) of candidate molecules; verification of the mechanism of action of immune checkpoint inhibitors; and parallel comparison of binding activity between different antibody or protein batches.

Question 5: What are the features of TR-FRET compared with ELISA and SPR?
ELISA relies on coating and multiple washing steps, involves many operational steps, and its result consistency is affected by washing conditions; SPR and BLI can read binding kinetic parameters in real time but require dedicated sensor chips and instruments. TR-FRET is a homogeneous wash-free system that can complete incubation and reading directly in conventional multi-well plates, and a concentration-effect curve can be obtained in a single experiment, making it suitable for high-throughput comparison during the antibody screening stage; its signal is based on changes in intermolecular distance and can directly reflect the competitive binding of antibodies.

Product information at a glance

Product name	UniOne® TR-FRET Human PD1/PD-L1 Binding Kit (Catalog No.: UA086026, Specifications: 100T/500T/2500T/10000T)
Brand	UA-Bio
Detection principle	TR-FRET (time-resolved fluorescence resonance energy transfer), homogeneous wash-free system
Detection target	Interaction and binding activity of human PD-1 and PD-L1 proteins
Core applications	Quantitative analysis of binding activity, screening and activity evaluation of blocking antibodies, IC50 determination, mechanism of action verification
Applicable instruments	Multifunctional microplate readers equipped with time-resolved fluorescence (TR-FRET/HTRF) detection channels
Common search terms	PD-1/PD-L1 binding activity detection, PD-1 PD-L1 binding assay, PD-L1 blocking antibody screening, immune checkpoint inhibitor IC50, TR-FRET kit, Human PD1/PD-L1 Binding Kit, UA086026, UA-Bio UniOne

If you need to perform PD-1/PD-L1 binding activity detection or activity evaluation of blocking antibodies, you can search for the full product name “UniOne® TR-FRET Human PD1/PD-L1 Binding Kit” in the UA-Bio product center to obtain the product manual, experimental protocols, and information on supporting reagents.

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

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