Mouse PD-1 (Biotiny)/PD-L1 Inhibitor Screening Assay Kit

In recent years, research on cancer immunotherapy has focused on the PD-1/PD-L1 pathway. The transmembrane protein known as PD-1 (Programmed Death-1) was initially identified in 1992. It shares some similarities with other CD28 family members, including CTLA-4 and inducible costimulatory molecules, based on its amino acid sequence. Activated T cells, natural killer (NK) cells, B lymphocytes, macrophages, dendritic cells (DC), and monocytes are the main cells that express PD-1, which inhibits both adaptive and innate immune responses. In the tumor microenvironment, PD-1 expression is especially high on tumor-specific T cells. PD-1 has two essential tyrosine residues in its intracellular area and an N-terminal domain in its extracellular portion. Multiple transcription factors, such as NFAT, NOTCH, FOXO1, and IRF9, can regulate PD-1 expression upon antigen activation, regulating PD-1 expression in many immune cell types. For example, IFN-α can trigger PD-1 expression by IRF9 binding to its promoter, whereas NFATc binds to the promoter area of PD-1. Furthermore, the PD-1 promoter becomes more demethylated in the setting of persistent viral infections and some types of cancer, which boosts the expression of the protein in worn-out T cells. Tumor immune evasion and immune system tolerance are both influenced by this control of PD-1 expression. PD-L1 (Programmed Death Ligand 1), the main ligand for PD-1, is a 33 kDa type I transmembrane glycoprotein that is a member of the B7 family. Activated T cells, B cells, DCs, macrophages, and certain epithelial cells are among the immune cells that express PD-L1. IFN-γ and other inflammatory stimuli can trigger PD-L1 expression in the tumor microenvironment, which aids tumor cells in avoiding immune monitoring. The stimulation of signaling pathways like JAK-STAT, PI3K-AKT, MAPK, and NF-κB results in the upregulation of PD-L1. For instance, through the JAK-STAT pathway, IFN-γ increases the expression of PD-L1 in tumor cells, enabling them to evade T cell attacks. Through the reduction of inhibitory molecules like PTEN, the activation of the PI3K-AKT pathway also increases the expression of PD-L1, which in turn increases the ability of cancer cells to survive and proliferate. Similar to how the MAPK signaling system controls PD-L1 expression, blocking the MEK1/2 pathway dramatically lowers the amount of PD-L1 in kidney cancer cells.

Figure 1. Inhibition of T Cell Activation, Proliferation, Survival, and Cytotoxic Secretion by the PD-1/PD-L1 Axis (Source: Han Y, et al., 2020)

The PD-1/PD-L1 pathway plays a critical role in immune evasion in cancer. The PD-1/PD-L1 axis suppresses T cell activation, proliferation, and cytotoxicity, resulting in a decreased anti-tumor immune response. For instance, in breast cancer, lung cancer, and colorectal cancer, PD-L1 expression is often associated with tumor aggressiveness, differentiation status, and poor prognosis in patients. In breast cancer, PD-L1 expression is linked to tumor stem cell markers and epithelial-mesenchymal transition (EMT), further promoting malignant progression. In lung cancer, PD-L1 shows a positive correlation with factors such as smoking, male gender, advanced cancer stage, and squamous cell carcinoma histology, with high expression linked to adverse patient prognosis. Moreover, the expression of the PD-1/PD-L1 signaling axis in colorectal cancer is often associated with increased inflammatory factors and immune-suppressive cells, enabling tumor cells to effectively evade immune surveillance and inducing apoptosis in active T cells, thus escaping the immune system. Research is currently focused on the signaling pathways regulating the functions of PD-1/PD-L1. The PI3K/AKT signaling pathway is closely related to cell proliferation and survival; by inhibiting negative regulatory factors such as PTEN, this pathway can promote PD-L1 expression and tumor cell anti-apoptotic properties. The MAPK pathway regulates tumor cell growth and invasive capabilities through ERK and JNK. In tumors like colorectal cancer, inhibiting this pathway's activity can effectively reduce PD-L1 expression. Furthermore, the JAK-STAT signaling pathway enhances PD-L1 expression in cancer through various cytokines and growth factors, making PD-L1 a crucial regulator of tumor immune evasion.

Figure 2. Stages of the Cancer Immune Cycle Involving PD-1 Activity (Source: Pauken KE, et al., 2021)

In addition to the aforementioned signals, abnormal activation of the NF-κB and Hedgehog signaling pathways also promotes PD-L1 expression. The NF-κB pathway plays a role in cell proliferation and inflammatory responses, and it adapts the tumor microenvironment by regulating PD-L1 expression. Compounds like curcumin, which inhibit NF-κB, combined with anti-PD-L1 immune checkpoint blockade, can significantly suppress the proliferation of various cancer cells. The role of the Hedgehog signaling pathway in malignant tumors like gastric cancer further emphasizes the complexity of PD-L1 regulation in cancer, particularly regarding its involvement in mechanisms of treatment resistance in various cancers. The PD-1/PD-L1 pathway holds significant application value in cancer immunotherapy. Antibodies targeting PD-1 or PD-L1 have shown good efficacy in various cancers, including breast cancer, lung cancer, and colorectal cancer. By relieving immune suppression, PD-1/PD-L1 blockade therapies can activate T cells' cytotoxic effects against tumor cells, extending patient survival. However, not all patients respond well to PD-1/PD-L1 inhibitors; the heterogeneity of the tumor microenvironment and complex signaling networks may lead to variations in treatment efficacy. Therefore, an important future challenge in immunotherapy is how to effectively combine regulators of other signaling pathways and optimize the application strategies of PD-1/PD-L1 blockade.

PD-1/PD-L1 inhibitor screening kit
Biotinylated PD-1/PD-L1 assay kit
Mouse PD-1/PD-L1 ELISA kit