Immune Checkpoint Signaling Pathway

What is immune checkpoint?

Immune checkpoints are regulators of the immune system. These pathways are crucial for self-tolerance, which prevents the immune system from attacking cells indiscriminately. Therefore, an immune response is tightly regulated, and multiple mechanisms are in place to prevent autoimmune reactions to self-proteins. These mechanisms are implemented by immune checkpoint molecules.Checkpoint molecules include cytotoxic T lymphocyte antigen-4, programmed death-1, lymphocyte activation gene-3, T-cell immunoglobulin and mucin protein-3, and several others. The expression of these checkpoint molecules on T cells represents an important mechanism that the immune system uses to regulate responses to self-proteins. Recent clinical data show that these Checkpoint molecules play a critical role in objective tumor responses and improved overall survival.

Figure 1. Selected Immune Checkpoint Pathways.
Adopted from P.Sharma 2012 ASCO Annual Meeting.

Immune checkpoint signaling pathway

1. Pathway construction

There are two types of checkpoint molecules in the immune checkpoint signaling pathway. One is to activate the immune system and the other is to suppress the immune system. They are cell surface receptors that cause changes in cell function by binding to ligands secreted by other cells. The researchers completed the classification of these immune checkpoint molecules through the detection of animal defect model phenotypes.

Stimulatory checkpoint molecules

There are eight activation checkpoint molecules. Four stimulatory checkpoint molecules are members of the tumor necrosis factor (TNF) receptor superfamily - CD27, CD40, OX40, GITR and CD137. Another two stimulatory checkpoint molecules belongs to the B7-CD28 superfamily - CD28 itself and ICOS.

  • CD27 - This molecule supports antigen-specific expansion of naive T cells and is vital for the generation of T cell memory. It is also a memory marker of B cells. It's activity is governed by the transient availability of its ligand, CD70, on lymphocytes and dendritic cells.
  • CD28 - This molecule is constitutively expressed on almost all human CD4+ T cells and on around half of all CD8 T cells. Binding with its two ligands are CD80 and CD86, expressed on dendritic cells, prompts T cell expansion. CD28 was the target of the TGN1412 which caused severe inflammatory reactions.
  • CD40 - This molecule, found on a variety of immune system cells including antigen presenting cells has CD40L, otherwise known as CD154 and transiently expressed on the surface of activated CD4+ T cells, as its ligand. CD40 signaling is known to ‘license’ dendritic cells to mature and thereby trigger T-cell activation and differentiation.
  • CD122 - This molecule, which is the Interleukin-2 receptor beta sub-unit, is known to increase proliferation of CD8+ effector T cells.
  • CD137 - When this molecule, also called 4-1BB, is bound by CD137 ligand, the result is T-cell proliferation. CD137-mediated signaling is also known to protect T cells, and in particular, CD8+ T cells from activation-induced cell death.
  • OX40 - This molecule, also called CD134, has OX40L, or CD252, as its ligand. Like CD27, OX40 promotes the expansion of effector and memory T cells, however it is also noted for its ability to suppress the differentiation and activity of T-regulatory cells, and also for its regulation of cytokine production.
  • GITR - short for Glucocorticoid-Induced TNFR family related gene, prompts T cell expansion, including regulatory T cells expansion. The ligand for GITR is mainly expressed on antigen presenting cells.
  • ICOS - This molecule, short for Inducible T-cell costimulator, and also called CD278, is expressed on activated T cells. Its ligand is ICOSL, expressed mainly on B cells and dendritic cells. The molecule seems to be important in T cell effector function.

Inhibitory checkpoint molecules

Due to the superior performance of immune checkpoint inhibitors in the treatment of tumors, they have received more attention and research from scholars.

  • A2AR - The Adenosine A2A receptor is regarded as an important checkpoint in cancer therapy because adenosine in the immune microenvironment, leading to the activation of the A2a receptor, is negative immune feedback loop and the tumor microenvironment has relatively high concentrations of adenosine.
  • B7-H3 - also called CD276, was originally understood to be a co-stimulatory molecule but is now regarded as co-inhibitory.
  • B7-H4 - also called VTCN1, is expressed by tumor cells and tumor-associated macrophages and plays a role in tumour escape.
  • BTLA - This molecule, short for B and T Lymphocyte Attenuator and also called CD272, has HVEM (Herpesvirus Entry Mediator) as its ligand. Surface expression of BTLA is gradually downregulated during differentiation of human CD8+ T cells from the naive to effector cell phenotype, however tumor-specific human CD8+ T cells express high levels of BTLA.
  • CTLA-4 - short for Cytotoxic T-Lymphocyte-Associated protein 4 and also called CD152. Expression of CTLA-4 on regulatory T cells serves to control T cell proliferation.
  • IDO - short for Indoleamine 2, 3-dioxygenase, is a tryptophan catabolic enzyme with immune-inhibitory properties. IDO is known to suppress T and NK cells, generate and activate regulatory T cells and myeloid-derived suppressor cells, and promote tumour angiogenesis.
  • KIR - short for Killer-cell Immunoglobulin-like Receptor, is a receptor for MHC Class I molecules on Natural Killer cells.
  • LAG3 - short for Lymphocyte Activation Gene-3, works to suppress an immune response by action to regulatory T cells as well as direct effects on CD8+ T cells.
  • PD-1 - short for Programmed Death 1 (PD-1) receptor, has two ligands, PD-L1 and PD-L2. An advantage of targeting PD-1 is that it can restore immune function in the tumor microenvironment.
  • TIM-3 - short for T-cell Immunoglobulin domain and Mucin domain 3, expresses on activated human CD4+ T cells and regulates Th1 and Th17 cytokines. TIM-3 acts as a negative regulator of Th1/Tc1 function by triggering cell death upon interaction with its ligand, galectin-9.
  • VISTA (protein) - Short for V-domain Ig suppressor of T cell activation, VISTA is primarily expressed on hematopoietic cells so that consistent expression of VISTA on leukocytes within tumors may allow VISTA blockade to be effective across a broad range of solid tumors.

2. Pathway regulation


After the activation immune checkpoint molecule binds to its corresponding ligand, it sends an activation signal to the T cell. The T cell is moderately activated. Activation of CD4+ T cells occurs through the simultaneous engagement of the T-cell receptor and a co-stimulatory molecule (like CD28, or ICOS) on the T cell by the major histocompatibility complex (MHCII) peptide and co-stimulatory molecules on the APC. Both are required for production of an effective immune response. The signaling pathways downstream from co-stimulatory molecules usually engages the PI3K pathway generating PIP3 at the plasma membrane and recruiting PH domain containing signaling molecules like PDK1 that are essential for the activation of PKCθ, and eventual IL-2 production, Which in turn activates the entire immune system.


Under normal conditions, ligands that bind to inhibitory immune checkpoint molecules are secreted so that the immune system can be immunized against self-proteins. Some of these pathways have already been described, and some are still unclear. In the proved pathway, we know CTLA-4 is a homolog of CD28 and plays an important role in the development of peripheral tolerance to self-proteins. B7–1 (CD80) and B7–2 (CD86) are the main ligands for CTLA-4. Upon binding B7-1 or B7-2, CTLA-4 recruits the phosphatases SHP2 and PP2A via the YVKM motif in its cytoplasmic domain. SHP2 recruitment results in attenuation of TCR signaling by dephosphorylating the CD3ζ chain. PP2A recruitment results in downstream dephosphorylation of AKT, further dampening the T cell activation pathway. PD-1 ligation by PD-L1 or PD-L2 also recruits SHP2 to the ITSM domain, resulting in membrane proximal decreases in TCR signaling. LAG-3 signaling is dependent on interaction with its ligand, MHC II, as well as its intracellular KIEELE domain. TIM-3 binds to Galectin-9, as well as other ligands. In the absence of ligand binding, TIM-3 is associated with Bat3, protecting the cell from TIM-3–mediated inhibition and allowing for greater activation. However, once TIM-3 binds to Galectin-9, Y265 is phosphorylated and the interaction with Bat3 is disrupted, allowing TIM-3 to deliver inhibitory signals to the T cell. BTLA and CD160 bind to herpes virus entry mediator. BTLA contains an intracellular ITIM domain that may be important in signaling. 2B4 has four intracellular ITSM domains and binds to CD48, but further signaling mechanisms are poorly understood.

Cancer Immunotherapy

The first immune checkpoint receptor, cytotoxic T lymphocyte antigen 4 (CTLA-4), which was discovered and identified, first showed a close relationship with cancer treatment. In animal models, antibodies that block CTLA-4 can mediate the regression of established tumors. This observation directly resulted in the clinical of detection of CTLA-4 monoclonal antibodies in cancer patients. The main limitation of CTLA-4 blocking therapy is the high rate of toxic effects of the immune. Many of the adverse reactions that are present in patients treated with anti-CTLA-4, such as rash, colitis, thyroiditis, and hepatitis. Dozens of immune checkpoint molecules, the next direct clinical immune checkpoint is the inclusion of a programmed death molecule 1. It is an inhibitory receptor expressed on activated tumor-specific CD4-positive cells and CD8-positive T-lymphocytes, and its primary ligand, PD-L1, is also expressed in various types of cancer. Unlike early activated CTLA-4, which modulates systemic T lymphocyte immunity, the PD-1 checkpoint only regulates the activity of cytotoxic T lymphocyte migration into tumors. The expression of PD-L1 ligand is selective and is not over-expressed in normal inflammatory tissues. This makes it possible to block the biological effects of the PD-1 pathway drug, which is much less toxic than anti-CTLA-4. The initial success of cancer immunotherapy by directly activating anti-tumor immunity with tumor vaccines or recombinant cytokines or by injecting prepared tumor-specific immune cells has led to limited success. Until scientific research can better understand the interaction between cancer and the immune system, tumor immunology will take a huge leap forward.


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