Cancer remains one of the deadliest diseases worldwide. According to the latest data from the National Cancer Center, China saw approximately 4.064 million new cancer cases and 2.4135 million deaths in 2016. These staggering numbers highlight the immense threat cancer poses to global health and underscore the urgency of finding effective treatments.

The Importance of Immune Checkpoints

Cancer's ability to "hijack" immune checkpoints on immune cells is a significant reason for its combat difficulty. Recently, the rise of immunotherapy has brought new hope to cancer treatment. By targeting these immune checkpoints, scientists have developed a series of innovative therapies that offer patients new life and create substantial market value for pharmaceutical companies. For example, Merck's PD-1 inhibitor Keytruda (commonly known as K-drug) achieved impressive sales of $20.937 billion in 2022, showcasing the market potential and efficacy of immune checkpoint therapies.

Definition of Immune Checkpoints

Immune checkpoints (ICs) are crucial regulatory factors expressed on immune cells. Their primary function is to regulate the activation level of the immune system, keeping it within an appropriate range. This regulatory mechanism effectively defends against pathogen invasion and prevents excessive immune responses and autoimmune diseases. However, abnormal expression and function of immune checkpoint molecules can lead to various diseases, including cancer.

Categories and Key Members of Immune Checkpoints

As key regulators in the immune system, immune checkpoints play an essential role in maintaining immune balance and disease defense. Based on their functions, immune checkpoints can be divided into two categories: co-inhibitory immune checkpoints and co-stimulatory immune checkpoints. Each category includes multiple members, each with unique roles in immune regulation. Let's delve into these two categories and their representative members.

Figure 1. CTLA4 and PD1 interact with their respective ligands and antibodies. (Korman AJ, et al., 2022)

Co-inhibitory Immune Checkpoints

Co-inhibitory immune checkpoints act as the "brake system" of the immune system, activating to prevent excessive immune activation that could harm normal cells. Some important co-inhibitory immune checkpoints include:

1. PD-1 (Programmed Death-1)

Also known as CD279, PD-1 is one of the most studied immune checkpoints. It is primarily found in activated T cells, B cells, and NK cells. During the effector phase of T cell activation, PD-1 binds to its ligand PD-L1, promoting SHP2 phosphatase recruitment. This interaction inhibits T cell receptor (TCR)-mediated T cell proliferation and cytokine secretion. In 2002, Lieping Chen et al. first demonstrated the B7-H1 (PD-L1) pathway as a potential mechanism for tumor immune evasion. Various tumor cells can express high levels of PD-L1, binding to PD-1 on T cells to reduce T cell activity and induce apoptosis, leading to immune evasion. Nearly twenty drugs targeting PD-1/PD-L1 have been approved globally, including PD-1 monoclonal antibodies (pembrolizumab, nivolumab, cemiplimab) and PD-L1 monoclonal antibodies (atezolizumab, durvalumab).

2. CTLA-4 (Cytotoxic T-Lymphocyte Antigen 4)

Also known as CD152, CTLA-4 is the first reported immune checkpoint molecule, a type I transmembrane receptor protein expressed on activated T cells. CTLA-4 is homologous to the co-stimulatory receptor CD28, competitively binding to the shared ligands B7-1 (CD80) and B7-2 (CD86), interrupting CD28-mediated T cell activation and transmitting inhibitory signals. In 2011, the FDA approved the first immune checkpoint inhibitor, ipilimumab, for treating advanced melanoma. This drug blocks CTLA-4's inhibitory effect on T cell activation, maintaining T cell activation to kill cancer cells. Despite initial clinical success, the side effects and limited efficacy of ipilimumab led to the development of second-generation CTLA-4 inhibitors like tremelimumab, approved in 2022.

3. LAG-3 (Lymphocyte-Activation Gene 3)

Also known as CD223, LAG-3 is the third immune checkpoint applied in clinical settings, primarily expressed on activated T cells, NK cells, B cells, and plasmacytoid dendritic cells. LAG-3 binds with high affinity to MHC II molecules, negatively regulating T cell proliferation and effector T cell function. In 2022, the first LAG-3 monoclonal antibody, relatlimab, was approved by the FDA for use in combination with nivolumab to treat unresectable or metastatic melanoma in patients aged 12 and older, marking LAG-3 as the third clinical immune checkpoint.

4. TIGIT (T cell Immunoreceptor with Ig and ITIM domains)

TIGIT belongs to the poliovirus receptor (PVR) family and is expressed on activated T cells, NK cells, Treg cells, and helper T cells. TIGIT interacts with ligands CD155 and CD112 on antigen-presenting cells, mediating inhibitory effects on NK cell and T cell activation. Several global pharmaceutical companies are racing to develop TIGIT-targeting drugs. Roche's tiragolumab and BeiGene's ociperlimab have entered phase III clinical trials, highlighting this target's potential.

5. Other Co-inhibitory Immune Checkpoints

Targets such as BTLA, TIM-3, AA2R, CEACAM1, SIRPα, and CD200R are actively being researched, with corresponding inhibitors in clinical trial stages. Although no approved drugs exist yet, these targets represent the frontier of immune checkpoint research and hold promise for future cancer treatments.

Figure 2. The graphic highlights key milestones in immunotherapeutic development. (Korman AJ, et al., 2022)

Creative Biogene leverages extensive expertise in cell line engineering to provide checkpoint-stable cell lines that are rigorously tested and ideal for evaluating drug candidates. These cell lines ensure consistent high expression of checkpoint molecules for reliable and sensitive in vitro assays.

Partial Immune Checkpoint Stable Cell Line List

Co-stimulatory Immune Checkpoints

Unlike co-inhibitory checkpoints, co-stimulatory immune checkpoints promote T cell activation. Complete T cell activation requires TCR recognition of antigen-presenting cells (APCs) peptide-MHC complex and an additional co-stimulatory signal. The receptors and ligands mediating this co-stimulatory signal are the co-stimulatory immune checkpoints.

Clinical applications of co-stimulatory immune checkpoints mainly follow two approaches:

1. Developing antagonists to inhibit immune activation.

2. Developing agonists to activate the immune checkpoint signaling pathway, enhancing immune defense responses.

Some key co-stimulatory immune checkpoints include:

1. ICOS (Inducible T-cell Co-stimulator)

Also known as CD278, ICOS is part of the immunoglobulin superfamily and is expressed on activated T cells, regulating their proliferation and function. ICOS binds to its ligand ICOS-L (a B7 family member) on APCs and other cells, including tumor cells in the tumor microenvironment. Antibody drugs targeting ICOS include agonists (enhancing immune response) and antagonists (inhibiting ICOS-positive T cell accumulation and Treg-mediated immunosuppression), both in clinical trials.

2. 4-1BB (CD137)

Also known as TNFRSF9 or CD137, 4-1BB is a co-stimulatory receptor found on activated T cells, with its ligand 4-1BBL expressed on dendritic cells, macrophages, and B cells. 4-1BBL binding activates 4-1BB signaling, promoting T cell proliferation, differentiation, effector function, and survival. Monoclonal antibodies and bispecific antibodies targeting 4-1BB have shown antitumor efficacy in cancer patients, with several clinical trials underway. Additionally, CAR-T cell therapies incorporate the 4-1BB intracellular signaling domain, such as FDA-approved tisagenlecleucel (Kymriah), lisocabtagene maraleucel (Breyanzi), and idecabtagene vicleucel (Abecma).

3. OX40 (CD134)

Also known as TNFRSF4 or CD134, OX40 is a co-stimulatory molecule expressed on activated regulatory T cells, NKT cells, NK cells, and neutrophils. OX40 binds to its ligand OX40L (TNFSF4) on APCs, activating signaling pathways involving TRAFs and NF-κB, amplifying effector T cells, and inhibiting regulatory T cells. No OX40-targeting drugs have been approved yet, but several candidates are in clinical trials, highlighting their potential.

4. CD40L

Also known as CD154 or TNFSF5, CD40L is expressed on activated CD4+ T cells, B cells, and platelets. Under inflammatory conditions, it can be induced in monocytes, NK cells, mast cells, and basophils. CD40L binds to CD40 (TNFR family member) on APCs and T cells, playing a crucial role in T cell-dependent B cell activation, differentiation, and antibody production. Monoclonal antibodies targeting CD40 and CD40L are in clinical trials, with most focusing on CD40.

5. Other Co-stimulatory Immune Checkpoints

Targets like CD28, CD27, GITR, LIGHT, DNAM-1, 2B4, DC-SIGN, and DR3 are under active investigation, with potential therapeutic applications being explored. These checkpoints represent future directions in enhancing immune responses for cancer treatment.

Conclusion

In summary, immune checkpoints play a pivotal role in the immune system, balancing activation and inhibition to maintain immune homeostasis. While co-inhibitory checkpoints act as brakes to prevent excessive immune responses, co-stimulatory checkpoints serve as accelerators to enhance immune activity. Abnormalities in these checkpoints can lead to immune evasion by tumors and autoimmune diseases, making them prime targets for therapeutic intervention.

The development and clinical application of immune checkpoint inhibitors has revolutionized cancer treatment, offering new hope to patients. As research progresses, the identification of new targets and the refinement of existing therapies promise to further improve outcomes for cancer patients, making immune checkpoints a focal point in the fight against cancer.

Related Target or Gene List

Creative Biogene provides comprehensive services and solutions to support research on the aforementioned targets in immune regulation and cancer therapy, including gene editing, protein expression analysis, and functional assays.