What Are Immune Checkpoints?

Immune checkpoints are cell-surface proteins expressed on immune cells—most notably T cells—that act as “off switches” or “brakes” on the immune response. They are part of a broader system of co‑inhibitory and co‑stimulatory signals that fine-tune T‑cell activity. The most well‑characterized checkpoints include:

  • PD‑1 (Programmed Cell Death Protein 1) – Found on activated T cells, PD‑1 binds to its ligands PD‑L1 and PD‑L2 on normal cells, sending an inhibitory signal that reduces T‑cell proliferation, cytokine production, and cytotoxic activity. PD‑1 is induced after T‑cell activation and functions mainly in peripheral tissues.
  • CTLA‑4 (Cytotoxic T‑Lymphocyte‑Associated Protein 4) – Expressed on T cells, CTLA‑4 competes with the co‑stimulatory receptor CD28 for binding to B7 molecules on antigen‑presenting cells. By outcompeting CD28, CTLA‑4 dampens early T‑cell activation in lymph nodes. It is constitutively expressed on regulatory T cells.
  • LAG‑3 (Lymphocyte Activation Gene 3) – Another inhibitory receptor that synergizes with PD‑1 to limit T‑cell effector function. It is often upregulated in exhausted T cells within tumors. LAG‑3 binds to MHC class II molecules with higher affinity than CD4.
  • TIM‑3 (T‑cell Immunoglobulin and Mucin‑Domain Containing‑3) – Expressed on dysfunctional T cells, TIM‑3 binding leads to cell death or suppression and is implicated in resistance to PD‑1 blockade. Its ligands include galectin‑9, HMGB1, and phosphatidylserine.
  • TIGIT (T‑cell Immunoreceptor with Ig and ITIM Domains) – An inhibitory receptor that suppresses both T cell and natural killer (NK) cell activity, often co‑expressed with PD‑1 on tumor‑infiltrating lymphocytes. TIGIT competes with the co‑stimulatory receptor CD226 for binding to CD155 and CD112 on tumor cells.

These molecules are not merely “brakes” but part of an intricate regulatory circuit that maintains immune homeostasis. Under normal conditions, checkpoints help prevent reactions against self‑antigens and limit collateral damage during an infection. However, tumors quickly learn to exploit them. The discovery of these pathways began in the 1990s, with James Allison’s work on CTLA‑4 and Tasuku Honjo’s on PD‑1, earning them the Nobel Prize in Physiology or Medicine in 2018.

Physiological Role of Immune Checkpoints

In a healthy immune response, T cells recognize antigens presented by antigen‑presenting cells (APCs) via the T‑cell receptor. This “signal one” is not enough; a second co‑stimulatory signal (e.g., CD28:B7) is required for full activation. Checkpoints like CTLA‑4 act in the priming phase — in lymph nodes — by raising the threshold for T‑cell activation, preventing T cells from becoming over‑activated against harmless self‑antigens. In peripheral tissues, PD‑1 is induced after T‑cell activation and functions in the effector phase, turning off T cells that encounter PD‑L1 on normal cells, thereby protecting healthy tissue from immune attack and limiting the duration of inflammatory responses. This duality — central tolerance (CTLA‑4) and peripheral tolerance (PD‑1) — explains why blocking these pathways can lead to both powerful anti‑tumor immunity and immune‑related adverse events (irAEs).

Beyond T cells, checkpoints also regulate other immune cells. For example, PD‑1 is expressed on B cells, NK cells, and myeloid cells, influencing antibody production and innate immunity. CTLA‑4 is also found on regulatory T cells (Tregs), where it is required for their suppressive function. The interplay between these checkpoints ensures that immune responses are appropriately scaled and terminated.

How Cancer Exploits Immune Checkpoints

Tumors are not passive targets; they actively create an immunosuppressive microenvironment. One of the most common strategies is the overexpression of PD‑L1 on the surface of cancer cells and infiltrating immune cells. When PD‑1‑positive T cells enter the tumor, they encounter high levels of PD‑L1, which triggers the inhibitory pathway and effectively “turns off” the T cells. This shields the tumor from destruction. Additionally, tumors deploy several complementary evasion mechanisms:

  • Secretion of immunosuppressive cytokines – Molecules such as IL‑10, TGF‑β, and VEGF further dampen T‑cell function and promote an environment hostile to effector cells. These cytokines also recruit suppressive cells like Tregs and myeloid-derived suppressor cells (MDSCs).
  • Induction of regulatory T cells (Tregs) – Tumors recruit or induce Tregs that express high levels of CTLA‑4 and PD‑1, creating a local immunosuppressive network that suppresses other immune cells. Tregs consume IL‑2, further starving effector T cells.
  • Myeloid‑derived suppressor cells (MDSCs) – These immature myeloid cells produce arginase, reactive oxygen species, and other factors that inhibit T‑cell activity and promote tumor growth. They also express high levels of PD‑L1.
  • Downregulation of antigen presentation – Many tumors reduce expression of MHC class I molecules, making it harder for T cells to recognize them. Some tumors also mutate or lose components of the antigen‑processing machinery.
  • Metabolic competition – Tumor cells consume large amounts of glucose and tryptophan, starving T cells of essential nutrients. Lactic acid produced by glycolysis further suppresses T‑cell function.
  • Exosome‑mediated interference – Tumor‑derived exosomes carry PD‑L1 to distant sites, suppressing T cells systemically before they even reach the tumor.

These mechanisms often cooperate, forming a robust barrier that prevents effective immune surveillance. Checkpoint proteins such as LAG‑3, TIM‑3, and TIGIT are also frequently upregulated on exhausted T cells within the tumor, contributing to a state of profound T‑cell dysfunction. This exhaustive state is characterized by progressive loss of effector functions, increased expression of multiple inhibitory receptors, and altered transcriptional programs.

Immunotherapy: Blocking the Brakes

Checkpoint inhibitors are monoclonal antibodies designed to block inhibitory receptors or their ligands, thereby restoring T‑cell activity. The principle is simple: by preventing PD‑1 from binding to PD‑L1 (or CTLA‑4 from binding to B7), these drugs release the immune system’s brakes, allowing T cells to attack the tumor. The first approved checkpoint inhibitor, ipilimumab (anti‑CTLA‑4), was approved in 2011 for metastatic melanoma, marking a new era in cancer immunotherapy. Since then, several drugs have entered the clinic:

  • Ipilimumab (Yervoy) – Targets CTLA‑4; used mainly in melanoma and now in combination with other agents. Dosing is typically 3 mg/kg every 3 weeks for 4 doses.
  • Pembrolizumab (Keytruda) – Targets PD‑1; approved for a wide range of cancers including non‑small cell lung cancer, head and neck cancer, Hodgkin lymphoma, and many others. Often given at 200 mg or 2 mg/kg every 3 weeks, or 400 mg every 6 weeks.
  • Nivolumab (Opdivo) – Targets PD‑1; similarly broad approvals, often in combination with ipilimumab. Standard dosing is 240 mg every 2 weeks or 480 mg every 4 weeks.
  • Atezolizumab (Tecentriq) – Targets PD‑L1; used in bladder cancer, lung cancer, and triple‑negative breast cancer. Given at 840 mg, 1200 mg, or 1680 mg depending on schedule.
  • Durvalumab (Imfinzi) – Targets PD‑L1; approved for non‑small cell lung cancer after chemoradiation (PACIFIC trial).
  • Avelumab (Bavencio) – Targets PD‑L1; approved for Merkel cell carcinoma and urothelial carcinoma.
  • Cemiplimab (Libtayo) – Targets PD‑1; approved for cutaneous squamous cell carcinoma and lung cancer.
  • Lag‑3 targeting – The combination of nivolumab plus relatlimab (anti‑LAG‑3) was approved in 2022 for metastatic melanoma, with a fixed‑dose combination (nivolumab 480 mg + relatlimab 160 mg) every 4 weeks.

These antibodies are typically administered intravenously every two to six weeks, and treatment may continue for two years or longer if the patient responds. The response can be dramatic, with some patients achieving complete remission even in advanced disease. However, not all patients respond, and immune‑related side effects remain a significant challenge. The mechanisms of action differ slightly: CTLA‑4 blockade increases T‑cell priming and diversity, while PD‑1/PD‑L1 blockade reinvigorates exhausted T cells in the tumor microenvironment. Combination therapy leverages both effects.

Impact on Cancer Treatment

The introduction of checkpoint inhibitors has reshaped treatment paradigms across numerous malignancies. In melanoma, five‑year survival rates for advanced disease have risen from roughly 10% to over 50% with combination PD‑1/CTLA‑4 blockade. In non‑small cell lung cancer, pembrolizumab is now a standard first‑line agent for tumors expressing high levels of PD‑L1 (≥50% tumor proportion score). “Cures” are still rare, but durable remissions lasting years have become a reality for a subset of patients. The impact extends to less common cancers like Merkel cell carcinoma, hepatocellular carcinoma, and renal cell carcinoma, where checkpoint inhibitors have become cornerstones of therapy. The National Cancer Institute provides a comprehensive overview of approved checkpoint inhibitors and their indications.

Challenges: Adverse Events and Resistance

Because checkpoint inhibitors remove the brakes on the immune system, they can unleash an overactive immune response that attacks healthy organs. These immune‑related adverse events (irAEs) can range from mild rashes and colitis to severe pneumonitis, hepatitis, and endocrine dysfunction. Most irAEs are manageable with corticosteroids and withholding therapy, but they can be life‑threatening. Management guidelines have been refined, including algorithms for early recognition and treatment with immunosuppressive drugs like infliximab for refractory colitis. Furthermore, many tumors are intrinsically resistant to checkpoint blockade. Resistance mechanisms include:

  • Lack of pre‑existing T‑cell infiltration (“cold tumors”) – due to low mutational burden, poor antigen presentation, or physical barriers like dense stroma.
  • Activation of alternative immune checkpoints (e.g., TIM‑3, VISTA, LAG‑3) that compensate when PD‑1 is blocked.
  • Genetic alterations that interfere with antigen presentation (e.g., β2‑microglobulin loss) or interferon signaling (e.g., JAK1/2 mutations).
  • Severe immunosuppressive microenvironment driven by MDSCs, Tregs, or tumor‑associated macrophages.
  • Metabolic hostility – lactate accumulation, hypoxia, and nutrient deprivation.
  • WNT/β‑catenin pathway activation – associated with T‑cell exclusion.

Overcoming resistance is a major research focus. Combination strategies — pairing checkpoint inhibitors with chemotherapy, radiation, targeted therapy, or other immunomodulators — are being explored to convert “cold” tumors into “hot” ones. For example, combining PD‑1 inhibitors with anti‑angiogenic agents like bevacizumab has shown synergy in hepatocellular carcinoma and non‑small cell lung cancer.

Future Directions: Next‑Generation Checkpoints and Biomarkers

Research is moving beyond PD‑1 and CTLA‑4 to target additional checkpoints and combination approaches. Emerging targets include:

  • LAG‑3 – Already validated with relatlimab; clinical trials are investigating dual PD‑1/LAG‑3 blockade in multiple cancers, including melanoma, colorectal, and gastric cancers.
  • TIGIT – Anti‑TIGIT antibodies (e.g., tiragolumab) combined with atezolizumab have shown promise in lung cancer, especially in PD‑L1‑high patients. The CITYSCAPE trial reported improved progression‑free survival.
  • VISTA – An inhibitory checkpoint expressed on myeloid cells; early‑phase trials are ongoing, and its distinct expression pattern may offer a unique therapeutic window.
  • B7‑H3 and B7‑H4 – Co‑inhibitory ligands that are often overexpressed on tumors; antibody‑drug conjugates (ADCs) targeting them, such as enoblituzumab and others, are being developed.
  • CD39/CD73‑adenosine pathway – Extracellular adenosine generated from ATP by CD39 and CD73 suppresses T‑cell function. Small‑molecule inhibitors and antibodies are in clinical trials.
  • Bispecific antibodies – Molecules that simultaneously engage a tumor antigen (e.g., EGFR, PD‑L1) and a checkpoint receptor, potentially delivering the checkpoint blockade directly to the tumor site. Blinatumomab is an example but for CD19. Next‑generation bispecifics target PD‑1 and CTLA‑4 or PD‑1 and LAG‑3.

The FDA continues to approve novel checkpoint inhibitors and combination regimens — for example, the recent approval of nivolumab plus relatlimab for melanoma. Another frontier is identifying reliable biomarkers to predict who will benefit. PD‑L1 expression (measured by immunohistochemistry) is used in many settings, but it is an imperfect predictor. Tumor mutational burden (TMB), microsatellite instability (MSI‑H), and the composition of the tumor microenvironment (e.g., presence of CD8+ T cells, tertiary lymphoid structures) are increasingly used to guide patient selection. Gene expression signatures such as the T‑cell‑inflamed gene expression profile (GEP) are also being validated.

Combination with other immunotherapies, such as CAR‑T cell therapy or cancer vaccines, holds promise for further improving response rates. For example, personalized neoantigen vaccines designed to stimulate T cells against each patient’s unique tumor mutations are being tested alongside checkpoint inhibitors. A 2022 review in Nature Reviews Drug Discovery described the landscape of checkpoint inhibitors in clinical development and highlighted emerging targets. Additionally, oncolytic viruses like talimogene laherparepvec (T‑VEC) are being combined with checkpoint inhibitors to turn cold tumors hot.

Another exciting direction is the use of neoadjuvant checkpoint blockade before surgery. Several studies have shown that giving PD‑1 inhibitors before tumor resection increases pathological complete response rates and activates a broader T‑cell repertoire. This approach is being explored in lung cancer, melanoma, and bladder cancer. Finally, efforts to reduce toxicity through engineered antibodies with modified Fc regions or shorter half‑lives are underway, as well as the development of oral small‑molecule checkpoint inhibitors that may offer improved pharmacokinetics and tissue penetration.

Conclusion

Immune checkpoints are a cornerstone of modern cancer immunotherapy. They represent a remarkable example of how fundamental immunology can be translated into life‑saving therapies. The ability to release the immune system’s brakes has given new hope to patients with aggressive cancers and has permanently altered the oncology landscape. Yet many questions remain: why do some patients respond while others do not? How can we limit toxicities? Which combinations will be most effective? As research continues to uncover the complexity of immune regulation — from well‑known pathways like PD‑1 to newly discovered ones like TIGIT and VISTA — the future of cancer treatment will increasingly rely on precisely targeting these checkpoints, either alone or in rational combinations, to tip the balance in favor of a sustained anti‑tumor immune response. Cancer Research Institute offers detailed information for patients and researchers on checkpoint inhibitors. Understanding these molecular gates is not only essential for advancing immunotherapy but also for improving outcomes for millions of patients worldwide. The next decade promises to bring even more refined strategies, including biomarker‑driven patient selection, novel targets, and smarter combination regimens that maximize efficacy while minimizing harm.