Immunotherapy and Checkpoint Inhibitors

Cancer immunotherapy harnesses the patient's own immune system to attack tumour cells. Its most influential form, immune checkpoint blockade, uses antibodies against inhibitory receptors such as CTLA-4 and PD-1 to release the natural brakes on T cells, allowing them to recognize and kill cancer — an approach that produced durable responses in cancers once considered untreatable.

Definition

Immune checkpoint inhibitors are monoclonal antibodies that block inhibitory signalling pathways (such as CTLA-4 and PD-1/PD-L1) used by tumours to suppress T cells, thereby reactivating an antitumour immune response.

Scope

This topic covers the immunological rationale of checkpoint inhibition: how tumours evade immune attack, how blocking checkpoint pathways restores T-cell activity, the main antibody classes, the distinctive pattern of immune-related toxicity, and predictors of response. It is a conceptual reference and does not provide dosing or individualized treatment guidance.

Core questions

• How do tumours evade the immune system?
• What are immune checkpoints and how does blocking them work?
• Why do immune-related adverse events occur?
• Which patients are most likely to respond to checkpoint blockade?

Key concepts

• Cancer immunosurveillance and evasion
• T-cell co-inhibitory receptors
• CTLA-4 blockade
• PD-1 / PD-L1 blockade
• Immune-related adverse events
• Tumour mutational burden and PD-L1 expression
• Durable response and immune memory

Key theories

Immune checkpoint blockade

T-cell activation is normally restrained by inhibitory 'checkpoint' receptors; tumours exploit these to escape immune attack, and antibodies that block the checkpoints remove the restraint and unleash an antitumour T-cell response — a principle first demonstrated experimentally with CTLA-4 blockade.

Mechanisms

Effective T-cell attack on a tumour requires both antigen recognition and freedom from inhibitory signals. Tumours co-opt physiological checkpoint pathways — CTLA-4, which limits early T-cell priming, and PD-1 with its ligand PD-L1, which dampens T-cell activity in the tumour microenvironment — to suppress the immune response. Checkpoint-inhibitor antibodies block these interactions, restoring T-cell activation and proliferation against tumour antigens. Because the same pathways normally guard against autoimmunity, releasing them can provoke inflammation in healthy tissues, the characteristic immune-related adverse events. Response is more likely in tumours with higher mutational burden or PD-L1 expression, though these biomarkers are imperfect, and successful responses can be unusually durable, consistent with the establishment of immune memory.

Clinical relevance

Checkpoint inhibitors have become standard options across many cancers and are increasingly combined with chemotherapy, targeted agents, or each other. Understanding their immunology supports recognition of their distinctive benefit-and-toxicity profile and informed multidisciplinary care. This entry explains mechanisms and is not a basis for selecting agents or managing toxicity in any individual patient.

Evidence & guidelines

Checkpoint inhibitors entered practice through randomized trials in melanoma and other tumours, including combination CTLA-4 plus PD-1 blockade, and their use and the management of immune-related toxicity are structured by tumour-specific and toxicity-management guidelines (e.g., NCCN, ASCO, ESMO). This reference summarizes principles rather than reproducing those recommendations.

History

The field grew from the demonstration that blocking the inhibitory receptor CTLA-4 could enhance antitumour immunity in experimental models, work that, together with discovery of the PD-1 pathway, was recognized with a Nobel Prize. Anti-CTLA-4 and anti-PD-1 antibodies then showed durable benefit in advanced melanoma and a widening range of cancers, and combination checkpoint blockade further improved outcomes, establishing immuno-oncology as a major modality of systemic therapy.

Debates

Predicting who will respond

PD-L1 expression and tumour mutational burden are associated with benefit but neither reliably identifies responders or non-responders, leaving the search for robust predictive biomarkers an open and clinically important question.

Balancing efficacy against immune toxicity

Combination checkpoint blockade increases response rates but also the frequency and severity of immune-related adverse events, raising ongoing questions about how to weigh added efficacy against added harm.

Key figures

• James P. Allison
• Tasuku Honjo
• Antoni Ribas
• Jedd D. Wolchok
• Drew M. Pardoll

Seminal works

• leach-allison-1996
• ribas-wolchok-2018
• wolchok-2017

Frequently asked questions

How does immunotherapy differ from chemotherapy?

Chemotherapy attacks cancer cells directly, while checkpoint immunotherapy does not kill cells itself — it removes the brakes on the patient's own immune cells so they can recognize and destroy the tumour.

Why can immunotherapy cause inflammation in healthy organs?

The checkpoint pathways that the drugs block normally help prevent the immune system from attacking the body's own tissues, so releasing them can lead to immune-related inflammation in organs such as the skin, gut, or endocrine glands.