Cancer immunotherapy — the use of the immune system to recognize and destroy cancer cells — has transformed oncology over the past two decades. The concept is not new: William Coley began injecting bacterial extracts into tumors in the 1890s, observing occasional regressions. But the modern era of cancer immunotherapy began with the elucidation of immune checkpoint pathways and the development of drugs that release the brakes on anti-tumor immunity. The 2018 Nobel Prize in Physiology or Medicine awarded to James Allison and Tasuku Honjo for their discovery of immune checkpoint inhibitors underscored the magnitude of this breakthrough (Ribas & Wolchok, 2018, Science).
Why the immune system should fight cancer — but often doesn't
The immune system has inherent capacity to recognize and destroy cancer cells: tumor cells express neoantigens — mutated proteins not found in normal cells — that can be recognized by T cells; natural killer (NK) cells can detect and kill cells with reduced MHC class I expression (a common feature of cancer cells — the "missing self" hypothesis); and cancer immunosurveillance experiments demonstrate that immunodeficient mice develop more spontaneous tumors than immunocompetent mice. Yet cancers grow and metastasize because tumors evolve mechanisms of immune evasion — the process of "immunoediting" described by Schreiber, Old, and Smyth (2011, Science).
Tumor immune evasion mechanisms
Immune checkpoint upregulation — tumors express ligands (PD-L1, PD-L2) that engage inhibitory receptors (PD-1) on T cells → T cell exhaustion → impaired anti-tumor immunity; regulatory T cell (Treg) recruitment — tumors secrete chemokines and cytokines that recruit immunosuppressive Tregs to the tumor microenvironment; myeloid-derived suppressor cells (MDSCs) — tumors recruit MDSCs that suppress T cell function through arginase, nitric oxide synthase, and ROS production; immunosuppressive cytokines — tumors produce TGF-β, IL-10, and VEGF that create an immunosuppressive microenvironment; antigen loss — tumor cells lose neoantigen expression through immunoediting → escaping T cell recognition; and MHC downregulation — tumors reduce MHC class I expression → impaired antigen presentation → escape from CD8⁺ CTL killing (Vinay et al., 2015, Seminars in Cancer Biology).
Immune checkpoint inhibitors
Anti-CTLA-4 therapy
CTLA-4 (cytotoxic T-lymphocyte-associated protein 4) is an inhibitory receptor expressed on T cells: CTLA-4 competes with the co-stimulatory receptor CD28 for binding to B7 ligands (CD80/CD86) on antigen-presenting cells → CTLA-4 wins due to higher affinity → suppresses T cell activation; ipilimumab (Yervoy) — an anti-CTLA-4 monoclonal antibody — was the first checkpoint inhibitor approved (2011, for metastatic melanoma); and ipilimumab blocks the inhibitory CTLA-4 signal → enhanced T cell activation → anti-tumor immunity. James Allison's insight was that releasing this "brake" on T cells could unleash anti-tumor responses (Leach et al., 1996, Science).
Anti-PD-1/PD-L1 therapy
PD-1 (programmed cell death protein 1) is expressed on activated T cells; PD-L1 (and PD-L2) are expressed on tumor cells and other cells in the tumor microenvironment; PD-1/PD-L1 interaction sends an inhibitory signal → T cell exhaustion → impaired anti-tumor function; anti-PD-1 antibodies: nivolumab (Opdivo), pembrolizumab (Keytruda), cemiplimab (Libtayo); anti-PD-L1 antibodies: atezolizumab (Tecentriq), durvalumab (Imfinzi), avelumab (Bavencio); and these drugs have demonstrated efficacy across a remarkable range of cancers: melanoma, non-small cell lung cancer, renal cell carcinoma, Hodgkin lymphoma, bladder cancer, head and neck cancer, hepatocellular carcinoma, MSI-high/dMMR cancers (tissue-agnostic approval), and many others (Topalian et al., 2015, Cancer Cell).
CAR-T cell therapy
Chimeric antigen receptor (CAR) T cell therapy represents a personalized approach to cancer immunotherapy: patient T cells are collected by leukapheresis; T cells are genetically modified (using lentiviral or retroviral vectors) to express a chimeric antigen receptor — an engineered receptor combining: an extracellular antibody-derived antigen-binding domain (typically a single-chain variable fragment — scFv), a transmembrane domain, and intracellular signaling domains (CD3ζ chain + co-stimulatory domains such as CD28 or 4-1BB); the CAR-T cells are expanded in vitro and infused back into the patient; and the CAR enables T cells to recognize specific tumor surface antigens (independent of MHC) → direct cytotoxic killing of tumor cells. FDA-approved CAR-T therapies include: tisagenlecleucel (Kymriah — B-cell ALL and DLBCL), axicabtagene ciloleucel (Yescarta — DLBCL), lisocabtagene maraleucel (Breyanzi — DLBCL), brexucabtagene autoleucel (Tecartus — mantle cell lymphoma), idecabtagene vicleucel (Abecma — multiple myeloma), and ciltacabtagene autoleucel (Carvykti — multiple myeloma) (June et al., 2018, Science).
The future of cancer immunotherapy
The future directions of cancer immunotherapy include: combination checkpoint inhibitor therapy (anti-CTLA-4 + anti-PD-1 — already approved for several indications); bispecific antibodies that simultaneously engage T cells and tumor cells; tumor-infiltrating lymphocyte (TIL) therapy — expanded from resected tumors; oncolytic virus therapy (T-VEC — talimogene laherparepvec — already approved for melanoma); and personalized neoantigen vaccines (mRNA-based cancer vaccines — exploiting the mRNA platform developed for COVID-19 vaccines to create individualized cancer treatments).
Cancer immunotherapy has transformed the promise "the immune system can fight cancer" from a hope into a reality — producing durable, long-lasting responses in cancers that were previously death sentences within months. The revolution is still unfolding, and the next decade will likely bring even more transformative advances.
Immune-related adverse events (irAEs)
Checkpoint inhibitors, by releasing brakes on the immune system, can cause autoimmune-like side effects: any organ can be affected: dermatologic (rash, vitiligo, Stevens-Johnson syndrome), gastrointestinal (colitis — potentially life-threatening), hepatic (autoimmune hepatitis), endocrine (thyroiditis, hypophysitis, adrenal insufficiency, type 1 diabetes), pulmonary (pneumonitis), renal (nephritis), neurological (myasthenia gravis, Guillain-Barré), cardiac (myocarditis — rare but potentially fatal); irAE management: most respond to corticosteroids and checkpoint inhibitor interruption; severe irAEs may require additional immunosuppression (infliximab for colitis, mycophenolate for hepatitis); and interestingly, the development of certain irAEs (particularly vitiligo in melanoma patients) correlates with improved anti-tumor responses — suggesting a connection between autoimmunity and anti-tumor immunity (Postow et al., 2018, New England Journal of Medicine).
Biomarkers for immunotherapy response
Predicting which patients will respond to immunotherapy is a critical challenge: PD-L1 expression on tumor cells — used as a biomarker for anti-PD-1/PD-L1 therapy (measured by immunohistochemistry, with various scoring systems and cutoffs); however, PD-L1 expression is an imperfect predictor — some PD-L1-negative tumors respond, while some PD-L1-high tumors do not; tumor mutational burden (TMB) — tumors with higher mutation rates generate more neoantigens → more T cell targets → better response to checkpoint inhibitors; microsatellite instability (MSI-high)/deficient mismatch repair (dMMR) — these tumors have very high TMB → the first tissue-agnostic FDA approval for pembrolizumab was in MSI-H/dMMR tumors (regardless of cancer type); and tumor-infiltrating lymphocyte (TIL) density — tumors with pre-existing T cell infiltration ("hot" tumors) respond better than immunologically "cold" tumors (Gibney et al., 2016, Lancet Oncology).
Adoptive cell therapy beyond CAR-T
Beyond CAR-T cells, other adoptive cell therapies are being developed: tumor-infiltrating lymphocyte (TIL) therapy — T cells are isolated from the patient's resected tumor, expanded ex vivo (billions of cells), and re-infused; lifileucel (Amtagvi) was the first TIL therapy approved by the FDA (2024, for advanced melanoma); TIL therapy targets the full spectrum of tumor neoantigens — potentially overcoming the single-target limitation of CAR-T; TCR-engineered T cells — T cells are engineered with specific T cell receptors (TCRs) targeting known tumor antigens; tebentafusp (Kimmtrak) — a bispecific gp100-targeting TCR-CD3 fusion — was approved for uveal melanoma; and natural killer (NK) cell therapies — exploiting NK cells' innate ability to recognize and kill tumor cells without prior sensitization.
The cost of cancer immunotherapy
The cost of immunotherapy raises important ethical and practical questions: checkpoint inhibitors cost approximately $100,000-$250,000 per year of treatment; CAR-T cell therapy costs approximately $400,000-$500,000 per treatment (excluding hospitalization for side effects); and the value proposition is complex: for patients who achieve durable complete responses (potentially curative), the lifetime cost savings may be enormous (compared to years of traditional chemotherapy and palliative care); but the upfront costs strain healthcare budgets globally, and access disparities are significant — particularly in low- and middle-income countries where cancer incidence is rising fastest.
Tumor microenvironment and immunotherapy
The tumor microenvironment (TME) is critical to immunotherapy response: "hot" tumors — characterized by dense T cell infiltration, high PD-L1 expression, and IFN-γ signaling — respond best to checkpoint inhibitors; "cold" tumors — immunologically silent, with few infiltrating T cells, low neoantigen load, and active immune exclusion mechanisms — respond poorly; strategies to convert cold tumors to hot: radiation therapy (immunogenic cell death → release of tumor antigens → "in situ vaccination"), oncolytic viruses (selective tumor cell lysis → inflammatory response → T cell recruitment), intratumoral injection of immune stimulants (STING agonists, TLR agonists), and epigenetic therapies (HDAC inhibitors, DNA methyltransferase inhibitors — can upregulate antigen processing and MHC expression) (Galon & Bruni, 2019, Nature Reviews Drug Discovery).
Combination immunotherapy strategies
The future of cancer immunotherapy increasingly involves combinations: dual checkpoint blockade — anti-CTLA-4 + anti-PD-1 (ipilimumab + nivolumab — approved for melanoma, renal cell carcinoma, colorectal cancer MSI-H, hepatocellular carcinoma, non-small cell lung cancer); checkpoint inhibitors + chemotherapy — chemotherapy can enhance anti-tumor immunity through: immunogenic cell death, MHC upregulation on tumor cells, and depletion of immunosuppressive cells; checkpoint inhibitors + targeted therapy — anti-PD-1 + anti-VEGF (pembrolizumab + lenvatinib) → VEGF inhibition normalizes tumor vasculature → improves T cell infiltration; and checkpoint inhibitors + vaccines — personalized neoantigen vaccines prime the immune system against tumor-specific antigens → checkpoint inhibitors remove the brakes → synergistic anti-tumor immunity.
Immunotherapy in historically difficult cancers
Immunotherapy is making inroads in cancers traditionally considered immunotherapy-resistant: pancreatic cancer — largely immunotherapy-resistant due to dense fibrotic stroma and immunosuppressive microenvironment — but combination approaches (anti-PD-1 + gemcitabine, personalized mRNA vaccines) show early promise; glioblastoma — the blood-brain barrier limits T cell access, and the brain microenvironment is immunosuppressive — but intratumoral checkpoint inhibitors and CAR-T cells are being studied; prostate cancer — succeeded with one therapeutic cancer vaccine (sipuleucel-T/Provenge — autologous dendritic cell therapy) but has been largely resistant to checkpoint inhibitors; and small cell lung cancer — atezolizumab + chemotherapy showed modest but statistically significant improvement — demonstrating that even notoriously aggressive cancers can respond.
The immunotherapy revolution: perspective
The immunotherapy revolution in oncology represents a paradigm shift comparable to the introduction of targeted therapy: before immunotherapy, metastatic melanoma had a median survival of approximately 8-10 months; with combination checkpoint blockade, approximately 50% of patients are alive at 5 years; some patients achieve durable complete responses that may represent functional cures; and the principle — that activating the immune system against cancer can produce lasting remissions — has been validated across dozens of cancer types, fundamentally changing the practice of oncology.
Liquid biopsies and immunotherapy monitoring
Circulating biomarkers are transforming immunotherapy monitoring: circulating tumor DNA (ctDNA) — measuring cell-free DNA shed by tumors into the bloodstream: decreasing ctDNA during treatment predicts response; molecular response (ctDNA clearance) may identify patients who can safely discontinue therapy; early ctDNA dynamics (within weeks of starting treatment) can predict long-term outcomes; circulating tumor cells (CTCs) — enumeration and molecular characterization; soluble PD-L1 — elevated levels may predict poor prognosis; and the development of minimally invasive monitoring tools is critical for: early detection of resistance, treatment duration optimization, and immunotherapy response prediction — reducing the need for repeated tissue biopsies.
CAR-T cell side effects
CAR-T cell therapy, while potentially curative, produces unique and sometimes severe side effects: cytokine release syndrome (CRS): massive cytokine release (IL-6, IL-10, IFN-γ) by activated CAR-T cells → fever, hypotension, hypoxia, organ dysfunction — can be life-threatening; treatment: tocilizumab (anti-IL-6 receptor antibody) is remarkably effective at reversing CRS; immune effector cell-associated neurotoxicity syndrome (ICANS): confusion, aphasia, seizures, cerebral edema — mechanism not fully understood; B cell aplasia: anti-CD19 CAR-T cells eliminate normal B cells along with malignant B cells → hypogammaglobulinemia → requires immunoglobulin replacement; and on-target, off-tumor effects: CAR-T cells may attack normal tissues expressing the target antigen — a major challenge for solid tumor CAR-T development (where truly tumor-specific antigens are rare).
Cancer vaccines in the immunotherapy arsenal
Cancer vaccines are experiencing a renaissance driven by immunotherapy synergies and mRNA technology: sipuleucel-T (Provenge) — autologous dendritic cell vaccine for metastatic castration-resistant prostate cancer (the first FDA-approved cancer therapeutic vaccine — 2010); personalized neoantigen vaccines — using next-generation sequencing to identify patient-specific tumor mutations → synthesizing mRNA or peptide vaccines targeting those neoantigens → autogene cevumeran (BioNTech/Genentech mRNA neoantigen vaccine for pancreatic cancer) showed remarkable Phase 1 results (50% of patients showed T cell responses to individualized neoantigens → improved recurrence-free survival); BCG (Bacille Calmette-Guérin) for bladder cancer — the oldest and most established cancer immunotherapy (intravesical BCG instillation for non-muscle-invasive bladder cancer — standard of care since the 1970s); and in situ vaccination — using radiation, oncolytic viruses, or intratumoral immunostimulants to turn a tumor into its own vaccine → releasing antigens in an immunogenic context → generating systemic anti-tumor immunity (the "abscopal effect").
Cancer immunotherapy has evolved from a radical idea (the immune system can fight cancer) to a proven clinical reality (checkpoint inhibitors, CAR-T cells, and cancer vaccines are saving lives). The next frontier involves: bringing immunotherapy to cancers currently resistant to it, reducing the cost to enable global access, improving biomarker-guided patient selection, managing and minimizing immune-related toxicity, and combining multiple immunotherapy modalities for synergistic anti-tumor effects. The revolution is far from over.
Immunotherapy and the microbiome
Emerging research connects the gut microbiome to immunotherapy response: patients with diverse gut microbiomes (particularly those enriched in Bifidobacterium, Faecalibacterium, and Ruminococcaceae) show better responses to checkpoint inhibitors; antibiotic use before or during immunotherapy is associated with reduced response and worse outcomes — potentially through disruption of beneficial gut bacteria; fecal microbiota transplantation (FMT) from immunotherapy responders has been shown to enhance checkpoint inhibitor response in some non-responders; and dietary interventions (high-fiber diets) are being studied as microbiome-modulating strategies to improve immunotherapy efficacy (Routy et al., 2018, Science).
Immunotherapy and autoimmune disease
A paradox of checkpoint inhibitors is their relationship with pre-existing autoimmune disease: patients with autoimmune conditions were largely excluded from early checkpoint inhibitor trials; retrospective analyses suggest that approximately 30-50% of patients with pre-existing autoimmune disease experience flares when treated with checkpoint inhibitors; however, many patients can be successfully treated with careful monitoring and management; and the observation that checkpoint inhibitors can trigger autoimmunity has provided insights into autoimmune disease mechanisms — leading to research on the role of PD-1/PD-L1 in maintaining peripheral tolerance.
Immunotherapy in pediatric oncology
Immunotherapy has made important advances in pediatric cancer: CAR-T cell therapy (tisagenlecleucel) was first approved for pediatric B-cell ALL — producing complete remission rates of approximately 80% in relapsed/refractory patients who had exhausted all other options; checkpoint inhibitors are approved for pediatric Hodgkin lymphoma and MSI-H cancers; and pediatric cancers present unique immunotherapy challenges: lower mutational burden (fewer neoantigens), more embryonal tumor types, and developing immune systems — requiring adapted approaches.
Cancer immunotherapy has transformed from William Coley's bacterial extracts to precision-guided immune warfare — checkpoint inhibitors, engineered T cells, personalized vaccines, and bispecific antibodies. The fundamental insight — that the immune system inherently recognizes cancer as foreign but is suppressed by tumor-evolved escape mechanisms — has produced a paradigm shift in oncology that is still unfolding and will continue to produce breakthroughs for decades to come.
The immunotherapy revolution represents one of the most consequential advances in the history of medicine. The fundamental insight — that the immune system can be directed against cancer through checkpoint blockade, engineered cellular therapies, and targeted vaccination — has created a new paradigm that is saving lives that would have been lost just a decade ago. The work is far from over, but the proof of concept is incontrovertible: the immune system, properly unleashed, can fight cancer.
From Coley's toxins to CAR-T cells, from immune surveillance theory to precision immune checkpoint blockade — the journey of cancer immunotherapy represents one of the most remarkable stories of scientific persistence and eventual triumph in the history of medicine. The immune system is the most sophisticated defense system in biology — and we are only beginning to learn how to deploy it against cancer.
Immunotherapy has fundamentally changed what is possible in cancer treatment. The question is no longer whether the immune system can fight cancer — it can. The question now is how to extend that power to every patient, every tumor, and every setting where cancer threatens human life.