Vδ1 T cells belong to the gamma-delta T cell family, a small subset of lymphocytes that carry an unconventional T cell receptor built from delta and gamma chains rather than the familiar alpha and beta chains found on most T cells. What makes them attractive for cancer immunotherapy is their recognition strategy. Unlike conventional T cells, which scan cells for peptide fragments presented on major histocompatibility complex molecules, Vδ1 T cells detect stress-associated ligands and other danger signals that appear on the surface of transformed and infected cells. This MHC-independent recognition means that donor-derived Vδ1 T cells can, in principle, be infused into unrelated recipients without the severe rejection risks and graft-versus-host complications that complicate other allogeneic cell therapies. Their natural tendency to home into tissues adds a further advantage for attacking solid tumors, which are notoriously difficult for circulating immune cells to infiltrate.

Yet the same rarity that makes Vδ1 T cells immunologically convenient also makes them logistically challenging. They typically account for only a small fraction of the lymphocytes circulating in peripheral blood, so isolating enough of them for manufacturing has been a persistent obstacle. The research team, led by Shuntian Chen, Lujie Zhao, Xinrui Xie, Haoyuan Wu and senior author Hongling Wu, with institutions including the Guangzhou Institute of Respiratory Health at Guangzhou Medical University and the Guangzhou Institutes of Biomedicine and Health of the Chinese Academy of Sciences, set out to determine whether IL-18, a cytokine already known to shape the behavior of conventional T cells and natural killer cells, could unlock the proliferative potential of Vδ1 T cells as well.

The experimental design was straightforward but systematic. The researchers isolated primary human Vδ1 T cells from donor blood using magnetic bead separation and cultured them with antibodies against CD3, a coreceptor that provides T cells with an activation signal, together with interleukin-15, a growth factor widely used to drive the expansion of innate-like lymphocytes. Some cultures received IL-18 in addition; others did not. The team then tracked how fast the cells divided, what surface molecules they displayed, and how long they survived, using flow cytometry to quantify each of these parameters across the culture period.

The results on proliferation were striking. IL-18 significantly accelerated cell-cycle progression, pushing the Vδ1 T cells into division more rapidly and sustaining robust clonal expansion over the course of the culture. Crucially, this growth did not come at the cost of purity: the expanded populations retained a high proportion of genuine Vδ1 T cells, an important consideration for any manufacturing process in which overgrowth of contaminating cell types can render a product unusable. For a cell type whose scarcity has been the primary barrier to clinical translation, the ability to generate large, clean batches from a modest starting population represents a substantial advance in manufacturability.

Expansion alone, however, is not enough. Cells grown in the laboratory must remain functional when they encounter the hostile environment of a tumor, where nutrient deprivation, hypoxia and immunosuppressive signals conspire to exhaust or kill infiltrating lymphocytes. Here too, IL-18 left a measurable imprint. The cytokine-treated Vδ1 T cells showed enhanced survival during prolonged exposure to tumor cells, suggesting that IL-18 conditioning confers a degree of resistance to the apoptotic pressures of the tumor microenvironment. They also produced greater quantities of inflammatory cytokines, including interferon-gamma and tumor necrosis factor alpha, which can recruit and activate other arms of the immune response, and they migrated more effectively toward tumor-derived signals in directed migration assays.

One result complicates the otherwise rosy picture and reveals something important about how IL-18 reshapes these cells. In short-term laboratory killing assays, the IL-18-expanded Vδ1 T cells actually showed reduced contact-dependent cytotoxicity, meaning they were somewhat less efficient at directly killing tumor cells through cell-to-cell contact. This apparent deficit, however, did not translate into weaker antitumor activity in living animals. The researchers turned to RNA sequencing and quantitative PCR to probe the transcriptional shifts underlying these changes, finding alterations in gene programs associated with cell survival, inflammatory signaling and apoptosis regulation, including changes in genes such as BAX, a pro-apoptotic regulator, and death receptor pathway components. The molecular profile suggested that IL-18 redirects the cells away from a purely cytotoxic specialization and toward a more durable, inflammation-oriented and tissue-infiltrating state.

The decisive test came in an animal model. The team used mice bearing xenografts of A549 cells, a human lung carcinoma line derived from a non-small cell lung cancer, and compared the therapeutic effects of IL-18-conditioned Vδ1 T cells against control Vδ1 T cells expanded without the cytokine. Bioluminescence imaging was used to track tumor burden over time. The IL-18-expanded cells suppressed tumor growth more effectively and, most importantly, significantly prolonged the survival of the tumor-bearing mice. The in vivo superiority of the IL-18-conditioned product, despite its reduced immediate killing capacity in vitro, underscores a recurring lesson in cell therapy development: laboratory killing assays do not always predict therapeutic performance, and properties such as persistence, migration and cytokine output can matter more for solid tumor control than raw cytotoxicity measured over a few hours in a culture dish.

The translational implications are considerable. Because Vδ1 T cells can be used in an allogeneic, off-the-shelf format, an optimized expansion protocol could enable the production of standardized cell products from healthy donors, manufactured in advance and administered to many patients without the individualized engineering timelines required for autologous therapies such as CAR-T cells. The study also did not involve clinical research on human subjects; all animal work was approved by the Institutional Animal Care and Use Committee of the Guangzhou Institutes of Biomedicine and Health, and the findings remain at the preclinical stage. Moving from xenograft models to human trials will require demonstrating safety, dosing feasibility and efficacy in patients, and the authors note that the work was supported by the National Natural Science Foundation of China and the National Key Research and Development Program of China.

For the broader field of gamma-delta T cell immunotherapy, the study adds IL-18 to the toolkit of cytokines that can be deployed to tune cell products for specific therapeutic goals. Interleukin-15 has long served as the workhorse expansion factor for innate-like lymphocytes, but IL-18 appears to impart a complementary identity, one that favors longevity, inflammatory potency and tumor homing over immediate cytolytic function. As researchers continue to engineer gamma-delta T cells with chimeric antigen receptors and other enhancements, the ability to shape their baseline fitness through cytokine conditioning could prove essential. If the results hold up in clinical testing, a simple change to the culture recipe, the addition of a single signaling molecule, may help transform a scarce circulating curiosity into a scalable weapon against some of the hardest tumors to treat.

Subject of Research: IL-18-driven expansion and functional enhancement of human Vδ1 gamma-delta T cells for solid tumor immunotherapy

Article Title: IL‑18 improves the expansion and functional fitness of human Vδ1 T cells for superior solid tumor immunotherapy

Article References: IL‑18 improves the expansion and functional fitness of human Vδ1 T cells for superior solid tumor immunotherapy. (n.d.). https://doi.org/10.1186/s12967-026-09079-w

Image Credits: AI Generated

DOI: 10.1186/s12967-026-09079-w

Keywords: Vδ1 T cells, gamma-delta T cells, IL-18, interleukin-15, immunotherapy, solid tumors, lung cancer, adoptive cell therapy, cytokines, tumor microenvironment, allogeneic cell therapy, preclinical study

The rationale behind the trial was grounded in a genuine biological puzzle. In an earlier phase 1 study, inhaled recombinant human interleukin-15, or rhIL-15, proved safe in dogs with metastatic osteosarcoma and melanoma and produced durable responses in a subset of animals. Intriguingly, dogs that entered the trial with lower absolute lymphocyte counts were statistically more likely to benefit. The Davis team hypothesized that reducing lymphocytes before treatment might remove so-called cytokine sinks, circulating immune cells that soak up interleukin-15 before it can reach the natural killer cells and CD8-positive memory T cells the drug is meant to energize.

Lymphodepletion is a well-established strategy in human adoptive cell therapy, where chemotherapy is routinely used before infusing engineered T cells to improve their expansion and persistence. Doxorubicin, a mainstay chemotherapy agent in veterinary medicine, had previously been shown to reduce circulating T lymphocytes in tumor-bearing dogs. The researchers therefore designed a trial in which dogs received a standard dose of doxorubicin, 30 milligrams per square meter for larger animals or 1 milligram per kilogram for smaller ones, followed one week later by two weeks of inhaled rhIL-15 at 50 micrograms twice daily, delivered by nebulizer at home by trained owners.

The study population consisted of ten client-owned dogs with histologically or cytologically confirmed melanoma or bone tumors, all carrying at least one lung lesion of a centimeter or larger on thoracic radiographs. Five dogs had osteosarcoma, four had melanoma, and one had a multilobular tumor of bone. Eligibility criteria were strict: dogs needed adequate organ function, no prior rhIL-15 exposure, no recent chemotherapy or immunotherapy, and screening echocardiograms to rule out cardiac disease that could be worsened by doxorubicin. The trial was approved by the UC Davis institutional review boards, and all owners provided informed consent.

When the results came in, the numbers told a sobering story. Two dogs progressed before the Day 35 response assessment, leaving eight evaluable patients. One dog achieved a complete response that has now lasted more than two years, two had stable disease, and five progressed. That translates to an overall response rate of 10 percent and a clinical benefit rate of 30 percent, figures essentially indistinguishable from the phase 1 trial of inhaled rhIL-15 alone, which reported a response rate of 11 percent and a clinical benefit rate of 39 percent. The trial had been powered to detect a jump in response rate from 11 to 50 percent, so the null result is unlikely to be a statistical fluke of modest effect size.

The hematological data confirmed the team did achieve the biological precondition they set out to create. Seven days after doxorubicin, mean absolute lymphocyte counts fell from 1,452 to 1,023 cells per microliter, a statistically significant drop that brought the dogs to lymphocyte levels comparable to those seen in responding patients from the earlier trial. White blood cells, monocytes, and neutrophils also declined significantly, and one dog developed grade 4 neutropenia with fever that resolved within two days of hospitalization and supportive care. Crucially, adding doxorubicin did not increase the toxicity of the inhaled cytokine therapy itself, and no allergic reactions or pulmonary adverse events were attributed to rhIL-15.

Transcriptomic analysis added an unexpected layer of insight. Using the NanoString nCounter canine immuno-oncology panel covering 780 genes, the researchers profiled peripheral blood mononuclear cells collected before doxorubicin, after doxorubicin, and after two weeks of inhaled rhIL-15. While overall lymphocyte counts fell, the relative proportions of T cells, CD8-positive T cells, exhausted CD8-positive T cells, natural killer cells, and Th1 cells did not change significantly, indicating that doxorubicin depleted lymphocytes indiscriminately rather than selectively removing suppressive populations. The one clear casualty was the B cell compartment: B cell scores and B cell function genes, including CD19, CD79B, MS4A1, and immunoglobulin genes, dropped significantly and had not recovered by Day 21, consistent with the rapid proliferation rate of B cells making them especially vulnerable to chemotherapy.

Perhaps the most tantalizing clue came from the single dog that achieved a complete response. That patient initially appeared to progress at Day 35, with new lung lesions appearing on radiographs, but after receiving palliative radiation to its primary mandibular melanoma, every lung lesion resolved by week 15. The researchers suggest this may represent pseudoprogression, a well-recognized phenomenon in human immunotherapy where apparent tumor growth reflects immune cell infiltration rather than true malignancy, though an abscopal effect triggered by the radiation cannot be excluded. Transcriptomically, this dog trended toward higher T cell, CD8-positive T cell, and cytotoxic cell scores, and started with a lower exhausted CD8 T cell score than non-responders, hinting that baseline immune fitness may predict who benefits from inhaled rhIL-15.

The study carries important limitations that the authors acknowledge candidly. Ten patients is a small cohort, and two dogs did not survive to formal response assessment. The therapy used human rather than canine interleukin-15, which likely provoked neutralizing antibodies, although the two-week treatment window was chosen to minimize that risk. Target lesions smaller than the standard 2-centimeter RECIST threshold were permitted, matching the phase 1 design but introducing radiographic measurement imprecision. Tumor type may also have influenced outcomes, since one of the stable disease patients had a multilobular tumor of bone, an inherently slower-growing cancer.

Even so, the trial delivers a valuable double message. First, it definitively rules out standard-dose doxorubicin preconditioning as a way to enhance inhaled rhIL-15 efficacy, redirecting the field toward other combination strategies such as radiation, which has already shown promise with cytokine therapy in canine sarcoma and melanoma. Second, it confirms that durable, multi-year complete responses are achievable even in dogs with diffuse pulmonary metastatic disease, a population whose prognosis otherwise hovers below 80 days. For the dogs that respond, and for the human osteosarcoma and melanoma patients their disease so closely mirrors, that signal is worth chasing with better-designed combinations.

Subject of Research: Doxorubicin preconditioning and inhaled recombinant human IL-15 immunotherapy in dogs with pulmonary metastasis

Article Title: Evaluating the impact of doxorubicin preconditioning on the efficacy of inhaled recombinant human IL-15 immunotherapy in dogs with pulmonary metastasis

Article References: Luker, M. E., York, D., Al-Nadaf, S., Johnson, E. G., Withers, S. S., Cruz, S. M., Lejeune, A., Skorupski, K. A., Young, J., Cohen-Davidyan, T., Sparger, E. E., Culp, W. T., Murphy, W. J., Kent, M. S., Canter, R. J., & Rebhun, R. B. (2025). Evaluating the impact of doxorubicin preconditioning on the efficacy of inhaled recombinant human IL-15 immunotherapy in dogs with pulmonary metastasis. Veterinary Oncology, 2(1), Article 23. https://doi.org/10.1186/s44356-025-00040-5

Image Credits: AI Generated

DOI: 10.1186/s44356-025-00040-5

Keywords: immunotherapy, interleukin-15, doxorubicin, canine osteosarcoma, melanoma, lung metastasis, lymphodepletion, veterinary oncology, natural killer cells, CD8 T cells, pseudoprogression, transcriptomics

What makes natural killer cells so attractive as an anti-cancer weapon is their biology. Unlike cytotoxic T cells, which must be primed against a specific antigen before they can attack, NK cells are hardwired to detect and destroy transformed tissue on contact, without prior sensitization. They patrol the body constantly, integrating activating and inhibitory signals through a sophisticated array of germline-encoded receptors. When a cell loses the surface expression of major histocompatibility complex class I molecules, a common immune-evasion trick employed by tumors, the inhibitory brakes on NK cells are released and the killer cell delivers its lethal hit through the polarized release of lytic granules containing perforin and granzymes. NK cells also deploy activating receptors such as NKG2D and the natural cytotoxicity receptors NKp30, NKp44, and NKp46, which recognize stress-induced ligands upregulated on malignant cells. Beyond direct killing, they shape the broader immune response by secreting interferon-gamma and tumor necrosis factor alpha, which in turn promote the maturation of dendritic cells and support T cell priming, making them both executioners and orchestrators of antitumor immunity.

The liver, however, is a uniquely hostile arena for these cells. The review devotes considerable attention to the hepatocellular carcinoma tumor microenvironment, describing it as a formidable barrier that systematically disables NK cell function. Chronic exposure to transforming growth factor beta, an immunosuppressive cytokine abundantly produced in cirrhotic and tumor-bearing livers, downregulates activating receptors and blunts cytotoxicity. Persistent hypoxia, a hallmark of rapidly growing tumors with disorganized vasculature, further erodes NK cell metabolism and effector function. Perhaps most strikingly, the review highlights metabolic exhaustion driven by lipid accumulation: tumor cells and their supporting stroma flood the microenvironment with lactic acid and lipids, reprogramming NK cell metabolism and starving the cells of the glycolytic capacity they need to mount an effective attack. Studies cited in the review show that lactate dehydrogenase-associated lactic acid production directly blunts tumor immunosurveillance by both T and NK cells, and that fructose-1,6-bisphosphatase-induced inhibition of glycolysis cripples NK cell function during tumor progression.

The clinical evidence of NK cell dysfunction in liver cancer is equally sobering. Single-cell sequencing studies of human liver tumors have mapped the landscape of infiltrating immune cells and revealed that tumor-resident NK cells are phenotypically and functionally compromised. Tissue-resident CD56-bright NK cells enriched in the adult human liver, characterized by high Eomes and low T-bet expression, show reduced proinflammatory potential compared with their blood counterparts. In patients with hepatocellular carcinoma, myeloid-derived suppressor cells inhibit NK cell activity through the NKp30 receptor, and elevated levels of soluble NKG2D ligands and immunomodulatory molecules such as DKK3 further dampen antitumor responses. Exhaustion markers tell a similar story: NK cells expressing the inhibitory receptors TIGIT, TIM-3, CD96, and programmed death 1 correlate with disease progression in hepatitis B virus-related hepatocellular carcinoma, and high CD96 expression on NK cells predicts poorer prognosis. Downregulation of the PR/SET domain 10 protein has also been implicated as an underlying mechanism of NK cell dysfunction in these patients.

Against this backdrop, the review maps out the therapeutic strategies now being pursued to restore or supercharge NK cell activity. The most straightforward approach is cytokine-supported adoptive transfer: NK cells harvested from patients or healthy donors are expanded ex vivo using cytokines such as interleukin-15 and interleukin-21, then infused back in large numbers. Interleukin-15 has shown particular promise, with experimental work demonstrating that it can overcome hepatocellular carcinoma-induced NK cell dysfunction. The foundational proof of concept came from early clinical studies showing successful adoptive transfer and in vivo expansion of haploidentical NK cells in patients with cancer, and more recent trials have extended the approach to liver cancer specifically. A phase I trial of NK cell infusion in patients with recurrent hepatocellular carcinoma after liver transplantation reported that the treatment was well tolerated and showed preliminary efficacy, while an open-label phase I study protocol is testing adoptive immunotherapy with NK cells derived from peripheral blood CD34-positive stem cells to prevent recurrence after curative hepatectomy.

Genetic engineering has pushed the field considerably further. Chimeric antigen receptor-engineered NK cells, or CAR-NK cells, are equipped with synthetic receptors that redirect their killing capacity toward tumor-associated antigens. In hepatocellular carcinoma, the leading targets are glypican-3, or GPC3, a cell surface proteoglycan abundantly expressed on malignant hepatocytes, and CD147, a transmembrane glycoprotein implicated in tumor invasion. GPC3-specific CAR-NK cells have been developed and validated in preclinical models, and anti-CD147 chimeric antigen receptors have demonstrated efficacy against hepatocellular carcinoma cells. A first-in-human phase 1 trial of induced pluripotent stem cell-derived CD19-directed CAR-NK cells in B-cell lymphoma has provided an important clinical template for the broader platform, demonstrating that off-the-shelf, genetically defined NK cell products can be manufactured and safely administered. The review notes, however, that head-to-head comparisons suggest CAR-T cells can outperform CAR-NK cells in some CAR-mediated effector functions, underscoring that the technology still has room to mature.

A parallel and rapidly evolving strategy involves bispecific and trispecific killer cell engagers, known as BiKEs and TriKEs. These antibody-like molecules physically bridge NK cells to tumor cells while simultaneously delivering activating signals. Trispecific killer engagers incorporating an interleukin-15 linker have been shown to direct NK cells toward CD33-positive targets while inducing persistence, in vivo expansion, and enhanced function. Tri-specific engagers targeting mesothelin have been used to steer NK cells toward lung cancer, and the platform is now being applied to liver cancer: a GPC3-targeted NK engager bispecific antibody has entered a phase 1/2a clinical trial in patients with hepatocellular carcinoma, and novel NKp46-based engagers targeting GPC3 have undergone preclinical characterization. Recent work has also produced therapeutic bispecific antibodies targeting CD16A on NK cells and GPC3 on tumor cells, and tetravalent bispecific engagers designed to activate NK cells against cancer without mediating fratricide, a key safety consideration when the targeting antigen is also expressed on the therapeutic cells themselves.

Combination strategies form another pillar of the review. The multikinase inhibitor sorafenib, a longstanding standard of care in advanced hepatocellular carcinoma, has been shown to perpetuate cellular anticancer effector functions by modulating crosstalk between macrophages and NK cells, and preclinical studies of sorafenib combined with memory-like NK cells have reported encouraging antitumor activity in models of the disease. Xenograft experiments mapping the signaling pathways engaged by combined sorafenib and NK cell therapy have reinforced the mechanistic rationale. Lenvatinib, a newer tyrosine kinase inhibitor, appears to activate antitumor immunity by suppressing immunoinhibitory infiltrates in the tumor microenvironment, creating a permissive setting for adoptive cell therapy. Immune checkpoint blockade is also being explored in combination with NK cell approaches, with single-cell RNA sequencing in humanized mice revealing antitumor potency of NK and CD8-positive T cells when PD-1 and TIGIT are co-targeted. Even oncolytic viruses have entered the picture, with an engineered measles virus shown to enhance the antitumor responses of adoptively transferred NKG2D-positive immune cells in hepatocellular carcinoma models.

The review is candid about the knowledge gaps that still separate laboratory promise from routine clinical benefit. The complex interplay between NK cells and the tumor microenvironment remains incompletely understood, particularly the roles of hepatic stellate cells and cancer-associated fibroblasts, which secrete chemokines such as CXCL5 that promote immunosuppressive signaling. The metabolic reprogramming of NK cells in the lipid-rich liver environment, the mechanosensory properties that govern NK cell function in dense tumor tissue, and the education and licensing processes that calibrate NK cell responsiveness all demand further study. Chronic alcohol consumption, a major risk factor for liver disease, has been shown to impair peripheral NK cell development by reducing interleukin-15 availability, adding another layer of complexity in a patient population where such exposures are common. The authors argue that resolving these questions is essential for optimizing NK cell immunotherapy and improving prognostic outcomes.

Nevertheless, the trajectory of the field is unmistakably upward. From the NK-92 cell line, whose three decades of use have shaped much of modern NK cell research, to iPSC-derived CAR-NK products and GPC3-directed engagers now in clinical trials, the toolkit for deploying natural killer cells against hepatocellular carcinoma has expanded dramatically in a short period. The review’s synthesis suggests that the coming years will be defined by rational combinations, pairing engineered NK cells with kinase inhibitors, checkpoint blockade, and metabolic interventions designed to neutralize the tumor microenvironment’s defenses. For a cancer that has long resisted immunotherapeutic advances and continues to claim hundreds of thousands of lives each year, the renaissance in NK cell biology may represent the most credible path yet toward durable, off-the-shelf immunotherapy for liver cancer, and the scientific foundation laid by this comprehensive analysis offers researchers a clear map of both the opportunities and the obstacles that lie ahead.

Subject of Research: Natural killer cell-based immunotherapy for hepatocellular carcinoma

Article Title: Natural killer cell-based therapy for hepatocellular carcinoma (HCC): A literature review

Article References: Widowati, W., Wargasetia, T. L., Sadri, B., Nainggolan, I. M., Azis, R., Rifana, S. D., Rismani, E., & Vosough, M. (2026). Natural killer cell-based therapy for hepatocellular carcinoma (HCC): A literature review. Medical Oncology, 43(11), Article 293. https://doi.org/10.1007/s12032-026-03393-5

Image Credits: AI Generated

DOI: 10.1007/s12032-026-03393-5

Keywords: natural killer cells, hepatocellular carcinoma, immunotherapy, CAR-NK cells, tumor microenvironment, glypican-3, bispecific killer cell engagers, interleukin-15, liver cancer, adoptive cell therapy, NK cell dysfunction, sorafenib

The central concept guiding this research is the tumor microenvironment, or TME, the scaffold of extracellular matrix within which tumor cells and host immune cells communicate and compete. Many cancers are described as immunologically cold, meaning they contain few functional cytotoxic immune cells and resist the effects of immune checkpoint inhibitors, the landmark therapies that block inhibitory receptors such as PD-1 and PD-L1 to unleash CD8-positive T cells. Checkpoint inhibitors have transformed survival for many patients since emerging in the 1990s, but their efficacy is throttled by features of the TME, including low T cell numbers, T cell exhaustion, oxygen-starved tissue, and dense populations of immunosuppressive cells. The review’s authors, led by Campbell M. Johnston and Hongji Zhang of the University of Virginia’s Department of Surgery, argue that exercise directly counters many of these barriers, effectively warming cold tumors and sensitizing them to drugs that were previously powerless against them.

One of the most detailed lines of evidence concerns tumor-associated macrophages, or TAMs, immune cells that are abundant within cold tumors and frequently promote malignancy. TAMs exist along a spectrum from the M1 phenotype, which fights tumors, to the M2 phenotype, which secretes immunosuppressive molecules such as interleukin-4, interleukin-10, PD-L1, and transforming growth factor-beta that blunt effector T cell function and empower regulatory T cells. Worse, TAMs can push CD8-positive T cells into a terminally exhausted state from which they cannot recover, and hypoxia accelerates this process. In mouse models of glioblastoma, depleting TAMs increased the proportion of stem-like progenitor-exhausted T cells and improved responsiveness to anti-PD-1 therapy, underscoring how central these cells are to treatment failure. Preclinical studies now show that aerobic exercise can tip the balance, repolarizing macrophages from the pro-tumor M2 state toward the anti-tumor M1 state and reducing total M2 numbers within tumors.

The macrophage data are remarkably consistent across exercise modalities. Breast cancer-bearing mice that ran on treadmills before and after tumor inoculation showed fewer M2 macrophages within their tumors, while medium-intensity treadmill running increased the M1-to-M2 ratio in similar models. In melanoma-inoculated mice, swimming prevented M2 polarization and reduced interleukin-6 production, inhibiting tumor glycolysis and lowering lactic acid accumulation. Exercise also increased the production of major histocompatibility complex class II molecules on macrophages, sharpening their ability to activate T cells. These findings matter clinically because pharmacological strategies targeting macrophage biology, including inhibitors of transforming growth factor-beta, have struggled in human trials, failing to show clear benefit or producing severe adverse events. Exercise, by contrast, achieves a similar biological reprogramming without toxicity, offering a route around a therapeutic bottleneck that has frustrated drug developers.

Myeloid-derived suppressor cells, or MDSCs, represent a second immunosuppressive population that exercise appears to tame. These cells promote immune evasion by impairing chemokine secretion, recruiting regulatory T cells, and increasing PD-1 expression on T cells. Multiple preclinical studies show that physical activity delays MDSC accumulation and reduces their numbers within tumors. Mice exercised before and after breast carcinoma inoculation had significantly lower intratumoral MDSC levels than sedentary controls, and treadmill running started after tumor inoculation reduced splenic MDSCs, slowed tumor progression, and increased immune cell infiltration in mammary carcinoma models, with an inverse relationship between MDSC abundance and CD8-positive T cell presence. Crucially, these findings extend to humans. In newly diagnosed breast cancer patients, a single session of acute exercise increased natural killer and CD8-positive T cell levels while reducing MDSCs. In esophageal cancer patients who exercised during neoadjuvant chemotherapy, CD8-positive T cell counts rose while inflammatory biomarkers associated with MDSCs and TAMs fell significantly.

Hypoxia and disordered blood vessel growth form a third pillar of the exercise-immunotherapy connection. Tumors grow so erratically that their vasculature becomes tangled and inefficient, starving tissue of oxygen and stabilizing hypoxia-inducible factors that drive further abnormal angiogenesis. The resulting hypoxic environment excludes natural killer and CD8-positive T cells, inhibits dendritic cells and antigen presentation, recruits immunosuppressive cells, and pushes macrophages toward the pro-tumor M2 phenotype through a CXCL8-interleukin-10 signaling axis, all of which correlate with poor prognosis and resistance to checkpoint blockade. Exercise directly counters this vicious cycle. Melanoma-bearing mice that swam at low or moderate intensity showed significantly reduced expression of hypoxia and glycolysis genes, along with greater CD8-positive T cell infiltration and cytotoxicity. Daily high-intensity exercise lowered intratumoral hypoxic fractions in breast carcinoma models, and in a landmark clinical observation, pancreatic cancer patients who exercised during preoperative therapy showed increased tumor vascularity, while exercised mice bearing patient-derived pancreatic tumors displayed vascular remodeling, accelerated regression, and delayed regrowth.

Natural killer cells, the innate immune system’s front-line tumor killers, emerge as perhaps the cells most responsive to exercise. NK cells mobilize more readily into tumors in exercised animals, and work by Cho and colleagues showed that NK cells from exercised individuals kill target cells more efficiently, with their cytotoxicity actually enhanced under hypoxic conditions, a striking advantage given the oxygen-poor nature of tumors. This resilience stems from exercise-induced metabolic reprogramming that reduces mitochondrial oxidative stress and, through interleukin-15 signaling, lessens sensitivity to reactive oxygen species such as hydrogen peroxide within the tumor microenvironment. Clinical translation is already visible: men with localized prostate cancer who adhered strictly to high-intensity interval training showed significantly increased NK cell infiltration into their tumors. Perhaps most dramatic, Pedersen and colleagues found that voluntary wheel running reduced tumor volume by 66 percent in mice lacking functional T cells, an effect abolished when NK cell production was blocked, proving that NK cells alone can mediate exercise-driven tumor suppression.

CD8-positive T cells, the primary targets of checkpoint inhibitors, are recruited into exercised tumors through well-defined molecular routes. The chemokine receptor CXCR3, which binds CXCL9, CXCL10, and CXCL11, guides these cells into tumor tissue, and CXCR3 knockout mice show reduced T cell infiltration and blunted responses to PD-1 blockade. Recent work demonstrated that four weeks of preoperative treadmill running increased CXCL9 release and CXCR3-positive T cell recruitment in colorectal liver metastases, an effect lost in CXCL9-deficient mice. Exercise also suppresses CCL5, a chemokine that recruits regulatory T cells, TAMs, and MDSCs and correlates with poor prognosis, while boosting interleukin-15, a cytokine essential for T cell survival that appears to actively shift CD8-positive cells from circulation into tumors rather than merely raising their blood counts.

The combination studies provide the most compelling case. Melanoma-bearing mice treated with exercise plus anti-PD-1 therapy developed smaller tumors with more apoptotic cells, more cytotoxic T cells, and fewer regulatory T cells than mice receiving the drug alone. Similar synergy appeared in triple-negative breast cancer, where treadmill running improved therapeutic response, boosted T cell and NK cell activation, and cut MDSC numbers alongside anti-PD-1 treatment. In transgenic breast cancer models, adding running to anti-PD-1 therapy delayed tumor growth and improved control, even when both interventions began only after tumors reached a clinically relevant size. Pancreatic ductal adenocarcinoma, notoriously resistant to checkpoint inhibitors, responded to low-intensity treadmill running combined with anti-PD-1 when neither approach worked alone, an especially promising result for patients too ill for vigorous activity. Clinically, hepatocellular carcinoma patients who exercised regularly had significantly better overall and progression-free survival on combined lenvatinib and anti-PD-1 therapy, with matching results in mouse models. Randomized trials such as HI AIM and ERICA are now testing supervised exercise before and during immunotherapy infusions in lung cancer patients.

The authors are careful to note that clinical evidence remains limited and that major questions persist regarding optimal exercise modality, intensity, frequency, timing, and patient selection across cancer types, ages, sexes, disease stages, and body mass indices. Yet the biological coherence of the evidence is difficult to dismiss: exercise relieves hypoxia, normalizes vasculature, repolarizes macrophages, suppresses MDSCs and regulatory T cells, mobilizes NK cells, and drives cytotoxic T cells into tumors through defined chemokine axes, collectively converting cold tumors into inflamed, drug-responsive ones. If ongoing adequately powered trials with longitudinal immune profiling confirm these mechanisms in patients, structured physical activity could become one of the first universally accessible adjuncts to cancer immunotherapy, a prescription written not on a pharmacy pad but into the daily routines of patients fighting some of medicine’s most treatment-resistant cancers.

Subject of Research: How physical exercise modulates the tumor microenvironment to enhance cancer immunotherapy efficacy

Article Title: Physical Exercise in Immunotherapy

Article References: Johnston, C. M., Kim, S. J., Zhang, Y., Tsung, C., Kent, E., May, A., & Zhang, H. (2026). Physical Exercise in Immunotherapy. Sports Medicine – Open, 12(1), Article 130. https://doi.org/10.1186/s40798-026-01101-1

Image Credits: AI Generated

DOI: 10.1186/s40798-026-01101-1

Keywords: physical exercise, cancer immunotherapy, immune checkpoint inhibitors, tumor microenvironment, tumor-associated macrophages, myeloid-derived suppressor cells, natural killer cells, CD8 T cells, PD-1, tumor hypoxia, interleukin-15, cold tumors