The core idea is deceptively simple. Natural T cells do not rely on their antigen receptor alone; they depend on co-stimulatory receptors such as CD28 and ICOS, the inducible T cell co-stimulator, to fine-tune activation, survival and differentiation. The researchers reasoned that if a therapeutic T cell receptor could be fused directly to a co-stimulatory signaling domain, every encounter with a tumor cell would deliver not just a recognition signal but a survival and persistence signal at the same time. The construct they engineered, which they call TCR:ICOS, combines the extracellular variable and constant domains of a conventional T cell receptor with a CD28 transmembrane segment and intracellular domains drawn from both ICOS and CD3 epsilon. The result is a single hybrid receptor that couples antigen specificity to co-stimulation in one molecular unit.

The functional consequences in mice were dramatic. T cells carrying the TCR:ICOS receptor showed enhanced, antigen-specific production of inflammatory cytokines, the chemical weapons that help recruit and coordinate an immune attack. More importantly, the cells persisted far longer within tumors than their conventional counterparts, and this persistence translated into clinical outcomes: delayed recurrence of melanoma and, in a subset of animals, complete and durable cures. For a field in which the transience of engineered T cells is a central bottleneck, the demonstration that a receptor-level modification can extend the working lifespan of the cells inside a hostile tumor microenvironment is a significant proof of concept.

Beneath the phenotypic changes lies a defined signaling logic. The team found that TCR:ICOS activation engaged two major pathways, PI3K and NF-kappa B, while paradoxically restraining the activation of AKT, a kinase downstream of PI3K that is often associated with terminal differentiation and metabolic burnout in T cells. This combination appears to be the key to the durability. Sustained PI3K and NF-kappa B signaling supports inflammatory function and survival, while blunted AKT activity helps the cells avoid the hyperactive, exhausted state that typically shortens the life of tumor-infiltrating lymphocytes. When the researchers genetically ablated the ICOS-PI3K pathway, the long-term anti-tumor effects disappeared, confirming that this signaling axis is not a side effect but the mechanistic engine of the durable response.

Single-cell level analysis reinforced the picture of a fundamentally altered differentiation program. TCR:ICOS T cells were enriched for a stem-like state, a less differentiated, self-renewing condition that immunologists regard as the hallmark of long-lived, re-challenge-capable T cell populations. They also showed resistance to exhaustion, the progressive loss of function marked by inhibitory receptors such as PD1, LAG3 and TIM3 that plagues conventional tumor-infiltrating cells. In practical terms, the engineered cells behaved less like short-lived commandos and more like a renewable garrison, capable of maintaining pressure on the tumor over time rather than expending themselves in a single burst of activity.

Translating the concept from mouse to human T cells required an additional round of molecular engineering. Early versions of the hybrid receptor did not express efficiently on the surface of human cells, a common obstacle in receptor design where folding, assembly and trafficking can fail silently. The team solved this by identifying a single amino acid change in the cytosolic tail of the receptor that enabled functional surface expression. Crucially, the optimized receptor avoided two well-known hazards of introducing engineered T cell receptors into patient cells: mispairing with endogenous TCR chains, which can create unpredictable and potentially dangerous specificities, and competition for limited CD3 molecules, which can impair the function of the T cell’s native receptor complex.

The human-cell experiments went beyond a single antigen. The researchers showed that the optimized TCR:ICOS format sustained the functional performance of human T cells across repeated stimulation cycles and could be extended to multiple tumor antigens, including targets relevant to solid cancers such as NY-ESO-1 and ROPN1, without eroding T cell fitness. This uniform applicability matters for the field. Many receptor-engineering advances are idiosyncratic, working for one TCR or one antigen but failing when generalized. A format that preserves T cell quality across different specificities offers a modular platform: any tumor-reactive TCR could, in principle, be equipped with the same built-in co-stimulation and inherit the same durability advantages.

The findings arrive at a moment when the cell therapy field is intensely focused on next-generation designs. Chimeric antigen receptors, or CARs, already incorporate co-stimulatory domains such as CD28 or 4-1BB, and that choice profoundly shapes how CAR T cells persist and differentiate. TCR-based therapies, which can recognize intracellular tumor antigens presented by HLA molecules and therefore access a much larger pool of cancer targets, have lacked an equivalent, systematic way to embed co-stimulation. The TCR:ICOS design fills that gap, and its mechanism, favoring stem-like persistence through PI3K and NF-kappa B while restraining AKT, offers a template that other groups can rationally modify.

There are, of course, steps between a mouse melanoma model and approved therapy. The study was conducted in preclinical systems, and human trials will need to establish safety, particularly given that engineered co-stimulation could, in principle, amplify off-tumor reactivity if a TCR recognizes healthy tissue. Dosing, manufacturing consistency and the behavior of TCR:ICOS cells in the complex, immunosuppressive environment of human solid tumors all remain to be tested. The authors note that the work was supported by the Dutch Cancer Society and a Health Holland public-private partnership, and several team members report pending patents related to the receptor design, signaling commercial interest in bringing the platform toward clinical evaluation.

Even so, the study offers a compelling answer to one of adoptive cell therapy’s most persistent questions: how to make engineered T cells last. By fusing recognition and co-stimulation into a single receptor, the Rotterdam team has shown that durability can be designed into the therapeutic product itself rather than bolted on with cytokines, checkpoint blockade or lymphodepleting chemotherapy. If the format performs in human trials as it has in mice, TCR:ICOS could become a foundational component of fitter, longer-lived T cell products for solid tumors, turning a transient spark of immune attack into a sustained campaign against cancer.

Subject of Research: Engineering T cell receptors with built-in ICOS co-stimulation to extend the durability of adoptive T cell therapy against solid tumors

Article Title: T cell receptors equipped with ICOS provide T cells with durable anti-tumor response

Article References: Marraffa, A., Berrevoets, C., Mosiello, M., Coelho, R. M., Roelofs, D., van Brakel, M., Wijers, R., Kroese, K., Peeters, M. J., Dik, W. A., Kunert, A., Abbott, R. J., Hammerl, D., Schliehe, C., & Debets, R. (2026). T cell receptors equipped with ICOS provide T cells with durable anti-tumor response. Molecular Cancer, 25(1), Article 216. https://doi.org/10.1186/s12943-026-02765-9

Image Credits: AI Generated

DOI: 10.1186/s12943-026-02765-9

Keywords: adoptive T cell therapy, T cell receptor, ICOS, co-stimulation, T cell exhaustion, solid tumors, melanoma, PI3K signaling, NF-kappa B, stem-like T cells, gene engineering, Molecular Cancer

Chimeric antigen receptor T-cell therapy has transformed treatment for several blood cancers by genetically programming a patient’s T cells to recognize and destroy malignant cells. Yet the same approach has produced much less impressive results in solid tumors. According to a meta-analysis cited by the authors, CAR-T therapy has achieved a pooled objective response rate of approximately 9 percent across solid malignancies. The disparity reflects the fundamentally different biology of solid cancers. Unlike many blood cancers, solid tumors form dense physical structures, contain abnormal and poorly organized blood vessels, display patchy antigen expression and create local conditions that can disable incoming immune cells.

To reach and attack a solid tumor, CAR-T cells must first leave the bloodstream and cross abnormal vasculature and dense extracellular matrix. Once inside, they encounter low oxygen, limited glucose and amino acids, high concentrations of lactic acid and suppressive metabolites. Tumor-associated macrophages, regulatory T cells, myeloid-derived suppressor cells and cancer-associated fibroblasts further reinforce immune resistance. Signals such as transforming growth factor beta can inhibit T-cell activity, while persistent exposure to tumor antigen can push CAR-T cells into exhaustion. This state is marked by altered transcriptional and epigenetic programs, reduced cytokine production, impaired proliferation and declining cytotoxicity.

The review highlights a growing shift from maximizing activation toward building cellular resilience. Earlier CAR designs often focused on stronger intracellular signaling and costimulatory domains intended to produce rapid T-cell expansion. In solid tumors, however, excessive stimulation can accelerate exhaustion. Newer strategies attempt to preserve function over time by modifying the metabolic, epigenetic and signaling systems that regulate T-cell fitness. The authors discuss c-Jun overexpression as one method of restoring AP-1-dependent transcription, a pathway involved in T-cell activation and persistence. They also examine disruption of DNMT3A, an epigenetic regulator associated with the establishment of exhaustion-related cellular states.

Additional forms of “armoring” are designed to help CAR-T cells withstand suppression after they enter the tumor. Cytokine-armored cells may be engineered to produce or respond more effectively to interleukins such as IL-10, IL-15, IL-18 or IL-21, each of which can influence survival, proliferation or effector function in different ways. Other designs interfere directly with inhibitory signals. A dominant-negative TGF-β receptor, for example, can bind suppressive cues without transmitting the full inhibitory signal into the T cell. This principle is being explored in GPC3-targeted C-CAR031 for hepatocellular carcinoma, which has reportedly produced objective response rates of approximately 50–57 percent in early clinical reports. The authors caution that these findings remain based on conference abstracts pending full peer-reviewed publication.

Getting engineered T cells to the tumor is another major challenge. Many solid tumors secrete chemokines that do not match the receptors naturally expressed by circulating T cells, leaving therapeutic cells poorly recruited to the cancer site. Adding receptors such as CCR2b may improve recognition of tumor-associated chemokine gradients and increase infiltration. Researchers are also developing hypoxia-responsive CAR systems that use the low-oxygen conditions found inside tumors as a biological switch, helping restrict activation to the tumor microenvironment. These approaches could improve both delivery and safety by reducing activity in healthy tissues where the target antigen may be present at lower levels.

Synthetic biology is adding another layer of control. SynNotch systems use one receptor to detect an initial antigen and trigger production of a second CAR, creating a sequential activation process. Tmod “NOT-gate” designs are intended to activate against malignant cells while suppressing responses to healthy cells that carry a protective antigen. Drug-controlled CARs offer yet another safety mechanism, allowing clinicians to regulate T-cell activity with an externally administered compound. Together, these circuits seek to address antigen heterogeneity, one of the defining problems of solid tumors, in which not every cancer cell displays the same target and antigen loss can allow resistant clones to survive.

The review points to several clinical developments suggesting that these principles are beginning to translate into meaningful patient outcomes. In H3K27M-mutated diffuse midline glioma, intracerebroventricular administration of GD2-targeted CAR-T cells produced substantial tumor reductions, including a complete response that was sustained beyond 30 months. In advanced gastric cancer, the CLDN18.2-targeted therapy satricabtagene autoleucel, also known as satri-cel, was reported to outperform physician’s choice in a randomized Phase 2 trial. The treatment produced a progression-free survival hazard ratio of 0.37 and an overall survival hazard ratio of 0.69, results the authors describe as the first randomized evidence of CAR-T superiority over standard treatment in a solid malignancy.

The authors stress that the engineered cell is only one part of the therapeutic system. Conditioning chemotherapy, the phenotype of T cells at infusion, manufacturing time, the patient’s gut microbiome and systemic neuroendocrine signals may all influence whether CAR-T cells persist and remain functional. Manufacturing is also undergoing rapid change, with next-day production methods and experimental technologies designed to generate CAR-T cells directly inside the patient using targeted lipid nanoparticles or receptor-targeted lentiviral particles. These approaches could reduce production delays, infrastructure requirements and treatment costs, although their safety, regulatory status and clinical feasibility remain under investigation. The review ultimately calls for an integrated development model that combines delivery, resilience, logic and whole-patient biology. It also warns that long-term genomic stability after multiplex gene editing, the safety of sustained cytokine production and the complexity of increasingly sophisticated cell products must be established through larger studies, longer follow-up and prospective biomarker-guided trials. The central message is that solid-tumor CAR-T therapy may advance not through a single breakthrough, but through the coordinated solution of many biological problems that once appeared inseparable.

Subject of Research: Not applicable

Article Title: Engineering CAR-T cells for solid tumors: Overcoming the microenvironment through integrated design and clinical translation

News Publication Date: August 11, 2026

Web References: https://doi.org/10.18632/oncoscience.666

References: Oncoscience, Volume 13; Figure 6: https://www.oncoscience.us/article/666/text/#F6

Image Credits: Copyright © 2026 Onyekweli et al.; distributed under the Creative Commons Attribution License (CC BY 4.0).

Keywords: CAR-T cell therapy, solid tumors, tumor microenvironment, cancer immunotherapy, immunotherapy engineering, synthetic biology, T-cell exhaustion, clinical translation, oncology, cellular therapy