GL261 is the most widely used mouse glioma cell line for studying how the immune system interacts with brain tumors, and its popularity stems from a combination of practical convenience and genuine biological usefulness. Grown in C57BL/6 mice with fully functioning immune systems, GL261 tumors reproduce many of the hallmarks of human glioblastoma, including rapid growth, invasive spread, and the recruitment of immunosuppressive cells. But GL261 is also far more immunogenic than typical human glioblastoma, carrying thousands more mutations and expressing higher levels of molecules that flag tumor cells for immune attack. That mismatch has made the model both indispensable for early-stage immunotherapy testing and a frequent source of optimism that does not always translate to the clinic.
How the GL261 Model Came to Be
The GL261 tumor was originally created in 1970 by implanting pellets of the chemical carcinogen methylcholanthrene directly into the brains of C57BL/6 mice. Researchers then kept the resulting tumor alive by transferring small pieces from one mouse brain to the next. Stable cell lines that could be grown in a dish were not established until the 1990s, which is when the model really took off in laboratory use.1Neuro-Oncology Advances. Mouse models of glioblastoma for the evaluation of novel therapeutic strategies Because GL261 grows in the same mouse strain it originated from, the host’s immune system treats the tumor as something that arose naturally rather than as a foreign transplant. This is what makes it “syngeneic” and why it is valuable for studying immune responses that would be absent in immunodeficient mice carrying human tumor grafts.
Genetically, GL261 carries two mutations commonly discussed in glioblastoma research. It has a point mutation in the K-ras oncogene at codon 12 and a homozygous point mutation in the p53 tumor suppressor gene at codon 153.2PubMed Central. Detailed characterization of the mouse glioma 261 tumor model for experimental glioblastoma therapy The p53 mutation mirrors what is seen in a substantial portion of human glioblastomas and is associated with worse outcomes. The K-ras mutation and elevated c-myc expression, however, are not features of typical human glioblastoma, which is one of the genetic mismatches researchers have to keep in mind.1Neuro-Oncology Advances. Mouse models of glioblastoma for the evaluation of novel therapeutic strategies
Why GL261 Is Unusually Immunogenic
The single most important thing to understand about GL261 and immune research is that GL261 tumors are far easier for the mouse immune system to “see” than human glioblastomas are for the human immune system. Whole-exome sequencing has revealed over 4,900 non-synonymous mutations in GL261 cells, producing a large number of predicted neoantigens, the abnormal protein fragments that immune cells can recognize as foreign.3PubMed Central. Mouse models of glioblastoma for the evaluation of novel therapeutic strategies GL261 also expresses moderate levels of MHC class I molecules, the surface proteins that display those neoantigens to killer T cells.4PubMed Central. Technical choices significantly alter the adaptive immune response against immunocompetent murine gliomas in a model-dependent manner Human glioblastoma, by contrast, tends to have lower mutational burdens and frequently downregulates MHC-I to hide from immune detection.
This immunogenicity is dramatically illustrated by vaccination experiments. When mice are pre-vaccinated with irradiated GL261 cells before being challenged with live tumor, about 90% of them reject the tumor entirely.3PubMed Central. Mouse models of glioblastoma for the evaluation of novel therapeutic strategies That is an extraordinary rejection rate and one that has no parallel in human glioblastoma. It may explain why so many immunotherapy strategies look promising in GL261 studies but then disappoint in human clinical trials.
The Immunosuppressive Microenvironment Inside GL261 Tumors
Despite being immunogenic, GL261 tumors still manage to build a deeply immunosuppressive local environment, much like human glioblastomas do. The tumor microenvironment fills with immune cells that, instead of attacking the tumor, actively protect it. Understanding the choreography of these cells has been a major focus of GL261 research.
Detailed time-course studies of GL261 tumors in the brain show that different immune populations arrive and peak at different stages. Regulatory T cells, which dampen immune responses, peak early, around day 7 after implantation. M2-polarized macrophages, which promote tissue remodeling and suppress anti-tumor immunity, peak around day 14. Myeloid-derived suppressor cells and dendritic cells then surge later, around day 21.5PLOS ONE. Temporal and spatial modulation of the tumor and systemic immune response in the murine Gl261 glioma model The key takeaway is that immunosuppression is not a static wall; it shifts over the life of the tumor, which has implications for when different therapies might be most effective.
A chemokine called CCL2 plays a central organizing role in this immunosuppressive buildup. In GL261 tumors, tumor-associated macrophages and resident microglia are the main producers of CCL2, which then recruits both regulatory T cells and a subset of monocytic myeloid-derived suppressor cells into the tumor. When researchers used mice genetically lacking CCL2, the number of regulatory T cells in the tumor dropped by more than half, and monocytic suppressor cells fell by more than 80%. Granulocytic suppressor cells, a different subtype, were unaffected by CCL2 loss, showing that different immunosuppressive populations use different recruitment signals.6PubMed Central. CCL2 produced by the glioma microenvironment is essential for the recruitment of regulatory T cells and myeloid-derived suppressor cells
The enzyme IDO adds another layer. When GL261 tumors express IDO, they accumulate significantly more regulatory T cells and fewer CD8+ killer T cells compared to IDO-deficient tumors. This effect is tightly localized to the brain tumor itself; the ratios in the spleen and lymph nodes remain unchanged.7Clinical Cancer Research. IDO Expression in Brain Tumors Increases the Recruitment of Regulatory T Cells and Negatively Impacts Survival GL261 tumors also upregulate TGF-beta1, though interestingly this appears to be a reactive process triggered by activated T cells rather than something the tumor secretes on its own.8PubMed. Investigation of immunosuppressive mechanisms in a mouse glioma model
Regulatory B cells are another underappreciated player. In both GL261 and CT2A brain tumor models, these cells make up roughly 10% of bone marrow-derived immune cells infiltrating the tumor.9Cancer Immunology Research. Myeloid-Derived Suppressive Cells Promote B cell–Mediated Immunosuppression via Transfer of PD-L1 in Glioblastoma Glioblastoma-derived small extracellular vesicles can also expand suppressor cell populations and inhibit CD8+ T cell function remotely, adding a systemic dimension to the tumor’s immune evasion.10PubMed Central. LILRB2-containing small extracellular vesicles from glioblastoma promote tumor progression by promoting the formation and expansion of myeloid-derived suppressor cells
Checkpoint Blockade and Combination Strategies
GL261 has been the testing ground for many of the immune checkpoint inhibitors now used or attempted in human glioblastoma trials. The results in mice have been a mixed bag that, in hindsight, reflects the model’s unusual immunogenicity. A comprehensive preclinical analysis found that anti-PD-1 antibodies showed significant anti-tumor effects in GL261, while blocking IDO1 or CTLA-4 either failed or offered only marginal benefit.11PubMed Central. Preclinical efficacy of immune-checkpoint monotherapy does not recapitulate corresponding biomarkers-based clinical predictions in glioblastoma This disconnect between preclinical predictions and clinical reality is a recurring theme.
Combination therapies have looked more impressive. One landmark study paired anti-PD-1 antibodies with stereotactic radiation in GL261-bearing mice. The control mice survived a median of 25 days, anti-PD-1 alone bought 27 days, radiation alone 28 days, but the combination pushed median survival to 53 days. More striking, 15% to 40% of mice receiving both treatments were still alive at day 180, effectively long-term survivors. Analysis of the tumors showed that the combination increased infiltration by cytotoxic T cells while reducing regulatory T cells.12PubMed Central. Anti-PD-1 Blockade and Stereotactic Radiation Produce Long-Term Survival in Mice With Intracranial Gliomas These results helped motivate clinical trials combining radiation with checkpoint inhibitors for human glioblastoma patients, though the clinical benefits have so far been more modest.
The functionality of the antigen-processing machinery inside GL261 cells appears to be a prerequisite for these immune treatments to work. When GL261 tumors retain functional antigen processing and presentation, they develop a more cytotoxic immune microenvironment and respond to both dendritic cell vaccines and anti-PD-1 therapy.13PubMed Central. Functional antigen processing and presentation mechanism as a prerequisite factor of response to treatment with dendritic cell vaccines and anti-PD-1 in preclinical murine LLC1 and GL261 tumor models This finding underscores why tumors that downregulate these pathways are harder to treat with immunotherapy.
Vaccines, CAR-T Cells, and Oncolytic Viruses
GL261 has been used to test a wide range of immunotherapy approaches beyond checkpoint blockade. Tumor lysate vaccines made from GL261 cells have been shown to prolong survival in mice by enhancing the recruitment of antigen-presenting cells to the brain and boosting the activation of CD8+ T cells.14PubMed. Autologous tumor lysate vaccines enhance anti-glioma immunity and prolong survival in a GL261 glioblastoma mouse model These vaccine approaches take advantage of GL261’s high neoantigen load, which again raises the question of how well they would perform against the less immunogenic landscape of human tumors.
CAR-T cell therapy, where a patient’s own T cells are engineered to recognize a specific tumor target, has shown limited efficacy against GL261 when used alone. In one study, CAR-T cells targeting B7H3 failed to suppress GL261 growth by themselves. However, when combined with an oncolytic adenovirus engineered to produce the chemokine CXCL11, the combination dramatically improved outcomes. The virus increased CAR-T cell infiltration into the tumor, overcoming one of the central challenges in brain tumor immunotherapy: getting enough immune cells past the blood-brain barrier and into the tumor mass.15Molecular Therapy. Armed oncolytic adenovirus enhances the therapeutic efficacy of chimeric antigen receptor T cell therapy in glioblastoma
Oncolytic viruses, which selectively infect and kill cancer cells, have emerged as one of the more promising combination partners tested in GL261. Engineered vaccinia viruses expressing the immune-stimulating molecule IL-21 can shift the tumor’s macrophage population from the pro-tumor M2 type to the anti-tumor M1 type as early as seven days after treatment. When these viruses were combined with anti-PD-1 antibodies, the polarization shift was the most pronounced, and activated macrophages were detected in the spleen weeks later, suggesting a systemic immune response had been ignited.16Molecular Therapy – Oncolytics. An effective therapeutic regime for treatment of glioma using oncolytic vaccinia virus expressing IL-21 in combination with immune checkpoint inhibition More recently, retargeted oncolytic viruses carrying multiple immunomodulators, including IL-12, anti-PD-1, a bispecific T cell engager, and agents targeting myeloid cells, have shown that a single injection can improve survival in preclinical glioblastoma models by simultaneously activating T cells, natural killer cells, and myeloid cells.17Nature Cancer. Retargeted oncolytic viruses engineered to remodel the tumor microenvironment for glioblastoma immunotherapy
Immune Memory and Tumor Rechallenge
One of the more encouraging findings from GL261 studies is that successful treatment can generate lasting immune memory. Mice cured of GL261 tumors through adoptive transfer of activated lymph node cells rejected a second intracranial challenge with GL261 but not with an unrelated tumor, showing that the memory was tumor-specific.18PubMed. Treatment of murine gliomas by adoptive transfer of ex vivo activated tumor-draining lymph node cells Similar memory formation has been seen with an IL-15-based immunotherapy, where treated mice showed durable resistance to rechallenge.19PubMed Central. Therapeutic administration of IL-15 superagonist complex ALT-803 leads to long-term survival and durable antitumor immune response in a murine glioblastoma model
Memory formation is not automatic, though. In experiments using an oncolytic adenovirus called Delta-24-ACT, mice that survived after receiving the virus alone died upon rechallenge, indicating no lasting immune memory. Only when the virus was combined with agents targeting CD137 and PD-L1 did 100% of long-term survivors reject the second tumor.20PubMed Central. CD137 and PD-L1 targeting with immunovirotherapy induces a potent and durable antitumor immune response in glioblastoma models This finding illustrates that tumor clearance and immune memory are separate outcomes. A treatment can shrink a tumor without priming the immune system to remember it, which matters enormously for preventing recurrence.
Innate Immunity and the STING Pathway
Most immunotherapy research focuses on T cells, but the innate immune system also plays a role in GL261 studies. GL261 cells stand out among murine glioma lines for their strong response to agonists of the STING pathway, an innate immune sensing mechanism. When treated with STING agonists, GL261 cells released high levels of CXCL10, a chemokine that attracts T cells and natural killer cells. Other murine glioma lines such as CT-2A and mut3 did not respond.21PubMed Central. STING activation promotes robust immune response and NK cell–mediated tumor regression in glioblastoma models This responsiveness makes GL261 a useful model for testing STING-based therapies, but it also means results from GL261 may overestimate how well such approaches work in tumors where the STING pathway is silent. Broader efforts to regulate the polarization of glial cells through pathways including STING, NF-κB, and STAT3 are being explored as ways to convert “cold” glioblastomas into tumors the immune system can attack.22PubMed Central. Glioblastoma Immunotherapy Adjuvants for Glial Cell Polarization Regulation
The SB28 Problem and Translational Skepticism
The biggest criticism of GL261 in immunotherapy research is that its results often do not translate to humans. Much of this concern crystallized with the emergence of SB28, a newer syngeneic mouse glioma model that more closely mimics the immune-cold profile of human glioblastoma. Where GL261 has over 4,900 non-synonymous mutations, SB28 has just 108, yielding only 11 predicted neoantigens. SB28 also has low MHC-I expression and relatively few tumor-infiltrating T cells.3PubMed Central. Mouse models of glioblastoma for the evaluation of novel therapeutic strategies
The practical consequence is stark. When mice bearing SB28 tumors received combination checkpoint blockade with anti-PD-1 and anti-CTLA-4, there was no significant survival benefit. The same treatment in GL261-bearing mice produced significant survival advantages, with half the mice becoming long-term survivors.3PubMed Central. Mouse models of glioblastoma for the evaluation of novel therapeutic strategies Human glioblastoma has similarly resisted checkpoint blockade in large clinical trials, suggesting SB28 may be a more honest testing ground for immunotherapy. A third model, CT-2A, is also poorly immunogenic but has been less thoroughly characterized from an immunological standpoint, though its use is increasing.23Journal of Neurosurgery. Syngeneic murine glioblastoma models: reactionary immune changes and immunotherapy intervention outcomes
None of this means GL261 is useless. For studying the basic mechanics of immune-tumor interactions in the brain, for identifying combinations worth testing, and for understanding how immunosuppressive cells are recruited and organized, GL261 remains the most data-rich model available. The key is recognizing that positive results in GL261 should be viewed as a necessary but insufficient step before clinical development, not as evidence that a therapy will work in humans.
Sex Differences in Immune Cell Composition
One dimension of GL261 research that has gained attention is the discovery of sex-dependent differences in the immune microenvironment. When GL261 cells were implanted into male and female mice, the types of myeloid-derived suppressor cells that accumulated differed dramatically. Male mice showed a 5.5- to 6.5-fold increase in the ratio of monocytic to granulocytic suppressor cells within the tumor, driven by accumulation of monocytic suppressor cells. In females, monocytic suppressor cells in the tumor did not change, but granulocytic suppressor cells doubled in the blood, a peripheral rather than local pattern.24Cancer Discovery. Myeloid-Derived Suppressor Cell Subsets Drive Glioblastoma Growth in a Sex-Specific Manner These differences were consistent across both GL261 and SB28 models, suggesting they are driven by the host’s biology rather than the tumor itself. Given that human glioblastoma is more common and more aggressive in men, these findings offer a potential mechanistic explanation rooted in how the two sexes handle myeloid-derived immunosuppression differently.
The Gut-Brain Immune Axis
Some of the most surprising recent GL261 findings involve the gut microbiome. Researchers have shown that the composition of gut bacteria in mice influences whether anti-PD-1 therapy works against GL261 brain tumors. Experiments using gnotobiotic mice, animals raised without any microbes and then colonized with specific human fecal samples, confirmed that the microbial community in the gut shapes the immune response inside a brain tumor hundreds of anatomical miles away.25PubMed Central. Human microbiota influence the immune cell composition and gene expression in the tumor environment of a murine model of glioma
The amino acid tryptophan appears to be one mediator of this gut-brain immune connection. As gliomas grow, they alter the gut microbial landscape in ways that promote T cell sequestration in the bone marrow, reducing the pool of immune cells available to fight the tumor. Supplementing tryptophan reversed these microbial changes, boosted T cell circulation, and improved survival when combined with anti-PD-1 therapy. A specific bacterial species, Duncaniella dubosii, was identified as a key contributor to this immunomodulatory effect, and mice colonized with it under tryptophan supplementation showed significantly better responses to checkpoint blockade.26Cell Reports. Tryptophan and gut microbiota modulation in brain tumor immunotherapy These findings are still early-stage, but they open an entirely new avenue for improving glioblastoma immunotherapy by manipulating the gut rather than the tumor directly.
Meningeal Lymphatics and Tumor Antigen Drainage
The brain was long considered “immune-privileged,” meaning largely walled off from the body’s immune surveillance. The discovery of functional lymphatic vessels running through the meninges, the membranes surrounding the brain, upended that idea. GL261 studies have contributed to understanding how these meningeal lymphatics participate in anti-tumor immunity. Research using GL261 has shown that these vessels are involved in draining tumor antigens from the brain to lymph nodes, where immune responses are initiated.27PubMed Central. Meningeal lymphatics prime tumor immunity in glioblastoma When meningeal lymphatic drainage is impaired, the immune system’s ability to recognize and attack brain tumors is diminished. Conversely, enhancing lymphatic function could potentially improve responses to immunotherapy. This line of research is reshaping the basic framework through which scientists understand brain tumor immunity, and GL261 has been one of the models making it possible.