Towards Immunotherapy for Pediatric Brain Tumors
Authors: Stacie Shiqi Wang, Pratiti Bandopadhayay, Misty Rayna Jenkins
Journal: Trends in Immunology (2019)

Checkpoint inhibition has proved to be an impressive new form of immunotherapy, demonstrating efficacy in adult malignancies such as melanoma, renal cell carcinoma, and small cell lung cancer; however, it has failed to produce durable responses in pediatric brain cancer.CAR-T cells can be well tolerated when delivered via peripheral, intracranial, or intratumoral administration. An anti-GD2 CAR has recently demonstrated significant antitumor activity and efficacy in vivo in a mouse model of diffuse midline glioma.Antitumor effects are yet to be seen with putative vaccine therapies in pediatric HGG. Current approaches to treating pediatric brain tumors include investigating viruses (such as measles) to potentially increase the immunogenicity of the vaccine.Owing to antigen escape and tumor heterogeneity, targeting a single tumor-associated antigen with immunotherapy is unlikely to provide a durable response. Although some HGG-specific targets are in preclinical development, identifying additional novel tumor antigens will be essential.Pediatric brain tumors differ from their adult counterparts not only in clinical presentation but also in their driver genetic mutations and expression, unique developmental origins, and tumor microenvironment. Pediatric brain tumors are now the primary cause of cancer-related death in children, and new approaches are urgently needed.To apply immunotherapy to this poor-outcome group, the challenge now is to generate safe, sustained, effective, and tailored immune responses. This requires an intimate understanding of the unique tumor sites, appropriate model systems, and future research investigating additional candidate targets for immunotherapy. Pediatric brain tumors are the leading cause of childhood cancer-related death. Immunotherapy is a powerful new approach for treating some refractory cancers; applying this 'fourth pillar' of cancer treatment to pediatric brain tumors is an exciting but challenging prospect. This review offers new perspectives on moving towards successful immunotherapy for pediatric brain tumors, focusing on pediatric high-grade glioma (HGG), a subgroup with universally poor outcomes. We cover chimeric antigen receptor T cell (CAR-T) therapy, vaccine therapy, and checkpoint inhibition in this context, and focus on the need for intimately understanding the growing brain and its immune system. We highlight the challenges associated with the application of immunotherapy in pediatric neuro-oncology, as well as the tissue-specific challenges to be overcome, to achieve improved outcomes. Pediatric brain tumors are the leading cause of childhood cancer-related death. Immunotherapy is a powerful new approach for treating some refractory cancers; applying this 'fourth pillar' of cancer treatment to pediatric brain tumors is an exciting but challenging prospect. This review offers new perspectives on moving towards successful immunotherapy for pediatric brain tumors, focusing on pediatric high-grade glioma (HGG), a subgroup with universally poor outcomes. We cover chimeric antigen receptor T cell (CAR-T) therapy, vaccine therapy, and checkpoint inhibition in this context, and focus on the need for intimately understanding the growing brain and its immune system. We highlight the challenges associated with the application of immunotherapy in pediatric neuro-oncology, as well as the tissue-specific challenges to be overcome, to achieve improved outcomes. Recent advances in cancer immunotherapy have improved outcomes for several human cancers, and in some cases have produced dramatic responses in patients refractory to all other conventional modes of treatment [1.Wolchok J.D. et al.Overall survival with combined nivolumab and ipilimumab in advanced melanoma.N. Engl. J. Med. 2017; 377: 1345-1356Crossref PubMed Scopus (2024) Google Scholar, 2.Motzer R.J. et al.Nivolumab plus ipilimumab versus sunitinib in advanced renal-cell carcinoma.N. Engl. J. Med. 2018; 378: 1277-1290Crossref PubMed Scopus (2211) Google Scholar, 3.Maude S.L. et al.Tisagenlecleucel in children and young adults with B-cell lymphoblastic leukemia.N. Engl. J. Med. 2018; 378: 439-448Crossref PubMed Scopus (2278) Google Scholar]. Although the concept of strengthening, redirecting, or generating an immune response against malignancy is by no means new, cancer immunotherapy has only recently received worldwide attention. Immunotherapy has demonstrated remarkable success in improving overall survival in Phase II–III trials in tumor subtypes such as melanoma and leukemia [1.Wolchok J.D. et al.Overall survival with combined nivolumab and ipilimumab in advanced melanoma.N. Engl. J. Med. 2017; 377: 1345-1356Crossref PubMed Scopus (2024) Google Scholar, 4.Maude S.L. et al.Sustained remissions with CD19-specific chimeric antigen receptor (CAR)-modified T cells in children with relapsed/refractory ALL.J. Clin. Oncol. 2016; 34: 3011Crossref Google Scholar]. This novel approach has been a welcome addition to the oncologist's conventional armamentarium of surgery, chemotherapy, and radiation, especially in the relapsed and/or refractory disease setting. This rapid advancement in the immuno-oncology field occurs in a timely manner for childhood brain tumors, which urgently require a new approach. Tumors of the central nervous system (CNS) are the most common solid tumor of childhood, and represent the leading cause of childhood cancer-related death [5.Curtin S.C. et al.Declines in cancer death rates among children and adolescents in the United States, 1999–2014.NCHS Data Brief. 2016; 257: 1-8Google Scholar]. Although survival rates for childhood cancers overall have markedly improved over recent decades, outcomes in pediatric CNS tumors have lagged behind the dramatic gains achieved in hematological cancers. The overall survival rate in childhood acute lymphoblastic leukemia now exceeds 90% at 5 years [6.Siegel R.L. et al.Cancer statistics, 2018.CA Cancer J. Clin. 2018; 68: 7-30Crossref PubMed Scopus (6017) Google Scholar]. In stark contrast, children with pediatric high-grade glioma (HGG; see Glossary), including diffuse midline glioma (DMG), and glioblastoma multiforme (GBM), have a dismal 5 year overall survival of <20% [6.Siegel R.L. et al.Cancer statistics, 2018.CA Cancer J. Clin. 2018; 68: 7-30Crossref PubMed Scopus (6017) Google Scholar]. This occurs despite recent increased knowledge of the genomics of pediatric HGG and how this diverges markedly from adult brain tumors [7.Wu G. et al.The genomic landscape of diffuse intrinsic pontine glioma and pediatric non-brainstem high-grade glioma.Nat. Genet. 2014; 46: 444-450Crossref PubMed Scopus (650) Google Scholar], and multiple trials of differing combinations of radiation, chemotherapeutic agents, and novel adjuvants. Furthermore, given that current conventional treatments involve irradiation of the brain, the few survivors of pediatric brain tumors are often left with devastating morbidities, including endocrine disease, psychiatric, cognitive and developmental disorders, and neurological disease, as well as a high incidence of secondary tumors [8.Cai Y. et al.Second malignant neoplasms in childhood malignant brain tumour: a long-term population-based study.J. Paediatr. Child Health. 2012; 48: 990-996Crossref PubMed Scopus (11) Google Scholar, 9.Gunn M.E. et al.Late morbidity in long-term survivors of childhood brain tumors: a nationwide registry-based study in Finland.Neuro-Oncology. 2015; 17: 747-756Crossref PubMed Scopus (26) Google Scholar]. Recent novel advances in immunotherapy include immune checkpoint blockade, cell-based approaches including chimeric antigen receptor (CAR)-T therapy, and vaccine therapy. Recently, monoclonal antibodies targeting checkpoint inhibitors have shown remarkable clinical efficacy in advanced, chemotherapy-resistant adult tumors such as melanoma and renal cell carcinoma [1.Wolchok J.D. et al.Overall survival with combined nivolumab and ipilimumab in advanced melanoma.N. Engl. J. Med. 2017; 377: 1345-1356Crossref PubMed Scopus (2024) Google Scholar, 2.Motzer R.J. et al.Nivolumab plus ipilimumab versus sunitinib in advanced renal-cell carcinoma.N. Engl. J. Med. 2018; 378: 1277-1290Crossref PubMed Scopus (2211) Google Scholar], and CAR-T cells recognizing the B cell-expressed CD19 antigen have induced durable remissions in patients with relapsed and refractory hematological cancers such as acute lymphoblastic leukemia (ALL) [3.Maude S.L. et al.Tisagenlecleucel in children and young adults with B-cell lymphoblastic leukemia.N. Engl. J. Med. 2018; 378: 439-448Crossref PubMed Scopus (2278) Google Scholar, 10.Maude S.L. et al.Chimeric antigen receptor T cells for sustained remissions in leukemia.N. Engl. J. Med. 2014; 371: 1507-1517Crossref PubMed Scopus (3334) Google Scholar]. However, a key part of making immunotherapy more broadly applicable is achieving success and antitumor efficacy in other poor-prognosis tumors. Applying the different arms of immunotherapy to pediatric brain tumors – a unique, heterogeneous cancer type distinct from its adult counterparts – presents exciting opportunities that, to succeed, must also consider its specific challenges (Figure 1, Key Figure). This review focuses on recent advances in checkpoint inhibition, CAR-T cell therapy, and vaccine therapy in pediatric HGG, in both preclinical and clinical contexts. We discuss the challenges of the growing brain as a unique tumor site, and highlight approaches that may enable immunotherapies in these difficult-to-treat malignancies. Costimulation molecules play a crucial role in modulating and inhibiting T cell responses [11.Leach D.R. et al.Enhancement of antitumor immunity by CTLA-4 blockade.Science. 1996; 271: 734-1736Crossref Scopus (2602) Google Scholar]. Monoclonal antibodies targeting these inhibitory molecules have been shown to restore CD8+ T cell activation within the tumor microenvironment, and, in some cancers, can function as an effective cancer therapeutic [1.Wolchok J.D. et al.Overall survival with combined nivolumab and ipilimumab in advanced melanoma.N. Engl. J. Med. 2017; 377: 1345-1356Crossref PubMed Scopus (2024) Google Scholar, 2.Motzer R.J. et al.Nivolumab plus ipilimumab versus sunitinib in advanced renal-cell carcinoma.N. Engl. J. Med. 2018; 378: 1277-1290Crossref PubMed Scopus (2211) Google Scholar]. The first seminal discovery demonstrated that antibody-mediated blockade of cytotoxic T lymphocyte-associated protein 4 (CTLA-4) resulted in tumor regression in murine models of colon carcinoma and fibrosarcoma [11.Leach D.R. et al.Enhancement of antitumor immunity by CTLA-4 blockade.Science. 1996; 271: 734-1736Crossref Scopus (2602) Google Scholar]. This was followed by another breakthrough: the marked efficacy of antibodies blocking the checkpoints programmed death 1 (PD-1) and its major ligand PD-L1 [12.Iwai Y. et al.Involvement of PD-L1 on tumor cells in the escape from host immune system and tumor immunotherapy by PD-L1 blockade.Proc. Natl. Acad. Sci. U. S. A. 2002; 99: 12293-12297Crossref PubMed Scopus (2103) Google Scholar]. The success of checkpoint inhibition in adult solid tumors such as melanoma has been remarkable. In 2011, the FDA approved the use of ipilimumab (a CTLA-4 inhibitor) for the treatment of melanoma [13.Food and Drug Administration FDA Approves New Treatment for a Type of Late-Stage Skin Cancer. FDA, 2011Google Scholar]; since then, five different checkpoint inhibitors have been approved. The mutational burden of a tumor, namely the quantity of mutations per coding area of a tumor genome, may be key in predicting clinical responses to checkpoint inhibition in specific cancer types. One study [14.Le D.T. et al.PD-1 blockade in tumors with mismatch-repair deficiency.N. Engl. J. Med. 2015; 372: 2509-2520Crossref PubMed Scopus (5658) Google Scholar] first reported that a subset of patients with colorectal cancer exhibiting mismatch repair deficiency (and thus a higher mutational burden) exhibited a better response to PD-1 blockade by pembrolizumab than did patients with an intact DNA damage response pathway [14.Le D.T. et al.PD-1 blockade in tumors with mismatch-repair deficiency.N. Engl. J. Med. 2015; 372: 2509-2520Crossref PubMed Scopus (5658) Google Scholar]. In a recent systematic review of the literature where objective responses (ORs) were available, the ORs to PD1 or PD-L1 inhibitor monotherapy correlated with tumor mutational burden in 27 human cancers [15.Yarchoan M. et al.Tumor mutational burden and response rate to PD-1 inhibition.N. Engl. J. Med. 2017; 377: 2500-2501Crossref PubMed Scopus (1451) Google Scholar]. Although this association was particularly strong in lung cancers, melanoma, and colorectal cancer, GBM demonstrated a comparatively lower mutational load (<5 coding somatic mutations per megabyte) that correlated with lower objective responses to checkpoint inhibition [15.Yarchoan M. et al.Tumor mutational burden and response rate to PD-1 inhibition.N. Engl. J. Med. 2017; 377: 2500-2501Crossref PubMed Scopus (1451) Google Scholar]. In selected GBM patients with a DNA repair deficiency (and thus a higher mutational tumor burden), case studies have demonstrated responses to checkpoint inhibition [16.Bouffet E. et al.Immune checkpoint inhibition for hypermutant glioblastoma multiforme resulting from germline biallelic mismatch repair deficiency.J. Clin. Oncol. 2016; 34: 2206-2211Crossref PubMed Scopus (545) Google Scholar, 17.Johanns T.M. et al.Immunogenomics of hypermutated glioblastoma: a patient with germline POLE deficiency treated with checkpoint blockade immunotherapy.Cancer Discov. 2016; 6: 1230-1236Crossref PubMed Scopus (202) Google Scholar]. In a report of two pediatric siblings with hypermutant GBM as a result of germline biallelic mismatch repair deficiency (bMMRD) [16.Bouffet E. et al.Immune checkpoint inhibition for hypermutant glioblastoma multiforme resulting from germline biallelic mismatch repair deficiency.J. Clin. Oncol. 2016; 34: 2206-2211Crossref PubMed Scopus (545) Google Scholar], the authors found that bMMRD GBMs had a higher mutational load and a significantly higher number of predicted neoantigens compared to sporadic pediatric gliomas, and up to seven- to 16-fold that of other immune-responsive malignancies such as melanomas, lung cancers, and microsatellite-unstable GI cancers [16.Bouffet E. et al.Immune checkpoint inhibition for hypermutant glioblastoma multiforme resulting from germline biallelic mismatch repair deficiency.J. Clin. Oncol. 2016; 34: 2206-2211Crossref PubMed Scopus (545) Google Scholar]. Both patients were treated with anti-PD-1 inhibitor nivolumab, with marked clinical and radiological response [16.Bouffet E. et al.Immune checkpoint inhibition for hypermutant glioblastoma multiforme resulting from germline biallelic mismatch repair deficiency.J. Clin. Oncol. 2016; 34: 2206-2211Crossref PubMed Scopus (545) Google Scholar]. However, a recent systemic analysis demonstrated glioma to be a tumor subtype wherein clinical response to checkpoint inhibition did not correlate with mutational load [18.Samstein R.M. et al.Tumor mutational load predicts survival after immunotherapy across multiple cancer types.Nat. Genet. 2019; 51: 202-206Crossref PubMed Scopus (1483) Google Scholar], although it is unclear whether any children were included in this analysis. Although this association – or perhaps lack thereof – is unclear, we suggest that it remains relevant to consider checkpoint inhibition when treating children with germline mutations causing hypermutation, especially in the relapsed/refractory disease context. In pediatric patients with brain tumors without germline hypermutation syndromes, however, it is clear that responses to checkpoint inhibitors have not yet reported efficacy (Table 1). Although several trials in pediatric HGG initially reported disease stability in response to checkpoint inhibition (e.g., ClinicalTrials.gov NCT01952769 [19.Fried I. et al.Preliminary results of immune modulating antibody MDV9300 (pidilizumab) treatment in children with diffuse intrinsic pontine glioma.J. Neuro-Oncol. 2018; 136: 189-195Crossref PubMed Scopus (22) Google Scholar]), many of these patients were actually treated in combination with additional therapies such as radiotherapy that are known to confer disease stability and clinical responses. Therefore, we must be careful not to purport disease response to novel therapies when they are given as a single-arm trial as adjuncts to conventional therapy.Table 1Checkpoint Inhibitors in Children with Brain TumorsName/clinical trial identifierAgentPopulationPhaseStatusNCT02359565PembrolizumabRecurrent, progressive, or refractory:High-grade gliomaDiffuse midline gliomaHypermutated brain tumorsEpendymomaMedulloblastomaIRecruitingEstimated enrolment 110aEstimated enrolments as of May 10, 2019.NCT02793466DurvalumabGerm cell tumors (CNS and non-CNS)Diffuse midline gliomaIRecruitingEstimated enrolment 36aEstimated enrolments as of May 10, 2019.NCT03130959'Checkmate 908'NivolumabNivolumab in combination with ipilimumabHigh-grade primary CNS malignancies, including:Primary diffuse midline glioma (recruited after radiotherapy)Recurrent or progressive:High-grade gliomaMedulloblastomaEpendymomaOther tumors of the CNSIIActive, not recruitingEstimated enrolment 170aEstimated enrolments as of May 10, 2019.NCT02550249'Neo-nivo'Neoadjuvant NivolumabNewly diagnosed or recurrent glioblastoma patients requiring surgery:Following surgery, nivolumab continued until toxicity or progressionIICompleted enrolmentActual enrolment: 29NCT02502708Indoximod in combination with temozolomide, or with radiation followed by indoximod/ temozolomide combinationProgressive malignant brain tumors (up-front therapy in context of disease progression) including:Glioblastoma multiforme, all progressive gliomas, and gliosarcomaMedulloblastomaEpendymomaDiffuse intrinsic pontine glioma (DIPG)Other primary tumors of the CNSIRecruitingEstimated enrolment 115aEstimated enrolments as of May 10, 2019.NCT01952769Pidilizumab(anti-PD1 humanized immunoglobulin G1 monoclonal antibody)DIPG patients, enrolled following radiation therapyI/IIUnknown recruitment statusEstimated enrolment 50aEstimated enrolments as of May 10, 2019.a Estimated enrolments as of May 10, 2019. Open table in a new tab Whether checkpoint inhibition will be a valuable therapeutic option for pediatric brain cancer patients remains to be seen. Early results suggest that, without evidence of high tumor mutational load at a minimum, checkpoint inhibition may not be useful as a monotherapy in children with HGG. Future trials are awaited to investigate hypermutation as a biomarker in the pediatric setting, as well as the efficacy of checkpoint inhibition in combination with other immunotherapies. Chimeric antigen receptor (CAR)-T cell therapy, a promising form of adoptive cell therapy involving the use of patient-derived T cells, came to public and scientific attention shortly after checkpoint inhibitors, and contributed significantly to the huge surge of interest in the immuno-oncology world. CARs were first reported in the mid-1980s [20.Gross G. et al.Expression of immunoglobulin-T-cell receptor chimeric molecules as functional receptors with antibody-type specificity.Proc. Natl. Acad. Sci. U. S. A. 1989; 86: 10024-10028Crossref PubMed Scopus (933) Google Scholar]; however, they have only been successfully translated into clinical practice in recent years. CARs are hybrid receptors, fusing the specificity of an antibody-binding domain to the signaling machinery of, typically, T lymphocytes, and other immune cells, thereby redirecting an endogenous T cell towards the antigen of choice [21.Sadelain M. et al.The basic principles of chimeric antigen receptor design.Cancer Discov. 2013; 3: 388-398Crossref PubMed Scopus (796) Google Scholar]. These synthetic receptors are retrovirally or lentivirally integrated into patient-derived T cells, and reinfused into the patient: thus, they represent the powerful concept of a living cell therapy that is truly personalized to the patient. CAR design is necessarily complex: from the extracellular to the intracellular domain, they are designed with a short-chain variable fragment (ScFv) that binds directly to the antigen of choice, a hinge-linker region, a transmembrane domain, a costimulatory domain, and an intracellular CD3ζ signaling tail [21.Sadelain M. et al.The basic principles of chimeric antigen receptor design.Cancer Discov. 2013; 3: 388-398Crossref PubMed Scopus (796) Google Scholar]. The signaling tail of first-generation CARs was derived from CD3ζ and lacked a costimulatory domain, causing limited in vivo expansion and cytotoxicity [21.Sadelain M. et al.The basic principles of chimeric antigen receptor design.Cancer Discov. 2013; 3: 388-398Crossref PubMed Scopus (796) Google Scholar]. With the incorporation of CD28, and subsequently CD137 costimulatory domains, remarkable results with second-generation CARs were reported in various hematological malignancies, including relapsed B cell acute lymphoblastic leukemia, chronic lymphocytic lymphoma, and B cell non-Hodgkin lymphoma [22.June C.H. et al.CAR T cell immunotherapy for human cancer.Science. 2018; 359: 1361-1365Crossref PubMed Scopus (1178) Google Scholar]. Although a proportion of children with B cell acute lymphoblastic leukemia (B cell ALL) treated with anti-CD19 CAR-T cell therapy do relapse, the overall 70–90% complete remission (CR) response [3.Maude S.L. et al.Tisagenlecleucel in children and young adults with B-cell lymphoblastic leukemia.N. Engl. J. Med. 2018; 378: 439-448Crossref PubMed Scopus (2278) Google Scholar, 10.Maude S.L. et al.Chimeric antigen receptor T cells for sustained remissions in leukemia.N. Engl. J. Med. 2014; 371: 1507-1517Crossref PubMed Scopus (3334) Google Scholar, 22.June C.H. et al.CAR T cell immunotherapy for human cancer.Science. 2018; 359: 1361-1365Crossref PubMed Scopus (1178) Google Scholar] is impressive, given that many of these children with relapsed or refractory disease have no other curative treatment options available to them. Similarly to checkpoint inhibitors, the sudden success of CAR-T cell therapy in hematological malignancies ignited natural interest in translation for poor-prognosis tumors. Unlike checkpoint inhibition, that relies on the presence of an endogenous immune response, CAR-T cell therapy targets tumor specific antigens, and may be more useful for pediatric brain tumors which lack a high somatic tumor mutational burden. In 2016, one landmark study [23.Brown C.E. et al.Regression of glioblastoma after chimeric antigen receptor T-cell therapy.N. Engl. J. Med. 2016; 375: 2561-2569Crossref PubMed Scopus (880) Google Scholar] demonstrated the first proof-of-concept evidence that CAR-T cell therapy in brain tumors could lead to clinical and radiological regression. Specifically, repeated intraventricular and intrathecal administration of CAR-T cells targeting interleukin-13 receptor α2 (IL-13Rα2), a glioma-associated antigen, demonstrated marked tumor regression in multifocal sites in one adult patient with relapsed GBM; however, the patient relapsed after 6 months with an antigen-escaped tumor (IL-13Rα2-negative) [23.Brown C.E. et al.Regression of glioblastoma after chimeric antigen receptor T-cell therapy.N. Engl. J. Med. 2016; 375: 2561-2569Crossref PubMed Scopus (880) Google Scholar]. It should also be noted that, although this publication only reported on a single patient from a cohort of seven patients in a Phase I trial (ClinicalTrials.gov NCT02208362), the study is still recruiting and its findings are yet to be published. Despite the single relapse with an antigen-escaped tumor, this study did find that CAR-T cells could be delivered safely into the brain without severe neurotoxicity or cytokine release syndrome. Published clinical CAR-T cell trials for CNS glial tumors include CARs directed against target antigens HER2 [24.Ahmed N. et al.HER2-specific chimeric antigen receptor-modified virus-specific T cells for progressive glioblastoma: a phase 1 dose-escalation trial.JAMA Oncol. 2017; 3: 1094-1101Crossref PubMed Scopus (365) Google Scholar] and epidermal growth factor receptor variant III (EGFRvIII) [25.O'Rourke D.M. et al.A single dose of peripherally infused EGFRvIII-directed CAR T cells mediates antigen loss and induces adaptive resistance in patients with recurrent glioblastoma.Sci. Transl. Med. 2017; 9PubMed Google Scholar, 26.Goff S.L. et al.Pilot trial of adoptive transfer of chimeric antigen receptor-transduced T cells targeting EGFRvIII in patients with glioblastoma.J. Immunother. 2019; 42: 126-135Crossref PubMed Scopus (124) Google Scholar]. Notably, the first in human Phase I trial (ClinicalTrials.gov NCT02209376) of peripherally administered CAR-T cells directed against the EGFRvIII antigen detected these CAR-T cells in brain parenchymal tissue after surgical resection in adult glioblastoma patients, implying effective CAR-T cell trafficking into the brain [25.O'Rourke D.M. et al.A single dose of peripherally infused EGFRvIII-directed CAR T cells mediates antigen loss and induces adaptive resistance in patients with recurrent glioblastoma.Sci. Transl. Med. 2017; 9PubMed Google Scholar]. However, it is possible that tumor tissues analyzed in this study were obtained from areas where the blood–brain barrier (BBB) was already disrupted [25.O'Rourke D.M. et al.A single dose of peripherally infused EGFRvIII-directed CAR T cells mediates antigen loss and induces adaptive resistance in patients with recurrent glioblastoma.Sci. Transl. Med. 2017; 9PubMed Google Scholar], and it is therefore impossible to definitively conclude from this study that anti-EGFRvIII CAR-T cells could cross an intact BBB. Another recent peripherally administered anti-EGFRvIII CAR-T cell Phase I trial (ClinicalTrials.gov NCT01454596) attempted dose escalation with interleukin-2 (IL-2) support; however, two patients developed severe respiratory side effects due to the CAR-T cell infusion, with one treatment-related mortality being recorded at the highest dose administered [26.Goff S.L. et al.Pilot trial of adoptive transfer of chimeric antigen receptor-transduced T cells targeting EGFRvIII in patients with glioblastoma.J. Immunother. 2019; 42: 126-135Crossref PubMed Scopus (124) Google Scholar]. Both of these anti-EGFRvIII CAR-T cell trials [25.O'Rourke D.M. et al.A single dose of peripherally infused EGFRvIII-directed CAR T cells mediates antigen loss and induces adaptive resistance in patients with recurrent glioblastoma.Sci. Transl. Med. 2017; 9PubMed Google Scholar, 26.Goff S.L. et al.Pilot trial of adoptive transfer of chimeric antigen receptor-transduced T cells targeting EGFRvIII in patients with glioblastoma.J. Immunother. 2019; 42: 126-135Crossref PubMed Scopus (124) Google Scholar] failed to elicit significant antitumor responses, with lack of CAR-T cell persistence and antigen escape being notable issues. With regard to HER-2 CAR-T cell therapy, an open-label Phase I clinical trial [24.Ahmed N. et al.HER2-specific chimeric antigen receptor-modified virus-specific T cells for progressive glioblastoma: a phase 1 dose-escalation trial.JAMA Oncol. 2017; 3: 1094-1101Crossref PubMed Scopus (365) Google Scholar] (ClinicalTrials.gov NCT01109095) examining the impact of anti-Her2 CAR-T cell therapy in patients with recurrent GBM undertook a cautious dose-escalation strategy, given a previous patient death from HER-2 CAR-T cell therapy [27.Morgan R.A. et al.Case report of a serious adverse event following the administration of T cells transduced with a chimeric antigen receptor recognizing ERBB2.Mol. Ther. 2010; 18: 843-851Abstract Full Text Full Text PDF PubMed Scopus (1646) Google Scholar]. Although no dose-limiting toxic adverse events were reported, efficacy was poor, and only one patient of 17 was reported to have a partial response (PR) [24.Ahmed N. et al.HER2-specific chimeric antigen receptor-modified virus-specific T cells for progressive glioblastoma: a phase 1 dose-escalation trial.JAMA Oncol. 2017; 3: 1094-1101Crossref PubMed Scopus (365) Google Scholar]. It is possible that the lower CAR infusion doses limited the efficacy of the therapy; however, this is presently unclear and warrants further testing. It should be noted that these three published clinical trials [24.Ahmed N. et al.HER2-specific chimeric antigen receptor-modified virus-specific T cells for progressive glioblastoma: a phase 1 dose-escalation trial.JAMA Oncol. 2017; 3: 1094-1101Crossref PubMed Scopus (365) Google Scholar, 25.O'Rourke D.M. et al.A single dose of peripherally infused EGFRvIII-directed CAR T cells mediates antigen loss and induces adaptive resistance in patients with recurrent glioblastoma.Sci. Transl. Med. 2017; 9PubMed Google Scholar, 26.Goff S.L. et al.Pilot trial of adoptive transfer of chimeric antigen receptor-transduced T cells targeting EGFRvIII in patients with glioblastoma.J. Immunother. 2019; 42: 126-135Crossref PubMed Scopus (124) Google Scholar] examining CAR-T cell function in patients with GBM have been largely adult-based, with little focus on differentiating the treatment for the small subset of pediatric patients included in one [24.Ahmed N. et al.HER2-specific chimeric antigen receptor-modified virus-specific T cells for progressive glioblastoma: a phase 1 dose-escalation trial.JAMA Oncol. 2017; 3: 1094-1101Crossref PubMed Scopus (365) Google Scholar] of these trials. Although we have learned many lessons about CAR-T cell persistence and tumor heterogeneity causing antigen escape in these studies, we need to be mindful that the same physiology may not apply in the developing brain of a child, compared to adults. A recent key discovery [28.Mount C.W. et al.Potent antitumor efficacy of anti-GD2 CAR T cell