Generation of Anti-HIV CAR-T Cells for Preclinical Research

The inability of people living with HIV (PLWH) to eradicate human immunodeficiency virus (HIV) infection is due in part to the inadequate HIV-specific cellular immune response. The antiviral function of cytotoxic CD8+ T cells, which are crucial for HIV control, is impaired during chronic viral infection because of viral escape mutations, immune exhaustion, HIV antigen downregulation, inflammation, and apoptosis. In addition, some HIV-infected cells either localize to tissue sanctuaries inaccessible to CD8+ T cells or are intrinsically resistant to CD8+ T cell killing. The novel design of synthetic chimeric antigen receptors (CARs) that enable T cells to target specific antigens has led to the development of potent and effective CAR-T cell therapies. While initial clinical trials using anti-HIV CAR-T cells performed over 20 years ago showed limited anti-HIV effects, the improved CAR-T cell design, which enabled its success in treating cancer, has reinstated CAR-T cell therapy as a strategy for HIV cure with notable progress being made in the recent decade.Effective CAR-T cell therapy against HIV infection requires the generation of anti-HIV CAR-T cells with potent in vivo activity against HIV-infected cells. Preclinical evaluation of anti-HIV efficacy of CAR-T cells and their safety is fundamental for supporting the initiation of subsequent clinical trials in PLWH. For these preclinical studies, we developed a novel humanized mouse model supporting in vivo HIV infection, the development of viremia, and the evaluation of novel HIV therapeutics. Preclinical assessment of anti-HIV CAR-T cells using this mouse model involves a multistep process including peripheral blood mononuclear cells (PBMCs) harvested from human donors, T cell purification, ex vivo T cell activation, transduction with lentiviral vectors encoding an anti-HIV CAR, CAR-T cell expansion and infusion in mice intrasplenically injected with autologous PBMCs followed by the determination of CAR-T cell capacity for HIV suppression. Each of the steps described in the following protocol were optimized in the lab to maximize the quantity and quality of the final anti-HIV CAR-T cell products.

Preclinical development of a chimeric antigen receptor T cell therapy targeting FGFR4 in rhabdomyosarcoma

Pediatric patients with relapsed or refractory rhabdomyosarcoma (RMS) have dismal cure rates, and effective therapy is urgently needed. The oncogenic receptor tyrosine kinase fibroblast growth factor receptor 4 (FGFR4) is highly expressed in RMS and lowly expressed in healthy tissues. Here, we describe a second-generation FGFR4-targeting chimeric antigen receptor (CAR), based on an anti-human FGFR4-specific murine monoclonal antibody 3A11, as an adoptive T cell treatment for RMS. The 3A11 CAR T cells induced robust cytokine production and cytotoxicity against RMS cell lines in vitro. In contrast, a panel of healthy human primary cells failed to activate 3A11 CAR T cells, confirming the selectivity of 3A11 CAR T cells against tumors with high FGFR4 expression. Finally, we demonstrate that 3A11 CAR T cells are persistent in vivo and can effectively eliminate RMS tumors in two metastatic and two orthotopic models. Therefore, our study credentials CAR T cell therapy targeting FGFR4 to treat patients with RMS.

A stem cell epigenome is associated with primary nonresponse to CD19 CAR T cells in pediatric acute lymphoblastic leukemia

CD19 chimeric antigen receptor T-cell therapy (CD19-CAR) has changed the treatment landscape and outcomes for patients with pre-B-cell acute lymphoblastic leukemia (B-ALL). Unfortunately, primary nonresponse (PNR), sustained CD19+ disease, and concurrent expansion of CD19-CAR occur in 20% of the patients and is associated with adverse outcomes. Although some failures may be attributable to CD19 loss, mechanisms of CD19-independent, leukemia-intrinsic resistance to CD19-CAR remain poorly understood. We hypothesize that PNR leukemias are distinct compared with primary sensitive (PS) leukemias and that these differences are present before treatment. We used a multiomic approach to investigate this in 14 patients (7 with PNR and 7 with PS) enrolled in the PLAT-02 trial at Seattle Children's Hospital. Long-read PacBio sequencing helped identify 1 PNR in which 47% of CD19 transcripts had exon 2 skipping, but other samples lacked CD19 transcript abnormalities. Epigenetic profiling discovered DNA hypermethylation at genes targeted by polycomb repressive complex 2 (PRC2) in embryonic stem cells. Similarly, assays of transposase-accessible chromatin-sequencing revealed reduced accessibility at these PRC2 target genes, with a gain in accessibility of regions characteristic of hematopoietic stem cells and multilineage progenitors in PNR. Single-cell RNA sequencing and cytometry by time of flight analyses identified leukemic subpopulations expressing multilineage markers and decreased antigen presentation in PNR. We thus describe the association of a stem cell epigenome with primary resistance to CD19-CAR therapy. Future trials incorporating these biomarkers, with the addition of multispecific CAR T cells targeting against leukemic stem cell or myeloid antigens, and/or combined epigenetic therapy to disrupt this distinct stem cell epigenome may improve outcomes of patients with B-ALL.

Intraventricular B7-H3 CAR T Cells for Diffuse Intrinsic Pontine Glioma: Preliminary First-in-Human Bioactivity and Safety

Diffuse intrinsic pontine glioma (DIPG) remains a fatal brainstem tumor demanding innovative therapies. As B7-H3 (CD276) is expressed on central nervous system (CNS) tumors, we designed B7-H3-specific chimeric antigen receptor (CAR) T cells, confirmed their preclinical efficacy, and opened BrainChild-03 (NCT04185038), a first-in-human phase I trial administering repeated locoregional B7-H3 CAR T cells to children with recurrent/refractory CNS tumors and DIPG. Here, we report the results of the first three evaluable patients with DIPG (including two who enrolled after progression), who received 40 infusions with no dose-limiting toxicities. One patient had sustained clinical and radiographic improvement through 12 months on study. Patients exhibited correlative evidence of local immune activation and persistent cerebrospinal fluid (CSF) B7-H3 CAR T cells. Targeted mass spectrometry of CSF biospecimens revealed modulation of B7-H3 and critical immune analytes (CD14, CD163, CSF-1, CXCL13, and VCAM-1). Our data suggest the feasibility of repeated intracranial B7-H3 CAR T-cell dosing and that intracranial delivery may induce local immune activation.

SIGNIFICANCE

This is the first report of repeatedly dosed intracranial B7-H3 CAR T cells for patients with DIPG and includes preliminary tolerability, the detection of CAR T cells in the CSF, CSF cytokine elevations supporting locoregional immune activation, and the feasibility of serial mass spectrometry from both serum and CSF. This article is highlighted in the In This Issue feature, p. 1.

Place of care manufacturing of chimeric antigen receptor cells: opportunities and challenges

The landscape of therapeutic options for B cell malignancies has fundamentally changed with regulatory and marketing approval of chimeric antigen receptor (CAR)-engineered T cell products. The cell types used for CAR-T production, the length of time of manufacture, the stimulation matrix, and the nature of the gene vector used to transduce human T cells all are significant variables that require adequate quality control before infusion. Having approved products available to clinicians using a centralized production paradigm has not stopped innovation in investigator-initiated trials. Moreover, the high costs of the commercial products have been a significant wake-up call to those concerned about rising costs in health care, and the ability of developing nations, and nations with managed care systems to support these costs. Place-of-care manufacturing is a clear alternative to the approved products created in a centralized manufacturing approach. It is supported by continued technological innovation and the willingness of clinicians to develop new ways to decrease costs and make these curative therapies equitably available.

FGFR4-targeted chimeric antigen receptors (CARs) combined with anti-myeloid poly-pharmacy effectively treats orthotopic rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is the most common soft tissue cancer in children. Treatment outcomes, particularly for relapsed/refractory or metastatic disease, have not improved in decades. The current lack of novel therapies and low tumor mutational burden suggest that CAR T therapy could be a promising approach to treating RMS. Previous work identified Fibroblast Growth Factor Receptor 4 (FGFR4, CD334) as being specifically upregulated in RMS, making it a candidate target for CAR-T cells. We tested the feasibility of an FGFR4 targeted CAR for treating RMS using an NSG mouse with RH30 orthotopic (intramuscular) tumors. The first barrier we noted was that RMS tumors produce a collagen-rich stroma, replete with immunosuppressive myeloid cells, when T cell therapy is initiated. This stromal response is not seen in tumor-only xenografts. When scFV-based binders were selected from phage display, CARs targeting FGFR4 were not effective until our screening approach was refined to identify binders to the membrane-proximal domain of FGFR4. Having improved the CAR, we devised a pharmacologic strategy to augment CAR-T activity by inhibiting the myeloid component of the T cell-induced tumor stroma. The combined treatment of mice with anti-myeloid polypharmacy (targeting CSF1R, IDO1, iNOS, TGFbeta, PDL1, MIF and myeloid mis-differentiation) allowed FGFR4 CAR-T to successfully clear orthotopic RMS tumors, demonstrating that RMS tumors, even with very low copy number targets, can be targeted by CAR-T upon reversal of an immunosuppressive microenvironment.

Abstract 579: Anti-myeloid poly-pharmacy allows FGFR4-targeted chimeric antigen receptors to effectively treat an orthotopic model of rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is the most common soft tissue cancer in children, yet treatment outcomes, particularly for relapsed/refractory or metastatic disease, have not improved in decades. The lack of novel therapies and limited immune checkpoint blockade efficacy suggests that CAR-T therapy would be a promising therapeutic approach for RMS and other sarcomas. The key to CAR specificity is to guide engineered T cells to a molecular target that is tumor specific, expressed on the cell surface, and expressed at high enough levels for CAR-T activation. Previous work identified Fibroblast Growth Factor Receptor 4 (FGFR4, CD334) as being specifically and consistently upregulated in RMS, making it a candidate target for CAR-T cells. We tested the feasibility of an FGFR4 targeted CAR for treating RMS using an NSG mouse with RH30 orthotopic (intramuscular) tumors. A previous CAR designed to target FGFR4 was active in vitro but failed to control orthotopic tumors in the NSG mouse model. A new generation of FGFR4 binders was produced targeting the membrane proximal domain of FGFR4. When engineered as CARs, these binders exceeded the activity of previous generation binders in vitro with regard to cellular cytotoxicity and cytokine production. Nevertheless, new candidate binders failed to control orthotopic tumors in vivo. We then interrogated the specific tumor defenses employed by RMS to evade immune control. First, we optimized CAR signaling domains to target low density antigen. Quantitative flow analysis determined FGFR4 expression to be a very low 700 molecules per cell on in vivo RH30 tumors, as opposed to 2-3,000 in tissue culture. We also found that RMS tumors produced a collagen-rich stroma, replete with immunosuppressive myeloid cells. Stroma was induced by T cell therapy and absent in untreated mice. This stroma sequesters CAR-T cells, and produces an immune-excluded phenotype, as assessed by immunohistochemical analysis. Immunohistochemistry identified that M2 macrophages, and to a lesser degree MDSC, were the major cellular constituents of the therapy-induced stroma. RNA expression panel analysis (Nanostring) identified the induction of tumor defense-associated transcripts, including MIF, IDO1, and TGFβ, upon T cell therapy. Based on these results, we devised a strategy to augment CAR-T activity while removing the immunosuppressive barriers. The exposure of mice to anti-myeloid poly-pharmacy (targeting CSF1R (PLX3397), IDO1 (epacadostat), iNOS (L-NAME), TGFβ (SD208), PDL1 (αPD1 antibody), MIF (gene knockout), and myeloid misdifferentiation (ATRA)) allowed FGFR4 CAR-T to successfully clear orthotopic RMS tumors. Our results demonstrate that RMS tumors, even with low copy number targets, can be targeted by CAR-T upon reversal of an immunosuppressive microenvironment, modeling an approach to treating pediatric sarcomas with CAR-T therapy.

Citation Format: Peter M. Sullivan, Rajesh Kumar, Wei Li, Lingyang Wang, Yue Zhang, Sophie Jamet, Adam Cheuk, Javed Khan, Dimiter S. Dimitrov, Rimas J. Orentas. Anti-myeloid poly-pharmacy allows FGFR4-targeted chimeric antigen receptors to effectively treat an orthotopic model of rhabdomyosarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 579.

Abstract 3581: Multi-omic analysis identifies mechanisms of resistance to CD19 CAR T-cell therapy in children with acute lymphoblastic leukemia

Background: Acute lymphoblastic leukemia (ALL) is the most common childhood cancer. Despite the survival rate of 90% for newly diagnosed children with ALL, the outcome for relapsed patients is historically poor with a less than 30% survival. CD19 CAR T-cell therapy (CART19) has shown remarkable response rates, between 80-90% in relapsed/refractory disease. Little is known about antigen-independent factors that predict initial resistance to CART19. We hypothesized that leukemias that are resistant to CART19 are distinct from sensitive leukemias and that these differences can be detected prior to therapy.

Methods: To interrogate differences between resistant and sensitive leukemias, we obtained pre-treatment bone marrow aspirates (BMAs) from patients enrolled in a clinical trial at Seattle Children’s Hospital (PLAT-02). Samples were categorized based on patient response, with non-response defined as not achieving and maintaining minimal residual disease negativity at Day +63. Our study included 7 resistant and 7 sensitive leukemias as controls. We performed whole exome sequencing, bulk RNA-seq, PacBio-seq of the CD19 locus, array-based methylation, ATAC-seq, scRNA-seq, and CyTOF.

Results: We found that non-response to CART19 is independent of leukemic subtype. Despite blasts being CD19+ in all patients by flow cytometry, we identified alternative splicing of CD19 in one non-responder, while the remaining non-responders expressed high levels of wildtype CD19. We discovered a distinctive DNA methylation pattern in the non-responders characterized by hypermethylation of PRC2 targets in embryonic and cancer stem cells (p = 8.15E-25) Furthermore, using gene set enrichment analysis of ATAC-seq data, we found increased accessibility of chromatin at regions associated with stem cell proliferation (NES = 2.31; p < 0.0001) and cell cycling (NES = 2.27; p < 0.0001). We found a greater similarity between accessibility patterns of non-responders to hematopoietic progenitors, including hematopoietic stem cells (p = 0.037) and common myeloid progenitors (p = 0.047). These findings were supported by an increased frequency of cell subpopulations expressing a multi-lineage phenotype (CD19, CD20, CD33, CD34; p = 0.009). Moreover, we find decreased expression of antigen presentation and processing pathways across all leukemic cells relative to responders (p = 0.0001).

Conclusions: This study, one of the most comprehensive multi-omic analyses of samples from patients treated with CAR T-cells, identified resistance mechanisms that can be detected prior to treatment. We report the novel association of a stem cell phenotype, lineage plasticity, and decreased antigen presentation with resistance. These results support further refinement of eligibility for CART19 for children with leukemia and highlights the need for alternative of complimentary approaches for these patients.

Citation Format: Katherine E. Masih, Rebecca Gardner, Hsien-Chao Chou, Abdalla Abdelmaksoud, Young K. Song, Luca Mariani, Vineela Gangalapudi, Berkley E. Gryder, Ashley Wilson, Serifat O. Adebola, Benjamin Z. Stanton, Chaoyu Wang, Xinyu Wen, Gregoire Altan-Bonnet, Michael C. Kelly, Jun S. Wei, Martha L. Bulyk, Michael C. Jensen, Rimas J. Orentas, Javed Khan. Multi-omic analysis identifies mechanisms of resistance to CD19 CAR T-cell therapy in children with acute lymphoblastic leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3581.

Bryostatin Activates CAR T-Cell Antigen-Non-Specific Killing (CTAK), and CAR-T NK-Like Killing for Pre-B ALL, While Blocking Cytolysis of a Burkitt Lymphoma Cell Line

The advent of CAR-T cell therapy has changed the face of clinical care for relapsed and refractory pre-B-acute lymphocytic leukemia (B-ALL) and lymphoma. Although curative responses are reported, long-term cures remain below 50%. Different CAR T-cell leukemia targets appear to have different mechanisms of CAR-T escape. For CD22, therapeutic evasion is linked to down-modulation of the number CD22 proteins expressed on the extracellular aspect of the leukemia cell plasma membrane. Recently, pharmacologic agents known to induce cellular differentiation or epigenetic modification of leukemia have been shown to impact CD22 and CD19 expression levels on B-ALL, and thereby increase sensitivity to CAR-T mediated cytolysis. We explored the impact of epigenetic modifiers and differentiation agents on leukemia cell lines of B cell origin, as well as normal B cells. We confirmed the activity of bryostatin to increase CD22 expression on model cell lines. However, bryostatin does not change CD22 levels on normal B cells. Furthermore, bryostatin inhibited CAR-T mediated cytolysis of the Raji Burkitt lymphoma cell line. Bryostatin increased the cytolysis by CD22 CAR-T for B-ALL cell lines by at least three mechanisms: 1) the previously reported increase in CD22 target cell numbers on the cell surface, 2) the induction of NK ligands, and 3) the induction of ligands that sensitize leukemia cells to activated T cell antigen-non-specific killing. The opposite effect was seen for Burkitt lymphoma, which arises from a more mature B cell lineage. These findings should caution investigators against a universal application of agents shown to increase killing of leukemia target cells by CAR-T in a specific disease class, and highlights that activation of non-CAR-mediated killing by activated T cells may play a significant role in the control of disease. We have termed the killing of leukemia targets, by a set of cell-surface receptors that does not overlap with NK-like killing “CTAK,” CAR-T Cell antigen-non-specific killing.

Immuno-transcriptomic profiling of extracranial pediatric solid malignancies

We perform an immunogenomics analysis utilizing whole-transcriptome sequencing of 657 pediatric extracranial solid cancer samples representing 14 diagnoses, and additionally utilize transcriptomes of 131 pediatric cancer cell lines and 147 normal tissue samples for comparison. We describe patterns of infiltrating immune cells, T cell receptor (TCR) clonal expansion, and translationally relevant immune checkpoints. We find that tumor-infiltrating lymphocytes and TCR counts vary widely across cancer types and within each diagnosis, and notably are significantly predictive of survival in osteosarcoma patients. We identify potential cancer-specific immunotherapeutic targets for adoptive cell therapies including cell-surface proteins, tumor germline antigens, and lineage-specific transcription factors. Using an orthogonal immunopeptidomics approach, we find several potential immunotherapeutic targets in osteosarcoma and Ewing sarcoma and validated PRAME as a bona fide multi-pediatric cancer target. Importantly, this work provides a critical framework for immune targeting of extracranial solid tumors using parallel immuno-transcriptomic and -peptidomic approaches.

Proteomic Screens for Suppressors of Anoikis Identify IL1RAP as a Promising Surface Target in Ewing Sarcoma

Cancer cells must overcome anoikis (detachment-induced death) to successfully metastasize. Using proteomic screens, we found that distinct oncoproteins upregulate IL1 receptor accessory protein (IL1RAP) to suppress anoikis. IL1RAP is directly induced by oncogenic fusions of Ewing sarcoma, a highly metastatic childhood sarcoma. IL1RAP inactivation triggers anoikis and impedes metastatic dissemination of Ewing sarcoma cells. Mechanistically, IL1RAP binds the cell-surface system X_c ^- transporter to enhance exogenous cystine uptake, thereby replenishing cysteine and the glutathione antioxidant. Under cystine depletion, IL1RAP induces cystathionine gamma lyase (CTH) to activate the transsulfuration pathway for de novo cysteine synthesis. Therefore, IL1RAP maintains cyst(e)ine and glutathione pools, which are vital for redox homeostasis and anoikis resistance. IL1RAP is minimally expressed in pediatric and adult normal tissues, and human anti-IL1RAP antibodies induce potent antibody-dependent cellular cytotoxicity of Ewing sarcoma cells. Therefore, we define IL1RAP as a new cell-surface target in Ewing sarcoma, which is potentially exploitable for immunotherapy. SIGNIFICANCE: Here, we identify cell-surface protein IL1RAP as a key driver of metastasis in Ewing sarcoma, a highly aggressive childhood sarcoma. Minimal expression in pediatric and adult normal tissues nominates IL1RAP as a promising target for immunotherapy.See related commentary by Yoon and DeNicola, p. 2679.This article is highlighted in the In This Issue feature, p. 2659.

Towards access for all: 1st Working Group Report for the Global Gene Therapy Initiative (GGTI)

The gene and cell therapy field saw its first approved treatments in Europe in 2012 and the United States in 2017 and is projected to be at least a $10B USD industry by 2025. Despite this success, a massive gap exists between the companies, clinics, and researchers developing these therapeutic approaches, and their availability to the patients who need them. The unacceptable reality is a geographic exclusion of low-and middle-income countries (LMIC) in gene therapy development and ultimately the provision of gene therapies to patients in LMIC. This is particularly relevant for gene therapies to treat human immunodeficiency virus infection and hemoglobinopathies, global health crises impacting tens of millions of people primarily located in LMIC. Bridging this divide will require research, clinical and regulatory infrastructural development, capacity-building, training, an approval pathway and community adoption for success and sustainable affordability. In 2020, the Global Gene Therapy Initiative was formed to tackle the barriers to LMIC inclusion in gene therapy development. This working group includes diverse stakeholders from all sectors and has set a goal of introducing two gene therapy Phase I clinical trials in two LMIC, Uganda and India, by 2024. Here we report on progress to date for this initiative.

Combining Immunocytokine and Ex Vivo Activated NK Cells as a Platform for Enhancing Graft-Versus-Tumor Effects Against GD2+ Murine Neuroblastoma

Management for high-risk neuroblastoma (NBL) has included autologous hematopoietic stem cell transplant (HSCT) and anti-GD2 immunotherapy, but survival remains around 50%. The aim of this study was to determine if allogeneic HSCT could serve as a platform for inducing a graft-versus-tumor (GVT) effect against NBL with combination immunocytokine and NK cells in a murine model. Lethally irradiated C57BL/6 (B6) x A/J recipients were transplanted with B6 bone marrow on Day +0. On day +10, allogeneic HSCT recipients were challenged with NXS2, a GD2+ NBL. On days +14-16, mice were treated with the anti-GD2 immunocytokine hu14.18-IL2. In select groups, hu14.18-IL2 was combined with infusions of B6 NK cells activated with IL-15/IL-15Rα and CD137L ex vivo. Allogeneic HSCT alone was insufficient to control NXS2 tumor growth, but the addition of hu14.18-IL2 controlled tumor growth and improved survival. Adoptive transfer of ex vivo CD137L/IL-15/IL-15Rα activated NK cells with or without hu14.18-IL2 exacerbated lethality. CD137L/IL-15/IL-15Rα activated NK cells showed enhanced cytotoxicity and produced high levels of TNF-α in vitro, but induced cytokine release syndrome (CRS) in vivo. Infusing Perforin-/- CD137L/IL-15/IL-15Rα activated NK cells had no impact on GVT, whereas TNF-α-/- CD137L/IL-15/IL-15Rα activated NK cells improved GVT by decreasing peripheral effector cell subsets while preserving tumor-infiltrating lymphocytes. Depletion of Ly49H+ NK cells also improved GVT. Using allogeneic HSCT for NBL is a viable platform for immunocytokines and ex vivo activated NK cell infusions, but must be balanced with induction of CRS. Regulation of TNFα or activating NK subsets may be needed to improve GVT effects.

Locoregional infusion of HER2-specific CAR T cells in children and young adults with recurrent or refractory CNS tumors: an interim analysis

Locoregional delivery of chimeric antigen receptor (CAR) T cells has resulted in objective responses in adults with glioblastoma, but the feasibility and tolerability of this approach is yet to be evaluated for pediatric central nervous system (CNS) tumors. Here we show that engineering of a medium-length CAR spacer enhances the therapeutic efficacy of human erb-b2 receptor tyrosine kinase 2 (HER2)-specific CAR T cells in an orthotopic xenograft medulloblastoma model. We translated these findings into BrainChild-01 ( NCT03500991 ), an ongoing phase 1 clinical trial at Seattle Children's evaluating repetitive locoregional dosing of these HER2-specific CAR T cells to children and young adults with recurrent/refractory CNS tumors, including diffuse midline glioma. Primary objectives are assessing feasibility, safety and tolerability; secondary objectives include assessing CAR T cell distribution and disease response. In the outpatient setting, patients receive infusions via CNS catheter into either the tumor cavity or the ventricular system. The initial three patients experienced no dose-limiting toxicity and exhibited clinical, as well as correlative laboratory, evidence of local CNS immune activation, including high concentrations of CXCL10 and CCL2 in the cerebrospinal fluid. This interim report supports the feasibility of generating HER2-specific CAR T cells for repeated dosing regimens and suggests that their repeated intra-CNS delivery might be well tolerated and activate a localized immune response in pediatric and young adult patients.

Trispecific CD19-CD20-CD22-targeting duoCAR-T cells eliminate antigen-heterogeneous B cell tumors in preclinical models

A substantial number of patients with leukemia and lymphoma treated with anti-CD19 or anti-CD22 monoCAR-T cell therapy relapse because of antigen loss or down-regulation. We hypothesized that B cell tumor antigen escape may be overcome by a chimeric antigen receptor (CAR) design that simultaneously targets three B cell leukemia antigens. We engineered trispecific duoCAR-T cells with lentiviral vectors encoding two CAR open reading frames that target CD19, CD20, and CD22. The duoCARs were composed of a CAR with a tandem CD19- and CD20-targeting binder, linked by the P2A self-cleaving peptide to a second CAR targeting CD22. Multiple combinations of intracellular T cell signaling motifs were evaluated. The most potent duoCAR architectures included those with ICOS, OX40, or CD27 signaling domains rather than those from CD28 or 4-1BB. We identified four optimal binder and signaling combinations that potently rejected xenografted leukemia and lymphoma tumors in vivo. Moreover, in mice bearing a mixture of B cell lymphoma lines composed of parental triple-positive cells, CD19-negative, CD20-negative, and CD22-negative variants, only the trispecific duoCAR-T cells rapidly and efficiently rejected the tumors. Each of the monoCAR-T cells failed to prevent tumor progression. Analysis of intracellular signaling profiles demonstrates that the distinct signaling of the intracellular domains used may contribute to these differential effects. Multispecific duoCAR-T cells are a promising strategy to prevent antigen loss-mediated relapse or the down-regulation of target antigen in patients with B cell malignancies.

Abstract 1545: Development of FGFR4-targeted chimeric antigen receptors (CARs) for the treatment of rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is a soft tissue cancer commonly arising in muscle, and is the most common soft tissue cancer in children. Patients with metastatic RMS have a poor prognosis that has not improved in decades, highlighting the need for novel therapies. Pediatric tumors, and especially the fusion-gene driven alveolar RMS (aRMS) subset, have among the lowest mutational burden of any tumor, suggesting that checkpoint therapies alone will be insufficient to control tumors. CAR-T cells are genetically engineered T lymphocytes expressing an extracellular binding domain and intracellular T cell signaling domains. Upon engagement of the binding domain (often antibody-derived) to the antigenic target on a tumor cell, activation and degranulation occurs resulting in cell-mediated toxicity induced death of the tumor cells. The key to CAR specificity is to guide engineered T cells to a molecular target that is tumor specific, expressed on the cell surface, and expressed at high enough levels for CAR-T activation. Previous work identified fibroblast growth factor receptor 4 (FGFR4, CD334) as specifically upregulated in RMS tumors compared to normal tissue, making it a candidate for the selective targeting of CAR T cells. A previous CAR designed to target FGFR4 (m410) showed in vitro efficacy, and some control of tumor in a metastatic (intravenous) RMS model but failed to control orthotopic (intramuscular) RMS tumors in vivo. We have developed a new generation of FGFR4-binding moieties to be tested as new components of CARs. Our new binders were derived from both human F(ab)- and human VH-only (single chain domain antibodies, dAbs) phage display libraries. Binders were cloned into a CAR featuring a CD8 hinge and transmembrane domain, 4-1BB signaling domain, and CD3zeta signaling domain. FGFR4 CARs were screened for surface expression by flow cytometry, cytotoxicity against target and non-target cells, and cytokine production in response to RMS cell lines in vitro. Two F(ab)-based FGFR4-binders engineered into CARs were selected for further development based on high cellular cytotoxicity and tumor specificity. These will be tested for in vivo efficacy against metastatic and intramuscular RMS tumor models in NSG mice. Additionally, we have tested RMS tumor cell lines, with and without exposure to IFN-gamma, to model induced mechanisms of tumor defense against immune cell attack. Pathway analysis identified several potential mechanisms of tumor resistance to CAR T therapy which will be exploited to help armor engineered CAR-T cells to control RMS tumors. This data will determine whether our new FGFR4 CARs are sufficient for targeting and eliminating RMS tumors, provide insight into which armoring approaches are best suited to control RMS tumors, and thus develop a potent new therapeutic approach for rhabdomyosarcoma.​

Citation Format: Peter Sullivan, Rajesh Kumar, Wei Li, Lingyang Wang, Yue Zhang, Adam Cheuk, Nityashree Shivaprasad, Javed Khan, Dimiter S. Dimitrov, Rimas J. Orentas. Development of FGFR4-targeted chimeric antigen receptors (CARs) for the treatment of rhabdomyosarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1545.

Abstract 1546: Defining the immune microenvironment in Ewing's sarcoma to potentiate IL1RAP-targeted CAR-T immunotherapy

IL1RAP (Interleukin 1 receptor accessory protein) is differentially expressed in both Ewing sarcoma (EWS) and select hematologic malignancies (CML, AML). Using single domain antibody-like binders generated against IL1RAP to create chimeric antigen receptors (CARs) we demonstrated highly specific cytokine production and cytolytic activity against multiple EWS cell lines in vitro. Preliminary in vivo functionality of IL1RAP CAR-T indicates that additional approaches will be required to increase the efficacy of IL1RAP CAR-T for orthotopic EWS tumor models. To optimize CAR-T cell therapy for EWS, we used model cell lines to define tumor- and stromal-derived factors that must be overcome to obtain effective treatment. The human cell line TC71 (fusion gene positive) was administered both i.v. and i.m. to NSG mice, and subsequently treated with IL1RAP-specifc CAR-T cells. Tumor engraftment was not seen in the i.v. model, however the orthotopic i.m. model in NSG mice was successful, generating tumor and metastatic lesions surrounded by a collagen rich stromal element when CAR-T or control activated T cells were administered. This was not seen in control untreated tumors. Histochemical analysis of i.m. tumors demonstrated that T cells (CD3 positive) were confined to the tumor stromal boundary in proximity to cells of myeloid lineage (CD11b positive). We therefore analyzed cell surface and soluble factors produced by TC71 by flow cytometry, ELISA, and NanoString mRNA panels. TC71 was cultured alone or with interferon-gamma (IFNG), to model the presence of activated T cells and was found to constitutively express transcripts for both canonical and non-canonical chemokines: GPI, MIF, IL-32, TGFβ1; whereas CSF1, CXCL9, CXCL10 and CXCL11 were only induced by IFNG. MIF and TGF-β1 production were confirmed by ELISA. Flow cytometry revealed that numerous CAR-T targets (IL1RAP, CD276, EGFR and FGFR4) were expressed, as were the immune inhibitory molecules: PDL1, PDL2, Galectin-9, and CTLA-4. MHC class I and II and PDL1 were strongly induced by IFNG. We also observed transcripts for collagen related genes in TC71: COL16A1, 3A1, 5A3, LOXL1, P4HB and SERPINH1 both in vitro and in vivo. Our results indicate that tumor surface expression of immune inhibitory signals, PDL-1, PDL-2, CTLA4, as well as the soluble factors MIF, and TGF-b1, must be considered in armoring CAR-T for effective immune destruction of EWS, along with strategies designed to increase the transmigration of T cells through matrix proteins. Moreover, the chemokines produced by tumor alone, as well as those induced by IFNG, indicate a complex network for recruitment of local and bone marrow-derived stromal cells that create a physical as well as cellular barrier to adoptive immunotherapy which needs to be addressed for effective CAR-T therapy of this solid tumor.

Citation Format: Rajesh Kumar, Peter Sullivan, Wei Li, Haifeng Zhang, Virginia Hoglund, Lingyang Wang, Yue Zhang, Poul H. Sorensen, Dimiter S. Dimitrov, Rimas J. Orentas. Defining the immune microenvironment in Ewing's sarcoma to potentiate IL1RAP-targeted CAR-T immunotherapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1546.

Abstract 5001: Barriers to adoptive immunotherapy of pediatric cancer

As the adoptive immunotherapy with chimeric antigen modified T cells (CAR-T) has not proven effective in pediatric solid tumors, we established a model to define which tumor- and stromal-derived factors must be surmounted to obtain effective treatment. The human alveolar rhabdomyosarcoma cell line RH30 (fusion gene positive) was administered both i.v. and i.m. to NSG mice, and subsequently treated with a FGFR4-specifc CAR-T cells (expressing the M410 binder). Although some response was noted in the i.v. model, the i.m. tumor was entirely resistant. We observed that in both primary tumors and liver metastases RH30 was surrounded with a rich stromal element when CAR-T or control activated T cells were administered. Trichrome and H&E histochemical analysis of i.m. tumors confirmed the presence of a rich collagen-containing stromal element in T cell treated mice. This was not present in untreated tumors. We therefore analyzed cell surface and soluble factors produced by RH30 by RNA array analysis, flow cytometry and ELISA in cultured RH30 alone, or treated with interferon-gamma (IFNG) to model the presence of activated T cells. RH30 expressed transcripts for both canonical and non-canonical chemokines: CCL24, GPI, MIF, CCL26, CCL20, CCL2; as well as the cytokines: IL-15, IL-32, LIF, TGFB1, and CSF1. ELISA confirmed high expression of MIF, TGFB1, CCL24 and LIF. Transcripts for the chemokines CCL8, 9, 10, and 11 were expressed at low or undetectable levels, but were strongly induced by IFNG. And although class I MHC was present, and upregulated in the presence of IFNG, so was PDL1. Surface marker analysis by flow cytometry demonstrated that CD80, CD86 and CXCR2 are absent while the CAR-T targets CD276, EGFR, FGFR4 as well as CXCR4 are present. Flow cytometry also demonstrated consistent expression of PDL2, GAL-9, and CTLA4, and strong induction of HLA class I and PDL1 by IFNG treatment of RH30. We are currently evaluating CRISPR knock-out lines of each soluble factor for effects on gene expression, IFNG-induced marker expression, and tumor stroma in vivo. Our results indicate that in this model of rhabdomyosarcoma, tumor surface expression of glycoproteins that give an immune inhibitory signal, PDL1, PDL2, CTLA4, GAL-9, as well as soluble modulators known to be associated with tumor escape from immune control, MIF, TGFB, and LIF, all must be considered in armoring CAR-T for effective immune destruction of tumor. Moreover, the numerous chemokines produced by tumor alone, as well as induced by IFNG, indicate a complex network for recruitment of local and bone marrow-derived stromal cells that create a physical as well as cellular barrier to adoptive immunotherapy.

Citation Format: Rajesh Kumar, Nityashree Shivaprasad, Virginia Hoglund, Cosette LeCeil, Lingyan Wang, Yue Zhang, Dimiter S. Dimitrov, Javed Khan, Rimas J. Orentas. Barriers to adoptive immunotherapy of pediatric cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5001.

Abstract A11: A comprehensive and integrative omic analysis of multiply relapsed refractory pediatric pre-B cell acute lymphoblastic leukemia predicts response to CD19 CAR T-cell therapy

Acute lymphoblastic leukemia (ALL) is the most common childhood cancer with a peak incidence at 3-5 years of age. Despite the improved survival rate of 90% for newly diagnosed children with ALL, the outcome for patients with relapsed disease is poor with a less than 30% overall survival. CD19 CAR T-cell therapy has shown remarkable response rates in relapsed/refractory disease. Long-term survival analysis has shown that initial response rates exceed 80%. However, durable response rates at one year are closer to 40%. Little is known about factors predicting durable response to CAR T therapy. We hypothesize that patients with CD19 CAR T-cell resistant ALL have a distinct disease compared to responders to therapy that can be identified in pretreatment leukemia. Utilizing advanced genomic, epigenetic, proteomic, and single-cell (sc) techniques, we characterized patient bone marrow aspirates (BMA) to identify mechanisms of resistance. Patients enrolled in PLAT-02 at Seattle Children’s Hospital were categorized according to the durability of their response to CD19 CAR T therapy. To characterize the molecular and genomic alterations specific to the therapy-resistant ALLs, we performed comprehensive analyses on pre-treatment therapy-resistant and sensitive BMAs using whole-exome sequencing, RNA- BMAs seq, scRNA-seq, sc B cell receptor (BCR)-seq, methylation array, H3K27ac ChIP-seq, ATAC-seq, and CyTOF. Additionally, we developed murine patient-derived xenografts (PDXs) for future studies. Initial mutation analyses revealed 5 hotspot mutations (ABL1, 2 x KRAS, IKZF1, and EP300) and actionable fusion (2 ABL1, 2 ETV6, 2 ETV5, KMT2A). Interestingly, we identified a KMT2A-AFF1 fusion in a sensitive leukemia, which has been demonstrated to predispose patients to CD19 CAR T resistance through lineage switching. Additionally, we identified a novel CREBBP-fusion in leukemias resistant to CD19 CAR T-induced B-cell aplasia. Alterations of CREBBP have previously been associated with ALL that is refractory to conventional therapies. Integrated gene expression and epigenetic analyses are ongoing to identify genes or pathways associated with resistant disease. scRNA- and scBCR-seq data are being analyzed and integrated with CyTOF analyses to detect mixed lineage and gene expression-based heterogeneity that may predict clonal selection by CAR T pressure. Finally, we developed and genetically analyzed murine PDXs for 64% of the patient samples, establishing a valuable resource for future studies and developing novel therapies for resistant leukemias. This study is one of the most integrative and comprehensive genomic profiling approaches to identify the molecular traits of therapy-resistant ALL in patient samples. We hope to identify and develop crucial biomarkers predicting responsiveness to CAR T-cell therapy.

Citation Format: Katherine E. Masih, Rebecca Gardner, Berkley E. Gryder, Abdalla Abdelmaksoud, Ashley Wilson, Serifat Adebola, Benjamin Z. Stanton, Young K. Song, Justin Lack, Chaoyu Wang, Xinyu Wen, Zachary Rae, Adam Cheuk, Gregoire Altan-Bonnet, Michael Kelly, Jun S. Wei, Michael C. Jensen, Rimas J. Orentas, Javed Khan. A comprehensive and integrative omic analysis of multiply relapsed refractory pediatric pre-B cell acute lymphoblastic leukemia predicts response to CD19 CAR T-cell therapy [abstract]. In: Proceedings of the AACR Special Conference on the Advances in Pediatric Cancer Research; 2019 Sep 17-20; Montreal, QC, Canada. Philadelphia (PA): AACR; Cancer Res 2020;80(14 Suppl):Abstract nr A11.

Persistent Polyfunctional Chimeric Antigen Receptor T Cells That Target Glypican 3 Eliminate Orthotopic Hepatocellular Carcinomas in Mice

BACKGROUND AND AIMS

Glypican 3 (GPC3) is an oncofetal antigen involved in Wnt-dependent cell proliferation that is highly expressed in hepatocellular carcinoma (HCC). We investigated whether the functions of chimeric antigen receptors (CARs) that target GPC3 are affected by their antibody-binding properties.

METHODS

We collected peripheral blood mononuclear cells from healthy donors and patients with HCC and used them to create CAR T cells, based on the humanized YP7 (hYP7) and HN3 antibodies, which have high affinities for the C-lobe and N-lobe of GPC3, respectively. NOD/SCID/IL-2Rgc^null (NSG) mice were given intraperitoneal injections of luciferase-expressing (Luc) Hep3B or HepG2 cells and after xenograft tumors formed, mice were given injections of saline or untransduced T cells (mock control), or CAR (HN3) T cells or CAR (hYP7) T cells. In other NOD/SCID/IL-2Rgc^null (NSG) mice, HepG2-Luc or Hep3B-Luc cells were injected into liver, and after orthotopic tumors formed, mice were given 1 injection of CAR (hYP7) T cells or CD19 CAR T cells (control). We developed droplet digital polymerase chain reaction and genome sequencing methods to analyze persistent CAR T cells in mice.

RESULTS

Injections of CAR (hYP7) T cells eliminated tumors in 66% of mice by week 3, whereas CAR (HN3) T cells did not reduce tumor burden. Mice given CAR (hYP7) T cells remained tumor free after re-challenge with additional Hep3B cells. The CAR T cells induced perforin- and granzyme-mediated apoptosis and reduced levels of active β-catenin in HCC cells. Mice injected with CAR (hYP7) T cells had persistent expansion of T cells and subsets of polyfunctional CAR T cells via antigen-induced selection. These T cells were observed in the tumor microenvironment and spleen for up to 7 weeks after CAR T-cell administration. Integration sites in pre-infusion CAR (HN3) and CAR (hYP7) T cells were randomly distributed, whereas integration into NUPL1 was detected in 3.9% of CAR (hYP7) T cells 5 weeks after injection into tumor-bearing mice and 18.1% of CAR (hYP7) T cells at week 7. There was no common site of integration in CAR (HN3) or CD19 CAR T cells from tumor-bearing mice.

CONCLUSIONS

In mice with xenograft or orthoptic liver tumors, CAR (hYP7) T cells eliminate GPC3-positive HCC cells, possibly by inducing perforin- and granzyme-mediated apoptosis or reducing Wnt signaling in tumor cells. GPC3-targeted CAR T cells might be developed for treatment of patients with HCC.

EGFR806-CAR T cells selectively target a tumor-restricted EGFR epitope in glioblastoma

Targeting solid tumor antigens with chimeric antigen receptor (CAR) T cell therapy requires tumor specificity and tolerance toward variability in antigen expression levels. Given the relative paucity of unique cell surface proteins on tumor cells for CAR targeting, we have focused on identifying tumor-specific epitopes that arise as a consequence of target protein posttranslational modification. We designed a CAR using a mAb806-based binder, which recognizes tumor-specific untethered EGFR. The mAb806 epitope is also exposed in the EGFRvIII variant transcript. By varying spacer domain elements of the CAR, we structurally tuned the CAR to recognize low densities of EGFR representative of non-gene amplified expression levels in solid tumors. The appropriately tuned short-spacer 2nd generation EGFR806-CAR T cells showed efficient in vitro cytokine secretion and glioma cell lysis, which was competitively blocked by a short peptide encompassing the mAb806 binding site. Unlike the nonselective Erbitux-based CAR, EGFR806-CAR T cells did not target primary human fetal brain astrocytes expressing wild-type EGFR, but showed a similar level of activity compared to Erbitux-CAR when the tumor-specific EGFRvIII transcript variant was overexpressed in astrocytes. EGFR806-CAR T cells successfully treated orthotopic U87 glioma implants in NSG mice, with 50% of animals surviving to 90 days. With additional IL-2 support, all tumors were eradicate without recurrence after 90 days. In a novel human induced pluripotent stem cell (iPSC)-derived teratoma xenograft model, EGFR806-CAR T cells infiltrated but were not activated in EGFR+ epidermal cell nests as assessed by Granzyme B expression. These results indicate that EGFR806-CAR T cells effectively and selectively target EGFR-expressing tumor cells.

CD19 CAR T cell product and disease attributes predict leukemia remission durability

BACKGROUND

Chimeric antigen receptor (CAR) T cells can induce remission in highly refractory leukemia and lymphoma subjects, yet the parameters for achieving sustained relapse-free survival are not fully delineated.

METHODS

We analyzed 43 pediatric and young adult subjects participating in a Phase I trial of defined composition CD19CAR T cells (NCT02028455). CAR T cell phenotype, function and expansion, as well as starting material T cell repertoire, were analyzed in relation to therapeutic outcome (defined as achieving complete remission within 63 days) and duration of leukemia free survival and B cell aplasia.

RESULTS

These analyses reveal that initial therapeutic failures (n = 5) were associated with attenuated CAR T cell expansion and/or rapid attrition of functional CAR effector cells following adoptive transfer. The CAR T products were similar in phenotype and function when compared to products resulting in sustained remissions. However, the initial apheresed peripheral blood T cells could be distinguished by an increased frequency of LAG-3+/TNF-αlow CD8 T cells and, following adoptive transfer, the rapid expression of exhaustion markers. For the 38 subjects who achieved an initial sustained MRD-neg remission, remission durability correlated with therapeutic products having increased frequencies of TNF-α-secreting CAR CD8+ T cells, and was dependent on a sufficiently high CD19+ antigen load at time of infusion to trigger CAR T cell proliferation.

CONCLUSION

These parameters have the potential to prospectively identify patients at risk for therapeutic failure and support the development of approaches to boost CAR T cell activation and proliferation in patients with low levels of CD19 antigen.

TRIAL REGISTRATION

ClinicalTrials.gov NCT02028455.

FUNDING

Partial funding for this study was provided by Stand Up to Cancer & St. Baldrick's Pediatric Dream Team Translational Research Grant (SU2C-AACR-DT1113), RO1 CA136551-05, Alex Lemonade Stand Phase I/II Infrastructure Grant, Conquer Cancer Foundation Career Development Award, Washington State Life Sciences Discovery Fund, Ben Towne Foundation, William Lawrence & Blanche Hughes Foundation, and Juno Therapeutics, Inc., a Celgene Company.

A Unique Human Immunoglobulin Heavy Chain Variable Domain-Only CD33 CAR for the Treatment of Acute Myeloid Leukemia

Acute myeloid leukemia (AML) remains a challenging pediatric and adult disease. Given the elevated expression of the CD33 antigen on leukemic blasts, therapeutic approaches to AML now feature the approved antibody drug conjugate (Mylotarg, GO) and investigational CART cell approaches incorporating CD33-binding domains derived from humanized scFvs. We designed a functional chimeric antigen receptor utilizing a human targeting sequence, derived from a heavy chain variable domain, termed CAR33VH. Lentiviral-based expression vectors which encoded CAR constructs incorporating the novel binding domain (CAR33VH), or the My96 scFv control binder (My96CAR) in frame with a CD8 hinge and transmembrane domain, a 4-1BB costimulatory domain and a CD3 zeta activation domain, were transduced into primary human CD4⁺ and CD8⁺ T cells, and CAR expression was confirmed by flow cytometry. CAR33VH, similar to My96CAR, demonstrated robust and specific cytotoxicity in short-term and long-term co-incubation killing assays against CD33⁺ AML lines. In overnight cytokine release assays in which CAR T cells were challenged with the CD33⁺ tumor cells HL-60, MOLM-14 and KG-1a, CAR33VH elicited IFN-gamma, TNF-alpha and IL-2. This was seen with CD33⁺ cell lines, but not when CAR T were cultured alone. Studies with a CD33⁻ cell line engineered to stably express the full length CD33 variant 1, or the naturally occurring CD33 splice variant 2, revealed that both CAR33VH and My96CAR, target the V domain of CD33, suggesting a similar therapeutic profile. Colony-formation assays utilizing peripheral blood CD34⁺ hematopoietic stem cells treated with CAR33VH, My96CAR, or with an untransduced T cell control, yielded similar numbers of BFU-E erythroid and CFU-GM myeloid colonies, suggesting a lack of CAR-related overt toxicity. In an in vivo AML model, NSG mice engrafted with MOLM-14 cells stably expressing firefly luciferase, both CAR33VH and CARMy96 efficiently eliminated tumors. In conclusion, we demonstrate for the first time the feasibility and efficacy of employing human variable domain-only binder derived from a phage display library in an anti-AML CAR design. CAR33VH, comprised of a human heavy-chain variable fragment-only antigen binding domain, was efficient in tumor killing in vitro and in vivo, and showed comparable functionality to the scFv-based My96CAR.

Abstract 2561: Fully human immunoglobulin heavy chain only-derived CD33 CAR for the treatment of acute myeloid leukemia

CD33 antigen is a promising target expressed on non-solid cancers, including acute myeloid leukemia (AML). Present investigative approaches to treatment of CD33-positive AML include antibody drug conjugates (My96, Mylotarg®) and CART cells incorporating CD33-targeting domains derived from a humanized scFv. Here, we designed a chimeric antigen receptor utilizing targeting domain derived from a fully human CD33 fragment variable heavy chain sequence, termed CAR33VH, and examined its in vitro and in vivo potency against AML.

Primary human CD4+ CD8+ T cells derived from three healthy donors were transduced with lentiviral constructs (LV) encoding CAR33VH, or control CAR construct based on scFv My96 (My96CAR). Flow cytometric analysis revealed expression of CAR33VH at 32%-45%, and expression of My96CAR at 77% - 86%. When challenged with CD33+ AML tumor lines HL60 and MOLM-14 in vitro, both constructs demonstrated efficient target killing. CD33- tumor lines K562 and Reh were not sensitive to CAR killing, underscoring CAR specificity to CD33 antigen. Pro-inflammatory cytokines IFN gamma, TNF alpha and IL-2 in culture supernatants of CART cells incubated with CD33+ HL-60 and MOLM-14 tumors, but not with CD33- K562 cells overnight, were induced, as measured by ELISA. Long-term co-incubation assay of CART cells with HL-60 leukemia at E:T ratios 1:5 to 1: 0.04, suggested similar killing potency and persistence of CAR33VH to the positive control scFv-based CAR. In in vivo AML model, NSG mice engrafted with MOLM-14 cells stably expressing firefly luciferase, both CAR33VH, and My96CAR control were equally efficient in tumor elimination.

In conclusion, CAR33VH, comprised of a fully human heavy-chain variable fragment only antigen binding domain, was efficient in tumor killing in vitro and in vivo, and may be used clinically for treatment of CD33+ hematologic malignancies. To our knowledge, this is one of the first instances demonstrating the feasibility of employing heavy chain only binder sequence in CART design.

Citation Format: Dina Schneider, Ying Xiong, Weizao Chen, Zhongyu Zhu, Darong Wu, Jennifer Hwang, Dimiter S. Dimitrov, Boro Dropulic, Rimas J. Orentas. Fully human immunoglobulin heavy chain only-derived CD33 CAR for the treatment of acute myeloid leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2561.

Paired Expression Analysis of Tumor Cell Surface Antigens

Adoptive immunotherapy with antibody-based therapy or with T cells transduced to express chimeric antigen receptors (CARs) is useful to the extent that the cell surface membrane protein being targeted is not expressed on normal tissues. The most successful CAR-based (anti-CD19) or antibody-based therapy (anti-CD20) in hematologic malignancies has the side effect of eliminating the normal B cell compartment. Targeting solid tumors may not provide a similar expendable marker. Beyond antibody to Her2/NEU and EGFR, very few antibody-based and no CAR-based therapies have seen broad clinical application for solid tumors. To expand the way in which the surfaceome of solid tumors can be analyzed, we created an algorithm that defines the pairwise relative overexpression of surface antigens. This enables the development of specific immunotherapies that require the expression of two discrete antigens on the surface of the tumor target. This dyad analysis was facilitated by employing the Hotelling’s T-squared test (Hotelling–Lawley multivariate analysis of variance) for two independent variables in comparison to a third constant entity (i.e., gene expression levels in normal tissues). We also present a unique consensus scoring mechanism for identifying transcripts that encode cell surface proteins. The unique application of our bioinformatics processing pipeline and statistical tools allowed us to compare the expression of two membrane protein targets as a pair, and to propose a new strategy based on implementing immunotherapies that require both antigens to be expressed on the tumor cell surface to trigger therapeutic effector mechanisms. Specifically, we found that, for MYCN amplified neuroblastoma, pairwise expression of ACVR2B or anaplastic lymphoma kinase (ALK) with GFRA3, GFRA2, Cadherin 24, or with one another provided the strongest hits. For MYCN, non-amplified stage 4 neuroblastoma, neurotrophic tyrosine kinase 1, or ALK paired with GFRA2, GFRA3, SSK1, GPR173, or with one another provided the most promising paired-hits. We propose that targeting these markers together would increase the specificity and thereby the safety of CAR-based therapy for neuroblastoma.

Tumor Antigen and Receptor Densities Regulate Efficacy of a Chimeric Antigen Receptor Targeting Anaplastic Lymphoma Kinase

We explored the utility of targeting anaplastic lymphoma kinase (ALK), a cell surface receptor overexpressed on pediatric solid tumors, using chimeric antigen receptor (CAR)-based immunotherapy. T cells expressing a CAR incorporating the single-chain variable fragment sequence of the ALK48 mAb linked to a 4-1BB-CD3ζ signaling domain lysed ALK-expressing tumor lines and produced interferon-gamma upon antigen stimulation but had limited anti-tumor efficacy in two xenograft models of human neuroblastoma. Further exploration demonstrated that cytokine production was highly dependent upon ALK target density and that target density of ALK on neuroblastoma cell lines was insufficient for maximal activation of CAR T cells. In addition, ALK CAR T cells demonstrated rapid and complete antigen-induced loss of receptor from the T cell surface via internalization. Using a model that simultaneously modulated antigen density and CAR expression, we demonstrated that CAR functionality is regulated by target antigen and CAR density and that low expression of either contributes to limited anti-tumor efficacy of the ALK CAR. These data suggest that stoichiometric relationships between CAR receptors and target antigens may significantly impact the anti-tumor efficacy of CAR T cells and that manipulation of these parameters could allow precise tuning of CAR T cell activity.

A tandem CD19/CD20 CAR lentiviral vector drives on-target and off-target antigen modulation in leukemia cell lines

Background

Clinical success with chimeric antigen receptor (CAR)- based immunotherapy for leukemia has been accompanied by the associated finding that antigen-escape variants of the disease are responsible for relapse. To target hematologic malignancies with a chimeric antigen receptor (CAR) that targets two antigens with a single vector, and thus potentially lessen the chance of leukemic escape mutations, a tandem-CAR approach was investigated.

Methods

Antigen binding domains from the FMC63 (anti-CD19) and Leu16 (anti-CD20) antibodies were linked in differing configurations to transmembrane and T cell signaling domains to create tandem-CARs. Expression on the surface of primary human T cells was induced by transduction with a single lentiviral vector (LV) encoding the tandem-CAR. Tandem-CARs were compared to single antigen targeting CARs in vitro and in vivo, and to an admixture of transduced cells expressing each CAR in vivo in immunodeficient (NSG) disease-bearing mice.

Results

Tandem constructs efficient killed the Raji leukemia cell line both in vitro and in vivo. Tandem CARs generated less cytokine than the CD20 CAR, but similar to CD19 CARs, on their own. In co-culture experiments at low effector to target ratios with both single- and tandem- CAR-T cells, a rapid down-modulation of full-length CD19 expression was seen on leukemia targets. There also was a partial down-modulation of CD22, and to a lesser degree, of CD20. Our data also highlight the extreme sensitivity of the NALM-6 cell line to general lymphocyte-mediated cytotoxicity. While single and tandem constructs were effective in vivo in a standard setting, in a high-disease burden setting, the tandem CAR proved both effective and less toxic than an admixture of transduced T cell populations expressing single CARs.

Conclusion

Tandem CARs are equally effective in standard disease models to single antigen specificity CARs, and may be both more effective and less toxic in a higher disease burden setting. This may be due to optimized cell killing with more moderate cytokine production. The rapid co-modulation of CD19, CD20, and CD22 may account for the ability to rapidly evolve escape mutants by selecting for leukemic clones that not require these target antigens for continued expansion.

Electronic supplementary material

The online version of this article (doi:10.1186/s40425-017-0246-1) contains supplementary material, which is available to authorized users.

Reduction of MDSCs with All-trans Retinoic Acid Improves CAR Therapy Efficacy for Sarcomas

Genetically engineered T cells expressing CD19-specific chimeric antigen receptors (CAR) have shown impressive activity against B-cell malignancies, and preliminary results suggest that T cells expressing a first-generation disialoganglioside (GD2)-specific CAR can also provide clinical benefit in patients with neuroblastoma. We sought to assess the potential of GD2-CAR therapies to treat pediatric sarcomas. We observed that 18 of 18 (100%) of osteosarcomas, 2 of 15 (13%) of rhabdomyosarcomas, and 7 of 35 (20%) of Ewing sarcomas expressed GD2. T cells engineered to express a third-generation GD2-CAR incorporating the 14g2a-scFv with the CD28, OX40, and CD3ζ signaling domains (14g2a.CD28.OX40.ζ) mediated efficient and comparable lysis of both GD2⁺ sarcoma and neuroblastoma cell lines in vitro However, in xenograft models, GD2-CAR T cells had no antitumor effect against GD2⁺ sarcoma, despite effectively controlling GD2⁺ neuroblastoma. We observed that pediatric sarcoma xenografts, but not neuroblastoma xenografts, induced large populations of monocytic and granulocytic murine myeloid-derived suppressor cells (MDSC) that inhibited human CAR T-cell responses in vitro Treatment of sarcoma-bearing mice with all-trans retinoic acid (ATRA) largely eradicated monocytic MDSCs and diminished the suppressive capacity of granulocytic MDSCs. Combined therapy using GD2-CAR T cells plus ATRA significantly improved antitumor efficacy against sarcoma xenografts. We conclude that retinoids provide a clinically accessible class of agents capable of diminishing the suppressive effects of MDSCs, and that co-administration of retinoids may enhance the efficacy of CAR therapies targeting solid tumors. Cancer Immunol Res; 4(10); 869-80. ©2016 AACR.

Abstract 2648: Chimeric antigen receptor T-cell therapy against anaplastic lymphoma kinase (ALK) is limited by target antigen density and CAR surface expression

ALK is overexpressed on the surface of neuroblastoma (NB) and is associated with high risk disease. We developed a second generation CAR based on a monoclonal antibody against ALK. ALK CAR T-cells significantly delay the growth of human NB cell lines in murine xenograft models, but animals eventually succumb to tumors.

In order to understand how target antigen density limits the effectiveness of this CAR, we created a library of NALM-6 B-cell leukemias with variable amounts of ALK expressed on the surface of each clone. In vitro, ALK CAR T-cells lyse high ALK expressing leukemias but have reduced activity against leukemias with low expression of ALK. In co-culture assays, we found that there is a minimum target antigen density required for CAR T-cells to produce appreciable amounts of Th1 cytokines. This threshold of ALK expression is above the expression on human NB lines. There is also a threshold target antigen density at which maximum cytokine production occurs. Above this density, no additional cytokines are produced. In xenograft models, ALK CAR T-Cells are significantly more effective against high ALK expressing leukemia than against low ALK expressing leukemia. To our knowledge, this is the first report of greater in vivo efficacy of CAR T-cells against tumors with higher antigen expression.

The level of surface expression of the CAR on T-cells also alters the function of CAR T-cells. We have identified a phenomenon in which both ALK and CD19 CAR T-cells lose surface expression of CAR over time in culture. Additionally, both the ALK CAR and the CD19 CAR downregulate quickly after T-cells are exposed to antigen. Using a fluorescently tagged CD19 CAR, we demonstrate that CARs are rapidly internalized upon antigen encounter.

We created an assay to understand the interplay of target density and CAR surface expression. We transduced T-cells with different amounts of supernatant to achieve different ALK CAR surface densities and then exposed these T-cells to varying amounts of immobilized protein-L in order to crosslink the CAR. T-cell activation (measured by an NFAT reporter) is greatest when both CAR surface expression and target antigen density are highest. As both variables are decreased, there is a significant loss of T-cell activity. Thus, diminished CAR surface density due to antigen-dependent and independent CAR downmodulation may limit CAR T-cell efficacy, especially in the context of low tumor antigen expression.

In conclusion, we have created a novel CAR targeting ALK that demonstrates in vivo efficacy against NB. Efficacy is limited by low target antigen density on NB and low CAR surface expression. We have identified phenomena in which CAR T-cells lose surface expression of CAR over time and also quickly internalize their receptors in response to antigen. These factors contribute to the efficacy of other CAR T-cells and this data provides important insights into future CAR target selection.

Citation Format: Robbie G. Majzner, Alec J. Walker, Meera Murgai, Ling Zhang, Adrienne H. Long, Kelsey M. Wanhainen, Rimas J. Orentas, Crystal L. Mackall. Chimeric antigen receptor T-cell therapy against anaplastic lymphoma kinase (ALK) is limited by target antigen density and CAR surface expression. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2648.

Abstract 4702: 4-1BB costimulation ameliorates exhaustion and prolongs in vivo persistence of chimeric antigen receptor (CAR) expressing T cells

Published reports have demonstrated high response rates of CD19 chimeric antigen receptor (CAR) modified T cells against acute lymphoblastic leukemia. CARs incorporating either CD28 or 4-1BB costimulatory domains mediated significant anti-leukemic effects. However CD19-41BBz CAR T cells could be detected for months and induced prolonged B cell depletion, whereas most patients treated with CD19-28z CAR T cells had no detectable CAR+ T cells by day 28, and B cell recovery was routinely observed. We report that CAR T cells incorporating the CD28 versus 4-1BB costimulatory domains show differential susceptibilities to T cell exhaustion. Using the tonically signaling 14g2a-based GD2 CAR as a model system, we identify that GD2-28z CARs rapidly acquire an exhaustion phenotype, characterized by high level expression of PD1, TIM3 and LAG3, high expression of exhaustion related transcription factors Blimp1 and T-bet, poor proliferative capacity, poor cytokine production, and poor anti-tumor efficacy in vivo. Interestingly, second generation GD2-28z CARs induce greater exhaustion than first generation GD2-z CARs, implicating CD28 costimulation in the induction of exhaustion. In contrast, GD2-41BBz CARs express lower levels of PD1 and TIM3, show increased cytokine production, improved persistence in vivo, and improved anti-tumor efficacy against both osteosarcoma and neuroblastoma xenografts. Similarly, CD19-41BBz CARs have improved persistence and lower expression of exhaustion markers compared to CD19-28z CARs following leukemia challenge. Finally, CD22-BBz CARs show lower exhaustion marker expression, improved persistence, and improved anti-leukemia efficacy in immunodeficient mice compared to CD22-28z CARs. Gene expression experiments are underway to further delineate the mechanism by which 4-1BB signaling ameliorates exhaustion in the setting of chronic CAR signaling. Together, this data demonstrates that exhaustion is a critical factor limiting the efficacy of CAR expressing T cells. CD28 and 4-1BB have opposing effects on exhaustion in CAR T cells, with CD28 signaling augmenting the phenotype while 4-1BB signaling mitigating exhaustion. This data is the first to demonstrate differential effects of costimulatory pathways on T cell exhaustion, likely explains differential persistence observed in the clinical trials of CAR therapeutics, and provides important insights for CAR design for future clinical applications.

Citation Format: Adrienne H. Long, Waleed M. Haso, Jillian P. Smith, Alec J. Walker, Terry J. Fry, Rimas J. Orentas, Crystal L. Mackall. 4-1BB costimulation ameliorates exhaustion and prolongs in vivo persistence of chimeric antigen receptor (CAR) expressing T cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4702. doi:10.1158/1538-7445.AM2015-4702

Abstract 3140: Antigen-induced downmodulation is associated with diminished efficacy of a novel chimeric antigen receptor targeting anaplastic lymphoma kinase

CD19 directed Chimeric Antigen Receptor (CAR) T cell therapies have shown clinical responses against B cell malignancies. However, CARs targeting other tumor-associated antigens have been less successful. Factors limiting CAR efficacy are not well understood. Anaplastic lymphoma kinase (ALK) is a cell-surface protein overexpressed in a large fraction of neuroblastomas, the most common extracranial solid tumor of childhood. We constructed MSGV1.ALK.BBZ retroviral vectors using single-chain variable fragment (scFv) sequences from murine monoclonal antibodies targeting ALK and transduced these into human PBMCs. ALK-CAR T cells lysed ALK+ tumor lines, but produced limited amounts of IFN-g and IL-2 upon co-culture with ALK+ tumor cells compared to CD19-CARs co-cultured with CD19+ targets. In order to identify potential differences between ALK-CAR and CD19-CAR induced signaling we employed a reporter system in which GFP expression was directed by an NFAT-responsive promoter and could be visualized by flow cytometry. T cells were co-transduced with lentiviral NFAT-GFP and MSGV1.ALK.BBZ or MSGV1.CD19.BBZ. Transduced T cells were co-cultured with tumor lines expressing ALK or CD19 antigen. GFP expression was induced within 4h of antigen exposure in double transduced T cells and persisted for at least 24h. The magnitude of GFP expression correlated positively with antigen density on tumor targets. Additionally, >50% of GFP+ cells retained CD19-CAR surface expression after antigen encounter. However, we observed drastically reduced surface ALK-CAR expression on GFP+ T cells after encounter with tumor targets. Upon further examination, we noticed that ALK-CARs were down-modulated from the T cell surface within 1h of exposure to antigen, and remained internalized for at least 24h. The magnitude of NFAT translocation in ALK-CAR T cells, as measured by GFP intensity, was also markedly lower than the magnitude of NFAT translocation in CD19-CAR T cells. These results suggest that limited numbers of CARs remaining on the T cell surface after initial antigen encounter may contribute to diminished ALK-CAR T cell efficacy, possibly by preventing temporal summation of repeated CAR signaling. Work is underway to prevent CAR down-modulation after antigen encounter, and to evaluate the impact of increased surface CAR retention on CAR functionality. In summary, optimal design of new CAR therapeutics requires a better understanding of essential factors limiting CAR efficacy. Through evaluation of a novel CAR targeting ALK, we have identified CAR down-modulation as one factor that may influence CAR-T efficacy, and may be amenable to modulation in order to improve CAR functionality.

Citation Format: Alec J. Walker, Ling Zhang, Adrienne H. Long, Rimas J. Orentas, Crystal L. Mackall. Antigen-induced downmodulation is associated with diminished efficacy of a novel chimeric antigen receptor targeting anaplastic lymphoma kinase. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3140. doi:10.1158/1538-7445.AM2015-3140

4-1BB costimulation ameliorates T cell exhaustion induced by tonic signaling of chimeric antigen receptors

Chimeric antigen receptors (CARs) targeting CD19 have mediated dramatic antitumor responses in hematologic malignancies, but tumor regression has rarely occurred using CARs targeting other antigens. It remains unknown whether the impressive effects of CD19 CARs relate to greater susceptibility of hematologic malignancies to CAR therapies, or superior functionality of the CD19 CAR itself. We show that tonic CAR CD3-ζ phosphorylation, triggered by antigen-independent clustering of CAR single-chain variable fragments, can induce early exhaustion of CAR T cells that limits antitumor efficacy. Such activation is present to varying degrees in all CARs studied, except the highly effective CD19 CAR. We further determine that CD28 costimulation augments, whereas 4-1BB costimulation reduces, exhaustion induced by persistent CAR signaling. Our results provide biological explanations for the antitumor effects of CD19 CARs and for the observations that CD19 CAR T cells incorporating the 4-1BB costimulatory domain are more persistent than those incorporating CD28 in clinical trials.

Emerging Immunotherapies for Cancer and Their Potential for Application in Pediatric Oncology

After decades of basic research, immune-based therapeutics for the treatment of cancer are showing evidence of efficacy in clinical trials; several immunotherapeutics already incorporated into standard treatment regimens. Intensive research is underway to improve the efficacy of immunotherapeutics and to expand the application of immunotherapy to a wider array of cancers. The therapeutic options that comprise immunotherapy for cancer are vast and span monoclonal antibodies, tumor vaccines, adoptive cellular therapies, as well as therapies aimed at reversing immunosuppression and enhancing immune reactivity globally and/or locally within the tumor microenvironment. In pediatric cancer, monoclonal antibodies have demonstrated efficacy in hematologic malignancies, and neuroblastoma and bispecific antibodies that activate resident T cells, as well as adoptive cell therapy, have shown recent exciting results for the treatment of acute lymphoblastic leukemia in childhood. This review discusses the basic principles of tumor immunology driving clinical development of new immunotherapies, describes immunotherapeutics with demonstrated efficacy and several currently in clinical trials, and highlight agents that seem to be most promising for the treatment of pediatric cancer.

Lessons learned from a highly-active CD22-specific chimeric antigen receptor

CD22 is an attractive target for the development of immunotherapeutic approaches for the therapy of B-cell malignancies. In particular, an m971 antibody-derived, second generation chimeric antigen receptor (CAR) that targets CD22 holds significant therapeutic promise. The key aspect for the development of such a highly-active CAR was its ability to target a membrane-proximal epitope of CD22.

Abstract 2943: 14g2a-based GD2-specific chimeric antigen receptors (CARs) constitutively signal, leading to rapidly induced T-cell exhaustion and poor antitumor efficacy in vivo

Chimeric antigen receptors (CARs) provide a promising new approach for generating T cells for the adoptive immunotherapy of cancer. In both pre-clinical and clinical studies, we have observed potent anti-tumor effects using CD19 CAR T cells (CART) to treat B cell malignancies. However, we observe less success when targeting solid tumors with GD2 CART. We hypothesized that the observed differences could be due to a more hostile microenvironment in solid tumors, and/or variable CAR potency. To address these issues, we created a CD19+GD2+ osteosarcoma (143B-CD19). This double positive solid tumor line allowed us to directly compare the in vivo efficacy of the GD2 and CD19 CARs, normalized for tumor type. In vitro 51Cr release demonstrated that CD19 and GD2 CART were equally active against 143B-CD19. However, in vivo models showed a significant difference in anti-tumor efficacy. NSG mice were injected with 143B-CD19 and treated with control, GD2 or CD19 CART 14d later. GD2 CART failed to persist in mice and did not inhibit tumor outgrowth. However, CD19 CART did persist and cleared the mice of all CD19+ solid tumor disease. Therefore, we conclude that the solid tumor microenvironment is not a barrier to effective CART therapies, and hypothesize that poor GD2 CAR potency leads to poor in vivo efficacy.

Further comparison of GD2 and CD19 CART demonstrated that excessive, rather than insufficient, signaling by the GD2 CAR contributes to the poor in vivo efficacy. GD2 CART have increased expression of activation makers (CD25, CD127, CD137) and increased size compared to CD19 CART following in vitro expansion. Despite this increased activation, GD2 CART expand poorly, have high rates of apoptosis, produce IL2, TNFa and IFNg poorly, and express high levels of PD1, TIM3, LAG3, and BLIMP1. This phenotype is consistent with T cell exhaustion, a state of poor effector function due to chronic antigen stimulation. This phenotype is prevented by knocking out GD2 CAR signaling through point mutations in the CD3 zeta and CD28 domains of the receptor, implicating tonic signaling through the CAR during in vitro expansion as the source of T cell exhaustion. Interestingly, we did not identify GD2 expression in the culture system. Point mutations in the CAR antigen-binding site, though blocking GD2 binding, did not prevent exhaustion. Thus, we postulate that constitutive receptor signaling may occur via interactions between the framework regions of the CAR, leading to exhaustion. This exhausted phenotype was recapitulated with CD19 CART when expanded in the presence of a crosslinking anti-idiotype antibody, suggesting that over-activation via other CARs also has the potential to limit efficacy. Ongoing experiments focus on elucidating the molecular pathways that link CAR signaling and induction of T cell exhaustion, as well as identifying the structural features of the GD2 CAR that lead to excessive signaling.

Citation Format: Adrienne Long, Rimas J. Orentas, Crystal L. Mackall. 14g2a-based GD2-specific chimeric antigen receptors (CARs) constitutively signal, leading to rapidly induced T-cell exhaustion and poor antitumor efficacy in vivo. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2943. doi:10.1158/1538-7445.AM2014-2943

Abstract 652: Development and characterization of anti-FGFR4 monoclonal antibodies as therapeutic agents for human rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma of childhood with two major subtypes embryonal (ERMS) and alveolar (ARMS), and current treatment modalities have yielded event free 5-year survival in only 30% of the patients with high-risk disease. Therefore, there is an absolute need for novel strategies and to identify and validate clinically relevant targets for the treatment of RMS. The fibroblast growth factor receptor 4 (FGFR4) is a very attractive therapeutic target because: 1) the FGFR4 gene is over expressed in RMS, 2) it is crucial for survival, proliferation, metastasis and drug resistance, 3) activating mutations in the kinase domain lead to aggressive growth and poor survival in patients with alveolar RMS and 4) genetic or pharmacologic inhibition of FGFR4 signaling inhibited tumor growth in vitro and in vivo. Monoclonal antibodies (mAbs) against specific antigens expressed on cancer cell surface have gained importance as potential therapeutic agents that may be used either alone or in combination with chemotherapeutic drugs. We have developed several mAbs against human FGFR4 protein from rabbit and mouse (by hybridoma technology), and from human immunoglobulin libraries (by recombinant DNA technology). In the present study, we report the development and characterization of some of these mAbs. Biacore analysis of these antibodies showed low nM affinity to purified extracellular domain of FGFR4 (FGFR4-ECD). The ability of these mAbs to bind the native molecule was demonstrated by specific binding to RMS cell lines of both subtypes, and dose response curves exhibited higher binding in ARMS cells than ERMS cells. More importantly, significant binding was noticed in freshly isolated tumor cells from a breast metastatic nodule of a patient with ARMS. The anti-FGFR4 mAbs also bound to transfected cell line expressing FGFR4 on the surface, but not the vector control (FGFR4 negative) cell line indicating the specificity of the reaction. Furthermore, cell surface FGFR4 can mediate internalization of the bound antibody upon incubation at 37C for as little as 30 min and maximum internalization was observed at 2 h. Receptor mediated internalization of the bound mAb was inhibited by a chemical inhibitor, and ARMS cells showed more internalization than ERMS. Together, these observations support the contention that anti- FGFR4 mAb can be used as a vehicle to deliver a cytotoxic payload in the form of small molecule drugs and toxins. Ongoing investigations are aimed at developing anti-FGFR4 mAbs and their derivatives as potential therapeutic agents for the treatment of patients with RMS.

This study is supported in part by the intramural research program of the National Cancer Institute, National Institutes of Health, grants from St. Baldrick's Foundation and Stand Up To Cancer - St. Baldrick's Pediatric Dream Team Translational Cancer Research Grant, Grant Number SU2C-AACR-DT11-13.

Citation Format: Sivasubramanian Baskar, Zhongyu Zhu, Ramon Lorenzo Labitigan, Michelle Ovanesian, Rimas J. Orentas, Samuel Q. Li, Yohe E. Marielle, John Shern, Dimiter S. Dimitrov, John Maris, Crystal Mackall, Khan Javed. Development and characterization of anti-FGFR4 monoclonal antibodies as therapeutic agents for human rhabdomyosarcoma. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 652. doi:10.1158/1538-7445.AM2014-652

Abstract 2799: Adoptive immunotherapy for pediatric solid tumors with CAR-T cells (chimeric antigen receptor bearing T cells) targeting ALK (anaplastic lymphoma kinase, CD246)

Neuroblastoma is the most common non-CNS tumor in children. High-risk disease has a dire prognosis and new treatment strategies are need. The identification of unique cell-surface proteins expressed on tumor cells, yet not expressed on normal tissues, has been challenging for pediatric malignancies. However, identification of such antigens allows for the development of new immune-based therapies featuring Ig-like binding motifs. The cell surface tyrosine kinase ALK (CD246, anaplastic lymphoma kinase) is a promising target for neuroblastoma in that it is expressed in either native, mutated, or over-expressed forms on the plasma membrane surface of the tumor cell and may contribute to disease progression or severity. We identified antibodies that bind to extracellular domains of ALK, sequenced their variable regions, and used this sequence information to construct chimeric antigen receptors (CARs). Primary human T lymphocytes were then transduced with retroviral gene vectors expressing a series of ALK-specific CAR. We tested CARs that incorporated both different structural and signaling motifs. Transduced T cells demonstrated ALK-specific cytolytic activity against ALK-expressing tumors and produced Th1 cytokines upon culture in the presence of tumor. In exploring different iterations of CAR domain structure we found that the scFv domains created from the heavy and light variable domains of ALK-specific immunoglobulin could be interchanged with respect to their orientation in the context of CAR tertiary protein structure. Moreover, ALK-specific scFv functioned whether expressed in a short format, that is as a single domain proximal to the T cell membrane, or in a long format, that is extended away from the plasma membrane using an IgG1-derived spacer domain composed of CH2 and CH3. Using a xenogeneic NSG mouse model for neuroblastoma, human ALK-specific CAR-expressing T cells were found to eradicate ALK-positive tumor, when IL-7 was included to support T cell persistence. These data argue for the continued evaluation of ALK-specific CARs in pre-clinical studies.

Citation Format: Alec Walker, Paola Lopomo, William Babbitt, Marc Vigny, Crystal L. Mackall, Rimas J. Orentas. Adoptive immunotherapy for pediatric solid tumors with CAR-T cells (chimeric antigen receptor bearing T cells) targeting ALK (anaplastic lymphoma kinase, CD246). [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2799. doi:10.1158/1538-7445.AM2014-2799

Bioinformatic description of immunotherapy targets for pediatric T-cell leukemia and the impact of normal gene sets used for comparison

Pediatric lymphoid leukemia has the highest cure rate of all pediatric malignancies, yet due to its prevalence, still accounts for the majority of childhood cancer deaths and requires long-term highly toxic therapy. The ability to target B-cell ALL with immunoglobulin-like binders, whether anti-CD22 antibody or anti-CD19 CAR-Ts, has impacted treatment options for some patients. The development of new ways to target B-cell antigens continues at rapid pace. T-cell ALL accounts for up to 20% of childhood leukemia but has yet to see a set of high-value immunotherapeutic targets identified. To find new targets for T-ALL immunotherapy, we employed a bioinformatic comparison to broad normal tissue arrays, hematopoietic stem cells (HSC), and mature lymphocytes, then filtered the results for transcripts encoding plasma membrane proteins. T-ALL bears a core T-cell signature and transcripts encoding TCR/CD3 components and canonical markers of T-cell development predominate, especially when comparison was made to normal tissue or HSC. However, when comparison to mature lymphocytes was also undertaken, we identified two antigens that may drive, or be associated with leukemogenesis; TALLA-1 and hedgehog interacting protein. In addition, TCR subfamilies, CD1, activation and adhesion markers, membrane-organizing molecules, and receptors linked to metabolism and inflammation were also identified. Of these, only CD52, CD37, and CD98 are currently being targeted clinically. This work provides a set of targets to be considered for future development of immunotherapies for T-ALL.

A transplantable TH-MYCN transgenic tumor model in C57Bl/6 mice for preclinical immunological studies in neuroblastoma

Current multimodal treatments for patients with neuroblastoma (NBL), including anti-disialoganglioside (GD2) monoclonal antibody (mAb) based immunotherapy, result in a favorable outcome in around only half of the patients with advanced disease. To improve this, novel immunocombinational strategies need to be developed and tested in autologous preclinical NBL models. A genetically well-explored autologous mouse model for NBL is the TH-MYCN model. However, the immunobiology of the TH-MYCN model remains largely unexplored. We developed a mouse model using a transplantable TH-MYCN cell line in syngeneic C57Bl/6 mice and characterized the immunobiology of this model. In this report, we show the relevance and opportunities of this model to study immunotherapy for human NBL. Similar to human NBL cells, syngeneic TH-MYCN-derived 9464D cells endogenously express the tumor antigen GD2 and low levels of MHC Class I. The presence of the adaptive immune system had little or no influence on tumor growth, showing the low immunogenicity of the NBL cells. In contrast, depletion of NK1.1+ cells resulted in enhanced tumor outgrowth in both wild-type and Rag1(-/-) mice, showing an important role for NK cells in the natural anti-NBL immune response. Analysis of the tumor infiltrating leukocytes ex vivo revealed the presence of both tumor associated myeloid cells and T regulatory cells, thus mimicking human NBL tumors. Finally, anti-GD2 mAb mediated NBL therapy resulted in ADCC in vitro and delayed tumor outgrowth in vivo. We conclude that the transplantable TH-MYCN model represents a relevant model for the development of novel immunocombinatorial approaches for NBL patients.

Abstract 3974: Evaluating the susceptibility of solid tumors to chimeric antigen receptor modified T cell therapies

Adoptive cell therapy with tumor infiltrating lymphocytes (TIL) has been successfully used as a treatment in metastatic melanoma. However, TIL are not present in the majority of solid cancers. To overcome this barrier, T cells can be transduced with chimeric antigen receptors (CARs) to produce tumor specific T cells for adoptive immunotherapy. CD19 CAR T cell therapies have demonstrated activity against B cell malignancies in both pre-clinical and clinical studies, but CARs targeting solid tumors have been less active in clinical trials and we observed less success when targeting solid tumors with GD2 CAR therapies in preclinical models. We hypothesize that the observed difference in anti-tumor efficacy may be due to 1) a more hostile microenvironment within solid tumors compared to hematologic malignancies, and/or 2) variable CAR potency. To normalize for differences in CAR potency we created a CD19 expressing osteosarcoma cell line, thus allowing use of the well-characterized CD19 CAR to explore the susceptibility of solid malignancies to adoptive immunotherapy.

The GD2 expressing 143B osteosarcoma cell line was transfected to express CD19 (143-CD19) and susceptibility to GD2 and CD19 CAR therapies was compared. 51Cr release experiments demonstrated that both GD2 and CD19 CAR T cells were similarly cytolytically active against 143B and 143B-CD19 cells, respectively. The in vivo susceptibility was then compared using two xenograft models. NSG mice were injected IM with 5e5 143B or 143B-CD19. The first was an early treatment model, where 3 days following tumor inoculation, mice received 5e6 GD2 CAR, CD19 CAR or mock T cells IV. In mice treated with GD2 CAR T cells, no difference in tumor growth or survival was observed. However, mice inoculated with 143B-CD19 and treated with CD19 CAR T cells showed improved survival (p<0.01) and complete eradication of their CD19+ disease. CD19 CAR T cells were next tested in a late treatment model: NSG mice were injected with 1e6 143B-CD19. On day 19, mice were treated IV with 1e7 CD19 CAR or mock T cells. By day 28, tumors of CD19 CAR treated mice had shrunk to 50% the size of day 19 tumors, while mock treated mice had progressed to 2 cm x 2 cm (p<0.01). Thus, the CD19 CAR induced impressive regression of established >1 cm x 1 cm 143B-CD19 tumors. We further observed that human solid tumors expand murine myeloid derived suppressor cells in this system, which bear a hallmark phenotype and suppress human T cells. We conclude that despite the presence of an immunosuppressive solid tumor microenvironment, appropriate choice of antigen and CAR design can lead to regression in established solid tumors. Our ongoing work focuses on using this platform to more clearly define the CAR T cell characteristics that correlate to anti-solid tumor efficacy in vivo, as well as explore how modulating the immunosuppressive solid tumor microenvironment can enhance efficacy of otherwise ineffective CAR treatments.

Citation Format: Adrienne H. Long, Steven L. Highfill, Waleed M. Haso, Rimas J. Orentas, Crystal L. Mackall. Evaluating the susceptibility of solid tumors to chimeric antigen receptor modified T cell therapies. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3974. doi:10.1158/1538-7445.AM2013-3974

Reading the tea leaves of tumor-mediated immunosuppression

Polyphenol E, available as Polyphenon E, is a green tea extract whose activity can be benchmarked to the presence of specific catechins such as epigallocatechin 3-gallate (EGCG). Herein, Polyphenon E is shown to reverse myeloid-derived suppressor cell activity, linking the activity of a natural product extract to cell-mediated immunity.

Identification of cell surface proteins as potential immunotherapy targets in 12 pediatric cancers

Technological advances now allow us to rapidly produce CARs and other antibody-derived therapeutics targeting cell surface receptors. To maximize the potential of these new technologies, relevant extracellular targets must be identified. The Pediatric Oncology Branch of the NCI curates a freely accessible database of gene expression data for both pediatric cancers and normal tissues, through which we have defined discrete sets of over-expressed transcripts in 12 pediatric cancer subtypes as compared to normal tissues. We coupled gene expression profiles to current annotation databases (i.e., Affymetrix, Gene Ontology, Entrez Gene), in order to categorize transcripts by their sub-cellular location. In this manner we generated a list of potential immune targets expressed on the cell surface, ranked by their difference from normal tissue. Global differences from normal between each of the pediatric tumor types studied varied, indicating that some malignancies expressed transcript sets that were more highly diverged from normal tissues than others. The validity of our approach is seen by our findings for pre-B cell ALL, where targets currently in clinical trials were top-ranked hits (CD19, CD22). For some cancers, reagents already in development could potentially be applied to a new disease class, as exemplified by CD30 expression on sarcomas. Moreover, several potential new targets shared among several pediatric solid tumors are herein identified, such as MCAM (MUC18), metadherin (MTDH), and glypican-2 (GPC2). These targets have been identified at the mRNA level and are yet to be validated at the protein level. The safety of targeting these antigens has yet to be demonstrated and therefore the identified transcripts should be considered preliminary candidates for new CAR and therapeutic antibody targets. Prospective candidate targets will be evaluated by proteomic analysis including Westerns and immunohistochemistry of normal and tumor tissues.