Serial Activation of the Inducible Caspase 9 Safety Switch After Human Stem Cell Transplantation

Small molecule-regulated switches to provide functional control of CAR T cells within the patient

INTRODUCTION

CAR T cells have generated great excitement due to their remarkable clinical response rates in selected hematologic malignancies. However, these engineered immune cells are living drugs which are hard to control after administration.

AREAS COVERED

We discuss small molecule-regulated switch systems which can potentially be used to control CAR T cell function within the patient, as well as the most important obstacles in the CAR T cell field, which might be overcome with those switch systems.

EXPERT OPINION

There is an urgent need to develop advanced switch systems. Once available, we expect that they will open up new avenues for future CAR T cell generations.

Suicide gene-enabled cell therapy: A novel approach to scalable human pluripotent stem cell quality control

There are an increasing number of cell therapy approaches being studied and employed world-wide. An emerging area in this field is the use of human pluripotent stem cell (hPSC) products for the treatment of injuries/diseases that cannot be effectively managed through current approaches. However, as with any cell therapy, vast numbers of functional and safe cells are required. Bioreactors provide an attractive avenue to generate clinically relevant cell numbers with decreased labour and decreased batch to batch variation. Yet, current methods of performing quality control are not readily scalable to the cell densities produced during bioreactor scale-up. One potential solution is the application of inducible/controllable suicide genes that can trigger cell death in unwanted cell types. These types of approaches have been demonstrated to increase the quality and safety of the resultant cell products. In this review, we will provide background on these approaches and how they could be used together with bioreactor technology to create effective bioprocesses for the generation of high quality and safe hPSCs for use in regenerative medicine approaches.

In vitro systems to study inborn errors of immunity using human induced pluripotent stem cells

In the last two decades, the exponential progress in the field of genetics could reveal the genetic impact on the onset and progression of several diseases affecting the immune system. This knowledge has led to the discovery of more than 400 monogenic germline mutations, also known as “inborn errors of immunity (IEI)”. Given the rarity of various IEI and the clinical diversity as well as the limited available patients’ material, the continuous development of novel cell-based in vitro models to elucidate the cellular and molecular mechanisms involved in the pathogenesis of these diseases is imperative. Focusing on stem cell technologies, this review aims to provide an overview of the current available in vitro models used to study IEI and which could lay the foundation for new therapeutic approaches. We elaborate in particular on the use of induced pluripotent stem cell-based systems and their broad application in studying IEI by establishing also novel infection culture models. The review will critically discuss the current limitations or gaps in the field of stem cell technology as well as the future perspectives from the use of these cell culture systems.

Combinatorial suicide gene strategies for the safety of cell therapies

Gene-modified cellular therapies carry inherent risks of severe and potentially fatal adverse events, including the expansion of alloreactive cells or malignant transformation due to insertional mutagenesis. Strategies to mitigate uncontrolled proliferation of gene-modified cells include co-transfection of a suicide gene, such as the inducible caspase 9 safety switch (ΔiC9). However, the activation of the ΔiC9 fails to completely eliminate all gene-modified cells. Therefore, we tested a two suicide gene system used independently or together, with the goal of complete cell elimination. The first approach combined the ΔiC9 with an inducible caspase 8, ΔiC8, which lacks the endogenous prodomain. The rationale was to use a second caspase with an alternative and complementary mechanism of action. Jurkat cells co-transduced to co-express the ΔiC8, activatable by a BB homodimerizer, and the ΔiC9 activatable by the rapamycin analog sirolimus were used in a model to estimate the degree of inducible cell elimination. We found that both agents could activate each caspase independently, with enhanced elimination with superior reduction in cell regrowth of gene-modified cells when both systems were activated simultaneously. A second approach was employed in parallel, combining the ΔiC9 with the RQR8 compact suicide gene. RQR8 incorporates a CD20 mimotope, targeted by the anti-CD20 monoclonal antibody rituxan, and the QBend10, a ΔCD34 selectable marker. Likewise, enhanced cell elimination with superior reduction in cell regrowth was observed when both systems were activated together. A dose-titration effect was also noted utilizing the BB homodimerizer, whereas sirolimus remained very potent at minimal concentrations. Further in vivo studies are needed to validate these novel combination systems, which may play a role in future cancer therapies or regenerative medicine.

Characterization of stem-cell-derived islets during differentiation and after implantation

Recapitulation of embryonic pancreatic development has enabled development of methods for in vitro islet cell differentiation using human pluripotent stem cells (hPSCs), which have the potential to cure diabetes. Advanced methods for optimal generation of stem-cell-derived islets (SC-islets) has enabled successful diabetes reversal in rodents and shown promising early clinical trial outcomes. The main impediment for use of SC-islets is concern about safety because of off-target growth resulting from contaminated residual cells. In this review, we summarize the different endocrine and non-endocrine cell populations that have been described to emerge throughout β cell differentiation and after transplantation. We discuss the most recent approaches to enrich endocrine populations and remove off-target cells. Finally, we discuss the critical quality control and release criteria testing that we anticipate will be required prior to transplantation to ensure product safety.

Sequential Stem Cell-Kidney Transplantation in Schimke Immuno-osseous Dysplasia

Lifelong immunosuppression is required for allograft survival after kidney transplantation but may not ultimately prevent allograft loss resulting from chronic rejection. We developed an approach that attempts to abrogate immune rejection and the need for post-transplantation immunosuppression in three patients with Schimke immuno-osseous dysplasia who had both T-cell immunodeficiency and renal failure. Each patient received sequential transplants of αβ T-cell-depleted and CD19 B-cell-depleted haploidentical hematopoietic stem cells and a kidney from the same donor. Full donor hematopoietic chimerism and functional ex vivo T-cell tolerance was achieved, and the patients continued to have normal renal function without immunosuppression at 22 to 34 months after kidney transplantation. (Funded by the Kruzn for a Kure Foundation.).

Inducible caspase-9 suicide gene under control of endogenous oct4 to safeguard mouse and human pluripotent stem cell therapy

Pluripotent stem cells (PSCs) are promising in regenerative medicine. A major challenge of PSC therapy is the risk of teratoma formation because of the contamination of undifferentiated stem cells. Constitutive promoters or endogenous SOX2 promoters have been used to drive inducible caspase-9 (iCasp9) gene expression but cannot specifically eradicate undifferentiated PSCs. Here, we inserted iCasp9 gene into the endogenous OCT4 locus of human and mouse PSCs without affecting their pluripotency. A chemical inducer of dimerization (CID), AP1903, induced iCasp9 activation, which led to the apoptosis of specific undifferentiated PSCs in vitro and in vivo. Differentiated cell lineages survived because of the silence of the endogenous OCT4 gene. Human and mouse PSCs were controllable when CID was administrated within 2 weeks after PSC injection in immunodeficient mice. However, an interval longer than 2 weeks caused teratoma formation and mouse death because a mass of somatic cells already differentiated from the PSCs. In conclusion, we have developed a specific and efficient PSC suicide system that will be of value in the clinical applications of PSC-based therapy.

Tumorigenicity Risk of iPSCs in vivo: Nip it in the Bud

In 2006, Takahashi and Yamanaka first created induced pluripotent stem cells from mouse fibroblasts via the retroviral introduction of genes encoding the transcription factors Oct3/4, Sox2, Klf44, and c-Myc. Since then, the future clinical application of somatic cell reprogramming technology has become an attractive research topic in the field of regenerative medicine. Of note, considerable interest has been placed in circumventing ethical issues linked to embryonic stem cell research. However, tumorigenicity, immunogenicity, and heterogeneity may hamper attempts to deploy this technology therapeutically. This review highlights the progress aimed at reducing induced pluripotent stem cells tumorigenicity risk and how to assess the safety of induced pluripotent stem cells cell therapy products.

Current Limitations and Perspectives of Chimeric Antigen Receptor-T-Cells in Acute Myeloid Leukemia

Simple Summary

Acute myeloid leukemia (AML) is the most frequent type of acute leukemia in adults. Allogeneic hematopoietic cell transplantation (allo-HCT) has been the only potentially curative treatment for the majority of patients. The ability of chimeric antigen receptor (CAR)-modified T-cell therapy directed against the CD19 antigen to induce durable remissions in patients with acute lymphoblastic leukemia (ALL) has provided optimism that this novel treatment paradigm can be extrapolated to AML. In this review, we provide an overview of candidate target antigens for CAR-T-cells in AML, an update on recent progress in preclinical and clinical development of investigational CAR-T-cell products, and discuss challenges for the clinical implementation of CAR-T-cell therapy in AML.

Abstract

Adoptive transfer of gene-engineered chimeric antigen receptor (CAR)-T-cells has emerged as a powerful immunotherapy for combating hematologic cancers. Several target antigens that are prevalently expressed on AML cells have undergone evaluation in preclinical CAR-T-cell testing. Attributes of an ‘ideal’ target antigen for CAR-T-cell therapy in AML include high-level expression on leukemic blasts and leukemic stem cells (LSCs), and absence on healthy tissues, normal hematopoietic stem and progenitor cells (HSPCs). In contrast to other blood cancer types, where CAR-T therapies are being similarly studied, only a rather small number of AML patients has received CAR-T-cell treatment in clinical trials, resulting in limited clinical experience for this therapeutic approach in AML. For curative AML treatment, abrogation of bulk blasts and LSCs is mandatory with the need for hematopoietic recovery after CAR-T administration. Herein, we provide a critical review of the current pipeline of candidate target antigens and corresponding CAR-T-cell products in AML, assess challenges for clinical translation and implementation in routine clinical practice, as well as perspectives for overcoming them.

Inclusion of the Inducible Caspase 9 Suicide Gene in CAR Construct Increases Safety of CAR.CD19 T Cell Therapy in B-Cell Malignancies

T cells engineered with chimeric antigen receptor (CAR-T cells) are an effective treatment in patients with relapsed/refractory B-cell precursor acute lymphoblastic leukemia or B-cell non-Hodgkin lymphoma. Despite the reported exciting clinical results, the CAR-T cell approach needs efforts to improve the safety profile, limiting the occurrence of adverse events in patients given this treatment. Besides the most common side effects, such as cytokine release syndrome and CAR-T cell–related encephalopathy syndrome, another potential issue involves the inadvertent transduction of leukemia B cells with the CAR construct during the manufacturing process, thus leading to the possibility of a peculiar mechanism of antigen masking and treatment resistance. In this study, we investigated whether the inclusion of the inducible caspase 9 (iC9) suicide gene in the CAR construct design could be an effective safety switch to control malignant CAR+ B cells, ultimately counteracting this serious adverse event. iC9 is a suicide gene able to be activated through binding with an otherwise inert small biomolecule, known as AP1903. The exposure of iC9.CAR.CD19-DAUDI lymphoma and iC9.CAR.CD19-NALM-6 leukemia cells in vitro to 20 nM of AP1903 resulted into the prompt elimination of CAR⁺ B-leukemia/lymphoma cell lines. The results obtained in the animal model corroborate in vitro data, since iC9.CAR.CD19⁺ tumor cells were controlled in vivo by the activation of the suicide gene through administration of AP1903. Altogether, our data indicate that the inclusion of the iC9 suicide gene may result in a safe CAR-T cell product, even when manufacturing starts from biological materials characterized by heavy leukemia blast contamination.

Depletion of T cells via Inducible Caspase 9 Increases Safety of Adoptive T-Cell Therapy Against Chronic Hepatitis B

T-cell therapy with T cells that are re-directed to hepatitis B virus (HBV)-infected cells by virus-specific receptors is a promising therapeutic approach for treatment of chronic hepatitis B and HBV-associated cancer. Due to the high number of target cells, however, side effects such as cytokine release syndrome or hepatotoxicity may limit safety. A safeguard mechanism, which allows depletion of transferred T cells on demand, would thus be an interesting means to increase confidence in this approach. In this study, T cells were generated by retroviral transduction to express either an HBV-specific chimeric antigen receptor (S-CAR) or T-cell receptor (TCR), and in addition either inducible caspase 9 (iC9) or herpes simplex virus thymidine kinase (HSV-TK) as a safety switch. Real-time cytotoxicity assays using HBV-replicating hepatoma cells as targets revealed that activation of both safety switches stopped cytotoxicity of S-CAR- or TCR-transduced T cells within less than one hour. In vivo, induction of iC9 led to a strong and rapid reduction of transferred S-CAR T cells adoptively transferred into AAV-HBV-infected immune incompetent mice. One to six hours after injection of the iC9 dimerizer, over 90% reduction of S-CAR T cells in the blood and the spleen and of over 99% in the liver was observed, thereby limiting hepatotoxicity and stopping cytokine secretion. Simultaneously, however, the antiviral effect of S-CAR T cells was diminished because remaining S-CAR T cells were mostly non-functional and could not be restimulated with HBsAg. A second induction of iC9 was only able to deplete T cells in the liver. In conclusion, T cells co-expressing iC9 and HBV-specific receptors efficiently recognize and kill HBV-replicating cells. Induction of T-cell death via iC9 proved to be an efficient means to deplete transferred T cells in vitro and in vivo containing unwanted hepatotoxicity.

Stem Cells as a Source of Pancreatic Cells for Production of 3D Bioprinted Bionic Pancreas in the Treatment of Type 1 Diabetes

Type 1 diabetes (T1D) is the third most common autoimmune disease which develops due to genetic and environmental risk factors. Often, intensive insulin therapy is insufficient, and patients require a pancreas or pancreatic islets transplant. However, both solutions are associated with many possible complications, including graft rejection. The best approach seems to be a donor-independent T1D treatment strategy based on human stem cells cultured in vitro and differentiated into insulin and glucagon-producing cells (β and α cells, respectively). Both types of cells can then be incorporated into the bio-ink used for 3D printing of the bionic pancreas, which can be transplanted into T1D patients to restore glucose homeostasis. The aim of this review is to summarize current knowledge about stem cells sources and their transformation into key pancreatic cells. Last, but not least, we comment on possible solutions of post-transplant immune response triggered stem cell-derived pancreatic cells and their potential control mechanisms.

Identification of dual positive CD19+/CD3+ T cells in a leukapheresis product undergoing CAR transduction: a case report

Background

Chimeric antigen receptor (CAR) therapy and hematopoietic stem cell transplantation (HSCT) are therapeutics for relapsed acute lymphocytic leukemia (ALL) that are increasingly being used in tandem. We identified a non-physiologic CD19+/CD3+ T-cell population in the leukapheresis product of a patient undergoing CAR T-cell manufacturing who previously received a haploidentical HSCT, followed by infusion of a genetically engineered T-cell addback product. We confirm and report the origin of these CD19+/CD3+ T cells that have not previously been described in context of CAR T-cell manufacturing. We additionally interrogate the fate of these CD19-expressing cells as they undergo transduction to express CD19-specific CARs.

Main body

We describe the case of a preteen male with multiply relapsed B-ALL who was treated with sequential cellular therapies. He received an αβ T-cell depleted haploidentical HSCT followed by addback of donor-derived T cells genetically modified with a suicide gene for iCaspase9 and truncated CD19 for cell tracking (RivoCel). He relapsed 6 months following HSCT and underwent leukapheresis and CAR T-cell manufacturing. During manufacturing, we identified an aberrant T-cell population dually expressing CD19 and CD3. We hypothesized that these cells were RivoCel cells and confirmed using flow cytometry and PCR that the identified cells were in fact RivoCel cells and were eliminated with iCaspase9 activation. We additionally tracked these cells through CD19-specific CAR transduction and notably did not detect T cells dually positive for CD19 and CD19-directed CARs. The most likely rationale for this is in vitro fratricide of the CD19+ ‘artificial’ T-cell population by the CD19-specific CAR+ T cells in culture.

Conclusions

We report the identification of CD19+/CD3+ cells in an apheresis product undergoing CAR transduction derived from a patient previously treated with a haploidentical transplant followed by RivoCel addback. We aim to bring attention to this cell phenotype that may be recognized with greater frequency as CAR therapy and engineered αβhaplo-HSCT are increasingly coupled. We additionally suggest consideration towards using alternative markers to CD19 as a synthetic identifier for post-transplant addback products, as CD19-expression on effector T cells may complicate subsequent treatment using CD19-directed therapy.

Precision installation of a highly efficient suicide gene safety switch in human induced pluripotent stem cells

Human pluripotent stem cells, including induced pluripotent stem cells (iPSCs) and embryonic stem cells, hold great promise for cell‐based therapies, but safety concerns that complicate consideration for routine clinical use remain. Installing a “safety switch” based on the inducible caspase‐9 (iCASP9) suicide gene system should offer added control over undesirable cell replication or activity. Previous studies utilized lentiviral vectors to integrate the iCASP9 system into T cells and iPSCs. This method results in random genomic insertion of the suicide switch and inefficient killing of the cells after the switch is “turned on” with a small molecule (eg, AP1903). To improve the safety and efficiency of the iCASP9 system for use in iPSC‐based therapy, we precisely installed the system into a genomic safe harbor, the AAVS1 locus in the PPP1R12C gene. We then evaluated the efficiencies of different promoters to drive iCASP9 expression in human iPSCs. We report that the commonly used EF1α promoter is silenced in iPSCs, and that the endogenous promoter of the PPP1R12C gene is not strong enough to drive high levels of iCASP9 expression. However, the CAG promoter induces strong and stable iCASP9 expression in iPSCs, and activation of this system with AP1903 leads to rapid killing and complete elimination of iPSCs and their derivatives, including MSCs and chondrocytes, in vitro. Furthermore, iPSC‐derived teratomas shrank dramatically or were completely eliminated after administration of AP1903 in mice. Our data suggest significant improvements on existing iCASP9 suicide switch technologies and may serve as a guide to other groups seeking to improve the safety of stem cell‐based therapies.

T-Cell Receptor-Based Immunotherapy for Hematologic Malignancies

Adoptive immunotherapy with engineered T cells is at the forefront of cancer treatment. T cells can be engineered to express T-cell receptors (TCRs) specific for tumor-associated antigens (TAAs) derived from intracellular or cell surface proteins. T cells engineered with TCRs (TCR-T) allow for targeting diverse types of TAAs, including proteins overexpressed in malignant cells, those with lineage-restricted expression, cancer-testis antigens, and neoantigens created from abnormal, malignancy-restricted proteins. Minor histocompatibility antigens can also serve as TAAs for TCR-T to treat relapsed hematologic malignancies after allogeneic hematopoietic cell transplantation. Moreover, TCR constructs can be modified to improve safety and enhance function and persistence of TCR-T. Transgenic T-cell receptor therapies targeting 3 different TAAs are in early-phase clinical trials for treatment of hematologic malignancies. Preclinical studies of TCR-T specific for many other TAAs are underway and offer great promise as safe and effective therapies for a wide range of cancers.

Genetically Modified T-Cell Therapy for Osteosarcoma: Into the Roaring 2020s

T-cell immunotherapy may offer an approach to improve outcomes for patients with osteosarcoma who fail current therapies. In addition, it has the potential to reduce treatment-related complications for all patients. Generating tumor-specific T cells with conventional antigen-presenting cells ex vivo is time-consuming and often results in T-cell products with a low frequency of tumor-specific T cells. Furthermore, the generated T cells remain sensitive to the immunosuppressive tumor microenvironment. Genetic modification of T cells is one strategy to overcome these limitations. For example, T cells can be genetically modified to render them antigen specific, resistant to inhibitory factors, or increase their ability to home to tumor sites. Most genetic modification strategies have only been evaluated in preclinical models; however, early clinical phase trials are in progress. In this chapter, we will review the current status of gene-modified T-cell therapy with special focus on osteosarcoma, highlighting potential antigenic targets, preclinical and clinical studies, and strategies to improve current T-cell therapy approaches.

Engineering for Success: Approaches to Improve Chimeric Antigen Receptor T Cell Therapy for Solid Tumors

While impressive clinical responses have been observed using chimeric antigen receptor (CAR) T cells targeting CD19+ hematologic malignancies, limited clinical benefit has been observed using CAR T cells for a variety of solid tumors. Results of clinical studies have highlighted several obstacles which CAR T cells face in the context of solid tumors, including insufficient homing to tumor sites, lack of expansion and persistence, encountering a highly immunosuppressive tumor microenvironment, and heterogeneous antigen expression. In this review, we review clinical outcomes and discuss strategies to improve the antitumor activity of CAR T cells for solid tumors.

Chimeric antigen receptor T-cell approaches to HIV cure

PURPOSE OF REVIEW

Combination antiretroviral therapy (ART) has enabled tremendous progress in suppressing HIV replication in infected patients. However, ART alone cannot eradicate HIV and its latent, persisting reservoirs. Novel approaches are needed to eradicate the virus or achieve functional cure in the absence of ART.

RECENT FINDINGS

Adoptive T-cell therapies were initially tested in HIV-infected individuals with limited efficiency. Benefiting from new and improved methodologies, an increasing array of CAR T-cell therapies has been successfully developed in the cancer immunotherapy field, demonstrating promising new avenues that could be applied to HIV. Numerous studies have characterized various HIV-specific CAR constructs, types of cytolytic effector cells, and CAR-expressing cells' trafficking to the reservoir compartments, warranting further in-vivo efforts. Notably, the ability of CAR cells to persist and function in low-antigen environments in vivo, that is, in ART-suppressed patients, remains unclear.

SUMMARY

Despite promising results in preclinical studies, only a handful of clinical trials have been initiated worldwide. Several obstacles remain prior to successful application of HIV-specific CAR T-cell therapies in patients. In this review, we survey the current state of the field, and address paths towards realizing the goal of an efficacious HIV CAR T-cell product.

Regulated ex vivo regional gene therapy for bone repair using an inducible caspase-9 suicide gene system

In order to adapt ex vivo regional gene therapy for clinical applications in orthopaedic surgery, safety issues must be considered. In this study we developed a suicide approach using a dual gene expression two step transcriptional amplification lentiviral vector (LV-TSTA) encoding BMP-2 and an inducible caspase 9 (iC9) system that selectively induces apoptosis upon activation with a chemical inducer of dimerization (CID). Transduction of rat bone marrow stromal cells (RBMSCs) with LV-TSTA-iC9/BMP-2 led to abundant BMP-2 production (90.3 ± 7.9 ng/24 h/10⁶ cells) in vitro and stimulated bone formation in a mouse muscle pouch in the absence of CID. Moreover it was shown that CID could be used to selectively induce apoptosis in iC9-transduced cells both in vitro and in vivo. Double exposure to serial dilutions of CID decreased in vitro production of BMP-2 by 85-87% and Luc activity by 97-99% in iC9/BMP-2 or iC9/Luc-transduced cells respectively. Early administration of CID (Days 0-1 post-op) in mice implanted with iC9/BMP-2-transduced RBMSCs was effective in blocking bone formation, indicating that CID was toxic to the transduced cells. In iC9/Luc-implanted mice, late administration of two doses of CID (Days 27-28 post-op) significantly reduced the luciferase signal. The current study provides proof of concept for the potential clinical application of regulated gene therapy to promote bone repair.

A Versatile Safeguard for Chimeric Antigen Receptor T-Cell Immunotherapies

CAR T-cell therapies hold great promise for treating a range of malignancies but are however challenged by the complexity of their production and by the adverse events related to their activity. Here we report the development of the CubiCAR, a tri-functional CAR architecture that enables CAR T-cell detection, purification and on-demand depletion by the FDA-approved antibody Rituximab. This novel architecture has the potential to streamline the manufacturing of CAR T-cells, allow their tracking and improve their overall safety.

A Rapamycin-Activated Caspase 9-Based Suicide Gene

Engineered T cell therapies show considerable promise in the treatment of refractory malignancies. Given the ability of engineered T cells to engraft and persist for prolonged periods along with unpredicted toxicities, incorporation of a suicide gene to allow selective depletion after administration is desirable. Rapamycin is a safe and widely available immunosuppressive pharmaceutical that acts by heterodimerization of FKBP12 with the FRB fragment of mTOR. The apical caspase caspase 9 is activated by homodimerization through its CARD domain. We developed a rapamycin-induced caspase 9 suicide gene. First, we showed that caspase 9 could be activated by a two-protein format with replacement of the CARD domain with both FRB and FKBP12. We next identified an optimal compact single-protein rapamycin caspase 9 (rapaCasp9) by fusing both FRB and FKBP12 with the catalytic domain of caspase 9. Functionality of rapaCasp9 when co-expressed with a CD19 CAR was demonstrated in vitro and in vivo.

Development of T-cell immunotherapy for hematopoietic stem cell transplantation recipients at risk of leukemia relapse

Leukemia relapse remains the major cause of allogeneic hematopoietic stem cell transplantation (HCT) failure, and the prognosis for patients with post-HCT relapse is poor. There is compelling evidence that potent selective antileukemic effects can be delivered by donor T cells specific for particular minor histocompatibility (H) antigens. Thus, T-cell receptors (TCRs) isolated from minor H antigen-specific T cells represent an untapped resource for developing targeted T-cell immunotherapy to manage post-HCT leukemic relapse. Recognizing that several elements may be crucial to the efficacy and safety of engineered T-cell immunotherapy, we developed a therapeutic transgene with 4 components: (1) a TCR specific for the hematopoietic-restricted, leukemia-associated minor H antigen, HA-1; (2) a CD8 coreceptor to promote function of the class I-restricted TCR in CD4⁺ T cells; (3) an inducible caspase 9 safety switch to enable elimination of the HA-1 TCR T cells in case of toxicity; and (4) a CD34-CD20 epitope to facilitate selection of the engineered cell product and tracking of transferred HA-1 TCR T cells. The T-cell product includes HA-1 TCR CD4⁺ T cells to augment the persistence and function of the HA-1 TCR CD8⁺ T cells and includes only memory T cells; naive T cells are excluded to limit the potential for alloreactivity mediated by native TCR coexpressed by HA-1 TCR T cells. We describe the development of this unique immunotherapy and demonstrate functional responses to primary leukemia by CD4⁺ and CD8⁺ T cells transduced with a lentiviral vector incorporating the HA-1 TCR transgene construct.

Treatment of Acute Myeloid Leukemia with T Cells Expressing Chimeric Antigen Receptors Directed to C-type Lectin-like Molecule 1

The successful immunotherapy of acute myeloid leukemia (AML) has been hampered because most potential antigenic targets are shared with normal hematopoietic stem cells (HSCs), increasing the risk of sustained and severe hematopoietic toxicity following treatment. C-type lectin-like molecule 1 (CLL-1) is a membrane glycoprotein expressed by >80% of AML but is absent on normal HSCs. Here we describe the development and evaluation of CLL-1-specific chimeric antigen receptor T cells (CLL-1.CAR-Ts) and we demonstrate their specific activity against CLL-1⁺ AML cell lines as well as primary AML patient samples in vitro. CLL-1.CAR-Ts selectively reduced leukemic colony formation in primary AML patient peripheral blood mononuclear cells compared to control T cells. In a human xenograft mouse model, CLL-1.CAR-Ts mediated anti-leukemic activity against disseminated AML and significantly extended survival. By contrast, the colony formation of normal progenitor cells remained intact following CLL-1.CAR-T treatment. Although CLL-1.CAR-Ts are cytotoxic to mature normal myeloid cells, the selective sparing of normal hematopoietic progenitor cells should allow full myeloid recovery once CLL-1.CAR-T activity terminates. To enable elective ablation of the CAR-T, we therefore introduced the inducible caspase-9 suicide gene system and we show that exposure to the activating drug rapidly induced a controlled decrease of unwanted CLL-1.CAR-T activity against mature normal myeloid cells.

Designing and building oncolytic viruses

Oncolytic viruses (OVs) are engineered and/or evolved to propagate selectively in cancerous tissues. They have a dual mechanism of action; direct killing of infected cancer cells cross-primes anticancer immunity to boost the killing of uninfected cancer cells. The goal of the field is to develop OVs that are easily manufactured, efficiently delivered to disseminated sites of cancer growth, undergo rapid intratumoral spread, selectively kill tumor cells, cause no collateral damage and pose no risk of transmission in the population. Here we discuss the many virus engineering strategies that are being pursued to optimize delivery, intratumoral spread and safety of OVs derived from different virus families. With continued progress, OVs have the potential to transform the paradigm of cancer care.

In Vitro Pre-Clinical Validation of Suicide Gene Modified Anti-CD33 Redirected Chimeric Antigen Receptor T-Cells for Acute Myeloid Leukemia

Background

Approximately fifty percent of patients with acute myeloid leukemia can be cured with current therapeutic strategies which include, standard dose chemotherapy for patients at standard risk of relapse as assessed by cytogenetic and molecular analysis, or high-dose chemotherapy with allogeneic hematopoietic stem cell transplant for high-risk patients. Despite allogeneic hematopoietic stem cell transplant about 25% of patients still succumb to disease relapse, therefore, novel strategies are needed to improve the outcome of patients with acute myeloid leukemia.

Methods and findings

We developed an immunotherapeutic strategy targeting the CD33 myeloid antigen, expressed in ~ 85–90% of patients with acute myeloid leukemia, using chimeric antigen receptor redirected T-cells. Considering that administration of CAR T-cells has been associated with cytokine release syndrome and other potential off-tumor effects in patients, safety measures were here investigated and reported. We genetically modified human activated T-cells from healthy donors or patients with acute myeloid leukemia with retroviral supernatant encoding the inducible Caspase9 suicide gene, a ΔCD19 selectable marker, and a humanized third generation chimeric antigen receptor recognizing human CD33. ΔCD19 selected inducible Caspase9-CAR.CD33 T-cells had a 75±3.8% (average ± standard error of the mean) chimeric antigen receptor expression, were able to specifically lyse CD33⁺ targets in vitro, including freshly isolated leukemic blasts from patients, produce significant amount of tumor-necrosis-factor-alpha and interferon-gamma, express the CD107a degranulation marker, and proliferate upon antigen specific stimulation. Challenging ΔCD19 selected inducible Caspase9-CAR.CD33 T-cells with programmed-death-ligand-1 enriched leukemia blasts resulted in significant killing like observed for the programmed-death-ligand-1 negative leukemic blasts fraction. Since the administration of 10 nanomolar of a non-therapeutic dimerizer to activate the suicide gene resulted in the elimination of only 76.4±2.0% gene modified cells in vitro, we found that co-administration of the dimerizer with either the BCL-2 inhibitor ABT-199, the pan-BCL inhibitor ABT-737, or mafosfamide, resulted in an additive effect up to complete cell elimination.

Conclusions

This strategy could be investigated for the safety of CAR T-cell applications, and targeting CD33 could be used as a ‘bridge” therapy for patients coming to allogeneic hematopoietic stem cell transplant, as anti-leukemia activity from infusing CAR.CD33 T-cells has been demonstrated in an ongoing clinical trial. Albeit never performed in the clinical setting, our future plan is to investigate the utility of iC9-CAR.CD33 T-cells as part of the conditioning therapy for an allogeneic hematopoietic stem cell transplant for acute myeloid leukemia, together with other myelosuppressive agents, whilst the activation of the inducible Caspase9 suicide gene would grant elimination of the infused gene modified T-cells prior to stem cell infusion to reduce the risk of engraftment failure as the CD33 is also expressed on a proportion of the donor stem cell graft.

Improving the safety of T-Cell therapies using an inducible caspase-9 gene

Adoptive transfer of T cells can be an effective anticancer treatment. However, uncontrolled or unpredictable immediate or persistent toxic effects are a source of concern. The ability to conditionally eliminate aberrant cells in vivo is therefore becoming a critical step for the successful translation of this approach to the clinic. We review the evolution of safety systems, focusing on a suicide switch that can be expressed stably and efficiently in human T cells without impairing phenotype, function, or antigen specificity. This system is based on the fusion of human caspase-9 to a modified human FK-binding protein, allowing conditional dimerization in the presence of an otherwise bio-inert small molecule drug. When exposed to the synthetic dimerizing drug, the inducible caspase-9 becomes activated, resulting in the rapid apoptosis of cells expressing this construct. We have illustrated the clinical feasibility and efficacy of this approach after haploidentical hematopoietic stem cell transplant. Here we review the benefits and limitations of the approach.

Current status of ex vivo gene therapy for hematological disorders: a review of clinical trials in Japan around the world

Gene therapies are classified into two major categories, namely, in vivo and ex vivo. Clinical trials of human gene therapy began with the ex vivo techniques. Based on the initial successes of gene-therapy clinical trials, these approaches have spread worldwide. The number of gene therapy trials approved worldwide increased gradually starting in 1989, reaching 116 protocols per year in 1999, and a total of 2210 protocols had been approved by 2015. Accumulating clinical evidence has demonstrated the safety and benefits of several types of gene therapy, with the exception of serious adverse events in several clinical trials. These painful experiences were translated backward to basic science, resulting in the development of several new technologies that have influenced the recent development of ex vivo gene therapy in this field. To date, six gene therapies have been approved in a limited number of countries worldwide. In Japan, clinical trials of gene therapy have developed under the strong influence of trials in the US and Europe. Since the initial stages, 50 clinical trials have been approved by the Japanese government. In this review, the history and current status of clinical trials of ex vivo gene therapy for hematological disorders are introduced and discussed.