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The production of autologous T cells expressing a chimaeric antigen receptor (CAR) is time-consuming, costly and occasionally unsuccessful. T-cell-derived induced pluripotent stem cells (TiPS) are a promising source for the generation of ‘off-the-shelf’ CAR T cells, but the in vitro differentiation of TiPS often yields T cells with suboptimal features. Here we show that the premature expression of the T-cell receptor (TCR) or a constitutively expressed CAR in TiPS promotes the acquisition of an innate phenotype, which can be averted by disabling the TCR and relying on the CAR to drive differentiation. Delaying CAR expression and calibrating its signalling strength in TiPS enabled the generation of human TCR– CD8αβ+ CAR T cells that perform similarly to CD8αβ+ CAR T cells from peripheral blood, achieving effective tumour control on systemic administration in a mouse model of leukaemia and without causing graft-versus-host disease. Driving T-cell maturation in TiPS in the absence of a TCR by taking advantage of a CAR may facilitate the large-scale development of potent allogeneic CD8αβ+ T cells for a broad range of immunotherapies.

Table providing information on scFv sequences CARs. Figures providing data on CAR T cell phenotype, BMMSC survival, target antigen expression, cytotoxic mediator measurements, Survivin and Mcl-1 expression in MM cells, effector cell survival, and in vivo efficacy of BCMA-CAR T cells.

Purpose:The microenvironment of multiple myeloma (MM) can critically impair therapy outcome, including immunotherapies. In this context, we have earlier demonstrated that bone marrow mesenchymal stromal cells (BMMSC) protect MM cells against the lytic machinery of MM-reactive cytotoxic T cells (CTL) and daratumumab-redirected natural killer (NK) cells through the upregulation of antiapoptotic proteins Survivin and Mcl-1 in MM cells. Here, we investigated the significance of this mode of immune escape on T cells engineered to express chimeric antigen receptors (CAR T cells).Experimental Design:We tested the cytolytic ability of a panel of 10 BCMA-, CD38-, and CD138-specific CAR T cells with different affinities against a model MM cell line and against patient-derived MM cells in the presence versus absence of BMMSCs.Results:Although BMMSCs hardly protected MM cells from lysis by high-affinity, strongly lytic BCMA- and CD38-CAR T cells, they significantly protected against lower affinity, moderately lytic BCMA-, CD38-, and CD138-specific CAR T cells in a cell–cell contact-dependent manner. Overall, there was a remarkable inverse correlation between the protective ability of BMMSCs and the lytic activity of all CAR T cells, which was dependent on CAR affinity and type of costimulation. Furthermore, BMMSC-mediated resistance against CAR T cells was effectively modulated by FL118, an inhibitor of antiapoptotic proteins Survivin, Mcl-1, and XIAP.Conclusions:These results extend our findings on the negative impact of the microenvironment against immunotherapies and suggest that outcome of CAR T cell or conventional CTL therapies could benefit from inhibition of antiapoptotic proteins upregulated in MM cells through BMMSC interactions.

Supplementary methods and data

Purpose:Targeting nonspecific, tumor-associated antigens (TAA) with chimeric antigen receptors (CAR) requires specific attention to restrict possible detrimental on-target/off-tumor effects. A reduced affinity may direct CAR-engineered T (CAR-T) cells to tumor cells expressing high TAA levels while sparing low expressing normal tissues. However, decreasing the affinity of the CAR-target binding may compromise the overall antitumor effects. Here, we demonstrate the prime importance of the type of intracellular signaling on the function of low-affinity CAR-T cells.Experimental Design:We used a series of single-chain variable fragments (scFv) with five different affinities targeting the same epitope of the multiple myeloma–associated CD38 antigen. The scFvs were incorporated in three different CAR costimulation designs and we evaluated the antitumor functionality and off-tumor toxicity of the generated CAR-T cells in vitro and in vivo.Results:We show that the inferior cytotoxicity and cytokine secretion mediated by CD38 CARs of very low–affinity (Kd < 1.9 × 10−6 mol/L) bearing a 4-1BB intracellular domain can be significantly improved when a CD28 costimulatory domain is used. Additional 4-1BB signaling mediated by the coexpression of 4-1BBL provided the CD28-based CD38 CAR-T cells with superior proliferative capacity, preservation of a central memory phenotype, and significantly improved in vivo antitumor function, while preserving their ability to discriminate target antigen density.Conclusions:A combinatorial costimulatory design allows the use of very low–affinity binding domains (Kd < 1 μmol/L) for the construction of safe but also optimally effective CAR-T cells. Thus, very-low-affinity scFvs empowered by selected costimulatory elements can enhance the clinical potential of TAA-targeting CARs.

Purpose: The microenvironment of multiple myeloma (MM) can critically impair therapy outcome, including immunotherapies. In this context, we have earlier demonstrated that bone marrow mesenchymal stromal cells (BMMSC) protect MM cells against the lytic machinery of MM-reactive cytotoxic T cells (CTL) and daratumumab-redirected natural killer (NK) cells through the upregulation of antiapoptotic proteins Survivin and Mcl-1 in MM cells. Here, we investigated the significance of this mode of immune escape on T cells engineered to express chimeric antigen receptors (CAR T cells). Experimental Design: We tested the cytolytic ability of a panel of 10 BCMA-, CD38-, and CD138-specific CAR T cells with different affinities against a model MM cell line and against patient-derived MM cells in the presence versus absence of BMMSCs. Results: Although BMMSCs hardly protected MM cells from lysis by high-affinity, strongly lytic BCMA- and CD38-CAR T cells, they significantly protected against lower affinity, moderately lytic BCMA-, CD38-, and CD138-specific CAR T cells in a cell–cell contact-dependent manner. Overall, there was a remarkable inverse correlation between the protective ability of BMMSCs and the lytic activity of all CAR T cells, which was dependent on CAR affinity and type of costimulation. Furthermore, BMMSC-mediated resistance against CAR T cells was effectively modulated by FL118, an inhibitor of antiapoptotic proteins Survivin, Mcl-1, and XIAP. Conclusions: These results extend our findings on the negative impact of the microenvironment against immunotherapies and suggest that outcome of CAR T cell or conventional CTL therapies could benefit from inhibition of antiapoptotic proteins upregulated in MM cells through BMMSC interactions.

International audience; Immunotherapy with gene engineered CAR and TCR transgenic T-cells is a transformative treatment in cancer medicine. There is a rich pipeline with target antigens and sophisticated technologies that will enable establishing this novel treatment not only in rare hematological malignancies, but also in common solid tumors. The T2EVOLVE consortium is a public private partnership directed at accelerating the preclinical development of and increasing access to engineered T-cell immunotherapies for cancer patients. A key ambition in T2EVOLVE is to assess the currently available preclinical models for evaluating safety and efficacy of engineered T cell therapy and developing new models and test parameters with higher predictive value for clinical safety and efficacy in order to improve and accelerate the selection of lead T-cell products for clinical translation. Here, we review existing and emerging preclinical models that permit assessing CAR and TCR signaling and antigen binding, the access and function of engineered T-cells to primary and metastatic tumor ligands, as well as the impact of endogenous factors such as the host immune system and microbiome. Collectively, this review article presents a perspective on an accelerated translational development path that is based on innovative standardized preclinical test systems for CAR and TCR transgenic T-cell products.

Despite promising clinical results in a small subset of malignancies, therapies based on engineered chimeric antigen receptor and T-cell receptor T cells are associated with serious adverse events, including cytokine release syndrome and neurotoxicity. These toxicities are sometimes so severe that they significantly hinder the implementation of this therapeutic strategy. For a long time, existing preclinical models failed to predict severe toxicities seen in human clinical trials after engineered T-cell infusion. However, in recent years, there has been a concerted effort to develop models, including humanized mouse models, which can better recapitulate toxicities observed in patients. The Accelerating Development and Improving Access to CAR and TCR-engineered T cell therapy (T2EVOLVE) consortium is a public–private partnership directed at accelerating the preclinical development and increasing access to engineered T-cell therapy for patients with cancer. A key ambition in T2EVOLVE is to design new models and tools with higher predictive value for clinical safety and efficacy, in order to improve and accelerate the selection of lead T-cell products for clinical translation. Herein, we review existing preclinical models that are used to test the safety of engineered T cells. We will also highlight limitations of these models and propose potential measures to improve them.

B-cell maturation antigen (BCMA)–specific chimeric antigen receptor (CAR) T cells have substantial therapeutic potential in multiple myeloma (MM), but most patients eventually relapse. Determinants of response and mechanisms of resistance are most likely multifactorial and include MM-related factors, premanufacturing T-cell characteristics, CAR T-cell–related features, and several components of the immunosuppressive microenvironment. Efforts to improve the potency and safety of CAR T-cell therapy include optimizing CAR design, combinatorial approaches to enhance persistence and activity, treatment of less heavily pretreated patients, and dual-antigen targeting to prevent antigen escape. We expect that these rationally designed strategies will contribute to further improvement in the clinical outcome of patients with MM.Significance:Although BCMA-specific CAR T-cell therapies are highly effective in heavily pretreated patients with MM, there has been, until now, no indication of a plateau in the survival curves. In this review, we provide an overview of the determinants of response and the mechanisms that contribute to the development of treatment failure after initial remission (acquired resistance). A better understanding of these mechanisms, underlying lack of disease response, and acquired resistance may lead to further improvements in the effectiveness of CAR T-cell therapy.

Purpose of Review In the rapidly developing field of adoptive cell immunotherapy, there is urgent need for discoveries that would improve outcomes, extend the applicability, and reduce the costs. Induced pluripotent stem cells (iPSC) can be a source of broadly applicable cellular immunotherapeutics, which have been manufactured, validated, and banked in advance, and can be applied across HLA barriers. Here, we discuss the recent advances and challenges in the generation of iPSC-derived cellular products for cancer therapy. Recent Findings iPSCs can be differentiated to functional tumor-specific T and NK cells in vitro with demonstrable in vitro and in vivo anti-tumor activity. Genetic modifications employed at the iPSC level can deliver desirable immunotherapeutic attributes to the generated immune effectors. iPSC-NK cells are currently evaluated in a clinical setting and pre-clinical testing of iPSC-T cells shows promising results but their production seems more challenging. Summary The use of iPSCs for the generation of tumor-targeting T/NK cells constitutes a feasible strategy to overcome limitations in manufacturing, efficacy, and applicability of cellular therapeutics.

Due to the CD1d restricted recognition of altered glycolipids, V&alpha, 24-invariant natural killer T (iNKT) cells are excellent tools for cancer immunotherapy with a significantly reduced risk for graft-versus-host disease when applied as off-the shelf-therapeutics across Human Leukocyte Antigen (HLA) barriers. To maximally harness their therapeutic potential for multiple myeloma (MM) treatment, we here armed iNKT cells with chimeric antigen receptors (CAR) directed against the MM-associated antigen CD38 and the plasma cell specific B cell maturation antigen (BCMA). We demonstrate that both CD38- and BCMA-CAR iNKT cells effectively eliminated MM cells in a CAR-dependent manner, without losing their T cell receptor (TCR)-mediated cytotoxic activity. Importantly, iNKT cells expressing either BCMA-CARs or affinity-optimized CD38-CARs spared normal hematopoietic cells and displayed a Th1-like cytokine profile, indicating their therapeutic utility. While the costimulatory domain of CD38-CARs had no influence on the cytotoxic functions of iNKT cells, CARs containing the 4-1BB domain showed a better expansion capacity. Interestingly, when stimulated only via CD1d+ dendritic cells (DCs) loaded with &alpha, galactosylceramide (&alpha, GalCer), both CD38- and BCMA-CAR iNKT cells expanded well, without losing their CAR- or TCR-dependent cytotoxic activities. This suggests the possibility of developing an off-the-shelf therapy with CAR iNKT cells, which might even be boostable in vivo by administration &alpha, GalCer pulsed DCs.

Background: The biology and treatment response of human multiple myeloma (MM) cells in vivo is influenced by interactions with mesenchymal bone marrow stromal cells (BMSCs). For several key BMSC-derived cytokines (including IL-6) only the human, not murine, form optimally interacts with their respective receptor(s) on human MM cells. To better simulate the treatment responses of human MM cells in the BM, "humanized" BM-like niches in vivo have been engineered with biocompatible ceramic scaffolds "functionalized" via osteogenic differentiation of human mesenchymal BMSCs and implanted subcutaneously in immunocompromised mice. Aim: To determine if the patterns of genetic dependencies elucidated through in vitro CRISPR-based functional genomic studies are recapitulated when human MM cells are grown in mice within BM-like scaffolds with "humanized" mesenchymal stromal compartment. Methods: Cas9+ MM cell lines KMS11 and XG7 were transduced with a library of 1372 single-guide RNAs (sgRNAs) targeting 184 genes of interest (4 sgRNAs/gene), including 89 genes preferentially essential for MM cell lines compared to other neoplasms in vitro; broad-spectrum oncogenic targets (e.g. KRAS, MYC); tumor suppressor genes (e.g. PTEN); genes with limited in vitro essentiality in MM cells, but significantly higher expression vs. non-MM lines (e.g. ZFP91, ZBP1); and 155 olfactory receptor (OR) genes (typically not expressed or biologically inactive in tumor cells) as "DNA cutting" control sgRNAs. MM cells transduced with this focused CRISPR knockout (KO) library were injected into "humanized" scaffolds implanted subcutaneously (s.c.) in NSG mice. Tumors were collected when mice reached criteria for euthanasia and next-generation sequencing quantified the changes in sgRNA distribution at the end vs. start of experiment. Results: A large majority of genes defined by in vitro CRISPR KO screens as MM-preferential dependencies were also essential for MM cells in BM-like scaffolds. Among 57 MM-preferential dependencies in vitro which were plausible dependencies for KMS11 cells (e.g. CERES scores -0.4) for KMS11 cells, 29 genes exhibited in vivo essentiality for both KMS11 and XG7 cells: several of these latter "in vivo dependencies" are recurrently essential for other MM lines in vitro (e.g. ZBTB38, UBE2J1, TCF3, DNAJB11), while also others have limited if any in vitro essentiality (e.g. ZBP1, PYGO2) across MM despite significantly higher transcript levels vs. other neoplasias. Notably, several genes with increased essentiality in the "humanized" BM scaffolds vs. in vitro also had stronger in vivo dependency in the BM scaffolds vs. when growth of the same MM cells as s.c. plasmacytomas (e.g. BCL2, PIM2, UBE2J1, SYVN1, ALG9, AMFR). Co-culture with BMSCs or IL-6 treatment induces several of these transcripts in MM cells suggesting that increased dependency of MM on these genes in the "humanized" BM model is due at least partly to its human cytokines. Conclusions: This study provides evidence for significant impact of the "humanized" BM-like niche on the patterns of genetic dependencies for human MM cells. Most genes preferentially essential for MM cells in vitro remain essential for their in vivo growth in the "humanized" BM model. Notably, several genes that do not meet criteria for dependency in vitro show such metrics in "humanized" BM scaffolds, but not sc plasmacytomas. These observations highlight important implications of the "humanized" BM-like in vivo model for current and future efforts to define and prioritize therapeutic targets for MM. Disclosures Downey-Kopyscinski: Rancho BioSciences, LLC: Current Employment. Tsherniak:Tango Therapeutics: Consultancy; Cedilla Therapeutics: Consultancy. Boise:AstraZeneca: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Genetech: Membership on an entity's Board of Directors or advisory committees. Mitsiades:FIMECS: Consultancy, Honoraria; Takeda: Other: employment of a relative; Fate Therapeutics: Consultancy, Honoraria; Janssen/Johnson & Johnson: Research Funding; Arch Oncology: Research Funding; TEVA: Research Funding; Sanofi: Research Funding; Karyopharm: Research Funding; EMD Serono: Research Funding; Abbvie: Research Funding; Ionis Pharmaceuticals, Inc.: Consultancy, Honoraria.

Summary RAG2 severe combined immune deficiency (RAG2-SCID) is a lethal disorder caused by the absence of functional T and B cells due to a differentiation block. Here, we generated induced pluripotent stem cells (iPSCs) from a RAG2-SCID patient to study the nature of the T cell developmental blockade. We observed a strongly reduced capacity to differentiate at every investigated stage of T cell development, from early CD7−CD5− to CD4+CD8+. The impaired differentiation was accompanied by an increase in CD7−CD56+CD33+ natural killer (NK) cell-like cells. T cell receptor D rearrangements were completely absent in RAG2SCID cells, whereas the rare T cell receptor B rearrangements were likely the result of illegitimate rearrangements. Repair of RAG2 restored the capacity to induce T cell receptor rearrangements, normalized T cell development, and corrected the NK cell-like phenotype. In conclusion, we succeeded in generating an iPSC-based RAG2-SCID model, which enabled the identification of previously unrecognized disorder-related T cell developmental roadblocks., Graphical Abstract, Highlights • RAG2-SCID cells show impaired differentiation at several stages of T cell development • RAG2-SCID T and NK cells fail to undergo legitimate RAG-driven TCR rearrangements • RAG2-SCID cells exhibit a skewed differentiation toward NK cell-like cells • RAG2-SCID phenotype is rescued by gene correction, In this article, Mikkers and colleagues model RAG2-SCID using iPSCs and show that the capacity of RAG2-SCID cells to go through T cell development is hampered at multiple transitions from the earliest stage onwards. As a consequence RAG2 mutant cells generate more CD7−CD56+ CD33+ cells with NK cell properties.

Immunotherapeutic strategies targeting the rare leukemic stem cell compartment might provide salvage to the high relapse rates currently observed in acute myeloid leukemia (AML). We applied gene expression profiling for comparison of leukemic blasts and leukemic stem cells with their normal counterparts. Here, we show that the T-cell receptor γ chain alternate reading frame protein (TARP) is over-expressed in de novo pediatric (n=13) and adult (n=17) AML sorted leukemic stem cells and blasts compared to hematopoietic stem cells and normal myeloblasts (15 healthy controls). Moreover, TARP expression was significantly associated with a fms-like tyrosine kinase receptor-3 internal tandem duplication in pediatric AML. TARP overexpression was confirmed in AML cell lines (n=9), and was found to be absent in B-cell acute lymphocytic leukemia (n=5) and chronic myeloid leukemia (n=1). Sequencing revealed that both a classical TARP transcript, as described in breast and prostate adenocarcinoma, and an AML-specific alternative TARP transcript, were present. Protein expression levels mostly matched transcript levels. TARP was shown to reside in the cytoplasmic compartment and showed sporadic endoplasmic reticulum co-localization. TARP-T-cell receptor engineered cytotoxic T-cells in vitro killed AML cell lines and patient leukemic cells co-expressing TARP and HLA-A*0201. In conclusion, TARP qualifies as a relevant target for immunotherapeutic T-cell therapy in AML. ispartof: HAEMATOLOGICA vol:105 issue:5 pages:1306-1316 ispartof: location:Italy status: published

INTRODUCTION: Multiple myeloma (MM) is generally an incurable hematological malignancy with heterogeneous overall survival rates ranging from a few months to more than 10 years. Survival is especially poor for patients who developed disease that is refractory to immunomodulatory drugs and proteasome inhibitors. Areas covered: This review will discuss the importance of CD38-targeting antibodies for the treatment of MM patients to improve their outcome. Expert commentary: Intense immuno-oncological laboratory research has resulted in the development of functionally active monoclonal antibodies against cell surface markers present on MM cells. In this respect, CD38-targeting antibodies such as daratumumab, MOR202, and isatuximab, have high single agent activity in heavily pretreated MM patients by virtue of their pleiotropic mechanisms of action including Fc-dependent effector mechanisms and immunomodulatory activities. Importantly, CD38-targeting antibodies are well tolerated, with infusion reactions as most frequent adverse event. Altogether, this makes them attractive combination partners with other anti-MM agents. Daratumumab is already approved as monotherapy and in combination with lenalidomide-dexamethasone as well as bortezomib-dexamethasone in pretreated MM patients. Furthermore, results from studies evaluating CD38-targeting antibodies in newly diagnosed MM patients are also promising, indicating that CD38-targeting antibodies will be broadly used in MM, resulting in further improvements in survival.

T cells armed with a chimeric antigen receptor, CAR T cells, have shown extraordinary activity against certain B lymphocyte malignancies, when targeted towards the CD19 B cell surface marker. These results have led to the regulatory approval of two CAR T cell approaches. Translation of this result to the solid tumor setting has been problematic until now. A number of differences between liquid and solid tumors are likely to cause this discrepancy. The main ones of these are undoubtedly the uncomplicated availability of the target cell within the blood compartment and the abundant expression of the target molecule on the cancerous cells in the case of hematological malignancies. Targets expressed by solid tumor cells are hard to engage due to the non-adhesive and abnormal vasculature, while conditions in the tumor microenvironment can be extremely immunosuppressive. Targets in the tumor vasculature are readily reachable by CAR T cells and reside outside the immunosuppressive tumor microenvironment. It is therefore hypothesized that targeting CAR T cells towards the tumor vasculature of solid tumors may share the excellent effects of CAR T cell therapy with that against hematological malignancies. A few reports have shown promising results. Suggestions are provided for further improvement.

Background: CAR-T cells are a tremendous breakthrough in the treatment of certain type of blood cancer, demonstrating impressive and durable responses. However, mechanisms of resistance and relapse have been reported. Mechanisms contributing to relapse after CAR-T therapy include the downregulation of the CAR-T target antigen and the rapid extinguishing of CAR-T cells. We have developed a personalized cancer vaccine whereby patient derived tumor cells are fused to autologous dendritic cells (DC). DC/tumor vaccine induces a broad anti-tumor immunity capable of preventing relapse but may not be effective in patients with advanced disease. Aims: We sought to overcome relapse and resistance to CAR-T therapy to improve the current response rate to CAR-T cells. To this end, we combined CAR-T cells treatment with our personalized DC/tumor vaccine. We postulated that the DC/tumor vaccine would demonstrate synergy with CAR-T cells in a setting where CAR-T cells reduce the bulky disease and the fusion vaccine prevents relapse by expanding CAR-T cells and tumor antigen specific lymphocytes. Methods: To investigate the effects of the CAR-T/fusion vaccine combination, we used the A20 murine B-cell lymphoma model. Syngeneic T cells were obtained from BALB/c mice and retrovirally transduced with a second-generation CAR construct composed of an antigen binding domain that recognizes murine CD19, the CD3ζ domain, 4-1BB as costimulatory molecule and GFP (m19BBz-GFP). GFP expression was used to assess gene-transfer efficiency and monitor the CAR-transduced T cells. The DC/tumor vaccine was obtained by PEG-mediated fusions of A20 cells and BALB/c DC. In the in vitro experiments, T cells transduced with the m19BBz-GFP CAR or non-transduced T cells were co-cultured in the presence or the absence of the DC/A20 vaccine for 3 days. Tumor killing was measured by quantifying Firefly luciferase activity (WT A20) or Renilla luciferase activity (CD19- A20). Vaccine was removed before starting the killing assay. In the in vivo experiment, B-cell lymphoma was induced in BALB/c mice by tail vein injection of A20 cells. The mice were lymphodepleted and treated with m19BBz-GFP CAR-T. The mice were then subcutaneous injected with the DC/A20 fusion vaccine or with PBS (control group). CD8+ T-cells specific for the A20 idiotype epitope were quantified by MHC Class I tetramer analysis. Results: In vitro co-culture of CAR-T cells and DC/A20 vaccine induced a memory-like CAR-T phenotype and strongly improved CAR-T persistence (measured as % of GFP+ T cells in culture). These results were confirmed in vivo where increased CAR-T percentage was observed in the bone marrow and spleen of vaccinated mice with respect to the unvaccinated control group. Moreover, in the vaccinated mice we detected CD8+ T-cells specific for the A20 idiotype epitope demonstrating the expansion of tumor-specific lymphocytes in response to the fusion vaccine. To assess whether the vaccine-induced persistence of CAR-T cells was translated in a higher tumor killing capacity, we performed an in vitro killing assay. To mimic the presence of CD19- clones in the tumor bulk, we used a mixture of WT A20 and CD19-A20 as CAR-T target cells. Enhanced killing capacity against CD19+ tumors was induced by education of the CAR-T with the vaccine. No effects on the CAR-T killing capacity against CD19- A20 were elicited by the fusion vaccine. However, when the same killing assay was performed using as effector cells a mixture of CAR-T and naïve T cells or a mixture of CAR-T and vaccine-educated T cells, we observed that the addition of vaccine-educated T cell to the CAR-T, strongly reduced both A20 WT and A20 CD19- in the cultures. Thus, vaccine-educated T cells are able to kill tumor cells independently from the presence or the absence of the CAR-T target antigen on the tumor. Conclusions: The combination of CAR-T and DC/tumor vaccine, increasing the persistence of CAR-T cells and evoking a polyclonal T cell response against tumor antigens in the non-CAR-transduced T cells, may represent a novel therapeutic strategy to overcame therapeutic resistance and improve current response rate to CAR-T therapy. Disclosures Capelletti: Caris Life Sciences: Current Employment. Stroopinsky: The Blackstone Group: Consultancy. Kufe: REATA: Consultancy, Current equity holder in publicly-traded company; Genus Oncology: Current equity holder in publicly-traded company; Hillstream BioPharma: Current equity holder in publicly-traded company; Canbas: Consultancy. Themeli: Fate Therapeutics: Patents & Royalties. Rosenblatt: Attivare Therapeutics: Consultancy; Imaging Endpoints: Consultancy; Karyopharm: Membership on an entity's Board of Directors or advisory committees; Parexel: Consultancy; Wolters Kluwer Health: Consultancy, Patents & Royalties; Bristol-Myers Squibb: Research Funding. Sadelain: Minerva Biotechnologies: Patents & Royalties; Juno Therapeutics: Patents & Royalties; Fate Therapeutics: Other: Provision of Services (uncompensated), Patents & Royalties; Mnemo Therapeutics: Patents & Royalties; Takeda Pharmaceuticals: Other: Provision of Services, Patents & Royalties; Ceramedix: Patents & Royalties; NHLBI Gene Therapy Resource Program: Other: Provision of Services (uncompensated); St. Jude Children's Research Hospital: Other: Provision of Services; Atara Biotherapeutics: Patents & Royalties. Avigan: Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding; Pharmacyclics: Research Funding; Kite Pharma: Consultancy, Research Funding; Juno: Membership on an entity's Board of Directors or advisory committees; Partner Tx: Membership on an entity's Board of Directors or advisory committees; Karyopharm: Membership on an entity's Board of Directors or advisory committees; Bristol-Myers Squibb: Membership on an entity's Board of Directors or advisory committees; Aviv MedTech Ltd: Membership on an entity's Board of Directors or advisory committees; Takeda: Membership on an entity's Board of Directors or advisory committees; Legend Biotech: Membership on an entity's Board of Directors or advisory committees; Chugai: Membership on an entity's Board of Directors or advisory committees; Janssen: Consultancy; Parexcel: Consultancy; Takeda: Consultancy; Sanofi: Consultancy.

Chimeric antigen receptor (CAR) T cells are highly successful in the treatment of hematologic malignancies. We recently generated affinity-optimized CD38CAR T cells, which effectively eliminate multiple myeloma (MM) cells with little or no toxicities against nonmalignant hematopoietic cells. The lack of universal donors and long manufacturing times however limit the broad application of CAR T cell therapies. Natural killer (NK) cells generated from third party individuals may represent a viable source of “off the shelf” CAR-based products, as they are not associated with graft-versus-host disease unlike allogeneic T cells. We therefore explored the preclinical anti-MM efficacy and potential toxicity of the CD38CAR NK concept by expressing affinity-optimized CD38CARs in KHYG-1 cells, an immortal NK cell line with excellent expansion properties. KHYG-1 cells retrovirally transduced with the affinity-optimized CD38CARs expanded vigorously and mediated effective CD38-dependent cytotoxicity towards CD38high MM cell lines as well as primary MM cells ex vivo. Importantly, the intermediate affinity CD38CAR transduced KHYG-1 cells spared CD38neg or CD38int nonmalignant hematopoietic cells, indicating an optimal tumor nontumor discrimination. Irradiated, short living CD38CAR KHYG-1 cells also showed significant anti-MM effects in a xenograft model with a humanized bone marrow-like niche. Finally, CD38CAR KHYG-1 cells effectively eliminated primary MM cells derived from patients who are refractory to CD38 antibody daratumumab. Taken together, the results of this proof-of-principle study demonstrate the potential value of engineering affinity-optimized CD38CARs in NK cells to establish effective anti-MM effects, with an excellent safety profile, even in patients who failed to response to most advanced registered myeloma therapies, such as daratumumab.

Introduction CAR T cells have demonstrated unique potency for tumor cytoreduction and durable response in hematological malignancies. However, disease relapse remains a concern due to the emergence of antigen negative variants, tolerization of CAR T cell populations and lack of T cell persistence. We postulated that vaccination with a personalized cancer vaccine, that we developed, in which patient derived tumor cells are fused with autologous dendritic cells would enhance CAR T cell efficacy through the expansion of T cell clonal populations and the vaccine mediated enhancement of T cell activation and persistence. In the present study, we examined the potential synergy between CAR T cells targeting CD19 and syngeneic DC/tumor fusions. Methods/Results CD19 CAR T cells and DC/tumor fusions were studied in the context of a murine A20 lymphoma model, and their combination effectively lysed A20 cells in a CTL assay in a higher percentage than CAR T cells alone (20% vs 34%). Moreover, we examined the interaction of vaccine and CAR T cells ex vivo using the IncuCyte S3 Live-Cell Analysis System which allows for live cell visualization of lysis of A20 cells over time. We studied the impact of combining vaccine educated and CAR T cells as well as an individual T cell population that underwent sequential vaccine mediated stimulation followed by transduction with the CD19 CAR. Even in this assay, coculture with either combined vaccine educated and CAR T cells or sequentially vaccine educated, and transduced T cells demonstrated the highest levels of cytotoxicity that was maintained over time. Enhanced lysis by combined vaccine stimulation and CAR T cells was similarly demonstrated in another tumor cell line, 5TGM1, a multiple myeloma cell line transduced to express CD19. On the contrary wild type 5TGM1 cells were recognized by the DC/tumor fusion stimulated cells in contrast to CAR T cells alone (40% vs. 8%). Finally, we examined the capacity of vaccine educated T cells in conjunction with CAR T cells to target A20 cells in an immunocompetent murine model. Mice were challenged with 1 × 106 A20 Mcherry-Luc and after lymphoma engraftment animals were treated with 3 × 106 T cells consisting of CAR T cells, vaccine educated T cells or the combination. Serial bioluminescence imaging demonstrated greatest reduction in tumor burden using combined CAR T and vaccine educated T cells with 4/5 animals still disease free at day 13 after tumor challenge. Conclusions Combined therapy with T cells stimulated by DC/tumor fusions and CAR T cells exhibited potent lysis of murine lymphoma and myeloma cells compared to CAR T cells or vaccine educated T cells alone in in vitro and immunocompetent murine models. These findings suggest potent synergy between these modalities that may overcome recognized pathways of resistance including the broadening of the tumor specific response and vaccine mediated activation of CAR T cell populations.

The tumor microenvironment of multiple myeloma (MM) is known to play a critical role in disease pathogenesis, MM cell survival, and drug resistance. In addition, we have previously demonstrated the impact of the microenvironment also on immunotherapies. We discovered that bone marrow mesenchymal stromal cells (BMMSCs) protected MM cells from HLA restricted CD4 and CD8 cytotoxic T cells and NK cell-mediated daratumumab dependent cytotoxicity through a cell-adhesion mediated immune resistance (CAM-IR). This CAM-IR corresponded with the upregulation of anti-apoptotic proteins Survivin and Mcl-1 in MM cells and could be modulated by small inhibitors of these molecules. We now extended our studies to investigate the impact of the microenvironment on novel immunotherapeutic approaches such as chimeric antigen receptor-transduced T cells (CAR T cells) and death receptor mediated antibody treatments, which can induce direct apoptosis in MM cells. We also investigated the possibility to modulate CAM-IR with small molecule inhibitors of Survivin, XIAP, and Mcl-1. To this end, we tested a panel of MM reactive CAR T cells directed against CD38, BCMA, and CD138, with different target affinities, for their potential to kill MM cells in the absence or presence of BMMSCs. We observed no effect of BMMSCs on the cytotoxic capacity of BCMA- and CD38-targeting CAR T cells with high affinity for the target and that were capable of inducing high levels of MM cell lysis at very low effector to target ratios. In contrast, BMMSCs effectively protected MM cells against killing by BCMA- and CD38-targeting CAR T cells with relative low affinity, or CD138-targeting CAR T cells that were less powerful in their lytic activity. Taken together, we discovered a significant inverse correlation between the lytic capacity of the CAR T cells and the extent of BMMSC-mediated protection. Additionally, we found that BMMSCs protected MM cells from apoptosis induction by death receptor 5 (DR5; or TRAIL receptor 2) antibodies. In all cases of BMMSC-mediated protection against CAR T cells as well as DR5 antibodies or daratumumab, the protection could be abrogated by an inhibitor of Survivin, XIAP, and Mcl-1. This indicates that BMMSC-mediated protection against CAR T cell or antibody-mediated lysis is indeed associated with an upregulation of anti-apoptotic proteins, similarly to the well-described cell adhesion mediated drug resistance. In conclusion, our results confirm the potential negative impact of the tumor microenvironment in the development of an adaptive resistance of MM cells against immunotherapies. Our data further suggest that this microenvironmental shielding of MM cells can be overcome either by increasing the avidity of immune killer cells or through combination of immunotherapy with inhibitors of anti-apoptotic mediators. Figure 1 Disclosures Li: Canget Bio Tekpharma LLC: Membership on an entity's Board of Directors or advisory committees. Zweegman:Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding; Takeda: Membership on an entity's Board of Directors or advisory committees, Research Funding; Janssen Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Research Funding. Themeli:Covagen: Consultancy. Van De Donk:Janssen Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Research Funding; Bristol-Myers Squibb: Membership on an entity's Board of Directors or advisory committees, Research Funding; AMGEN: Membership on an entity's Board of Directors or advisory committees, Research Funding; Takeda: Membership on an entity's Board of Directors or advisory committees; Servier: Membership on an entity's Board of Directors or advisory committees; Celgene Corporation: Membership on an entity's Board of Directors or advisory committees, Research Funding; Bayer: Membership on an entity's Board of Directors or advisory committees; Roche: Membership on an entity's Board of Directors or advisory committees. Mutis:Aduro: Research Funding; Celgene: Research Funding; BMS: Research Funding; Amgen: Research Funding; Onkimmune: Research Funding; Janssen Pharmaceuticals: Research Funding; Novartis: Research Funding.

Introduction: CAR T cells have demonstrated unique potency for tumor cytoreduction and the potential for durable response in patients with advanced hematological malignancies. However, disease relapse remains a significant concern due to the emergence of antigen negative variants, tolerization of CAR T cell populations and lack of T cell persistence. We have developed a personalized cancer vaccine in which patient derived tumor cells are fused with autologous dendritic cells such that a broad array of tumor antigens is expressed in the context of DC mediated co-stimulation. Vaccination of patients with acute leukemia and multiple myeloma has been associated with the durable expansion of tumor specific lymphocytes in the bone marrow and peripheral blood, targeting of residual disease, and durable remission. We postulated that vaccination with DC/tumor fusions would enhance CAR T cell efficacy through the expansion of T cell clonal populations targeting tumor cells via the native TCR and the vaccine mediated enhancement of T cell activation and persistence. In addition, ex vivo engineered CAR T cells provide a substrate of functionally competent T cells with cytoreductive capacity in the setting of advanced disease. In the present study, we examined the potential synergy between CAR T cells targeting CD19 and syngeneic DC/tumor fusions. Methods/Results: CAR T cells and DC/tumor fusions were studied in the context of a murine A20 lymphoma model. CD19 CAR T cells were established through retroviral transduction of a CD19 CAR construct expressing CD28 and 41BBL syngeneic DC/A20 fusions were generated as previously described. Vaccine stimulated T cells were generated by coculturing splenocyte derived T cells with syngeneic DC/A20 fusion cells over a period of three days in a 10:1 ratio in the presence of low dose IL2. While CD19 CAR T cells effectively lysed a subset of A20 cells in a CTL, the addition of vaccine educated T cells increased the percentage of tumor cells undergoing CTL mediated lysis (20% vs 34%). We subsequently examined the interaction of vaccine and CAR T cells ex vivo using the IncuCyte S3 Live-Cell Analysis System which allows for live cell visualization of lysis of A20 cells over time. We studied the impact of combining vaccine educated and CAR T cells as well as an individual T cell population that underwent sequential vaccine mediated stimulation followed by transduction with the CD19 CAR. While vaccine educated and CAR T cells demonstrated potent lysis of A20 cells over time, coculture with either combined vaccine educated and CAR T cells or sequentially vaccine educated and transduced T cells demonstrated the highest levels of cytotoxicity that was maintained over time (1786 and 2338 signal overlap count per image at 23 hours compared to 123 of the control). Enhanced lysis by combined vaccine stimulation and CAR T cells was similarly demonstrated in another tumor cell line, 5TGM1, a multiple myeloma cell line transduced to express CD19. Cytotoxic killing of the 5TGM1-CD19 cells was most pronounced when combining vaccine educated and CAR T cells as compared to CAR T cells alone (33% vs 14%). Consistent with the broad targeting of vaccine educated as compared to the CAR T cell population, wild type 5TGM1 cells were recognized by the DC/tumor fusion stimulated cells in contrast to CAR T cells alone (40% vs. 8%). We subsequently examined the capacity of vaccine educated T cells in conjunction with CAR T cells to target A20 cells in an immunocompetent murine model. Mice were challenged with 1 x 10(6) A20 Mcherry-Luc and lymphoma engraftment was demonstrated at Day 7. Animals were then treated with 3 x 10(6) T cells consisting of CAR T cells, vaccine educated T cells or the combination. Serial bioluminescence imaging demonstrated greatest reduction in tumor burden using combined CAR T and vaccine educated T cells with 4/5 animals without BLI evidence of disease at day 13 after tumor challenge. Conclusions: In in vitro and immunocompetent murine models, we have demonstrated that combined therapy with T cells stimulated by DC/tumor fusions and CAR T cells exhibited potent lysis of murine lymphoma and myeloma cells as compared to the efficacy of CAR T cells or vaccine educated T cells alone. These findings suggest potent synergy between these modalities that may overcome recognized pathways of resistance including the broadening of the tumor specific response and vaccine mediated activation of CAR T cell populations. Disclosures Themeli: Covagen: Consultancy. Mutis:Janssen Research and Development: Research Funding; Celgene: Research Funding; Onkimmune: Research Funding; Genmab: Research Funding. Munshi:Adaptive: Consultancy; Amgen: Consultancy; Oncopep: Consultancy; Janssen: Consultancy; Celgene: Consultancy; Takeda: Consultancy; Abbvie: Consultancy. Kufe:Genus Oncology: Equity Ownership; Reata Pharmaceuticals: Consultancy, Equity Ownership, Honoraria; Nanogen Therapeutics: Equity Ownership, Membership on an entity's Board of Directors or advisory committees; Hillstream BioPharma: Equity Ownership; Victa BioTherapeutics: Consultancy, Equity Ownership, Honoraria, Membership on an entity's Board of Directors or advisory committees; Canbas: Consultancy, Honoraria. Rosenblatt:BMS: Research Funding; Amgen: Other: Advisory Board; Merck: Other: Advisory Board; BMS: Other: Advisory Board ; Parexel: Consultancy; Imaging Endpoint: Consultancy; Partner Tx: Other: Advisory Board; Dava Oncology: Other: Education; Celgene: Research Funding. Sadelain:Fate Therapeutics: Consultancy, Patents & Royalties; Memorial Sloan Kettering Cancer Center: Employment; Juno Therapeutics: Consultancy, Patents & Royalties, Research Funding. Avigan:Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding; Pharmacyclics: Research Funding; Juno: Membership on an entity's Board of Directors or advisory committees; Partners Tx: Membership on an entity's Board of Directors or advisory committees; Partner Tx: Membership on an entity's Board of Directors or advisory committees; Karyopharm: Membership on an entity's Board of Directors or advisory committees; Bristol-Myers Squibb: Membership on an entity's Board of Directors or advisory committees; Janssen: Consultancy; Parexel: Consultancy; Takeda: Consultancy.

T cells expressing chimeric antigen receptors (CARs) are a formidable platform for the study and application of synthetic biology approaches to study customized and flexible control of cellular functions. Recent reports in the journal Cell provide a new twist on combinatorial antigen targeting, profiting from the singular cleavage and signaling of the Notch receptor to conditionally express CARs.

The unphysiological formation of biological chimeras after allogeneic hematopoietic cell transplantation is not free of consequences. Recent findings suggest that in the transplant recipient some epithelial cells reveal, unexpectedly, donor-derived genotype and/or acquire genomic alterations. Since both phenomena are presented in the host epithelium, one could argue that they might be etiologically linked through a common background mechanism. We recently proposed that the incessant charge of the transplant recipient with donor-DNA and its integration in host epithelium by horizontal DNA transference may indeed be operative in the generation of epithelial cells with donor derived genome. On the other hand, the incessant incorporation of the foreign DNA into the host genome may result in genomic alterations. Lymphocyteepithelial interactions between the two genetically distinct cell populations in the transplant recipient should be investigated more precisely not only in cellular but also in molecular level.

Summary Epigenetic regulation of key transcriptional programs is a critical mechanism that controls hematopoietic development, and, thus, aberrant expression patterns or mutations in epigenetic regulators occur frequently in hematologic malignancies. We demonstrate that the Polycomb protein L3MBTL1, which is monoallelically deleted in 20q- myeloid malignancies, represses the ability of stem cells to drive hematopoietic-specific transcriptional programs by regulating the expression of SMAD5 and impairing its recruitment to target regulatory regions. Indeed, knockdown of L3MBTL1 promotes the development of hematopoiesis and impairs neural cell fate in human pluripotent stem cells. We also found a role for L3MBTL1 in regulating SMAD5 target gene expression in mature hematopoietic cell populations, thereby affecting erythroid differentiation. Taken together, we have identified epigenetic priming of hematopoietic-specific transcriptional networks, which may assist in the development of therapeutic approaches for patients with anemia., Highlights • L3MBTL1 is a chromatin-binding protein that represses SMAD5 expression • Lack of L3MBTL1 primes the hematopoietic development of pluripotent stem cells • L3MBTL1 regulates erythroid differentiation, Nimer, Perna, and colleagues report that the Polycomb protein L3MBTL1, deleted in 20q- myeloid malignancies, plays a regulatory role during the differentiation of human iPS cells. Upon lentiviral knockdown of L3MBTL1, iPS cells stay poised for hematopoietic development and upregulate BMP/SMAD target gene expression, leading to impaired neural development.

Chimeric Antigen Receptors (CARs) are engineered transmembrane proteins consisting of an antibody (ab)-derived antigen recognition domain linked to intracellular T cell signaling domains. Cytotoxic T cells endowed with tumor-reactive CARs are highly promising tools for immunotherapy of cancer. There are however only a few truly tumor specific molecules that can be targeted by CARs, a drawback for the broad application of CAR T cell therapy. Indeed, when we recently aimed at targeting CD38high multiple myeloma (MM) with T cells transduced with high affinity CD38CARs, we observed that they not only lysed the CD38high MM cells but also CD38+ normal hematopoietic cells, pointing towards potential safety issues of such tumor-associated, but not entirely tumor-specific CARs. Therefore, using CD38 as a model for antigen we now tested whether it would be possible to reduce the on target, off-tumor effects of such CARs by optimizing their target cell affinity. To this end, we generated a new panel of CD38 abs through the "light chain exchange" method, in which heavy chains of two high affinity CD38 abs were combined with 176 different germ line light chains. This approach revealed around 100 new abs, which displayed 10- >1000 fold lower affinity to CD38 as compared to the parental abs. After categorizing them in three classes based on CD38 binding affinity, we used 8 abs from each class to generate 24 different CD38-CAR constructs. Testing the cytotoxic activity of T cells transduced with these CD38-CARs against CD38++ MM cell lines, primary MM cells and CD38+ normal hematopoietic cells in vitro and in vivo demonstrated that CD38-CAR T cells with ca. 1000 fold lower affinity to CD38 could still effectively lyse CD38++ MM cells while there was little or no cytotoxicity against CD38+ healthy hematopoietic cells. The results of this study reveal that it is possible to reduce the on-target off-tumor effects of CARs by optimizing their affinity. Thus, tailored affinity of the ab binding domain may open up new roads for CAR therapy. Disclosures van de Donk: Janssen: Research Funding; Celgene: Research Funding; BMS: Research Funding; Amgen: Research Funding. Lokhorst:Janssen: Membership on an entity's Board of Directors or advisory committees, Research Funding; Genmab: Research Funding. Mutis:Celgene: Research Funding; Genmab: Research Funding; Janssen: Membership on an entity's Board of Directors or advisory committees, Research Funding.

T cell therapies have demonstrated long-term efficacy and curative potential for the treatment of some cancers. However, their use is limited by damage to bystander tissues, as seen in graft-versus-host disease after donor lymphocyte infusion, or " on-target, off-tumor " toxicities incurred in some engineered T cell therapies. Nonspecific immunosuppression and irreversible T cell elimination are currently the only means to control such deleterious responses, but at the cost of abrogating therapeutic benefits or causing secondary complications. On the basis of the physiological paradigm of immune inhibitory receptors, we designed antigen-specific inhibitory chimeric antigen receptors (iCARs) to preemptively constrain T cell responses. We demonstrate that CTLA-4- or PD-1-based iCARs can selectively limit cytokine secretion, cytotoxicity, and proliferation induced through the endogenous T cell receptor or an activating chimeric receptor. The initial effect of the iCAR is temporary, thus enabling T cells to function upon a subsequent encounter with the antigen recognized by their activating receptor. iCARs thus provide a dynamic, self-regulating safety switch to prevent, rather than treat, the consequences of inadequate T cell specificity.

Progress in adoptive T-cell therapy for cancer and infectious diseases1–2 is hampered by the lack of readily available, antigen-specific, human T lymphocytes. Pluripotent stem cells could provide an unlimited source of T lymphocytes, but the therapeutic potential of human pluripotent stem cell–derived lymphoid cells generated to date remains uncertain3–6. Here we combine induced pluripotent stem cell (iPSC)7 and chimeric antigen receptor (CAR)8 technologies to generate human T cells targeted to CD19, an antigen expressed by malignant B cells, in tissue culture. These iPSC-derived, CAR-expressing T cells display a phenotype resembling that of innate γδ T cells. Similar to CAR-transduced, peripheral blood γδ T cells, the iPSC–derived T cells potently inhibit tumor growth in a xenograft model. This approach of generating therapeutic human T cells ‘in the dish’ may be useful for cancer immunotherapy and other medical applications.

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) in humans, following hematoablative treatment, results in biological chimeras. In this case, the transplanted hematopoietic, immune cells and their derivatives can be considered the donor genotype, while the other tissues are the recipient genotype. The first sequel, which has been recognized in the development of chimerical organisms after allo-HSCT, is the graft versus host (GvH) reaction, in which the new developed immune cells from the graft recognize the host’s epithelial cells as foreign and mount an inflammatory response to kill them. There is now accumulating evidence that this chronic inflammatory tissue stress may contribute to clinical consequences in the transplant recipient. It has been recently reported that host epithelial tissue acquire genomic alterations and display a mutator phenotype that may be linked to the occurrence of a GvH reaction. The current review discusses existing data on this recently discovered phenomenon and focuses on the possible pathogenesis, clinical significance and therapeutic implications.

The ability to differentiate T cells of defined specificity and function from T-cell derived induced pluripotent stem cells (TiPSC) may be useful for the treatment of a range of pathologies, including cancer, infection and immune deficits. We recently reported that genetic engineering of TiPSC to express a Chimeric Antigen Receptor (CAR) is an effective strategy to combine the unlimited availability of TiPSC and facilitate reprogramming of the T cell specificity and functional potential. In vivo, CD19-retargeted human TiPS-derived CAR T cells (CARTiPSC-T) display therapeutic potency in a lymphoma model. Surprisingly, despite expression of the endogenous αβ TCR, the CARTiPSC-T cells possessed innate-like phenotype and function, most similar to γδ T cells. Although innate T cells have anti-tumor activity, they lack some vital features for therapeutic efficacy including long-term memory and in vivo persistence, which characterize mature CD8+ and CD4+ αβ TCR T cells. Additional research into the mechanisms underlying in vitro T cell differentiation of TiPSC is required to improve the development of mature TCRαβ T cells and facilitate the employment of their therapeutic potential. T cell lineage determination depends in part on the balance between Notch and TCR signaling, we are investigating their respective roles, as well as that of the CAR, in determining lineage commitment. We hypothesize that the combined CAR and early CD3/TCRαβ expression disrupts the TCR/Notch signaling balance, prohibiting mature TCRαβ T cell development. To study the effects of the different Notch ligands, TiPSC were differentiated on OP9 cells expressing one of four Notch ligands (DLL1, DLL4, Jagged-1 or Jagged-2). Preliminary data suggests that DLL4 is able to facilitate T cell development to CD4/CD8 double positive (DP) stage, however, this is hindered by CAR expression. To determine the role of early TCRαβ expression, elimination of TCRαβ expression was facilitated by targeted disruption of the TCRα constant region using the CRISPR/Cas9 system. Elimination of early TCRαβ expression showed improved development towards DP stage of TiPSC on OP9-DLL1. TiPSC are a valuable system for the study of human T cell differentiation. In addition, and most importantly they are further amenable to genetic engineering with TCRs or CARs, which may be useful for the generation of therapeutic “off-the-shelf”, antigen-specific T cells. Citation Format: Sjoukje J.C. van der Stegen, Maria Themeli, Justin Eyquem, Jorge Mansilla-Soto, Michel Sadelain. T-cell development from T cell-derived induced pluripotent stem cell. [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 2309.

Human pluripotent stem cells (hPSCs) hold great promise for cell therapy. However, a major concern is the risk of tumor formation by residual undifferentiated cells contaminating the hPSC-derived cell product. Suicide genes could safeguard against such adverse events by enabling elimination of cells gone astray, but the efficacy of this approach has not yet been thoroughly tested. Here, we engineered a lentivirally encoded herpes simplex virus thymidine kinase (HSVtk) with expression restricted to undifferentiated hPSCs through regulation by the let7 family of miRNAs. We show that induced pluripotent stem cells (iPSCs) expressing a let7-regulated HSVtk transgene are selectively killed by ganciclovir (GCV), whereas differentiated cells are fully protected. However, in contrast to previous studies, we find that in vivo GCV administration results in longer latency but does not prevent teratoma formation by iPSCs expressing either a constitutive or a let7-regulated HSVtk, without evidence of silencing of the HSVtk. Clonal analyses of iPSCs expressing HSVtk revealed frequent emergence of GCV resistance which, at least in some cases, could be attributed to preexisting inactivating mutations in the HSVtk coding sequence, selected for upon GCV treatment. Our findings have important consequences for the future use of suicide genes in hPSC-based cell therapies.