Function of Novel Anti-CD19 Chimeric Antigen Receptors with Human Variable Regions Is Affected by Hinge and Transmembrane Domains

Limitations in the Design of Chimeric Antigen Receptors for Cancer Therapy

Cancer therapy has entered a new era, transitioning from unspecific chemotherapeutic agents to increasingly specific immune-based therapeutic strategies. Among these, chimeric antigen receptor (CAR) T cells have shown unparalleled therapeutic potential in treating refractory hematological malignancies. In contrast, solid tumors pose a much greater challenge to CAR T cell therapy, which has yet to be overcome. As this novel therapeutic modality matures, increasing effort is being invested to determine the optimal structure and properties of CARs to facilitate the transition from empirical testing to the rational design of CAR T cells. In this review, we highlight how individual CAR domains contribute to the success and failure of this promising treatment modality and provide an insight into the most notable advances in the field of CAR T cell engineering.

A safe and potent anti-CD19 CAR T cell therapy

Anti-CD19 chimeric antigen receptor (CAR) T cell therapies can cause severe cytokine-release syndrome (CRS) and neurotoxicity, impeding their therapeutic application. Here we generated a new anti-CD19 CAR molecule (CD19-BBz(86)) derived from the CD19-BBz prototype bearing co-stimulatory 4-1BB and CD3ζ domains. We found that CD19-BBz(86) CAR T cells produced lower levels of cytokines, expressed higher levels of antiapoptotic molecules and proliferated more slowly than the prototype CD19-BBz CAR T cells, although they retained potent cytolytic activity. We performed a phase 1 trial of CD19-BBz(86) CAR T cell therapy in patients with B cell lymphoma (ClinicalTrials.gov identifier NCT02842138 ). Complete remission occurred in 6 of 11 patients (54.5%) who each received a dose of 2 × 10⁸-4 × 10⁸ CD19-BBz(86) CAR T cells. Notably, no neurological toxicity or CRS (greater than grade 1) occurred in any of the 25 patients treated. No significant elevation in serum cytokine levels after CAR T cell infusion was detected in the patients treated, including in those who achieved complete remission. CD19-BBz(86) CAR T cells persistently proliferated and differentiated into memory cells in vivo. Thus, therapy with the new CD19-BBz(86) CAR T cells produces a potent and durable antilymphoma response without causing neurotoxicity or severe CRS, representing a safe and potent anti-CD19 CAR T cell therapy.

Chimeric Antigen Receptors Incorporating D Domains Targeting CD123 Direct Potent Mono- and Bi-specific Antitumor Activity of T Cells

Chimeric antigen receptor (CAR) T cell therapies have demonstrated impressive initial response rates in hematologic malignancies. However, relapse rates are significant, and robust efficacies in other indications, such as solid tumors, will likely require novel therapeutic strategies and CAR designs. To that end, we sought to develop simple, highly selective targeting domains (D domains) that could be incorporated into complex, multifunctional therapeutics. Herein, we describe the identification and characterization of D domains specific for CD123, a therapeutic target for hematologic malignancies, including acute myelogenous leukemia (AML). CARs comprised of these D domains mediate potent T cell activation and cytolysis of CD123-expressing target cells and induce complete durable remission in two AML xenograft models. We describe a strategy of engineering less immunogenic D domains through the identification and removal of putative T cell epitopes and investigate the binding kinetics and affinity requirements of the resultant D domain CARs. Finally, we extended the utility of D domains by generating functional, bi-specific CARs comprised of a CD123-specific D domain and a CD19-specific scFv. The properties of D domains suggest that this class of targeting domain may facilitate the development of multi-functional CARs where conventional, scFv-based designs may be suboptimal.

Chimeric antigen receptor T-cell therapy for B-cell non-Hodgkin lymphoma: opportunities and challenges

B-cell non-Hodgkin lymphoma (NHL) is the most frequent hematologic malignancy. Despite the refinement of chemoimmunotherapy, a substantial number of patients experience chemorefractory disease. Anti-CD19 chimeric antigen receptor (CAR) T-cell therapy is considered the most promising and effective therapy to overcome chemorefractory B-cell NHL. Based on the promising results obtained from pivotal trials, the US Food and Drug Administration and European Medicines Agency approved anti-CD19 CAR T-cell therapy for relapsed/refractory diffuse large B-cell lymphoma. Nonetheless, there remain several controversial issues and problems awaiting solutions, including optimal management of toxicities, overcoming relapsed/refractory disease after CAR T-cell therapy, and improving CAR-T manufacturing platform. Definite unmet medical needs among patients with chemorefractory B-cell NHL still exist. CAR T-cell therapy might be a game changer that can defeat chemorefractory B-cell NHL, and further clinical development is warranted. In this review, we summarize the recent clinical developments, clinical implications, and perspectives of CAR T-cell therapy, focusing on B-cell NHL.

An immunoproteomic approach to characterize the CAR interactome and signalosome

Adoptive transfer of T cells that express a chimeric antigen receptor (CAR) is an approved immunotherapy that may be curative for some hematological cancers. To better understand the therapeutic mechanism of action, we systematically analyzed CAR signaling in human primary T cells by mass spectrometry. When we compared the interactomes and the signaling pathways activated by distinct CAR-T cells that shared the same antigen-binding domain but differed in their intracellular domains and their in vivo antitumor efficacy, we found that only second-generation CARs induced the expression of a constitutively phosphorylated form of CD3ζ that resembled the endogenous species. This phenomenon was independent of the choice of costimulatory domains, or the hinge/transmembrane region. Rather, it was dependent on the size of the intracellular domains. Moreover, the second-generation design was also associated with stronger phosphorylation of downstream secondary messengers, as evidenced by global phosphoproteome analysis. These results suggest that second-generation CARs can activate additional sources of CD3ζ signaling, and this may contribute to more intense signaling and superior antitumor efficacy that they display compared to third-generation CARs. Moreover, our results provide a deeper understanding of how CARs interact physically and/or functionally with endogenous T cell molecules, which will inform the development of novel optimized immune receptors.

Lymphocytes in Cellular Therapy: Functional Regulation of CAR T Cells

Lymphocytes especially autologous T cells have been used for the treatment of numerous indications including cancers, autoimmune disorders and infectious diseases. Very recently, FDA approved Chimeric Antigen Receptor T cells (CAR T cells) therapy for relapse and refractory CD19+ B cell acute lymphoblastic leukemia (r/r B-ALL) and r/r diffuse large B cell lymphoma (r/r DLBCL) upon their remarkable success in multiple Phase I-II clinical trials. While CAR T cells are considered as major breakthrough in the field of cancer immunotherapy, the regulation of CAR T cells remains poorly understood. In this review we will discuss the strategies that regulate the CAR T cells efficacy and persistence with focus on roles of different structural component of CAR construct. Different domains of CAR construct, for example, antigen binding domain, hinge, transmembrane, and signaling domain as well as immune-regulatory cytokines have significant impact on CAR T cell efficacy. Finally, this review will highlight the strategies that will promote CAR T cells efficacy and will reduce the toxicity.

Recent advances in CAR T-cell toxicity: Mechanisms, manifestations and management

Chimeric antigen receptor (CAR) T-cell therapy is an effective new treatment for hematologic malignancies. Two CAR T-cell products are now approved for clinical use by the U.S. FDA: tisagenlecleucel for pediatric acute lymphoblastic leukemia (ALL) and adult diffuse large B-cell lymphoma subtypes (DLBCL), and axicabtagene ciloleucel for DLBCL. CAR T-cell therapies are being developed for multiple myeloma, and clear evidence of clinical activity has been generated. A barrier to widespread use of CAR T-cell therapy is toxicity, primarily cytokine release syndrome (CRS) and neurologic toxicity. Manifestations of CRS include fevers, hypotension, hypoxia, end organ dysfunction, cytopenias, coagulopathy, and hemophagocytic lymphohistiocytosis. Neurologic toxicities are diverse and include encephalopathy, cognitive defects, dysphasias, seizures, and cerebral edema. Our understanding of the pathophysiology of CRS and neurotoxicity is continually improving. Early and peak levels of certain cytokines, peak blood CAR T-cell levels, patient disease burden, conditioning chemotherapy, CAR T-cell dose, endothelial activation, and CAR design are all factors that may influence toxicity. Multiple grading systems for CAR T-cell toxicity are in use; a universal grading system is needed so that CAR T-cell products can be compared across studies. Guidelines for toxicity management vary among centers, but typically include supportive care, plus immunosuppression with tocilizumab or corticosteroids administered for severe toxicity. Gaining a better understanding of CAR T-cell toxicities and developing new therapies for these toxicities are active areas of laboratory research. Further clinical investigation of CAR T-cell toxicity is also needed. In this review, we present guidelines for management of CRS and CAR neurotoxicity.

Dawn of Chimeric Antigen Receptor T Cell Therapy in Non-Hodgkin Lymphoma

Two Chimeric Antigen Receptor (CAR) T cell therapies are now approved for the treatment of relapsed and refractory large cell lymphomas, with many others under development. The dawn of CAR T cell therapy in non-Hodgkin Lymphoma (NHL) has been characterized by rapid progress and high response rates, with a subset of patients experiencing durable benefit. In this review, we describe commercially available and investigational CAR T cell therapies, including product characteristics and clinical outcomes. We review patient selection, with an emphasis on sequencing cell therapy options in the refractory setting. Finally, we discuss durability of response, highlighting mechanisms of escape and investigational approaches to prevent and treat relapse after CAR T cell therapy.

Checkpoint Blockade Reverses Anergy in IL-13Rα2 Humanized scFv-Based CAR T Cells to Treat Murine and Canine Gliomas

We generated two humanized interleukin-13 receptor α2 (IL-13Rα2) chimeric antigen receptors (CARs), Hu07BBz and Hu08BBz, that recognized human IL-13Rα2, but not IL-13Rα1. Hu08BBz also recognized canine IL-13Rα2. Both of these CAR T cell constructs demonstrated superior tumor inhibitory effects in a subcutaneous xenograft model of human glioma compared with a humanized EGFRvIII CAR T construct used in a recent phase 1 clinical trial (ClinicalTrials.gov: NCT02209376). The Hu08BBz demonstrated a 75% reduction in orthotopic tumor growth using low-dose CAR T cell infusion. Using combination therapy with immune checkpoint blockade, humanized IL-13Rα2 CAR T cells performed significantly better when combined with CTLA-4 blockade, and humanized EGFRvIII CAR T cells’ efficacy was improved by PD-1 and TIM-3 blockade in the same mouse model, which was correlated with the levels of checkpoint molecule expression in co-cultures with the same tumor in vitro. Humanized IL-13Rα2 CAR T cells also demonstrated benefit from a self-secreted anti-CTLA-4 minibody in the same mouse model. In addition to a canine glioma cell line (J3T), canine osteosarcoma lung cancer and leukemia cell lines also express IL-13Rα2 and were recognized by Hu08BBz. Canine IL-13Rα2 CAR T cell was also generated and tested in vitro by co-culture with canine tumor cells and in vivo in an orthotopic model of canine glioma. Based on these results, we are designing a pre-clinical trial to evaluate the safety of canine IL-13Rα2 CAR T cells in dog with spontaneous IL-13Rα2-positive glioma, which will help to inform a human clinical trial design for glioblastoma using humanized scFv-based IL-13Rα2 targeting CAR T cells.

T Cells Genetically Modified to Express an Anti-B-Cell Maturation Antigen Chimeric Antigen Receptor Cause Remissions of Poor-Prognosis Relapsed Multiple Myeloma

Purpose Therapies with novel mechanisms of action are needed for multiple myeloma (MM). T cells can be genetically modified to express chimeric antigen receptors (CARs), which are artificial proteins that target T cells to antigens. B-cell maturation antigen (BCMA) is expressed by normal and malignant plasma cells but not normal essential cells. We conducted the first-in-humans clinical trial, to our knowledge, of T cells expressing a CAR targeting BCMA (CAR-BCMA). Patients and Methods Sixteen patients received 9 × 106 CAR-BCMA T cells/kg at the highest dose level of the trial; we are reporting results of these 16 patients. The patients had a median of 9.5 prior lines of MM therapy. Sixty-three percent of patients had MM refractory to the last treatment regimen before protocol enrollment. T cells were transduced with a γ-retroviral vector encoding CAR-BCMA. Patients received CAR-BCMA T cells after a conditioning chemotherapy regimen of cyclophosphamide and fludarabine. Results The overall response rate was 81%, with 63% very good partial response or complete response. Median event-free survival was 31 weeks. Responses included eradication of extensive bone marrow myeloma and resolution of soft-tissue plasmacytomas. All 11 patients who obtained an anti-MM response of partial response or better and had MM evaluable for minimal residual disease obtained bone marrow minimal residual disease-negative status. High peak blood CAR+ cell levels were associated with anti-MM responses. Cytokine-release syndrome toxicities were severe in some cases but were reversible. Blood CAR-BCMA T cells were predominantly highly differentiated CD8+ T cells 6 to 9 days after infusion. BCMA antigen loss from MM was observed. Conclusion CAR-BCMA T cells had substantial activity against heavily treated relapsed/refractory MM. Our results should encourage additional development of CAR T-cell therapies for MM.

Pre-clinical development of chimeric antigen receptor T-cell immunotherapy: Implications of design for efficacy and safety

Following the landmark approvals by the United States Food and Drug Administration, the adoptive transfer of CD19-directed chimeric antigen receptor (CAR) T-cells has now entered mainstream clinical practice for patients with chemotherapy-resistant or refractory B-cell malignancies. These approvals have followed on from a prolonged period of pre-clinical evaluation, informing the design of clinical trials that have demonstrated unprecedented efficacy in this difficult to treat patient population. However, the delivery of autologous CAR-engineered T-cell therapy is complex, costly and not without significant risk. Here we summarize the key themes of CAR T-cell preclinical development and highlight a number of innovative strategies designed to further address toxicity and improve efficacy. In concert with the emerging promise of precision genome editing, it is hoped these next generation products will increase the repertoire of clinical applications of CAR T-cell therapy in malignant and perhaps other disease settings.

Programming CAR-T cells to kill cancer

T cells engineered to express chimeric antigen receptors (CARs) that are specific for tumour antigens have led to high complete response rates in patients with haematologic malignancies. Despite this early success, major challenges to the broad application of CAR-T cells as cancer therapies remain, including treatment-associated toxicities and cancer relapse with antigen-negative tumours. Targeting solid tumours with CAR-T cells poses additional obstacles because of the paucity of tumour-specific antigens and the immunosuppressive effects of the tumour microenvironment. To overcome these challenges, T cells can be programmed with genetic modules that increase their therapeutic potency and specificity. In this Review Article, we survey major advances in the engineering of next-generation CAR-T therapies for haematologic cancers and solid cancers, with particular emphasis on strategies for the control of CAR specificity and activity and on approaches for improving CAR-T-cell persistence and overcoming immunosuppression. We also lay out a roadmap for the development of off-the-shelf CAR-T cells.

Chimeric Antigen Receptor-T Cell Therapy: Practical Considerations for Implementation in Europe

Chimeric antigen receptor (CAR)-T cell therapy is a new class of cellular immunotherapies that involves ex vivo genetic modification of T cells to incorporate an engineered CAR. After infusion into the patient, the CAR-expressing T cells recognize specific tumor targets and induce an immune response against them. The technology utilized is fundamentally different from previously available cancer treatments. Currently, most CAR-T cell therapies use autologous T cells. Tisagenlecleucel (formerly CTL019) is an anti-CD19 CAR-T cell therapy that was recently approved in the United States for the treatment of pediatric and young adult patients with relapsed/refractory B-cell acute lymphoblastic leukemia (B-ALL). Tisagenlecleucel has shown robust in vivo expansion and long-term persistence, clinically meaningful durable response and remission rates, and overall survival benefit in pediatric and young adult patients with relapsed/refractory B-ALL and in relapsed/refractory diffuse large B-cell lymphoma. Common adverse events (AEs) include cytokine release syndrome, which may require hospitalization and admission to an intensive care unit, neurological toxicities, and B-cell aplasia. These AEs are manageable when treated by an appropriately trained team. Additional research is required to further develop AE management protocols. In this review, we describe regulatory requirements, clinical considerations, and site-level requirements for clinical study implementation of CAR-T cell therapy in Europe. We also provide a case study of the European experience from the first global clinical trial for tisagenlecleucel, which may serve as a useful starting point for investigators and clinicians looking to implement CAR-T cell therapy at their institutions.

Enhancing CAR T cell persistence through ICOS and 4-1BB costimulation

Successful tumor eradication by chimeric antigen receptor-expressing (CAR-expressing) T lymphocytes depends on CAR T cell persistence and effector function. We hypothesized that CD4+ and CD8+ T cells may exhibit distinct persistence and effector phenotypes, depending on the identity of specific intracellular signaling domains (ICDs) used to generate the CAR. First, we demonstrate that the ICOS ICD dramatically enhanced the in vivo persistence of CAR-expressing CD4+ T cells that, in turn, increased the persistence of CD8+ T cells expressing either CD28- or 4-1BB-based CARs. These data indicate that persistence of CD8+ T cells was highly dependent on a helper effect provided by the ICD used to redirect CD4+ T cells. Second, we discovered that combining ICOS and 4-1BB ICDs in a third-generation CAR displayed superior antitumor effects and increased persistence in vivo. Interestingly, we found that the membrane-proximal ICD displayed a dominant effect over the distal domain in third-generation CARs. The optimal antitumor and persistence benefits observed in third-generation ICOSBBz CAR T cells required the ICOS ICD to be positioned proximal to the cell membrane and linked to the ICOS transmembrane domain. Thus, CARs with ICOS and 4-1BB ICD demonstrate increased efficacy in solid tumor models over our current 4-1BB-based CAR and are promising therapeutics for clinical testing.