Enhancing the Potency and Specificity of Engineered T Cells for Cancer Treatment

Emerging Approaches for Solid Tumor Treatment Using CAR-T Cell Therapy

Cancer immunotherapy is becoming more important in the clinical setting, especially for cancers resistant to conventional chemotherapy, including targeted therapy. Chimeric antigen receptor (CAR)-T cell therapy, which uses patient’s autologous T cells, combined with engineered T cell receptors, has shown remarkable results, with five US Food and Drug Administration (FDA) approvals to date. CAR-T cells have been very effective in hematologic malignancies, such as diffuse large B cell lymphoma (DLBCL), B cell acute lymphoblastic leukemia (B-ALL), and multiple myeloma (MM); however, its effectiveness in treating solid tumors has not been evaluated clearly. Therefore, many studies and clinical investigations are emerging to improve the CAR-T cell efficacy in solid tumors. The novel therapeutic approaches include modifying CARs in multiple ways or developing a combination therapy with immune checkpoint inhibitors and chemotherapies. In this review, we focus on the challenges and recent advancements in CAR-T cell therapy for solid tumors.

Engineering living therapeutics with synthetic biology

The steadfast advance of the synthetic biology field has enabled scientists to use genetically engineered cells, instead of small molecules or biologics, as the basis for the development of novel therapeutics. Cells endowed with synthetic gene circuits can control the localization, timing and dosage of therapeutic activities in response to specific disease biomarkers and thus represent a powerful new weapon in the fight against disease. Here, we conceptualize how synthetic biology approaches can be applied to programme living cells with therapeutic functions and discuss the advantages that they offer over conventional therapies in terms of flexibility, specificity and predictability, as well as challenges for their development. We present notable advances in the creation of engineered cells that harbour synthetic gene circuits capable of biological sensing and computation of signals derived from intracellular or extracellular biomarkers. We categorize and describe these developments based on the cell scaffold (human or microbial) and the site at which the engineered cell exerts its therapeutic function within its human host. The design of cell-based therapeutics with synthetic biology is a rapidly growing strategy in medicine that holds great promise for the development of effective treatments for a wide variety of human diseases.

A Novel Treatment for Ewing's Sarcoma: Chimeric Antigen Receptor-T Cell Therapy

Ewing's sarcoma (EWS) is a malignant and aggressive tumor type that predominantly occurs in children and adolescents. Traditional treatments such as surgery, radiotherapy and chemotherapy, while successful in the early disease stages, are ineffective in patients with metastases and relapses who often have poor prognosis. Therefore, new treatments for EWS are needed to improve patient's outcomes. Chimeric antigen receptor (CAR)-T cells therapy, a novel adoptive immunotherapy, has been developing over the past few decades, and is increasingly popular in researches and treatments of various cancers. CAR-T cell therapy has been approved by the Food and Drug Administration (FDA) for the treatment of leukemia and lymphoma. Recently, this therapeutic approach has been employed for solid tumors including EWS. In this review, we summarize the safety, specificity and clinical transformation of the treatment targets of EWS, and point out the directions for further research.

Chimeric Antigen Receptor T-Cell Therapy: The Light of Day for Osteosarcoma

Simple Summary

As a novel immunotherapy, chimeric antigen receptor (CAR) T-cell therapy has achieved encouraging results in leukemia and lymphoma. Furthermore, CAR-T cells have been explored in the treatment of osteosarcoma (OS). However, there is no strong comprehensive evidence to support their efficacy. Therefore, we reviewed the current evidence on CAR-T cells for OS to demonstrate their feasibility and provide new options for the treatment of OS.

Abstract

Osteosarcoma (OS) is the most common malignant bone tumor, arising mainly in children and adolescents. With the introduction of multiagent chemotherapy, the treatments of OS have remarkably improved, but the prognosis for patients with metastases is still poor, with a five-year survival rate of 20%. In addition, adverse effects brought by traditional treatments, including radical surgery and systemic chemotherapy, may seriously affect the survival quality of patients. Therefore, new treatments for OS await exploitation. As a novel immunotherapy, chimeric antigen receptor (CAR) T-cell therapy has achieved encouraging results in treating cancer in recent years, especially in leukemia and lymphoma. Furthermore, researchers have recently focused on CAR-T therapy in solid tumors, including OS. In this review, we summarize the safety, specificity, and clinical transformation of the targets in treating OS and point out the direction for further research.

A chimeric antigen receptor with antigen-independent OX40 signaling mediates potent antitumor activity

Although chimeric antigen receptor (CAR)-modified T cells have shown great success in the treatment of B cell malignancies, this approach has limited efficacy in patients with solid tumors. Various modifications in CAR structure have been explored to improve this efficacy, including the incorporation of two costimulatory domains. Because costimulatory signals are transduced together with T cell receptor signals during T cell activation, we engineered a type of CAR-T cells with a costimulatory signal that was activated independently from the tumor antigen to recapitulate physiological stimulation. We screened 12 costimulatory receptors to identify OX40 as the most effective CAR-T function enhancer. Our data indicated that these new CAR-T cells showed superior proliferation capability compared to current second-generation CAR-T cells. OX40 signaling reduced CAR-T cell apoptosis through up-regulation of genes encoding Bcl-2 family members and enhanced proliferation through increased activation of the NF-κB (nuclear factor κB), MAPK (mitogen-activated protein kinase), and PI3K-AKT (phosphoinositide 3-kinase to the kinase AKT) pathways. OX40 signaling not only enhanced the cytotoxicity of CAR-T cells but also reduced exhaustion markers, thereby maintaining their function in immunosuppressive tumor microenvironments. In mouse tumor models and in patients with metastatic lymphoma, these CAR-T cells exhibited robust amplification and antitumor activity. Our findings provide an alternative option for CAR-T optimization with the potential to overcome the challenge of treating solid tumors.

Navigating CAR-T cells through the solid-tumour microenvironment

The adoptive transfer of T cells that are engineered to express chimeric antigen receptors (CARs) has shown remarkable success in treating B cell malignancies but only limited efficacy against other cancer types, especially solid tumours. Compared with haematological diseases, solid tumours present a unique set of challenges, including a lack of robustly expressed, tumour-exclusive antigen targets as well as highly immunosuppressive and metabolically challenging tumour microenvironments that limit treatment safety and efficacy. Here, we review protein- and cell-engineering strategies that seek to overcome these obstacles and produce next-generation T cells with enhanced tumour specificity and sustained effector function for the treatment of solid malignancies.

The safety and clinical effects of administering a multiantigen-targeted T cell therapy to patients with multiple myeloma

Multiple myeloma (MM) is an almost always incurable malignancy of plasma cells. Despite the advent of new therapies, most patients eventually relapse or become treatment-refractory. Consequently, therapies with nonoverlapping mechanisms of action that are nontoxic and provide long-term benefit to patients with MM are greatly needed. To this end, we clinically tested an autologous multitumor-associated antigen (mTAA)-specific T cell product for the treatment of patients with high-risk, relapsed or refractory MM. In this study, we expanded polyclonal T cells from 23 patients with MM. T cells whose native T cell receptors were reactive toward five myeloma-expressed target TAAs (PRAME, SSX2, MAGEA4, Survivin, and NY-ESO-1) were enriched ex vivo. To date, we have administered escalating doses of these nonengineered mTAA-specific T cells (0.5 × 10⁷ to 2 × 10⁷ cells/m²) to 21 patients with MM, 9 of whom were at high risk of relapse after a median of 3 lines of prior therapy and 12 with active, relapsed or refractory disease after a median of 3.5 prior lines. The cells were well tolerated, with only two transient, grade III infusion-related adverse events. Furthermore, patients with active relapsed or refractory myeloma enjoyed a longer than expected progression-free survival and responders included three patients who achieved objective responses concomitant with detection of functional TAA-reactive T cell clonotypes derived from the infused mTAA product.

Engineered Cytokine Signaling to Improve CAR T Cell Effector Function

Adoptive immunotherapy with T cells genetically modified to express chimeric antigen receptors (CARs) is a promising approach to improve outcomes for cancer patients. While CAR T cell therapy is effective for hematological malignancies, there is a need to improve the efficacy of this therapeutic approach for patients with solid tumors and brain tumors. At present, several approaches are being pursued to improve the antitumor activity of CAR T cells including i) targeting multiple antigens, ii) improving T cell expansion/persistence, iii) enhancing homing to tumor sites, and iv) rendering CAR T cells resistant to the immunosuppressive tumor microenvironment (TME). Augmenting signal 3 of T cell activation by transgenic expression of cytokines or engineered cytokine receptors has emerged as a promising strategy since it not only improves CAR T cell expansion/persistence but also their ability to function in the immunosuppressive TME. In this review, we will provide an overview of cytokine biology and highlight genetic approaches that are actively being pursued to augment cytokine signaling in CAR T cells.

Hypoxia-inducible CAR expression: An answer to the on-target/off-tumor dilemma?

On-target/off-tumor toxicity is one of the major concerns regarding CAR T-cell therapy. Kosti et al.1 demonstrate that this form of toxicity can be prevented by designing a CAR whose expression is controlled by oxygen levels in the tumor environment.

Virus-specific T cells for malignancies - then, now and where to?

Purpose of Review

Virus-associated malignancies are a global health burden, constituting 10-12% of cancers worldwide. As these tumors express foreign viral antigens that can elicit specific T cell responses, virus-directed immunotherapies are a promising treatment strategy. Specifically, adoptive cell transfer of virus-specific T cells (VSTs) has demonstrated the potential to eradicate cancers associated with certain viruses.

Recent Findings

Initial studies in 1990s first showed that VSTs specific for the Epstein-Barr virus (EBVSTs) can induce complete remissions in patients with post-transplant lymphoproliferative disease. Since then, studies have validated the specificity and safety of VSTs in multiple lymphomas and solid malignancies. However, challenges remain to optimize this platform for widespread use, including enhancing potency and persistence, overcoming the immunosuppressive tumor microenvironment, and streamlining manufacturing processes that comply with regulatory requirements.

Summary

This review focuses on data from clinical trials evaluating VSTs directed against three viruses (EBV, HPV and MCPyV), as well as recent preclinical and clinical advances, and potential future directions.

Synthetic biology in the clinic: engineering vaccines, diagnostics, and therapeutics

Synthetic biology is a design-driven discipline centered on engineering novel biological functions through the discovery, characterization, and repurposing of molecular parts. Several synthetic biological solutions to critical biomedical problems are on the verge of widespread adoption and demonstrate the burgeoning maturation of the field. Here, we highlight applications of synthetic biology in vaccine development, molecular diagnostics, and cell-based therapeutics, emphasizing technologies approved for clinical use or in active clinical trials. We conclude by drawing attention to recent innovations in synthetic biology that are likely to have a significant impact on future applications in biomedicine.

Dilemma and Challenge of Immunotherapy for Pancreatic Cancer

Pancreatic cancer is a tumor with a high degree of malignancy, morbidity, and mortality. Immunotherapy is another important treatment for pancreatic cancer in addition to surgery and chemotherapy, but its application in pancreatic cancer is very limited, which is related to the unique biological behavior of pancreatic cancer and the tumor microenvironment. The immunosuppressive microenvironment of pancreatic cancer is highly heterogeneous and presents challenges for immunotherapy. The transformation of tumor immunosuppressive microenvironment contributes to the response to tumor immunotherapy, such that the tumor undergoes functional reprogramming to change from immunologically "cold" to immunologically "hot." In this review, we summarized the research and progress in immunotherapy for pancreatic cancer, including immune checkpoint inhibitors, vaccines, adoptive T cell therapy, oncolytic viruses, and immunomodulators, and suggest that individualized, combination, and precise therapy should be the main direction of future immunotherapy in pancreatic cancer.

Mechanisms Governing Immunotherapy Resistance in Pancreatic Ductal Adenocarcinoma

Pancreatic ductal adenocarcinoma (PDA) is a lethal malignancy with an overall 5-year survival rate of 10%. Disease lethality is due to late diagnosis, early metastasis and resistance to therapy, including immunotherapy. PDA creates a robust fibroinflammatory tumor microenvironment that contributes to immunotherapy resistance. While previously considered an immune privileged site, evidence demonstrates that in some cases tumor antigen-specific T cells infiltrate and preferentially accumulate in PDA and are central to tumor cell clearance and long-term remission. Nonetheless, PDA can rapidly evade an adaptive immune response using a myriad of mechanisms. Mounting evidence indicates PDA interferes with T cell differentiation into potent cytolytic effector T cells via deficiencies in naive T cell priming, inducing T cell suppression or promoting T cell exhaustion. Mechanistic research indicates that immunotherapy combinations that change the suppressive tumor microenvironment while engaging antigen-specific T cells is required for treatment of advanced disease. This review focuses on recent advances in understanding mechanisms limiting T cell function and current strategies to overcome immunotherapy resistance in PDA.

pH-responsive antibodies for therapeutic applications

Therapeutic antibodies are instrumental in improving the treatment outcome for certain disease conditions. However, to enhance their efficacy and specificity, many efforts are continuously made. One of the approaches that are increasingly explored in this field are pH-responsive antibodies capable of binding target antigens in a pH-dependent manner. We reviewed suitability and examples of these antibodies that are functionally modulated by the tumor microenvironment. Provided in this review is an update about antigens targeted by pH-responsive, sweeping, and recycling antibodies. Applicability of the pH-responsive antibodies in the engineering of chimeric antigen receptor T-cells (CAR-T) and in improving drug delivery to the brain by the enhanced crossing of the blood-brain barrier is also discussed. The pH-responsive antibodies possess strong treatment potential. They emerge as next-generation programmable engineered biologic drugs that are active only within the targeted biological space. Thus, they are valuable in targeting acidified tumor microenvironment because of improved spatial persistence and reduced on-target off-tumor toxicities. We predict that the programmable pH-dependent antibodies become powerful tools in therapies of cancer.

Precision Tools in Immuno-Oncology: Synthetic Gene Circuits for Cancer Immunotherapy

Engineered mammalian cells for medical purposes are becoming a clinically relevant reality thanks to advances in synthetic biology that allow enhanced reliability and safety of cell-based therapies. However, their application is still hampered by challenges including time-consuming design-and-test cycle iterations and costs. For example, in the field of cancer immunotherapy, CAR-T cells targeting CD19 have already been clinically approved to treat several types of leukemia, but their use in the context of solid tumors is still quite inefficient, with additional issues related to the adequate quality control for clinical use. These limitations can be overtaken by innovative bioengineering approaches currently in development. Here we present an overview of recent synthetic biology strategies for mammalian cell therapies, with a special focus on the genetic engineering improvements on CAR-T cells, discussing scenarios for the next generation of genetic circuits for cancer immunotherapy.

Programming CAR T cells to enhance anti-tumor efficacy through remodeling of the immune system

Chimeric antigen receptor (CAR) T cells have been indicated effective in treating B cell acute lymphoblastic leukemia and non-Hodgkin lymphoma and have shown encouraging results in preclinical and clinical studies. However, CAR T cells have achieved minimal success against solid malignancies because of the additional obstacles of their insufficient migration into tumors and poor amplification and persistence, in addition to antigen-negative relapse and an immunosuppressive microenvironment. Various preclinical studies are exploring strategies to overcome the above challenges. Mobilization of endogenous immune cells is also necessary for CAR T cells to obtain their optimal therapeutic effect given the importance of the innate immune responses in the elimination of malignant tumors. In this review, we focus on the recent advances in the engineering of CAR T cell therapies to restore the immune response in solid malignancies, especially with CAR T cells acting as cellular carriers to deliver immunomodulators to tumors to mobilize the endogenous immune response. We also explored the sensitizing effects of conventional treatment approaches, such as chemotherapy and radiotherapy, on CAR T cell therapy. Finally, we discuss the combination of CAR T cells with biomaterials or oncolytic viruses to enhance the anti-tumor outcomes of CAR T cell therapies in solid tumors.

Engineering Next-Generation CAR-T Cells for Better Toxicity Management

Immunoadoptive therapy with genetically modified T lymphocytes expressing chimeric antigen receptors (CARs) has revolutionized the treatment of patients with hematologic cancers. Although clinical outcomes in B-cell malignancies are impressive, researchers are seeking to enhance the activity, persistence, and also safety of CAR-T cell therapy-notably with a view to mitigating potentially serious or even life-threatening adverse events like on-target/off-tumor toxicity and (in particular) cytokine release syndrome. A variety of safety strategies have been developed by replacing or adding various components (such as OFF- and ON-switch CARs) or by combining multi-antigen-targeting OR-, AND- and NOT-gate CAR-T cells. This research has laid the foundations for a whole new generation of therapeutic CAR-T cells. Here, we review the most promising CAR-T cell safety strategies and the corresponding preclinical and clinical studies.

MyD88/CD40 signaling retains CAR T cells in a less differentiated state

Chimeric antigen receptor (CAR) T cell therapy for solid tumors has shown limited efficacy in early-phase clinical studies. The majority of CARs encode CD28 and/or 41BB costimulatory endodomains, and we explored whether MyD88 and CD40 (MC) costimulatory endodomains in CARs could improve their antitumor activity. We generated CD28-, 41BB-, and MC-CAR T cells and demonstrated that MC-CAR T cells have greater proliferative capacity and antitumor activity in repeat stimulation assays and in tumor models in vivo. Transcriptomic analysis revealed that MC-CAR T cells expressed higher levels of MYB and FOXM1, key cell cycle regulators, and were activated at baseline. After stimulation, MC-CAR T cells remained in a less differentiated state than CD28- and 41BB-CAR T cells as judged by low levels of transcription factor TBET and B lymphocyte induced maturation protein 1 expression and lower cytolytic activity in comparison with CD28- and 41BB-CAR T cells. Thus, including MyD88 and CD40 signaling domains in CARs may improve current CAR T cell therapy approaches for solid tumors.

Clinical CAR-T Cell and Oncolytic Virotherapy for Cancer Treatment

Immunotherapy has recently garnered success with the induction of clinical responses in tumors, which are traditionally associated with poor outcomes. Chimeric antigen receptor T (CAR-T) cells and oncolytic viruses (OVs) have emerged as promising cancer immunotherapy agents. Herein, we provide an overview of the current clinical status of CAR-T cell and OV therapies. While preclinical studies have demonstrated curative potential, the benefit of CAR-T cells and OVs as single-agent treatments remains limited to a subset of patients. Combinations of different targeted therapies may be required to achieve efficient, durable responses against heterogeneous tumors, as well as the microenvironment. Using a combinatorial approach to take advantage of the unique features of CAR-T cells and OVs with other treatments can produce additive therapeutic effects. This review also discusses ongoing clinical evaluations of these combination strategies for improved outcomes in treatment of resistant malignancies.

Route of 41BB/41BBL Costimulation Determines Effector Function of B7-H3-CAR.CD28ζ T Cells

B7-H3 is actively being explored as an immunotherapy target for pediatric patients with solid tumors using monoclonal antibodies or T cells expressing chimeric antigen receptors (CARs). B7-H3-CARs containing a 41BB costimulatory domain are currently favored by several groups based on preclinical studies. In this study, we initially performed a detailed analysis of T cells expressing B7-H3-CARs with different hinge/transmembrane (CD8α versus CD28) and CD28 or 41BB costimulatory domains (CD8α/CD28, CD8α/41BB, CD28/CD28, CD28/41BB). Only subtle differences in effector function were observed between CAR T cell populations in vitro. However, CD8α/CD28-CAR T cells consistently outperformed other CAR T cell populations in three animal models, resulting in a significant survival advantage. We next explored whether adding 41BB signaling to CD8α/CD28-CAR T cells would further enhance effector function. Surprisingly, incorporating 41BB signaling into the CAR endodomain had detrimental effects, while expressing 41BBL on the surface of CD8α/CD28-CAR T cells enhanced their ability to kill tumor cells in repeat stimulation assays. Furthermore, 41BBL expression enhanced CD8α/CD28-CAR T cell expansion in vivo and improved antitumor activity in one of four evaluated models. Thus, our study highlights the intricate interplay between CAR hinge/transmembrane and costimulatory domains. Based on our study, we selected CD8α/CD28-CAR T cells expressing 41BBL for early phase clinical testing.

Emerging vistas in CAR T-cell therapy: challenges and opportunities in solid tumors

INTRODUCTION

Despite advances in modern evidence-based medicine, cancer remains a major cause of global disease-associated mortality. CAR T-cell therapy is a major histocompatibility complex (MHC)-independent immunotherapy involving adoptive cell transfer. Cancer immunotherapy witnessed a major breakthrough with the US FDA approval of the first chimeric antigen receptor (CAR) T-cell therapy Kymriah^TM (tisagenlecleucel) for relapsed or refractory (R/R) acute lymphoblastic leukemia (ALL) in August 2017 followed by approval of Yescarta® (axicabtagene ciloleucel) for R/R non-Hodgkin's lymphoma (NHL) in October 2017.

AREAS COVERED

We review the potential of CAR T-cell therapy which, despite showing great promise in hematological malignancies, faces significant challenges in targeting solid tumors. We address these challenges and discuss proposed strategies to overcome them in solid tumors. We highlight the potential of CAR T-cell therapy as cancer precision medicine and briefly discuss the 'financial toxicity' of CAR T-cell therapy.

EXPERT OPINION

Taken together, we discuss various strategies to circumvent the limitations of CAR T-cell therapy in solid tumors. Despite the rapid advances in CAR NK-cell therapies, there is immense scope for CAR T-cell therapy in solid tumors. We provide a synthetic review of CAR T-cell therapy that will drive future research and harness its full potential in cancer precision medicine for solid tumors.

Intelligent cell-based therapies for cancer and autoimmune disorders

Synthetic biology, when combined with immunoengineering (designated synthetic immunology), has enabled the invention of an arsenal of genetically encoded synthetic devices and systems to reprogram cells for therapeutic purposes. The engineered intelligent cells can serve as 'living' drugs to treat a wide range of human diseases including cancer, disorders of the immune system, and infectious diseases. As the most successful example, cells of the immune system engineered with chimeric antigen receptors (CARs) have shown curative potentials for the treatment of hematological malignancies. We present herein emerging approaches of designing smart CARs to improve their safety, specificity and efficacy in cellular immunotherapy, and describe latest advances in applying CAR-engineered immune cells to target cancer and autoimmune diseases.

Engineering AvidCARs for combinatorial antigen recognition and reversible control of CAR function

T cells engineered to express chimeric antigen receptors (CAR-T cells) have shown impressive clinical efficacy in the treatment of B cell malignancies. However, the development of CAR-T cell therapies for solid tumors is hampered by the lack of truly tumor-specific antigens and poor control over T cell activity. Here we present an avidity-controlled CAR (AvidCAR) platform with inducible and logic control functions. The key is the combination of (i) an improved CAR design which enables controlled CAR dimerization and (ii) a significant reduction of antigen-binding affinities to introduce dependence on bivalent interaction, i.e. avidity. The potential and versatility of the AvidCAR platform is exemplified by designing ON-switch CARs, which can be regulated with a clinically applied drug, and AND-gate CARs specifically recognizing combinations of two antigens. Thus, we expect that AvidCARs will be a highly valuable platform for the development of controllable CAR therapies with improved tumor specificity.

Regulatory T cell therapy: Current and future design perspectives

Regulatory T cells (Tregs) maintain immune equilibrium by suppressing immune responses through various multistep contact dependent and independent mechanisms. Cellular therapy using polyclonal Tregs in transplantation and autoimmune diseases has shown promise in preclinical models and clinical trials. Although novel approaches have been developed to improve specificity and efficacy of antigen specific Treg based therapies, widespread application is currently restricted. To date, design-based approaches to improve the potency and persistence of engineered chimeric antigen receptor (CAR) Tregs are limited. Here, we describe currently available Treg based therapies, their advantages and limitations for implementation in clinical studies. We also examine various strategies for improving CAR T cell design that can potentially be applied to CAR Tregs, such as identifying co-stimulatory signalling domains that enhance suppressive ability, determining optimal scFv affinity/avidity, and co-expression of accessory molecules. Finally, we discuss the importance of tailoring CAR Treg design to suit the individual disease.

Innovative strategies to advance CAR T cell therapy for solid tumors

Current cancer treatment strategies have been advanced by chimeric antigen receptor (CAR) cell therapy, a rapidly emerging cellular immunotherapy. The numerous revolutionary achievements of CAR T cells against hematological malignancies initiated an upsurge in research on translating this therapy into a treatment for solid tumors. Unfortunately, no equivalent success has yet been achieved in treating solid tumors. The main challenges posed by solid tumors have gradually been recognized and include a lack of unique antigen targets, antigen heterogeneity, limited infiltration into the tumor, and an immunosuppressive tumor microenvironment. Surmounting the limitations of solid tumors remains critical in popularizing CAR T cell applications. Various approaches to augmenting the efficiency of CAR T cells through directly optimizing CAR constructs or through innovative combination strategies such as vaccines, biomaterials, and oncolytic virus have arisen. In addition to describing the main obstacles that restrict the promotion of CAR T cells, this paper focuses on reviewing new ongoing strategies to circumvent these limitations.

T-Cell Gene Therapy in Cancer Immunotherapy: Why It Is No Longer Just CARs on The Road

T-cells have a natural ability to fight cancer cells in the tumour microenvironment. Due to thymic selection and tissue-driven immunomodulation, these cancer-fighting T-cells are generally low in number and exhausted. One way to overcome these issues is to genetically alter T-cells to improve their effectiveness. This process can involve introducing a receptor that has high affinity for a tumour antigen, with two promising candidates known as chimeric-antigen receptors (CARs), or T-cell receptors (TCRs) with high tumour specificity. This review focuses on the editing of immune cells to introduce such novel receptors to improve immune responses to cancer. These new receptors redirect T-cells innate killing abilities to the appropriate target on cancer cells. CARs are modified receptors that recognise whole proteins on the surface of cancer cells. They have been shown to be very effective in haematological malignancies but have limited documented efficacy in solid cancers. TCRs recognise internal antigens and therefore enable targeting of a much wider range of antigens. TCRs require major histocompatibility complex (MHC) restriction but novel TCRs may have broader antigen recognition. Moreover, there are multiple cell types which can be used as targets to improve the “off-the-shelf” capabilities of these genetic engineering methods.

Engineering Strategies to Enhance TCR-Based Adoptive T Cell Therapy

T cell receptor (TCR)-based adoptive T cell therapies (ACT) hold great promise for the treatment of cancer, as TCRs can cover a broad range of target antigens. Here we summarize basic, translational and clinical results that provide insight into the challenges and opportunities of TCR-based ACT. We review the characteristics of target antigens and conventional αβ-TCRs, and provide a summary of published clinical trials with TCR-transgenic T cell therapies. We discuss how synthetic biology and innovative engineering strategies are poised to provide solutions for overcoming current limitations, that include functional avidity, MHC restriction, and most importantly, the tumor microenvironment. We also highlight the impact of precision genome editing on the next iteration of TCR-transgenic T cell therapies, and the discovery of novel immune engineering targets. We are convinced that some of these innovations will enable the field to move TCR gene therapy to the next level.

CAR-T Cells Hit the Tumor Microenvironment: Strategies to Overcome Tumor Escape

Chimeric antigen receptor (CAR) T cell therapies have demonstrated remarkable efficacy for the treatment of hematological malignancies. However, in patients with solid tumors, objective responses to CAR-T cell therapy remain sporadic and transient. A major obstacle for CAR-T cells is the intrinsic ability of tumors to evade immune responses. Advanced solid tumors are largely composed of desmoplastic stroma and immunosuppressive modulators, and characterized by aberrant cell proliferation and vascularization, resulting in hypoxia and altered nutrient availability. To mount a curative response after infusion, CAR-T cells must infiltrate the tumor, recognize their cognate antigen and perform their effector function in this hostile tumor microenvironment, to then differentiate and persist as memory T cells that confer long-term protection. Fortunately, recent advances in synthetic biology provide a wide set of tools to genetically modify CAR-T cells to overcome some of these obstacles. In this review, we provide a comprehensive overview of the key tumor intrinsic mechanisms that prevent an effective CAR-T cell antitumor response and we discuss the most promising strategies to prevent tumor escape to CAR-T cell therapy.

Evaluation of targetable biomarkers for chimeric antigen receptor T-cell (CAR-T) in the treatment of pancreatic cancer: a systematic review and meta-analysis of preclinical studies

One of the cutting edge techniques for treating cancer is the use of the patient's immune system to prevail cancerous disease. The versatility of the chimeric antigen receptor (CAR) T-cell approach in conjugation with promising treatments in haematological cancer has led to countless cases of research literature for the treatment of solid cancer. A systematic search of online databases as well as gray literature and reference lists of retrieved studies were carried out up to March 2019 to identify experimental animal studies that investigated the antigens targeted by CAR T-cell for pancreatic cancer treatment. Studies were evaluated for methodological quality using the SYstematic Review Center for Laboratory Animal Experimentation bias risk tool (SYRCLE's ROB tool). Pooled cytotoxicity ratio/percentage and 95% confidence intervals were calculated using the inverse-variance method while random-effects meta-analysis was used, taking into account conceptual heterogeneity. Heterogeneity was assessed with the Cochran Q statistic and quantified with the I2 statistic using Stata 13.0. Of the 485 identified studies, 56 were reviewed in-depth with 16 preclinical animal studies eligible for inclusion in the systematic review and 11 studies included in our meta-analysis. CAR immunotherapy significantly increased the cytotoxicity assay (percentage: 65%; 95% CI: 46%, 82%). There were no evidence for significant heterogeneity across studies [P = 0.38 (Q statistics), I2 = 7.14%] and for publication bias. The quality assessment of included studies revealed that the evidence was moderate to low quality and none of studies was judged as having a low risk of bias across all domains. CAR T-cell therapy is effective for pancreatic cancer treatment in preclinical animal studies. Further high-quality studies are needed to confirm our finding and a standard approach of this type of studies is necessary according to our assessment.

Engineering bionic T cells: signal 1, signal 2, signal 3, reprogramming and the removal of inhibitory mechanisms

Gene engineering and combinatorial approaches with other cancer immunotherapy agents may confer capabilities enabling full tumor rejection by adoptive T cell therapy (ACT). The provision of proper costimulatory receptor activity and cytokine stimuli, along with the repression of inhibitory mechanisms, will conceivably make the most of these treatment strategies. In this sense, T cells can be genetically manipulated to become refractory to suppressive mechanisms and exhaustion, last longer and differentiate into memory T cells while endowed with the ability to traffic to malignant tissues. Their antitumor effects can be dramatically augmented with permanent or transient gene transfer maneuvers to express or delete/repress genes. A combination of such interventions seeks the creation of the ultimate bionic T cell, perfected to seek and destroy cancer cells upon systemic or local intratumor delivery.

Signaling from T cell receptors (TCRs) and chimeric antigen receptors (CARs) on T cells

T cells react to foreign or self-antigens through T cell receptor (TCR) signaling. Several decades of research have delineated the mechanism of TCR signal transduction and its impact on T cell performance. This knowledge provides the foundation for chimeric antigen receptor T cell (CAR-T cell) technology, by which T cells are redirected in a major histocompatibility complex-unrestricted manner. TCR and CAR signaling plays a critical role in determining the T cell state, including exhaustion and memory. Given its artificial nature, CARs might affect or rewire signaling differently than TCRs. A better understanding of CAR signal transduction would greatly facilitate improvements to CAR-T cell technology and advance its usefulness in clinical practice. Herein, we systematically review the knowns and unknowns of TCR and CAR signaling, from the contact of receptors and antigens, proximal signaling, immunological synapse formation, and late signaling outcomes. Signaling through different T cell subtypes and how signaling is translated into practice are also discussed.

Metabolic regulation of CAR T cell function by the hypoxic microenvironment in solid tumors

The field of immunometabolism has attracted growing attention as an area at the heart of immune regulation. Upon activation, T cells undergo significant metabolic changes allowing them to mediate effector responses. The advent of chimeric antigen receptor T cell-adoptive therapy has shown some striking clinical efficacy but fails to induce sufficient antitumor response in many patients. Solid tumors put up significant opposition creating a microenvironment deficient of oxygen and glucose, depriving T cells of energy and pushing them to exhaustion. Here, we focus on immune suppressive mechanisms related to hypoxia in the tumor microenvironment and the resulting metabolic changes in T cells. New therapeutic approaches such as generating chimeric antigen receptor T cells able to withstand the challenging solid tumor microenvironment are needed.

'Off-the-shelf' allogeneic CAR T cells: development and challenges

Autologous chimeric antigen receptor (CAR) T cells have changed the therapeutic landscape in haematological malignancies. Nevertheless, the use of allogeneic CAR T cells from donors has many potential advantages over autologous approaches, such as the immediate availability of cryopreserved batches for patient treatment, possible standardization of the CAR-T cell product, time for multiple cell modifications, redosing or combination of CAR T cells directed against different targets, and decreased cost using an industrialized process. However, allogeneic CAR T cells may cause life-threatening graft-versus-host disease and may be rapidly eliminated by the host immune system. The development of next-generation allogeneic CAR T cells to address these issues is an active area of research. In this Review, we analyse the different sources of T cells for optimal allogeneic CAR-T cell therapy and describe the different technological approaches, mainly based on gene editing, to produce allogeneic CAR T cells with limited potential for graft-versus-host disease. These improved allogeneic CAR-T cell products will pave the way for further breakthroughs in the treatment of cancer.

Immunometabolism and Its Potential to Improve the Current Limitations of Immunotherapy

The last century of research in tumor immunology has culminated in the advent of immunotherapy, most notably immune checkpoint inhibitors. These drugs have shown encouraging results across a multitude of malignancies and have shifted the paradigm of cancer treatment. However, no more than 40% of patients treated with these immune checkpoint blockade inhibitors respond. Thus, resistance is a barrier to therapy that remains poorly understood. All cells require energy and biosynthetic precursors for survival, growth, and functioning, where multiple metabolic pathways allow for flexibility in how nutrients are utilized. A defining hallmark of many cancers is altered cellular metabolism, creating an imbalanced demand for nutrients within the tumor microenvironment. Immunometabolism is increasingly understood to be vital to the functions and phenotypes of a myriad of immune cell subsets. In tumors, the high demand for nutrients by the tumor drives competition between tumor cells and infiltrating immune cells, culminating in dysfunctional immune responses. This chapter discusses the recent successes in cancer immunotherapy and highlights challenges to therapy. We also outline the major metabolic processes involved in the generation of an immune response, how this can become dysregulated in the context of the tumor microenvironment, and how this contributes to resistance to immunotherapy. Finally, we explore the potential for targeting immunometabolic pathways to improve immunotherapy, and examine current trials targeting various aspects of metabolism in an attempt to improve the outcomes from immunotherapy.

Cytotoxic CD8+ Lymphocytes in the Tumor Microenvironment

In the tumor microenvironment, CD8⁺ T cells play a major role in tumor immunity. CD8⁺ T cells differentiate to cytotoxic T cells, traffic into the tumor microenvironment, and exhibit cytotoxicity against tumor cells. These processes have both positive and negative effects. Enhancements in the cytotoxic activity of tumor antigen-specific cytotoxic T cells in the tumor microenvironment are crucial for the development of cancer immunotherapy. To achieve this, several immunotherapies, including cancer vaccines, T cells engineered to express chimeric antigen receptors (CAR T cells), and bispecific T-cell engagers (BiTEs), have been developed. In contrast to cancer vaccines, CAR T cells, and BiTEs, immune checkpoint inhibitors enhance the activity of cytotoxic T cells by inhibiting the negative regulators of T cells.The total number, type, and activity of tumor antigen-specific cytotoxic T cells in the tumor microenvironment need to be clarified, particularly for the development of companion diagnostics to identify patients for whom these therapies are effective. Therefore, technologies including TCR repertoire, single-cell, and T-cell cytotoxicity analyses using BiTEs have been developed.Based on these and future innovations, the generation of effective cancer immunotherapies is anticipated.

CAR T Cell Therapy Progress and Challenges for Solid Tumors

The past two decades have marked the beginning of an unprecedented success story for cancer therapy through redirecting antitumor immunity [1]. While the mechanisms that control the initial and ongoing immune responses against tumors remain a strong research focus, the clinical development of technologies that engage the immune system to target and kill cancer cells has become a translational research priority. Early attempts documented in the late 1800s aimed at sparking immunity with cancer vaccines were difficult to interpret but demonstrated an opportunity that more than 100 years later has blossomed into the current field of cancer immunotherapy. Perhaps the most recent and greatest illustration of this is the widespread appreciation that tumors actively shut down antitumor immunity, which has led to the emergence of checkpoint pathway inhibitors that re-invigorate the body's own immune system to target cancer [2, 3]. This class of drugs, with first FDA approvals in 2011, has demonstrated impressive durable clinical responses in several cancer types, including melanoma, lung cancer, Hodgkin's lymphoma, and renal cell carcinoma, with the ongoing investigation in others. The biology and ultimate therapeutic successes of these drugs led to the 2018 Nobel Prize in Physiology or Medicine, awarded to Dr. James Allison and Dr. Tasuku Honjo for their contributions to cancer therapy [4]. In parallel to the emerging science that aided in unleashing the body's own antitumor immunity with checkpoint pathway inhibitors, researchers were also identifying ways to re-engineer antitumor immunity through adoptive cellular immunotherapy approaches. Chimeric antigen receptor (CAR)-based T cell therapy has achieved an early head start in the field, with two recent FDA approvals in 2017 for the treatment of B-cell malignancies [5]. There is an explosion of preclinical and clinical efforts to expand the therapeutic indications for CAR T cell therapies, with a specific focus on improving their clinical utility, particularly for the treatment of solid tumors. In this chapter, we will highlight the recent progress, challenges, and future perspectives surrounding the development of CAR T cell therapies for solid tumors.

Redirecting T cells to treat solid pediatric cancers

The capacity of single-agent therapy with immune checkpoint inhibitors to control solid cancers by unleashing preexisting local antitumor T cell responses has renewed interest in the broader use of T cells as anticancer therapeutics. At the same time, durable responses of refractory B-lineage malignancies to chimeric-receptor engineered T cells illustrate that T cells can be effectively redirected to cancers that lack preexisting tumor antigen-specific T cells, as most typical childhood cancers. This review summarizes strategies by which T cells can be modified to recognize defined antigens, with a focus on chimeric-receptor engineering. We provide an overview of candidate target antigens currently investigated in advanced preclinical and early clinical trials in pediatric malignancies and discuss the prerequisites for an adequate in vivo function of engineered T cells in the microenvironment of solid tumors and intrinsic and extrinsic limitations of current redirected T cell therapies. We further address innovative solutions to recruit therapeutic T cells to tumors, overcome the unreliable and heterogenous expression of most known tumor-associated antigens, and prevent functional inactivation of T cells in the hostile microenvironment of solid childhood tumors.

CAR-armed cell therapy for gliomas

Chimeric antigen receptor (CAR)-armed cell therapy has developed rapidly in recent years, especially in the treatment of leukemia. However, the treatment methods for solid tumors represented by glioma have not achieved the ideal therapeutic effect. This situation necessitates learning from chimeric antigen receptor T cell (CAR-T) treatment in other malignancies and discovering the differences between gliomas and other solid tumors. The current design idea is to enhance the targeting, regulatory effects, and adaptation of CAR-armed cells. This review traced not only clinical trials, but also several animal experiments, which might promote the development of CAR-T treatment in glioma. Furthermore, we have discussed the obstacles to CAR-T in the treatment of glioma and the current possible solutions.

Multi-antigen-targeted chimeric antigen receptor T cells for cancer therapy

The approval of two chimeric antigen receptor-modified T cell types by the US Food and Drug Administration (FDA) for the treatment of hematologic malignancies is a milestone in immunotherapy; however, the application of CAR-T cells has been limited by antigen escape and on-target, off-tumor toxicities. Therefore, it may be a potentially effective strategy to select appropriate targets and to combine multi-antigen-targeted CAR-T cells with "OR", "AND" and "NOT" Boolean logic gates. We summarize the current limitations of CAR-T cells as well as the efficacy and safety of logic-gated CAR-T cells in antitumor therapy. This review will help to explore more optimized strategies to expand the CAR-T cell therapeutic window.

Expanding CAR T cells in human platelet lysate renders T cells with in vivo longevity

BACKGROUND

Pre-clinical and clinical studies have shown that the infusion of CAR T cells with a naive-like (T_N) and central memory (T_CM) phenotype is associated with prolonged in vivo T cell persistence and superior anti-tumor effects. To optimize the maintenance of such populations during the in vitro preparation process, we explored the impact of T cell exposure to both traditional [fetal bovine serum (FBS), human AB serum (ABS)] and non-traditional [human platelet lysate (HPL) - a xeno-free protein supplement primarily used for the production of clinical grade mesenchymal stromal / stem cells (MSCs)] serum supplements.

METHODS

Second generation chimeric antigen receptor with CD28 and CD3ζ endodomain targeting prostate stem cell antigen (PSCA) (P28z) or CD19 (1928z) were constructed and used for this study. After retroviral transduction, CAR T cells were divided into 3 conditions containing either FBS, ABS or HPL and expanded for 7 days. To evaluate the effect of different sera on CAR T cell function, we performed a series of in vitro and in vivo experiments.

RESULTS

HPL-exposed CAR T cells exhibited the less differentiated T cell phenotype and gene signature, which displayed inferior short-term killing abilities (compared to their FBS- or ABS-cultured counterparts) but superior proliferative and anti-tumor effects in long-term in vitro coculture experiments. Importantly, in mouse xenograft model, HPL-exposed CAR T cells outperformed their ABS or FBS counterparts against both subcutaneous tumor (P28z T cells against Capan-1^PSCA) and systemic tumor (1928z T cells against NALM6). We further observed maintenance of less differentiated T cell phenotype in HPL-exposed 1928z T cells generated from patient's PBMCs with superior anti-tumor effect in long-term in vitro coculture experiments.

CONCLUSIONS

Our study highlights the importance of serum choice in the generation of CAR T cells for clinical use.

Advances in Chimeric Antigen Receptor T-Cell Therapies for Solid Tumors

In 2017, the US Food and Drug Administration approved the first two novel cellular immunotherapies using synthetic, engineered receptors known as chimeric antigen receptors (CARs), tisagenlecleucel (Kymriah) and axicabtagene ciloleucel (Yescarta), expressed by patient-derived T cells for the treatment of hematological malignancies expressing the B-cell surface antigen CD19 in both pediatric and adult patients. This approval marked a major milestone in the use of antigen-directed "living drugs" for the treatment of relapsed or refractory blood cancers, and with these two approvals, there is increased impetus to expand not only the target antigens but also the tumor types that can be targeted. This state-of-the-art review will focus on the challenges, advances, and novel approaches being used to implement CAR T-cell immunotherapy for the treatment of solid tumors.

Current status and hurdles for CAR-T cell immune therapy

Chimeric antigen receptor T (CAR-T) cells have emerged as novel and promising immune therapies for the treatment of multiple types of cancer in patients with hematological malignancies. There are several key components critical for development and application of CAR-T therapy. First, the design of CAR vectors can considerably affect several aspects of the physiological functions of these T cells. Moreover, despite the wide use of γ-retrovirus and lentivirus in mediating gene transfer into T cells, optimal CAR delivery systems are also being developed and evaluated. In addition, several classes of mouse models have been used to evaluate the efficacies of CAR-T cells; however, each model has its own limitations. Clinically, although surprising complete remission (CR) rates were observed in acute lymphoblastic leukemia (ALL), lymphoma, and multiple myeloma (MM), there is still a lack of specific targets for acute myeloid leukemia (AML). Leukemia relapse remains a major challenge, and its mechanism is presently under investigation. Cytokine release syndrome (CRS) and neurotoxicity are life-threatening adverse effects that need to be carefully treated. Several factors that compromise the activities of anti-solid cancer CAR-T cells have been recognized, and further improvements targeting these factors are the focus of the development of novel CAR-T cells. Overcoming the current hurdles will lead to optimal responses of CAR-T cells, thus paving the way for their wide clinical application.

Research progress and design optimization of CAR-T therapy for pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is a highly malignant cancer with limited treatment options. Chimeric antigen receptor T cells (CAR-T) are genetically engineered T cells that can specifically kill tumor cells without major histocompatibility complex restriction. Encouraging progress in CAR-T therapy for PDAC has been made in preclinical and early phase clinical trials. Challenges in CAR-T therapy for solid tumors still exist, including immunosuppressive microenvironment, interstitial barrier, poor chemotaxis, and the "on-target, off-tumor" effect. Applying neoantigens of PDAC as targets for CAR-T therapy, recognizing the CAR-T subgroup with better antitumor effect, and designing a CAR-T system targeting stroma of PDAC may contribute to develop a powerful CAR-T therapy for PDAC in the future.

NextGen cell-based immunotherapies in cancer and other immune disorders

T lymphocyte and other cell therapies have the potential to transform how we treat cancers and other diseases that have few therapeutic options. Here, we review the current progress in engineered T cell therapies and look to the future of what will establish cell therapy as the next pillar of medicine. The tools of synthetic biology along with fundamental knowledge in cell biology and immunology have enabled the development of approaches to engineer cells with enhanced capacity to recognize and treat disease safely and effectively. This along with new modes of engineering cells with CRISPR and strategies to make universal 'off-the-shelf' cell therapies will provide more rapid, flexible, and cheaper translation to the clinic.

Outsmarting and outmuscling cancer cells with synthetic and systems immunology

Adoptive T-cell therapy has shown remarkable clinical efficacy in treating refractory hematological cancers. However, challenges presented by solid tumors impede the applicability of adoptive T-cell therapy to the majority of cancers. In order to engineer effective T-cell therapies targeting solid tumors, two synergistic design criteria-T-cell therapeutic programs and anti-tumor T-cell chassis-should be taken into consideration. Recent advances in synthetic biology have enabled genetic programming of therapeutic sense-and-respond modalities in T cells. Furthermore, systems-level integration of multi-omics datum have allowed researchers to holistically profile robust anti-tumor T-cell populations. In this review, we feature novel strategies that can be incorporated into adoptive T-cell therapy design-ushering in a new paradigm of solid tumor treatment options.

Genetic Modification Strategies to Enhance CAR T Cell Persistence for Patients With Solid Tumors

Immunotherapy with chimeric antigen receptor (CAR) T cells offers a promising method to improve cure rates and decrease morbidities for patients with cancer. In this regard, CD19-specific CAR T cell therapies have achieved dramatic objective responses for a high percent of patients with CD19-positive leukemia or lymphoma. Most patients with solid tumors however, have experienced transient or no benefit from CAR T cell therapies. Novel strategies are therefore needed to improve CAR T cell function for patients with solid tumors. One obstacle for the field is limited CAR T cell persistence after infusion into patients. In this review we highlight genetic engineering strategies to improve CAR T cell persistence for enhancing antitumor activity for patients with solid tumors.

Bispecific T-cell engagers: Towards understanding variables influencing the in vitro potency and tumor selectivity and their modulation to enhance their efficacy and safety

Bispecific molecules redirecting the cytotoxicity of T-cells are a growing class of therapeutics with numerous molecules being tested in clinical trials. However, it has been a long way since the proof of concept studies in the mid 1980's. In the process we have learnt about the impact of different variables related to the bispecific molecule and the target antigen on the potency of this type of drugs. This work reviews the insights gained and how that knowledge has been used to design more potent bispecific T-cell engagers. The more recent advancement of antibodies with this modality into safety studies in non-human primates and as well as in clinical studies has revealed potential toxicity liabilities for the mode of action. Modifications in existing antibody formats and new experimental molecules designed to mitigate these problems are discussed.