Generation of Potent T-cell Immunotherapy for Cancer Using DAP12-Based, Multichain, Chimeric Immunoreceptors

Strategies to Improve Chimeric Antigen Receptor Therapies for Neuroblastoma

Chimeric antigen receptors (CARs) are among the curative immunotherapeutic approaches that exploit the antigen specificity and cytotoxicity function of potent immune cells against cancers. Neuroblastomas, the most common extracranial pediatric solid tumors with diverse characteristics, could be a promising candidate for using CAR therapies. Several methods harness CAR-modified cells in neuroblastoma to increase therapeutic efficiency, although the assessment has been less successful. Regarding the improvement of CARs, various trials have been launched to overcome insufficient capacity. However, the reasons behind the inadequate response against neuroblastoma of CAR-modified cells are still not well understood. It is essential to update the present state of comprehension of CARs to improve the efficiency of CAR therapies. This review summarizes the crucial features of CARs and their design for neuroblastoma, discusses challenges that impact the outcomes of the immunotherapeutic competence, and focuses on devising strategies currently being investigated to improve the efficacy of CARs for neuroblastoma immunotherapy.

CAR T-cell therapy for triple-negative breast cancer: Where we are

Triple-negative breast cancer (TNBC) is the most complex and challenging breast cancer subtype to treat, and chemotherapy remains the standard of care. Clinically, TNBC has a relatively high rate of recurrence and poor prognosis, which leads to a significant effort to discover novel strategies to treat patients with these tumors. Currently, chimeric antigen receptor (CAR) T cell-based immunotherapy redirects the patient's immune system directly to recognize and eradicate tumor-associated antigens (TAAs) expressing tumor cells being explored as a treatment for TNBC. A steadily increasing research in CAR T-cell therapy targeting different TAAs in TNBC has reported. In this review, we introduce the CAR technology and summarize the potential TAAs, available CARs, the antitumor activity, and the related toxicity of CARs currently under investigation for TNBC. We also highlight the potential strategies to prevent/reduce potential "on target, off tumor" toxicity induced by CAR T-cell therapy. This review will help to explore proper targets to expand further the CAR T-cell therapy for TNBCs in the clinic.

T Cell Activation Machinery: Form and Function in Natural and Engineered Immune Receptors

The impressive success of chimeric antigen receptor (CAR)-T cell therapies in treating advanced B-cell malignancies has spurred a frenzy of activity aimed at developing CAR-T therapies for other cancers, particularly solid tumors, and optimizing engineered T cells for maximum clinical benefit in many different disease contexts. A rapidly growing body of design work is examining every modular component of traditional single-chain CARs as well as expanding out into many new and innovative engineered immunoreceptor designs that depart from this template. New approaches to immune cell and receptor engineering are being reported with rapidly increasing frequency, and many recent high-quality reviews (including one in this special issue) provide comprehensive coverage of the history and current state of the art in CAR-T and related cellular immunotherapies. In this review, we step back to examine our current understanding of the structure-function relationships in natural and engineered lymphocyte-activating receptors, with an eye towards evaluating how well the current-generation CAR designs recapitulate the most desirable features of their natural counterparts. We identify key areas that we believe are under-studied and therefore represent opportunities to further improve our grasp of form and function in natural and engineered receptors and to rationally design better therapeutics.

T Cells Expressing NKG2D CAR with a DAP12 Signaling Domain Stimulate Lower Cytokine Production While Effective in Tumor Eradication

Cytokine-related toxicity associated with the use of highly active chimeric antigen receptor T cells (CAR-T cells) is a significant clinical problem. By fusing the natural killer group 2D (NKG2D) ectodomain to 4-1BB and the DAP12 cytoplasmic domain containing only one immunoreceptor tyrosine-based activation motif, we have developed a 2nd-generation (2nd-Gen) NKG2D CAR for stable expression in human T cells. When compared to T cells modified with NKG2D CAR containing the commonly used CD3ζ activation domain, T cells expressing the NKG2D-DAP12 CAR stimulated lower level release of interferon gamma (IFN-γ), tumor necrosis factor alpha (TNF-α), and interleukin (IL)-2 during tumor cell lysis and their proliferative activity was lower upon repeated antigen stimulation, although no difference between the two CARs was observed in mediating in vitro tumor cell lysis. In tumor-bearing NSG mice, both types of CAR-T cells displayed similar anti-tumor activity, being able to completely eradicate established solid tumor xenografts. However, treatment with the NKG2D-CD3ζ CAR-T cells led to the death of most mice from xenogeneic graft versus host disease starting 30 days post-CAR-T cell injection, which was associated with a higher level of cytokine release, whereas all the mice treated with the NKG2D-DAP12 CAR-T cells survived well. Thus, the incorporation of the DAP12 activation domain in a CAR design may possibly provide a potential clinical advantage in mitigating the risk of cytokine release syndrome (CRS).

Current Advances in CAR T Cell Therapy for Malignant Mesothelioma

Malignant mesothelioma is a relatively rare malignancy arising in the body's serosal surfaces, with malignant pleural mesothelioma (MPM) being the most common type. It is characterized by local spread within the thorax, poor prognosis and resistance to treatment. The development of various immunotherapeutic options has provided a new way- and hope- in treating cancer patients. Chimeric antigen receptor (CAR) T cell therapy has been proven very successful in treating hematological cancers, like leukemias and lymphomas, and its use is now being tested in solid tumors. CARs that recognize and bind to a specific tumor-associated antigen on the tumor's cell surface, are engineered and transduced into T cells. Interaction of the CAR T cell with the tumor then results in T cell activation and subsequent tumor cell lysis. In this review, we provide a current update on our previous comprehensive study summarizing the CAR T cell preclinical studies and clinical trials in MM, and discuss the future perspectives of CAR T cell therapy in this disease.

CAR-T design: Elements and their synergistic function

Chimeric antigen receptor (CAR) T cells use re-engineered cell surface receptors to specifically bind to and lyse oncogenic cells. Two clinically approved CAR-T–cell therapies have significant clinical efficacy in treating CD19-positive B cell cancers. With widespread interest to deploy this immunotherapy to other cancers, there has been great research activity to design new CAR structures to increase the range of targeted cancers and anti-tumor efficacy. However, several obstacles must be addressed before CAR-T–cell therapies can be more widely deployed. These include limiting the frequency of lethal cytokine storms, enhancing T-cell persistence and signaling, and improving target antigen specificity. We provide a comprehensive review of recent research on CAR design and systematically evaluate design aspects of the four major modules of CAR structure: the ligand-binding, spacer, transmembrane, and cytoplasmic domains, elucidating design strategies and principles to guide future immunotherapeutic discovery.

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.

Ligand-Induced Degradation of a CAR Permits Reversible Remote Control of CAR T Cell Activity In Vitro and In Vivo

Chimeric antigen receptor (CAR)-modified T cells are endowed with novel antigen specificity and are most often administered to patients without an engineered mechanism to control the CAR T cells once infused. "Suicide switches" such as the small molecule-controlled, inducible caspase-9 (iCas9) system afford the ability to selectively eliminate engineered T cells; however, these approaches are designed for all-or-none, irreversible termination of an ongoing immune response. In order to permit reversible and adjustable modulation, we have created a CAR that is capable of on-demand downregulation by fusing the CAR to a previously developed ligand-induced degradation (LID) domain. Addition of a small molecule ligand triggers exposure of a cryptic degron within the LID domain, resulting in proteasomal degradation of the CAR-LID fusion protein and loss of CAR on the surface of T cells. This fusion construct allowed for reversible and "tunable" inhibition of CAR T cell activity in vitro. Delivery of the triggering molecule in CAR-LID-treated tumor-bearing mice temporarily reduced CAR activity through modulation of CAR surface expression. The ability to more flexibly modulate CAR T cell expression through a small molecule provides a platform for controlling possible adverse side effects, as well as preclinical investigations of CAR T cell biology.

A Humanized Lym-1 CAR with Novel DAP10/DAP12 Signaling Domains Demonstrates Reduced Tonic Signaling and Increased Antitumor Activity in B-Cell Lymphoma Models

PURPOSE

The murine Lym-1 mAb targets a discontinuous epitope (Lym-1 epitope) on several subtypes of HLA-DR, which is upregulated in a majority of human B-cell lymphomas and leukemias. Unlike CD19, the Lym-1 epitope does not downregulate upon crosslinking, which may provide an advantage as a target for CAR T-cell therapy. Lym-1 CAR T cells with a conventional 4-1BB and CD3ζ (BB3z) signaling domain exhibited impaired ex vivo expansion. This study aimed to identify the underlying mechanisms and develop strategies to overcome this effect.

EXPERIMENTAL DESIGN

A functional humanized Lym-1 antibody (huLym-1-B) was identified and its scFv form was used for CAR design. To overcome observed impaired expansion in vitro, a huLym-1-B CAR using DAP10 and DAP12 (DAP) signaling domains was evaluated for ex vivo expansion and in vivo function.

RESULTS

Impaired expansion in huLym-1-B-BB3z CAR T cells was shown to be due to ligand-dependent suboptimal CAR signaling caused by interaction of the CAR binding domain and the surface of human T cells. Using the novel DAP signaling domain construct, the effects of suboptimal CAR signaling were overcome to produce huLym-1-B CAR T cells with improved expansion ex vivo and function in vivo. In addition, the Lym-1 epitope does not significantly downregulate in response to huLym-1-B-DAP CAR T cells both ex vivo and in vivo.

CONCLUSIONS

DAP intracellular domains can serve as signaling motifs for CAR, and this new construct enables nonimpaired production of huLym-1-B CAR T cells with potent in vivo antitumor efficacy.

4-1BB costimulation promotes CAR T cell survival through noncanonical NF-κB signaling

Clinical response to chimeric antigen receptor (CAR) T cell therapy is correlated with CAR T cell persistence, especially for CAR T cells that target CD19⁺ hematologic malignancies. 4-1BB-costimulated CAR (BBζ) T cells exhibit longer persistence after adoptive transfer than do CD28-costimulated CAR (28ζ) T cells. 4-1BB signaling improves T cell persistence even in the context of 28ζ CAR activation, which indicates distinct prosurvival signals mediated by the 4-1BB cytoplasmic domain. To specifically study signal transduction by CARs, we developed a cell-free, ligand-based activation and ex vivo culture system for CD19-specific CAR T cells. We observed greater ex vivo survival and subsequent expansion of BBζ CAR T cells when compared to 28ζ CAR T cells. We showed that only BBζ CARs activated noncanonical nuclear factor κB (ncNF-κB) signaling in T cells basally and that the anti-CD19 BBζ CAR further enhanced ncNF-κB signaling after ligand engagement. Reducing ncNF-κB signaling reduced the expansion and survival of anti-CD19 BBζ T cells and was associated with a substantial increase in the abundance of the most pro-apoptotic isoforms of Bim. Although our findings do not exclude the importance of other signaling differences between BBζ and 28ζ CARs, they demonstrate the necessary and nonredundant role of ncNF-κB signaling in promoting the survival of BBζ CAR T cells, which likely underlies the engraftment persistence observed with this CAR design.

A general view of CD33+ leukemic stem cells and CAR-T cells as interesting targets in acute myeloblatsic leukemia therapy

Acute myeloblastic leukemia (AML) is the most frequent acute leukemia in adulthood with very poor overall survival rates. In the past few decades, significant progresses had led to the findings of new therapeutic approaches and the better understanding of the molecular complexity of this hematologic malignancy. Leukemic stem cells (LSCs) play a key role in the initiation, progression, regression, and drug resistance of different types of leukemia. The cellular and molecular characteristics of LSCs and their mechanism in the development of leukemia had not yet been specified. Therefore, determining their cellular and molecular characteristics and creating new approaches for targeted therapy of LSCs is crucial for the future of leukemia research. For this reason, the recognition of surface maker targets on the cell surface of LSCs has attracted much attention. CD33 has been detected on blasts in most AML patients, making them an interesting target for AML therapy. Genetic engineering of T cells with chimeric antigen receptor (CAR-T cell therapy) is a novel therapeutic strategy. It extends the range of antigens available for use in adoptive T-cell immunotherapy. This review will focus on CAR-T cell approaches as well as monoclonal antibody (mAB)-based therapy, the two antibody-based therapies utilized in AML treatment.

Advances and Challenges of CAR T Cells in Clinical Trials

As a specifically programmable, living immunotherapeutic drug, chimeric antigen receptor (CAR)-modified T cells are providing an alternative treatment option for a broad variety of diseases including so far refractory cancer. By recognizing a tumor-associated antigen, the CAR triggers an anti-tumor response of engineered patient's T cells achieving lasting remissions in the treatment of leukemia and lymphoma. During the last years, significant progress was made in optimizing the CAR design, in manufacturing CAR-engineered T cells, and in the clinical management of patients showing promise to establish adoptive CAR T cell therapy as an effective treatment option in the forefront.

Engineering strategies to overcome the current roadblocks in CAR T cell therapy

T cells genetically engineered to express chimeric antigen receptors (CARs) have proven — and impressive — therapeutic activity in patients with certain subtypes of B cell leukaemia or lymphoma, with promising efficacy also demonstrated in patients with multiple myeloma. Nevertheless, various barriers restrict the efficacy and/or prevent the widespread use of CAR T cell therapies in these patients as well as in those with other cancers, particularly solid tumours. Key challenges relating to CAR T cells include severe toxicities, restricted trafficking to, infiltration into and activation within tumours, suboptimal persistence in vivo, antigen escape and heterogeneity, and manufacturing issues. The evolution of CAR designs beyond the conventional structures will be necessary to address these limitations and to expand the use of CAR T cells to a wider range of malignancies. Investigators are addressing the current obstacles with a wide range of engineering strategies in order to improve the safety, efficacy and applicability of this therapeutic modality. In this Review, we discuss the innovative designs of novel CAR T cell products that are being developed to increase and expand the clinical benefits of these treatments in patients with diverse cancers.

Chimeric antigen receptor (CAR) T cell therapy, the first approved therapeutic approach with a genetic engineering component, holds substantial promise in the treatment of a range of cancers but is nevertheless limited by various challenges, including toxicities, intrinsic and acquired resistance mechanisms, and manufacturing issues. In this Review, the authors describe the innovative approaches to the engineering of CAR T cell products that are providing solutions to these challenges and therefore have the potential to considerably improve the safety and effectiveness of treatment.

Chimeric antigen receptor (CAR) T cells have induced remarkable responses in patients with certain haematological malignancies, yet various barriers restrict the efficacy and/or prevent the widespread use of this treatment.

Investigators are addressing these challenges with engineering strategies designed to improve the safety, efficacy and applicability of CAR T cell therapy.

CARs have modular components, and therefore the optimal molecular design of the CAR can be achieved through many variations of the constituent protein domains.

Toxicities currently associated with CAR T cell therapy can be mitigated using engineering strategies to make CAR T cells safer and that potentially broaden the range of tumour-associated antigens that can be targeted by overcoming on-target, off-tumour toxicities.

CAR T cell efficacy can be enhanced by using engineering strategies to address the various challenges relating to the unique biology of diverse haematological and solid malignancies.

Strategies to address the manufacturing challenges can lead to an improved CAR T cell product for all patients.

The promise of chimeric antigen receptor (CAR) T cell therapy in multiple myeloma

A cure for multiple myeloma (MM), a malignancy of plasma cells, remains elusive. Nearly all myeloma patients will eventually relapse and develop resistance to currently available treatments. There is an unmet medical need to develop novel and effective therapies that can induce sustained responses. Early phase clinical trials using chimeric antigen receptor (CAR) T cell therapy have shown great promise in the treatment of relapsed and/or refractory MM. In this review article, we provide an overview of the CAR constructs, the gene transfer vector systems, and approaches for T cell activation and expansion. We then summarize the outcomes of several early phase clinical trials of CAR T cell therapy in MM and the novel CAR T targets that are under development. Finally, we explore the potential mechanisms that result in disease relapse after CAR T therapy and propose future directions in CAR T therapy in MM.

Recent perspective on CAR and Fcγ-CR T cell immunotherapy for cancers: Preclinical evidence versus clinical outcomes

The chimeric antigen receptor T cell (CAR-T cell) immunotherapy currently represents a hot research trend and it is expected to revolutionize the field of cancer therapy. Promising outcomes have been achieved using CAR-T cell therapy for haematological malignancies. Despite encouraging results, several challenges still pose eminent hurdles before being fully recognized. Directing CAR-T cells to target a single tumour associated antigen (TAA) as the case in haematological malignancies might be much simpler than targeting the extensive inhibitory microenvironments associated with solid tumours. This review focuses on the basic principles involved in development of CAR-T cells, emphasizing the differences between humoral IgG, T-cell receptors, CAR and Fcγ-CR constructs. It also highlights the complex inhibitory network that is usually associated with solid tumours, and tackles recent advances in the clinical studies that have provided great hope for the future use of CAR-T cell immunotherapy. While current Fcγ-CR T cell immunotherapy is in pre-clinical stage, is expected to provide a sound therapeutic approach to add to existing classical chemo- and radio-therapeutic modalities.

Phase I Study of Lentiviral-Transduced Chimeric Antigen Receptor-Modified T Cells Recognizing Mesothelin in Advanced Solid Cancers

This phase I study investigated the safety and activity of lentiviral-transduced chimeric antigen receptor (CAR)-modified autologous T cells redirected against mesothelin (CART-meso) in patients with malignant pleural mesothelioma, ovarian carcinoma, and pancreatic ductal adenocarcinoma. Fifteen patients with chemotherapy-refractory cancer (n = 5 per indication) were treated with a single CART-meso cell infusion. CART-meso cells were engineered by lentiviral transduction with a construct composed of the anti-mesothelin single-chain variable fragment derived from the mouse monoclonal antibody SS1 fused to intracellular signaling domains of 4-1BB and CD3zeta. Patients received 1-3 × 10⁷ or 1-3 × 10⁸ CART-meso cells/m² with or without 1.5 g/m² cyclophosphamide. Lentiviral-transduced CART-meso cells were well tolerated; one dose-limiting toxicity (grade 4, sepsis) occurred at 1-3 × 10⁷/m² CART-meso without cyclophosphamide. The best overall response was stable disease (11/15 patients). CART-meso cells expanded in the blood and reached peak levels by days 6-14 but persisted transiently. Cyclophosphamide pre-treatment enhanced CART-meso expansion but did not improve persistence beyond 28 days. CART-meso DNA was detected in 7/10 tumor biopsies. Human anti-chimeric antibodies (HACA) were detected in the blood of 8/14 patients. CART-meso cells were well tolerated and expanded in the blood of all patients but showed limited clinical activity. Studies evaluating a fully human anti-mesothelin CAR are ongoing.

TREM1/Dap12-based CAR-T cells show potent antitumor activity

Aim: Chimeric antigen receptor-engineered T (CAR-T) cells have gained huge success in treating hematological malignancies, yet the CD3ζ-based CAR-T therapies have not shown comparable clinical benefits in solid tumors. We designed an alternative chimeric immunoreceptor in which a single-chain variable fragment was fused to the transmembrane-cytoplasmic domains of triggering receptor expressed on myeloid (TREM1), which may show potent antitumor activity. Methods: To generate TREM1/DNAX activation protein of 12 kDa (Dap12)-based CAR-T cells, TREM1 along with DAP12 was transduced into T cells. Results: TREM1/Dap12-based CAR-T cells showed more lysis in vitro and a similar antitumor effect in mouse models compared with CD19BBζ CAR-T cells. Conclusion: In this study, we designed a TREM1/Dap12-based CAR, which was not reported previously and demonstrated that TREM1/Dap12-based CAR-T cells had potent antitumor activity in vitro and in vivo.

Synthetic TRuC receptors engaging the complete T cell receptor for potent anti-tumor response

T cells expressing CD19-targeting chimeric antigen receptors (CARs) reveal high efficacy in the treatment of B cell malignancies. Here, we report that T cell receptor fusion constructs (TRuCs) comprising an antibody-based binding domain fused to T cell receptor (TCR) subunits can effectively reprogram an intact TCR complex to recognize tumor surface antigens. Unlike CARs, TRuCs become a functional component of the TCR complex. TRuC-T cells kill tumor cells as potently as second-generation CAR-T cells, but at significant lower cytokine release and despite the absence of an extra co-stimulatory domain. TRuC-T cells demonstrate potent anti-tumor activity in both liquid and solid tumor xenograft models. In several models, TRuC-T cells are more efficacious than respective CAR-T cells. TRuC-T cells are shown to engage the signaling capacity of the entire TCR complex in an HLA-independent manner.

Chimeric Antigen Receptor (CAR) T Cell Therapy in Acute Myeloid Leukemia (AML)

Despite high response rates after initial chemotherapy in patients with acute myeloid leukemia (AML), relapses occur frequently, resulting in a five-year-survival by <30% of the patients. Hitherto, allogeneic hemotopoietic stem cell transplantation (allo-HSCT) is the best curative treatment option in intermediate and high risk AML. It is the proof-of-concept for T cell-based immunotherapies in AML based on the graft-versus-leukemia (GvL)-effect, but it also bears the risk of graft-versus-host disease. CD19-targeting therapies employing chimeric antigen receptor (CAR) T cells are a breakthrough in cancer therapy. A similar approach for myeloid malignancies is highly desirable. This article gives an overview on the state-of-the art of preclinical and clinical studies on suitable target antigens for CAR T cell therapy in AML patients.

CAR T Cells: A Snapshot on the Growing Options to Design a CAR

Adoptive cell therapy of malignant diseases with chimeric antigen receptor (CAR) modified T cells rapidly advanced from pre-clinical models to commercial approvals within 2 decades. CARs redirect patient's T cells towards cancer cells and activate the engineered cells for a cytolytic attack resulting in the destruction of the cognate target cell. CAR T cells have demonstrated their powerful capacities in inducing complete and lasting remissions of leukemia/lymphoma in an increasing number of trials worldwide. Since the early 90's, the design of CARs went through various steps of optimization until the very recent developments which include CARs with logic gating in the recognition of antigen patterns on target cells and TRUCKs with a target recognition induced delivery of immune modulating agents. Here we review the generations in CAR design, the impact of specific modifications, the strategies to improve the safety of CAR T cell therapy, and the challenges to adapt the CAR design for broader applications.

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.

Improving CART-Cell Therapy of Solid Tumors with Oncolytic Virus-Driven Production of a Bispecific T-cell Engager

T cells expressing chimeric antigen receptors (CART) have shown significant promise in clinical trials to treat hematologic malignancies, but their efficacy in solid tumors has been limited. Oncolytic viruses have the potential to act in synergy with immunotherapies due to their immunogenic oncolytic properties and the opportunity of incorporating therapeutic transgenes in their genomes. Here, we hypothesized that an oncolytic adenovirus armed with an EGFR-targeting, bispecific T-cell engager (OAd-BiTE) would improve the outcome of CART-cell therapy in solid tumors. We report that CART cells targeting the folate receptor alpha (FR-α) successfully infiltrated preestablished xenograft tumors but failed to induce complete responses, presumably due to the presence of antigen-negative cancer cells. We demonstrated that OAd-BiTE-mediated oncolysis significantly improved CART-cell activation and proliferation, while increasing cytokine production and cytotoxicity, and showed an in vitro favorable safety profile compared with EGFR-targeting CARTs. BiTEs secreted from infected cells redirected CART cells toward EGFR in the absence of FR-α, thereby addressing tumor heterogeneity. BiTE secretion also redirected CAR-negative, nonspecific T cells found in CART-cell preparations toward tumor cells. The combinatorial approach improved antitumor efficacy and prolonged survival in mouse models of cancer when compared with the monotherapies, and this was the result of an increased BiTE-mediated T-cell activation in tumors. Overall, these results demonstrated that the combination of a BiTE-expressing oncolytic virus with adoptive CART-cell therapy overcomes key limitations of CART cells and BiTEs as monotherapies in solid tumors and encourage its further evaluation in human trials. Cancer Immunol Res; 6(5); 605-16. ©2018 AACR.

The development of CAR design for tumor CAR-T cell therapy

In recent years, the chimeric antigen receptor modified T cells (Chimeric antigen receptor T cells, CAR-T) immunotherapy has developed rapidly, which has been considered the most promising therapy. Efforts to enhance the efficacy of CAR-based anti-tumor therapy have been made, such as the improvement of structures of CAR-T cells, including the development of extracellular antigen recognition receptors, intracellular co-stimulatory molecules and the combination application of CARs and synthetic small molecules. In addition, effects on the function of the CAR-T cells that the space distance between the antigen binding domains and tumor targets and the length of the spacer domains have are also being investigated. Given the fast-moving nature of this field, it is necessary to make a summary of the development of CAR-T cells. In this review, we mainly focus on the present design strategies of CAR-T cells with the hope that they can provide insights to increase the anti-tumor efficacy and safety.

High-Affinity GD2-Specific CAR T Cells Induce Fatal Encephalitis in a Preclinical Neuroblastoma Model

The GD2 ganglioside, which is abundant on the surface of neuroblastoma cells, is targeted by an FDA-approved therapeutic monoclonal antibody and is an attractive tumor-associated antigen for cellular immunotherapy. Chimeric antigen receptor (CAR)-modified T cells can have potent antitumor activity in B-cell malignancies, and trials to harness this cytolytic activity toward GD2 in neuroblastoma are under way. In an effort to enhance the antitumor activity of CAR T cells that target GD2, we generated variant CAR constructs predicted to improve the stability and the affinity of the GD2-binding, 14G2a-based, single-chain variable fragment (scFv) of the CAR and compared their properties in vivo We included the E101K mutation of GD2 scFv (GD2-E101K) that has enhanced antitumor activity against a GD2⁺ human neuroblastoma xenograft in vivo However, this enhanced antitumor efficacy in vivo was concomitantly associated with lethal central nervous system (CNS) toxicity comprised of extensive CAR T-cell infiltration and proliferation within the brain and neuronal destruction. The encephalitis was localized to the cerebellum and basal regions of the brain that display low amounts of GD2. Our results highlight the challenges associated with target antigens that exhibit shared expression on critical normal tissues. Despite the success of GD2-specific antibody therapies in the treatment of neuroblastoma, the fatal neurotoxicity of GD2-specific CAR T-cell therapy observed in our studies suggests that GD2 may be a difficult target antigen for CAR T-cell therapy without additional strategies that can control CAR T-cell function within the CNS. Cancer Immunol Res; 6(1); 36-46. ©2017 AACR.

Chimeric antigen receptor transduced T cells: Tuning up for the next generation

Chimeric antigen receptor (CAR) T cell therapy has recently achieved impressive clinical outcome in patients with CD19-positive hematologic malignancies. Extrapolation of CAR T cell treatment to solid tumors, however, has not yet yielded similar results. This might be due to intrinsic causes, e.g. insufficient CAR T cell activation or CAR toxicity as well as extrinsic factors displaying an unfavorable tumor environment for CAR T cells by raising physical and chemical barriers. In this review, we discuss the advantages as well as major obstacles of CAR T cell therapy, particularly in the context of solid tumors, and focus on efforts and novel strategies in CAR T cell development.

Emerging targets in cancer immunotherapy

The first generation of immune checkpoint inhibitors (anti-CTLA-4 and anti-PD-1/PD-L1) targeted natural immune homeostasis pathways, co-opted by cancers, to drive anti-tumor immune responses. These agents led to unprecedented results in patients with previously incurable metastatic disease and may become first-line therapies for some advanced cancers. However, these agents are efficacious in only a minority of patients. Newer strategies are becoming available that target additional immunomodulatory mechanisms to activate patients' own anti-tumor immune responses. Herein, we present a succinct summary of emerging immune targets with reported pre-clinical efficacy that have progressed to active investigation in clinical trials. These emerging targets include co-inhibitory and co-stimulatory markers of the innate and adaptive immune system. In this review, we discuss: 1) T lymphocyte markers: Lymphocyte Activation Gene 3 [LAG-3], T-cell Immunoglobulin- and Mucin-domain-containing molecule 3 [TIM-3], V-domain containing Ig Suppressor of T cell Activation [VISTA], T cell ImmunoGlobulin and ITIM domain [TIGIT], B7-H3, Inducible T-cell Co-stimulator [ICOS/ICOS-L], CD27/CD70, and Glucocorticoid-Induced TNF Receptor [GITR]; 2) macrophage markers: CD47/Signal-Regulatory Protein alpha [SIRPα] and Indoleamine-2,3-Dioxygenase [IDO]; and 3) natural killer cell markers: CD94/NKG2A and the Killer Immunoglobulin-like receptor [KIR] family. Finally, we briefly highlight combination strategies and potential biomarkers of response and resistance to these cancer immunotherapies.

Chimeric Antigen Receptor (CAR) T Cell Therapy for Malignant Pleural Mesothelioma (MPM)

Cancer immunotherapy has now become a recognized approach to treating cancers. In addition to checkpoint blockade, adoptive T cell transfer (ACT) using chimeric antigen receptors (CARs) has shown impressive clinical outcomes in leukemias and is now being explored in solid tumors. CARs are engineered receptors, stably or transiently transduced into T cells, that aim to enhance T cell effector function by recognizing and binding to a specific tumor-associated antigen. In this review, we provide a summary of CAR T cell preclinical studies and clinical trials for malignant pleural mesothelioma (MPM), a rare, locally invasive pleural cancer with poor prognosis. We list other attractive potential targets for CAR T cell therapy for MPM, and discuss augmentation strategies of CAR T cell therapy with other forms of immunotherapy in this disease.

Cancer Immunotherapy: Whence and Whither

The current concepts and practice of cancer immunotherapy evolved from classical experiments that distinguished "self" from "non-self" and the finding that humoral immunity is complemented by cellular immunity. Elucidation of the biology underlying immune checkpoints and interactions between ligands and ligand receptors that govern the immune system's ability to recognize tumor cells as foreign has led to the emergence of new strategies that mobilize the immune system to reverse this apparent tolerance. Some of these approaches have led to new therapies such as the use of mAbs to interfere with the immune checkpoint. Others have exploited molecular technologies to reengineer a subset of T cells to directly engage and kill tumor cells, particularly those of B-cell malignancies. However, before immunotherapy can become a more effective method of cancer care, there are many challenges that remain to be addressed and hurdles to overcome. Included are manipulation of tumor microenvironment (TME) to enhance T effector cell infiltration and access to the tumor, augmentation of tumor MHC expression for adequate presentation of tumor associated antigens, regulation of cytokines and their potential adverse effects, and reduced risk of secondary malignancies as a consequence of mutations generated by the various forms of genetic engineering of immune cells. Despite these challenges, the future of immunotherapy as a standard anticancer therapy is encouraging. Mol Cancer Res; 15(6); 635-50. ©2017 AACR.

Driving gene-engineered T cell immunotherapy of cancer

Chimeric antigen receptor (CAR) gene-engineered T cell therapy holds the potential to make a meaningful difference in the lives of patients with terminal cancers. For decades, cancer therapy was based on biophysical parameters, with surgical resection to debulk, followed by radiation and chemotherapy to target the rapidly growing tumor cells, while mostly sparing quiescent normal tissues. One breakthrough occurred with allogeneic bone-marrow transplant for patients with leukemia, which provided a sometimes curative therapy. The field of adoptive cell therapy for solid tumors was established with the discovery that tumor-infiltrating lymphocytes could be expanded and used to treat and even cure patients with metastatic melanoma. Tumor-specific T-cell receptors (TCRs) were identified and engineered into patient peripheral blood lymphocytes, which were also found to treat tumors. However, these were limited by patient HLA-restriction. Close behind came generation of CAR, combining the exquisite recognition of an antibody with the effector function of a T cell. The advent of CD19-targeted CARs for treating patients with multiple forms of advanced B-cell malignancies met with great success, with up to 95% response rates. Applying CAR treatment to solid tumors, however, has just begun, but already certain factors have been made clear: the tumor target is of utmost importance for clinicians to do no harm; and solid tumors respond differently to CAR therapy compared with hematologic ones. Here we review the state of clinical gene-engineered T cell immunotherapy, its successes, challenges, and future.

Can Targeting Stroma Pave the Way to Enhanced Antitumor Immunity and Immunotherapy of Solid Tumors?

Solid tumors are complex organ-like structures. The potential of normal neighboring cells to contribute to the initiation, progression, and metastasis of epithelial-derived carcinomas has long been appreciated. However, the role of host cells has proven complex. Through multiple local and systemic mechanisms, nontransformed host cells can promote transition from a tumor-resistant to tumor-permissive environment, drive neoplastic transformation of epithelial cells, promote tumor growth, progression, and metastasis, but also constrain tumorigenesis. This complexity reflects the spatially and temporally dynamic involvement of multiple cell types and processes, including the development and recruitment of inflammatory, immune, endothelial, and mesenchymal stromal cells, and the remodeling of extracellular matrix. Our mechanistic understanding, as well as our ability to translate advances in our understanding of these mechanisms for therapeutic benefit, is rapidly advancing. Further insights will depend on delineating pathways that mediate the communication networks between inflammatory and immune cells with tumor and mesenchymal stromal cells and extracellular matrix. Here, we discuss the diversity of mesenchymal stromal cell populations and how context can dictate either their promotion or constraint of tumorigenesis. We review evidence for plasticity that allows for reprogramming of stromal cells and how tumor immunogenicity and desmoplasia influence the balance of immune-independent and immune-dependent regulation of tumor growth. The pivotal roles of matrix and mesenchymal stromal cells in modulating inflammation, antitumor immunity, and the efficacy of immune-based therapies are discussed. These concepts have emerged from data obtained from tumors of multiple organs, but we focus mostly on studies of pancreatic ductal adenocarcinomas.

CAR T Cell Therapy for Solid Tumors

The field of cancer immunotherapy has been re-energized by the application of chimeric antigen receptor (CAR) T cell therapy in cancers. These CAR T cells are engineered to express synthetic receptors that redirect polyclonal T cells to surface antigens for subsequent tumor elimination. Many CARs are designed with elements that augment T cell persistence and activity. To date, CAR T cells have demonstrated tremendous success in eradicating hematologic malignancies (e.g., CD19 CARs in leukemias). However, this success has yet to be extrapolated to solid tumors, and the reasons for this are being actively investigated. We characterize some of the challenges that CAR T cells have to surmount in the solid tumor microenvironment and new approaches that are being considered to overcome these hurdles.

Chimeric Antigen Receptor T-Cells: New Approaches to Improve Their Efficacy and Reduce Toxicity

The durable remission of B-cell leukemia and lymphoma following chimeric antigen receptor (CAR) T-cell therapy has brought this new form of adoptive immunotherapy to center stage with the expectation that CAR T-cell therapy may provide similar efficacy in other hematologic and solid cancers. Herein, we review recent advances in the areas of CAR design that improve CAR T-cell proliferation, engraftment, and efficacy, as well as clinical application strategies that are designed to improve clinical efficacy while reducing the risk of toxicity and broaden patient access to this promising form of cancer immunotherapy.

Efficient tumor regression by adoptively transferred CEA-specific CAR-T cells associated with symptoms of mild cytokine release syndrome

Carcinoembryonic antigen (CEA) is a cell surface antigen highly expressed in various cancer cell types and in healthy tissues. It has the potential to be a target for chimeric antigen receptor (CAR)-modified T-cell therapy; however, the safety of this approach in terms of on-target/off-tumor effects needs to be determined. To address this issue in a clinically relevant model, we used a mouse model in which the T cells expressing CEA-specific CAR were transferred into tumor-bearing CEA-transgenic (Tg) mice that physiologically expressed CEA as a self-antigen. The adoptive transfer in conjunction with lymphodepleting and myeloablative preconditioning mediated significant tumor regression but caused weight loss in CEA-Tg, but not in wild-type mice. The weight loss was not associated with overt inflammation in the CEA-expressing gastrointestinal tract but was associated with malnutrition, reflected in elevated systemic levels of cytokines linked to anorexia, which could be controlled by the administration of an anti-IL-6 receptor monoclonal antibody without compromising efficacy. The apparent relationship between lymphodepleting and myeloablative preconditioning, efficacy, and off-tumor toxicity of CAR-T cells would necessitate the development of CEA-specific CAR-T cells with improved signaling domains that require less stringent preconditioning for their efficacy. Taken together, these results suggest that CEA-specific CAR-based adoptive T-cell therapy may be effective for patients with CEA⁺ solid tumors. Distinguishing the fine line between therapeutic efficacy and off-tumor toxicity would involve further modifications of CAR-T cells and preconditioning regimens.

Reengineering chimeric antigen receptor T cells for targeted therapy of autoimmune disease

Ideally, therapy for autoimmune diseases should eliminate pathogenic autoimmune cells while sparing protective immunity, but feasible strategies for such an approach have been elusive. Here, we show that in the antibody-mediated autoimmune disease pemphigus vulgaris (PV), autoantigen-based chimeric immunoreceptors can direct T cells to kill autoreactive B lymphocytes through the specificity of the B cell receptor (BCR). We engineered human T cells to express a chimeric autoantibody receptor (CAAR), consisting of the PV autoantigen, desmoglein (Dsg) 3, fused to CD137-CD3ζ signaling domains. Dsg3 CAAR-T cells exhibit specific cytotoxicity against cells expressing anti-Dsg3 BCRs in vitro and expand, persist, and specifically eliminate Dsg3-specific B cells in vivo. CAAR-T cells may provide an effective and universal strategy for specific targeting of autoreactive B cells in antibody-mediated autoimmune disease.

CAR models: next-generation CAR modifications for enhanced T-cell function

T cells genetically targeted with a chimeric antigen receptor (CAR) to B-cell malignancies have demonstrated tremendous clinical outcomes. With the proof in principle for CAR T cells as a therapy for B-cell malignancies being established, current and future research is being focused on adapting CAR technology to other cancers, as well as enhancing its efficacy and/or safety. The modular nature of the CAR, extracellular antigen-binding domain fused to a transmembrane domain and intracellular T-cell signaling domains, allows for optimization by replacement of the various components. These modifications are creating a whole new class of therapeutic CARs. In this review, we discuss the recent major advances in CAR design and how these modifications will impact its clinical application.

Chimeric antigen receptor T-cell therapy for solid tumors

Chimeric antigen receptor (CAR) T cells are engineered constructs composed of synthetic receptors that direct T cells to surface antigens for subsequent elimination. Many CAR constructs are also manufactured with elements that augment T-cell persistence and activity. To date, CAR T cells have demonstrated tremendous success in eradicating hematological malignancies (e.g., CD19 CARs in leukemias). This success is not yet extrapolated to solid tumors, and the reasons for this are being actively investigated. Here in this mini-review, we discuss some of the key hurdles encountered by CAR T cells in the solid tumor microenvironment.

Design of chimeric antigen receptors with integrated controllable transient functions

The ability to control T cells engineered to permanently express chimeric antigen receptors (CARs) is a key feature to improve safety. Here, we describe the development of a new CAR architecture with an integrated switch-on system that permits to control the CAR T-cell function. This system offers the advantage of a transient CAR T-cell for safety while letting open the possibility of multiple cytotoxicity cycles using a small molecule drug.

CAR T-cell immunotherapy: The path from the by-road to the freeway?

Chimeric antigen receptors are genetically encoded artificial fusion molecules that can re-program the specificity of peripheral blood polyclonal T-cells against a selected cell surface target. Unparallelled clinical efficacy has recently been demonstrated using this approach to treat patients with refractory B-cell malignancy. However, the approach is technically challenging and can elicit severe toxicity in patients. Moreover, solid tumours have largely proven refractory to this approach. In this review, we describe the important structural features of CARs and how this may influence function. Emerging clinical experience is summarized in both solid tumours and haematological malignancies. Finally, we consider the particular challenges imposed by solid tumours to the successful development of CAR T-cell immunotherapy, together with a number of innovative strategies that have been developed in an effort to reverse the balance in favour of therapeutic benefit.

Current status of chimeric antigen receptor therapy for haematological malignancies

The field of adoptive cell transfer includes chimeric antigen receptor (CAR) engineered T cells, constructs that emerged from basic research into principles of immunology and have transformed into clinically effective therapies for haematological malignancies. T cells engineered to express these artificial receptors hold great promise, but also carry significant risk. While permanent genetic modification of mature T cells appears safe, modulating their in vivo function is difficult, partly because the robust response can trigger other arms of the immune system. Suicide systems and toxicity management with cytokine blockade or signal transduction modulators have emerged as a new frontier in this field, a far cry from early problems getting CAR T cells to work at all. Currently, clinical trials in patients with relapsed or refractory B cell malignancies treated with CD19-specific CAR T cells have induced durable remissions in adults and children. Results from these trials indicate that more work needs to be done to understand biomarkers of efficacy, the role of T cell persistence and how to integrate this care into standard practice. Cell therapy will not be a 'one size fits all' class of medicine, and here we will discuss the development of this therapy and important questions for its future.

Mesothelin-Targeted CARs: Driving T Cells to Solid Tumors

Chimeric antigen receptors (CAR) are synthetic receptors that target T cells to cell-surface antigens and augment T-cell function and persistence. Mesothelin is a cell-surface antigen implicated in tumor invasion, which is highly expressed in mesothelioma and lung, pancreas, breast, ovarian, and other cancers. Its low-level expression in mesothelia, however, commands thoughtful therapeutic interventions. Encouragingly, recent clinical trials evaluating active immunization or immunoconjugates in patients with pancreatic adenocarcinoma or mesothelioma have shown responses without toxicity. Altogether, these findings and preclinical CAR therapy models using either systemic or regional T-cell delivery argue favorably for mesothelin CAR therapy in multiple solid tumors.

SIGNIFICANCE

Recent success obtained with adoptive transfer of CAR T cells targeting CD19 in patients with refractory hematologic malignancies has generated much enthusiasm for T-cell engineering and raises the prospect of implementing similar strategies for solid tumors. Mesothelin is expressed in a wide range and a high percentage of solid tumors, which we review here in detail. Mesothelin CAR therapy has the potential to treat multiple solid malignancies.