Inducible caspase-9 suicide gene controls adverse effects from alloreplete T cells after haploidentical stem cell transplantation. Blood. 2015;125:4103-4113. [PMID: 25977584 DOI: 10.1182/blood-2015-02-628354]

Augmenting the landscape of chimeric antigen receptor T-cell therapy

INTRODUCTION

The inception of recombinant DNA technology and live cell genomic alteration have paved the path for the excellence of cell and gene therapies and often provided the first curative treatment for many indications. The approval of the first Chimeric Antigen Receptor (CAR) T-cell therapy was one of the breakthrough innovations that became the headline in 2017. Currently, the therapy is primarily restricted to a few nations, and the market is growing at a CAGR (current annual growth rate) of 11.6% (2022-2032), as opposed to the established bio-therapeutic market at a CAGR of 15.9% (2023-2030). The limited technology democratization is attributed to its autologous nature, lack of awareness, therapy inclusion criteria, high infrastructure cost, trained personnel, complex manufacturing processes, regulatory challenges, recurrence of the disease, and long-term follow-ups.

AREAS COVERED

This review discusses the vision and strategies focusing on the CAR T-cell therapy democratization with mitigation plans. Further, it also covers the strategies to leverage the mRNA-based CAR T platform for building an ecosystem to ensure availability, accessibility, and affordability to the community.

EXPERT OPINION

mRNA-guided CAR T cell therapy is a rapidly growing area wherein a collaborative approach among the stakeholders is needed for its success.

CAR-T lymphocyte-based cell therapies; mechanistic substantiation, applications and biosafety enhancement with suicide genes: new opportunities to melt side effects

Immunotherapy has made significant strides in cancer treatment with strategies like checkpoint blockade antibodies and adoptive T cell transfer. Chimeric antigen receptor T cells (CAR-T) have emerged as a promising approach to combine these strategies and overcome their limitations. This review explores CAR-T cells as a living drug for cancer treatment. CAR-T cells are genetically engineered immune cells designed to target and eliminate tumor cells by recognizing specific antigens. The study involves a comprehensive literature review on CAR-T cell technology, covering structure optimization, generations, manufacturing processes, and gene therapy strategies. It examines CAR-T therapy in haematologic cancers and solid tumors, highlighting challenges and proposing a suicide gene-based mechanism to enhance safety. The results show significant advancements in CAR-T technology, particularly in structure optimization and generation. The manufacturing process has improved for broader clinical application. However, a series of inherent challenges and side effects still need to be addressed. In conclusion, CAR-T cells hold great promise for cancer treatment, but ongoing research is crucial to improve efficacy and safety for oncology patients. The proposed suicide gene-based mechanism offers a potential solution to mitigate side effects including cytokine release syndrome (the most common toxic side effect of CAR-T therapy) and the associated neurotoxicity.

Clinical grade multiparametric cell sorting and gene-marking of regulatory T cells

BACKGROUND AIMS

Regulatory T cells (Tregs) are the main mediators of peripheral tolerance. Treg-directed therapy has shown promising results in preclinical studies of diverse immunopathologies. At present, the clinical applicability of adoptive Treg transfer is limited by difficulties in generating Tregs at sufficient cell dose and purity.

METHODS

We developed a Good Manufacturing Practice (GMP) compliant method based on closed-system multiparametric Fluorescence-Activated Cell Sorting (FACS) to purify Tregs, which are then expanded in vitro and gene-marked with a clinical grade retroviral vector to enable in vivo fate tracking. Following small-scale optimization, we conducted four clinical-scale processing runs.

RESULTS

We showed that Tregs could be enriched to 87- 92% purity following FACS-sorting, and expanded and transduced to yield clinically relevant cell dose of 136-732×106 gene-marked cells, sufficient for a cell dose of at least 2 × 106 cells/kg. The expanded Tregs were highly demethylated in the FOXP3 Treg-specific demethylated region (TSDR), consistent with bona fide natural Tregs. They were suppressive in vitro, but a small percentage could secrete proinflammatory cytokines, including interferon-γ and interleukin-17A.

CONCLUSIONS

This study demonstrated the feasibility of isolating, expanding and gene-marking Tregs in clinical scale, thus paving the way for future phase I trials that will advance knowledge about the in vivo fate of transferred Tregs and its relationship with concomitant Treg-directed pharmacotherapy and clinical response.

Control of the antitumour activity and specificity of CAR T cells via organic adapters covalently tethering the CAR to tumour cells

On-target off-tumour toxicity limits the anticancer applicability of chimaeric antigen receptor (CAR) T cells. Here we show that the tumour-targeting specificity and activity of T cells with a CAR consisting of an antibody with a lysine residue that catalytically forms a reversible covalent bond with a 1,3-diketone hapten can be regulated by the concentration of a small-molecule adapter. This adapter selectively binds to the hapten and to a chosen tumour antigen via a small-molecule binder identified via a DNA-encoded library. The adapter therefore controls the formation of a covalent bond between the catalytic antibody and the hapten, as well as the tethering of the CAR T cells to the tumour cells, and hence the cytotoxicity and specificity of the cytotoxic T cells, as we show in vitro and in mice with prostate cancer xenografts. Such small-molecule switches of T-cell cytotoxicity and specificity via an antigen-independent 'universal' CAR may enhance the control and safety profile of CAR-based cellular immunotherapies.

Synthetic Biology Meets Ca2+ Release-Activated Ca2+ Channel-Dependent Immunomodulation

Many essential biological processes are triggered by the proximity of molecules. Meanwhile, diverse approaches in synthetic biology, such as new biological parts or engineered cells, have opened up avenues to precisely control the proximity of molecules and eventually downstream signaling processes. This also applies to a main Ca2+ entry pathway into the cell, the so-called Ca2+ release-activated Ca2+ (CRAC) channel. CRAC channels are among other channels are essential in the immune response and are activated by receptor-ligand binding at the cell membrane. The latter initiates a signaling cascade within the cell, which finally triggers the coupling of the two key molecular components of the CRAC channel, namely the stromal interaction molecule, STIM, in the ER membrane and the plasma membrane Ca2+ ion channel, Orai. Ca2+ entry, established via STIM/Orai coupling, is essential for various immune cell functions, including cytokine release, proliferation, and cytotoxicity. In this review, we summarize the tools of synthetic biology that have been used so far to achieve precise control over the CRAC channel pathway and thus over downstream signaling events related to the immune response.

Enhancing the safety of CAR-T cell therapy: Synthetic genetic switch for spatiotemporal control

Chimeric antigen receptor T (CAR-T) cell therapy is a promising and precise targeted therapy for cancer that has demonstrated notable potential in clinical applications. However, severe adverse effects limit the clinical application of this therapy and are mainly caused by uncontrollable activation of CAR-T cells, including excessive immune response activation due to unregulated CAR-T cell action time, as well as toxicity resulting from improper spatial localization. Therefore, to enhance controllability and safety, a control module for CAR-T cells is proposed. Synthetic biology based on genetic engineering techniques is being used to construct artificial cells or organisms for specific purposes. This approach has been explored in recent years as a means of achieving controllability in CAR-T cell therapy. In this review, we summarize the recent advances in synthetic biology methods used to address the major adverse effects of CAR-T cell therapy in both the temporal and spatial dimensions.

Updates and Challenges in ENS Cell Therapy for the Treatment of Neurointestinal Diseases

Neurointestinal diseases represent a significant challenge in clinical management with current palliative approaches failing to overcome disease and treatment-related morbidity. The recent progress with cell therapy to restore missing or defective components of the gut neuromusculature offers new hope for potential cures. This review discusses the progress that has been made in the sourcing of putative stem cells and the studies into their biology and therapeutic potential. We also explore some of the practical challenges that must be overcome before cell-based therapies can be applied in the clinical setting. Although a number of obstacles remain, the rapid advances made in the enteric neural stem cell field suggest that such therapies are on the near horizon.

Engineering Challenges and Opportunities in Autologous Cellular Cancer Immunotherapy

The use of a patient's own immune or tumor cells, manipulated ex vivo, enables Ag- or patient-specific immunotherapy. Despite some clinical successes, there remain significant barriers to efficacy, broad patient population applicability, and safety. Immunotherapies that target specific tumor Ags, such as chimeric Ag receptor T cells and some dendritic cell vaccines, can mount robust immune responses against immunodominant Ags, but evolving tumor heterogeneity and antigenic downregulation can drive resistance. In contrast, whole tumor cell vaccines and tumor lysate-loaded dendritic cell vaccines target the patient's unique tumor antigenic repertoire without prior neoantigen selection; however, efficacy can be weak when lower-affinity clones dominate the T cell pool. Chimeric Ag receptor T cell and tumor-infiltrating lymphocyte therapies additionally face challenges related to genetic modification, T cell exhaustion, and immunotoxicity. In this review, we highlight some engineering approaches and opportunities to these challenges among four classes of autologous cell therapies.

Cellular Strategies for Separating GvHD from GvL in Haploidentical Transplantation

GvHD still remains, despite the continuous improvement of transplantation platforms, a fearful complication of transplantation from allogeneic donors. Being able to separate GvHD from GvL represents the greatest challenge in the allogeneic transplant setting. This may be possible through continuous improvement of cell therapy techniques. In this review, current cell therapies are taken into consideration, which are based on the use of TCR alpha/beta depletion, CD45RA depletion, T regulatory cell enrichment, NK-cell-based immunotherapies, and suicide gene therapies in order to prevent GvHD and maximally amplify the GvL effect in the setting of haploidentical transplantation.

Comparing the Benefits and Drawbacks of Stem Cell Therapy Based on the Cell Origin or Manipulation Process: Addressing Immunogenicity

Mesenchymal stem cells (MSCs) are effective in treating autoimmune diseases and managing various conditions, such as engraftment of allogeneic islets. Additionally, autologous and HLA-matched allogeneic MSCs can aid in the engraftment of human allogeneic kidneys with or without low doses of tacrolimus, respectively. However, HLA alloantigens are problematic because cell therapy uses more HLA-mismatched allogeneic cells than autologous for convenience and standardization. In particular, HLA-mismatched MSCs showed increased Ag-specific T/B cells and reduced viability faster than HLA-matched MSCs. In CRISPR/Cas9-based cell therapy, Cas9 induce T cell activation in the recipient's immune system. Interestingly, despite their immunogenicity being limited to the cells with foreign Ags, the accumulation of HLA alloantigen-sensitized T/B cells may lead to allograft rejection, suggesting that alloantigens may have a greater scope of adverse effects than foreign Ags. To avoid alloantigen recognition, the β2-microglobulin knockout (B2MKO) system, eliminating class-I MHC, was able to avoid rejection by alloreactive CD8 T cells compared to controls. Moreover, universal donor cells in which both B2M and Class II MHC transactivator (CIITA) were knocked out was more effective in avoiding immune rejection than single KO. However, B2MKO and CIITA KO system remain to be controlled and validated for adverse effects such as the development of tumorigenicity due to deficient Ag recognition by CD8 T and CD4 T cells, respectively. Overall, better HLA-matching or depletion of HLA alloantigens prior to cell therapy can reduce repetitive transplantation through the long-term survival of allogeneic cell therapy, which may be especially important for patients seeking allogeneic transplantation.

A simeprevir-inducible molecular switch for the control of cell and gene therapies

Chemical inducer of dimerization (CID) modules can be used effectively as molecular switches to control biological processes, and thus there is significant interest within the synthetic biology community in identifying novel CID systems. To date, CID modules have been used primarily in engineering cells for in vitro applications. To broaden their utility to the clinical setting, including the potential to control cell and gene therapies, the identification of novel CID modules should consider factors such as the safety and pharmacokinetic profile of the small molecule inducer, and the orthogonality and immunogenicity of the protein components. Here we describe a CID module based on the orally available, approved, small molecule simeprevir and its target, the NS3/4A protease from hepatitis C virus. We demonstrate the utility of this CID module as a molecular switch to control biological processes such as gene expression and apoptosis in vitro, and show that the CID system can be used to rapidly induce apoptosis in tumor cells in a xenograft mouse model, leading to complete tumor regression.

Universal cell donor lines: A review of the current research

Human embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) hold promise for transplantation medicine. Diverse human leukocyte antigen (HLA) profiles necessitate autologous cells or multiple cell lines for therapeutics, incurring time and cost. Advancements in CRISPR-Cas9 and cellular therapies have led to the conceptualization of "off-the-shelf" universal cell donor lines, free of immune rejection. Overcoming immune rejection is a challenge. This review outlines strategies to modulate the major histocompatibility complex (MHC) to generate a universal cell donor line. Upon bypassing MHC mismatch, multifaceted approaches are required to generate foreign host-tolerated cells. Universal cells harbor risks, namely immune escape and tumor formation. To mitigate, we review safety mechanisms enabling donor cell inactivation or removal. Achieving a universal cell line would reduce treatment wait time, eliminate donor search, and reduce graft-versus-host disease risk without immunosuppression. The pursuit of universally tolerated cells is under way, ready to transform transplantation and regenerative medicine.

Challenges and new technologies in adoptive cell therapy

Adoptive cell therapies (ACTs) have existed for decades. From the initial infusion of tumor-infiltrating lymphocytes to the subsequent specific enhanced T cell receptor (TCR)-T and chimeric antigen receptor (CAR)-T cell therapies, many novel strategies for cancer treatment have been developed. Owing to its promising outcomes, CAR-T cell therapy has revolutionized the field of ACTs, particularly for hematologic malignancies. Despite these advances, CAR-T cell therapy still has limitations in both autologous and allogeneic settings, including practicality and toxicity issues. To overcome these challenges, researchers have focused on the application of CAR engineering technology to other types of immune cell engineering. Consequently, several new cell therapies based on CAR technology have been developed, including CAR-NK, CAR-macrophage, CAR-γδT, and CAR-NKT. In this review, we describe the development, advantages, and possible challenges of the aforementioned ACTs and discuss current strategies aimed at maximizing the therapeutic potential of ACTs. We also provide an overview of the various gene transduction strategies employed in immunotherapy given their importance in immune cell engineering. Furthermore, we discuss the possibility that strategies capable of creating a positive feedback immune circuit, as healthy immune systems do, could address the flaw of a single type of ACT, and thus serve as key players in future cancer immunotherapy.

Proximity-inducing modalities: the past, present, and future

Living systems use proximity to regulate biochemical processes. Inspired by this phenomenon, bifunctional modalities that induce proximity have been developed to redirect cellular processes. An emerging example of this class is molecules that induce ubiquitin-dependent proteasomal degradation of a protein of interest, and their initial development sparked a flurry of discovery for other bifunctional modalities. Recent advances in this area include modalities that can change protein phosphorylation, glycosylation, and acetylation states, modulate gene expression, and recruit components of the immune system. In this review, we highlight bifunctional modalities that perform functions other than degradation and have great potential to revolutionize disease treatment, while also serving as important tools in basic research to explore new aspects of biology.

Identifying highly active anti-CCR4 CAR T cells for the treatment of T-cell lymphoma

A challenge when targeting T-cell lymphoma with chimeric antigen receptor (CAR) T-cell therapy is that target antigens are often shared between T cells and tumor cells, resulting in fratricide between CAR T cells and on-target cytotoxicity on normal T cells. CC chemokine receptor 4 (CCR4) is highly expressed in many mature T-cell malignancies, such as adult T-cell leukemia/lymphoma (ATLL) and cutaneous T-cell lymphoma (CTCL), and has a unique expression profile in normal T cells. CCR4 is predominantly expressed by type-2 and type-17 helper T cells (Th2 and Th17) and regulatory T cells (Treg), but it is rarely expressed by other T helper (Th) subsets and CD8+ cells. Although fratricide in CAR T cells is generally thought to be detrimental to anticancer functions, in this study, we demonstrated that anti-CCR4 CAR T cells specifically depleted Th2 and Tregs, while sparing CD8+ and Th1 T cells. Moreover, fratricide increased the percentage of CAR+ T cells in the final product. CCR4-CAR T cells were characterized by high transduction efficiency, robust T-cell expansion, and rapid fratricidal depletion of CCR4-positive T cells during CAR transduction and expansion. Furthermore, mogamulizumab-based CCR4-CAR T cells induced superior antitumor efficacy and long-term remission in mice engrafted with human T-cell lymphoma cells. In summary, CCR4-depleted anti-CCR4 CAR T cells are enriched in Th1 and CD8+ T cells and exhibit high antitumor efficacy against CCR4-expressing T-cell malignancies.

Novel chimeric antigen receptor T cell-based immunotherapy: a perspective for triple-negative breast cancer

Triple-negative breast cancer (TNBC) is highly aggressive and does not express estrogen receptor (ER), progesterone (PR), or human epidermal growth factor receptor 2 (HER2). It has a poor prognosis, and traditional endocrine and anti-HER2 targeted therapies have low efficacy against it. In contrast, surgery, radiotherapy, and/or systemic chemotherapy are relatively effective at controlling TNBC. The resistance of TNBC to currently available clinical therapies has had a significantly negative impact on its treatment outcomes. Hence, new therapeutic options are urgently required. Chimeric antigen receptor T cell (CAR-T) therapy is a type of immunotherapy that integrates the antigen specificity of antibodies and the tumor-killing effect of T cells. CAR-T therapy has demonstrated excellent clinical efficacy against hematological cancers. However, its efficacy against solid tumors such as TNBC is inadequate. The present review aimed to investigate various aspects of CAR-T administration as TNBC therapy. We summarized the potential therapeutic targets of CAR-T that were identified in preclinical studies and clinical trials on TNBC. We addressed the limitations of using CAR-T in the treatment of TNBC in particular and solid tumors in general and explored key strategies to overcome these impediments. Finally, we comprehensively examined the advancement of CAR-T immunotherapy as well as countermeasures that could improve its efficacy as a TNBC treatment and the prognosis of patients with this type of cancer.

GD2-CART01 for Relapsed or Refractory High-Risk Neuroblastoma

BACKGROUND

Immunotherapy with chimeric antigen receptor (CAR)-expressing T cells that target the disialoganglioside GD2 expressed on tumor cells may be a therapeutic option for patients with high-risk neuroblastoma.

METHODS

In an academic, phase 1-2 clinical trial, we enrolled patients (1 to 25 years of age) with relapsed or refractory, high-risk neuroblastoma in order to test autologous, third-generation GD2-CAR T cells expressing the inducible caspase 9 suicide gene (GD2-CART01).

RESULTS

A total of 27 children with heavily pretreated neuroblastoma (12 with refractory disease, 14 with relapsed disease, and 1 with a complete response at the end of first-line therapy) were enrolled and received GD2-CART01. No failure to generate GD2-CART01 was observed. Three dose levels were tested (3-, 6-, and 10×10⁶ CAR-positive T cells per kilogram of body weight) in the phase 1 portion of the trial, and no dose-limiting toxic effects were recorded; the recommended dose for the phase 2 portion of the trial was 10×10⁶ CAR-positive T cells per kilogram. Cytokine release syndrome occurred in 20 of 27 patients (74%) and was mild in 19 of 20 (95%). In 1 patient, the suicide gene was activated, with rapid elimination of GD2-CART01. GD2-targeted CAR T cells expanded in vivo and were detectable in peripheral blood in 26 of 27 patients up to 30 months after infusion (median persistence, 3 months; range, 1 to 30). Seventeen children had a response to the treatment (overall response, 63%); 9 patients had a complete response, and 8 had a partial response. Among the patients who received the recommended dose, the 3-year overall survival and event-free survival were 60% and 36%, respectively.

CONCLUSIONS

The use of GD2-CART01 was feasible and safe in treating high-risk neuroblastoma. Treatment-related toxic effects developed, and the activation of the suicide gene controlled side effects. GD2-CART01 may have a sustained antitumor effect. (Funded by the Italian Medicines Agency and others; ClinicalTrials.gov number, NCT03373097.).

Chimeric antigen receptor T (CAR-T) cells: Novel cell therapy for hematological malignancies

Over the last decade, the emergence of several novel therapeutic approaches has changed the therapeutic perspective of human malignancies. Adoptive immunotherapy through chimeric antigen receptor T cell (CAR-T), which includes the engineering of T cells to recognize tumor-specific membrane antigens and, as a result, death of cancer cells, has created various clinical benefits for the treatment of several human malignancies. In particular, CAR-T-cell-based immunotherapy is known as a critical approach for the treatment of patients with hematological malignancies such as acute lymphoblastic leukemia (ALL), multiple myeloma (MM), chronic lymphocytic leukemia (CLL), acute myeloid leukemia (AML), Hodgkin lymphoma (HL), and non-Hodgkin's lymphoma (NHL). However, CAR-T-cell therapy of hematological malignancies is associated with various side effects. There are still extensive challenges in association with further progress of this therapeutic approach, from manufacturing and engineering issues to limitations of applications and serious toxicities. Therefore, further studies are required to enhance efficacy and minimize adverse events. In the current review, we summarize the development of CAR-T-cell-based immunotherapy and current clinical antitumor applications to treat hematological malignancies. Furthermore, we will mention the current advantages, disadvantages, challenges, and therapeutic limitations of CAR-T-cell therapy.

Tuning CARs: recent advances in modulating chimeric antigen receptor (CAR) T cell activity for improved safety, efficacy, and flexibility

Cancer immunotherapies utilizing genetically engineered T cells have emerged as powerful personalized therapeutic agents showing dramatic preclinical and clinical results, particularly in hematological malignancies. Ectopically expressed chimeric antigen receptors (CARs) reprogram immune cells to target and eliminate cancer. However, CAR T cell therapy's success depends on the balance between effective anti-tumor activity and minimizing harmful side effects. To improve CAR T cell therapy outcomes and mitigate associated toxicities, scientists from different fields are cooperating in developing next-generation products using the latest molecular cell biology and synthetic biology tools and technologies. The immunotherapy field is rapidly evolving, with new approaches and strategies being reported at a fast pace. This comprehensive literature review aims to provide an up-to-date overview of the latest developments in controlling CAR T cell activity for improved safety, efficacy, and flexibility.

Pathogen-specific T Cells: Targeting Old Enemies and New Invaders in Transplantation and Beyond

Adoptive immunotherapy with virus-specific cytotoxic T cells (VSTs) has evolved over the last three decades as a strategy to rapidly restore virus-specific immunity to prevent or treat viral diseases after solid organ or allogeneic hematopoietic cell-transplantation (allo-HCT). Since the early proof-of-principle studies demonstrating that seropositive donor-derived T cells, specific for the commonest pathogens post transplantation, namely cytomegalovirus or Epstein-Barr virus (EBV) and generated by time- and labor-intensive protocols, could effectively control viral infections, major breakthroughs have then streamlined the manufacturing process of pathogen-specific T cells (pSTs), broadened the breadth of target recognition to even include novel emerging pathogens and enabled off-the-shelf administration or pathogen-naive donor pST production. We herein review the journey of evolution of adoptive immunotherapy with nonengineered, natural pSTs against infections and virus-associated malignancies in the transplant setting and briefly touch upon recent achievements using pSTs outside this context.

Harnessing synthetic biology to engineer organoids and tissues

The development of an organism depends on intrinsic genetic programs of progenitor cells and their spatiotemporally complex extrinsic environment. Ex vivo generation of organoids from progenitor cells provides a platform for recapitulating and exploring development. Current approaches rely largely on soluble morphogens or engineered biomaterials to manipulate the physical environment, but the emerging field of synthetic biology provides a powerful toolbox to genetically manipulate cell communication, adhesion, and even cell fate. Applying these modular tools to organoids should lead to a deeper understanding of developmental principles, improved organoid models, and an enhanced capability to design tissues for regenerative purposes.

How I treat high-risk acute myeloid leukemia using preemptive adoptive cellular immunotherapy

Allogeneic hematopoietic stem cell transplantation (alloHSCT) is a potentially curative treatment for patients with high-risk acute leukemias, but unfortunately disease recurrence remains the major cause of death in these patients. Infusion of donor lymphocytes (DLI) has the potential to restore graft-versus-leukemia immunologic surveillance; however, efficacy varies across different hematologic entities. Although relapsed chronic myeloid leukemia, transplanted in chronic phase, has proven remarkably susceptible to DLI, response rates are more modest for relapsed acute myeloid leukemia and acute lymphoblastic leukemia. To prevent impending relapse, a number of groups have explored administering DLI preemptively on detection of measurable residual disease (MRD) or mixed chimerism. Evidence for the effectiveness of this strategy, although encouraging, comes from only a few, mostly single-center retrospective, nonrandomized studies. This article seeks to (1) discuss the available evidence supporting this approach while highlighting some of the inherent challenges of MRD-triggered treatment decisions post-transplant, (2) portray other forms of postremission cellular therapies, including the role of next-generation target-specific immunotherapies, and (3) provide a practical framework to support clinicians in their decision-making process when considering preemptive cellular therapy for this difficult-to-treat patient population.

Immunotherapy associated central nervous system complications in primary brain tumors

Advances clarifying the genetics and function of the immune system within the central nervous system (CNS) and brain tumor microenvironment have led to increasing momentum and number of clinical trials using immunotherapy for primary brain tumors. While neurological complications of immunotherapy in extra-cranial malignancies is well described, the CNS toxicities of immunotherapy in patients with primary brain tumors with their own unique physiology and challenges are burgeoning. This review highlights the emerging and unique CNS complications associated with immunotherapy including checkpoint inhibitors, oncolytic viruses, adoptive cell transfer/chimeric antigen receptor (CAR) T cell and vaccines for primary brain tumors, as well as reviews modalities that have been currently employed or are undergoing investigation for treatment of such toxicities.

Ex vivo and in vivo T-cell depletion in allogeneic transplantation: towards less or non-cytotoxic conditioning regimens

INTRODUCTION

Although tremendous progress has been made since the introduction of allogeneic hematopoietic stem cell transplantation (HSCT) decades ago, there are still many obstacles to overcome. A major obstacle is the presence of T-lymphocytes in the recipient and in the donor. Recipient-derived T-lymphocytes not eliminated by the conditioning regimen are a major barrier and can lead to mixed chimerism or to complete rejection of the graft. Donor-derived T-lymphocytes can induce severe acute and chronic Graft-versus-Host Disease (GvHD).

AREAS COVERED

Currently published strategies for in vivo depletion of recipient-derived T-lymphocytes are discussed including the increase of the intensity of the conditioning regimen, the addition of anti-thymocyte globulin (ATG) or the anti-CD52 monoclonal antibody Campath. For the depletion or tolerization of the donor-derived T-lymphocytes, ex vivo-T-cell depletion methods, such as positive selection of CD34+ stem cells, negative depletion of CD3+ or TcRαβ+ T-lymphocytes or the use of post-transplant cyclophosphamide (PTCy) have been developed.

EXPERT COMMENTARY

All these currently used approaches have their disadvantages and new approaches should be investigated. In this review, we discuss current and propose new possible strategies to overcome the HLA barrier by using more specific T-cell directed therapies and/or by the combinations of current methods.

Combinatorial suicide gene strategies for the safety of cell therapies

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

Long-term follow-up for the development of subsequent malignancies in patients treated with genetically modified IECs

Subsequent malignancies are well-documented complications in long-term follow-up of cancer patients. Recently, genetically modified immune effector (IE) cells have shown benefit in hematologic malignancies and are being evaluated in clinical trials for solid tumors. Although the short-term complications of IE cells are well described, there is limited literature summarizing long-term follow-up, including subsequent malignancies. We retrospectively reviewed data from 340 patients treated across 27 investigator-initiated pediatric and adult clinical trials at our center. All patients received IE cells genetically modified with γ-retroviral vectors to treat relapsed and/or refractory hematologic or solid malignancies. In a cumulative 1027 years of long-term follow-up, 13 patients (3.8%) developed another cancer with a total of 16 events (4 hematologic malignancies and 12 solid tumors). The 5-year cumulative incidence of a first subsequent malignancy in the recipients of genetically modified IE cells was 3.6% (95% confidence interval, 1.8% to 6.4%). For 11 of the 16 subsequent tumors, biopsies were available, and no sample was transgene positive by polymerase chain reaction. Replication-competent retrovirus testing of peripheral blood mononuclear cells was negative in the 13 patients with subsequent malignancies tested. Rates of subsequent malignancy were low and comparable to standard chemotherapy. These results suggest that the administration of IE cells genetically modified with γ retroviral vectors does not increase the risk for subsequent malignancy.

Emerging approaches to improve allogeneic hematopoietic cell transplantation outcomes for nonmalignant diseases

Many congenital or acquired nonmalignant diseases (NMDs) of the hematopoietic system can be potentially cured by allogeneic hematopoietic cell transplantation (HCT) with varying types of donor grafts, degrees of HLA matching, and intensity of conditioning regimens. Unique features that distinguish the use of allogeneic HCT in this population include higher rates of graft failure, immune-mediated cytopenias, and the potential to achieve long-term disease-free survival in a mixed chimerism state. Additionally, in contrast to patients with hematologic malignancies, a priority is to completely avoid graft-versus-host disease in patients with NMD because there is no theoretical beneficial graft-versus-leukemia effect that can accompany graft-versus-host responses. In this review, we discuss the current approach to each of these clinical issues and how emerging novel therapeutics hold promise to advance transplant care for patients with NMDs.

Chimeric Antigen Receptor (CAR) Regulatory T-Cells in Solid Organ Transplantation

Solid organ transplantation is the treatment of choice for various end-stage diseases, but requires the continuous need for immunosuppression to prevent allograft rejection. This comes with serious side effects including increased infection rates and development of malignancies. Thus, there is a clinical need to promote transplantation tolerance to prevent organ rejection with minimal or no immunosuppressive treatment. Polyclonal regulatory T-cells (Tregs) are a potential tool to induce transplantation tolerance, but lack specificity and therefore require administration of high doses. Redirecting Tregs towards mismatched donor HLA molecules by modifying these cells with chimeric antigen receptors (CAR) would render Tregs far more effective at preventing allograft rejection. Several studies on HLA-A2 specific CAR Tregs have demonstrated that these cells are highly antigen-specific and show a superior homing capacity to HLA-A2+ allografts compared to polyclonal Tregs. HLA-A2 CAR Tregs have been shown to prolong survival of HLA-A2+ allografts in several pre-clinical humanized mouse models. Although promising, concerns about safety and stability need to be addressed. In this review the current research, obstacles of CAR Treg therapy, and its potential future in solid organ transplantation will be discussed.

An ISCT Stem Cell Engineering Committee Position Statement on Immune Reconstitution: the importance of predictable and modifiable milestones of immune reconstitution to transplant outcomes

Allogeneic stem cell transplantation is a potentially curative therapy for some malignant and non-malignant disease. There have been substantial advances since the approaches first introduced in the 1970s, and the development of approaches to transplant with HLA incompatible or alternative donors has improved access to transplant for those without a fully matched donor. However, success is still limited by morbidity and mortality from toxicity and imperfect disease control. Here we review our emerging understanding of how reconstitution of effective immunity after allogeneic transplant can protect from these events and improve outcomes. We provide perspective on milestones of immune reconstitution that are easily measured and modifiable.

T cells expressing CD5/CD7 bispecific chimeric antigen receptors with fully human heavy-chain-only domains mitigate tumor antigen escape

Bispecific chimeric antigen receptor T-cell (CAR-T) therapies have shown promising results in clinical trials for advanced B-cell malignancies. However, it is challenging to broaden the success of bispecific CAR-T therapies to treat refractory/relapse (r/r) T-cell leukemia/lymphoma because targeting multiple T-cell-expressing antigens leads to exacerbated CAR-T cell fratricide and potential safety concerns. Fully human heavy chain variable (FHVH) antibodies that specifically target CD5 or CD7 were screened and constructed to CD5/CD7 bispecific CARs. A truncated Epidermal growth factor receptor were integrated into CAR constructs to address safety concerns. To tackle the fratricidal issue of CAR-T cells targeting T-cell-pan marker(s), CRISPR/Cas9-based CD5 and CD7 genes knockout were performed before lentiviral transduction of bispecific CARs. Functional comparison between different bispecific CAR structures: tandem CARs and dual CAR were performed in vitro and in vivo to determine the optimal construct suitable for addressing T-cell malignancy antigen escape in clinical setting. Knockout of CD5 and CD7 prevents fratricide of CD5/CD7 bispecific CAR-T cells, and FHVH-derived CD5/CD7 bispecific CAR-T cells demonstrate potent antitumor activity in vitro and in vivo. The fratricide-resistant FHVH-derived CD5/CD7 bispecific CAR-T cells have potent antitumor activity against T-cell malignancies, and tandem CARs are more effective than dual CAR in preventing tumor escape in heterogeneous leukemic cells. The meaningful clinical efficacy and safety of tandem CD5/CD7 CAR-T cells deserve to be explored urgently.

Engineering CAR T cells for enhanced efficacy and safety

Despite its success in treating hematologic malignancies, chimeric antigen receptor (CAR) T cell therapy faces two major challenges which hinder its broader applications: the limited effectiveness against solid tumors and the nonspecific toxicities. To address these concerns, researchers have used synthetic biology approaches to develop optimization strategies. In this review, we discuss recent improvements on the CAR and other non-CAR molecules aimed to enhance CAR T cell efficacy and safety. We also highlight the development of different types of inducible CAR T cells that can be controlled by environmental cues and/or external stimuli. These advancements are bringing CAR T therapy one step closer to safer and wider applications, especially for solid tumors.

Improving CAR-T immunotherapy: Overcoming the challenges of T cell exhaustion

Chimeric antigen receptor (CAR) T cell therapy has emerged as a cancer treatment with enormous potential, demonstrating impressive antitumor activity in the treatment of hematological malignancies. However, CAR T cell exhaustion is a major limitation to their efficacy, particularly in the application of CAR T cells to solid tumors. CAR T cell exhaustion is thought to be due to persistent antigen stimulation, as well as an immunosuppressive tumor microenvironment, and mitigating exhaustion to maintain CAR T cell effector function and persistence and achieve clinical potency remains a central challenge. Here, we review the underlying mechanisms of exhaustion and discuss emerging strategies to prevent or reverse exhaustion through modifications of the CAR receptor or CAR independent pathways. Additionally, we discuss the potential of these strategies for improving clinical outcomes of CAR T cell therapy.

Design and characterization of a salicylic acid-inducible gene expression system for Jurkat cells

With continued progress in cell and gene therapies, there is an immediate need for exogenously tunable gene expression systems with safe and predictable behavior in specific human cell types. Here, we demonstrate the ability of the salicylic acid (SA)-inducible MarR repressor protein from Escherichia coli to regulate target gene expression in a human T lymphocyte cell line. Two lentiviral vectors, one encoding an enhanced green fluorescent protein (EGFP) reporter cassette and the other a repressor cassette, were sequentially transduced into Jurkat cells, using fluorescence-activated cell sorting (FACS) to isolate stable Jurkat progeny. As a result, EGFP expression was repressed by MarR and was inducible upon the addition of SA (~1.3 fold). This represents the first example of functional expression of bacterial MarR in mammalian cells, and opens the possibility for further development of regulated, SA-tunable gene expression system for T-cells.

Advances in Allogeneic Cancer Cell Therapy and Future Perspectives on “Off-the-Shelf” T Cell Therapy Using iPSC Technology and Gene Editing

The concept of allogeneic cell therapy was first presented over 60 years ago with hematopoietic stem cell transplantation. However, complications such as graft versus host disease (GVHD) and regimen-related toxicities remained as major obstacles. To maximize the effect of graft versus leukemia, while minimizing the effect of GVHD, donor lymphocyte infusion was utilized. This idea, which was used against viral infections, postulated that adoptive transfer of virus-specific cytotoxic T lymphocytes could reconstitute specific immunity and eliminate virus infected cells and led to the idea of banking third party cytotoxic T cells (CTLs). T cell exhaustion sometimes became a problem and difficulty arose in creating robust CTLs. However, the introduction of induced pluripotent stem cells (iPSCs) lessens such problems, and by using iPSC technology, unlimited numbers of allogeneic rejuvenated CTLs with robust and proliferative cytotoxic activity can be created. Despite this revolutionary concept, several concerns still exist, such as immunorejection by recipient cells and safety issues of gene editing. In this review, we describe approaches to a feasible “off-the-shelf” therapy that can be distributed rapidly worldwide. We also offer perspectives on the future of allogeneic cell cancer immunotherapy.

The Implementation of TNFRSF Co-Stimulatory Domains in CAR-T Cells for Optimal Functional Activity

The Tumor Necrosis Factor Receptor Superfamily (TNFRSF) is a large and important immunoregulatory family that provides crucial co-stimulatory signals to many if not all immune effector cells. Each co-stimulatory TNFRSF member has a distinct expression profile and a unique functional impact on various types of cells and at different stages of the immune response. Correspondingly, exploiting TNFRSF-mediated signaling for cancer immunotherapy has been a major field of interest, with various therapeutic TNFRSF-exploiting anti-cancer approaches such as 4-1BB and CD27 agonistic antibodies being evaluated (pre)clinically. A further application of TNFRSF signaling is the incorporation of the intracellular co-stimulatory domain of a TNFRSF into so-called Chimeric Antigen Receptor (CAR) constructs for CAR-T cell therapy, the most prominent example of which is the 4-1BB co-stimulatory domain included in the clinically approved product Kymriah. In fact, CAR-T cell function can be clearly influenced by the unique co-stimulatory features of members of the TNFRSF. Here, we review a select group of TNFRSF members (4-1BB, OX40, CD27, CD40, HVEM, and GITR) that have gained prominence as co-stimulatory domains in CAR-T cell therapy and illustrate the unique features that each confers to CAR-T cells.

Optimizing allogeneic grafts in hematopoietic stem cell transplantation

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is widely used in the treatment of hematological diseases. It is well known that allogeneic grafts play a key role in predicting transplantation prognosis. Hematopoietic stem cells (HSCs) are a functional part of grafts and are capable of reconstructing hematopoiesis and immunity, but purified HSCs have not been identified or isolated to date. In clinical practice, allogeneic grafts have been optimized to improve transplantation outcomes. The optimized grafts are considered to engraft successfully, reconstruct immunity rapidly, and exert a graft-vs-leukemia (GVL) effect without causing severe graft-vs-host disease (GvHD). In the last several decades, considerable efforts have been made in searching for optimized grafts based on different graft manipulation approaches and different graft sources. Currently, there is no uniform standard for optimized grafts in allogeneic transplantation. In the future, sorting out the cellular elements responsible for the effects of allo-HSCT might be a research direction for further optimization of grafts. In this review, we propose the concept of optimized grafts and summarize the recent advances made in the process of optimizing grafts.

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

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

Trial Watch: Adoptive TCR-Engineered T-Cell Immunotherapy for Acute Myeloid Leukemia

Simple Summary

Acute myeloid leukemia (AML) is a type of blood cancer with an extremely grim prognosis. This is due to the fact that the majority of patients will relapse after frontline treatment. Overall survival of relapsed AML is very low, and treatment options are few. T lymphocytes harnessed with antitumor T-cell receptors (TCRs) can produce objective clinical responses in certain cancers, such as melanoma, but have not entered the main road for AML. In this review, we describe the current status of the field of TCR-T-cell therapies for AML.

Abstract

Despite the advent of novel therapies, acute myeloid leukemia (AML) remains associated with a grim prognosis. This is exemplified by 5-year overall survival rates not exceeding 30%. Even with frontline high-intensity chemotherapy regimens and allogeneic hematopoietic stem cell transplantation, the majority of patients with AML will relapse. For these patients, treatment options are few, and novel therapies are urgently needed. Adoptive T-cell therapies represent an attractive therapeutic avenue due to the intrinsic ability of T lymphocytes to recognize tumor cells with high specificity and efficiency. In particular, T-cell therapies focused on introducing T-cell receptors (TCRs) against tumor antigens have achieved objective clinical responses in solid tumors such as synovial sarcoma and melanoma. However, contrary to chimeric antigen receptor (CAR)-T cells with groundbreaking results in B-cell malignancies, the use of TCR-T cells for hematological malignancies is still in its infancy. In this review, we provide an overview of the status and clinical advances in adoptive TCR-T-cell therapy for the treatment of AML.

Real-world experience: Introduction of T cell replete haploidentical transplantations in a single center

Objectives

The aim of this study was to describe real-world data on outcomes of T cell replete haploidentical hematopoietic stem cell transplantation (HSCT) after the introduction of this modality in a single center and to compare them with different donor types.

Method

Outcomes of 30 consecutive patients with hematological malignancies that received T cell replete haploidentical HSCT with posttransplantation cyclophosphamide (PTCY) from 2016 to 2018 in our center were analyzed and compared to the outcome of human leukocyte antigen (HLA)-related and unrelated matched donor HSCT (n = 97) and to a historical cohort of T cell depleted haploidentical HSCT (n = 11).

Results

One year graft-versus-host-free, relapse-free survival in haploidentical HSCT was comparable with other donor types (haplo 40%, matched related donor [MRD] 33%, matched unrelated donor [MUD] 25%, p = 0.55). Non relapse mortality was high in haploidentical HSCT (50%), mostly due to infectious complications. However, relapse rates were only 3%, and OS and progression-free survival after 1 year were 47% and thereby also similar to HLA-matched HSCT in our center (MRD 53%, MUD 48%).

Conclusion

Our data show that T cell replete haploidentical HSCT has similar outcomes to HLA identical HSCT after introduction in our center. More strict adaptation on infection prevention was a crucial aspect of our learning curve. Overall, this type of transplantation is a feasible option when lacking an HLA-identical donor. This option has advantages over an unrelated donor as it brings less logistical challenges than MUD transplantations.

European Myeloma Network perspective on CAR T-Cell therapies for multiple myeloma

Chimeric antigen receptor (CAR) T cells (CAR-T) have dramatically changed the treatment landscape of B-cell malignancies, providing a potential cure for relapsed/refractory patients. Long-term responses in patients with acute lymphoblastic leukemia and non Hodgkin lymphomas have encouraged further development in myeloma. In particular, B-cell maturation antigen (BCMA)-targeted CAR-T have established very promising results in heavily pre-treated patients. Moreover, CAR-T targeting other antigens (i.e., SLAMF7 and CD44v6) are currently under investigation. However, none of these current autologous therapies have been approved, and despite high overall response rates across studies, main issues such as long-term outcome, toxicities, treatment resistance, and management of complications limit as yet their widespread use. Here, we critically review the most important pre-clinical and clinical findings, recent advances in CAR-T against myeloma, as well as discoveries in the biology of a still incurable disease, that, all together, will further improve safety and efficacy in relapsed/refractory patients, urgently in need of novel treatment options.

Optimization of CAR-T Cell-Based Therapies Using Small-Molecule-Based Safety Switches

Chimeric antigen receptor T cell therapy has demonstrated antileukemia efficacy. However, this therapeutic approach is hampered by severe cytokine release syndrome, which is a major impediment to its widespread application in the clinic. The safety of this approach can be improved by engineering a rapid and reversible "off" or "on" safety switch for CAR-T cells. Cutting-edge investigations combining the advantages of genetic engineering and chemical technology have led to the invention of small-molecule-based safety switches for CAR-T cells. Small molecules such as FITC, folate, rimiducid, rapamycin, proteolysis-targeting chimera (PROTAC) compounds, and dasatinib are being investigated to design such safety switches. Optimized CAR-T cells may have enhanced therapeutic efficiency with fewer adverse effects. Herein we summarize and classify current novel small-molecule-based safety switches for CAR-T cells that aim to provide pharmacological control over the activities and toxicities associated with CAR-T cell-based cancer immunotherapies.

Efficient Genetic Safety Switches for Future Application of iPSC-Derived Cell Transplants

Induced pluripotent stem cell (iPSC)-derived cell products hold great promise as a potential cell source in personalized medicine. As concerns about the potential risk of graft-related severe adverse events, such as tumor formation from residual pluripotent cells, currently restrict their applicability, we established an optimized tool for therapeutic intervention that allows drug-controlled, specific and selective ablation of either iPSCs or the whole graft through genetic safety switches. To identify the best working system, different tools for genetic iPSC modification, promoters to express safety switches and different safety switches were combined. Suicide effects were slightly stronger when the suicide gene was delivered through lentiviral (LV) vectors compared to integration into the AAVS1 locus through TALEN technology. An optimized HSV-thymidine kinase and the inducible Caspase 9 both mediated drug-induced, efficient in vitro elimination of transgene-positive iPSCs. Choice of promoter allowed selective elimination of distinct populations within the graft: the hOct4 short response element restricted transgene expression to iPSCs, while the CAGs promoter ubiquitously drove expression in iPSCs and their progeny. Remarkably, both safety switches were able to prevent in vivo teratoma development and even effectively eliminated established teratomas formed by LV CAGs-transgenic iPSCs. These optimized tools to increase safety provide an important step towards clinical application of iPSC-derived transplants.

Humanized Mouse Models for the Advancement of Innate Lymphoid Cell-Based Cancer Immunotherapies

Innate lymphoid cells (ILCs) are a branch of the immune system that consists of diverse circulating and tissue-resident cells, which carry out functions including homeostasis and antitumor immunity. The development and behavior of human natural killer (NK) cells and other ILCs in the context of cancer is still incompletely understood. Since NK cells and Group 1 and 2 ILCs are known to be important for mediating antitumor immune responses, a clearer understanding of these processes is critical for improving cancer treatments and understanding tumor immunology as a whole. Unfortunately, there are some major differences in ILC differentiation and effector function pathways between humans and mice. To this end, mice bearing patient-derived xenografts or human cell line-derived tumors alongside human genes or human immune cells represent an excellent tool for studying these pathways in vivo. Recent advancements in humanized mice enable unparalleled insights into complex tumor-ILC interactions. In this review, we discuss ILC behavior in the context of cancer, the humanized mouse models that are most commonly employed in cancer research and their optimization for studying ILCs, current approaches to manipulating human ILCs for antitumor activity, and the relative utility of various mouse models for the development and assessment of these ILC-related immunotherapies.

Prodrugs and prodrug-activated systems in gene therapy

The inclusion of genes that control cell fate (so-called suicide, or kill-switch, genes) into gene therapy vectors is based on a compelling rationale for the safe and selective elimination of aberrant transfected cells. Prodrug-activated systems were developed in the 1980s and 1990s and rely on the enzymatic conversion of non-active prodrugs to active metabolites that lead to cell death. Although considerable effort and ingenuity has gone into vector design for gene therapy, less attention has been directed at the efficacy or associated adverse effects of the prodrug systems employed. In this review, we discuss prodrug systems employed in clinical trials and consider their role in the field of gene therapy. We highlight potential drawbacks associated with the use of specific prodrugs, such as systemic toxicity of the activated compound, the paucity of data on biodistribution of prodrugs, bystander effects, and destruction of genetically modified cells, and how these can inform future advances in cell therapies.

Engineered TCR-T Cell Immunotherapy in Anticancer Precision Medicine: Pros and Cons

This review provides insight into the role of engineered T-cell receptors (TCRs) in immunotherapy. Novel approaches have been developed to boost anticancer immune system, including targeting new antigens, manufacturing new engineered or modified TCRs, and creating a safety switch for endo-suicide genes. In order to re-activate T cells against tumors, immune-mobilizing monoclonal TCRs against cancer (ImmTAC) have been developed as a novel class of manufactured molecules which are bispecific and recognize both cancer and T cells. The TCRs target special antigens such as NY-ESO-1, AHNAK^S2580F or ERBB2^H473Y to boost the efficacy of anticancer immunotherapy. The safety of genetically modified T cells is very important. Therefore, this review discusses pros and cons of different approaches, such as ImmTAC, Herpes simplex virus thymidine kinase (HSV-TK), and inducible caspase-9 in cancer immunotherapy. Clinical trials related to TCR-T cell therapy and monoclonal antibodies designed for overcoming immunosuppression, and recent advances made in understanding how TCRs are additionally examined. New approaches that can better detect antigens and drive an effective T cell response are discussed as well.

Arming Immune Cells for Battle: A Brief Journey through the Advancements of T and NK Cell Immunotherapy

Simple Summary

This review is intended to provide an overview on the history and recent advances of T cell and natural killer (NK) cell-based immunotherapy. While the thymus was discovered as the origin of T cells in the 1960s, and NK cells were first described in 1975, the clinical application of adoptive cell therapies (ACT) only began in the early 1980s with the first lymphokine activated killer (LAK) cell product for the treatment of cancer patients. Over the past decades, further immunotherapies have been developed, including ACT using cytokine-induced killer (CIK) cells, products based on the NK cell line NK-92 as well as specific T and NK cell preparations. Recent advances have successfully improved the effectiveness of T, NK, CIK or NK-92 cells towards tumor-targeting antigens generated by genetic engineering of the immune cells. Herein, we summarize the promising development of ACT over the past decades in the fight against cancer.

Abstract

The promising development of adoptive immunotherapy over the last four decades has revealed numerous therapeutic approaches in which dedicated immune cells are modified and administered to eliminate malignant cells. Starting in the early 1980s, lymphokine activated killer (LAK) cells were the first ex vivo generated NK cell-enriched products utilized for adoptive immunotherapy. Over the past decades, various immunotherapies have been developed, including cytokine-induced killer (CIK) cells, as a peripheral blood mononuclear cells (PBMCs)-based therapeutic product, the adoptive transfer of specific T and NK cell products, and the NK cell line NK-92. In addition to allogeneic NK cells, NK-92 cell products represent a possible “off-the-shelf” therapeutic concept. Recent approaches have successfully enhanced the specificity and cytotoxicity of T, NK, CIK or NK-92 cells towards tumor-specific or associated target antigens generated by genetic engineering of the immune cells, e.g., to express a chimeric antigen receptor (CAR). Here, we will look into the history and recent developments of T and NK cell-based immunotherapy.

Neurological complications of cancer immunotherapy (CAR T cells)

Chimeric antigen receptor (CAR) T cell therapy has become an indispensable tool in the treatment of advanced malignancy, however, it is associated with significant neurologic toxicity. The pathophysiology of CAR T-cell associated neurotoxicity is incompletely understood, and the specific risk factors have only recently begun to be characterized. Despite a growing clinical experience with CAR T cell therapy, the unpredictability of neurologic symptoms remains a source of great anxiety for patients and practitioners alike, and a major limitation for more widespread adoption of this important treatment modality. The purpose of this review is to familiarize clinicians with the typical clinical manifestations and salient features of CAR T cell associated neurotoxicity. We place an emphasis on highlighting the clinical and laboratory markers that may be helpful for predicting clinical course, allowing teams to anticipate necessary supportive measures. We will also review the appropriate diagnostic workup for CAR T cell neurotoxicity and current treatment recommendations.

Genome editing of donor-derived T-cells to generate allogenic chimeric antigen receptor-modified T cells: Optimizing αβ T cell-depleted haploidentical hematopoietic stem cell transplantation

Allogeneic hematopoietic stem cell transplantation is an effective therapy for high-risk leukemias. In children, graft manipulation based on the selective removal of αβT cells and B cells has been shown to reduce the risk of acute and chronic graft-versus-host disease, thus allowing the use of haploidentical donors which expands the population of recipients in whom allogeneic hematopoietic stem cell transplantation can be used. Leukemic relapse, however, remains a challenge. T cells expressing chimeric antigen receptors can potently eliminate leukemia, including those in the central nervous system. We hypothesized that by engineering the donor αβT cells that are removed from the graft by genome editing to express a CD19-specific chimeric antigen receptor, while simultaneously inactivating the T-cell receptor, we could create a therapy that enhances the anti-leukemic efficacy of the stem cell transplant without increasing the risk of graft-versus-host disease. Using genome editing with Cas9 ribonucleoprotein and adeno-associated virus serotype 6, we integrated a CD19-specific chimeric antigen receptor inframe into the TRAC locus. More than 90% of cells lost T-cell receptor expression, while >75% expressed the chimeric antigen receptor. The initial product was further purified with less than 0.05% T-cell receptorpositive cells remaining. In vitro, the chimeric antigen receptor T cells efficiently eliminated target cells and produced high cytokine levels when challenged with CD19⁺ leukemia cells. In vivo, the gene-modified T cells eliminated leukemia without causing graft-versus-host disease in a xenograft model. Gene editing was highly specific with no evidence of off-target effects. These data support the concept that the addition of αβ T-cell-derived, genome-edited T cells expressing CD19-specific chimeric antigen receptors could enhance the anti-leukemic efficacy of αβT-celldepleted haploidentical hematopoietic stem cell transplantation without increasing the risk of graft-versus-host disease.

The role of small molecules in cell and gene therapy

Cell and gene therapies have achieved impressive results in the treatment of rare genetic diseases using gene corrected stem cells and haematological cancers using chimeric antigen receptor T cells. However, these two fields face significant challenges such as demonstrating long-term efficacy and safety, and achieving cost-effective, scalable manufacturing processes. The use of small molecules is a key approach to overcome these barriers and can benefit cell and gene therapies at multiple stages of their lifecycle. For example, small molecules can be used to optimise viral vector production during manufacturing or used in the clinic to enhance the resistance of T cell therapies to the immunosuppressive tumour microenvironment. Here, we review current uses of small molecules in cell and gene therapy and highlight opportunities for medicinal chemists to further consolidate the success of cell and gene therapies.