Production and first-in-man use of T cells engineered to express a HSVTK-CD34 sort-suicide gene

Challenges of CRISPR/Cas-Based Cell Therapy for Type 1 Diabetes: How Not to Engineer a "Trojan Horse"

Type 1 diabetes mellitus (T1D) is an autoimmune disease caused by the destruction of insulin-producing β-cells in the pancreas by cytotoxic T-cells. To date, there are no drugs that can prevent the development of T1D. Insulin replacement therapy is the standard care for patients with T1D. This treatment is life-saving, but is expensive, can lead to acute and long-term complications, and results in reduced overall life expectancy. This has stimulated the research and development of alternative treatments for T1D. In this review, we consider potential therapies for T1D using cellular regenerative medicine approaches with a focus on CRISPR/Cas-engineered cellular products. However, CRISPR/Cas as a genome editing tool has several drawbacks that should be considered for safe and efficient cell engineering. In addition, cellular engineering approaches themselves pose a hidden threat. The purpose of this review is to critically discuss novel strategies for the treatment of T1D using genome editing technology. A well-designed approach to β-cell derivation using CRISPR/Cas-based genome editing technology will significantly reduce the risk of incorrectly engineered cell products that could behave as a "Trojan horse".

Enrichment strategies to enhance genome editing

Genome editing technologies hold great promise for numerous applications including the understanding of cellular and disease mechanisms and the development of gene and cellular therapies. Achieving high editing frequencies is critical to these research areas and to achieve the overall goal of being able to manipulate any target with any desired genetic outcome. However, gene editing technologies sometimes suffer from low editing efficiencies due to several challenges. This is often the case for emerging gene editing technologies, which require assistance for translation into broader applications. Enrichment strategies can support this goal by selecting gene edited cells from non-edited cells. In this review, we elucidate the different enrichment strategies, their many applications in non-clinical and clinical settings, and the remaining need for novel strategies to further improve genome research and gene and cellular therapy studies.

CRISISS: A Novel, Transcriptionally and Post-Translationally Inducible CRISPR/Cas9-Based Cellular Suicide Switch

With the ever-increasing developing rate of gene and cellular therapy applications and growing accessibility due to products receiving regulatory approval, the need for effective and reliable safety mechanisms to prevent or eliminate potentially fatal side effects is of the utmost importance. In this study, we present the CRISPR-induced suicide switch (CRISISS) as a tool to eliminate genetically modified cells in an inducible and highly efficient manner by targeting Cas9 to highly repetitive Alu retrotransposons in the human genome, causing irreparable genomic fragmentation by the Cas9 nuclease and resulting cell death. The suicide switch components, including expression cassettes for a transcriptionally and post-translationally inducible Cas9 and an Alu-specific single-guide RNA, were integrated into the genome of target cells via Sleeping-Beauty-mediated transposition. The resulting transgenic cells did not show signs of any impact on overall fitness when uninduced, as unintended background expression, background DNA damage response and background cell killing were not observed. When induced, however, a strong expression of Cas9, a strong DNA damage response and a rapid halt of cell proliferation coupled with near complete cell death within four days post-induction were seen. With this proof-of-concept study, we present a novel and promising approach for a robust suicide switch with potential utility for gene and cell therapy in the future.

Spotlight on CYP4B1

The mammalian cytochrome P450 monooxygenase CYP4B1 can bioactivate a wide range of xenobiotics, such as its defining/hallmark substrate 4-ipomeanol leading to tissue-specific toxicities. Similar to other members of the CYP4 family, CYP4B1 has the ability to hydroxylate fatty acids and fatty alcohols. Structural insights into the enigmatic role of CYP4B1 with functions in both, xenobiotic and endobiotic metabolism, as well as its unusual heme-binding characteristics are now possible by the recently solved crystal structures of native rabbit CYP4B1 and the p.E310A variant. Importantly, CYP4B1 does not play a major role in hepatic P450-catalyzed phase I drug metabolism due to its predominant extra-hepatic expression, mainly in the lung. In addition, no catalytic activity of human CYP4B1 has been observed owing to a unique substitution of an evolutionary strongly conserved proline 427 to serine. Nevertheless, association of CYP4B1 expression patterns with various cancers and potential roles in cancer development have been reported for the human enzyme. This review will summarize the current status of CYP4B1 research with a spotlight on its roles in the metabolism of endogenous and exogenous compounds, structural properties, and cancer association, as well as its potential application in suicide gene approaches for targeted cancer therapy.

Process Development for Adoptive Cell Therapy in Academia: A Pipeline for Clinical-Scale Manufacturing of Multiple TCR-T Cell Products

Cellular immunotherapies based on T cell receptor (TCR) transfer are promising approaches for the treatment of cancer and chronic viral infections. The discovery of novel receptors is expanding considerably; however, the clinical development of TCR-T cell therapies still lags. Here we provide a pipeline for process development and clinical-scale manufacturing of TCR-T cells in academia. We utilized two TCRs specific for hepatitis C virus (HCV) as models because of their marked differences in avidity and functional profile in TCR-redirected cells. With our clinical-scale pipeline, we reproduced the functional profile associated with each TCR. Moreover, the two TCR-T cell products demonstrated similar yield, purity, transduction efficiency as well as phenotype. The TCR-T cell products had a highly reproducible yield of over 1.4 × 10⁹ cells, with an average viability of 93%; 97.8–99% of cells were CD3+, of which 47.66 ± 2.02% were CD8+ T cells; the phenotype was markedly associated with central memory (CD62L+CD45RO+) for CD4+ (93.70 ± 5.23%) and CD8+ (94.26 ± 4.04%). The functional assessments in 2D and 3D cell culture assays showed that TCR-T cells mounted a polyfunctional response to the cognate HCV peptide target in tumor cell lines, including killing. Collectively, we report a solid strategy for the efficient large-scale manufacturing of TCR-T cells.

Application and Design of Switches Used in CAR

Among the many oncology therapies, few have generated as much excitement as CAR-T. The success of CAR therapy would not have been possible without the many discoveries that preceded it, most notably, the Nobel Prize-winning breakthroughs in cellular immunity. However, despite the fact that CAR-T already offers not only hope for development, but measurable results in the treatment of hematological malignancies, CAR-T still cannot be safely applied to solid tumors. The reason for this is, among other things, the lack of tumor-specific antigens which, in therapy, threatens to cause a lethal attack of lymphocytes on healthy cells. In the case of hematological malignancies, dangerous complications such as cytokine release syndrome may occur. Scientists have responded to these clinical challenges with molecular switches. They make it possible to remotely control CAR lymphocytes after they have already been administered to the patient. Moreover, they offer many additional capabilities. For example, they can be used to switch CAR antigenic specificity, create logic gates, or produce local activation under heat or light. They can also be coupled with costimulatory domains, used for the regulation of interleukin secretion, or to prevent CAR exhaustion. More complex modifications will probably require a combination of reprogramming (iPSc) technology with genome editing (CRISPR) and allogenic (off the shelf) CAR-T production.

A novel CD34-derived hinge for rapid and efficient detection and enrichment of CAR T cells

Immunotherapy including chimeric antigen receptor (CAR) T cell therapy has revolutionized modern cancer therapy and has achieved remarkable remission and survival rates for several malignancies with historically dismal outcomes. The hinge of the CAR connects the antigen binding to the transmembrane domain and can be exploited to confer features to CAR T cells including additional stimulation, targeted elimination or detection and enrichment of the genetically modified cells. For establishing a novel hinge derived from human CD34, we systematically tested CD34 fragments of different lengths, all containing the binding site of the QBend-10 monoclonal antibody, in a FMC63-based CD19 CAR lentiviral construct. A final construct of 99 amino acids called C6 proved to be the best candidate for flow cytometry-based detection of CAR T cells and >95% enrichment of genetically modified T cells on MACS columns. The C6 hinge was functionally indistinguishable from the commonly used CD8α hinge in vitro as well as in in vivo experiments in NSG mice. We also showed that the C6 hinge can be used for a variety of different CARs and mediates high killing efficacy without unspecific activation by target antigen-negative cells, thus making C6 ideally suited as a universal hinge for CARs for clinical applications.

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.

Retroviral gene therapy in Germany with a view on previous experience and future perspectives

Gene therapy can be used to restore cell function in monogenic disorders or to endow cells with new capabilities, such as improved killing of cancer cells, expression of suicide genes for controlled elimination of cell populations, or protection against chemotherapy or viral infection. While gene therapies were originally most often used to treat monogenic diseases and to improve hematopoietic stem cell transplantation outcome, the advent of genetically modified immune cell therapies, such as chimeric antigen receptor modified T cells, has contributed to the increased numbers of patients treated with gene and cell therapies. The advancement of gene therapy with integrating retroviral vectors continues to depend upon world-wide efforts. As the topic of this special issue is "Spotlight on Germany," the goal of this review is to provide an overview of contributions to this field made by German clinical and research institutions. Research groups in Germany made, and continue to make, important contributions to the development of gene therapy, including design of vectors and transduction protocols for improved cell modification, methods to assess gene therapy vector efficacy and safety (e.g., clonal imbalance, insertion sites), as well as in the design and conduction of clinical gene therapy trials.

Toward a Scalable Purification Protocol of GaLV-TR-Pseudotyped Lentiviral Vectors

Lentiviral vectors (LV) that are used in research and development as well as in clinical trials are in majority vesicular stomatitis virus G glycoprotein (VSVg) pseudotyped. The predominance of this pseudotype choice for clinical gene therapy studies is largely due to a lack of purification schemes for pseudotypes other than VSVg. In this study, we report for the first time the development of a new downstream process protocol allowing high-yield production of stable and infectious gibbon ape leukemia virus (GaLV)-TR-LV particles. We identified critical conditions in tangential flow filtration (TFF) and chromatographic steps for preserving the infectivity/functionality of LV during purification. This was carried out by identifying for each step, the critical parameters affecting LV infectivity, including pH, salinity, presence of stabilizers, temperature, and by defining the optimal order of these steps. A three-step process was developed for GaLV-TR-LV purification consisting of one TFF and two chromatographic steps (ion-exchange chromatography and size exclusion chromatography) permitting recoveries of >27% of infectious particles. With this process, purified GaLV-pseudotyped LV enabled the transduction of 70% human CD34⁺ cells in the presence of the Vectofusin-1 peptide, whereas in the same conditions nonpurified vector transduced only 9% of the cells (multiplicity of infection 20). Our protocol will allow for the first time the purification of GaLV-TR-LV that are biologically active, stable, and with sufficient recovery in the perspective of preclinical studies and clinical applications. Obviously, further optimizations are required to improve final vector yields.

Comprehensive Characterization of a Next-Generation Antiviral T-Cell Product and Feasibility for Application in Immunosuppressed Transplant Patients

Viral infections have a major impact on morbidity and mortality of immunosuppressed solid organ transplant (SOT) patients because of missing or failure of adequate pharmacologic antiviral treatment. Adoptive antiviral T-cell therapy (AVTT), regenerating disturbed endogenous T-cell immunity, emerged as an attractive alternative approach to combat severe viral complications in immunocompromised patients. AVTT is successful in patients after hematopoietic stem cell transplantation where T-cell products (TCPs) are manufactured from healthy donors. In contrast, in the SOT setting TCPs are derived from/applied back to immunosuppressed patients. We and others demonstrated feasibility of TCP generation from SOT patients and first clinical proof-of-concept trials revealing promising data. However, the initial efficacy is frequently lost long-term, because of limited survival of transferred short-lived T-cells indicating a need for next-generation TCPs. Our recent data suggest that Rapamycin treatment during TCP manufacture, conferring partial inhibition of mTOR, might improve its composition. The aim of this study was to confirm these promising observations in a setting closer to clinical challenges and to deeply characterize the next-generation TCPs. Using cytomegalovirus (CMV) as model, our next-generation Rapamycin-treated (Rapa-)TCP showed consistently increased proportions of CD4⁺ T-cells as well as CD4⁺ and CD8⁺ central-memory T-cells (T_CM). In addition, Rapamycin sustained T-cell function despite withdrawal of Rapamycin, showed superior T-cell viability and resistance to apoptosis, stable metabolism upon activation, preferential expansion of T_CM, partial conversion of other memory T-cell subsets to T_CM and increased clonal diversity. On transcriptome level, we observed a gene expression profile denoting long-lived early memory T-cells with potent effector functions. Furthermore, we successfully applied the novel protocol for the generation of Rapa-TCPs to 19/19 SOT patients in a comparative study, irrespective of their history of CMV reactivation. Moreover, comparison of paired TCPs generated before/after transplantation did not reveal inferiority of the latter despite exposition to maintenance immunosuppression post-SOT. Our data imply that the Rapa-TCPs, exhibiting longevity and sustained T-cell memory, are a reasonable treatment option for SOT patients. Based on our success to manufacture Rapa-TCPs from SOT patients under maintenance immunosuppression, now, we seek ultimate clinical proof of efficacy in a clinical study.

Molecular Approaches to Safe and Controlled Engineered T-cell Therapy

Chimeric antigen receptor-modified T-cell therapy (CAR-T therapy) is one of the fastest developing areas of immuno-oncology. Over the past decade, it has revolutionized the cell therapy modality and expedited its pace of development, from optimization of the structure of chimeric antigen receptors and animal model experiments to successful clinical application. The initial designs of the CAR configuration focused on increasing T-cell activation, cytotoxicity, and persistence. However, the first attempts to treat patients with CAR T cells have demonstrated the need for increased safety and controlled activation of genetically modified T cells. Herein, we summarize the different molecular approaches to engineering chimeric antigen receptors for reducing the potential clinical risks of T-cell therapy.

Cell-mediated enzyme prodrug cancer therapies

Cell-directed gene therapy is a promising new frontier for the field of targeted cancer therapies. Here we discuss the current pre-clinical and clinical use of cell-mediated enzyme prodrug therapy (EPT) directed against solid tumors and avenues for further development. We also discuss some of the challenges encountered upon translating these therapies to clinical trials. Upon sufficient development, cell-mediated enzyme prodrug therapy has the potential to maximize the distribution of therapeutic enzymes within the tumor environment, localizing conversion of prodrug to active drug at the tumor sites thereby decreasing off-target toxicities. New combinatorial possibilities are also promising. For example, when combined with viral gene-delivery vehicles, this may result in new hybrid vehicles that attain heretofore unmatched levels of therapeutic gene expression within the tumor.

Phenotypic correction of Fanconi anemia cells in the murine bone marrow after carrier cell mediated delivery of lentiviral vector

Fanconi anemia (FA) is an autosomal-recessive disorder associated with hematopoietic failure and it is a candidate for hematopoietic stem cell (HSC)-directed gene therapy. However, the characteristically reduced HSC numbers found in FA patients, their ineffective mobilization from the marrow, and re-oxygenation damage during ex vivo manipulation have precluded clinical success using conventional in vitro approaches. We previously demonstrated that lentiviral vector (LV) particles reversibly attach to the cell surface where they gain protection from serum complement neutralization. We reasoned that cellular delivery of LV to the bone marrow niche could avoid detrimental losses during FA HSC mobilization and in vitro modification. Here, we demonstrate that a VSV-G pseudotyped lentivector, carrying the FANCC transgene, can be transmitted from carrier to bystander cells. In cell culture and transplantation models of FA, we further demonstrate that LV carrier cells migrate along SDF-1α gradients and transfer vector particles that stably integrate and phenotypically correct the characteristic DNA alkylator sensitivity in murine and human FA-deficient target bystander cells. Altogether, we demonstrate that cellular homing mechanisms can be harnessed for the functional phenotype correction in murine FA hematopoietic cells.

Optimized human CYP4B1 in combination with the alkylator prodrug 4-ipomeanol serves as a novel suicide gene system for adoptive T-cell therapies

Engineering autologous or allogeneic T cells to express a suicide gene can control potential toxicity in adoptive T-cell therapies. We recently reported the development of a novel human suicide gene system that is based on an orphan human cytochrome P450 enzyme, CYP4B1, and the naturally occurring alkylator prodrug 4-ipomeanol. The goal of this study was to systematically develop a clinically applicable self-inactivating lentiviral vector for efficient co-expression of CYP4B1 as an ER-located protein with two distinct types of cell surface proteins, either MACS selection genes for donor lymphocyte infusions after allogeneic stem cell transplantation or chimeric antigen receptors for retargeting primary T cells. The U3 region of the myeloproliferative sarcoma virus in combination with the T2A site was found to drive high-level expression of our CYP4B1 mutant with truncated CD34 or CD271 as MACS suitable selection markers. This lentiviral vector backbone was also well suited for co-expression of CYP4B1 with a codon-optimized CD19 chimeric antigen receptor (CAR) construct. Finally, 4-ipomeanol efficiently induced apoptosis in primary T cells that co-express mutant CYP4B1 and the divergently located MACS selection and CAR genes. In conclusion, we here developed a clinically suited lentiviral vector that supports high-level co-expression of cell surface proteins with a potent novel human suicide gene.

Advances in PET Detection of the Antitumor T Cell Response

Positron emission tomography (PET) is a powerful noninvasive imaging technique able to measure distinct biological processes in vivo by administration of a radiolabeled probe. Whole-body measurements track the probe accumulation providing a means to measure biological changes such as metabolism, cell location, or tumor burden. PET can also be applied to both preclinical and clinical studies providing three-dimensional information. For immunotherapies (in particular understanding T cell responses), PET can be utilized for spatial and longitudinal tracking of T lymphocytes. Although PET has been utilized clinically for over 30 years, the recent development of additional PET radiotracers have dramatically expanded the use of PET to detect endogenous or adoptively transferred T cells in vivo. Novel probes have identified changes in T cell quantity, location, and function. This has enabled investigators to track T cells outside of the circulation and in hematopoietic organs such as spleen, lymph nodes, and bone marrow, or within tumors. In this review, we cover advances in PET detection of the antitumor T cell response and areas of focus for future studies.

Molecular Determinants of Vectofusin-1 and Its Derivatives for the Enhancement of Lentivirally Mediated Gene Transfer into Hematopoietic Stem/Progenitor Cells

Gene delivery into hCD34+ hematopoietic stem/progenitor cells (HSPCs) using human immunodeficiency virus, type 1-derived lentiviral vectors (LVs) has several promising therapeutic applications. Numerous clinical trials are currently underway. However, the efficiency, safety, and cost of LV gene therapy could be ameliorated by enhancing target cell transduction levels and reducing the amount of LV used on the cells. Several transduction enhancers already exist, such as fibronectin fragments or cationic compounds. Recently, we discovered Vectofusin-1, a new transduction enhancer, also called LAH4-A4, a short histidine-rich amphipathic peptide derived from the LAH4 family of DNA transfection agents. Vectofusin-1 enhances the infectivity of lentiviral and γ-retroviral vectors pseudotyped with various envelope glycoproteins. In this study, we compared a family of Vectofusin-1 isomers and showed that Vectofusin-1 remains the lead peptide for HSPC transduction enhancement with LVs pseudotyped with vesicular stomatitis virus glycoproteins and also with modified gibbon ape leukemia virus glycoproteins. By comparing the capacity of numerous Vectofusin-1 variants to promote the modified gibbon ape leukemia virus glycoprotein-pseudotyped lentiviral vector infectivity of HSPCs, the lysine residues on the N-terminal extremity of Vectofusin-1, a hydrophilic angle of 140° formed by the histidine residues in the Schiffer-Edmundson helical wheel representation, hydrophobic residues consisting of leucine were all found to be essential and helped to define a minimal active sequence. The data also show that the critical determinants necessary for lentiviral transduction enhancement are partially different from those necessary for efficient antibiotic or DNA transfection activity of LAH4 derivatives. In conclusion, these results help to decipher the action mechanism of Vectofusin-1 in the context of hCD34+ cell-based gene therapy.

Engineered T cell therapies

Alongside advancements in gene therapy for inherited immune disorders, the need for effective alternative therapeutic options for other conditions has resulted in an expansion in the field of research for T cell gene therapy. T cells are easily obtained and can be induced to divide robustly ex vivo, a characteristic that allows them to be highly permissible to viral vector-mediated introduction of transgenes. Pioneering clinical trials targeting cancers and infectious diseases have provided safety and feasibility data and important information about persistence of engineered cells in vivo. Here, we review clinical experiences with γ-retroviral and lentiviral vectors and consider the potential of integrating transposon-based vectors as well as specific genome editing with designer nucleases in engineered T cell therapies.

Lentivirus technologies for modulation of the immune system

Lentiviral vectors (LVV) are important tools for the treatment of immune system disorders. Integration of therapeutic genetic material into the haematopoietic stem cell compartment using LVV can mediate long-term correction of haematopoietic lineages, thereby correcting disease phenotypes. Twenty years of vector development have successfully brought LVV to the clinic, with follow up studies of clinical trials treating primary immunodeficiencies now being reported. Results have demonstrated clear improvements in the quality of life for patients with a number of conditions in the absence of the severe adverse events observed in earlier retroviral gene therapy trials. Growing interest in gene modified adoptive T cell transfer as an alternative strategy has driven further technology innovation, including characterisation of novel viral envelopes. We will also discuss the progression of gene editing technology to preclinical investigations in models of immune deficiency.

Adoptive T-cell therapy for cancer in the United kingdom: a review of activity for the British Society of Gene and Cell Therapy annual meeting 2015

Adoptive T-cell therapy is delivering objective clinical responses across a number of cancer indications in the early phase clinical setting. Much of this clinical activity is taking place at major clinical academic centers across the United States. This review focuses upon cancer-focused cell therapy activity within the United Kingdom as a contribution to the 2015 British Society of Gene and Cell Therapy annual general meeting. This overview reflects the diversity and expansion of clinical and preclinical studies within the United Kingdom while considering the background context of this work against new infrastructural developments and the requirements of nationalized healthcare delivery within the UK National Health Service.

[(18)F]FHBG PET/CT Imaging of CD34-TK75 Transduced Donor T Cells in Relapsed Allogeneic Stem Cell Transplant Patients: Safety and Feasibility

Described herein is a first-in-man attempt to both genetically modify T cells with an imagable suicide gene and track these transduced donor T cells in allogeneic stem cell transplantation recipients using noninvasive positron emission tomography/computerized tomography (PET/CT) imaging. A suicide gene encoding a human CD34-Herpes Simplex Virus-1-thymidine kinase (CD34-TK75) fusion enabled enrichment of retrovirally transduced T cells (TdT), control of graft-versus-host disease and imaging of TdT migration and expansion in vivo in mice and man. Analysis confirmed that CD34-TK75-enriched TdT contained no replication competent γ-retrovirus, were sensitive to ganciclovir, and displayed characteristic retroviral insertion sites (by targeted sequencing). Affinity-purified CD34-TK75(+)-selected donor T cells (1.0-13 × 10(5))/kg were infused into eight patients who relapsed after allogeneic stem cell transplantation. Six patients also were administered 9-[4-((18)F)fluoro-3-hydroxymethyl-butyl]guanine ([(18)F]FHBG) to specifically track the genetically modified donor T cells by PET/CT at several time points after infusion. All patients were assessed for graft-versus-host disease, response to ganciclovir, circulating TdT cells (using both quantitative polymerase chain reaction and [(18)F]FHBG PET/CT imaging), TdT cell clonal expansion, and immune response to the TdT. This phase 1 trial demonstrated that genetically modified T cells and [(18)F]FHBG can be safely infused in patients with relapsed hematologic malignancies after allogeneic stem cell transplantation.

Immunotherapy of HCC metastases with autologous T cell receptor redirected T cells, targeting HBsAg in a liver transplant patient

HBV-DNA integration frequently occurs in HBV-related hepatocellular carcinoma (HCC), but whether HBV antigens are expressed in HCC cells and can be targeted by immune therapeutic strategies remains controversial. Here, we first characterized HBV antigen expression in HCC metastases, occurring in a patient who had undergone liver transplantation for HBV-related HCC. We then deployed for the first time in HCC autologous T cells, genetically modified to express an HBsAg specific T cell receptor, as therapy against chemoresistant extrahepatic metastases. We confirmed that HBV antigens were expressed in HCC metastases (but not in the donor liver) and demonstrated that tumour cells were recognized in vivo by lymphocytes, engineered to express an HBV-specific T cell receptor (TCR). Gene-modified T cells survived, expanded and mediated a reduction in HBsAg levels without exacerbation of liver inflammation or other toxicity. Whilst clinical efficacy was not established in this subject with end-stage metastatic disease, we confirm the feasibility of providing autologous TCR-redirected therapy against HCC and advocate this strategy as a novel therapeutic opportunity in hepatitis B-associated malignancies.

Suicide genes: monitoring cells in patients with a safety switch

Clinical trials increasingly incorporate suicide genes either as direct lytic agents for tumors or as safety switches in therapies based on genetically modified cells. Suicide genes can also be used as non-invasive reporters to monitor the biological consequences of administering genetically modified cells to patients and gather information relevant to patient safety. These genes can monitor therapeutic outcomes addressable by early clinical intervention. As an example, our recent clinical trial used (18)F-9-(4-fluoro-3-hydroxymethylbutyl)guanine ((18)FHBG) and positron emission tomography (PET)/CT scans to follow T cells transduced with herpes simplex virus thymidine kinase after administration to patients. Guided by preclinical data we ultimately hope to discern whether a particular pattern of transduced T cell migration within patients reflects early development of graft vs. host disease. Current difficulties in terms of choice of suicide gene, biodistribution of radiolabeled tracers in humans vs. animal models, and threshold levels of genetically modified cells needed for detection by PET/CT are discussed. As alternative suicide genes are developed, additional radiolabel probes suitable for imaging in patients should be considered.

Progress and prospects for engineered T cell therapies

Proof-of-concept studies have demonstrated the therapeutic potential of engineered T cells. Transfer of recombinant antigen-specific T cell receptors (TCR) and chimaeric antigen receptors (CARs) against tumour and viral antigens are under investigation by multiple approaches, including viral- and nonviral-mediated gene transfer into both autologous and allogeneic T cell populations. There have been notable successes recently using viral vector-mediated transfer of CARs specific for B cell antigens, but also reports of anticipated and unanticipated complications in these and other studies. We review progress in this promising area of cellular therapy, and consider developments in antigen receptor therapies including the application of emerging gene-editing technologies.