Infusion of suicide-gene-engineered donor lymphocytes after family haploidentical haemopoietic stem-cell transplantation for leukaemia (the TK007 trial): a non-randomised phase I-II study

Graft-versus-leukemia effect of HLA-haploidentical central-memory T-cells expanded with leukemic APCs and modified with a suicide gene

Allogeneic hematopoietic stem cell transplantation (HSCT) from a human leukocyte antigen (HLA)-haploidentical family donor (haplo-HSCT) is a readily available and potentially curative option for high-risk leukemia. In haplo-HSCT, alloreactivity plays a major role in the graft-versus-leukemia (GVL) effect, which, however, is frequently followed by relapse due to emerging leukemic cell variants that have lost the unshared HLA haplotype as a mechanism of immune escape. We report that stimulation of HLA-haploidentical donor T lymphocytes with leukemic antigen-presenting cells (L-APCs) expands a population of leukemia-reactive T cells, which, besides alloreactivity to unshared HLAs, contain leukemia-associated specificities restricted by shared HLAs. According to a preferential central-memory (T(CM)) phenotype and to high interleukin (IL)-7Rα expression, these T cells persist in vivo and sustain a major GVL effect in a clinically relevant xenograft model. Moreover, we demonstrate that modifying L-APC-expanded T cells to express the herpes simplex virus thymidine kinase (HSV-tk) suicide gene enables their elimination with the prodrug ganciclovir (GCV), therefore providing a safety switch in case of graft-versus-host disease (GVHD). These results warrant the clinical investigation of L-APC-expanded T cells modified with a suicide gene in the setting of haplo-HSCT.

Convergence of gene and cell therapy

Gene therapy and cell therapy have followed similar roller coaster paths of rising public expectations and disappointment over the past two decades. There is now reason to believe that momentum in the field has reached the point where the successes will be more frequent. The use of gene-modified cells has opened new avenues for engineering desired cell properties, for the use of cells as vehicles for gene delivery, and for tracking cells and controlling cell persistence after transplantation. Some notable recent clinical developments in cellular engineering by gene transfer offer lessons on how the field has emerged, and hint at additional future clinical applications.

Will T-cell therapy for cancer ever be a standard of care?

Cellular therapies for cancer are showing increasing efficacy but their introduction as a 'standard of care' for these disorders is hampered by technical, regulatory and financial concerns. This review identifies some of the major problems and suggests potential solutions.

Cytotoxic T lymphocytes for the treatment of viral infections and posttransplant lymphoproliferative disorders in transplant recipients

PURPOSE OF REVIEW

The continuous and successful expansion of organ transplants is unfortunately associated with increased incidence of severe opportunistic viral infections and Epstein-Barr virus (EBV)-related lymphomas secondary to immunosuppression. Here, we review the strengths and limitations of T-cell-based strategies used to treat viral infections in immunocompromised individuals.

RECENT FINDINGS

While current antiviral drugs are often suboptimal because of associated toxicities, a promising approach in the management of infections with viruses like cytomegalovirus (CMV), adenovirus (AdV) and EBV is the adoptive transfer of T cells targeting these viruses that can be directly isolated from the peripheral blood of the donor or expanded ex vivo prior to infusions in patients.

SUMMARY

T-cell-based immunotherapies are now being included in the clinical practice of transplant recipients to prevent and treat infections and complications associated with CMV, AdV and EBV. Improvement of current limitations will enable the extension of these approaches to all patients at risk and to other clinically relevant viruses and pathogens that are emerging as significant complications for immunocompromised patients.

Bacterial virulence proteins as tools to rewire kinase pathways in yeast and immune cells

Bacterial pathogens have evolved specific effector proteins that, by interfacing with host kinase signaling pathways, provide a mechanism to evade immune responses during infection^1,2. Although these effectors are responsible for pathogen virulence, we realized that they might also serve as valuable synthetic biology reagents for engineering cellular behavior. Here, we have exploited two effector proteins, the Shigella flexneri OspF protein³ and Yersinia pestis YopH protein⁴, to systematically rewire kinase-mediated responses in both yeast and mammalian immune cells. Bacterial effector proteins can be directed to selectively inhibit specific mitogen activated protein kinase (MAPK) pathways in yeast by artificially targeting them to pathway specific complexes. Moreover, we show that unique properties of the effectors generate novel pathway behaviors: OspF, which irreversibly inactivates MAPKs⁴, was used to construct a synthetic feedback circuit that displays novel frequency-dependent input filtering. Finally, we show that effectors can be used in T cells, either as feedback modulators to precisely tune the T cell response amplitude, or as an inducible pause switch that can temporarily disable T cell activation. These studies demonstrate how pathogens could provide a rich toolkit of parts to engineer cells for therapeutic or biotechnological applications.

T-cell therapy in the treatment of post-transplant lymphoproliferative disease

Post-transplant lymphoproliferative diseases (PTLD) associated with Epstein-Barr virus (EBV) infection often develop after organ and haematopoietic stem-cell transplantation. These lymphoproliferative diseases are tumours that usually express all latent EBV viral proteins, and are therefore amenable to T-cell-based immune therapies, such as donor lymphocyte infusions and the adoptive transfer of EBV-specific cytotoxic T lymphocytes. In this Review, we describe current approaches of T-cell-based therapies to treat PTLD, and describe strategies that improve the feasibility of such treatment.

Genetic modification of human T lymphocytes for the treatment of hematologic malignancies

Modern chemotherapy regimens and supportive care have produced remarkable improvements in the overall survival of patients with hematologic malignancies. However, the development of targeted small molecules, monoclonal antibodies, and biological therapies that demonstrate greater efficacy and lower toxicity remains highly desirable in hematology, and oncology in general. In the context of biological therapies, T-lymphocyte based treatments have enormous potential. Donor lymphocyte infusion in patients relapsed after allogeneic hematopoietic stem cell transplant pioneered the concept that T lymphocytes can effectively control tumor growth, and this was then followed by the development of cell culture strategies to generate T lymphocytes with selective activity against tumor cells. Over the past decade, it has become clear that the adoptive transfer of ex vivo expanded antigen-specific cytotoxic T lymphocytes promotes sustained antitumor effects in patients with virus-associated lymphomas, such as Epstein-Barr virus related post-transplant lymphomas and Hodgkin's lymphomas. Because of this compelling clinical evidence and the concomitant development of methodologies for robust gene transfer to human T lymphocytes, the field has rapidly evolved, offering new opportunities to extend T-cell based therapies. This review summarizes the most recent biological and clinical developments using genetically manipulated T cells for the treatment of hematologic malignancies.

Immunotherapeutic approaches to improve graft-versus-tumor effect and reduce graft-versus-host disease

The therapeutic efficacy of allogeneic stem cell transplantation is mainly based on the alloreactive immune response of the graft against the host. However, the graft-versus-host process can be viewed as a double-edged sword since it is responsible for both the beneficial graft-versus-tumor effect and the deleterious graft-versus-host disease. During the last two decades, intensive research has been focused on the development of novel immunotherapeutic methods aimed to dissociate graft-versus-host disease from graft-versus-tumor effect. A brief description of these efforts is discussed in this review.

Thymidine kinase suicide gene-mediated ganciclovir ablation of autologous gene-modified rhesus hematopoiesis

Despite the genotoxic complications encountered in clinical gene therapy trials for primary immunodeficiency diseases targeting hematopoietic cells with integrating vectors; this strategy holds promise for the cure of several monogenic blood, metabolic and neurodegenerative diseases. In this study, we asked whether the inclusion of a suicide gene in a standard retrovirus vector would allow elimination of vector-containing stem and progenitor cells and their progeny in vivo following transplantation, using our rhesus macaque transplantation model. Following stable engraftment with autologous CD34(+) cells transduced with a retrovirus vector encoding a highly sensitive modified Herpes simplex virus thymidine kinase SR39, the administration of the antiviral prodrug ganciclovir (GCV) was effective in completely eliminating vector-containing cells in all hematopoietic lineages in vivo. The sustained absence of vector-containing cells over time, without additional GCV administration, suggests that the ablation of TkSR39 GCV-sensitive cells occurred in the most primitive hematopoietic long-term repopulating stem or progenitor cell compartment. These results are a proof-of-concept that the inclusion of a suicide gene in integrating vectors, in addition to a therapeutic gene, can provide a mechanism for later elimination of vector-containing cells, thereby increasing the safety of gene transfer.

Targeted immunotherapy of cancer with CAR T cells: achievements and challenges

The adoptive transfer of chimeric antigen receptor (CAR)-expressing T cells is a relatively new but promising approach in the field of cancer immunotherapy. This therapeutic strategy is based on the genetic reprogramming of T cells with an artificial immune receptor that redirects them against targets on malignant cells and enables their destruction by exerting T cell effector functions. There has been an explosion of interest in the use of CAR T cells as an immunotherapy for cancer. In the pre-clinical setting, there has been a considerable focus upon optimizing the structural and signaling potency of the CAR while advances in bio-processing technology now mean that the clinical testing of these gene-modified T cells has become a reality. This review will summarize the concept of CAR-based immunotherapy and recent clinical trial activity and will further discuss some of the likely future challenges facing CAR-modified T cell therapies.

T-cell suicide gene therapy prompts thymic renewal in adults after hematopoietic stem cell transplantation

The genetic modification of T cells with a suicide gene grants a mechanism of control of adverse reactions, allowing safe infusion after partially incompatible hematopoietic stem cell transplantation (HSCT). In the TK007 clinical trial, 22 adults with hematologic malignancies experienced a rapid and sustained immune recovery after T cell-depleted HSCT and serial infusions of purified donor T cells expressing the HSV thymidine kinase suicide gene (TK+ cells). After a first wave of circulating TK+ cells, the majority of T cells supporting long-term immune reconstitution did not carry the suicide gene and displayed high numbers of naive lymphocytes, suggesting the thymus-dependent development of T cells, occurring only upon TK+ -cell engraftment. Accordingly, after the infusions, we documented an increase in circulating TCR excision circles and CD31+ recent thymic emigrants and a substantial expansion of the active thymic tissue as shown by chest tomography scans. Interestingly, a peak in the serum level of IL-7 was observed after each infusion of TK+ cells, anticipating the appearance of newly generated T cells. The results of the present study show that the infusion of genetically modified donor T cells after HSCT can drive the recovery of thymic activity in adults, leading to immune reconstitution.

Enforced IL-10 expression confers type 1 regulatory T cell (Tr1) phenotype and function to human CD4(+) T cells

Type 1 regulatory T (Tr1) cells are an inducible subset of CD4(+) Tr cells characterized by high levels of interleukin (IL)-10 production and regulatory properties. Several protocols to generate human Tr1 cells have been developed in vitro. However, the resulting population includes a significant fraction of contaminating non-Tr1 cells, representing a major bottleneck for clinical application of Tr1 cell therapy. We generated an homogeneous IL-10-producing Tr1 cell population by transducing human CD4(+) T cells with a bidirectional lentiviral vector (LV) encoding for human IL-10 and the marker gene, green fluorescent protein (GFP), which are independently coexpressed. The resulting GFP(+) LV-IL-10-transduced human CD4(+) T (CD4(LV-IL-10)) cells expressed, upon T-cell receptor (TCR) activation, high levels of IL-10 and concomitant low levels of IL-4, and markers associated with IL-10. Moreover, CD4(LV-IL-10) T cells displayed typical Tr1 features: the anergic phenotype, the IL-10, and transforming growth factor (TGF)-β dependent suppression of allogeneic T-cell responses, and the ability to suppress in a cell-to-cell contact independent manner in vitro. CD4(LV-IL-10) T cells were able to control xeno graft-versus-host disease (GvHD), demonstrating their suppressive function in vivo. These results show that constitutive over-expression of IL-10 in human CD4(+) T cells leads to a stable cell population that recapitulates the phenotype and function of Tr1 cells.

Emerging concepts in haematopoietic cell transplantation

Haematopoietic cell transplantation (HCT) is the most widely used form of cellular therapy. It is the only known cure for some haematological malignancies and has recently been used in additional clinical settings, such as allograft tolerance induction and treatment of autoimmune diseases. Recent advances have enabled HCT in a wider range of patients with improved outcomes. This Review summarizes the latest developments in this therapy, focusing on issues that will affect future advancement.

Haploidentical allogeneic hematopoietic cell transplantation in adults using CD3/CD19 depletion and reduced intensity conditioning: a phase II study

BACKGROUND

We report a prospective multicenter phase II study of haploidentical hematopoietic stem cell transplantation using CD3/CD19-depleted grafts after reduced intensity conditioning with fludarabine, thiotepa, melphalan and OKT-3.

DESIGN AND METHODS

Sixty-one adults with a median age of 46 years (range 19-65 years) have been enrolled. Diagnoses were acute myeloid leukemia (n=38), acute lymphoblastic leukemia (n=8), non-Hodgkin's lymphoma (n=6), myeloma (n=4), chronic myeloid leukemia (n=3), chronic lymphatic leukemia (n=1) and myelodysplastic syndrome (n=1). Patients were considered high risk because of refractory disease (n=18), cytogenetics (n=6), complete remission (≥ 2) (n=9), chemosensitive relapse in partial remission (n=4) or relapse after prior hematopoietic stem cell transplantation (n=15 allogeneic, n=8 autologous, n=1 both). At haploidentical hematopoietic stem cell transplantation, 30 patients were in complete remission and 31 in partial remission. Grafts contained a median of 7.0 × 10(6) (range 3.2-22) CD34(+) cells/kg, 4.2 × 10(4) (range 0.6-44) CD3(+) T cells/kg and 2.7 × 10(7) (range 0.00-37.3) CD56(+) cells/kg.

RESULTS

Engraftment was rapid with a median of 12 days to granulocytes more than 0.5 × 10(9)/L (range 9-50 days) and 11 days to platelets more than 20 × 10(9) (range 7-38 days). Incidence of grade IIIV acute graft-versus-host-disease and chronic graft-versus-host-disease was 46% and 18%, respectively. Non-relapse mortality on Day 100 was 23% and 42% at two years. Cumulative incidence of relapse/progression at two years was 31%. Kaplan-Meier estimated 1-year and 2-year overall survival with median follow up of 869 days (range 181-1932) is 41% and 28%, respectively.

CONCLUSIONS

This regimen allows successful haploidentical hematopoietic stem cell transplantation with reduced intensity conditioning in high-risk patients lacking a suitable donor. (clinicaltrials.gov identifier:NCT00202917).

Improving TCR Gene Therapy for Treatment of Haematological Malignancies

Adoptive immunotherapy using TCR gene modified T cells may allow separation of beneficial Graft versus tumour responses from harmful GvHD. Improvements to this include methods to generate high avidity or high affinity TCR, improvements in vector design and reduction in mispairing. Following adoptive transfer, TCR transduced T cells must be able to survive and persist in vivo to give most effective antitumour responses. Central memory or naive T cells have both been shown to be more effective than effector cells at expanding and persisting in vivo. Lymphodepletion may enhance persistence of transferred T cell populations. TCR gene transfer can be used to redirect CD4 helper T cells, and these could be used in combination with CD8+ tumour specific T cells to provide help for the antitumour response. Antigen specific T regulatory T cells can also be generated by TCR gene transfer and could be used to suppress unwanted alloresponses.

Retroviral integrations in gene therapy trials

γ-Retroviral and lentiviral vectors allow the permanent integration of a therapeutic transgene in target cells and have provided in the last decade a delivery platform for several successful gene therapy (GT) clinical approaches. However, the occurrence of adverse events due to insertional mutagenesis in GT treated patients poses a strong challenge to the scientific community to identify the mechanisms at the basis of vector-driven genotoxicity. Along the last decade, the study of retroviral integration sites became a fundamental tool to monitor vector–host interaction in patients overtime. This review is aimed at critically revising the data derived from insertional profiling, with a particular focus on the evidences collected from GT clinical trials. We discuss the controversies and open issues associated to the interpretation of integration site analysis during patient's follow up, with an update on the latest results derived from the use of high-throughput technologies. Finally, we provide a perspective on the future technical development and on the application of these studies to address broader biological questions, from basic virology to human hematopoiesis.

The chemokine system in allogeneic stem-cell transplantation: a possible therapeutic target?

Further improvements in allogeneic stem-cell transplantation will probably depend on a better balance between immunosuppression to control graft-versus-host disease and immunological reconstitution sufficient to ensure engraftment, reduction of infection-related mortality and maintenance of post-transplant antileukemic immune reactivity. The chemokine network is an important part of the immune system, and, in addition, CXCL12/CXCR4 seem to be essential for granulocyte colony-stimulating factor-induced stem-cell mobilization. Partial ex vivo graft T-cell depletion based on the expression of specific chemokine receptors involved in T-cell recruitment to graft-versus-host disease target organs may also become a future therapeutic strategy; an alternative approach could be pharmacological inhibition (single-receptor inhibitors or dual-receptor inhibitors) in vivo of specific chemokine receptors involved in this T-cell recruitment. Future clinical studies should therefore be based on a better characterization of various immunocompetent cells, including their chemokine receptor profile, both in the allografts and during post-transplant reconstitution.

Genotoxicity assay for gene therapy vectors in tumor prone Cdkn2a⁻/⁻ mice

Integrative viral vectors are able to efficiently transduce hematopoietic stem progenitor cells allowing stable transgene expression in the entire hematopoietic system upon transplant in conditioned recipients. For these reasons, integrative vectors based on γ-retroviruses and lentiviruses have been successfully used in gene therapy clinical trials for the treatment of genetic diseases, especially blood disorders. However, in different γ-retroviral-based clinical trials, vector integration into the host cell genome triggered oncogenesis by a mechanism called insertional mutagenesis. Thus, a thorough reassessment of the safety of available gene transfer systems is a crucial outstanding issue for the whole gene therapy field. Sensitive preclinical models of vector genotoxicity are instrumental to achieve a more detailed understanding of the factors that modulate the risks of insertional mutagenesis. Here, we will describe the methodologies used to address the mutagenesis risk of vector integration using a murine in vivo genotoxicity assay based on transduction and transplantation of tumor-prone hematopoietic stem and progenitor cells.

Inducible apoptosis as a safety switch for adoptive cell therapy

BACKGROUND

Cellular therapies could play a role in cancer treatment and regenerative medicine if it were possible to quickly eliminate the infused cells in case of adverse events. We devised an inducible T-cell safety switch that is based on the fusion of human caspase 9 to a modified human FK-binding protein, allowing conditional dimerization. When exposed to a synthetic dimerizing drug, the inducible caspase 9 (iCasp9) becomes activated and leads to the rapid death of cells expressing this construct.

METHODS

We tested the activity of our safety switch by introducing the gene into donor T cells given to enhance immune reconstitution in recipients of haploidentical stem-cell transplants. Patients received AP1903, an otherwise bioinert small-molecule dimerizing drug, if graft-versus-host disease (GVHD) developed. We measured the effects of AP1903 on GVHD and on the function and persistence of the cells containing the iCasp9 safety switch.

RESULTS

Five patients between the ages of 3 and 17 years who had undergone stem-cell transplantation for relapsed acute leukemia were treated with the genetically modified T cells. The cells were detected in peripheral blood from all five patients and increased in number over time, despite their constitutive transgene expression. A single dose of dimerizing drug, given to four patients in whom GVHD developed, eliminated more than 90% of the modified T cells within 30 minutes after administration and ended the GVHD without recurrence.

CONCLUSIONS

The iCasp9 cell-suicide system may increase the safety of cellular therapies and expand their clinical applications. (Funded by the National Heart, Lung, and Blood Institute and the National Cancer Institute; ClinicalTrials.gov number, NCT00710892.).

Haploidentical hematopoietic transplantation: current status and future perspectives

For patients with hematologic malignancies at high risk of relapse who do not have matched donors, a suitable alternative stem cell source is the HLA-haploidentical 2 or 3-loci mismatched family donor who is readily available for nearly all patients. Transplantation across the major HLA barrier is associated with strong T-cell alloreactions, which were originally manifested as a high incidence of severe GVHD and graft rejection. The present review shows how these obstacles to successful transplantation were overcome in the last 15 years, making full haplotype-mismatched transplantation a clinical reality that provides similar outcomes to transplantation from matched unrelated donors. The review also discusses the advantages and drawbacks of current options for full haplotype-mismatched transplantation and highlights innovative approaches for re-building immunity after transplantation and improving survival.

Optimization manufacture of virus- and tumor-specific T cells

Although ex vivo expanded T cells are currently widely used in pre-clinical and clinical trials, the complexity of manufacture remains a major impediment for broader application. In this review we discuss current protocols for the ex vivo expansion of virus- and tumor-specific T cells and describe our experience in manufacture optimization using a gas-permeable static culture flask (G-Rex). This innovative device has revolutionized the manufacture process by allowing us to increase cell yields while decreasing the frequency of cell manipulation and in vitro culture time. It is now being used in good manufacturing practice (GMP) facilities for clinical cell production in our institution as well as many others in the US and worldwide.

Moving to tolerance: clinical application of T regulatory cells

Decreasing the incidence of chronic rejection and reducing the need for life-long immunosuppression remain important goals in clinical transplantation. In this article, we will review how regulatory T cells (Treg) came to be recognized as an attractive way to prevent or treat allograft rejection, the ways in which Treg can be manipulated or expanded in vivo, and the potential of in vitro expanded/generated Treg for cellular therapy. We will describe the first regulatory T cell therapies that have been or are in the process of being conducted in the clinic as well as the safety concerns of such therapies and how outcomes may be measured.

Strategies to optimize the outcome of children given T-cell depleted HLA-haploidentical hematopoietic stem cell transplantation

The most advanced frontier of allogeneic hematopoietic stem cell transplantation (allo-HSCT) is represented by the use of an HLA-partially matched relative as donor. In this type of transplantation, donor-derived natural killer (NK) cells, which are alloreactive toward recipient cells, significantly contribute to the eradication of leukemia blasts. Alloreactive NK cells may also kill host dendritic cells and T lymphocytes, thus preventing graft-versus-host disease and graft rejection, respectively. Sophisticated strategies of adoptive infusion of T-cell lines/clones specific for the most life-threatening pathogens (namely cytomegalovirus, Epstein-Barr virus, Aspergillus and Adenovirus) have been envisaged, and successfully tested in a few pilot trials, to protect the recipient in the early post-transplantation period. In these patients, also ex-vivo expanded mesenchymal stromal cells have been shown to be beneficial for preventing graft failure. Novel and effective strategies aimed at further augmenting the graft-versus-leukemia effect and at optimizing prevention/treatment of opportunistic/viral infections are warranted.

Chimeric antigen receptor (CAR)-engineered lymphocytes for cancer therapy

INTRODUCTION

Chimeric antigen receptors (CARs) usually combine the antigen binding site of a monoclonal antibody with the signal activating machinery of a T cell, freeing antigen recognition from MHC restriction and thus breaking one of the barriers to more widespread application of cellular therapy. Similar to treatment strategies employing monoclonal antibodies, T cells expressing CARs are highly targeted, but additionally offer the potential benefits of active trafficking to tumor sites, in vivo expansion and long-term persistence. Furthermore, gene transfer allows the introduction of countermeasures to tumor immune evasion and of safety mechanisms.

AREAS COVERED

The basic structure of so-called first and later generation CARs and their potential advantages over other immune therapy systems. How these molecules can be grafted into immune cells (including retroviral and non-retroviral transduction methods) and strategies to improve the in vivo persistence and function of immune cells expressing CARs. Examples of tumor-associated antigens that have been targeted in preclinical models and clinical experience with these modified cells. Safety issues surrounding CAR gene transfer into T cells and potential solutions to them.

EXPERT OPINION

Because of recent advances in immunology, genetics and cell processing, CAR-modified T cells will likely play an increasing role in the cellular therapy of cancer, chronic infections and autoimmune disorders.

Innovative platforms for haploidentical stem cell transplantation: the role of unmanipulated donor graft

We exploited the dual positive effects of rapamycin to prevent GvHD and control malignant cells upon infusion of unmanipulated grafts from family haploidentical donors to patients affected by advanced hematological malignancies. Preliminary results on 45 patients show the feasibility of this platform with an appreciable low rate of GvHD.

Gammaretroviral vectors: biology, technology and application

Retroviruses are evolutionary optimized gene carriers that have naturally adapted to their hosts to efficiently deliver their nucleic acids into the target cell chromatin, thereby overcoming natural cellular barriers. Here we will review—starting with a deeper look into retroviral biology—how Murine Leukemia Virus (MLV), a simple gammaretrovirus, can be converted into an efficient vehicle of genetic therapeutics. Furthermore, we will describe how more rational vector backbones can be designed and how these so-called self-inactivating vectors can be pseudotyped and produced. Finally, we will provide an overview on existing clinical trials and how biosafety can be improved.

Ex vivo gene transfer and correction for cell-based therapies

Cell-based therapies are fast-growing forms of personalized medicine that make use of the steady advances in stem cell manipulation and gene transfer technologies. In this Review, I highlight the latest developments and the crucial challenges for this field, with an emphasis on haematopoietic stem cell gene therapy, which is taken as a representative example given its advanced clinical translation. New technologies for gene correction and targeted integration promise to overcome some of the main hurdles that have long prevented progress in this field. As these approaches marry with our growing capacity for genetic reprogramming of mammalian cells, they may fulfil the promise of safe and effective therapies for currently untreatable diseases.

Clinical tolerance in allogeneic hematopoietic stem cell transplantation

Allogeneic hematopoietic stem cell transplantation (HSCT) has been a curative therapeutic option for a wide range of immune hematologic malignant and non-malignant disorders including genetic diseases and inborn errors. Once in the host, allogeneic transplanted cells have not only to ensure myeloid repopulation and immunological reconstitution but also to acquire tolerance to host human leukocyte antigens via central or peripheral mechanisms. Peripheral tolerance after allogeneic HSCT depends on several regulatory mechanisms aimed at blocking alloimmune reactivity while preserving immune responses to pathogens and tumor antigens. Patients transplanted with HSCT represent an ideal model system in humans to identify and characterize the key cellular and molecular players underlying these mechanisms. The knowledge gained from these studies has allowed the development of novel therapeutic strategies aimed at inducing long-term peripheral tolerance, which can be applicable not only in allogeneic HSCT but also in autoimmune diseases and solid-organ transplantation. In the present review, we describe Type 1 regulatory T cells, initially discovered and characterized in chimeric patients transplanted with human leukocyte antigen-mismatched HSCT, and how their presence correlates to tolerance induction and maintenance. Furthermore, we summarize different cell therapy approaches with regulatory T cells, designed to facilitate tolerance induction, minimizing pharmaceutical interventions.

Induced pluripotent stem cells for regenerative cardiovascular therapies and biomedical discovery

The discovery of induced pluripotent stem cells (iPSC) has, in the short time since their discovery, revolutionized the field of stem cell biology. This technology allows the generation of a virtually unlimited supply of cells with pluripotent potential similar to that of embryonic stem cells (ESC). However, in contrast to ESC, iPSC are not subject to the same ethical concerns and can be easily generated from living individuals. For the first time, patient-specific iPSC can be generated and offer a supply of genetically identical cells that can be differentiated into all somatic cell types for potential use in regenerative therapies or drug screening and testing. As the techniques for generation of iPSC lines are constantly evolving, new uses for human iPSC are emerging from in-vitro disease modeling to high throughput drug discovery and screening. This technology promises to revolutionize the field of medicine and offers new hope for understanding and treatment of numerous diseases.

IL-7 receptor expression identifies suicide gene-modified allospecific CD8+ T cells capable of self-renewal and differentiation into antileukemia effectors

In allogeneic hematopoietic cell transplantation (HSCT), donor T lymphocytes mediate the graft-versus-leukemia (GVL) effect, but induce graft-versus-host disease (GVHD). Suicide gene therapy-that is, the genetic induction of a conditional suicide phenotype into donor T cells-allows dissociating the GVL effect from GVHD. Genetic modification with retroviral vectors after CD3 activation reduces T-cell alloreactivity. We recently found that alloreactivity is maintained when CD28 costimulation, IL-7, and IL-15 are added. Herein, we used the minor histocompatibility (mH) antigens HA-1 and H-Y as model alloantigens to directly explore the antileukemia efficacy of human T cells modified with the prototypic suicide gene herpes simplex virus thymidine kinase (tk) after activation with different stimuli. Only in the case of CD28 costimulation, IL-7, and IL-15, the repertoire of tk(+) T cells contained HA-1- and H-Y-specific CD8(+) cytotoxic T cells (CTL) precursors. Thymidine kinase-positive HA-1- and H-Y-specific CTLs were capable of self-renewal and differentiation into potent antileukemia effectors in vitro, and in vivo in a humanized mouse model. Self-renewal and differentiation coincided with IL-7 receptor expression. These results pave the way to the clinical investigation of T cells modified with a suicide gene after CD28 costimulation, IL-7, and IL-15 for a safe and effective GVL effect.

Genetically modified donor leukocyte transfusion and graft-versus-leukemia effect after allogeneic stem cell transplantation

Seven patients with acute myeloid leukemia (AML) and two patients with chronic myelogenous leukemia (CML) were transplanted from HLA-identical sibling donors with CD34(+) cell-enriched stem cells (HSCTs) without further immunosuppression. The myeloablative standard transplantation protocol was adapted to include transfusion of gene-modified donor T cells after HSCT. Donor T cells were transduced with the replication-deficient retrovirus SFCMM-3, which expresses herpes simplex thymidine kinase (HSV-Tk) and a truncated version of low-affinity nerve growth factor receptor (ΔLNGFR) for selection and characterization of transduced cells. Transduced T cells were detectable in all patients during follow-up for up to 5 years after transfusion. Proteomic screening for development of acute graft-versus-host disease (aGvHD) was applied to five of the seven patients with AML. No positivity for the aGvHD grade II-specific proteomic pattern was observed. Only one patient developed aGvHD grade I. To date, three of the patients with AML relapsed; one responded to three escalating transfusions of lymphocytes from the original donor and is in complete remission. Two were retransplanted with non-T cell-depleted peripheral blood stem cells from their original donors and died after retransplantation of septic complications or relapse, respectively. In one patient with CML, loss of bcr-abl gene expression was observed after an expansion of transduced cells. Seven of nine patients are alive and in complete remission.

Immunotherapy through T-cell receptor gene transfer induces severe graft-versus-host disease

Evaluation of: Bendle GM, Linnemann C, Hooijkaas AI et al.: Lethal graft-versus-host disease in mouse models of T cell receptor gene therapy. Nat. Med. 16(5), 565-570 (2010). Graft-versus-host disease is commonly associated with allogeneic hematopoietic cell transplantation, as it is the major complication. This article reports that, after immunotherapy with lymphocytes that have been transduced with T-cell receptor (TCR) genes of known specificity, graft-versus-host disease can occur through TCR gene transfer. This autoimmune pathology occurs through the formation of self-reactive TCRs as a result of one chain of the transduced TCR cross-pairing with an endogenous TCR. Certain adjustments in the design of gene therapy vectors may help reduce the risk of such autoimmune phenomena.

Ex vivo gene transfer for improved adoptive immunotherapy of cancer

Adoptive immunotherapy is an appealing approach to cancer treatment, with the potential for more precise targeting and reduced toxicity. While early clinical trial data using adoptive T cells against post-transplant virus-associated hematologic malignancies, lymphoma and melanoma have been promising, treating other solid tumors has proven to be more challenging. Adoptive lymphocytes have been genetically modified in many ways to improve activity and circumvent tumor evasion, including transfer of transgenic T-cell receptors and chimeric antigen receptors to redirect T cell and natural killer cell antigen specificity. Gene transfer may also allow expression of homeostatic cytokines or their receptors to overcome the lack of stimulatory signals or expression of dominant-negative receptors for inhibitory cytokines to compensate for an immunosuppressive tumor milieu. In addition, suicide genes can install a 'safety switch' on adoptively transferred cells to allow ablation if necessary. Although further refinement and validation are necessary, these genetic modification strategies offer hope for significant improvements in cancer immunotherapy.

Lentiviral vectors for induction of self-differentiation and conditional ablation of dendritic cells

Development of lentiviral vectors (LVs) in the field of immunotherapy and immune regeneration will strongly rely on biosafety of the gene transfer. We demonstrated previously the feasibility of ex vivo genetic programming of mouse bone marrow precursors with LVs encoding granulocyte macrophage colony-stimulating factor (GM-CSF) and interleukin-4 (IL-4), which induced autonomous differentiation of long-lived dendritic cells (DCs), referred to as self-differentiated myeloid-derived antigen-presenting-cells reactive against tumors (SMART-DCs). Here, LV biosafety was enhanced by using a DC-restricted and physiological promoter, the major histocompatibility complex (MHC) II promoter, and including co-expression of the herpes simplex virus-thymidine kinase (sr39HSV-TK) conditional suicide gene. Tricistronic vectors co-expressing sr39HSV-TK, GM-CSF and IL-4 transcriptionally regulated by the MHCII promoter or the ubiquitous cytomegalovirus (CMV) promoter were compared. Despite the different gene transfer effects, such as the kinetics, levels of transgene expression and persistency of integrated vector copies, both vectors induced highly viable SMART-DCs, which persisted for at least 70 days in vivo and could be ablated with the pro-drug Ganciclovir (GCV). SMART-DCs co-expressing the tyrosine-related protein 2 melanoma antigen administered subcutaneously generated antigen-specific, anti-melanoma protective and therapeutic responses in the mouse B16 melanoma model. GCV administration after immunotherapy did not abrogate DC vaccination efficacy. This demonstrates proof-of-principle of genetically programmed DCs that can be ablated pharmacologically.

Gene-modified T lymphocytes in the setting of hematopoietic cell transplantation: potential benefits and possible risks

INTRODUCTION

Allogeneic hematopoietic cell transplantation (HCT) is a consolidated treatment for several hematologic malignancies. Donor T lymphocytes can mediate a graft versus tumor (GVT) effect and control opportunistic infections but can also cause severe graft versus host disease (GVHD). Gene-transfer strategies are appealing tools to modulate T cell functions when infused after HCT.

AREAS COVERED

The current and potential future applications of T cell gene-transfer approaches to HCT. This review is not limited to GVHD control but covers the issues of GVT and immune reconstitution. Clinical data are used to discuss more general issues, perspectives and concerns common to gene-modification of T cells. An overview of the results and limitations emerging from clinical trials with herpes simplex virus-thymidine kinase (HSV-TK) engineered lymphocytes is provided. The review provides perspectives on additional gene-transfer strategies, currently at preclinical level or that have just entered clinical trials, to increase the efficacy and safety of HCT.

EXPERT OPINION

Gene-transfer can positively interfere with T cell functions after HCT. TK-lymphocytes have proven effective in controlling GVHD while retaining an acceptable GVT effect. Strategies exploiting new suicide molecules or engineered T cell receptors (TCRs) should be further explored to address current limitations with TK-lymphocytes and augment the efficacy and specificity of GVT and antiviral activity.

Retroviral vectors for gene therapy

Since their first clinical trial 20 years ago, retroviral (gretroviral and lentiviral) vectors have now been used in more than 350 gene-therapy studies. Retroviral vectors are particularly suited for gene-correction of cells due to long-term and stable expression of the transferred transgene(s), and also because little effort is required for their cloning and production. Several monogenic inherited diseases, mostly immunodeficiencies, can now be successfully treated. The occurrence of insertional mutagenesis in some studies allowed extensive analysis of integration profiles of retroviral vectors, as well as the design of lentiviral vectors with increased safety properties. These new-generation vectors will enable us to continue the successful story of gene therapy, and treat more patients and even more complex diseases.

Tregs prevent GVHD and promote immune reconstitution in HLA-haploidentical transplantation

Hastening posttransplantation immune reconstitution is a key challenge in human leukocyte antigen (HLA)-haploidentical hematopoietic stem-cell transplantation (HSCT). In experimental models of mismatched HSCT, T-regulatory cells (Tregs) when co-infused with conventional T cells (Tcons) favored posttransplantation immune reconstitution and prevented lethal graft-versus-host disease (GVHD). In the present study, we evaluated the impact of early infusion of Tregs, followed by Tcons, on GVHD prevention and immunologic reconstitution in 28 patients with high-risk hematologic malignancies who underwent HLA-haploidentical HSCT. We show for the first time in humans that adoptive transfer of Tregs prevented GVHD in the absence of any posttransplantation immunosuppression, promoted lymphoid reconstitution, improved immunity to opportunistic pathogens, and did not weaken the graft-versus-leukemia effect. This study provides evidence that Tregs are a conserved mechanism in humans.

Ex-vivo gene therapy restores LEKTI activity and corrects the architecture of Netherton syndrome-derived skin grafts

Netherton syndrome (NS) is a debilitating congenital skin disorder caused by mutations in the SPINK5 gene encoding the lymphoepithelial Kazal-type-related inhibitor (LEKTI). It is characterized by defective keratinization, recurrent infections, and hypernatraemic dehydration with a mortality rate of about 10% in the first year of life. Currently, there are no curative treatments for NS. We have developed a HIV-1 based, self-inactivating lentiviral vector to express SPINK5 in keratinocytes as part of an ex-vivo gene therapy strategy for NS. High transduction efficiency was achieved in NS keratinocytes and reconstitution of LEKTI expression was confirmed in previously deficient cells. These genetically corrected keratinocytes were further tested in an in vitro organotypic culture (OTC) system and in vivo mouse/human skin engraftment model. Results showed correction of epidermal architecture in both OTCs and regenerated skin grafts. Importantly, the results from corrected skin grafts indicated that even where detectable LEKTI expression was restored to a limited numbers of cells, a wider bystander benefit occurred around these small populations. As LEKTI is a secreted protein, the genetically modified graft may provide not only an immediate local protective barrier, but also act as a source of secreted LEKTI providing a generalized benefit following ex-vivo gene therapy.

Alternative donors hematopoietic stem cells transplantation for adults with acute myeloid leukemia: Umbilical cord blood or haploidentical donors?

Use of allogeneic transplantation for patients with acute myeloid leukemia (AML) depends mainly on the risk of the disease, and HLA matched donor availability. In patients with high-risk leukemia, in the absence of a HLA (human leukocyte antigen) matched donor, alternative donors such as unrelated umbilical cord blood (UCB) or haploidentical donor (haplo) have been currently used. Both strategies have important advantages such as shorter time to transplant, which is particularly relevant to patients requiring urgent transplantation, and tolerance of HLA mismatched graft that make possible that a donor can be found for virtually all patients. However, in spite of higher incidence of graft failure in UCB transplatation recipients and higher relapse incidence after haplo transplants, final outcomes seem to be comparable with HLA matched unrelated hematopoietic stem cell transplantation (bone marrow or peripheral blood). Therefore, the complexity of choosing the best alternative donor will depend on urgency of the transplantation, status and risk of the disease, donor criteria and center experience. Here we review the current status of UCBT and haplo transplants to treat adults with high-risk acute myeloid leukemia and we discuss the main issues associated with the use of both hematopoietic stem cell transplant approaches.

Haploidentical hematopoietic stem-cell transplantation in adults

Haploidentical hematopoietic stem-cell transplantation is an alternative transplant strategy for patients without an HLA-matched donor. Still, only half of patients who might benefit from transplantation are able to find an HLA-matched related or unrelated donor. Haploidentical donor is readily available for many patients in need of immediate stem-cell transplantation. Historical experience with haploidentical stem-cell transplantation has been characterised by a high rejection rate, graft-versus-host disease, and transplant-related mortality. Important advances have been made in this field during the last 20 years. Many drawbacks of haploidentical transplants such as graft failure and significant GVHD have been overcome due to the development of new extensive T cell depletion methods with mega dose stem-cell administration. However, prolonged immune deficiency and an increased relapse rate remain unresolved problems of T cell depletion. New approaches such as partial ex vivo or in vivo alloreactive T cell depletion and posttransplant cell therapy will allow to improve immune reconstitution in haploidentical transplants. Results of unmanipulated stem-cell transplantation with using ATG and combined immunosuppression in mismatched/haploidentical transplant setting are promising. This paper focuses on recent advances in haploidentical hematopoietic stem-cell transplantation for hematologic malignancies.

Genetic modification of T cells

Adoptively transferred T cells have shown activity in treating viral infections after hemopoietic transplantation and anti-tumor activity against some malignancies such as melanoma and lymphoma. Current research focuses on defining the optimum type of cell for transfer to improve persistence and genetically modifying infused T cells to augment function, overcome tumor evasion strategies and allow ablation should adverse effects occur.