Gene therapy in peripheral blood lymphocytes and bone marrow for ADA- immunodeficient patients

In vivo tracking of T cells in humans unveils decade-long survival and activity of genetically modified T memory stem cells

A definitive understanding of survival and differentiation potential in humans of T cell subpopulations is of paramount importance for the development of effective T cell therapies. In particular, uncovering the dynamics in vivo in humans of the recently described T memory stem cells (TSCM) would be crucial for therapeutic approaches that aim at taking advantage of a stable cellular vehicle with precursor potential. We exploited data derived from two gene therapy clinical trials for an inherited immunodeficiency, using either retrovirally engineered hematopoietic stem cells or mature lymphocytes to trace individual T cell clones directly in vivo in humans. We compared healthy donors and bone marrow-transplanted patients, studied long-term in vivo T cell composition under different clinical conditions, and specifically examined TSCM contribution according to age, conditioning regimen, disease background, cell source, long-term reconstitution, and ex vivo gene correction processing. High-throughput sequencing of retroviral vector integration sites (ISs) allowed tracing the fate of more than 1700 individual T cell clones in gene therapy patients after infusion of gene-corrected hematopoietic stem cells or mature lymphocytes. We shed light on long-term in vivo clonal relationships among different T cell subtypes, and we unveiled that TSCM are able to persist and to preserve their precursor potential in humans for up to 12 years after infusion of gene-corrected lymphocytes. Overall, this work provides high-resolution tracking of T cell fate and activity and validates, in humans, the safe and functional decade-long survival of engineered TSCM, paving the way for their future application in clinical settings.

Outcomes in two Japanese adenosine deaminase-deficiency patients treated by stem cell gene therapy with no cytoreductive conditioning

OBJECTIVE

We here describe treatment outcomes in two adenosine deaminase (ADA)-deficiency patients (pt) who received stem cell gene therapy (SCGT) with no cytoreductive conditioning. As this protocol has features distinct from those of other clinical trials, its results provide insights into SCGT for ADA deficiency.

PATIENTS AND METHODS

Pt 1 was treated at age 4.7 years, whereas pt 2, who had previously received T-cell gene therapy, was treated at age 13 years. Bone marrow CD34(+) cells were harvested after enzyme replacement therapy (ERT) was withdrawn; following transduction of ADA cDNA by the γ-retroviral vector GCsapM-ADA, they were administered intravenously. No cytoreductive conditioning, at present considered critical for therapeutic benefit, was given before cell infusion. Hematological/immunological reconstitution kinetics, levels of systemic detoxification, gene-marking levels, and proviral insertion sites in hematopoietic cells were assessed.

RESULTS

Treatment was well tolerated, and no serious adverse events were observed. Engraftment of gene-modified repopulating cells was evidenced by the appearance and maintenance of peripheral lymphocytes expressing functional ADA. Systemic detoxification was moderately achieved, allowing temporary discontinuation of ERT for 6 and 10 years in pt 1 and pt 2, respectively. Recovery of immunity remained partial, with lymphocyte counts in pts 1 and 2, peaked at 408/mm(3) and 1248/mm(3), approximately 2 and 5 years after SCGT. Vector integration site analyses confirmed that hematopoiesis was reconstituted with a limited number of clones, some of which were shown to have myelo-lymphoid potential.

CONCLUSIONS

Outcomes in SCGT for ADA-SCID are described in the context of a unique protocol, which used neither ERT nor cytoreductive conditioning. Although proven safe, immune reconstitution was partial and temporary. Our results reiterate the importance of cytoreductive conditioning to ensure greater benefits from SCGT.

Nanotherapy for Cancer: Targeting and Multifunctionality in the Future of Cancer Therapies

Cancer continues to be a prevalent and lethal disease, despite advances in tumor biology research and chemotherapy development. Major obstacles in cancer treatment arise from tumor heterogeneity, drug resistance, and systemic toxicities. Nanoscale delivery systems, or nanotherapies, are increasing in importance as vehicles for antineoplastic agents because of their potential for targeting and multifunctionality. We discuss the current field of cancer therapy and potential strategies for addressing obstacles in cancer treatment with nanotherapies. Specifically, we review the strategies for rationally designing nanoparticles for targeted, multimodal delivery of therapeutic agents.

HSV-sr39TK positron emission tomography and suicide gene elimination of human hematopoietic stem cells and their progeny in humanized mice

Engineering immunity against cancer by the adoptive transfer of hematopoietic stem cells (HSC) modified to express antigen-specific T-cell receptors (TCR) or chimeric antigen receptors generates a continual supply of effector T cells, potentially providing superior anticancer efficacy compared with the infusion of terminally differentiated T cells. Here, we demonstrate the in vivo generation of functional effector T cells from CD34-enriched human peripheral blood stem cells modified with a lentiviral vector designed for clinical use encoding a TCR recognizing the cancer/testes antigen NY-ESO-1, coexpressing the PET/suicide gene sr39TK. Ex vivo analysis of T cells showed antigen- and HLA-restricted effector function against melanoma. Robust engraftment of gene-modified human cells was demonstrated with PET reporter imaging in hematopoietic niches such as femurs, humeri, vertebrae, and the thymus. Safety was demonstrated by the in vivo ablation of PET signal, NY-ESO-1-TCR-bearing cells, and integrated lentiviral vector genomes upon treatment with ganciclovir, but not with vehicle control. Our study provides support for the efficacy and safety of gene-modified HSCs as a therapeutic modality for engineered cancer immunotherapy. Cancer Res; 74(18); 5173-83. ©2014 AACR.

The case for transmissible antivirals to control population-wide infectious disease

Infectious disease control faces significant challenges including: how to therapeutically target the highest-risk populations, circumvent behavioral barriers, and overcome pathogen persistence and resistance mechanisms. We review a recently proposed solution to overcome these challenges: antivirals that transmit by 'piggybacking' on viral replication. These proposed antivirals, termed 'therapeutic interfering particles' (TIPs), are engineered molecular parasites of viruses that are designed to steal replication resources from the wild type virus. Depriving viruses of crucial replication machinery, TIPs would reduce viral loads. As obligate parasites, TIPs would transmit via the same risk factors and transmission routes as wild type viruses, automatically reaching high-risk populations, and thereby substantially limiting viral transmission even in resource-poor settings. Design issues and ethical/safety considerations of this proposed intervention are discussed.

Concise review: lessons learned from clinical trials of gene therapy in monogenic immunodeficiency diseases

Thirty years ago, retroviral transfer of genetic material into hematopoietic stem and progenitor cells (HSC/Ps) led to predictions that this technology would transform modern medicine [Nature 1983;305:556-558; Nature 1984;310:476-480]. Studies in several immunodeficiency diseases in the past 15 years have demonstrated clear proof of principle that gene therapy can have long-lasting, potentially curative effects without the need to search for allogeneic donors and without risk of graft-versus-host disease. Improvement in gene transfer efficiency for target HSC/Ps brought to light issues of insertional mutagenesis caused by transfer vectors, resulting in oncogene transactivation and leukemias. Lessons from these adverse events have now led to a new generation of vectors, refinements in conditioning regimens, and manufacturing, which are paving the way for expanded applications of the current technology and recent emphasis on gene targeting/genome editing as the next advancements in the field.

Adoptive immunotherapy for cancer or viruses

Adoptive immunotherapy, or the infusion of lymphocytes, is a promising approach for the treatment of cancer and certain chronic viral infections. The application of the principles of synthetic biology to enhance T cell function has resulted in substantial increases in clinical efficacy. The primary challenge to the field is to identify tumor-specific targets to avoid off-tumor, on-target toxicity. Given recent advances in efficacy in numerous pilot trials, the next steps in clinical development will require multicenter trials to establish adoptive immunotherapy as a mainstream technology.

[A therapeutic approach towards chronic granulomatous disease]

Chronic granulomatous disease (CGD) is a primary immunodeficiency (PID) characterized by the inability of phagocytes to produce reactive oxygen intermediates (ROIs) due to a defect in the NADPH oxidase complex. Recent studies have revealed that ROIs are involved in inflammatory signaling in phagocytes, illuminating the underlying mechanisms of hyper-inflammation in CGD. CGD patients frequently suffer from CGD-associated bowel inflammation, granuloma, and life-threatening infections. Based on the discovery of the regulatory function of ROIs in the immune response, therapeutic methods for excessive inflammation focusing on inflammatory cytokines are being developed for CGD. Although hematopoietic stem cell (HSC) transplantation (HSCT) is a curative therapy for CGD, successful transplants greatly depend on HSC source selection and the degree of matching of potential donors. Gene therapy trials for PID have been performed on over 120 patients with no HLA identical donor for HSCT, and have demonstrated clinical benefits. Genotoxicity in HSC gene therapy trials has expanded our knowledge on the mechanisms of vector-associated clonal expansion of gene-modified cells, which will advance gene therapy development using self-inactivating retrovirus and lentivirus vectors. We will discuss the complications of HSCT for CGD. We will then outline the status of gene therapy approaches in the treatment of CGD.

Gene therapy as a new treatment option for inherited monogenic diseases

BACKGROUND

Gene therapy, replacing a defective gene by a functional copy, has been in development for more than 40years. Initial efforts involved engineering viral vectors to deliver genes to the appropriate cells. Early successes in severe combined immunodeficiency (SCID) were later derailed by safety issues including host reaction to the vector and gene insertion near promoters that favored secondary leukemia.

METHODS

Systematic review of the literature using PubMed.gov with key word gene therapy from 1972 to March 2013. Google search with key word gene therapy.

RESULTS

Despite early setbacks, progresses for monogenic diseases continued unabated. Patients with SCIDs have been cured and the first gene therapy has been approved for lipoprotein lipase deficiency. Many clinical research studies are ongoing as part of systematic clinical development program with a view to have more gene therapies approved.

CONCLUSION

Our review highlights progresses and questions that remain to be answered to make gene therapy an integral part of our therapeutic arsenal.

Establishing diagnostic criteria for severe combined immunodeficiency disease (SCID), leaky SCID, and Omenn syndrome: the Primary Immune Deficiency Treatment Consortium experience

BACKGROUND

The approach to the diagnosis of severe combined immunodeficiency disease (SCID) and related disorders varies among institutions and countries.

OBJECTIVES

The Primary Immune Deficiency Treatment Consortium attempted to develop a uniform set of criteria for diagnosing SCID and related disorders and has evaluated the results as part of a retrospective study of SCID in North America.

METHODS

Clinical records from 2000 through 2009 at 27 centers in North America were collected on 332 children treated with hematopoietic stem cell transplantation (HCT), enzyme replacement therapy, or gene therapy for SCID and related disorders. Eligibility for inclusion in the study and classification into disease groups were established by using set criteria and applied by an expert review group.

RESULTS

Two hundred eighty-five (86%) of the patients were determined to be eligible, and 47 (14%) were not eligible. Of the 285 eligible patients, 84% were classified as having typical SCID; 13% were classified as having leaky SCID, Omenn syndrome, or reticular dysgenesis; and 3% had a history of enzyme replacement or gene therapy. Detection of a genotype predicting an SCID phenotype was accepted for eligibility. Reasons for noneligibility were failure to demonstrate either impaired lymphocyte proliferation or maternal T-cell engraftment. Overall (n = 332) rates of testing were as follows: proliferation to PHA, 77%; maternal engraftment, 35%; and genotype, 79% (mutation identified in 62%).

CONCLUSION

Lack of complete laboratory evaluation of patients before HCT presents a significant barrier to definitive diagnosis of SCID and related disorders and prevented inclusion of subjects in our observational HCT study. This lesson is critical for patient care, as well as the design of future prospective treatment studies for such children because a well-defined and consistent study population is important for precision in outcomes analysis.

Monogenic diseases that can be cured by liver transplantation

While the prevalence of most diseases caused by single-gene mutations is low and defines them as rare conditions, all together, monogenic diseases account for approximately 10 in every 1000 births according to the World Health Organisation. Orthotopic liver transplantation (LT) could offer a therapeutic option in monogenic diseases in two ways: by substituting for an injured liver or by supplying a tissue that can replace a mutant protein. In this respect, LT may be regarded as the correction of a disease at the level of the dysfunctional protein. Monogenic diseases that involve the liver represent a heterogeneous group of disorders. In conditions associated with predominant liver parenchymal damage (i.e., genetic cholestatic disorders, Wilson's disease, hereditary hemochromatosis, tyrosinemia, α1 antitrypsin deficiency), hepatic complications are the major source of morbidity and LT not only replaces a dysfunctional liver but also corrects the genetic defect and effectively cures the disease. A second group includes liver-based genetic disorders characterised by an architecturally near-normal liver (urea cycle disorders, Crigler-Najjar syndrome, familial amyloid polyneuropathy, primary hyperoxaluria type 1, atypical haemolytic uremic syndrome-1). In these defects, extrahepatic complications are the main source of morbidity and mortality while liver function is relatively preserved. Combined transplantation of other organs may be required, and other surgical techniques, such as domino and auxiliary liver transplantation, have been attempted. In a third group of monogenic diseases, the underlying genetic defect is expressed at a systemic level and liver involvement is just one of the clinical manifestations. In these conditions, LT might only be partially curative since the abnormal phenotype is maintained by extrahepatic synthesis of the toxic metabolites (i.e., methylmalonic acidemia, propionic acidemia). This review focuses on principles of diagnosis, management and LT results in both paediatric and adult populations of selected liver-based monogenic diseases, which represent examples of different transplantation strategies, driven by the understanding of the expression of the underlying genetic defect.

Current progress on gene therapy for primary immunodeficiencies

Primary immunodeficiencies have played a major role in the development of gene therapy for monogenic diseases of the bone marrow. The last decade has seen convincing evidence of long-term disease correction as a result of ex vivo viral vector-mediated gene transfer into autologous haematopoietic stem cells. The success of these early studies has been balanced by the development of vector-related insertional mutagenic events. More recently the use of alternative vector designs with self-inactivating designs, which have an improved safety profile has led to the initiation of a wave of new studies that are showing early signs of efficacy. The ongoing development of safer vector platforms and gene-correction technologies together with improvements in cell-transduction techniques and optimised conditioning regimes is likely to make gene therapy amenable for a greater number of PIDs. If long-term efficacy and safety are shown, gene therapy will become a standard treatment option for specific forms of PID.

History of gene therapy

Two decades after the initial gene therapy trials and more than 1700 approved clinical trials worldwide we not only have gained much new information and knowledge regarding gene therapy in general, but also learned to understand the concern that has persisted in society. Despite the setbacks gene therapy has faced, success stories have increasingly emerged. Examples for these are the positive recommendation for a gene therapy product (Glybera) by the EMA for approval in the European Union and the positive trials for the treatment of ADA deficiency, SCID-X1 and adrenoleukodystrophy. Nevertheless, our knowledge continues to grow and during the course of time more safety data has become available that helps us to develop better gene therapy approaches. Also, with the increased understanding of molecular medicine, we have been able to develop more specific and efficient gene transfer vectors which are now producing clinical results. In this review, we will take a historical view and highlight some of the milestones that had an important impact on the development of gene therapy. We will also discuss briefly the safety and ethical aspects of gene therapy and address some concerns that have been connected with gene therapy as an important therapeutic modality.

Gene therapy for PIDs: progress, pitfalls and prospects

Substantial progress has been made in the past decade in treating several primary immunodeficiency disorders (PIDs) with gene therapy. Current approaches are based on ex-vivo transfer of therapeutic transgene via viral vectors to patient-derived autologous hematopoietic stem cells (HSCs) followed by transplantation back to the patient with or without conditioning. The overall outcome from all the clinical trials targeting different PIDs has been extremely encouraging but not without caveats. Malignant outcomes from insertional mutagenesis have featured prominently in the adverse events associated with these trials and have warranted intense pre-clinical investigation into defining the tendencies of different viral vectors for genomic integration. Coupled with issues pertaining to transgene expression, the therapeutic landscape has undergone a paradigm shift in determining safety, stability and efficacy of gene therapy approaches. In this review, we aim to summarize the progress made in the gene therapy trials targeting ADA-SCID, SCID-X1, CGD and WAS, review the pitfalls, and outline the recent advancements which are expected to further enhance favourable risk benefit ratios for gene therapeutic approaches in the future.

Current translational and clinical practices in hematopoietic cell and gene therapy

Clinical trials over the last 15 years have demonstrated that cell and gene therapies for cancer, monogenic and infectious disease are feasible and can lead to long-term benefit for patients. However, these trials have been limited to proof-of-principle and were conducted on modest numbers of patients or over long periods of time. In order for these studies to move towards standard practice and commercialization, scalable technologies for the isolation, ex vivo manipulation and delivery of these cells to patients must be developed. Additionally, regulatory strategies and clinical protocols for the collection, creation and delivery of cell products must be generated. In this article we review recent progress in hematopoietic cell and gene therapy, describe some of the current issues facing the field and discuss clinical, technical and regulatory approaches used to navigate the road to product development.

Gene therapy for primary immunodeficiencies

For over 40 years, primary immunodeficiencies (PIDs) have featured prominently in the development and refinement of human allogeneic hematopoietic stem cell transplantation. More recently, ex vivo somatic gene therapy using autologous cells has provided remarkable evidence of clinical efficacy in patients without HLA-matched stem cell donors and in whom toxicity of allogeneic procedures is likely to be high. Together with improved preclinical models, a wealth of information has accumulated that has allowed development of safer, more sophisticated technologies and protocols that are applicable to a much broader range of diseases. In this review we summarize the status of these gene therapy trials and discuss the emerging application of similar strategies to other PIDs.

Decade-long safety and function of retroviral-modified chimeric antigen receptor T cells

The success of adoptive T cell gene transfer for treatment of cancer and HIV is predicated on generating a response that is both durable and safe. We report long-term results from three clinical trials to evaluate gammaretroviral vector-engineered T cells for HIV. The vector encoded a chimeric antigen receptor (CAR) composed of CD4 linked to the CD3ζ signaling chain (CD4ζ). CAR T cells were detected in 98% of samples tested for at least 11 years after infusion at frequencies that exceeded average T cell levels after most vaccine approaches. The CD4ζ transgene retained expression and function. There was no evidence of vector-induced immortalization of cells; integration site distributions showed no evidence of persistent clonal expansion or enrichment for integration sites near genes implicated in growth control or transformation. The CD4ζ T cells had stable levels of engraftment, with decay half-lives that exceeded 16 years, in marked contrast to previous trials testing engineered T cells. These findings indicate that host immunosuppression before T cell transfer is not required to achieve long-term persistence of gene-modified T cells. Further, our results emphasize the safety of T cells modified by retroviral gene transfer in clinical application, as measured in >500 patient-years of follow-up. Thus, previous safety issues with integrating viral vectors are hematopoietic stem cell or transgene intrinsic, and not a general feature of retroviral vectors. Engineered T cells are a promising form of synthetic biology for long-term delivery of protein-based therapeutics. These results provide a framework to guide the therapy of a wide spectrum of human diseases.

Gene therapy/bone marrow transplantation in ADA-deficient mice: roles of enzyme-replacement therapy and cytoreduction

Gene therapy (GT) for adenosine deaminase-deficient severe combined immune deficiency (ADA-SCID) can provide significant long-term benefit when patients are given nonmyeloablative conditioning and ADA enzyme-replacement therapy (ERT) is withheld before autologous transplantation of γ-retroviral vector-transduced BM CD34+ cells. To determine the contributions of conditioning and discontinuation of ERT to the therapeutic effects, we analyzed these factors in Ada gene knockout mice (Ada(-/-)). Mice were transplanted with ADA-deficient marrow transduced with an ADA-expressing γ-retroviral vector without preconditioning or after 200 cGy or 900 cGy total-body irradiation and evaluated after 4 months. In all tissues analyzed, vector copy numbers (VCNs) were 100- to 1000-fold greater in mice receiving 900 cGy compared with 200 cGy (P < .05). In mice receiving 200 cGy, VCN was similar whether ERT was stopped or given for 1 or 4 months after GT. In unconditioned mice, there was decreased survival with and without ERT, and VCN was very low to undetectable. When recipients were conditioned with 200 cGy and received transduced lineage-depleted marrow, only recipients receiving ERT (1 or 4 months) had detectable vector sequences in thymocytes. In conclusion, cytoreduction is important for the engraftment of gene-transduced HSC, and short-term ERT after GT did not diminish the capacity of gene-corrected cells to engraft and persist.

Biopump: Autologous skin-derived micro-organ genetically engineered to provide sustained continuous secretion of therapeutic proteins

A novel approach for sustained production of therapeutic proteins is described, using genetic modification of intact autologous micro-organ tissue explants from the subject's own skin. The skin-derived micro-organ can be maintained viable ex vivo for extended periods and is transduced with a transgene encoding a desired therapeutic protein, resulting in protein-secreting micro-organ (biopump (BP)). The daily protein production from each BP is quantified, enabling drug dosing by subcutaneous implantation of the requisite number of BPs into the patient to provide continuous production to the circulation of a known amount of the therapeutic protein. Each implanted BP remains localized and is accessible, to enable removal or ablation if needed. Examples from preclinical and clinical studies are presented, including use of associated virus vector 1 and helper-dependent adenoviral vectors producing BPs to provide long-term sustained secretion of recombinant interferon-α and erythropoietin.

Strategies for retrovirus-based correction of severe, combined immunodeficiency (SCID)

Severe combined immunodeficiencies (SCIDs) appear as optimal disease targets to challenge potential efficacy of gene therapy. Ex vivo, retrovirally mediated gene transfer into hematopoietic progenitor cells has been shown to provide sustained correction of two forms of SCID, that is, SCID-X1 and adenosine deaminase deficiencies. In the former case, however, genotoxicity was observed in a minority of patients as a consequence of retroviral integration into proto-oncogenes loci and transactivation. Design of vectors in which the enhancer element of retroviral LTR has been deleted and an internal promoter added (self-inactivated vectors) could provide both safe and efficient gene transfer as being presently tested.

Lentiviral vectors displaying modified measles virus gp overcome pre-existing immunity in in vivo-like transduction of human T and B cells

Gene transfer into quiescent T and B cells is important for gene therapy and immunotherapy approaches. Previously, we generated lentiviral vectors (LVs) pseudotyped with Edmonston (Ed) measles virus (MV) hemagglutinin (H) and fusion (F) glycoproteins (H/F-LVs), which allowed efficient transduction of quiescent human T and B cells. However, a major obstacle in the use of H/F-LVs in vivo is that most of the human population is vaccinated against measles. As the MV humoral immune response is exclusively directed against the H protein of MV, we mutated the two dominant epitopes in H, Noose, and NE. LVs pseudotyped with these mutant H-glycoproteins escaped inactivation by monoclonal antibodies (mAbs) but were still neutralized by human serum. Consequently, we took advantage of newly emerged MV-D genotypes that were less sensitive to MV vaccination due to a different glycosylation pattern. The mutation responsible was introduced into the H/F-LVs, already mutated for Noose and NE epitopes. We found that these mutant H/F-LVs could efficiently transduce quiescent lymphocytes in the presence of high concentrations of MV antibody-positive human serum. Finally, upon incubation with total blood, mimicking the in vivo situation, the mutant H/F-LVs escaped MV antibody neutralization, where the original H/F-LVs failed. Thus, these novel H/F-LVs offer perspectives for in vivo lymphocyte-based gene therapy and immunotherapy.

Nonmyeloablative conditioning regimen to increase engraftment of gene-modified hematopoietic stem cells in young rhesus monkeys

Immune responses to transgene products may lead to rejection of transduced cells, limiting successful gene therapy for genetic diseases. While moderate dosages of chemotherapeutic agents such as busulfan may increase hematopoietic stem cells (HSC) engraftment, they are not immune suppressive and do not abrogate immune responses to transgene products. Studies focused on nonmyeloablative conditioning with busulfan ± fludarabine in a clinically relevant monkey model to induce immune suppression to allow cells expressing a foreign transgene product to persist. Bone marrow CD34(+) HSC were transduced in two equal fractions using simian immunodeficiency virus (SIV)-based lentiviral vectors carrying a nonexpressed DNA sequence tag (NoN) and the green fluorescent protein (GFP) reporter gene. Post-transplant there was no evidence of elimination of cells containing the potentially immunogenic GFP gene; several recipients had stable persistence of cells, and no differences were detected with fludarabine, which was rapidly cleared. Antibodies and cellular immune responses to GFP developed in recipients with the highest levels of GFP-marked cells, although these cells were not eliminated. These studies establish a clinically relevant pediatric primate model to assess the effects of conditioning regimens on the engraftment of transduced HSC and the immune responses to cells expressing a foreign gene product.

A tale of two SCIDs

Hematopoietic stem cell (HSC) transplantation may be curative for severe combined immunodeficiency (SCID). However, for a majority of infants with SCID a suitable donor is not available, and even with a matched donor, allogeneic HSC transplantation itself carries potential complications such as graft-versus-host disease as well as side effects from myelosuppressive chemotherapy. In the past decade, substantial advances have been made in the transplantation of gene-modified autologous HSCs, especially for two forms of SCID: X-linked SCID (SCID-X1) and adenosine deaminase (ADA)-deficient SCID. Two new reports in this issue of Science Translational Medicine add to the accumulating findings from gene therapy trials in Italy, France, and the United States that show clinical benefits of this alternative treatment.

Hematopoietic stem cell gene therapy for adenosine deaminase-deficient severe combined immunodeficiency leads to long-term immunological recovery and metabolic correction

Genetic defects in the purine salvage enzyme adenosine deaminase (ADA) lead to severe combined immunodeficiency (SCID) with profound depletion of T, B, and natural killer cell lineages. Human leukocyte antigen-matched allogeneic hematopoietic stem cell transplantation (HSCT) offers a successful treatment option. However, individuals who lack a matched donor must receive mismatched transplants, which are associated with considerable morbidity and mortality. Enzyme replacement therapy (ERT) for ADA-SCID is available, but the associated suboptimal correction of immunological defects leaves patients susceptible to infection. Here, six children were treated with autologous CD34-positive hematopoietic bone marrow stem and progenitor cells transduced with a conventional gammaretroviral vector encoding the human ADA gene. All patients stopped ERT and received mild chemotherapy before infusion of gene-modified cells. All patients survived, with a median follow-up of 43 months (range, 24 to 84 months). Four of the six patients recovered immune function as a result of engraftment of gene-corrected cells. In two patients, treatment failed because of disease-specific and technical reasons: Both restarted ERT and remain well. Of the four reconstituted patients, three remained off enzyme replacement. Moreover, three of these four patients discontinued immunoglobulin replacement, and all showed effective metabolic detoxification. All patients remained free of infection, and two cleared problematic persistent cytomegalovirus infection. There were no adverse leukemic side effects. Thus, gene therapy for ADA-SCID is safe, with effective immunological and metabolic correction, and may offer a viable alternative to conventional unrelated donor HSCT.

Gene therapy finds its niche

Gene therapy is finally poised to make a contribution to the treatment of debilitating, highly penetrant genetic diseases that have proved intractable to other regimens.

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.

Chronic granulomatous disease: lessons from a rare disorder

Chronic granulomatous disease (CGD) is a rare primary immunodeficiency with X-linked or autosomal recessive inheritance involving defects in genes encoding phox proteins, which are the subunits of the phagocyte NADPH oxidase. This results in failure to produce superoxide anion and downstream antimicrobial oxidant metabolites and to activate antimicrobial proteases. Affected patients are susceptible to severe, life-threatening bacterial and fungal infections and excessive inflammation characterized by granulomatous enteritis resembling Crohn's disease and genitourinary obstruction. Early diagnosis of CGD and rapid treatment of infections are critical. Prophylaxis with antibacterial and mold-active antifungal agents and the administration of interferon-γ has significantly improved the natural history of CGD. Currently, the only cure is allogeneic hematopoietic cell transplant (HCT), although there remains controversy as to which patients with CGD should get a transplant. Allele-based HLA typing of alternative donors, improved supportive care measures, and use of reduced toxicity conditioning have resulted in event-free survival (EFS) of at least 80% even with an unrelated donor and even better when the patient has no active infections/inflammation. Gene correction of CGD would eliminate the risks of graft-versus-host disease (GVHD) and the immunoablative chemotherapy required for allogeneic HCT. Based on gene therapy trials in patients with SCID-X1, ADA-SCID, and the early experience with CGD, it is clear that at least some degree of myeloablation will be necessary for CGD as there is no inherent selective growth advantage for gene-corrected cells. Current efforts for gene therapy focus on use of lentivector constructs, which are thought to be safer from the standpoint of insertional mutagenesis and more efficient in transducing hematopoietic stem cells (HSCs).

Contributions of gene marking to cell and gene therapies

The first human genetic modification studies used replication-incompetent integrating vector vectors to introduce marker genes into T lymphocytes and subsequently into hematopoietic stem cells. Such studies have provided numerous insights into the biology of hematopoiesis and immune reconstitution and contributed to clinical development of gene and cell therapies. Tracking of hematopoietic reconstitution and analysis of the origin of residual malignant disease after hematopoietic transplantation has been possible via gene marking. Introduction of selectable marker genes has enabled preselection of specific T-cell populations for tumor and viral immunotherapy and reduced the threat of graft-versus-host disease, improving the survival of patients after allogeneic marrow transplantation. Marking studies in humans, murine xenografts, and large animals have helped optimize conditions for gene transfer into CD34(+) hematopoietic progenitors, contributing to the achievement of gene transfer efficiencies sufficient for clinical benefit in several serious genetic diseases such as X-linked severe combined immunodeficiency and adrenoleukodystrophy. When adverse events linked to insertional mutagenesis arose in clinical gene therapy trials for inherited immunodeficiencies, additional animal studies using gene-marking vectors have greatly increased our understanding of genotoxicity. The knowledge gained from these studies is being translated into new vector designs and clinical protocols, which we hope will continue to improve the efficiency, effectiveness and safety of these promising therapeutic approaches.

Hematopoietic stem cell transplantation for severe combined immune deficiency or what the children have taught us

It is now more than 40 years since the first successful allogeneic hematopoietic stem cell transplantation (HSCT) for a child with severe combined immunodeficiency (SCID). In the succeeding years, HSCT for SCID patients have represented only a small portion of the total number of allogeneic HSCT performed. Nevertheless, the clinical and biologic importance of the patients transplanted for SCID has continued. SCID patients were the first to be successfully transplanted with nonsibling related bone marrow, unrelated bone marrow, T-cell depleted HSCT, and genetically corrected (gene transfer) autologous HSC. Many of the biologic insights now widely applied to allogeneic HSCT were first identified in the transplantation of SCID patients. This article reviews the clinical and biologic lessons that have been learned from HSCT for SCID patients, and how the information has impacted the general field of allogeneic HSCT.

Gene therapy for primary immunodeficiencies

The concept of gene therapy emerged as a way of correcting monogenic inherited diseases by introducing a normal copy of the mutated gene into at least some of the patients' cells. Although this concept has turned out to be quite complicated to implement, it is in the field of primary immunodeficiencies (PIDs) that proof of feasibility has been undoubtedly achieved. There is now a strong rationale in support of gene therapy for at least some PIDs, as discussed in this article.

Microbead-assisted retroviral transduction for clinical application

Retroviral transduction is the most commonly used strategy to obtain long-term expression of therapeutic genes. To efficiently transduce mammalian cells, a recombinant fibronectin molecule, RetroNectin, is generally used to juxtapose viral particles and cells, and thereby enhance viral uptake. Although this strategy has become widely adopted, in particular for the genetic modification of hematopoietic cells, several limitations apply. For example, it requires the use of culture systems that allow protein coating, something that is not possible for many of the closed cell culture systems that are used in clinical trials. Furthermore, efficient transduction is obtained only when culture systems can be exposed to centrifugation, an approach termed spin transduction. Here, we describe a novel and more potent strategy for the transduction of T cells that can be applied on a clinical scale. We show that RetroNectin can efficiently be coated onto epoxy-modified paramagnetic beads. After a blocking step, these beads can subsequently bind retroviral particles from viral supernatants, rendering such supernatants largely devoid of functional viral particles. Addition of these virus-loaded beads to activated T cells results in efficient retroviral infection. Importantly, transduction does not require the use of culture systems that are compatible with protein coating, nor is it dependent on centrifugation of either the viral supernatant or the cells. Finally, cell growth, phenotype, and function of spin-transduced versus bead-transduced cells are comparable. Viral coating of microbeads should facilitate the production of genetically modified cells, in particular for use in clinical trials.

Autonomous targeting of infectious superspreaders using engineered transmissible therapies

Infectious disease treatments, both pharmaceutical and vaccine, face three universal challenges: the difficulty of targeting treatments to high-risk ‘superspreader’ populations who drive the great majority of disease spread, behavioral barriers in the host population (such as poor compliance and risk disinhibition), and the evolution of pathogen resistance. Here, we describe a proposed intervention that would overcome these challenges by capitalizing upon Therapeutic Interfering Particles (TIPs) that are engineered to replicate conditionally in the presence of the pathogen and spread between individuals — analogous to ‘transmissible immunization’ that occurs with live-attenuated vaccines (but without the potential for reversion to virulence). Building on analyses of HIV field data from sub-Saharan Africa, we construct a multi-scale model, beginning at the single-cell level, to predict the effect of TIPs on individual patient viral loads and ultimately population-level disease prevalence. Our results show that a TIP, engineered with properties based on a recent HIV gene-therapy trial, could stably lower HIV/AIDS prevalence by ∼30-fold within 50 years and could complement current therapies. In contrast, optimistic antiretroviral therapy or vaccination campaigns alone could only lower HIV/AIDS prevalence by <2-fold over 50 years. The TIP's efficacy arises from its exploitation of the same risk factors as the pathogen, allowing it to autonomously penetrate superspreader populations, maintain efficacy despite behavioral disinhibition, and limit viral resistance. While demonstrated here for HIV, the TIP concept could apply broadly to many viral infectious diseases and would represent a new paradigm for disease control, away from pathogen eradication but toward robust disease suppression.

We introduce a proposed intervention against infectious diseases that extends and optimizes the recognized benefit of ‘transmissible immunization’ that occurs with live-attenuated vaccines such as Oral Polio Vaccine (OPV), the vaccine chosen for the worldwide polio eradication campaign. The intervention proposed here is based upon Therapeutic Interfering Particles (TIPs) that are engineered to replicate only in the presence of the wildtype pathogen and act to inhibit the growth of the pathogen. Therefore TIPs ‘piggyback’ on the pathogen, leading to two important differences from live-attenuated vaccines: TIPs can only transmit from individuals already infected with wildtype pathogen, and TIPs could only revert to virulence in individuals already carrying the wild-type pathogen. Intriguingly, because TIPs spread between individuals using the same transmission routes as the pathogen, they automatically find their way to the populations at greatest risk of infection, thus circumventing the unsolved problem of how to identify superspreaders and target them for preventive measures. Based on clinical-trial data, we analyze the impact that TIP intervention would have on HIV/AIDS in sub-Saharan Africa and show that TIPs could lower HIV/AIDS prevalence more effectively than vaccines or drugs alone and, in fact, would effectively complement these other interventions.

Genotoxicity of retroviral hematopoietic stem cell gene therapy

INTRODUCTION

Retroviral vectors have been developed for hematopoietic stem cell (HSC) gene therapy and have successfully cured X-linked severe combined immunodeficiency (SCID-X1), adenosine deaminase deficiency (ADA-SCID), adrenoleukodystrophy, and Wiskott-Aldrich syndrome. However, in HSC gene therapy clinical trials, genotoxicity mediated by integrated vector proviruses has led to clonal expansion, and in some cases frank leukemia. Numerous studies have been performed to understand the molecular basis of vector-mediated genotoxicity with the aim of developing safer vectors and safer gene therapy protocols. These genotoxicity studies are critical to advancing HSC gene therapy.

AREAS COVERED

This review provides an introduction to the mechanisms of retroviral vector genotoxicity. It also covers advances over the last 20 years in designing safer gene therapy vectors, and in integration site analysis in clinical trials and large animal models. Mechanisms of retroviral-mediated genotoxicity, and the risk factors that contribute to clonal expansion and leukemia in HSC gene therapy are introduced.

EXPERT OPINION

Continued research on virus-host interactions and next-generation vectors should further improve the safety of future HSC gene therapy vectors and protocols.

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.

Integration profile of retroviral vector in gene therapy treated patients is cell-specific according to gene expression and chromatin conformation of target cell

The analysis of genomic distribution of retroviral vectors is a powerful tool to monitor ‘vector-on-host’ effects in gene therapy (GT) trials but also provides crucial information about ‘host-on-vector’ influences based on the target cell genetic and epigenetic state. We had the unique occasion to compare the insertional profile of the same therapeutic moloney murine leukemia virus (MLV) vector in the context of the adenosine deaminase-severe combined immunodeficiency (ADA-SCID) genetic background in two GT trials based on infusions of transduced mature lymphocytes (peripheral blood lymphocytes, PBL) or a single infusion of haematopoietic stem/progenitor cells (HSC). We found that vector insertions are cell-specific according to the differential expression profile of target cells, favouring, in PBL-GT, genes involved in immune system and T-cell functions/pathways as well as T-cell DNase hypersensitive sites, differently from HSC-GT. Chromatin conformations and histone modifications influenced integration preferences but we discovered that only H3K27me3 was cell-specifically disfavoured, thus representing a key epigenetic determinant of cell-type dependent insertion distribution. Our study shows that MLV vector insertional profile is cell-specific according to the genetic/chromatin state of the target cell both in vitro and in vivo in patients several years after GT.

Functional aliphatic polyesters for biomedical and pharmaceutical applications

Functional aliphatic polyesters are biodegradable polymers with many possibilities to tune physico-chemical characteristics such as hydrophilicity and degradation rate as compared to traditional polyesters (e.g. PLLA, PLGA and PCL), making the materials suitable for drug delivery or as scaffolds for tissue engineering. Lately, a large number of polyesters have been synthesized by homopolymerization of functionalized monomers or co-polymerization with other monomers mainly via ring-opening polymerization (ROP) of cyclic esters. This review presents the recent trends in the synthesis of these materials and their application for protein delivery and tissue engineering.

[Gene and cell therapy for primary immunodeficiency diseases]

Primary immunodeficiency diseases (PID) represents a group of inherited diseases where mutations in certain gene lead to certain levels of defects in patient immune systems. Among them, several types of PID, including severe combined immunodeficiecny (SCID), warrented development of new types of curative treatment other than allogeneic hematopoietic stem cell transplantation, eventually culiminating in successful stem cell gene therapy tials such as the cases for adenosine deaminase (ADA)-deficiency SCID patients. In this article, I will summarize the current status of stem cell gene therapy for PID, and discuss the problems such clinical trials have in the present forms of treatment, e.g., possible risks of leukemogenesis due to insertional mutagenesis by the use of therapeutic viral vectors. I also try to discuss the future of this type of experimental medicine aiming for the permanent cure of PID, including the one utilizing innovative technologies such as induced pluripotent stem cells.

The different extent of B and T cell immune reconstitution after hematopoietic stem cell transplantation and enzyme replacement therapies in SCID patients with adenosine deaminase deficiency

The lack of adenosine deaminase (ADA) leads to the accumulation of toxic metabolites, resulting in SCID. If the disease is left untreated, it is likely to have a fatal outcome in early infancy. Because hematopoietic stem cell transplantation (HSCT) and enzyme replacement therapy with pegylated bovine ADA (PEG-ADA) are both provided in our hospital, we undertook a retrospective longitudinal comparative study of the extent of lymphocyte recovery in two groups of treated ADA-SCID children. Together with classical immunological parameters, we quantified the output of the new B and T cells from the production sites using the κ-deleting recombination excision circle and TCR excision circle assay, and we monitored T cell repertoire diversification. We found that immune reconstitution was different following the two treatments. The stable production of κ-deleting recombination excision circle(+) lymphocytes sustained an increase in B cell number in HSCT-treated patients, whereas in PEG-ADA-treated patients, it was accompanied by a significant and progressive decrease in circulating CD19(+) lymphocytes, which never reached the levels observed in age-matched children. The mobilization of TCR excision circle(+) cells, though lower than in controls, was stable with time after HSCT treatment, leading to a constant peripheral T cell number and to the diversification of the T cell repertoire; however, it was compromised in children receiving prolonged PEG-ADA therapy, whose T cells showed progressively narrowing T cell repertoires.