Development of lentiviral gene therapy for Wiskott Aldrich syndrome

Recent advances in hematopoietic gene therapy for genetic disorders

Hematopoietic gene therapy is based on the transplantation of gene-modified autologous hematopoietic stem cells and since the inception of this approach, many technological and medical improvements have been achieved. This review focuses on the clinical studies that have used hematopoietic gene therapy to successfully treat several rare and severe genetic disorders of the blood or immune system as well as some non-hematological diseases. Today, in some cases hematopoietic gene therapy has progressed to the point of being equal to, or better than, allogeneic bone marrow transplant. In others, further improvements are needed to obtain more consistent efficacy or to reduce the risks posed by vectors or protocols. Several hematopoietic gene therapy products showing both long-term efficacy and safety have reached the market, but economic considerations challenge the possibility of patient access to novel disease-modifying therapies. © 2023 Published by Elsevier Masson SAS on behalf of French Society of Pediatrics.

Identification of a novel WAS mutation and the non-splicing effect of a second-site mutation in a Chinese pedigree with Wiskott-Aldrich syndrome

BACKGROUND

Wiskott-Aldrich syndrome (WAS) is a rare X-linked immunodeficiency disorder caused by abnormal expression of the WAS protein (WASp) due to mutations in the WAS gene, and is generally characterized by microthrombocytopenia, eczema, recurrent infections, and high susceptibility to autoimmune complications and hematological malignancies.

RESULTS

Herein, we identified a novel WAS mutation (c.158 T > C) using next-generation sequencing in a Chinese pedigree with WAS. The expression of WASp in the patients and their families was detected by flow cytometry and western blot analysis. To explore the exon-splicing effect of intron mutations and the correlation between the genotype and clinical phenotype, four groups of wild-type (WT), exon mutant, intron mutant, and combined mutant recombinant plasmids were transfected into COS-7 cells in vitro. The proband showed dramatically decreased WASp expression, while the female carriers showed a slightly lower level of WASp. The expression of products in the mutant and WT recombinant plasmids was detected by real-time fluorescence quantitative polymerase chain reaction (PCR), which showed a significant reduction in the combined mutant group than in the WT, exon mutant, and intron mutant groups. The length of the expression products in the four groups showed no differences, each containing 360 base pairs. Sequence analysis confirmed that the c.158 T > C mutation appeared in the exon mutant and combined mutant groups, whereas the intron variant c.273 + 14C > T caused no other sequence changes.

CONCLUSION

This study confirmed that the intron mutation did not affect the splicing of exons and excluded the influence of the double mutations at the transcription level on the severe clinical manifestations in the cousin. This in vitro study provided new insights into the pathogenesis of intronic mutations in WAS.

Hematopoietic Stem Cell Therapy for Wiskott-Aldrich Syndrome: Improved Outcome and Quality of Life

The Wiskott-Aldrich syndrome (WAS) is an X-linked disorder caused by mutations in the WAS gene resulting in congenital thrombocytopenia, eczema, recurrent infections and an increased incidence of autoimmune diseases and malignancies. Without curative therapies, affected patients have diminished life expectancy and reduced quality of life. Since WAS protein (WASP) is constitutively expressed only in hematopoietic stem cell-derived lineages, hematopoietic stem cell transplantation (HSCT) and gene therapy (GT) are well suited to correct the hematologic and immunologic defects. Advances in high-resolution HLA typing, new techniques to prevent GvHD allowing the use of haploidentical donors, and the introduction of reduced intensity conditioning regimens with myeloablative features have increased overall survival (OS) to over 90%. The development of GT for WAS has provided basic knowledge into vector selection and random integration of various viral vectors into the genome, with the possibility of inducing leukemogenesis. After trials and errors, inactivating lentiviral vectors carrying the WAS gene were successfully evaluated in clinical trials, demonstrating cure of the disease except for insufficient resolution of the platelet defect. Thus, 50 years of clinical evaluation, genetic exploration and extensive clinical trials, a lethal syndrome has turned into a curable disorder.

Gene Therapy for Primary Immunodeficiency

Over the past 3 decades, there has been significant progress in refining gene therapy technologies and procedures. Transduction of hematopoietic stem cells ex vivo using lentiviral vectors can now create a highly effective therapeutic product, capable of reconstituting many different immune system dysfunctions when reinfused into patients. Here, we review the key developments in the gene therapy landscape for primary immune deficiency, from an experimental therapy where clinical efficacy was marred by adverse events, to a commercialized product with enhanced safety and efficacy. We also discuss progress being made in preclinical studies for challenging disease targets and emerging gene editing technologies that are showing promising results, particularly for conditions where gene regulation is important for efficacy.

Toward Combined Cell and Gene Therapy for Genodermatoses

To date, more than 200 monogenic, often devastating, skin diseases have been described. Because of unmet medical needs, development of long-lasting and curative therapies has been consistently attempted, with the aim of correcting the underlying molecular defect. In this review, we will specifically address the few combined cell and gene therapy strategies that made it to the clinics. Based on these studies, what can be envisioned for the future is a patient-oriented strategy, built on the specific features of the individual in need. Most likely, a combination of different strategies, approaches, and advanced therapies will be required to reach the finish line at the end of the long and winding road hampering the achievement of definitive treatments for genodermatoses.

Managing Inflammatory Manifestations in Patients with Chronic Granulomatous Disease

Chronic granulomatous disease (CGD) is a primary immunodeficiency caused by lack of phagocyte nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, which results in inflammatory dysregulation and increased susceptibility to infections. Patients with CGD may develop severe obstructive disorders of the digestive tract as a result of their dysregulated inflammatory response. Despite a growing focus on inflammatory manifestations in CGD, the literature data on obstructive complications are far less extensive than those on infectious complications. Diagnosis and management of patients with concomitant predispositions to infections and hyperinflammation are particularly challenging. Although the inflammatory and granulomatous manifestations of CGD usually respond rapidly to steroid treatment, second-line therapies (immunosuppressants and biologics) may be required in refractory cases. Indeed, immunosuppressants (such as anti-tumor necrosis factor agents, thalidomide, and anakinra) have shown some efficacy, but the value of this approach is controversial, given the questionable risk-to-benefit ratio and the small numbers of patients treated to date. Significant progress in allogeneic hematopoietic stem cell transplantation (the only curative treatment for CGD) has been made through better supportive care and implementation of improved, reduced-intensity conditioning regimens. Gene therapy may eventually be an option for patients lacking a suitable donor; clinical trials with new, safer vectors are ongoing at a few centers.

Clinical applications of gene therapy for primary immunodeficiencies

Primary immunodeficiencies (PIDs) have represented a paradigmatic model for successes and pitfalls of hematopoietic stem cells gene therapy. First clinical trials performed with gamma retroviral vectors (γ-RV) for adenosine deaminase severe combined immunodeficiency (ADA-SCID), X-linked SCID (SCID-X1), and Wiskott-Aldrich syndrome (WAS) showed that gene therapy is a valid therapeutic option in patients lacking an HLA-identical donor. No insertional mutagenesis events have been observed in more than 40 ADA-SCID patients treated so far in the context of different clinical trials worldwide, suggesting a favorable risk-benefit ratio for this disease. On the other hand, the occurrence of insertional oncogenesis in SCID-X1, WAS, and chronic granulomatous disease (CGD) RV clinical trials prompted the development of safer vector construct based on self-inactivating (SIN) retroviral or lentiviral vectors (LVs). Here we present the recent results of LV-mediated gene therapy for WAS showing stable multilineage engraftment leading to hematological and immunological improvement, and discuss the differences with respect to the WAS RV trial. We also describe recent clinical results of SCID-X1 gene therapy with SIN γ-RV and the perspectives of targeted genome editing techniques, following early preclinical studies showing promising results in terms of specificity of gene correction. Finally, we provide an overview of the gene therapy approaches for other PIDs and discuss its prospects in relation to the evolving arena of allogeneic transplant.

Progress in gene therapy for primary immunodeficiencies using lentiviral vectors

PURPOSE OF REVIEW

This review gives an overview over the most recent progress in the field of lentiviral gene therapy for primary immunodeficiencies (PIDs). The history and state-of-the-art of lentiviral vector development are summarized and the recent advancements for a number of selected diseases are reviewed in detail. Past retroviral vector trials for these diseases, the most recent improvements of lentiviral vector platforms and their application in preclinical development as well as ongoing clinical trials are discussed.

RECENT FINDINGS

Main focus is on the preclinical studies and clinical trials for the treatment of Wiskott-Aldrich syndrome, chronic granulomatous disease, adenosine deaminase deficient severe combined immunodeficiency (ADA-SCID) and X-linked severe combined immunodeficiency with lentiviral gene therapy.

SUMMARY

Gene therapy for PIDs is an effective treatment, providing potential long-term clinical benefit for affected patients. Substantial progress has been made to make lentiviral gene therapy platforms available for a number of rare genetic diseases. Although many ongoing gene therapy trials are based on ex-vivo approaches with autologous hematopoietic stem cells, other approaches such as in-vivo gene therapy or gene-repair platforms might provide further advancement for certain PIDs.

Cancer gene discovery: exploiting insertional mutagenesis

Insertional mutagenesis has been used as a functional forward genetics screen for the identification of novel genes involved in the pathogenesis of human cancers. Different insertional mutagens have been successfully used to reveal new cancer genes. For example, retroviruses are integrating viruses with the capacity to induce the deregulation of genes in the neighborhood of the insertion site. Retroviruses have been used for more than 30 years to identify cancer genes in the hematopoietic system and mammary gland. Similarly, another tool that has revolutionized cancer gene discovery is the cut-and-paste transposons. These DNA elements have been engineered to contain strong promoters and stop cassettes that may function to perturb gene expression upon integration proximal to genes. In addition, complex mouse models characterized by tissue-restricted activity of transposons have been developed to identify oncogenes and tumor suppressor genes that control the development of a wide range of solid tumor types, extending beyond those tissues accessible using retrovirus-based approaches. Most recently, lentiviral vectors have appeared on the scene for use in cancer gene screens. Lentiviral vectors are replication-defective integrating vectors that have the advantage of being able to infect nondividing cells, in a wide range of cell types and tissues. In this review, we describe the various insertional mutagens focusing on their advantages/limitations, and we discuss the new and promising tools that will improve the insertional mutagenesis screens of the future.

Two cases of Wiskott-Aldrich syndrome in neonates due to gene mutations

Wiskott-Aldrich syndrome (WAS) is a rare X-linked recessive disease characterized by eczema, thrombocytopenia and immune deficiency. WAS gene mutations impair WAS protein function which cause WAS. The WAS-related disorders of X-linked thrombocytopenia (XLT) and X-linked congenital neutropenia (XLN) may have similar but less severe symptoms those are also caused by mutations of the same gene. We present two cases of WAS in neonates with WAS gene mutations. Early genetic diagnosis can help to the treatment and prevention this disease.

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.

Characteristics and outcome of early-onset, severe forms of Wiskott-Aldrich syndrome

On the basis of a nationwide database of 160 patients with Wiskott-Aldrich syndrome (WAS), we identified a subset of infants who were significantly more likely to be attributed with an Ochs score of 5 before the age of 2 (n = 26 of 47 [55%], P = 2.8 × 10(−7)). A retrospective analysis revealed that these patients often had severe refractory thrombocytopenia (n = 13), autoimmune hemolytic anemia (n = 15), and vasculitis (n = 6). One patient had developed 2 distinct cancers. Hemizygous mutations predictive of the absence of WAS protein were identified in 19 of the 24 tested patients, and the absence of WAS protein was confirmed in all 10 investigated cases. Allogeneic hematopoietic stem cell transplantation (HSCT) was found to be a curative treatment with a relatively good prognosis because it was successful in 17 of 22 patients. Nevertheless, 3 patients experienced significant disease sequelae and 4 patients died before HSCT. Therefore, the present study identifies a distinct subgroup of WAS patients with early-onset, life-threatening manifestations. We suggest that HSCT is a curative strategy in this subgroup of patients and should be performed as early in life as possible, even when a fully matched donor is lacking.

Gene therapy for primary immunodeficiencies: Part 2

Gene therapy has become an attractive alternative therapeutic strategy to allogeneic transplant for primary immunodeficiencies (PIDs) owing to known genetic defects. Clinical trials using gammaretroviral vectors have demonstrated the proof of principle of gene therapy for Wiskott-Aldrich syndrome (WAS) and chronic granulomatous disease (CGD), but have also highlighted limitations of the technology. New strategies based on vectors that can achieve more robust correction with less risk of insertional mutagenesis are being developed. In this review we present the status of gene therapy for WAS and CGD, and discuss the emerging application of similar strategies to a broader range of PIDs, such as IPEX syndrome.

Autoimmunity in wiskott-Aldrich syndrome: an unsolved enigma

Wiskott-Aldrich Syndrome (WAS) is a severe X-linked Primary Immunodeficiency that affects 1-10 out of 1 million male individuals. WAS is caused by mutations in the WAS Protein (WASP) expressing gene that leads to the absent or reduced expression of the protein. WASP is a cytoplasmic protein that regulates the formation of actin filaments in hematopoietic cells. WASP deficiency causes many immune cell defects both in humans and in the WAS murine model, the Was(-/-) mouse. Both cellular and humoral immune defects in WAS patients contribute to the onset of severe clinical manifestations, in particular microthrombocytopenia, eczema, recurrent infections, and a high susceptibility to develop autoimmunity and malignancies. Autoimmune diseases affect from 22 to 72% of WAS patients and the most common manifestation is autoimmune hemolytic anemia, followed by vasculitis, arthritis, neutropenia, inflammatory bowel disease, and IgA nephropathy. Many groups have widely explored immune cell functionality in WAS partially explaining how cellular defects may lead to pathology. However, the mechanisms underlying the occurrence of autoimmune manifestations have not been clearly described yet. In the present review, we report the most recent progresses in the study of immune cell function in WAS that have started to unveil the mechanisms contributing to autoimmune complications in WAS patients.

Preclinical safety and efficacy of human CD34(+) cells transduced with lentiviral vector for the treatment of Wiskott-Aldrich syndrome

Gene therapy with ex vivo-transduced hematopoietic stem/progenitor cells may represent a valid therapeutic option for monogenic immunohematological disorders such as Wiskott-Aldrich syndrome (WAS), a primary immunodeficiency associated with thrombocytopenia. We evaluated the preclinical safety and efficacy of human CD34⁺ cells transduced with lentiviral vectors (LV) encoding WAS protein (WASp). We first set up and validated a transduction protocol for CD34⁺ cells derived from bone marrow (BM) or mobilized peripheral blood (MPB) using a clinical grade, highly purified LV. Robust transduction of progenitor cells was obtained in normal donors and WAS patients' cells, without evidence of toxicity. To study biodistribution of human cells and exclude vector release in vivo, LV-transduced CD34⁺ cells were transplanted in immunodeficient mice, showing a normal engraftment and differentiation ability towards transduced lymphoid and myeloid cells in hematopoietic tissues. Vector mobilization to host cells and transmission to germline cells of the LV were excluded by different molecular assays. Analysis of vector integrations showed polyclonal integration patterns in vitro and in human engrafted cells in vivo. In summary, this work establishes the preclinical safety and efficacy of human CD34⁺ cells gene therapy for the treatment of WAS.

Gene therapy for the Wiskott-Aldrich syndrome

BACKGROUND

Wiskott-Aldrich syndrome (WAS) is a rare X-linked primary immunodeficiency (PID) characterized by micro-thrombocytopenia, recurrent infections, eczema, which is associated with a high incidence of auto-immunity and lymphoreticular malignancy. One of the first diseases to be successfully treated by allogeneic hematopoietic stem cell transplantation, WAS is currently the subject of several phase I/II gene therapy trials for patients without HLA-compatible donors.

PURPOSE OF REVIEW

This article reviews the preclinical and clinical data leading to the development of gene therapy of WAS with lentiviral vectors.

RECENT FINDINGS

A recent clinical trial using a conventional gammaretroviral vector has demonstrated the proof of principle of gene therapy in WAS, but has also highlighted a common limitation of the technology. Encouraging preclinical efficacy and safety results using refined lentiviral vectors, and the development of robust clinical-grade manufacturing processes have supported the initiation of several phase I/II new studies.

SUMMARY

WAS is amenable to hematopoietic stem cell gene therapy. New trials using lentiviral vectors are expected to improve efficacy and safety profiles. Beyond proof of principle, ongoing international efforts to coordinate trials of gene therapy for the WAS may also provide a model for the expedited development of new treatments for other rare diseases.

Development of novel efficient SIN vectors with improved safety features for Wiskott-Aldrich syndrome stem cell based gene therapy

Gene therapy is a promising therapeutic approach to treat primary immunodeficiencies. Indeed, the clinical trial for the Wiskott-Aldrich Syndrome (WAS) that is currently ongoing at the Hannover Medical School (Germany) has recently reported the correction of all affected cell lineages of the hematopoietic system in the first treated patients. However, an extensive study of the clonal inventory of those patients reveals that LMO2, CCND2 and MDS1/EVI1 were preferentially prevalent. Moreover, a first leukemia case was observed in this study, thus reinforcing the need of developing safer vectors for gene transfer into HSC in general. Here we present a novel self-inactivating (SIN) vector for the gene therapy of WAS that combines improved safety features. We used the elongation factor 1 alpha (EFS) promoter, which has been extensively evaluated in terms of safety profile, to drive a codon-optimized human WASP cDNA. To test vector performance in a more clinically relevant setting, we transduced murine HSPC as well as human CD34+ cells and also analyzed vector efficacy in their differentiated myeloid progeny. Our results show that our novel vector generates comparable WAS protein levels and is as effective as the clinically used LTR-driven vector. Therefore, the described SIN vectors appear to be good candidates for potential use in a safer new gene therapy protocol for WAS, with decreased risk of insertional mutagenesis.

The c.273+11dup genetic change in the WAS gene is a functionally neutral polymorphism

Several pediatric patients showing symptoms consistent with the Wiskott-Aldrich syndrome (WAS) were referred to us and turned out to display the c.273+11dup change in the WAS gene. It consisted of the insertion of one C in an unusual tract of 7C near the intron 2 donor splicing site of the WAS gene. In the patients, non-synonymous WAS mutations were found twice only and one mutation was elucidated in RUNX1. In the absence of a non-synonymous mutation in the WAS gene, the c.273+11dup change affected neither the levels nor the sequence of WAS mRNA. In the presence of a non-synonymous WAS mutation, the c.273+11dup alteration failed to worsen the expected phenotype. Minor splicing abnormalities concerning exon 10 were observed both in WAS patients, and in healthy individuals carrying or not carrying the c.273+11dup. The c.273+11dup change was encountered four times in 107 normal male and female controls (172 alleles tested: 2.3%), and eight times in a series of 248 male patients (248 alleles tested: 3.2%). We conclude that the presence of the additional C in the WAS gene is a functionally neutral polymorphism.

Lentiviral-mediated gene therapy leads to improvement of B-cell functionality in a murine model of Wiskott-Aldrich syndrome

BACKGROUND

Wiskott-Aldrich syndrome (WAS) is an X-linked primary immunodeficiency characterized by thrombocytopenia, eczema, infections, autoimmunity, and lymphomas. Transplantation of hematopoietic stem cells from HLA-identical donors is curative, but it is not available to all patients. We have developed a gene therapy (GT) approach for WAS by using a lentiviral vector encoding for human WAS promoter/cDNA (w1.6W) and demonstrated its preclinical efficacy and safety.

OBJECTIVE

To evaluate B-cell reconstitution and correction of B-cell phenotype in GT-treated mice.

METHODS

We transplanted Was(-/-) mice sublethally irradiated (700 rads) with lineage marker-depleted bone marrow wild-type cells, Was(-/-) cells untransduced or transduced with the w1.6W lentiviral vector and analyzed B-cell reconstitution in bone marrow, spleen, and peritoneum.

RESULTS

Here we show that WAS protein(+) B cells were present in central and peripheral B-cell compartments from GT-treated mice and displayed the strongest selective advantage in the splenic marginal zone and peritoneal B1 cell subsets. After GT, splenic architecture was improved and B-cell functions were restored, as demonstrated by the improved antibody response to pneumococcal antigens and the reduction of serum IgG autoantibodies.

CONCLUSION

WAS GT leads to improvement of B-cell functions, even in the presence of a mixed chimerism, further validating the clinical application of the w1.6W lentiviral vector.

COS-1 cells as packaging host for production of lentiviruses

We present a protocol for in vitro production of recombinant lentiviruses using COS-1 African green monkey kidney epithelial cells and HEK293T human embryonic kidney epithelial cells as packaging cells. COS-1 and HEK293T express SV40 large T antigen, amplifying transfected circular plasmids harboring SV40 replication origin. Support protocols for evaluation of transfection efficiency by in situ β-galactosidase enzyme activity assay and titer of infection-capable virions are also provided. Advantages of using COS-1 packaging cells over the standard HEK293T cells for contamination-sensitive applications or automated processing are discussed.

Large-scale manufacture and characterization of a lentiviral vector produced for clinical ex vivo gene therapy application

From the perspective of a pilot clinical gene therapy trial for Wiskott-Aldrich syndrome (WAS), we implemented a process to produce a lentiviral vector under good manufacturing practices (GMP). The process is based on the transient transfection of 293T cells in Cell Factory stacks, scaled up to harvest 50 liters of viral stock per batch, followed by purification of the vesicular stomatitis virus glycoprotein-pseudotyped particles through several membrane-based and chromatographic steps. The process leads to a 200-fold volume concentration and an approximately 3-log reduction in protein and DNA contaminants. An average yield of 13% of infectious particles was obtained in six full-scale preparations. The final product contained low levels of contaminants such as simian virus 40 large T antigen or E1A sequences originating from producer cells. Titers as high as 2 × 10(9) infectious particles per milliliter were obtained, generating up to 6 × 10(11) infectious particles per batch. The purified WAS vector was biologically active, efficiently expressing the genetic insert in WAS protein-deficient B cell lines and transducing CD34(+) cells. The vector introduced 0.3-1 vector copy per cell on average in CD34(+) cells when used at the concentration of 10(8) infectious particles per milliliter, which is comparable to preclinical preparations. There was no evidence of cellular toxicity. These results show the implementation of large-scale GMP production, purification, and control of advanced HIV-1-derived lentiviral technology. Results obtained with the WAS vector provide the initial manufacturing and quality control benchmarking that should be helpful to further development and clinical applications.

Introduction to gene therapy: a clinical aftermath

Despite three decades of huge progress in molecular genetics, in cloning of disease causative gene as well as technology breakthroughs in viral biotechnology, out of thousands of gene therapy clinical trials that have been initiated, only very few are now reaching regulatory approval. We shall review some of the major hurdles, and based on the current either positive or negative examples, we try to initiate drawing a learning curve from experience and possibly identify the major drivers for future successful achievement of human gene therapy trials.

Quantification of lentiviral vector copy numbers in individual hematopoietic colony-forming cells shows vector dose-dependent effects on the frequency and level of transduction

Lentiviral vectors are effective tools for gene transfer and integrate variable numbers of proviral DNA copies in variable proportions of cells. The levels of transduction of a cellular population may therefore depend upon experimental parameters affecting the frequency and/or the distribution of vector integration events in this population. Such analysis would require measuring vector copy numbers (VCN) in individual cells. To evaluate the transduction of hematopoietic progenitor cells at the single-cell level, we measured VCN in individual colony-forming cell (CFC) units, using an adapted quantitative PCR (Q-PCR) method. The feasibility, reproducibility and sensitivity of this approach were tested with characterized cell lines carrying known numbers of vector integration. The method was validated by correlating data in CFC with gene expression or with calculated values, and was found to slightly underestimate VCN. In spite of this, such Q-PCR on CFC was useful to compare transduction levels with different infection protocols and different vectors. Increasing the vector concentration and re-iterating the infection were two different strategies that improved transduction by increasing the frequency of transduced progenitor cells. Repeated infection also augmented the number of integrated copies and the magnitude of this effect seemed to depend on the vector preparation. Thus, the distribution of VCN in hematopoietic colonies may depend upon experimental conditions including features of vectors. This should be carefully evaluated in the context of ex vivo hematopoietic gene therapy studies.

Gene therapy of chronic granulomatous disease: the engraftment dilemma

The potential of gene therapy as a curative treatment for monogenetic disorders has been clearly demonstrated in a series of recent Phase I/II clinical trials. Among primary immunodeficiencies, gene transfer into hematopoietic stem (HSC)/progenitor cells has resulted in the long-term correction of immune and metabolic defects in treated patients. In most cases, successes were augmented by a recognized biological selection for successfully treated cells in vivo, perhaps even to some extent at the HSC level. In contrast, similar achievements have not turned into reality for immunodeficiencies in which gene-transduced cells lack selective advantages in vivo. This is the case for chronic granulomatous disease (CGD), a primary immunodeficiency, characterized by deficient antimicrobial activity in phagocytic cells. Several attempts to correct CGD by gene transfer in combination with bone marrow conditioning have resulted in low-level long-term engraftment and transient clinical benefits despite high levels of gene marking and high numbers of reinfused cells. This review summarizes the data from clinical trials for CGD and provides some insights into treatment options that may lead to a successful application of gene therapy for CGD.

Recent advances in lentiviral vector development and applications

Lentiviral vectors (LVs) have emerged as potent and versatile vectors for ex vivo or in vivo gene transfer into dividing and nondividing cells. Robust phenotypic correction of diseases in mouse models has been achieved paving the way toward the first clinical trials. LVs can deliver genes ex vivo into bona fide stem cells, particularly hematopoietic stem cells, allowing for stable transgene expression upon hematopoietic reconstitution. They are also useful to generate induced pluripotent stem cells. LVs can be pseudotyped with distinct viral envelopes that influence vector tropism and transduction efficiency. Targetable LVs can be generated by incorporating specific ligands or antibodies into the vector envelope. Immune responses toward the transgene products and transduced cells can be repressed using microRNA-regulated vectors. Though there are safety concerns regarding insertional mutagenesis, their integration profile seems more favorable than that of gamma-retroviral vectors (gamma-RVs). Moreover, it is possible to minimize this risk by modifying the vector design or by employing integration-deficient LVs. In conjunction with zinc-finger nuclease technology, LVs allow for site-specific gene correction or addition in predefined chromosomal loci. These recent advances underscore the improved safety and efficacy of LVs with important implications for clinical trials.

Hematopoietic stem cell gene therapy for adenosine deaminase deficient-SCID

Gene therapy is a highly attractive strategy for many types of inherited disorders of the immune system. Adenosine deaminase (ADA) deficient-severe combined immunodeficiency (SCID) has been the target of several clinical trials based on the use of hematopoietic stem/progenitor cells engineered with retroviral vectors. The introduction of a low intensity conditioning regimen has been a crucial factor in achieving stable engrafment of hematopoietic stem cells and therapeutic levels of ADA-expressing cells. Recent studies have demonstrated that gene therapy for ADA-SCID has favorable safety profile and is effective in restoring normal purine metabolism and immune functions. Stem cell gene therapy combined with appropriate conditioning regimens might be extended to other genetic disorders of the hematopoietic system.

Wiskott-Aldrich syndrome protein is an effector of Kit signaling

The pleiotropic receptor tyrosine kinase Kit can provide cytoskeletal signals that define cell shape, positioning, and migration, but the underlying mechanisms are less well understood. In this study, we provide evidence that Kit signals through Wiskott-Aldrich syndrome protein (WASP), the central hematopoietic actin nucleation-promoting factor and regulator of the cytoskeleton. Kit ligand (KL) stimulation resulted in transient tyrosine phosphorylation of WASP, as well as interacting proteins WASP-interacting protein and Arp2/3. KL-induced filopodia in bone marrow-derived mast cells (BMMCs) were significantly decreased in number and size in the absence of WASP. KL-dependent regulation of intracellular Ca(2+) levels was aberrant in WASP-deficient BMMCs. When BMMCs were derived from WASP-heterozygous female mice using KL as a growth factor, the cultures eventually developed from a mixture of WASP-positive and -negative populations into a homogenous WASP-positive culture derived from the WASP-positive progenitors. Thus, WASP expression conferred a selective advantage to the development of Kit-dependent hematopoiesis consistent with the selective advantage of WASP-positive hematopoietic cells observed in WAS-heterozygous female humans. Finally, KL-mediated gene expression in wild-type and WASP-deficient BMMCs was compared and revealed that approximately 30% of all Kit-induced changes were WASP dependent. The results indicate that Kit signaling through WASP is necessary for normal Kit-mediated filopodia formation, cell survival, and gene expression, and provide new insight into the mechanism in which WASP exerts a strong selective pressure in hematopoiesis.

Recent advances in understanding the pathophysiology of Wiskott-Aldrich syndrome

Wiskott-Aldrich syndrome (WAS) is a severe X-linked immunodeficiency caused by mutations in the gene encoding for WASP, a key regulator of signaling and cytoskeletal reorganization in hematopoietic cells. Mutations in WASP result in a wide spectrum of clinical manifestations ranging from the relatively mild X-linked thrombocytopenia to the classic full-blown WAS phenotype characterized by thrombocytopenia, immunodeficiency, eczema, and high susceptibility to developing tumors and autoimmune manifestations. The life expectancy of patients affected by severe WAS is reduced, unless they are successfully cured by bone marrow transplantation from related identical or matched unrelated donors. Because many patients lack a compatible bone marrow donor, the administration of WAS gene–corrected autologous hematopoietic stem cells could represent an alternative therapeutic approach. In the present review, we focus on recent progress in understanding the molecular and cellular mechanisms contributing to the pathophysiology of WAS. Although molecular and cellular studies have extensively analyzed the mechanisms leading to defects in T, B, and dendritic cells, the basis of autoimmunity and thrombocytopenia still remains poorly understood. A full understanding of these mechanisms is still needed to further implement new therapeutic strategies for this peculiar immunodeficiency.

Advantages of COS-1 monkey kidney epithelial cells as packaging host for small-volume production of high-quality recombinant lentiviruses

The HEK293T human embryonic kidney cells have been used widely as a packaging host for transfection-based production of recombinant lentiviruses. The present study describes advantages of using COS-1 African green monkey kidney cells versus HEK293T cells as a packaging host for small-volume production of high-quality recombinant lentiviruses. The particle performance index, defined as the ratio of infection-competent viral particles to the total number of particles, was three- to four-fold greater in transfection supernatants generated using COS-1 cells than that generated using HEK293T cells. Adhesion of HEK293T cells to the cell culture-treated plastic surface was weak, causing significant HEK293T cell contamination in the transfection supernatants produced by laboratory automation using the 96-well cell culture plates. In contrast, COS-1 cells adhered strongly to the plastic surface, and cell contamination was not detected in the transfection supernatants. These results suggest that COS-1 cells may be a useful alternative packaging host for use for automated generation of large numbers of high-quality lentivirus reagents, particularly because they eliminate the need for additional purification steps to remove viral particles from cell culture supernatant.

New insights into the biology of Wiskott-Aldrich syndrome (WAS)

The Wiskott-Aldrich syndrome (WAS) is an X-linked immunodeficiency disease with a characteristic clinical phenotype that includes thrombocytopenia with small platelets, eczema, recurrent infections due to immunodeficiency, and an increased incidence of autoimmune manifestations and malignancies. The identification of the molecular defect in the WAS gene has broadened the clinical spectrum of disease to include chronic or intermittent X-linked thrombocytopenia (XLT), a relatively mild form of WAS, and X-linked neutropenia (XLN) due to an arrest of myelopoiesis. The pathophysiological mechanisms relate to defective actin polymerization in hematopoietic cells as a result of deficient or dysregulated activity of the WAS protein (WASp). The severity of disease is variable and somewhat predictable from genotype. Treatment strategies therefore range from conservative through to early definitive intervention by using allogeneic hematopoietic stem cell transplantation and potentially somatic gene therapy. All aspects of the condition from clinical presentation to molecular pathology and basic cellular mechanisms have been reviewed recently.

Ten years of gene therapy for primary immune deficiencies

Gene therapy with hematopoietic stem cells (HSC) is an attractive therapeutic strategy for several forms of primary immunodeficiencies. Current approaches are based on ex vivo gene transfer of the therapeutic gene into autologous HSC by vector-mediated gene transfer. In the past decade, substantial progress has been achieved in the treatment of severe combined immundeficiencies (SCID)-X1, adenosine deaminase (ADA)-deficient SCID, and chronic granulomatous disease (CGD). Results of the SCID gene therapy trials have shown long-term restoration of immune competence and clinical benefit in over 30 patients. The inclusion of reduced-dose conditioning in the ADA-SCID has allowed the engraftment of multipotent gene-corrected HSC at substantial level. In the CGD trial significant engraftment and transgene expression were observed, but the therapeutic effect was transient. The occurrence of adverse events related to insertional mutagenesis in the SCID-X1 and CGD trial has highlighted the limitations of current retroviral vector technology. For future applications the risk-benefit evaluation should include the type of vector employed, the disease background and the nature of the transgene. The use of self-inactivating lentiviral vectors will provide significant advantages in terms of natural gene regulation and reduction in the potential for adverse mutagenic events. Following recent advances in preclinical studies, lentiviral vectors are now being translated into new clinical approaches, such as Wiskott-Aldrich Syndrome.

WASP confers selective advantage for specific hematopoietic cell populations and serves a unique role in marginal zone B-cell homeostasis and function

Development of hematopoietic cells depends on a dynamic actin cytoskeleton. Here we demonstrate that expression of the cytoskeletal regulator WASP, mutated in the Wiskott-Aldrich syndrome, provides selective advantage for the development of naturally occurring regulatory T cells, natural killer T cells, CD4(+) and CD8(+) T lymphocytes, marginal zone (MZ) B cells, MZ macrophages, and platelets. To define the relative contribution of MZ B cells and MZ macrophages for MZ development, we generated wild-type and WASP-deficient bone marrow chimeric mice, with full restoration of the MZ. However, even in the presence of MZ macrophages, only 10% of MZ B cells were of WASP-deficient origin. We show that WASP-deficient MZ B cells hyperproliferate in vivo and fail to respond to sphingosine-1-phosphate, a crucial chemoattractant for MZ B-cell positioning. Abnormalities of the MZ compartment in WASP(-/-) mice lead to aberrant uptake of Staphylococcus aureus and to a reduced immune response to TNP-Ficoll. Moreover, WASP-deficient mice have increased levels of "natural" IgM antibodies. Our findings reveal that WASP regulates both development and function of hematopoietic cells. We demonstrate that WASP deficiency leads to an aberrant MZ that may affect responses to blood-borne pathogens and peripheral B-cell tolerance.

Gene therapy for primary immunodeficiencies

Primary immunodeficiencies are a group of disorders that are highly amenable to gene therapy because of their defined pathophysiology and the accessibility of the hematopoietic system to molecular intervention. The development of this new therapeutic modality has been driven by the established morbidity and mortality associated with conventional allogeneic stem cell transplantation, particularly in the human leukocyte antigen-mismatched setting. Recently, several clinical studies have shown that gamma retroviral gene transfer technology can produce major beneficial therapeutic effects, but, as for all cellular and pharmacologic treatment approaches, with a finite potential for toxicity. Newer developments in vector design showing promise in overcoming these issues are likely to establish gene therapy as an efficacious strategy for many forms of primary immunodeficiencies.

Gene therapy of inherited immunodeficiencies

BACKGROUND

Primary immunodeficiencies (PID) are a group of inherited diseases that affect the development or activity of the immune system. In severe cases allogeneic haematopoietic stem cell transplantation has proved to be a successful curative modality but it is limited by toxicity and reduced efficacy in mismatched donor settings.

OBJECTIVE

Gene therapy for PID has been developed as an alternative strategy and has entered the clinical arena. In this review we discuss the outcomes of recent gene therapy trials and some of the problems that remain to be tackled.

METHODS

Results from clinical trials for X-linked severe combined immunodeficiency (SCID-X1), adenosine deaminase deficient SCID (ADA-SCID), and X-linked chronic granulomatous disease (X-CGD) are discussed. In addition, other conditions are highlighted such as the Wiskott Aldrich Syndrome (WAS) for which gene therapy has shown considerable promise in preclinical studies, and are currently being translated into novel clinical approaches.

RESULTS/CONCLUSION

Whilst these encouraging results demonstrate that gene therapy can be used successfully to treat monogenic PID, the occurrence of vector-related side effects has highlighted the need for accurate assessment of the associated risks and a requirement for improvements in vector design.