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.

Restoration of photoreceptor ultrastructure and function in retinal degeneration slow mice by gene therapy.

The gene Prph2 encodes a photoreceptor-specific membrane glycoprotein, peripherin-2 (also known as peripherin/rds), which is inserted into the rims of photoreceptor outer segment discs in a complex with rom-1 (ref. 2). The complex is necessary for the stabilization of the discs, which are renewed constantly throughout life, and which contain the visual pigments necessary for photon capture. Mutations in Prph2 have been shown to result in a variety of photoreceptor dystrophies, including autosomal dominant retinitis pigmentosa and macular dystrophy. A common feature of these diseases is the loss of photoreceptor function, also seen in the retinal degeneration slow (rds or Prph2 Rd2/Rd2) mouse, which is homozygous for a null mutation in Prph2. It is characterized by a complete failure to develop photoreceptor discs and outer segments, downregulation of rhodopsin and apoptotic loss of photoreceptor cells. The electroretinograms (ERGs) of Prph2Rd2/Rd2 mice have greatly diminished a-wave and b-wave amplitudes, which decline to virtually undetectable concentrations by two months. Subretinal injection of recombinant adeno-associated virus (AAV) encoding a Prph2 transgene results in stable generation of outer segment structures and formation of new stacks of discs containing both perpherin-2 and rhodopsin, which in many cases are morphologically similar to normal outer segments. Moreover, the re-establishment of the structural integrity of the photoreceptor layer also results in electrophysiological correction. These studies demonstrate for the first time that a complex ultrastructural cell defect can be corrected both morphologically and functionally by in vivo gene transfer.

Scale-up and manufacturing of clinical-grade self-inactivating γ-retroviral vectors by transient transfection.

The need for γ-retroviral (gRV) vectors with a self-inactivating (SIN) design for clinical application has prompted a shift in methodology of vector manufacturing from the traditional use of stable producer lines to transient transfection-based techniques. Herein, we set out to define and optimize a scalable manufacturing process for the production of gRV vectors using transfection in a closed-system bioreactor in compliance with current good manufacturing practices (cGMP). The process was based on transient transfection of 293T cells on Fibra-Cel disks in the Wave Bioreactor. Cells were harvested from tissue culture flasks and transferred to the bioreactor containing Fibra-Cel in the presence of vector plasmid, packaging plasmids and calcium-phosphate in Dulbecco's modified Eagle's medium and 10% fetal bovine serum. Virus supernatant was harvested at 10-14 h intervals. Using optimized procedures, a total of five ecotropic cGMP-grade gRV vectors were produced (9 liters each) with titers up to 3.6 × 10(7) infectious units per milliliter on 3T3 cells. One GMP preparation of vector-like particles was also produced. These results describe an optimized process for the generation of SIN viral vectors by transfection using a disposable platform that allows for the generation of clinical-grade viral vectors without the need for cleaning validation in a cost-effective manner.

Critical variables affecting clinical-grade production of the self-inactivating gamma-retroviral vector for the treatment of X-linked severe combined immunodeficiency.

Patients with X-linked severe combined immunodeficiency (SCID-X1) were successfully cured following gene therapy with a gamma-retroviral vector (gRV) expressing the common gamma chain of the interleukin-2 receptor (IL2RG). However, 5 of 20 patients developed leukemia from activation of cellular proto-oncogenes by viral enhancers in the long-terminal repeats (LTR) of the integrated vector. These events prompted the design of a gRV vector with self-inactivating (SIN) LTRs to enhance vector safety. Herein we report on the production of a clinical-grade SIN IL2RG gRV pseudotyped with the Gibbon Ape Leukemia Virus envelope for a new gene therapy trial for SCID-X1, and highlight variables that were found to be critical for transfection-based large-scale SIN gRV production. Successful clinical production required careful selection of culture medium without pre-added glutamine, reduced exposure of packaging cells to cell-dissociation enzyme, and presence of cations in wash buffer. The clinical vector was high titer; transduced 68-70% normal human CD34(+) cells, as determined by colony-forming unit assays and by xenotransplantation in immunodeficient NOD.CB17-Prkdc(scid)/J (nonobese diabetic/severe combined immunodeficiency (NOD/SCID)) and NOD.Cg-Prkdc(scid) Il2rg(tm1Wjl)/SzJ (NOD/SCID gamma (NSG))) mice; and resulted in the production of T cells in vitro from human SCID-X1 CD34(+) cells. The vector was certified and released for the treatment of SCID-X1 in a multi-center international phase I/II trial.

Long-term safety and efficacy of lentiviral hematopoietic stem/progenitor cell gene therapy for Wiskott-Aldrich syndrome.

Patients with Wiskott-Aldrich syndrome (WAS) lacking a human leukocyte antigen-matched donor may benefit from gene therapy through the provision of gene-corrected, autologous hematopoietic stem/progenitor cells. Here, we present comprehensive, long-term follow-up results (median follow-up, 7.6 years) (phase I/II trial no. NCT02333760 ) for eight patients with WAS having undergone phase I/II lentiviral vector-based gene therapy trials (nos. NCT01347346 and NCT01347242 ), with a focus on thrombocytopenia and autoimmunity. Primary outcomes of the long-term study were to establish clinical and biological safety, efficacy and tolerability by evaluating the incidence and type of serious adverse events and clinical status and biological parameters including lentiviral genomic integration sites in different cell subpopulations from 3 years to 15 years after gene therapy. Secondary outcomes included monitoring the need for additional treatment and T cell repertoire diversity. An interim analysis shows that the study meets the primary outcome criteria tested given that the gene-corrected cells engrafted stably, and no serious treatment-associated adverse events occurred. Overall, severe infections and eczema resolved. Autoimmune disorders and bleeding episodes were significantly less frequent, despite only partial correction of the platelet compartment. The results suggest that lentiviral gene therapy provides sustained clinical benefits for patients with WAS.

Progress and prospects: gene therapy for inherited immunodeficiencies.

Haematopoietic stem cell transplantation (HSCT) is now widely used to treat primary immunodeficiencies (PID). For patients with specific disorders (severe combined immunodeficiency (SCID)-X1, adenosine deaminase deficiency (ADA)-SCID, X-chronic granulomatous disease (CGD) and Wiskott-Aldrich Syndrome (WAS)) who lack a suitable human leukocyte antigen (HLA)-matched donor, gene therapy has offered an important alternative treatment option. The success of gene therapy can be attributed, in part, to the selective advantage offered to gene-corrected cells, the avoidance of graft-versus-host disease and to the use of pre-conditioning in patients with chemotherapy to facilitate engraftment of corrected cells. Adverse events have been encountered and this has led to detailed characterization of retroviral vector integration profiles. A new generation of self-inactivating retroviral and lentiviral vectors have been designed to address these safety concerns, and are at an advanced stage of preparation for the next phase of clinical testing.

Validation of a mutated PRE sequence allowing high and sustained transgene expression while abrogating WHV-X protein synthesis: application to the gene therapy of WAS.

The woodchuck hepatitis virus posttranscriptional regulatory element (WPRE) is widely used in retroviral gene transfer vectors. However, this element contains an open-reading frame (ORF) encoding a truncated peptide of the woodchuck hepatitis virus X protein (WHX). Because we are developing a lentiviral vector for the gene therapy of Wiskott-Aldrich syndrome (WAS), we evaluated whether the WPRE was needed in the gene transfer cassette and tested the possibility of replacing it with a mutated derivative. The transcriptional activity of the WPRE was undetectable in the context of the lentiviral vector but the element was capable of translating a polypeptide. This capability was abrogated by mutating the WHX ORF translation start. The WPRE was required to express high levels of the transgene and for that, the native form or mutated derivatives functioned equivalently. The vector using a WAS gene promoter and the mut6 WPRE induced long-term expression of the WAS transgene in vivo, correcting cytoskeletal defects, thymocyte and B-cell numbers and improved the colitis of WAS-null mice. By providing additional evidence of efficacy of this WAS lentiviral vector with improved safety features, our results validate a mutated WPRE, which should be useful in future gene therapy applications.

Transgene optimization significantly improves SIN vector titers, gp91phox expression and reconstitution of superoxide production in X-CGD cells.

Gene therapy has proven to be of potential value for the correction of inherited hematopoietic disorders. However, the occurrence of severe side effects in some of the clinical trials has questioned the safety of this approach and has hampered the use of long terminal repeat-driven vectors for the treatment of a large number of patients. The development of self-inactivating (SIN) vectors with reduced genotoxicity provides an alternative to the currently used vectors. Our initial attempts to use SIN vectors for the correction of a myeloid disorder, chronic granulomatous disease, failed due to low vector titers and poor transgene expression. The optimization of the transgene cDNA (gp91(phox)) resulted in substantially increased titers and transgene expression. Most notably, transgene optimization significantly improved expression of a second cistron located downstream of gp91(phox). Thus, optimization of the transgene sequence results in higher expression levels and increased therapeutic index allowing the use of low vector copy numbers per transduced cell and weaker internal promoters.

Efficient gene delivery to the adult and fetal CNS using pseudotyped non-integrating lentiviral vectors.

Non-integrating lentiviral vectors show considerable promise for gene therapy applications as they persist as long-term episomes in non-dividing cells and diminish risks of insertional mutagenesis. In this study, non-integrating lentiviral vectors were evaluated for their use in the adult and fetal central nervous system of rodents. Vectors differentially pseudotyped with vesicular stomatitis virus, rabies and baculoviral envelope proteins allowed targeting of varied cell populations. Efficient gene delivery to discrete areas of the brain and spinal cord was observed following stereotactic administration. Furthermore, after direct in utero administration (E14), sustained and strong expression was observed 4 months into adulthood. Quantification of transduction and viral copy number was comparable when using non-integrating lentivirus and conventional integrating vector. These data support the use of non-integrating lentiviral vectors as an effective alternative to their integrating counterparts in gene therapy applications, and highlight their potential for treatment of inherited and acquired neurological disorders.

Lentiviral transduction of the murine lung provides efficient pseudotype and developmental stage-dependent cell-specific transgene expression.

Gene transfer for cystic fibrosis (CF) airway disease has been hampered by the lung's innate refractivity to pathogen infection. We hypothesized that early intervention with an integrating gene transfer vector capable of transducing the lung via the lumen may be a successful therapeutic approach. An HIV-based lentiviral vector pseudotyped with the baculovirus gp64 envelope was applied to the fetal, neonatal or adult airways. Fetal intra-amniotic administration resulted in transduction of approximately 14% of airway epithelial cells, including both ciliated and non-ciliated epithelia of the upper, mid and lower airways; there was negligible alveolar or nasal transduction. Following neonatal intra-nasal administration we observed significant transduction of the airway epithelium (approximately 11%), although mainly in the distal lung, and substantial alveolar transduction. This expression was still detectable at 1 year after application. In the adult, the majority of transduction was restricted to the alveoli. In contrast, vesicular stomatitis virus glycoprotein pseudotyped virus transduced only alveoli after adult and neonatal application and no transduction was observed after fetal administration. Repeat administration did not increase transduction levels of the conducting airway epithelia. These data demonstrate that application at early developmental stages in conjunction with an appropriately pseudotyped virus provides efficient, high-level transgene expression in the murine lung. This may provide a modality for treatment for lung disease in CF.

WASP and WIP regulate podosomes in migrating leukocytes.

Podosomes are specialized adhesion sites found in rapidly migrating and invasive cells, most notably in cells from the myeloid lineage that participate in immune surveillance and phagocyte defence mechanisms. In this review, we describe the nature of leukocyte podosomes and the regulation of their turnover during migration by the key regulatory molecules Wiskott-Aldrich syndrome protein and WASP-interacting protein.

Ectopic retroviral expression of LMO2, but not IL2Rgamma, blocks human T-cell development from CD34+ cells: implications for leukemogenesis in gene therapy.

The occurrence of leukemia in a gene therapy trial for SCID-X1 has highlighted insertional mutagenesis as an adverse effect. Although retroviral integration near the T-cell acute lymphoblastic leukemia (T-ALL) oncogene LIM-only protein 2 (LMO2) appears to be a common event, it is unclear why LMO2 was preferentially targeted. We show that of classical T-ALL oncogenes, LMO2 is most highly transcribed in CD34+ progenitor cells. Upon stimulation with growth factors typically used in gene therapy protocols transcription of LMO2, LYL1, TAL1 and TAN1 is most prominent. Therefore, these oncogenes may be susceptible to viral integration. The interleukin-2 receptor gamma chain (IL2Rgamma), which is mutated in SCID-X1, has been proposed as a cooperating oncogene to LMO2. However, we found that overexpressing IL2Rgamma had no effect on T-cell development. In contrast, retroviral overexpression of LMO2 in CD34+ cells caused severe abnormalities in T-cell development, but B-cell and myeloid development remained unaffected. Our data help explain why LMO2 was preferentially targeted over many of the other known T-ALL oncogenes. Furthermore, during T-cell development retrovirus-mediated expression of IL2Rgamma may not be directly oncogenic. Instead, restoration of normal IL7-receptor signaling may allow progression of T-cell development to stages where ectopic LMO2 expression causes aberrant thymocyte growth.

Molecular Evidence of Genome Editing in a Mouse Model of Immunodeficiency.

Genome editing is the introduction of directed modifications in the genome, a process boosted to therapeutic levels by designer nucleases. Building on the experience of ex vivo gene therapy for severe combined immunodeficiencies, it is likely that genome editing of haematopoietic stem/progenitor cells (HSPC) for correction of inherited blood diseases will be an early clinical application. We show molecular evidence of gene correction in a mouse model of primary immunodeficiency. In vitro experiments in DNA-dependent protein kinase catalytic subunit severe combined immunodeficiency (Prkdc scid) fibroblasts using designed zinc finger nucleases (ZFN) and a repair template demonstrated molecular and functional correction of the defect. Following transplantation of ex vivo gene-edited Prkdc scid HSPC, some of the recipient animals carried the expected genomic signature of ZFN-driven gene correction. In some primary and secondary transplant recipients we detected double-positive CD4/CD8 T-cells in thymus and single-positive T-cells in blood, but no other evidence of immune reconstitution. However, the leakiness of this model is a confounding factor for the interpretation of the possible T-cell reconstitution. Our results provide support for the feasibility of rescuing inherited blood disease by ex vivo genome editing followed by transplantation, and highlight some of the challenges.

Dendritic cells: the bare bones of immunity.

Dendritic cells are specialized antigen-presenting cells, critical for initiating and regulating immune responses. Two new studies demonstrate the importance of coordinated cytoskeletal regulation for their normal function.

T cell suicide gene therapy to aid haematopoietic stem cell transplantation.

Graft versus host disease (GVHD) is a T cell mediated phenomenon that arises following allogeneic haematopoietic stem cell transplantation, and may be particularly severe in the context of human leukocyte antigen (HLA) mismatched procedures. Although GVHD can be largely abrogated through T cell depletion, such measures result in loss of graft potency and reduced anti-viral and anti-leukaemic effects. The genetic modification of T cells to carry a suicide gene mechanism has been advocated as means of allowing T cells to be harnessed for their beneficial effects, and safely eliminated in the event of significant GVHD. The feasibility of the strategy has been demonstrated in clinical studies using T cells modified by retroviral transduction to encode the herpes simplex thymidine kinase (HSVTK) gene to treat patients with haematological malignancies. However, a number of limitations associated with current protocols have become apparent. Most notably, the process of retroviral transduction, which requires pre-activation of T cells, appears to impair subsequent functional potential. Efforts are now directed towards circumventing the pre-activation requirements of retroviral vectors by using alternative lentiviral systems, in association with improved suicide gene/prodrug combinations.

A defined window for efficient gene marking of severe combined immunodeficient-repopulating cells using a gibbon ape leukemia virus-pseudotyped retroviral vector.

We have investigated the minimal time required for efficient transduction of human hematopoietic repopulating cells using a surrogate nonobese diabetic (NOD)/severe combined immunodeficient (SCID) xenoengraftment assay. Cord blood CD34+ cells were transduced to high levels over 24-48 hr in the presence of Flt-3 ligand, stem cell factor, interleukin 3, and interleukin 6. Under these conditions, high levels of NOD/SCID repopulating activity were preserved, but the levels of gene marking in engrafting cell populations measured by expression of a reporter transgene were low. Extension of the transduction period by 24 hr (total culture period, 72 hr) under the same cytokine conditions resulted in high levels of gene marking, but on closer analysis expression was limited predominantly to the myeloid population. Efficient transduction of both lymphoid and myeloid lineages could be achieved only if the transduction protocol was extended by a further 24 hr (total culture period, 96 hr), suggesting that myeloid lineage-committed precursors are capable of repopulation, and that over shorter time periods transduction is largely restricted to this population. This adds to the emerging evidence of heterogeneity within the SRC compartment, and has important implications for the interpretation of this assay in stem cell transplantation and gene transfer studies.

Adeno-associated virus gene transfer to mouse retina.

Ocular gene transfer may provide a means for arresting the retinal degeneration characteristic of many inherited causes of blindness, including retinitis pigmentosa (RP). Previously, we have shown in immunodeficient animals that recombinant adeno-associated virus (rAAV) mediates transduction of photoreceptors as well as the retinal pigment epithelium (RPE) following subretinal injection. In this study we extend these observations and show that highly purified recombinant AAV vectors encoding the reporter gene LacZ transduce photoreceptors in an immunocompetent mouse strain following subretinal injection and efficiently transduce ganglion cells after intravitreal injection. Levels of transduction increase over time. Sublethal gamma-irradiation is shown to facilitate this process.

High-titer recombinant adeno-associated virus production from replicating amplicons and herpes vectors deleted for glycoprotein H.

Production of high-titer rAAV is essential for in vivo clinical application. One limiting factor may be the failure of existing systems to replicate the packaging genome in such a way that expression of Rep and Cap proteins is coordinately amplified. DISC-HSV (disabled single-cycle virus) is a genetically modified herpes simplex virus (HSV) that by deletion of glycoprotein H (gH) is infectious only if propagated in a complementing cell line. In this study, we have used DISC-HSV as a helper for rAAV replication, and have simulated to some extent the amplication of the rep and cap genomes seen in wtAAV infection by incorporating both these and vector sequences in HSV amplicons. Facilitated production of AAV Rep and Cap proteins translates into a considerably improved recovery of rAAV, which transduces cells of the neuroretina in vivo with high efficiency. The potential for contamination with infectious herpes particles is eliminated by the use of noncomplementing (gH-) cell lines to propagate the virus, and by standard purification methods. The use of DISC-HSV and herpes-derived amplicons for production of rAAV may be a useful strategy for future in vivo studies and for clinical application.

Lipid-mediated enhancement of transfection by a nonviral integrin-targeting vector.

Nonviral vectors consisting of integrin-targeting peptide/DNA (ID) complexes have the potential for widespread application in gene therapy. The transfection efficiency of this vector, however, has been limited by endosomal degradation. We now report that lipofectin (L) incorporated into the ID complexes enhances integrin-mediated transfection, increasing luciferase expression by more than 100-fold. The transfection efficiency of Lipofectin/Integrin-binding peptide/DNA (LID) complexes, assessed by beta-galactosidase reporter gene expression and X-gal staining, was improved from 1% to 10% to over 50% for three different cell lines, and from 0% to approximately 25% in corneal endothelium in vitro. Transfection complexes have been optimized with respect to their transfection efficiency and we have investigated their structure, function, and mode of transfection. Both ID and LID complexes formed particles, unlike the fibrous network formed by lipofectin/DNA complexes (LD). Integrin-mediated transfection by LID complexes was demonstrated by the substantially lower transfection efficiency of LKD complexes in which the integrin-biding peptide was substituted for K16 (K). Furthermore, the transfection efficiency of complexes was shown to be dependent on the amount of integrin-targeting ligand in the complex. Finally, a 34% reduction in integrin-mediated transfection efficiency by LID complexes was achieved with a competing monoclonal antibody. The role of lipofectin in LID complexes appears, therefore, to be that of a co-factor, enhancing the efficiency of integrin-mediated transfection. The mechanism of enhancement is likely to involve a reduction in the extent of endosomal degradation of DNA.