IL-3 or IL-7 increases ex vivo gene transfer efficiency in ADA-SCID BM CD34+ cells while maintaining in vivo lymphoid potential

Multiple Integrated Non-clinical Studies Predict the Safety of Lentivirus-Mediated Gene Therapy for β-Thalassemia

Gene therapy clinical trials require rigorous non-clinical studies in the most relevant models to assess the benefit-to-risk ratio. To support the clinical development of gene therapy for β-thalassemia, we performed in vitro and in vivo studies for prediction of safety. First we developed newly GLOBE-derived vectors that were tested for their transcriptional activity and potential interference with the expression of surrounding genes. Because these vectors did not show significant advantages, GLOBE lentiviral vector (LV) was elected for further safety characterization. To support the use of hematopoietic stem cells (HSCs) transduced by GLOBE LV for the treatment of β-thalassemia, we conducted toxicology, tumorigenicity, and biodistribution studies in compliance with the OECD Principles of Good Laboratory Practice. We demonstrated a lack of toxicity and tumorigenic potential associated with GLOBE LV-transduced cells. Vector integration site (IS) studies demonstrated that both murine and human transduced HSCs retain self-renewal capacity and generate new blood cell progeny in the absence of clonal dominance. Moreover, IS analysis showed an absence of enrichment in cancer-related genes, and the genes targeted by GLOBE LV in human HSCs are well known sites of integration, as seen in other lentiviral gene therapy trials, and have not been associated with clonal expansion. Taken together, these integrated studies provide safety data supporting the clinical application of GLOBE-mediated gene therapy for β-thalassemia.

Twenty-Five Years of Gene Therapy for ADA-SCID: From Bubble Babies to an Approved Drug

Twenty-five years have passed since first attempts of gene therapy (GT) in children affected by severe combined immunodeficiency (SCID) due to adenosine deaminase (ADA) defect, also known by the general public as bubble babies. ADA-SCID is fatal early in life if untreated. Unconditioned hematopoietic stem cell (HSC) transplant from matched sibling donor represents a curative treatment but is available for few patients. Enzyme replacement therapy can be life-saving, but its chronic use has many drawbacks. This review summarizes the history of ADA-SCID GT over the last 25 years, starting from first pioneering studies in the early 1990s using gamma-retroviral vectors, based on multiple infusions of genetically corrected autologous peripheral blood lymphocytes. HSC represented the ideal target for gene correction to guarantee production of engineered multi-lineage progeny, but it required a decade to achieve therapeutic benefit with this approach. Introduction of low-intensity conditioning represented a crucial step in achieving stable gene-corrected HSC engraftment and therapeutic levels of ADA-expressing cells. Recent clinical trials demonstrated that gamma-retroviral GT for ADA-SCID has a favorable safety profile and is effective in restoring normal purine metabolism and immune functions in patients >13 years after treatment. No abnormal clonal proliferation or leukemia development have been observed in >40 patients treated experimentally in five different centers worldwide. In 2016, the medicinal product Strimvelis™ received marketing approval in Europe for patients affected by ADA-SCID without a suitable human leukocyte antigen-matched related donor. Positive safety and efficacy results have been obtained in GT clinical trials using lentiviral vectors encoding ADA. The results obtained in last 25 years in ADA-SCID GT development fundamentally contributed to improve patients' prognosis, together with earlier diagnosis thanks to newborn screening. These advances open the way to further clinical development of GT as treatment for broader applications, from inherited diseases to cancer.

Developmental and Functional Control of Natural Killer Cells by Cytokines

Natural killer (NK) cells are effective in combating infections and tumors and as such are tempting for adoptive transfer therapy. However, they are not homogeneous but can be divided into three main subsets, including cytotoxic, tolerant, and regulatory NK cells, with disparate phenotypes and functions in diverse tissues. The development and functions of such NK cells are controlled by various cytokines, such as fms-like tyrosine kinase 3 ligand (FL), kit ligand (KL), interleukin (IL)-3, IL-10, IL-12, IL-18, transforming growth factor-β, and common-γ chain family cytokines, which operate at different stages by regulating distinct signaling pathways. Nevertheless, the specific roles of each cytokine that regulates NK cell development or that shapes different NK cell functions remain unclear. In this review, we attempt to describe the characteristics of each cytokine and the existing protocols to expand NK cells using different combinations of cytokines and feeder cells. A comprehensive understanding of the role of cytokines in NK cell development and function will aid the generation of better efficacy for adoptive NK cell treatment.

Inborn errors of the development of human natural killer cells

PURPOSE OF REVIEW

Inborn errors of human natural killer (NK) cells may affect the development of these cells, their function, or both. There are two broad categories of genetic defects of NK cell development, depending on whether the deficiency is apparently specific to NK cells or clearly affects multiple hematopoietic lineages. We review here recent progress in the genetic dissection of these NK deficiencies (NKDs).

RECENT FINDINGS

Patients with severe combined immunodeficiencies bearing mutations of adenosine deaminase, adenylate kinase 2, interleukin-2 receptor gamma chain, and Janus kinase 3 genes present NKDs and are prone to a broad range of infections. Patients with GATA binding protein 2 deficiency are susceptible to both mycobacterial and viral infections, and display NKDs and a lack of monocytes. Rare patients with mini chromosomal maintenance 4 deficiency display an apparently selective NKD associated with viral infections, but they also display various nonhematopoietic phenotypes, including adrenal insufficiency and growth retardation.

SUMMARY

These studies have initiated a genetic dissection of the development of human NK cells. Further studies are warranted, including the search for genetic causes of NKD in particular. This research may lead to the discovery of molecules specifically controlling the development of NK cells and to improvements in our understanding of the hitherto elusive function of these cells in humans.

Preclinical demonstration of lentiviral vector-mediated correction of immunological and metabolic abnormalities in models of adenosine deaminase deficiency

Gene transfer into autologous hematopoietic stem cells by γ-retroviral vectors (gRV) is an effective treatment for adenosine deaminase (ADA)-deficient severe combined immunodeficiency (SCID). However, current gRV have significant potential for insertional mutagenesis as reported in clinical trials for other primary immunodeficiencies. To improve the efficacy and safety of ADA-SCID gene therapy (GT), we generated a self-inactivating lentiviral vector (LV) with a codon-optimized human cADA gene under the control of the short form elongation factor-1α promoter (LV EFS ADA). In ADA(-/-) mice, LV EFS ADA displayed high-efficiency gene transfer and sufficient ADA expression to rescue ADA(-/-) mice from their lethal phenotype with good thymic and peripheral T- and B-cell reconstitution. Human ADA-deficient CD34(+) cells transduced with 1-5 × 10(7) TU/ml had 1-3 vector copies/cell and expressed 1-2x of normal endogenous levels of ADA, as assayed in vitro and by transplantation into immune-deficient mice. Importantly, in vitro immortalization assays demonstrated that LV EFS ADA had significantly less transformation potential compared to gRV vectors, and vector integration-site analysis by nrLAM-PCR of transduced human cells grown in immune-deficient mice showed no evidence of clonal skewing. These data demonstrated that the LV EFS ADA vector can effectively transfer the human ADA cDNA and promote immune and metabolic recovery, while reducing the potential for vector-mediated insertional mutagenesis.

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.

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.

Ex vivo gene therapy with lentiviral vectors rescues adenosine deaminase (ADA)–deficient mice and corrects their immune and metabolic defects

Adenosine deaminase (ADA) deficiency is caused by a purine metabolic dysfunction, leading to severe combined immunodeficiency (SCID) and multiple organ damage. To investigate the efficacy of ex vivo gene therapy with self-inactivating lentiviral vectors (LVs) in correcting this complex phenotype, we used an ADA–/– mouse model characterized by early postnatal lethality. LV-mediated ADA gene transfer into bone marrow cells combined with low-dose irradiation rescued mice from lethality and restored their growth, as did transplantation of wild-type bone marrow. Mixed chimerism with multilineage engraftment of transduced cells was detected in the long term in animals that underwent transplantation. ADA activity was normalized in lymphocytes and partially corrected in red blood cells (RBCs), resulting in full metabolic detoxification and prevention of severe pulmonary insufficiency. Moreover, gene therapy restored normal lymphoid differentiation and immune functions, including antigen-specific antibody production. Similar degrees of detoxification and immune reconstitution were obtained in mice treated early after birth or after 1 month of enzyme-replacement therapy, mimicking 2 potential applications for ADA-SCID. Overall, this study demonstrates the efficacy of LV gene transfer in correcting both the immunological and metabolic phenotypes of ADA-SCID and supports the future clinical use of this approach.

Genotoxicity of retroviral integration in hematopoietic cells

The experience of the past 3 years, since the first case of leukemia was reported in a child cured of X-linked severe combined immunodeficiency (X-SCID) by gene therapy, indicates that the potential genotoxicity of retroviral integration in hematopoietic cells will remain a consideration in evaluating the relative risks versus benefits of gene therapy for specific blood disorders. Although many unique variables may have contributed to an increased risk in X-SCID patients, clonal dominance or frank neoplasia in animal models, clonal dominance in humans with chronic granulomatous disease, and the ability of retroviral integration to immortalize normal bone marrow cells or convert factor-dependent cells to factor independence suggest that transduction of cells with an integrating retrovirus has the potential for altering their subsequent biologic behavior. The selective pressure imposed during in vitro culture or after engraftment may uncover a growth or survival advantage for cells in which an integration event has affected gene expression. Such cells then carry the risk that subsequent mutations may lead to neoplastic evolution of individual clones. Balancing that risk is that the vast majority of integration events seem to be neutral and that optimizing vector design may diminish the probability of altering gene expression by an integrated vector genome. Several cell culture systems and animal models designed to empirically evaluate the safety of vector systems are being developed and should provide useful data for weighing the relative risks and benefits for specific diseases and patient populations. Gene therapy interventions continue to have enormous potential for the treatment of disorders of the hematopoietic system. The future of such efforts seems bright as we continue to evolve and improve various strategies to make such interventions both effective and as safe as possible.

Primary immune effects of eukaryotic expression plasmids encoding two hyperactive mutants of human soluble B lymphocyte stimulator

B lymphocyte stimulator (BLyS), a ligand belonging to the tumor necrosis factor (TNF) family, plays a critical role in regulating survival and activation of peripheral B cell populations during humeral immune responses. Among the TNF family members, BLyS is unique in that it contains an unpaired Cys residue (Cys146) at the corresponding position where some other members have about 37.5% (6/16) Ala or 37.5% (6/16) Val. Here, with eukaryotic expression vector pcDNA3.1(-), we mutated Cys146 to Ala or Val and constructed two mutant eukaryotic expression plasmids of the human soluble BLyS, pcDNA3.1BY-A and pcDNA3.1BY-V. Following repetitive subcutaneous injection of these expression plasmids in BALb/C mice, the wild-type and mutant BLyS proteins were detectable in the blood of treated animals over several weeks. In addition, the expression of these proteins induced specific IgG but not IgM responses. The implications of the results are discussed.