In vivo retroviral gene transfer by direct intrafemoral injection results in correction of the SCID phenotype in Jak3 knock-out animals

Evolving Gene Therapy in Primary Immunodeficiency

Prior to the first successful bone marrow transplant in 1968, patients born with severe combined immunodeficiency (SCID) invariably died. Today, with a widening availability of newborn screening, major improvements in the application of allogeneic procedures, and the emergence of successful hematopoietic stem and progenitor cell (HSC/P) gene therapy, the majority of these children can be identified and cured. Here, we trace key steps in the development of clinical gene therapy for SCID and other primary immunodeficiencies (PIDs), and review the prospects for adoption of new targets and technologies.

Hemophilia A gene therapy via intraosseous delivery of factor VIII-lentiviral vectors

Current treatment of hemophilia A (HemA) patients with repeated infusions of factor VIII (FVIII; abbreviated as F8 in constructs) is costly, inconvenient, and incompletely effective. In addition, approximately 25 % of treated patients develop anti-factor VIII immune responses. Gene therapy that can achieve long-term phenotypic correction without the complication of anti-factor VIII antibody formation is highly desired. Lentiviral vector (LV)-mediated gene transfer into hematopoietic stem cells (HSCs) results in stable integration of FVIII gene into the host genome, leading to persistent therapeutic effect. However, ex vivo HSC gene therapy requires pre-conditioning which is highly undesirable for hemophilia patients. The recently developed novel methodology of direct intraosseous (IO) delivery of LVs can efficiently transduce bone marrow cells, generating high levels of transgene expression in HSCs. IO delivery of E-F8-LV utilizing a ubiquitous EF1α promoter generated initially therapeutic levels of FVIII, however, robust anti-FVIII antibody responses ensued neutralized functional FVIII activity in the circulation. In contrast, a single IO delivery of G-FVIII-LV utilizing a megakaryocytic-specific GP1bα promoter achieved platelet-specific FVIII expression, leading to persistent, partial correction of HemA in treated animals. Most interestingly, comparable therapeutic benefit with G-F8-LV was obtained in HemA mice with pre-existing anti-FVIII inhibitors. Platelets is an ideal IO delivery vehicle since FVIII stored in α-granules of platelets is protected from high-titer anti-FVIII antibodies; and that even relatively small numbers of activated platelets that locally excrete FVIII may be sufficient to promote efficient clot formation during bleeding. Additionally, combination of pharmacological agents improved transduction of LVs and persistence of transduced cells and transgene expression. Overall, a single IO infusion of G-F8-LV can generate long-term stable expression of hFVIII in platelets and correct hemophilia phenotype for long term. This approach has high potential to permanently treat FVIII deficiency with and without pre-existing anti-FVIII antibodies.

Migration of bone marrow progenitor cells in the adult brain of rats and rabbits

Neurogenesis takes place in the adult mammalian brain in three areas: Subgranular zone of the dentate gyrus (DG); subventricular zone of the lateral ventricle; olfactory bulb. Different molecular markers can be used to characterize the cells involved in adult neurogenesis. It has been recently suggested that a population of bone marrow (BM) progenitor cells may migrate to the brain and differentiate into neuronal lineage. To explore this hypothesis, we injected recombinant SV40-derived vectors into the BM and followed the potential migration of the transduced cells. Long-term BM-directed gene transfer using recombinant SV40-derived vectors leads to expression of the genes delivered to the BM firstly in circulating cells, then after several months in mature neurons and microglial cells, and thus without central nervous system (CNS) lesion. Most of transgene-expressing cells expressed NeuN, a marker of mature neurons. Thus, BM-derived cells may function as progenitors of CNS cells in adult animals. The mechanism by which the cells from the BM come to be neurons remains to be determined. Although the observed gradual increase in transgene-expressing neurons over 16 mo suggests that the pathway involved differentiation of BM-resident cells into neurons, cell fusion as the principal route cannot be totally ruled out. Additional studies using similar viral vectors showed that BM-derived progenitor cells migrating in the CNS express markers of neuronal precursors or immature neurons. Transgene-positive cells were found in the subgranular zone of the DG of the hippocampus 16 mo after intramarrow injection of the vector. In addition to cells expressing markers of mature neurons, transgene-positive cells were also positive for nestin and doublecortin, molecules expressed by developing neuronal cells. These cells were actively proliferating, as shown by short term BrdU incorporation studies. Inducing seizures by using kainic acid increased the number of BM progenitor cells transduced by SV40 vectors migrating to the hippocampus, and these cells were seen at earlier time points in the DG. We show that the cell membrane chemokine receptor, CCR5, and its ligands, enhance CNS inflammation and seizure activity in a model of neuronal excitotoxicity. SV40-based gene delivery of RNAi targeting CCR5 to the BM results in downregulating CCR5 in circulating cells, suggesting that CCR5 plays an important role in regulating traffic of BM-derived cells into the CNS, both in the basal state and in response to injury. Furthermore, reduction in CCR5 expression in circulating cells provides profound neuroprotection from excitotoxic neuronal injury, reduces neuroinflammation, and increases neuronal regeneration following this type of insult. These results suggest that BM-derived, transgene-expressing, cells can migrate to the brain and that they become neurons, at least in part, by differentiating into neuron precursors and subsequently developing into mature neurons.

Intraosseous delivery of lentiviral vectors targeting factor VIII expression in platelets corrects murine hemophilia A

Intraosseous (IO) infusion of lentiviral vectors (LVs) for in situ gene transfer into bone marrow may avoid specific challenges posed by ex vivo gene delivery, including, in particular, the requirement of preconditioning. We utilized IO delivery of LVs encoding a GFP or factor VIII (FVIII) transgene directed by ubiquitous promoters (a MND or EF-1α-short element; M-GFP-LV, E-F8-LV) or a platelet-specific, glycoprotein-1bα promoter (G-GFP-LV, G-F8-LV). A single IO infusion of M-GFP-LV or G-GFP-LV achieved long-term and efficient GFP expression in Lineage(-)Sca1(+)c-Kit(+) hematopoietic stem cells and platelets, respectively. While E-F8-LV produced initially high-level FVIII expression, robust anti-FVIII immune responses eliminated functional FVIII in circulation. In contrast, IO delivery of G-F8-LV achieved long-term platelet-specific expression of FVIII, resulting in partial correction of hemophilia A. Furthermore, similar clinical benefit with G-F8-LV was achieved in animals with pre-existing anti-FVIII inhibitors. These findings further support platelets as an ideal FVIII delivery vehicle, as FVIII, stored in α-granules, is protected from neutralizing antibodies and, during bleeding, activated platelets locally excrete FVIII to promote clot formation. Overall, a single IO infusion of G-F8-LV was sufficient to correct hemophilia phenotype for long term, indicating that this approach may provide an effective means to permanently treat FVIII deficiency.

Rapamycin relieves lentiviral vector transduction resistance in human and mouse hematopoietic stem cells

Transplantation of genetically modified hematopoietic stem cells (HSCs) is a promising therapeutic strategy for genetic diseases, HIV, and cancer. However, a barrier for clinical HSC gene therapy is the limited efficiency of gene delivery via lentiviral vectors (LVs) into HSCs. We show here that rapamycin, an allosteric inhibitor of the mammalian target of rapamycin complexes, facilitates highly efficient lentiviral transduction of mouse and human HSCs and dramatically enhances marking frequency in long-term engrafting cells in mice. Mechanistically, rapamycin enhanced postbinding endocytic events, leading to increased levels of LV cytoplasmic entry, reverse transcription, and genomic integration. Despite increasing LV copy number, rapamycin did not significantly alter LV integration site profile or chromosomal distribution in mouse HSCs. Rapamycin also enhanced in situ transduction of mouse HSCs via direct intraosseous infusion. Collectively, rapamycin strongly augments LV transduction of HSCs in vitro and in vivo and may prove useful for therapeutic gene delivery.

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.

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.

A novel lentiviral vector targets gene transfer into human hematopoietic stem cells in marrow from patients with bone marrow failure syndrome and in vivo in humanized mice

In vivo lentiviral vector (LV)-mediated gene delivery would represent a great step forward in the field of gene therapy. Therefore, we have engineered a novel LV displaying SCF and a mutant cat endogenous retroviral glycoprotein, RDTR. These RDTR/SCF-LVs outperformed RDTR-LVs for transduction of human CD34(+) cells (hCD34(+)). For in vivo gene therapy, these novel RDTR/SCF-displaying LVs can distinguish between the target hCD34(+) cells of interest and nontarget cells. Indeed, they selectively targeted transduction to 30%-40% of the hCD34(+) cells in cord blood mononuclear cells and in the unfractionated BM of healthy and Fanconi anemia donors, resulting in the correction of CD34(+) cells in the patients. Moreover, RDTR/SCF-LVs targeted transduction to CD34(+) cells with 95-fold selectivity compared with T cells in total cord blood. Remarkably, in vivo injection of the RDTR/SCF-LVs into the BM cavity of humanized mice resulted in the highly selective transduction of candidate hCD34(+)Lin(-) HSCs. In conclusion, this new LV will facilitate HSC-based gene therapy by directly targeting these primitive cells in BM aspirates or total cord blood. Most importantly, in the future, RDTR/SCF-LVs might completely obviate ex vivo handling and simplify gene therapy for many hematopoietic defects because of their applicability to direct in vivo inoculation.

Correction of Fanconi Anemia Group C Hematopoietic Stem Cells Following Intrafemoral Gene Transfer

The main cause of morbidity and mortality in Fanconi anemia patients is the development of bone marrow (BM) failure; thus correction of hematopoietic stem cells (HSCs) through gene transfer approaches would benefit FA patients. However, gene therapy trials for FA patients using ex vivo transduction protocols have failed to provide long-term correction. In addition, ex vivo cultures have been found to be hazardous for FA cells. To circumvent negative effects of ex vivo culture in FA stem cells, we tested the corrective ability of direct injection of recombinant lentiviral particles encoding FancC-EGFP into femurs of FancC(-/-) mice. Using this approach, we show that FancC(-/-) HSCs were efficiently corrected. Intrafemoral gene transfer of the FancC gene prevented the mitomycin C-induced BM failure. Moreover, we show that intrafemoral gene delivery into aplastic marrow restored the bone marrow cellularity and corrected the remaining HSCs. These results provide evidence that targeting FA-deficient HSCs directly in their environment enables efficient and long-term correction of BM defects in FA.

In situ (in vivo) gene transfer into murine bone marrow stem cells

Adult bone marrow stem cell is an ideal target for gene therapy of genetic diseases, selected malignant diseases, and AIDS. The in vivo approach of lentivirus vector (LV)-mediated stem cell gene transfer by intrafemoral (IF) injection can take full advantage of any source of stem cells residing in the bone cavity. Such an approach may avoid several difficulties encountered by ex vivo hematopoietic stem cell (HSC) gene transfer. We have shown that both HSC and mesenchymal stem/progenitor cells (MSC) can be genetically modified successfully by a single "in situ" IF injection in their natural "niche" in mice without any preconditioning. This approach may provide a novel application for treatment of human diseases, and represent an interesting new tool to study adult stem cell plasticity and the nature of unperturbed hematopoiesis.

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.

Hematopoietic stem cell transplant in mice by intra-femoral injection

In traditional bone marrow transplantation, the majority of peripherally introduced stem cells are trapped in peripheral organs, such as the lung and liver. The frequency of cells homed in bone marrow by such method is extremely low. This circumstance adds difficulty to the research of hematopoietic stem cell (HSC), a rare population to begin with. By introducing HSC directly into bone marrow cavity, the peripheral loss of HSC can be minimized. Thus, intra-femoral injection of HSC is a useful method for HSC study.

Delivering genes to the organ-localized immune system: long-term results of direct intramarrow transduction

We studied the distribution of transgene-expressing cells after direct gene transfer into the bone marrow (BM). Rats received direct injection into the femoral BM of SV(Nef-FLAG), a Tag-deleted recombinant SV40 carrying a marker gene (FLAG epitope). Controls received an unrelated rSV40 or saline. Blood cells (5%) and femoral marrow cells (25%) expressed FLAG throughout. FLAG expression was assessed in different organs at 1, 4 and 16 months. FLAG+ macrophages were seen throughout the body, and were prominent in the spleen. FLAG+ cells were common in pulmonary alveoli. The former included alveolar macrophages and type II pneumocytes. These cells were not detected at 1 month, occasional at 4 months and common at 16 months after intramarrow injection. Rare liver cells were positive for both FLAG and ferritin, indicating that some hepatocytes also expressed this BM-delivered transgene. Control animals were negative. Thus: (a) fixed tissue phagocytes may be accessible to gene delivery by intramarrow transduction of their progenitors; (b) transduced BM-resident cells or their derivatives may migrate to other organs (lungs) and may differentiate into epithelial cells; and (c) intramarrow injection of rSV40s does not detectably transduce parenchymal cells of other organs.

Rat bone marrow progenitor cells transduced in situ by rSV40 vectors differentiate into multiple central nervous system cell lineages

Using bone marrow-directed gene transfer, we tested whether bone marrow-derived cells may function as progenitors of central nervous system (CNS) cells in adult animals. SV40-derived gene delivery vectors were injected directly into femoral bone marrow, and we examined transgene expression in blood and brain for 0-16 months thereafter by immunostaining for FLAG epitope marker. An average of 5% of peripheral blood cells and 25% of femoral marrow cells were FLAG(+) throughout the study. CNS FLAG-expressing cells were mainly detected in the dentate gyrus (DG) and periventricular subependymal zone (PSZ). Although absent before 1 month and rare at 4 months, DG and PSZ FLAG(+) cells were abundant 16 months after bone marrow injection. Approximately 5% of DG cells expressed FLAG, including neurons (48.6%) and microglia (49.7%), and occasional astrocytes (1.6%), as determined by double immunostaining for FLAG and lineage markers. These data suggest that one or more populations of cells resident within adult bone marrow can migrate to the brain and differentiate into CNS-specific cells.

In vivo gene transfer into adult stem cells in unconditioned mice by in situ delivery of a lentiviral vector

The potential of in vivo lentivirus-mediated bone marrow stem cell gene transfer by bone cavity injection, which could take full advantage of any source of stem cells present there, has not been previously explored. Such an approach may avoid several difficulties encountered by ex vivo hematopoietic stem cell (HSC) gene transfer. We sought to determine if efficient gene transfer could be achieved in HSC and mesenchymal stem/progenitor cells (MSC) by intrafemoral injection of a lentivirus vector in mice. Four months after injection, up to 12% GFP-expressing cells were observed in myeloid and lymphoid subpopulations. Significant transduction efficiencies were seen in Lin(-)c-kit(+)Sca1(+) HSC/progenitors and CFU with multilineage potential, which were also confirmed by duplex PCR analysis of progenitor-derived colonies. Four months after secondary BMT, we observed 8.1 to 15% vector(+) CFU in all recipients. Integration analysis by LAM-PCR demonstrated that multiple transduced clones contributed to hematopoiesis in these animals. We also showed that GFP-expressing MSC retained multilineage differentiation potential, with 2.9 to 8.8% GFP-containing CFU-fibroblasts detected in both injected and BMT recipients. Our data provide evidence that adult stem cells in bone marrow can be efficiently transduced "in situ" by in vivo vector administration without preconditioning. This approach could lead to a novel application for treatment of human diseases.

Perspectives of gene therapy for primary immunodeficiencies

PURPOSE OF REVIEW

Standard therapies for patients with severe primary immunodeficiencies include bone marrow transplantation and, for adenosine deaminase deficiency, enzyme replacement. In the last decade, gene therapy has been developed as an alternative for these conditions. We summarize the recent advances in gene therapy for primary immunodeficiencies and discuss the unexpected occurrence of leukemia in a gene therapy trial for X-linked severe combined immunodeficiency.

RECENT FINDINGS

Eight of 10 infants with X-linked severe combined immunodeficiency who received autologous hematopoietic stem cells transduced with a retroviral vector carrying the IL2RG complementary DNA achieved immune reconstitution. However, the two youngest patients developed leukemic expansions of gene-corrected cells. The first case had proliferation of a gamma delta T cell clone, and the second case had three alpha beta T cell clones derived from a single transduced progenitor. Leukemic cells in both patients aberrantly expressed the LIM domain only-2 transcription factor due to retroviral vector insertions in this locus. After receiving anti-leukemic treatment one patient achieved a lasting remission, but the other relapsed. Four adenosine deaminase deficient severe combined immunodeficiency patients also developed functional immunity after receiving autologous hematopoietic stem cells transduced with the adenosine deaminase gene complementary DNA following submyeloablative chemotherapy. Chronic granulomatous disease, Wiskott-Aldrich syndrome, JAK3 deficiency and RAG2 deficiency are other immunodeficiencies being studied as candidates for gene therapy.

SUMMARY

Gene therapy is a promising therapeutic option for some primary immunodeficiencies, especially when cells expressing the correct gene have a selective advantage. More clinical trials with closer patient monitoring are under way to define which patients may benefit from this approach, and strategies are being developed to understand and ultimately reduce the risk of leukemia secondary to retroviral vector insertion.

Gene therapy for HIV infection: what does it need to make it work?

The efficacy of antiviral drug therapy for HIV infection is limited by toxicity and viral resistance. Thus, alternative therapies need to be explored. Several gene therapeutic strategies for HIV infection have been developed, but in clinical testing therapeutically effective levels of the transgene product were not achieved. This review focuses on the determinants of therapeutic efficacy and discusses the potential and also the limits of current gene therapy approaches for HIV infection.

Gene therapy in primary immunodeficiencies

Primary immunodeficiencies are a group of disorders that are highly amenable to gene therapy due to their defined molecular biology and pathophysiology. 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 demonstrated that conventional gene transfer technology can produce major beneficial therapeutic effects, but as for all cellular and pharmacologic treatment approaches, with a finite potential for toxicity. New strategies to overcome these issues are likely to establish gene therapy as an efficacious strategy for many forms of primary immunodeficiencies.

In vivo gene transfer into rat bone marrow progenitor cells using rSV40 viral vectors

Hematopoietic stem cell (HSC) gene transfer has been attempted almost entirely ex vivo and has been limited by cytokine-induced loss of self-renewal capacity and transplantation-related defects in homing and engraftment. Here, we attempted to circumvent such limitations by injecting vectors directly into the bone marrow (BM) to transduce HSCs in their native environment. Simian virus 40 (SV40)-derived gene delivery vectors were used because they transduce resting CD34+ cells very efficiently. Rats received SV-(Nef-FLAG), carrying FLAG marker epitope--or a control recombinant SV40 (rSV40)--directly into both femoral marrow cavities. Intracellular transgene expression by peripheral blood (PB) or BM cells was detected by cytofluorimetry. An average of 5.3% PB leukocytes expressed FLAG for the entire study--56 weeks. Transgene expression was sustained in multiple cell lineages, including granulocytes (average, 3.3% of leukocytes, 20.4% of granulocytes), CD3+ T lymphocytes (average, 0.53% of leukocytes, 1% of total T cells), and CD45R+ B lymphocytes, indicating gene transfer to long-lived progenitor cells with multilineage capacity. An average of 15% of femoral marrow cells expressed FLAG up to 16.5 months after transduction. Thus, direct intramarrow administration of rSV40s yields efficient gene transfer to rat BM progenitor cells and may be worthy of further investigation.

In vivo transduction of hematopoietic stem cells after neonatal intravenous injection of an amphotropic retroviral vector in mice

Hematopoietic stem cells (HSC) are important targets for gene therapy. Most protocols involve ex vivo modification, in which HSC are transduced in vitro and injected into the recipient. An in vivo delivery method might simplify HSC gene therapy. We previously demonstrated that iv injection of an amphotropic retroviral vector (RV) into newborn mice resulted in long-term expression from hepatocytes. The goal of this study was to determine if HSC were also transduced. After neonatal administration of 1 x 10(10) transducing units/kg of RV, peripheral blood cells had approximately 0.1 copy of RV per cell for up to 22 months. At 18 months, RV sequences were detected in T, B, and myeloid cells from bone marrow (BM). Unfractionated BM was transplanted into naive recipients after total body irradiation. Recipients maintained similar levels of the RV in their blood cells for 10 months, at which time RV sequences were present at the same integration site in all lineages of cells from BM. We conclude that neonatal iv injection of RV results in transduction of HSC in mice, which might be used for BM-directed gene therapy. Transduction of blood cells after liver-directed neonatal gene therapy might have adverse effects in patients, although no leukemias developed here.

Expression profiling of murine acute promyelocytic leukemia cells reveals multiple model-dependent progression signatures

Leukemia results from the expansion of self-renewing hematopoietic cells that are thought to contain mutations that contribute to disease initiation and progression. Studies of the gene expression profiles of human acute myeloid leukemia samples has allowed their classification based on the presence of translocations and French-American-British subtypes, but it is not yet clear whether their molecular signatures reflect the initiating mutations or mutations acquired during progression. To begin to address this question, we examined the expression profiles of normal murine promyelocyte-enriched samples, nontransformed murine promyelocytes expressing human promyelocytic leukemia-retinoic acid receptor alpha (PML-RARalpha) fusion gene, and primary acute promyelocytic leukemia cells. The expression profile of nontransformed cells expressing PML-RARalpha was remarkably similar to that of wild-type promyelocytes. In contrast, the expression profiles of fully transformed cells from three acute promyelocytic leukemia model systems were all different, suggesting that the expression signature of acute promyelocytic leukemia cells reflects the genetic changes that contributed to progression. To further evaluate these progression events, we compared two high-penetrance acute promyelocytic leukemia models that both commonly acquire an interstitial deletion of chromosome 2 during progression. The two models exhibited distinct gene expression profiles, suggesting that the dominant molecular signatures of murine acute promyelocytic leukemia can be influenced by several independent progression events.