Sustained multilineage gene persistence and expression in dogs transplanted with CD34(+) marrow cells transduced by RD114-pseudotype oncoretrovirus vectors

Generation of a neutralizing antibody against RD114-pseudotyped viral vectors

The feline endogenous RD114 glycoprotein has proved to be an attractive envelope to pseudotype both retroviral and lentiviral vectors. As a surface protein, its detection on packaging cells as well as viral particles would be useful in different fields of its use. To address this, we generated a monoclonal antibody against RD114 by immunization of rats, termed 22F10. Once seroconversion was confirmed, purified 22F10 was cloned into murine Fc and characterized with a binding affinity of 10nM. The antibody was used to detect RD114 and its variant envelopes on different stable viral packaging cell lines (FLYRD18 and WinPac-RD). 22F10 was also shown to prevent the infections of different strains of RD-pseudotyped vectors but not related envelope glycoproteins by blocking cell surface receptor binding. We are the first to report the neutralization of viral particles by a monoclonal αRD114 antibody.

Large animal models for foamy virus vector gene therapy

Foamy virus (FV) vectors have shown great promise for hematopoietic stem cell (HSC) gene therapy. Their ability to efficiently deliver transgenes to multi-lineage long-term repopulating cells in large animal models suggests they will be effective for several human hematopoietic diseases. Here, we review FV vector studies in large animal models, including the use of FV vectors with the mutant O6-methylguanine-DNA methyltransferase, MGMTP140K to increase the number of genetically modified cells after transplantation. In these studies, FV vectors have mediated efficient gene transfer to polyclonal repopulating cells using short ex vivo transduction protocols designed to minimize the negative effects of ex vivo culture on stem cell engraftment. In this regard, FV vectors appear superior to gammaretroviral vectors, which require longer ex vivo culture to effect efficient transduction. FV vectors have also compared favorably with lentiviral vectors when directly compared in the dog model. FV vectors have corrected leukocyte adhesion deficiency and pyruvate kinase deficiency in the dog large animal model. FV vectors also appear safer than gammaretroviral vectors based on a reduced frequency of integrants near promoters and also near proto-oncogenes in canine repopulating cells. Together, these studies suggest that FV vectors should be highly effective for several human hematopoietic diseases, including those that will require relatively high percentages of gene-modified cells to achieve clinical benefit.

In vivo selection of autologous MGMT gene-modified cells following reduced-intensity conditioning with BCNU and temozolomide in the dog model

Chemotherapy with 1,3-bis (2-chloroethyl)-1-nitrosourea (BCNU) and temozolomide (TMZ) is commonly used for the treatment of glioblastoma multiforme (GBM) and other cancers. In preparation for a clinical gene therapy study in patients with glioblastoma, we wished to study whether these reagents could be used as a reduced-intensity conditioning regimen for autologous transplantation of gene-modified cells. We used an MGMT(P140K)-expressing lentivirus vector to modify dog CD34(+) cells and tested in four dogs whether these autologous cells engraft and provide chemoprotection after transplantation. Treatment with O(6)-benzylguanine (O6BG)/TMZ after transplantation resulted in gene marking levels up to 75%, without significant hematopoietic cytopenia, which is consistent with hematopoietic chemoprotection. Retrovirus integration analysis showed that multiple clones contribute to hematopoiesis. These studies demonstrate the ability to achieve stable engraftment of MGMT(P140K)-modified autologous hematopoietic stem cells (HSCs) after a novel reduced-intensity conditioning protocol using a combination of BCNU and TMZ. Furthermore, we show that MGMT(P140K)-HSC engraftment provides chemoprotection during TMZ dose escalation. Clinically, chemoconditioning with BCNU and TMZ should facilitate engraftment of MGMT(P140K)-modified cells while providing antitumor activity for patients with poor prognosis glioblastoma or alkylating agent-sensitive tumors, thereby supporting dose-intensified chemotherapy regimens.

Combination of HOXB4 and Delta-1 ligand improves expansion of cord blood cells

Umbilical cord blood (UCB) is an attractive cell source for hematopoietic cell transplantation (HCT). Here we examine whether the combination of homeobox B4 (HOXB4) and Delta-1 ligand (DL) synergize when used together. Monkey and human UCB CD34(+) cells were transduced with a HOXB4-expressing gammaretroviral vector and cultured with DL. Individual and combined effects of HOXB4 and DL were assessed by colony-forming unit assays, flow cytometry, and nonobese diabetic/severe combined immune deficienct mouse transplantation. The presence of DL yielded higher percentage of CD34(+) and CD7(+) cells and lower percentages of CD14(+) cells than non-DL cultures. Furthermore, HOXB4 yielded higher percentages of CD34(+) and CD14(+) cells than non-HOXB4 cultures. Interestingly, coculture with DL-expressing OP9 cells resulted in better maintenance of HOXB4 than culture in DL-conditioned medium. Culture of HOXB4-transduced human cells in the presence of DL yielded enhanced generation of repopulating cells with higher levels of engraftment of human CD45(+), CD34(+), CD3(+), CD20(+), and CD41(+) cells compared with either factor individually. Our results demonstrate enhanced generation of hematopoietic progenitors by combining HOXB4 and DL; addition of DL further enhances expansion of multipotent cells capable of repopulating lymphoid and megakaryocyte lineages, which is not observed with HOXB4 alone.

Efficient and stable MGMT-mediated selection of long-term repopulating stem cells in nonhuman primates

HSC transplantation using genetically modified autologous cells is a promising therapeutic strategy for various genetic diseases, cancer, and HIV. However, for many of these conditions, the current efficiency of gene transfer to HSCs is not sufficient for clinical use. The ability to increase the percentage of gene-modified cells following transplantation is critical to overcoming this obstacle. In vivo selection with mutant methylguanine methyltransferase (MGMTP140K) has been proposed to overcome low gene transfer efficiency to HSCs. Previous studies have shown efficient in vivo selection in mice and dogs but only transient selection in primates. Here, we report efficient and stable MGMTP140K-mediated multilineage selection in both macaque and baboon nonhuman primate models. Treatment consisting of both O6-benzylguanine (O6BG) and N,N'-bis(2-chloroethyl)-N-nitroso-urea (BCNU) stably increased the percentage of transgene-expressing cells from a range of initial levels of engrafted genetically modified cells, with the longest follow-up after drug treatment occurring over 2.2 years. Drug treatment was well tolerated, and selection occurred in myeloid, lymphoid, and erythroid cells as well as platelets. Retrovirus integration site analysis before and after drug treatments confirmed the presence of multiple clones. These nonhuman primate studies closely model a clinical setting and should have broad applications for HSC gene therapy targeting human diseases of malignant, genetic, and infectious nature, including HIV.

Large animal models of hematopoietic stem cell gene therapy

Large animal models have been instrumental in advancing hematopoietic stem cell (HSC) gene therapy. Here we review the advantages of large animal models, their contributions to the field of HSC gene therapy and recent progress in this field. Several properties of human HSCs including their purification, their cell-cycle characteristics, their response to cytokines and the proliferative demands placed on them after transplantation are more similar in large animal models than in mice. Progress in the development and use of retroviral vectors and ex vivo transduction protocols over the last decade has led to efficient gene transfer in both dogs and nonhuman primates. Importantly, the approaches developed in these models have translated well to the clinic. Large animals continue to be useful to evaluate the efficacy and safety of gene therapy, and dogs with hematopoietic diseases have now been cured by HSC gene therapy. Nonhuman primates allow evaluation of aspects of transplantation as well as disease-specific approaches such as AIDS (acquired immunodeficiency syndrome) gene therapy that can not be modeled well in the dog. Finally, large animal models have been used to evaluate the genotoxicity of viral vectors by comparing integration sites in hematopoietic repopulating cells and monitoring clonality after transplantation.

Foamy and lentiviral vectors transduce canine long-term repopulating cells at similar efficiency

Foamy viral vectors and lentiviral vectors are attractive gene transfer vectors for hematopoietic stem cell gene therapy because they both efficiently transduce stem cells using rapid ex vivo transduction protocols designed to maintain engraftment potential. Here we directly compared the ability of foamy and lentiviral vectors to transduce long-term hematopoietic repopulating cells in the dog model, using a competitive repopulation assay with vectors that express enhanced yellow or green fluorescent proteins (EY/GFP). Mobilized canine peripheral blood CD34(+) cells were divided into two fractions and exposed to either foamy (EGFP) or lentiviral (EYFP) vectors at a multiplicity of infection of 5 in an 18-hr transduction protocol and then reinfused after conditioning with 920 cGy of total body irradiation. Both dogs studied had rapid neutrophil engraftment and multilineage engraftment of transduced cells. Marking was similar for both vectors, particularly at later time points, indicating that both vector types transduce long-term repopulating cells at similar frequencies.

Cocal-pseudotyped lentiviral vectors resist inactivation by human serum and efficiently transduce primate hematopoietic repopulating cells

Lentiviral vectors are established as efficient and convenient vehicles for gene transfer. They are almost always pseudotyped with the envelope glycoprotein of vesicular stomatitis virus (VSV-G) due to the high titers that can be achieved, their stability, and broad tropism. We generated a novel cocal vesiculovirus envelope glycoprotein plasmid and compared the properties of lentiviral vectors pseudotyped with cocal, VSV-G, and a modified feline endogenous retrovirus envelope glycoprotein (RD114/TR). Cocal-pseudotyped lentiviral vectors can be produced at titers as high as with VSV-G, have a broad tropism, and are stable, allowing for efficient concentration by centrifugation. Additionally, cocal vectors are more resistant to inactivation by human serum than VSV-G-pseudotyped vectors, and efficiently transduce human CD34(+) nonobese diabetic/severe combined immunodeficient (NOD/SCID) mouse-repopulating cells (SRCs), and long-term primate hematopoietic repopulating cells. These studies establish the potential of cocal-pseudotyped lentiviral vectors for a variety of scientific and therapeutic gene transfer applications, including in vivo gene delivery and hematopoietic stem cell (HSC) gene therapy.

Detection of replication competent retrovirus and lentivirus

Retroviral vectors based on murine leukemia viruses (MuLV) have been used in clinical investigations for over a decade. Alternative retroviruses, most notably vectors based on HIV-1 and other lentiviruses, are now entering into clinical trials. Although vectors are designed to be replication defective, recombination events during vector production could lead to the generation of replication competent retroviruses (RCR) or replication competent lentiviruses (RCL). Careful screening of vector prior to human use must insure that patients are not inadvertently exposed to RCR or RCL. We describe methods capable of detecting low levels of virus contamination and discuss the current regulatory guidelines for screening gene therapy products intended for human use.

Investigation of various methods for the cryopreservation of canine bone marrow-derived CD34(+) cells

Optimal condition for the cryopreservation of canine CD34(+) cells was explored. Canine bone marrow CD34(+) cells were isolated from 5 healthy dogs by a magnetic- activated cell-sorting system using a monoclonal antibody specific to canine CD34. These cells were cryopreserved by 4 different methods: 2 different cryoprotectant solutions-solutions A (fetal bovine serum containing 10% dimethylsulfoxide (DMSO) and B (physiological saline containing 5% DMSO, 6% hydroxyethyl starch, and 4% bovine serum albumin)-were used in combination with 2 different freezing procedures-in a rate-controlled programmed freezer (PF) and in an ordinary freezing container. The cell viability, cell recovery rates, and colony-forming unit (CFU) recovery rates were examined following cryopreservation for 1 week, 4 weeks, and 6 months. The values of these parameters were significantly higher for the CD34(+) cells that had been frozen in Solution B than for those that had been frozen in Solution A, regardless of the freezing procedure employed. The highest CFU recovery rate following cryopreservation for 6 months corresponded to the cells that had been cryopreserved with Solution B and frozen in a PF. In conclusion, cryopreservation with Solution B in a PF proved to be the most efficient of the 4 cryopreservation procedures investigated in terms of maintaining the quality of canine bone marrow-derived CD34(+) cells. This method will be useful for clinical applications involving the use of canine bone marrow-derived CD34(+) cells.

Preserve mesenchymal stem cell research

Hematopoietic stem cell (HSC) graft cell dose impacts significantly on allogeneic transplant. Similarly, HSC gene therapy outcome is affected by loss of repopulating cells during culture required for ex vivo retrovirus transduction. Stromal cell-derived factor-1 (SDF-1) and its receptor CXCR4 play a central role in marrow trafficking of HSCs, and maneuvers that enhance CXCR4 activation might positively impact outcome in settings of limiting graft dose. CD26/dipeptidyl peptidase IV (DPP-IV) is an ectoenzyme protease that cleaves SDF-1, thus reducing CXCR4 activation. We show that injection of irradiated nonobese diabetic/severe combined immunodeficiency (NOD/SCID) mice with >or=2 micromol Diprotin A (a tripeptide specific inhibitor of CD26 protease activity) at the time of transplant of human granulocyte colony-stimulating factor (G-CSF) mobilized CD34(+) peripheral blood cells (CD34(+) PBCs) results in a >3.4-fold enhancement of engraftment of human cells. We also show that CD26 on residual stromal cells in the irradiated recipient marrow milieu, and not any CD26 activity in the human CD34(+) PBC graft itself, plays the critical role in regulating receptivity of this environment for the incoming graft. Human marrow stromal cells also express CD26, raising the possibility that Diprotin A treatment could significantly enhance engraftment of HSCs in humans in settings of limiting graft dose just as we observed in the NOD/SCID mouse human xenograft model.

Unique integration profiles in a canine model of long-term repopulating cells transduced with gammaretrovirus, lentivirus, or foamy virus

Recent advances have allowed for improved retrovirus-mediated gene transfer, and therapeutic benefits have been described in patients. These successes have shown the potential of hematopoietic stem cell (HSC) gene therapy, but treatment-related leukemia and benign expansion of gene-modified clones have shifted the attention toward safety. The delayed onset of adverse events in gene therapy clinical trials emphasizes the importance of long-term integration site studies in large animal models. We have addressed safety by characterizing the genomic location of 555 integration sites of the three most commonly used integrating retroviral vectors, that is, gammaretrovirus, lentivirus, and foamy virus, in long-term repopulating cells from dogs. Gammaretroviral integrants showed the most significant frequency of occurrence very close (<2.5 kb) to transcription start sites, but a substantial portion of all three retroviral integrants were within 50 kb. Importantly, gammaretroviral integrants were found more frequently in and near proto-oncogenes, suggesting this retroviral system may be the most prone to adverse gene activation. These data suggest that gammaretroviral vectors may have the highest intrinsic risk, but also emphasize that no vector system can be defined as "safe" based solely on integration profile.

Improved short-term engraftment of lentivirally versus gammaretrovirally transduced allogeneic canine repopulating cells

Gammaretroviral vectors require cell division for efficient transduction. Thus, extended cell culture times are necessary for efficient transduction with gammaretroviral vectors, which in turn can lead to stem cell loss and impaired engraftment. Lentiviral vectors transduce nondividing cells and are therefore able to transduce stem cells in short transduction protocols. Here, we compared the short-term engraftment of lentivirally and gammaretrovirally transduced canine allogeneic DLA-matched littermate cells. A reduced conditioning regimen of 400 cGy total body irradiation was used in preparation for clinical studies. Two dogs received a graft of gammaretrovirally transduced CD34-selected cells. CD34(+) cells were prestimulated for 30 h and then exposed twice to concentrated RD114 pseudotype vector. Three dogs received lentivirally transduced CD34-selected cells. Cells were transduced overnight with concentrated VSV-G pseudotype lentiviral vector. The animals in the lentiviral group showed a significantly faster granulocyte recovery. VNTR analysis 40-50 days after transplantation revealed higher donor chimerism for the lentiviral group compared to the retroviral group. These data suggest that short lentiviral transduction protocols may be superior to extended gammaretroviral transduction protocols with respect to engraftment potential of transduced CD34(+) hematopoietic repopulating cells.

Marking of peripheral T-lymphocytes by retroviral transduction and transplantation of CD34+ cells in a canine X-linked severe combined immunodeficiency model

A retrovirus vector containing an enhanced green fluorescent protein complimentary DNA (EGFP cDNA) was used to mark and dynamically follow vector-expressing cells in the peripheral blood of bone marrow transplanted X-linked severe combined immunodeficient dogs. CD34(+) cells isolated from young normal dogs were transduced, using a 2 day protocol, with an amphotropic retroviral vector that expressed enhanced green fluorescent protein (EGFP) and the canine common gamma chain (gammac) cDNAs. Following transplantation of the transduced cells, normal donor peripheral blood lymphocytes (PBL) appeared by 1 month post-bone marrow transplant (BMT) and rescued three of five treated dogs from their lethal immunodeficiency. PCR and flow cytometric analysis of post-BMT PBL documented the peripheral EGFP expressing cells as CD3(+) T cells, which varied from 0% to 28%. Sorting of EGFP(+) and EGFP(-) peripheral blood T cells from two dogs, followed by vector PCR analysis, showed no evidence of vector shutdown. EGFP expression in B cells or monocytes was not detected. These marking experiments demonstrate that the transduction protocol did not abolish the lymphoid engraftment capability of ex vivo transduced canine CD34(+) cells and supports the potential utility of the MSCV retroviral vector for gene transfer to XSCID affected canine hematopoietic progenitor cells (HPC).

Induction of chemoresistance in a cultured canine cell line by retroviral transduction of the canine multidrug resistance 1 gene

OBJECTIVE

To induce chemoresistance in a normal canine cell line through the transduction of the canine multidrug resistance 1 gene (mdr1).

SAMPLE POPULATION

Madin-Darby canine kidney (MDCK) epithelial cell line.

PROCEDURES

The full-length canine mdr1 cDNA clone isolated in our laboratory was inserted into a Moloney murine leukemia virus-based vector to construct the retroviral vector, pLNC-cMDR1. After retroviral transduction of pLNC-cMDR1 into MDCK cells, the expression and function of the P-glycoprotein, a product of mdr1, were assessed by immunoblotting, measurement of rhodamine123 (Rh123) retention, and drug sensitivity assays.

RESULTS

P-glycoprotein was strongly expressed in cells transduced with pLNC-cMDR1. This P-glycoprotein was fully functional, as demonstrated by the decreased Rh123 retention and the increased resistance to chemotherapeutic drugs. Measured as 50% inhibitory concentrations, resistance increased 59 times to vincristine and 25 times to doxorubicin in MDCK cells after transduction of pLNC-cMDR1.

CONCLUSIONS AND CLINICAL RELEVANCE

Transduction of canine mdr1 is an effective method for inducing chemoresistance in normal canine cells. This system may be applicable to the induction of drug resistance in hematopoietic cells.

Optimized Transduction of Canine Paediatric CD34+ Cells Using an MSCV-based Bicistronic Vector

We have used a murine MSCV-based bicistronic retroviral vector, containing the common gamma chain (gammac) and enhanced green fluorescent protein (EGFP) cDNAs, to optimize retroviral transduction of canine cells, including an adherent canine thymus fibroblast cell line, Cf2Th, as well as normal canine CD34(+) bone marrow (BM) cells. Both canine cell types were shown to express Ram-1 (the amphotropic retroviral receptor) mRNA. Supernatants containing infectious viruses were produced using both stable (PA317) and transient (Phoenix cells) amphotropic virus producer cell lines. Centrifugation (spinfection) combined with the addition of polybrene produced the highest transduction efficiencies, infecting approximately 75% of Cf2Th cells. An average of 11% of highly enriched canine CD34(+) cells could be transduced in a protocol that utilized spinfection and plates coated with the fibronectin fragment CH-296 (Retronectin). Indirect assays showed the vector-encoded canine gammac cDNA produced a gammac protein that was expressed on the cell surface of transduced cells. This strategy may result in the transduction of sufficient numbers of CD34(+) BM cells to make the treatment of canine X-linked severe combined immunodeficiency and other canine genetic diseases feasible.

Foamy-virus-mediated gene transfer to canine repopulating cells

Foamy virus (FV) vectors are particularly attractive gene-transfer vectors for stem-cell gene therapy because they form a stable transduction intermediate in quiescent cells and can efficiently transduce hematopoietic stem cells. Here, we studied the use of FV vectors to transduce long-term hematopoietic repopulating cells in the dog, a clinically relevant large animal model. Mobilized canine peripheral blood (PB) CD34+ cells were transduced with an enhanced green fluorescent protein (EGFP)-expressing FV vector in an 18-hour transduction protocol. All 3 dogs studied had rapid neutrophil engraftment to greater than 500/microL with a median of 10 days. Transgene expression was detected in all cell lineages (B cells, T cells, granulocytes, red blood cells, and platelets), indicating multilineage engraftment of transduced cells. Up to 19% of blood cells were EGFP+, and this was confirmed at the DNA level by real-time polymerase chain reaction (PCR) and Southern blot analysis. These transduction rates were higher than the best results we obtained previously with lentiviral vectors in a similar transduction protocol. Integration site analysis also demonstrated polyclonal repopulation and the transduction of multipotential hematopoietic repopulating cells. These data suggest that FV vectors should be useful for stem-cell gene therapy, particularly for applications in which short transduction protocols are critical.

WHIM syndrome myelokathexis reproduced in the NOD/SCID mouse xenotransplant model engrafted with healthy human stem cells transduced with C-terminus-truncated CXCR4

WHIM(warts, hypogammaglobulinemia, recurrent bacterial infection, and myelokathexis) syndrome is a rare immunodeficiency caused in many cases by autosomal dominant C-terminal truncation mutations in the chemokine receptor CXCR4. A prominent and unexplained feature of WHIM is myelokathexis (hypercellularity with apoptosis of mature myeloid cells in bone marrow and neutropenia). We transduced healthy human CD34(+) peripheral blood-mobilized stem cells (PBSCs) with retrovirus vector encoding wild-type (wt) CXCR4 or WHIM-type mutated CXCR4 and studied these cells ex vivo in culture and after engraftment in a nonobese diabetic/severe combined immunodeficiency (NOD/SCID) mouse xenograft model. Neither wt CXCR4 nor mutated CXCR4 transgene expression itself enhanced apoptosis of neutrophils arising in transduced PBSC cultures even with stimulation by a CXCR4 agonist, stromal cell-derived factor-1 (SDF-1 [CXCL12]). Excess wt CXCR4 expression by transduced human PBSCs enhanced marrow engraftment, but did not affect bone marrow (BM) apoptosis or the release of transduced leukocytes into PB. However, mutated CXCR4 transgene expression further enhanced BM engraftment, but was associated with a significant increase in apoptosis of transduced cells in BM and reduced release of transduced leukocytes into PB. We conclude that increased apoptosis of mature myeloid cells in WHIM is secondary to a failure of marrow release and progression to normal myeloid cell senescence, and not a direct effect of activation of mutated CXCR4.

Correction of the disease phenotype in canine leukocyte adhesion deficiency using ex vivo hematopoietic stem cell gene therapy

Canine leukocyte adhesion deficiency (CLAD) represents the canine counter-part of the human disease leukocyte adhesion deficiency (LAD). Defects in the leukocyte integrin CD18 adhesion molecule in both CLAD and LAD lead to recurrent, life-threatening bacterial infections. We evaluated ex vivo retroviral-mediated gene therapy in CLAD using 2 nonmyeloablative conditioning regimens--200 cGy total body irradiation (TBI) or 10 mg/kg busulfan--with or without posttransplantation immunosuppression. In 6 of 11 treated CLAD dogs, therapeutic levels of CD18(+) leukocytes were achieved. Conditioning with either TBI or busulfan allowed long-term engraftment, and immunosuppression was not required for efficacy. The percentage of CD18(+) leukocytes in the peripheral blood progressively increased over 6 to 8 months after infusion to levels ranging from 1.26% to 8.37% at 1-year follow-up in the 6 dogs. These levels resulted in reversal or moderation of the severe CLAD phenotype. Linear amplification-mediated polymerase chain reaction assays indicated polyclonality of insertion sites. These results describe ex vivo hematopoietic stem cell gene transfer in a disease-specific, large animal model using 2 clinically applicable conditioning regimens, and they provide support for the use of nonmyeloablative conditioning regimens in preclinical protocols of retroviral-mediated gene transfer for nonmalignant hematopoietic diseases such as LAD.

Hematopoietic stem cell transduction and amplification in large animal models

Progress in retroviral gene transfer to large animal hematopoietic stem cells (HSCs) has led to efficient, reproducible long-term marking in both canine and nonhuman primate models. Successes for HSC gene therapy have occurred in the severe combined immunodeficiency setting, in which transduced cells have a selective advantage. However, for most diseases, the therapeutic transgene does not confer a sufficient survival advantage, and increasing the percentage of gene-marked cells in vivo will be necessary to observe a therapeutic effect. In vivo amplification should expand the potential of HSC gene therapy, and progress in this area has benefited greatly from the use of large animal models where efficacy and toxicity have often not correlated with results in murine models. To date, the best results have been observed with O(6)-methylguanine-DNA methyltransferase (MGMT) selection, with which increases in gene-marked repopulating cells have been maintained long-term, likely because of the toxicity of 1,3-bis-(2-chloroethyl)-1-nitrosourea and temozolomide to quiescent HSCs. Using MGMT selection, long-term marking levels exceeding 50% can now be routinely attained with minimal toxicity. There is cause to be optimistic that HSC gene therapy with in vivo amplification will soon allow the treatment of several genetic and infectious diseases.

Retroviral vector production in the National Gene Vector Laboratory at Indiana University

The National Gene Vector Laboratory (NGVL) is a US National Institutes of Health initiative charged with providing clinical grade vectors for gene therapy trials. The program was started in 1995 and Indiana University has served as the production site for retroviral vectors and is also accepting applications for production of lentiviral vectors. The facility is designed to produce vectors for Phase I and Phase II clinical trials with the specific mandate to facilitate investigator-initiated research for academic institutions. To date, the facility has generated over 30 Master Cell Banks for gene therapy investigators throughout the United States. This required the facility to develop a system that can adapt to the varied needs of investigators, most of whom request different vector backbones, packaging cell lines, final product volumes, and media. In this review, we will illustrate some of the experiences of the Indiana University NGVL during the generation of retroviral vectors using murine-based packaging cell lines.

Enhanced function with decreased internalization of carboxy-terminus truncated CXCR4 responsible for WHIM syndrome

OBJECTIVE

WHIM (warts, hypogammaglobulinemia, recurrent bacterial infection, myelokathexis) syndrome is an autosomal dominant immune deficiency with severe chronic neutropenia and marrow neutrophil apoptosis. Carboxy-termini truncating mutations in the chemokine receptor CXCR4 have been identified in WHIM patients. We created a retrovirus encoding mutated CXCR4 (truncating point mutation 1000C-->T [R334X] inherited heterozygously in several WHIM patients) in order to transducer healthy human CD34 stem cells and K562 to overexpress mutated CXCR4 and determined its effect on receptor responses to stromal-derived factor-1 (SDF1).

METHODS

Retrovirus vector was engineered to coexpress WHIM-associated R334X mutated CXCR4 together with green fluorescent protein (GFP). Control vectors included similar constructs with wild-type CXCR4 (WT-CXCR4) or only GFP. CD34+ cells and K562 were transduced with these vectors. Populations of 100% transduced K562 were established by sorting GFP+ cells by flow cytometry. We performed migration and calcium flux assays of transduced CD34+ cells and transduced/sorted K562. We also examined receptor recycling in response to SDF1.

RESULTS

Healthy human CD34+ cells and/or human erythroleukemia K562 cells transduced to express mutated CXCR4, WT-CXCR4, or GFP alone demonstrated that mutated CXCR4 was associated with enhanced calcium flux and enhanced migration. There was also decreased receptor internalization and enhanced recovery of surface mutated CXCR4 in response to SDF1 compared with WT-CXCR4.

CONCLUSION

We propose that decreased internalization of WHIM-associated mutated CXCR4 leads to prolongation/enhancement of signaling in response to SDF1 and that this may provide the biochemical basis for the autosomal dominant abnormalities of cell trafficking and function associated with WHIM syndrome.

Improved transduction of human sheep repopulating cells by retrovirus vectors pseudotyped with feline leukemia virus type C or RD114 envelopes

Gene therapy for hematopoietic diseases has been hampered by the low frequency of transduction of human hematopoietic stem cells (HSCs) with retroviral vectors pseudotyped with amphotropic envelopes. We hypothesized that transduction could be increased by the use of retroviral vectors pseudotyped with envelopes that recognize more abundant cellular receptors. The levels of mRNA encoding the receptors of the feline retroviruses, RD114 and feline leukemia virus type C (FeLV-C), were significantly higher than the level of gibbon ape leukemia virus (GaLV) receptor mRNA in cells enriched for human HSCs (Lin- CD34+ CD38-). We cotransduced human peripheral blood CD34+ cells with equivalent numbers of FeLV-C and GALV or RD114 and GALV-pseudotyped retroviruses for injection into fetal sheep. Analysis of DNA from peripheral blood and bone marrow from recipient sheep demonstrated that FeLV-C- or RD114-pseudotyped vectors were present at significantly higher levels than GALV-pseudotyped vectors. Analysis of individual myeloid colonies demonstrated that retrovirus vectors with FeLV-C and RD114 pseudotypes were present at 1.5 to 1.6 copies per cell and were preferentially integrated near known genes We conclude that the more efficient transduction of human HSCs with either FeLV-C- or RD114-pseudotyped retroviral particles may improve gene transfer in human clinical trials.

In Vivo Retroviral Mediated Gene Transfer into Bladder Urothelium Results in Preferential Transduction of Tumoral Cells

OBJECTIVES

Superficial bladder tumours are at high risk for recurrence, relapse after resection, escape to intravesical immunotherapy and they may become invasive. New therapeutics are therefore needed to achieve cure. Thus, gene therapy is an attractive new treatment modality for malignant bladder tumours. The purpose of this study was to evaluate the feasibility and the efficiency of retroviral mediated reporter gene transfer into malignant urothelial cells both in vitro and in vivo.

METHODS

We evaluated the feasibility of the transfection of bladder tumour with direct intravesical instillation of a defective retrovirus. The vector was derived from LXSN. The efficiency of transduction with the Moloney Leukaemia Murine virus-based vector, amphotrophic retroviral vector, was monitored through the expression of two marker genes (nls-LacZ and NeoR). The canine animal was chosen since it can present with spontaneous bladder carcinomas mimicking human pathology. Primary cultures of two normal canine bladder urothelium and two canine primary bladder tumours were first studied. We then investigated in vivo, in two normal and two spontaneous tumour bearing dogs, the transduction of urothelial cells following direct intravesical instillation of 2.10(4) to 3.10(6) of the retroviral vector.

RESULTS

Transduced cells were evidenced in all primary cultures of canine normal urothelium and transitional cell carcinoma. Bladder biopsies from sound dogs instilled with the viral solution showed long lasting transduction up to 60 days long. Bladder cryosections from tumour-bearing dogs displayed transduction of superficial layers of urothelial cancer cells without passing through lamina propria. In vivo transduction was evidenced in 1 to 15% (mean 5%) of the cells in the tumours and preferentially addressed malignant cells. Normal epithelium either originating from sound or tumour-bearing animals was not transduced.

CONCLUSION

These results demonstrate for the first time the feasibility of in vivo retroviral transduction of bladder cancer using a clinically relevant procedure.

Strategies to expand transduced hematopoietic stem cells in vivo

Data in mice suggest that in vivo selection strategies will expand the numbers of transduced hematopoietic stem cells (HSC) to levels sufficient for clinical therapies, and it is argued that comparable strategies will benefit larger animals and humans. To test this assumption, we performed virtual gene therapy in mouse and cat, species in which the in vivo kinetics of HSC are defined. In the simulated experiments, 10% of HSC and 50% of short-term repopulating cells were transduced with a gene allowing a conditional replication or apoptosis advantage. After transplantation, differentiation proceeded stochastically and contributions of transduced cells were tracked for 2 years. Fifty independent transplantations were simulated per species for each analysis. When transduced HSC had a 2-fold increased chance of replication (self-renewal) extending for 4, 10, or 20 weeks after transplantation, or a 5-fold replication advantage extending for 4 weeks, results in mice were far better than in cat, a larger animal, with slower baseline HSC cell cycle kinetics. Similarly, when transduced HSC had a 2-, 4-, or 10-fold decreased chance of apoptosis, extending for 20 or more weeks after transplantation, the murine studies were poor predictors of feline results. Simulation may allow one to optimize and/or understand the limitations of a gene therapy strategy.

Polyclonal chemoprotection against temozolomide in a large-animal model of drug resistance gene therapy

Incorporation of drug resistance genes into gene vectors has 2 important roles in stem cell gene therapy: increasing the proportion of gene-corrected cells in vivo (ie, in vivo selection) and marrow protection to permit higher or more tightly spaced doses of chemotherapy in the treatment of malignant diseases. We studied in a clinically relevant canine model of gene therapy the P140K mutant of the drug resistance gene methylguanine methyltransferase (MGMT), which encodes a DNA-repair enzyme that confers resistance to the combination of the MGMT inhibitor O(6)-benzylguanine (O(6)BG) and nitrosourea drugs such as carmustine and methylating agents such as temozolomide. Two dogs received MGMT(P140K)-transduced autologous CD34(+)-selected cells. After stable engraftment, gene marking in granulocytes was between 3% and 16% in the 2 animals, respectively. Repeated administration of O(6)BG and temozolomide resulted in a multilineage increase in gene-modified repopulating cells with marking levels of greater than 98% in granulocytes. MGMT(P140K) overexpression prevented the substantial myelosuppression normally associated with this drug combination. Importantly, hematopoiesis remained polyclonal throughout the course of the study. Extrahematopoietic toxicity was minimal, and no signs of myelodysplasia or leukemia were detected. These large-animal data support the evaluation of MGMT(P140K) in conjunction with O(6)BG and temozolomide in clinical trials.

Genetic manipulation of hematopoietic stem cells

Over the past two decades, the ability to transfer genes into hematopoietic stem cells (HSCs) has provided new insights into the behavior of individual stem cells and offered a novel approach for the treatment of various inherited or acquired disorders. At present, gene transfer into HSCs has been achieved mainly using modified retroviruses. While retrovirus-based vectors could efficiently transduce murine HSCs, extrapolation of these methods to large mammals and human clinical trials resulted in very low numbers of gene-marked engrafted cells. In addition, in vitro progenitor assays used to optimize gene transfer procedures were found to poorly predict the outcome of stem cell gene transfer. The focus rapidly turned to the development of superior and more relevant preclinical assays in human stem cell gene transfer research. Xenogeneic transplant models and large animal transplantation system have been invaluable. The development of better assays for evaluating human gene therapy protocols and a better understanding of stem cell and vector biology has culminated over the past decade in multiple strategies to improve gene transfer efficiency into HSCs. Improved gene transfer vectors, optimization of cytokine combination, and incorporation of a recombinant fragment of fibronectin during transduction are examples of novel successful additions to the early gene transfer protocols that have contributed to the first unequivocal clinical benefits resulting from genetic manipulation of HSC.

Direct comparison of steady-state marrow, primed marrow, and mobilized peripheral blood for transduction of hematopoietic stem cells in dogs

The optimal stem cell source for stem cell gene therapy has not been defined. Most gene transfer studies have used peripheral blood or marrow repopulating cells collected after administration of granulocyte colony-stimulating factor and stem cell factor (G-CSF/SCF). For clinical applications, however, growth factor administration may not be feasible. Thus, in the current study we used a competitive repopulation assay in the dog to directly compare transduction efficiency of steady-state marrow, G-CSF/SCF-primed marrow, and G-CSF/SCF-mobilized peripheral blood. Cells from all three sources were transduced, cryopreserved, and thawed together before infusion into myeloablated dogs. Gene marking in hematopoietic repopulating cells was assessed by polymerase chain reaction. While primed marrow resulted in the highest long-term marking levels, steady-state marrow was transduced at least as efficiently as mobilized peripheral blood in all three dogs. These results suggest that steady-state marrow may be an appropriate source for genetic modification of hematopoietic stem cells.

Canine long-term bone marrow culture neutrophil production and functionality

This in vitro study has been conducted to determine the optimal experimental conditions under which to produce canine neutrophils in long-term bone marrow cultures (LTBMC), establish functional parameters of neutrophils obtained from LTBMC and peripheral blood and to ascertain whether these cells display physiological similarities. Our aim is to provide an experimental model, enabling a correlation between hemopoietic injury and neutrophil functionality. The authors demonstrate for the first time that canine neutrophils grown in cultures are able to produce oxyradicals capable of killing bacterial products. Moreover, culture-grown neutrophils contain gelatinase granules, a marker of terminal neutrophil differentiation, and express a specific surface antigen. The results described in this article illustrate the development of a dynamic system that mimics physiological hemopoiesis.

Efficient lentiviral gene transfer to canine repopulating cells using an overnight transduction protocol

The use of lentiviral vectors for the transduction of hematopoietic stem cells has evoked much interest owing to their ability to stably integrate into the genome of nondividing cells. However, published large animal studies have reported highly variable gene transfer rates of typically less than 1%. Here we report the use of lentiviral vectors for the transduction of canine CD34+ hematopoietic repopulating cells using a very short, 18-hour transduction protocol. We compared lentiviral transduction of hematopoietic repopulating cells from either stem cell factor (SCF)– and granulocyte-colony stimulating factor (G-CSF)–primed marrow or mobilized peripheral blood in a competitive repopulation assay in 3 dogs. All dogs engrafted rapidly within 9 days. Transgene expression was detected in all lineages (B cells, T cells, granulocytes, and red blood cells as well as platelets) indicating multilineage engraftment of transduced cells, with overall long-term marking levels of up to 12%. Gene transfer levels in mobilized peripheral blood cells were slightly higher than in primed marrow cells. In conclusion, we show efficient lentiviral transduction of canine repopulating cells using an overnight transduction protocol. These results have important implications for the design of stem cell gene therapy protocols, especially for those diseases in which the maintenance of stem cells in culture is a major limitation.

Long-Term Clinical and Molecular Follow-up of Large Animals Receiving Retrovirally Transduced Stem and Progenitor Cells: No Progression to Clonal Hematopoiesis or Leukemia

There has been significant progress toward clinically relevant levels of retroviral gene transfer into hematopoietic stem cells (HSC), and the therapeutic potential of HSC-based gene transfer has been convincingly demonstrated in children with severe combined immunodeficiency syndrome (SCID). However, the subsequent development of leukemia in two children with X-linked SCID who were apparently cured after transplantation of retrovirally corrected CD34+ cells has raised concerns regarding the safety of gene therapy approaches utilizing integrating vectors. Nonhuman primates and dogs represent the best available models for gene transfer safety and efficacy and are particularly valuable for evaluation of long-term effects. We have followed 42 rhesus macaques, 23 baboons, and 17 dogs with significant levels of gene transfer for a median of 3.5 years (range 1-7) after infusion of CD34+ cells transduced with retroviral vectors expressing marker or drug-resistance genes. None developed abnormal hematopoiesis or leukemia. Integration site analysis confirmed stable, polyclonal retrovirally marked hematopoiesis, without progression toward mono- or oligoclonality over time. These results suggest that retroviral integrations using replication-incompetent vectors, at copy numbers achieved using standard protocols, are unlikely to result in leukemogenesis and that patient- or transgene-specific factors most likely contributed to the occurrence of leukemia in the X-SCID gene therapy trial.

Efficient gene transfer into rhesus repopulating hematopoietic stem cells using a simian immunodeficiency virus-based lentiviral vector system

High-titer, HIV-1-based lentiviral vector particles were found to transduce cytokine-mobilized rhesus macaque CD34(+) cells and clonogenic progenitors very poorly (< 1%), reflecting the postentry restriction in rhesus cells to HIV infection. To overcome this barrier, we developed a simian immunodeficiency virus (SIV)-based vector system. A single exposure to a low concentration of amphotropic pseudotyped SIV vector particles encoding the green fluorescent protein (GFP) resulted in gene transfer into 68% +/- 1% of rhesus bulk CD34(+) cells and 75% +/- 1% of clonogenic progenitors. Polymerase chain reaction (PCR) analysis of DNA from individual hematopoietic colonies confirmed these relative transduction efficiencies. To evaluate SIV vector-mediated stem cell gene transfer in vivo, 3 rhesus macaques underwent transplantation with transduced, autologous cytokine-mobilized peripheral blood CD34(+) cells following myeloablative conditioning. Hematopoietic reconstitution was rapid, and an average of 18% +/- 8% and 15% +/- 7% GFP-positive granulocytes and monocytes, respectively, were observed 4 to 6 months after transplantation, consistent with the average vector copy number of 0.19 +/- 0.05 in peripheral blood leukocytes as determined by real-time PCR. Vector insertion site analysis demonstrated polyclonal reconstitution with vector-containing cells. SIV vectors appear promising for evaluating gene therapy approaches in nonhuman primate models.

Methylguanine methyltransferase-mediated in vivo selection and chemoprotection of allogeneic stem cells in a large-animal model

Clinical application of gene therapy for genetic and malignant diseases has been limited by inefficient stem cell gene transfer. Here we studied in a clinically relevant canine model whether genetic chemoprotection mediated by a mutant of the DNA-repair enzyme methylguanine methyltransferase could circumvent this limitation. We hypothesized that genetic chemoprotection might also be used to enhance allogeneic stem cell transplantation, and thus we evaluated methylguanine methyltransferase-mediated chemoprotection in an allogeneic setting. We demonstrate that gene-modified allogeneic canine CD34+ cells can engraft even after low-dose total body irradiation conditioning. We also show that cytotoxic drug treatment produced a significant and sustained multilineage increase in gene-modified repopulating cells. Marking in granulocytes rose to levels of up to 98%, the highest in vivo marking reported to date to our knowledge in any large-animal or human study. Increases in transgene-expressing cells after in vivo selection provided protection from chemotherapy-induced myelosuppression, and proviral integration site analysis demonstrated the protection of multiple repopulating clones. Drug treatment also resulted in an increase in donor chimerism. These data demonstrate that durable, therapeutically relevant in vivo selection and chemoprotection of gene-modified cells can be achieved in a large-animal model and suggest that chemoprotection can also be used to enhance allogeneic stem cell transplantation.

Methylguanine methyltransferase-mediated in vivo selection and chemoprotection of allogeneic stem cells in a large-animal model

Clinical application of gene therapy for genetic and malignant diseases has been limited by inefficient stem cell gene transfer. Here we studied in a clinically relevant canine model whether genetic chemoprotection mediated by a mutant of the DNA-repair enzyme methylguanine methyltransferase could circumvent this limitation. We hypothesized that genetic chemoprotection might also be used to enhance allogeneic stem cell transplantation, and thus we evaluated methylguanine methyltransferase-mediated chemoprotection in an allogeneic setting. We demonstrate that gene-modified allogeneic canine CD34+ cells can engraft even after low-dose total body irradiation conditioning. We also show that cytotoxic drug treatment produced a significant and sustained multilineage increase in gene-modified repopulating cells. Marking in granulocytes rose to levels of up to 98%, the highest in vivo marking reported to date to our knowledge in any large-animal or human study. Increases in transgene-expressing cells after in vivo selection provided protection from chemotherapy-induced myelosuppression, and proviral integration site analysis demonstrated the protection of multiple repopulating clones. Drug treatment also resulted in an increase in donor chimerism. These data demonstrate that durable, therapeutically relevant in vivo selection and chemoprotection of gene-modified cells can be achieved in a large-animal model and suggest that chemoprotection can also be used to enhance allogeneic stem cell transplantation.

Prospects and RISC score of viral gene therapy for sarcoma

Soft tissue sarcomas are a challenge for medical oncology and gene therapy. Protective and sensitising approaches that target normal and malignant tissue, respectively, both have their role for opening the therapeutic window. Recent data show that an intensive maintenance chemotherapy significantly reduces metastatic spread and improves disease-free survival in selected patient groups. However, delays of treatment due to cytopenia are frequent. Cytostatic drug resistance gene transfer to haematopoietic progenitor cells using retroviral vectors may allow further improvement of therapy results. In recent years, retroviral vector design, transduction techniques and engraftment capability of transduced cells have been optimised. Safety considerations of retroviral gene transfer have attracted public attention and can be addressed by analysis of genomic vector integration sites. A data bank project, 'retroviral insertion estimate of chromosomal integration' (RISC), containing > 200 integration sequences, has been set up by the authors' group to recognise critical genomic regions and genes involved with possible transforming capacity. Monitoring these parameters will allow the selection of the most suitable vectors for clinical application. Sarcoma cells seem to be highly susceptible to a variety of vectors, such as recombinant adeno-associated virus-2 (rAAV-2) vectors, adenoviral vectors or oncolytic herpes simplex viruses. Results from the first clinical trials with adenoviral vectors encoding for cytokines are promising. The other systems await further development towards clinical applications. Perspectives for further research are discussed in this review.

Gene therapy for platelet disorders: studies with Glanzmann's thrombasthenia

Current research aimed at correcting platelet defects are designed to further our knowledge in the use of hematopoietic stem cells for gene therapies of hemorrhagic disorders. Information gained from these studies may be directly applicable to treatment of disorders affecting platelets (e.g. Glanzmann's thrombasthenia, Bernard Soulier syndrome, gray platelet syndrome, and von Willebrand disease) as well as other disorders affecting distinct hematopoietic cell lineages. This work specifically addresses three questions: (i) can bone marrow stem cells be given sufficient genetic information to induce abnormal megakaryocytes to synthesize transgene products that help newly formed platelets to participate in normal hemostasis? (ii) can the newly synthesized receptor be maintained as a platelet-specific protein at therapeutic levels for a reasonable period of time? and (iii) will newly expressed proteins be tolerated by the immune system or become a target for B- and T-cell mediated immunity resulting in the premature destruction and clearing of the genetically altered megakaryocytes and platelets? Answers to these questions should indicate the feasibility of targeting platelets with genetic therapies that will in turn enable better management of patients with inherited bleeding disorders. The long-range benefit of this research will be an improved understanding of the regulation of protein expression during normal megakaryocytopoiesis, and the accumulation of additional scientific knowledge about normal platelet function and the way in which platelets and other cells recognize and interact with each other.

Direct comparison of RD114-pseudotyped versus amphotropic-pseudotyped retroviral vectors for transduction of rhesus macaque long-term repopulating cells

Recently, RD114 (feline endogenous retrovirus envelope protein)-pseudotyped retroviral particles have been shown to transduce human NOD/SCID repopulating cells efficiently. In this study, we compared directly transduction of repopulating cells with RD114-pseudotyped vector to that with standard amphotropic vector in the rhesus macaque model. G-CSF/SCF-mobilized CD34(+) rhesus peripheral blood cells were cultured in the presence of SCF, Flt-3 ligand, and MGDF on Retronectin-coated flasks. To assess directly the ability of the two pseudotypes to transduce primitive cells, both vectors were added simultaneously to the target cells every 24 h, for a total of four exposures in 96 h. The cells were reinfused after the animals received 1000 cGy total body irradiation. At the end of transduction, gene marking efficiency of CFU was higher with amphotropic LNL6 vector (mean 88.4%) vs RD114-G1Na vector (mean 18.5%). After long-term engraftment in three animals, total neo gene marking levels were 4-5% in PBMNCs and 1.5-4% in granulocytes. The RD114-G1Na marking levels were consistently higher in granulocytes than in mononuclear cells, while amphotropic LNL6 marking levels were higher in PBMNCs than in granulocytes. The differential gene marking patterns suggest that RD114 and amphotropic vectors may target distinct progenitor or stem cell populations. There was no clear advantage for RD114-pseudotyped vectors in this predictive preclinical model in terms of overall long-term marking levels; however, optimization of transduction conditions by increasing m.o.i. or inducing the receptor could potentially improve results with this novel vector system.

Induction of cytotoxic T-lymphocyte responses to enhanced green and yellow fluorescent proteins after myeloablative conditioning

Lentiviral vectors are increasingly being used for transferring genes into hematopoietic stem cells (HSCs) due to their ability to transduce nondividing cells. Whereas results in in vitro studies and the nonobese diabetic/severe combined immunodeficiency (NOD/SCID) model have been highly encouraging, studies in large animals have not confirmed the superior transduction of HSCs using lentiviral vectors versus oncoretroviral vectors. In contrast to the stable gene marking we have consistently achieved with oncoretroviral vectors in animals that received myeloablative conditioning, we observed the complete disappearance of genetically modified enhanced green or yellow fluorescent protein-expressing cells in 5 baboons that received transplants of HSCs transduced with lentiviral vectors alone or in combination with oncoretroviral vectors. Immune responses to transgene products have been found to be involved in the disappearance of gene-modified cells after nonmyeloablative conditioning. Thus, we examined whether the disappearance of gene-modified cells after ablative conditioning may be due to an immune response. In 4 of 5 animals, cytotoxic T lymphocytes specific for the transgene protein were readily detected, demonstrating that immune reactions were responsible for the disappearance of the gene-marked cells in the animals. In summary, we report the induction of transgene-specific immune responses after transplantation of lentivirally transduced repopulating cells in a myeloablative setting.

Gene therapy with viral vectors

A key factor in the success of gene therapy is the development of gene delivery systems that are capable of efficient gene transfer in a broad variety of tissues, without causing any pathogenic effect. Currently, viral vectors based on many different viruses have been developed, and their performance and pathogenicity has been evaluated in animal models. The results of these studies form the basis for the first clinical trials for correcting genetic disorders using retroviral, adenoviral, and adeno-associated viral vectors. Even though the results of these trials are encouraging, vector development is still required to improve and refine future treatment of hereditary disorders.

Novel therapeutic approaches in sickle cell disease

In this update, selected clinical features of sickle cell disease and their management are reviewed. In addition, the current status of interventions that have curative potential for sickle cell disease is discussed, with particular attention focused on indications, methodology, recent results, and challenges to wider clinical application. In Section I, Dr. Nienhuis describes recent improvements in vector technology, safety, and replacement gene expression that are creating the potential for clinical application of this technology. In Section II, Dr. Vichinsky reviews our current understanding of the pathophysiology and treatment of pulmonary injury in sickle cell disease. The acute and chronic pulmonary complications of sickle cell disease, modulators and predictors of severity, and conventional and novel treatment of these complications are discussed. In Section III, Dr. Walters reviews the current status of hematopoietic cell transplantation for sickle cell disease. Newer efforts to expand its availability by identifying alternate sources of stem cells and by reducing the toxicity of transplantation are discussed.