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Amphotropic helper-free retroviral vectors containing either the bacterial neomycin phosphotransferase gene (NEO) or a mutant dihydrofolate reductase gene (DHFR*) were used to infect canine hematopoietic progenitor cells. In previous experiments, successful transfer and expression of both genes in canine CFU-GM were achieved after 24-hour cocultivation with virus-producing cells. The average rate of gene expression was 10% (6% to 16%) as measured by the number of CFU-GM resistant to either the aminoglycoside G418 or methotrexate. In an attempt to increase the efficiency of gene transfer, marrow was cocultured for 24 hours with either NEO or DHFR* virus-producing packaging cells and then kept in long-term marrow culture fed three times with virus-containing supernatant (2 to 5 x 10(6) CFU/mL). After six days, cells were harvested and cultured in CFU-GM assay with and without a selective agent. The average rate of gene expression in CFU- GM in five independent experiments was 46% and ranged from 19% to 87%. In conclusion, the efficiency of gene transfer into canine hematopoietic progenitor cells has been increased fourfold by combining cocultivation with long-term marrow culture as compared with results obtained with cocultivation only.

Inherited diseases might be treated by introducing normal genes into a patient's somatic tissues to correct the genetic defects. In the case of hemophilia resulting from a missing clotting factor, the required gene could be introduced into any cell as long as active factor reached the circulation. We previously showed that retroviral vectors can efficiently transfer genes into normal skin fibroblasts and that the infected cells can produce high levels of a therapeutic product in vitro. In the current study, we examined the ability of skin fibroblasts to secrete active clotting factor after infection with different retroviral vectors encoding human clotting factor IX. Normal human fibroblasts infected with one vector secreted greater than 3 micrograms factor IX/10(6) cells/24 h. Of this protein, greater than 70% was structurally and functionally indistinguishable from human factor IX derived from normal plasma. This suggests that infected autologous fibroblasts might provide therapeutic levels of factor IX if transplanted into patients suffering from hemophilia B. By transplanting normal diploid fibroblasts infected with the factor IX vectors, we showed that human factor IX can be produced and is circulated at readily detectable levels in rats and mice.

Foamy viruses are retroviruses of the spumavirus family that are often isolated from primary cultures of primate cells. We previously constructed vectors based on human foamy virus (HFV) and found that they were able to transduce a wide variety of vertebrate cells by integration of the vector genome. Here we show that several types of hematopoietic cells are efficiently transduced by an HFV vector that encodes alkaline phosphatase (AP). These cell types include transformed cell lines and primary hematopoietic progenitors from mice, baboons, and humans. The transduction rates of HFV vectors compare favorably with those obtained by murine leukemia virus vectors, which suggests that HFV vectors may be effective in the treatment of hematologic diseases by gene therapy.

Amphotropic helper-free retrovirus vectors containing the bacterial neomycin phosphotransferase gene (neo) and the human adenosine deaminase gene (adenosine aminohydrolase, EC 3.5.4.4; ADA) were used to transduce canine marrow cells. In one approach, dogs were treated for 7 days with recombinant human granulocyte colony-stimulating factor to stimulate hematopoietic cell division. Bone marrow cells were collected and transduced by 24 hours of cocultivation on vector-producing cells followed by incubation in a vector-containing long-term marrow culture system for 4 days. Transduced autologous marrow (0.4 to 1.0 x 10(8) cells/kg) was infused into dogs administered otherwise lethal total body irradiation (TBI) of 920 cGy. Two of four dogs engrafted, and their marrows showed intermittently between 1% and 11% G418-resistant colony-forming unit granulocyte-macrophage (CFU-GM) colonies for up to 2 years after transplantation. In a different experimental approach, autologous marrow, obtained at the time of the PB neutrophil nadir 7 days after a single cyclophosphamide injection (40 mg/kg intravenously), was cocultivated for 24 hours on vector-producing cells and infused at doses of 0.06 to 0.18 x 10(8) cells/kg into dogs administered 920 cGy TBI. One of three dogs engrafted, and the marrow showed intermittently 1% to 10% G418-resistant CFU-GM colonies for at least 2 years. Culture results were confirmed by polymerase chain reaction (PCR) showing the presence of the neo gene in marrow cells, peripheral blood (PB) granulocytes, and PB and lymph node lymphocytes. Dilution experiments indicated that up to 10% of marrow, lymph node, and PB cells contained the neo gene, consistent with the culture results. Samples harboring the neo gene also contained the gene for human ADA. However, repeated analyses of PB and marrow cells for human ADA gene expression by starch gel electrophoresis were negative. PB samples of all dogs were free of helper virus, and no long-term side effects from the transduction were observed.

A high titer retroviral vector was used to transfer a human adenosine deaminase (h-ADA) cDNA into murine bone marrow cells in vitro. The h- ADA cDNA was linked to the retroviral promoter, and the vector also contained a neomycin phosphotransferase gene as a selectable marker. Infected marrow was transplanted into syngeneic W/Wv recipients, and h- ADA expression was monitored for 5.5 months. Several weeks after transplantation, h-ADA was detected in the erythrocytes of all nine recipients, eight of which expressed levels equal to the endogenous enzyme. This level of expression persisted in two of six surviving mice, while expression in three others stabilized at lower, but readily detectable, levels. Only one mouse had no detectable h-ADA after 5.5 months. Vector DNA sequences with common integration sites were found in hematopoietic and lymphoid tissues of the mice at 5.5 months, providing evidence that hematopoietic stem cells had been infected. Furthermore, all mice transplanted with marrow that had been selected in G418 before infusion had multiple vector copies per genome. While this category included the two highest h-ADA expressors, it also included the negative mouse. Thus, multiple copies of the vector were not sufficient to guarantee long-term h-ADA expression. Mice were monitored for “helper virus” infections with an assay designed to detect a wide range of replication-competent retroviruses, including those endogenous to the mouse genome. No helper virus was detected in the two highest h-ADA expressors, ruling out helper-assisted vector spread as a cause of the high h-ADA expression. These results help provide a foundation for the development of somatic gene therapy techniques to be used in the treatment of human disease.

Adenosine deaminase (ADA) deficiency is associated with a fatal severe combined immunodeficiency. Because most patients do not have a suitable marrow donor, the introduction of a normal ADA gene into the patient's marrow cells is a potentially useful alternative therapy. To identify vectors that provide optimal gene expression in human hematopoietic cells, we investigated retroviral vectors containing the ADA gene under the transcriptional control of the promoter/enhancers of Moloney murine leukemia virus, the simian virus 40 early region, the cytomegalovirus immediate-early gene, the lymphotropic papovavirus, and the human beta- globin gene. ADA expression from these vectors was monitored in the ADA- human histiocytic lymphoma cell line DHL-9, and in the multipotential chronic myeloid leukemia cell line K562. ADA expression in infected K562 cells was also measured after induction of megakaryoblastic differentiation by phorbol ester, and after induction of erythroid differentiation by sodium n-butyrate or hemin. In these hematopoietic cell lines, the vectors that contained ADA controlled by either the Moloney murine leukemia virus promoter (LASN) or the cytomegalovirus promoter (LNCA) expressed ADA at much higher levels than the other vectors tested. Furthermore, in K562 cells infected with LASN and LNCA vectors, induction of terminal differentiation resulted in the same or higher level expression of ADA. These cell lines have permitted the evaluation of transduced gene expression in proliferating and differentiating hematopoietic cells that provide a model for bone marrow-targeted gene therapy.

Successful retroviral gene transfer into murine hematopoietic stem cells indicates the potential for somatic gene therapy in the treatment of certain human hereditary diseases. We developed a canine model to test the applicability of these techniques to a preclinical model of human marrow transplantation. Previously we reported that canine CFU-GM could be infected with retroviral vectors carrying either the gene for a mutant dihydrofolate reductase (DHFR) or neomycin phosphotransferase (NEO). This study reports six lethally irradiated dogs transplanted with autologous marrow cocultivated with retroviral vector-producing cells. This procedure conferred drug resistance to 3% to 13% of the CFU- GM. Three dogs infected with either the NEO or DHFR virus engrafted, but we detected no drug-resistant CFU-GM. Three dogs were given marrow infected with a DHFR virus and received methotrexate (MTX) as in vivo selection; all three had evidence of engraftment. In the surviving dog, we detected 0.03% to 0.1% MTX-resistant CFU-GM at 3 to 5 weeks posttransplant during in vivo selection. These results indicate that we can reconstitute lethally irradiated dogs with autologous marrow exposed to retroviral vectors and suggest that gene transfer into hematopoietic cells is feasible on a large scale. However, the low- level transient gene expression indicates that considerable obstacles remain before human gene therapy can be considered.

Leukocyte adherence deficiency (LAD) is an inherited immunodeficiency disease caused by defects in the CD18 leukocyte integrin subunit. Transduction of CD18 into hematopoietic cells from children with LAD represents a potential therapy for this disorder. In an attempt to maximize transfer and expression of CD18, we evaluated retroviral vectors with and without the neomycin selectable marker, with a modified tRNA primer binding site designed to prevent inhibition of gene expression, and with two different viral envelope proteins produced by using the amphotropic retrovirus packaging cell line PA317 or the gibbon ape leukemia virus packaging cell line PG13. The vectors were tested using transducing K562/CD11b cells and LAD Epstein-Barr virus (EBV) B cells and measuring levels of cell-surface CD11/CD18 expression by fluorescence-activated cell sorter analysis. The best results were obtained with vectors made using PG13 packaging cells, for which about 25% of the K562 cells exposed once to the vectors expressed surface CD11b/CD18 and about 25% of the LAD EBV B cells exposed three times over a 3-day period to the vectors expressed surface CD11a/CD18. In contrast, transduction of cells under similar conditions with retroviral vectors produced using PA317 producer cells yielded less than 2% of the K562 cells and less than 4% of the LAD EBV B cells expressing the CD11/CD18 heterodimer on the cell surface. The presence or absence of the neomycin resistance gene or the modified tRNA primer had no effect on CD18 gene transfer rate or expression level. The increase in transduction with PG13 vectors correlated with Northern blotting and reverse transcription-polymerase chain reaction studies that indicated that both K562 cells and the LAD EBV B cells express transcripts for the gibbon ape leukemia virus receptor at higher levels than for the amphotropic virus receptor. These findings indicate that the transduction efficiency of retroviral packaging cell lines correlates with receptor gene expression in the target cells and that vectors made using PG13 cells may be efficacious for gene therapy for LAD and other diseases in which gene transfer to hematopoietic cells is required.

Genetically marked peripheral blood progenitor cells were used to investigate their contribution to long-term hematopoietic reconstitution after autologous marrow and peripheral blood cell transplantation. After autologous marrow harvest and cryopreservation, canine peripheral blood progenitor cells were mobilized in three dogs by treatment with recombinant canine stem cell factor for 8 days. Peripheral blood mononuclear cells were collected and enriched for major histocompatibility complex (MHC) class II antigen-positive cells by avidin-biotin immunoadsorption, thereby enriching for repopulating cells. Subsequently, the cells were cocultivated for 24 hours on irradiated vector-producing packaging cells (PA317/LN), followed by an 11-day incubation in a vector containing long-term marrow culture system. On the day of transplantation, the animals were irradiated with 9.2 Gy total body irradiation (TBI), and transduced peripheral blood cells and untransduced cryopreserved marrow cells were infused within 2 hours of TBI. All three dogs engrafted. Two dogs are long-term survivors showing intermittently G418-resistant marrow-derived colony- forming unit granulocyte-macrophage colonies at a median of 1% and 2%, respectively (range, 1% to 10%), for now up to 48 weeks after transplantation. Neo-specific sequences were detected by polymerase chain reaction in peripheral blood granulocytes for now up to 65 weeks and in peripheral blood lymphocytes for up to 75 weeks after transplantation. Peripheral blood samples of the dogs were free of helper virus and no side effects from the transduction were observed. One of the three dogs died from chronic canine distemper sclerosing encephalitis on day 84, whereas the other two dogs are alive at 15 and 17 months. Our data show successful retroviral transduction of canine peripheral blood repopulating cells. Long-term persistence of marked myeloid and lymphoid cells after transplantation suggests that peripheral blood contains repopulating cells that contribute to long- term hematopoietic reconstitution after otherwise lethal TBI.