Patent Abstract:
materials and methods for transferring nucleic acid encoding a polypeptide for treating a disease or disorder into populations of quiescent cells such as haematopoietic stem cells , using retroviral packaging cell lines and retroviral particles expressing and displaying a growth factor such as stem cell factor on the cell surface or as a fusion with a viral envelope protein . the present invention also relates compositions comprising the retroviral packaging cell lines and viral particles , and their use in methods of medical treatment , in vivo and ex vivo .

Detailed Description:
bone marrow cells were harvested and stem cells selected using the sfu technique transduced with retroviral vectors and incubated in long term bone marrow culture all as described in the original application . tf - 1 cells were maintained in rpmi medium supplemented with 10 % fetal calf serum , 2 mm glutamine , penicillin and streptomycin . recombinant human gm - csf was added to 200 pg / ml for routine passage and recombinant human scf at 25 ng / ml was used for short term growth support . packaging and producer cell lines were cultured under the conditions described below and irradiated ( 500 rads ) prior to use in culture experiments . staining of cells for proliferating cell nuclear antigen ( pcna ) was performed on cells spun onto microscope slides using a cytospin ( shandon instruments ) and fixed in methanol , using a mouse anti - human monoclonal antibody ( dako ) directly conjugated to fitc . cells were incubated for 1 hour at room temperature before the antibody solution was washed away with pbs , the slides air - dried , cover - slipped using citifluor and the fluorescence viewed under uv light using a zeiss microscope . a retroviral producer line established in the packaging line am12 and containing the retroviral vector genome nlslacz ( 12 ) was used as a target to introduce the plasmid pjp2 by calcium phosphate transfection as described in the original submission . cells were selected for the presence of the scf - encoding plasmid using the linked histinidol resistance marker , as described previously . cells were washed twice in pbs pelleted and resuspended in media without growth factor and incubated overnight . cytokines and 3 h thymidine ( 10 μci ) were added to the media and the cells incubated for a further 24 hours . the cells were then harvested - using a titertek cell harvester and the incorporated radioactivity counted using a scintillation counter . for assessing retroviral gene transfer , tf - 1 cells were deposited onto microscope slides using a cytospin ( shandon instruments ) and fixed in pbs buffer containing 0 . 5 % glutaraldehyde . β - galactosidase activity was detected by staining the cells in situ with 0 . 1 % x - gal ( 5chloro - 4bromo - 3indolyl - β - d - galactopyranoside .) in pbs buffer containing , 0 . 01 % sodium deoxycholate , 0 . 02 % np40 , 2 mm mgcl 2 , 5 mm potassium ferricyanide and 5 mm potassium ferrocyanide for 1 - 2 hours at 37 ° c . for 3 h thymidine labelling tf - 1 cells were transduced with retroviral vector as described above in rpmi media to which 10 μci of 3 h thymidine was added . after overnight labelling , the cells were deposited onto microscope slides and stained for β - galactosidase activity before being washed dried and dipped in photographic emulsion . the emulsion was allowed to air dry overnight and the slides were exposed at − 70 ° c . in a light tight box for 3 days . to visualise the 3 h thymidine incorporation the emulsion was developed and fixed using standard x - ray film developer and fixer and cells producing silver grains were assessed by light and dark field light microscopy . retroviral transduction of quiescent target cells with retroviral producer cells expressing surface growth factor ( scf ); the establishment of a model system . to demonstrate the feasibility of using producer cells that express surface growth factor to transduce quiescent cells with retroviral vectors , we have developed a model system using the growth factor - dependent cell line tf - 1 ( 12 ). this cell line was developed from an erythroleukaemic patient and probably equates to cell arrested at an early stage of megakaryocyte development . the cells can only be grown in the presence of growth factors , the most usual being il - 3 or gm - csf to which they respond very sensitively . they also are capable of dividing in response to scf though this response is significantly weaker . these cells can easily be rendered quiescent by withdrawal of growth factor support . we used these cells as targets for retroviral transduction , following induction of quiescence . in order to facilitate this analysis , we constructed a retroviral producer line expressing surface scf ( see fig4 ). this was done by transfecting the same plasmid we had constructed before containing the hscf cdna ( plasmid pjp2 , fig3 ), into retroviral producer cells transducing a retroviral vector encoding the gene for β - galactosidase ( nlslacz ). this bacterial enzyme can be used in conjunction with synthetic substrates to produce a blue staining reaction when active enzyme is present . accordingly , in these experiments , successfully transduced cells will stain blue following retrovirally mediated gene transfer . the resulting producer line was identified as lacjp . immunofluorescent staining of the lacjp cell line with anti - scf antibody showed the presence of the surface bound scf . we compared the proliferative response of tf - 1 cells to soluble scf and the membrane bound growth factor . to start this analysis we titrated soluble scf to against tf - 1 cells to establish the optimum concentration of scf ( fig5 ). tf - 1 cells were then grown in the presence of soluble recombinant human scf or in the presence of the lac jp cell line or the parent packaging cell line ( am12 ). these initial experiments quickly revealed , however , that the retroviral packaging cell line produced a growth factor that was a potent mitogen for the tf - 1 target cells ( fig6 ). it was previously described in the literature that packaging cells lines were sources of the cytokine il - 6 . we hypothesised that mitogen secreted by the retroviral packaging cells might be il - 6 . to test this we attempted to block the response to il - 6 in our cultures by the addition of neutralising antibody to the cytokine . titration of the neutralising antibody ( fig7 ) demonstrated that it was possible to completely eradicate the il - 6 mediated proliferative response of tf - 1 cells when co - cultured with retroviral producer cells . all the subsequent experiments were therefore performed in the presence of inhibitory amounts of anti - il - 6 . we then tested the ability of our modified producer cells to facilitate transduction of quiescent target cells . tf - 1 cells in exponential growth were removed from growth factor and incubated overnight to allow them to become quiescent . tf - 1 cells were then co - cultured overnight with the retroviral producer cells expressing surface scf or the parent producer line . following co - culture , the tf - 1 cells were removed collected onto microscope slides and stained for β - galactosidase production . cells that were co - cultured on the parent producer line showed no evidence for retroviral transduction . in contrast , approximately 3 % of cells that were co - cultured with the scf producer cells were found to be positively staining . these were confirmed as cycling tf - 1 cells by doubly - labelling the cells with titrated thymidine ( 3 h ). this radioactive nucleoside becomes incorporated into the dna of dividing cells and can be detected by autoradiographic deposition of silver grains in a photographic emulsion into which the slides have been dipped . as expected , many of the transduced tf - 1 cells also showed the presence of silver grains indicating that cell division had taken place . thus , these experiments show that expression of a surface bound growth factor by retroviral producer cell lines is able to facilitate the retroviral transduction of a quiescent target cell population and therefore enable the retroviral - mediated transfer of genes to cells that would normally be refractory to this technique . retroviral transduction using populations of cells isolated from umbilical cord blood . we carried out a transduction experiment similar to those described above using haematopoietic progenitor cells obtained from human umbilical cord blood . progenitors were selected from cord blood using the macs ™ ( miltenyi biotech ) system to isolate cd34 + progenitor cells ( 15 ). the cd34 + population in cord blood is made up extensively of quiescent cells ( 13 , 16 ). these cells were transduced in vitro as described above by co - culture for 48 hours in the presence of the nlslacz producers or the lacjp producers . the cord blood cells were then harvested and pelleted onto microscope slides and stained for β - galactosidase activity as described above . cells that had been exposed to the nlslacz producers had a low proportion of blue - staining cells (& lt ; 10 %), whereas those that had been exposed to the lacjp producers that expresses scf on its surface had a very high proportion of blue - stained cells (& gt ; 80 %). in addition , blue - staining colonies formed by subsequent growth of these populations of cells in semisolid media were only found in cultures derived from cells that had been co - cultured with the lacjp cell line . retroviral transduction of haematopoietic stem cells using a retroviral packaging cell line expressing surface bound scf . the cell line 1mi - δscf was constructed as follows : the parent producer cell line 1mi was derived from the am12 packaging cell line ( 1 ), by calcium phosphate - mediated dna transfection , using the retroviral vector encoding the p47 - phox cdna we described previously ( 2 ), with the exception that the neomycin resistance cassette was removed . the retroviral backbone is derived from the pbabe series of vectors described by morgenstern et al ( 3 ). high titre producer clones were then selected by “ dot blot ” analysis of successful transfectants . the 1mi producer line was then transfected as described above using the plasmid pjp2 ( fig3 ) encoding the membrane - associated form of the human stem cell factor ( scf ). cells expressing scf were selected using histidinol . individual clones were grown out and tested for expression of scf by immunofluorescence with a labelled anti - scf antibody . the plasmid pjp2 was constructed by insertion of an 816 bp hindiii to bamh1 , scf cdna fragment into the mammalian cell expression plasmid prep8 ( invitrogen corp ). the scf cdna was excised from the plasmid bssk : humgfδ28 , see wo92 / 00376 . bone marrow cells ( 10 mls ; approx 5 × 10 7 cells ) were aspirated from the iliac crest of normal volunteers under local anaesthesia . the cells were washed twice with sterile pbs , re - pelleted and layered onto the surface of a discontinuous ficoll gradient . cells were separated by centrifugation for 20 mins at 2500 rpm . mononuclear cells were removed from the interphase and washed with pbs . cells were then incubated in iscove &# 39 ; s dmem medium supplemented with 10 % fetal calf serum , 5 - fluorouracil ( 5 - fu ), stem cell factor ( scf ) and il - 3 , as described by beradi et al ( science 267 1995 ). following seven - days in selection ( see fig1 a ), the surviving cells were co - cultured for 48 hrs in the presence of the scf - producer line . following co - cultivation , they were removed from the producers and used to establish long term cultures ( ltc ) on heterologous irradiated human stroma , in mccoy &# 39 ; s medium modified for long term culture . after 4 weeks in ltc , cells were plated in semi - solid media containing cytokines ( stemgem ™), to allow colonies to develop . transduction was scored by pcr analysis of colonies ( fig1 b ). the pcr relied on a nested strategy using two upstream and one downstream primers . an initial round of 35 cycles of amplification using the most upstream primer and the downstream primers was performed . a small aliquot of this reaction was removed and re - amplified in a second reaction using the second upstream primer and the downstream primer . the most upstream primer is complementary in sequence to a region from the gag gene of the retroviral vector and the other two primers are complementary to different regions from the p47 - phox cdna sequence . the size of the initial product is 454 nucleotides and the nested product 180 nucleotides . this strategy ensures that the pcr product is specific for the retrovirally encoded p47 - phox gene and not the endogenous gene . the products of the pcr amplification were visualised under ultra - violet light ( 300 nm ) following separation by standard agarose gel electrophoresis on 2 % gels containing ethidium bromide . the data set out in table 1 shows that in five separate experiments on marrow cell cultures indicated that approximately 13 % and 63 % of colonies were positive for the presence of the retroviral genome , showing that the retroviral vector had succeeded in delivering the p47 - phox gene to the phscs . in addition , the data presented as experiment 2 was re - evaluated using a different , more sensitive , method ( southern blotting ), demonstrating an 11 - fold increase in the rate of transduction of the sfu - selected phsc following exposure to the retroviral producers that express stem cell factor on their surface . as no positive colonies were obtained from the controls in two experiments ( experiments 2 and 6 ), the increased rates of transduction can only be expressed as greater than 5 or 4 times better than the control respectively . these values can therefore be considered as the absolute minimum in terms of the benefit of using the scf - expressing producers ; the actual value may be considerably greater . a nuclear antigen referred to as “ proliferating cell nuclear antigen ” ( pcna ) a component of the dna replication machinery can be used as a marker of cells in the process or capable of undergoing cell division . as such , staining cells with antibody to this antigen can give information on the cycling status of cells in a chosen population . staining of phsc selected by the sfu technique with a monoclonal antibody to pcna ( pc10 ), revealed that the majority of the cells (˜ 90 %, fig1 a and table 2 ) were unlabelled and were therefore quiescent , as expected . these cells were then cultured for 48 hours in the presence of the scf - expressing producer line or the control parent cell line and stained with the pc10 antibody . cells that had been exposed to the scf producers showed a marked increase in the fraction of positively staining cells (˜ 50 % and table 2 ). plasmids were created encoding chimeric envelopes in which stem cell factor ( scf ) or flt3 ligand ( fl ) is fused to the first codon of the su envelope glycoprotein as a factor x a cleavable or non - cleavable n - terminal extension of the 4070a ( amphotropic ) murine leukaemia virus ( mlv ). scf and fl cdna was pcr amplified and tailed with sfil and not 1 restriction sites . the pcr products were cloned into existing chimeric envelope expression plasmids ea1 and exa1 ( 10 ) after digestion with the restriction enzymes sfil and not 1 . fig8 shows a diagrammatic representation of the plasmid constructs . the sequences of all the constructs was confirmed by dideoxysequencing . the chimeric envelopes and control amphotropic ( 4070a ) envelopes were expressed in telceb . 6 complementing cells , which express mlv gag - pol core particles and a nlslacz retroviral vector . envelope expression plasmids were transfected by calcium phosphate precipitation into the telceb . 6 cells . transfected cells were selected with phleomycin and blasticidin in dmem supplemented with 10 % foetal calf serum ( fcs ) and grown to confluency . viral supernatants were harvested from stably transfected cells after overnight incubation in either serum free dmem or dmem containing 10 % fcs and filtered with a 0 . 45 μm filter for use in infection or binding experiments . for immunoblotting , viral supernatants were filtered ( 0 . 45 μm ) and then pelleted by ultracentrifugation at 30 , 000 rpm in a − sw40 rotor for one hour at 4 ° c . the pelleted viral particles were re - suspended in 100 μl of phosphate buffered saline and stored at − 20 ° c . the murine cell line nih 3t3 was grown in dmem supplemented with 10 % fcs . the human scf receptor ( kit ) expressing cell line hmc - 1 was grown in iscove &# 39 ; s modified dulbecco &# 39 ; s eagle medium ( imdm ) supplemented with 10 % fcs and monothioglycerol . the human kit negative cell line k422 was grown in rpmi supplemented with 10 % fcs . ten μl of the pelleted viral particles were separated on a 10 % polyacrylamide gel under reducing conditions and subsequently transferred to nitrocellulose . the viral su proteins were detected using a primary goat anti - envelope antibody . blots were developed using a secondary anti - goat antibody conjugated to horseradish peroxidase and an enhanced chemiluminescence kit . target cells were plated into six - well plates at approximately 10 5 cells per well and incubated overnight at 37 ° c . ( adherent cells ) or plated into six well plates at approximately 10 6 cells per well one hour before infection ( suspension cells ). filtered viral supernatant in serum free medium was added to the target cells and incubated for 4 hours in the presence of 8 μg / ml polybrene . the retroviral supernatant was then removed from the target cells , the medium was replaced with the usual medium and the cells were placed at 37 ° c . for a further 48 - 72 hours . x - gal staining for detection of p - galactosidase activity was carried out as previously described ( 11 ). viral titre was calculated by counting blue stained colonies microscopically and expressed as enzyme forming units per ml ( adherent cells ) or percentage blue stained cells the viral pellets of both scf and fl displaying viruses were analysed by immunoblotting and a representative blot is shown in fig9 . the presence of chimeric viral envelope with a distinct mobility from that of the wild type virus is demonstrated . fig1 shows that upon treatment of the viral pellets with 4 μg / ml fx a protease , the fx a cleavage signal in the interdomain linker of the expressed envelope scfxa1 is correctly recognised and cleaved to yield a su cleavage product with identical mobility to the unmodified 4070a envelope su . viral supernatant harvested from cells transfected with the various constructs was used to infect mouse fibroblasts . gene delivery to the 3t3 cells was demonstrated for the amphotropic chimeras and was unaffected by protease cleavage . thus viral infectivity mediated by the underlying envelope was not blocked by display of these dimeric ligands . the infectivity of viruses incorporating the scfa1 and scfxa1 chimeric envelopes was tested on hmc - 1 cells — a human kit expressing cell line . both vectors were capable of binding to kit positive cells but gave very low titres compared to the wild type vector . when soluble scf was added as competitor to prevent the vectors from binding to kit , the infectivity was increased , suggesting that the reduced ability of these vectors to bind to kit positive cells was a consequence of targeted binding to scf receptors ( fig1 ). these vectors were then treated with factor x a protease prior to infection . pre - incubation with fxa had no effect on the titre of the scfa1 vector but restored the titre of scfxa1 to that of the wild - type amphotropic vector ( fig1 ). this effect of fx a cleavage did not occur on the kit negative cell line k422 . this data provides evidence that retroviral vectors displaying scf are capable of selectively binding to human cells expressing kit but that infection of kit positive cells cannot take place until the engineered binding domain has been cleaved . in this respect , this ligand - receptor systems resembles the egf system developed in our laboratory ( 11 ) and is potentially amenable to the two - step targeting strategies developed for egf . the above results show that good levels of transduction of phsc can be achieved using engineered retroviral packaging cells expressing human scf on their cell surface . thus , the results above indicate that the cells should be capable of simultaneously delivering both a growth signal and a retroviral vector to the target phsc . this simultaneous delivery of vector and growth signal should also have the advantage of increasing the effective retroviral titre , owing to the intimate association of producer and target cells . scf has been shown to have both soluble and membrane - bound forms . evidence acquired from the study of mice carrying a small intragenic deletion in the gene encoding the scf receptor has indicated that the membrane - bound form of the cytokine is essential for normal haematopoiesis . despite being able to synthesise a soluble scf retaining full biological activity , these mice are as badly affected as their counterparts who carry a complete deletion of the gene . while not wishing to be bound by any particular theory , we believe that the in vivo biological activities of the soluble and membrane - associated forms of the growth factor are distinct , and that normal haematopoiesis has an absolute requirement for the membrane - bound form of scf that cannot be substituted by the soluble form . it may also be that the expression of bound scf on the cell surface changes / reduces the extent to which other growth factors are expressed , and that this has a beneficial effect on transduction levels of the phsc . improved transduction rates using the method may be achieved using the synergistic action of additional cytokines . in this regard , scf is particularly noted for its property of interacting in this way with other growth factors , which has led to the suggestion that on its own it may not be a mitogen but acts as an anti - apoptotic factor . to assess this , similar experiments to those described above can be performed using additional cytokines added to the media in conjunction with our modified producers . ideally , we would hope to find conditions favouring self - renewal at the expense of differentiation . this would have the highly desirable consequence of enabling us to expand phsc numbers in culture . one factor thought possibly to act in this way is mip1 - α1 . there is also evidence that stem cell quiescence may be negatively influenced by tgf - β , antagonists of this molecule may therefore be beneficial in stimulating cells into cycle . of the positively acting cytokines , lif , the factor that blocks differentiation of mouse embryonal stem cells and il - 11 , a recently identified member of the same family of cytokines , are candidates for acting on stem cells , as is flt3 ligand , a molecule with a similar spectrum of activities to scf . as regards embodiments of the invention using growth factors displayed on the surface of retroviruses , the results presented above demonstrate that the dimeric ligands scf and fl can successfully be displayed on retroviruses and that these ligands retain the capability of binding to their receptors . we have demonstrated that , although infection cannot be mediated through the displayed ligand , if a protease cleavable linker is used to fuse the displayed ligand to the viral envelope , targeted infection can result . the modified viruses described here retain their capacity to infect cells through the ram1 receptor but other results from our laboratory indicate that it is possible to construct a cleavable linker which is capable of blocking infection through the natural viral receptor . in should therefore be possible to target retroviral gene transfer specifically to phsc . the above method describes a protocol which is potentially applicable to any clinical procedure requiring the transfer of genetic information to pluripotent haematopoeitic stem cells ( phsc ). as discussed above , this method is applicable for gene therapy of inherited haematopoeitic disorders , such as the immunodeficiencies , but it could also be applicable to conditions such as haemophilia , or other conditions requiring the synthesis of a pharmacologically active compound normally present in the serum . there are also potential applications in the field . of cancer therapy , primarily as a way of protecting cells from cytotoxic agents or radioprotecting them , thus giving them a survival advantage over non - treated bone marrow cells . the references mentioned in this application are all herein incorporated by reference . 5 1 . markowitz , d ., s . goff , and a . bank . 1988 . construction and use of a safe and efficient amphotropic packaging cell line . virology 167 : 400 . 2 . thrasher , a ., m . chetty , c . casimir , and a . w . segal . 1992 . restoration of superoxide generation to a chronic granulomatous disease - derived b - cell line by retrovirus mediated gene transfer . blood 80 : 1125 . 3 . morgenstern , j . p . and h . land . 1990 . advanced mammalian gene transfer : high titre retroviral vectors with multiple drug selection markers and a complementary helper - free packaging cell line . nucleic acids res . 18 : 3587 . 4 . anderson , d . m ., d . e . williams , r . tushinski , s . gimpel , j . eisenman , l . a . cannizzaro , m . aaronson , c . m . croce , k . huebner , and d . cosman . 1991 . alternate splicing of mrnas encoding human mast cell growth factor and localisation of the gene to chromosome 12q22 - 12q24 . cell growth differ 2 : 373 . 5 . beradi , a . c ., a . wang , j . d . levine , p . lopez , and d . t . scadden . 1995 . functional isolation and characterization of human hematopoietic stem cells . science 267 : 104 . 6 . mullis , k ., f . faloona , s . scharf , r . saiki , g . horn , and h . erlich . 1986 . specific enzymatic amplification of dna in vitro : the polymerase chain reaction . cold spring harb symp quant biol 51 : 263 . 7 . russell , s . j ., r . e . hawkins , and g . winter , 1993 . retroviral vectors displaying functional antibody fragments . 8 . valesia - wittman , s ., a . drynda , g . delange , m . aumailley , j . m . heard , o . danos , g . verdier , and f . l . cosset . 1994 . modifications in the binding domain of avian retrovirus envelope protein to redirect the host range of retroviral vectors . j . virol ., 68 : 4609 . 9 . kasahara , n ., a . m . dozy and y . n . kan . 1994 . tissue - specific targeting of retroviral vectors through ligand - receptor interactions . science , 266 : 1373 . 10 . nilson b h k , morling f j , cosset f - 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