Adeno-associated virus type 2-mediated transfer of ecotropic retrovirus receptor cDNA allows ecotropic retroviral transduction of established and primary human cells.

The cellular receptors that mediate binding and internalization of retroviruses have recently been identified. The concentration and accessibility of these receptors are critical determinants in accomplishing successful gene transfer with retrovirus-based vectors. Murine retroviruses containing ecotropic glycoproteins do not infect human cells since human cells do not express the receptor that binds the ecotropic glycoproteins. To enable human cells to become permissive for ecotropic retrovirus-mediated gene transfer, we have developed a recombinant adeno-associated virus type 2 (AAV) vector containing ecotropic retroviral receptor (ecoR) cDNA under the control of the Rous sarcoma virus (RSV) long terminal repeat (LTR) promoter (vRSVp-ecoR). Established human cell lines, such as HeLa and KB, known to be nonpermissive for murine ecotropic retroviruses, became permissive for infection by a retroviral vector containing a bacterial gene for resistance to neomycin (RV-Neo(r)), with a transduction efficiency of up to 47%, following transduction with vRSVp-ecoR, as determined by the development of colonies that were resistant to the drug G418, a neomycin analog. No G418-resistant colonies were present in cultures infected with either vRSVp-ecoR or RV-Neo(r) alone. Southern and Northern blot analyses revealed stable integration and long-term expression, respectively, of the transduced murine ecoR gene in clonal isolates of HeLa and KB cells. Similarly, ecotropic retrovirus-mediated Neo(r) transduction of primary human CD34+ hematopoietic progenitor cells from normal bone marrow was also documented, but only following infection with vRSVp-ecoR. The retroviral transduction efficiency was approximately 7% without prestimulation and approximately 14% with prestimulation of CD34+ cells with cytokines, as determined by hematopoietic clonogenic assays. No G418-resistant progenitor cell colonies were present in cultures infected with either vRSVp-ecoR or RV-Neo(r) alone. These results suggest that sequential transduction of primary human cells with two different viral vectors may overcome limitations encountered with a single vector. Thus, the combined use of AAV- and retrovirus-based vectors may have important clinical implications for ex vivo and in vivo human gene therapy.     

Impact of cell growth morphology on retroviral transduction: effect of contact inhibition

Retrovirus-mediated gene transfer is one of the most commonly used methods to deliver, integrate, and express the gene of interest because the retrovirus can insert the desired gene into the chromosome of the target cells with high stability. However, to deliver the gene successfully, the retrovirus requires active division to integrate reversely transcribed DNA into the chromosome of target cells. In this study, we focused on the effect of cell-cell contact inhibition on the efficiency of retroviral transduction with two anchorage-dependent cell lines: NIH 3T3 and 293 cells. These two cell lines have very different cell morphologies and growth patterns on surfaces. Human embryonic kidney epithelial 293 cells tend to stick together after dividing, while NIH 3T3 cells migrate to occupy available surface and spread. Experimental data indicate that the abatement of the transduction rate of 293 cells was initiated in the early stage of the culture, whereas effect of contact inhibition of NIH 3T3 cells on the transduction rate became dominating at the end of the culture period. Experimental results were also quantitatively illustrated by plotting normalized multiplicity of infection (MOI) versus normalized cell density. According to the outcomes, cell inoculation density plays an important role in optimizing the retroviral transduction rate. The optimal time of retroviral transduction should be confined to the accelerating growth phase for 293 cells and at the exponential growth phase for NIH 3T3 cells. The implication drawn from this study is that contact inhibition effect on retroviral transduction should be taken into account for large-scale gene transfer systems such as the microcarrier bioreactor.     

Focus Formation: A Cell-based Assay to Determine the Oncogenic Potential of a Gene

Malignant transformation of cells is typically associated with increased proliferation, loss of contact inhibition, acquisition of anchorage-independent growth potential, and the ability to form tumors in experimental animals¹. In NIH 3T3 cells, the Ras signal transduction pathway is known to trigger many of these events, what is known as Ras transformation. The introduction of an overexpressed gene in NIH 3T3 cells may promote morphological transformation and loss of contact inhibition, which can help determine the oncogenic potential of that gene of interest. An assay that provides a straightforward method to assess one aspect of the transforming potential of an oncogene is the Focus Formation Assay (FFA)². When NIH 3T3 cells divide normally in culture, they do so until they reach a confluent monolayer. However, in the presence of an overexpressed oncogene, these cells can begin to grow in dense, multilayered foci¹ that can be visualized and quantified by crystal violet or Hema 3 staining. In this article we describe the FFA protocol with retroviral transduction of the gene of interest into NIH 3T3 cells, and how to quantify the number of foci through staining. Retroviral transduction offers a more efficient method of gene delivery than transfection, and the use of an ecotropic murine retrovirus provides a biosafety control when working with potential human oncogenes.     

干细胞因子逆转录病毒表达载体的构建及体外性质研究

目的通过逆转录病毒介导两种类型人干细胞因子在NIH3T3细胞中稳定表达,并研究它们对白血病细胞的作用。方法用DNA重组技术构建并鉴定可溶型及膜结合型干细胞因子的重组逆转录病毒表达载体MSCV—PGK—GFP—sSCF、MSCV—PGK—GFP—mSCF,与空载体对照MSCV—PGK—GFP分别转染Phoenix细胞包装病毒,并感染NIH3T3细胞,流式分选术获得3种阳性细胞,CCK8法分别检测与其共培养的K562细胞的增殖情况。结果成功构建了sSCF、mSCF逆转录病毒表达载体;经Phoenix包装的重组及对照逆转录病毒成功感染NIH3T3细胞,获得了稳定表达细胞株NIH3T3-S、NIH3T3-M和对照细胞株NIH3T3-V。共培养中NIH3T3-S、NIH3T3-M均可促进K562细胞的增殖,且在低血清条件下,NIH3T3-M的作用高于NIH3T3-S。结论可溶性及膜结合SCF分别通过旁分泌和并置性作用促进白血病细胞的增殖。     

Improvement of retroviral vectors by coating with poly(ethylene glycol)-poly(L-lysine) block copolymer (PEG-PLL)

BACKGROUND: Although some cationic reagents, such as polybrene, improve gene transduction in vitro, their use in vivo is prohibited due to their toxicity to the exposed cells. This paper demonstrates that a new cationic reagent, poly(ethylene glycol)-poly(L-lysine) block copolymer (PEG-PLL), improves gene transduction with retroviral vectors without increasing cell toxicity. METHODS: A retroviral vector derived from the Moloney leukemia virus, containing the lacZ gene, was modified with PEG-PLL prior to transduction into NIH3T3, Lewis lung carcinoma, and primary cultured mouse brain cells. LacZ transduction efficacy was evaluated by counting the number of X-Gal-positive cells. RESULTS: We have demonstrated that PEG-PLL is able to stably modify the viral particle surface due to the affinity of the PEG moiety to the biomembrane, and neutralizes negative charges by the cationic nature of the poly-lysine residue. Thus, PEG-PLL increased the gene transduction efficiency and minimized cell toxicity because free PEG-PLL was removable by centrifugation. We have shown that PEG-PLL increased the viral gene transduction efficiency 3- to 7-fold with NIH3T3 or Lewis lung carcinoma cell lines without increasing cytotoxicity. It improved retroviral gene transduction efficacy even against labile cells, such as primary cultured brain cells. CONCLUSIONS: PEG-PLL is a novel reagent that improves retroviral gene transduction efficacy without increasing cytotoxicity.     

Mutational Analysis of the Fusion Peptide of Moloney Murine Leukemia Virus Transmembrane Protein p15E

Fusion peptides are hydrophobic sequences located at the N terminus of the transmembrane (TM) envelope proteins of the orthomyxoviruses and paramyxoviruses and several retroviruses. The Moloney murine leukemia virus TM envelope protein, p15E, contains a hydrophobic stretch of amino acids at its N terminus followed by a region rich in glycine and threonine residues. A series of single amino acid substitutions were introduced into this region, and the resulting proteins were examined for their abilities to be properly processed and transported to the cell surface and to induce syncytia in cells expressing the ecotropic receptor. One substitution in the hydrophobic core and several substitutions in the glycine/threonine-rich region that prevented both cell-cell fusion and the transduction of NIH 3T3 cells when incorporated into retroviral vector particles were identified. In addition, one mutation that enhanced the fusogenicity of the resulting envelope protein was identified. The fusion-defective mutants trans dominantly interfered with the ability of the wild-type envelope protein to cause syncytium formation in a cell-cell fusion assay, although no trans-dominant inhibition of transduction was observed. Certain substitutions in the hydrophobic core that prevented envelope protein processing were also found. These data indicate that the N-terminal region of p15E is important both for viral fusion and for the correct processing and cell surface expression of the viral envelope protein.     

Quantitative analysis of retroviral and lentiviral gene transfer to murine embryonic stem cells

The effectiveness of retrovirus or lentivirus transduction of embryonic stem (ES) cells is often limited because transgene expression is silenced or variegated. We wondered if other steps of transduction, in addition to gene expression, were restricted in ES cells. We quantitatively compared (1) the amount of virus binding, (2) the number of integrated transgenes, and (3) the resulting level of gene expression. We found that three- to fourfold fewer retroviruses and lentiviruses bound to R1 mES cells than to NIH 3T3 cells, suggesting that both types of viruses bind less efficiently to mES cells. Retroviruses and lentiviruses differed in the efficiency with which they completed post-binding steps of transduction. In R1 mES cells, we detected 3-fold fewer integrated retrovirus transgenes and 11-fold lower expression levels than in NIH 3T3 cells, which suggests that the primary limitation to retrovirus transduction may be low levels of transgene expression. In contrast, we detected 10-fold fewer integrated lentivirus transgenes and 8-fold lower expression levels in R1 mES cells than in NIH 3T3 cells, which suggests that lentivirus transduction may be limited by inefficient intracellular post-binding steps of transduction. The implications of our findings for developing improved viral vectors for transducing mES cells are discussed.     

Construction and isolation of a transmissible retrovirus containing the src gene of Harvey murine sarcoma virus and the thymidine kinase gene of herpes simplex virus type 1

We constructed lambda recombinants containing the Harvey murine sarcoma virus genome and the thymidine kinase (tk) gene of herpes simplex virus type 1 linked to each other. The tk gene was located in a position downstream from both the long terminal repeat and the src gene of Harvey murine sarcoma virus. The DNAs of the lambda recombinants were used to transfect NIH3T3 mouse fibroblasts in order to obtain Harvey murine sarcoma virus DNA-induced foci of transformed cells. The transformed foci were superinfected with a helper-independent retrovirus, and new individual retrovirus were isolated from the superinfected foci. The new viruses could induce focus formation on NIH3T3 cells and could convert NIH3T3(TK-) cells into TK+ cells by carrying the herpes simplex virus type 1 tk gene into the TK- cells. From virus-infected cells, we isolated nonproducer foci on NIH3T3 cells and TK+ transformants on NIH3T3(TK-) cells containing one such new viral genome coding for the dual properties. The new retroviral sequence in the nonproducer cells could be rescued into virus particles at high titers by superinfection with a helper-independent retrovirus. A hybridization analysis indicated that the recombinant virus contained both the Harvey murine sarcoma virus src sequence and the tk gene sequence in a single RNA species approximately 4.9 kilobases long. We concluded that retroviruses can be used as true vectors for genes other than genes that lead to oncogenesis.     

Construction of a defective retrovirus containing the human hypoxanthine phosphoribosyltransferase cDNA and its expression in cultured cells and mouse bone marrow.

Defective ecotropic and amphotropic retroviral vectors containing the cDNA for human hypoxanthine phosphoribosyltransferase (HPRT) were developed for efficient gene transfer and high-level cellular expression of HPRT. Helper cell clones which produced a high viral titer were generated by a simplified method which minimizes cell culture. We used the pZIP-NeoSV(X) vector containing a human hprt cDNA. Viral titers (1 X 10(3) to 5 X 10(4)/ml) of defective SVX HPRT B, a vector containing both the hprt and neo genes, were increased 3- to 10-fold by cocultivation of the ecotropic psi 2 and amphotropic PA-12 helper cells. Higher viral titers (8 X 10(5) to 7.5 X 10(6] were obtained when nonproducer NIH 3T3 cells or psi 2 cells carrying a single copy of SVX HPRT B were either transfected or infected by Moloney leukemia virus. The SVX HPRT B defective virus partially corrected the HPRT deficiency (4 to 56% of normal) of cultured rodent and human Lesch-Nyhan cells. However, instability of HPRT expression was detected in several infected clones. In these unstable variants, both retention and loss of the SVX HPRT B sequences were observed. In the former category, cells which became HPRT- (6-thioguanine resistant [6TGr]) also became G418s, indicative of a cis-acting down regulation of expression. Both hypoxanthine-aminopterin-thymidine resistance (HATr) and G418r could be regained by counterselection in hypoxanthine-aminopterin-thymidine. In vitro mouse bone marrow experiments indicated low-level expression of the neo gene in in vitro CFU assays. Individual CFU were isolated and pooled, and the human hprt gene was shown to be expressed. These studies demonstrated the applicability of vectors like SVX HPRT B for high-titer production of defective retroviruses required for hematopoietic gene transfer and expression.     

Proteoglycans secreted by packaging cell lines inhibit retrovirus infection.

Using a model recombinant retrovirus encoding the Escherichia coli lacZ gene, we have found that medium conditioned with NIH 3T3 cells and packaging cell lines derived from NIH 3T3 cells inhibits infection. Most of the inhibitory activity was greater than 100 kDa and was sensitive to chondroitinase ABC digestion, which is consistent with the inhibitor being a chondroitin sulfate proteoglycan. Proteoglycans secreted by NIH 3T3 cells and purified by anion-exchange chromatography inhibited amphotropic retrovirus infection. Pretreatment of amphotropic retrovirus stocks with chondroitinase ABC boosted the level of transduction efficiency by more than twofold. The implications of these findings with respect to retrovirus-cell interactions and the production of high-titer retroviral stocks are discussed.     

High-yield retroviral production using a temperature-modulated two-stage operation

For clinical trials, large amounts of high-titer retroviral supernatants are required. However, retroviral concentration is relatively low compared with other viral vectors. Moreover, less than half of retroviral vectors suspended in a collected supernatant are infectious because of their short half-lives. In this study, a culture medium of ecotropic retrovirus-producing GP + E86/LNCX cells in tissue culture dishes was circulated through a reservoir, which was arranged with an incubator or ice-bath stage. Titers determined from the retroviral supernatant circulated through an ice-cold reservoir increased for a week from the beginning of retroviral production, while the titers from static production with circulation through the 37 degrees C reservoir reached a plateau after 3 days of retroviral production. After 5 days, 10 times more infectious retroviruses were obtained by circulating and keeping the majority of supernatant longer in the cold reservoir than in the production vessel at 37 degrees C in comparison with the number collected from the static tissue culture dish without circulating the culture medium. Furthermore, the concentration of transduction inhibitors in the supernatant was decreased along with the retardation of retroviral decay at low temperature. The two-stage operation developed in this study should be easily applied to large-scale bioreactors for mass production of high-titer retroviral supernatants.     

A role for endogenous retroviral sequences in the regulation of lymphocyte activation

The genomes of most vertebrates contain numerous retroviral sequences, the great majority of which are non-infectious. These endogenous retroviral sequences are transcribed and translated in many host tissues, and are induced by mitogens. The function, if any, of endogenous retroviruses has been unclear. The transmembrane envelope proteins of some infectious type C retroviruses suppress lymphocyte activation, but it is unknown whether any endogenous type C retroviruses share this suppressive activity. To study the possible effects of murine endogenous retroviral expression, specific antisense oligonucleotides were synthesized complementary to type C retroviral sequences, and were cultured with murine spleen cells. If any of these endogenous retroviruses are suppressing lymphocyte activation, then inhibiting such endogenous retroviral-mediated suppression with antisense might result in lymphocyte stimulation. Three classes of endogenous type C retroviral sequences may be distinguished by antisense oligonucleotides (based on their homology to infectious retroviruses): ecotropic, xenotropic, and mink cell focus-forming (MCF). Antisense oligonucleotides to endogenous MCF envelope gene (env) initiation regions caused: i) doubling or tripling of spleen cell RNA synthesis, and ii) marked increases in lymphocyte surface Ia and Ig expression relative to control oligonucleotides. Antisense oligos to xenotropic or ecotropic env sequences or to endogenous MCF non-envelope sequences had no effect. These data suggest that endogenous MCF sequences exert an inhibitory influence on the murine immune system. Because endogenous MCF expression is inducible by immune stimuli, such expression could constitute an inhibitory feedback circuit that participates in the regulation of immune homeostasis.     

Evaluation of retroviral production systems using quantitative analysis

Because of the low titer of retroviral supernatant, it is necessary to develop and optimize large-scale retroviral production systems. To quantitatively determine the effect of a given operating condition (e.g., temperature and serum content) on producer cells' retrovirus-producing capacity, a mathematical model was used to analyze the static retroviral production system described by three processes: viral diffusion, decay, and generation. The analytical solutions of the defined model equations were fitted with experimental data to determine the specific retroviral production rate constant, which represents the competence of a retroviral production system. Two different retroviral production systems, inducible production of vesicular stomatitis virus G glycoprotein (VSV-G) pseudotyped retrovirus from 293GPG/EGFP cells and constant production of ecotropic retrovirus from GP+E86/LNCX cells, were employed to demonstrate the feasibility of the engineering analysis. Our results indicated that the time-variant specific retroviral production rate of 293GPG/EGFP cells reached its maximum value of 5.7 x 10(-)(3) CFU/cm(2).h.cell, and the constant specific retroviral production rate of GP+E86/LNCX cells was 1.49 x 10(-)(2) CFU/cm(2).h.cell for the whole period of production. Furthermore, the effects of serum concentration and temperature on the ecotropic retroviral production system were examined separately. Our results suggest that producing ecotropic retroviruses with 10% fetal bovine serum at 37 degrees C is the optimal operating conditions for the long-term production system used here.     

Inhibition of infectious murine leukemia virus production by Fv-4 env gene products exerting dominant negative effect on viral envelope glycoprotein

Fv-4 is a mouse gene that confers resistance against ecotropic murine leukemia virus (MLV) infection on mice. While receptor interference by the Fv-4 env gene product, Fv-4 Env, that can bind to the ecotropic MLV receptor has been shown to play an important role in the resistance, other mechanisms have also been suggested because it confers extremely efficient, complete resistance in vivo. Here, we have examined the effect of Fv-4 Env on infectious MLV production. Infectious MLV titers in supernatants obtained after transfection with a Friend MLV (FMLV) Env-expressing plasmid from MLV gag-pol producer cells harboring a retroviral vector were largely reduced by coexpression of Fv-4 Env. Syncytia formation mediated by R-peptide-deleted FMLV Env in NIH 3T3 cells was impaired by Fv-4 Env coexpression. Similarly, Fv-4 Env inhibited infectious amphotropic MLV production and syncytia formation mediated by R-peptide-deleted amphotropic MLV Env. Immunoprecipitation analysis revealed interaction of Fv-4 Env with amphotropic MLV Env as well as FMLV Env. These results indicate that Fv-4 Env inhibits infectious ecotropic and amphotropic MLV production by exerting dominant negative effect on MLV Env, suggesting contribution of this inhibitory effect to the resistance against ecotropic MLV infection in Fv-4-expressing mice.     

Involvement of somatically acquired ecotropic viruses in the immunogenicity of nude-transplanted NIH/3T3 transformed cell lines.

Immunogenic tumor variants were previously derived after transplantation in vivo into nude mice of NIH/3T3-transformed cell lines. Nude-passaged cell lines were rejected by immunocompetent H-2q NIH mice, were recognized by specific CTL clones, and expressed new retroviral Ag. The aim of the present work was to investigate whether somatically acquired proviral sequences were present in the genome of nude-passaged cells and to test directly for a causative relationship between murine leukemia virus (MuLV) expression and immunogenicity. Southern blot analysis of PstI-digested DNA indicated that in contrast to the parental NIH/3T3 transformed cell lines (pT, T12N/5a, NS-1) all the nude-passaged immunogenic variants (pT-nude, T12N/5a-nude, NS-1-nude) contained newly acquired ecotropic-related proviruses. Immediately after in vitro establishment, these tumors displayed multiple integration sites as assessed by analysis of 3' proviral-cellular junctions. Long term in vitro culture of one of the cell lines (pT-nude) resulted in a cell line (pT-nude/vitro) that was clonal or oligo-clonal with respect to viral integration. Northern blot analysis established that the new proviruses were actively transcribed in all the immunogenic variants. To assess whether the somatically acquired ecotropic proviral sequences encode for target structures recognized by specific CTL, obtained after immunization of NIH mice with pT-nude, the parental cell line pT was transfected with plasmids containing the entire AKV MuLV genome, the cloned AKV gag or env genes. Screening of transfectants for their ability to stimulate the production of TNF by anti-pT-nude effectors indicated that cells transfected with the entire ecotropic virus or with MuLV-env gene products could be recognized by an NIH anti-pT-nude CTL line and NIH anti-pT-nude Kq-restricted CTL clones as well as the immunizing target pT-nude.     

Retroviral vectors containing putative internal ribosome entry sites: development of a polycistronic gene transfer system and applications to human gene therapy.

Recombinant retroviral vectors producing multicistronic mRNAs were constructed. Picornavirus putative internal ribosome entry sites (IRES) were used to confer cap-independent translation of an internal cistron. Internal cistrons were engineered by ligation of various lengths of the IRES of encephalomyocarditis (EMC) virus or polio virus to the E. coli chloramphenicol acetyltransferase (CAT) gene. The IRES/CAT fusions were introduced into retroviral vectors 3' to the translation stop codon of the neomycin phosphotransferase (NEO) gene, and the molecular constructs transfected into retroviral vector packaging lines. Retroviral vector producer cells efficiently express the internal CAT gene product only when the full length IRES is used. Both the EMC/CAT and polio/CAT retroviral vectors produced high titer vector supernatant capable of productive transduction of target cells. To test the generality of this gene transfer system, a retroviral vector containing an IRES fusion to the human adenosine deaminase (ADA) gene was constructed. Producer cell supernatant was used to transduce NIH/3T3 cells, and transduced cells were shown to express NEO, and ADA. Novel three-gene-containing retroviral vectors were constructed by introducing the EMC/ADA fusion into either an existing internal-promoter-containing vector, or a polio/CAT bicistronic vector. Producer cell clones of the three-gene vectors synthesize all three gene products, were of high titer, and could productively transduce NIH/3T3 cells. By utilizing cap-independent translation units, IRES vectors can produce polycistronic mRNAs which enhance the ability of retroviral-mediated gene transfer to engineer cells to produce multiple foreign proteins.     

BALB/c myeloma retroviruses have mink cell focus-inducing activity.

We have determined the in vitro host range of the cloned MO-21 and FL-1 murine myeloma retroviruses grown in SC-1 cells that were originally isolated from cloned MOPC-21 and FLOPC-1 BALB/c plasmacytoma cell lines. These viruses are able to replicate in murine (BALB/3T3, NIH/3T3) as well as numerous heterologous cell lines. These myeloma retroviruses also exhibit mink cell focus-inducing activity. MO-21 and FL-1 shared interference patterns with each other, but their replication was not interfered with by ecotropic, xenotropic, or amphotropic viruses. The lack of cross-interference with ecotropic or xenotropic viruses distinguishes these isolates from other mink cell focus-inducing viruses.     

Detection of receptor-specific murine leukemia virus binding to cells by immunofluorescence analysis.

Four classes of murine leukemia virus (MuLV) which display distinct cellular tropisms and bind to different retrovirus receptors to initiate virus infection have been described. In the present study, we describe a rapid, sensitive immunofluorescence assay useful for characterizing the initial binding of MuLV to cells. By using the rat monoclonal antibody 83A25 (L. H. Evans, R. P. Morrison, F. G. Malik, J. Portis, and W. J. Britt, J. Virol. 64:6176-6183, 1990), which recognizes an epitope of the envelope gp70 molecule common to the different classes of MuLV, it is possible to analyse the binding of ecotropic, amphotropic, or xenotropic MuLV by using only a single combination of primary and secondary antibodies. The MuLV binding detected by this assay is envelope receptor specific and matches the susceptibility to infection determined for cells from a variety of species. The binding of amphotropic MuLV to NIH 3T3 cells was shown to be rapid, saturable, and temperature dependent. Chinese hamster ovary (CHO-K1) cells normally lack the ability to bind ecotropic virus and are not infectible by ecotropic vectors. Expression of the cloned ecotropic retrovirus receptor gene (Rec) in CHO-K1 cells confers high levels of ecotropic virus-specific binding and confers susceptibility to infection. Characterization of MuLV binding to primary cells may provide insight into the infectibility of cells by retroviruses and aid in the selection of appropriate vectors for gene transfer experiments.     

Gene transfer of human TCR in primary murine T cells is improved by pseudo-typing with amphotropic and ecotropic envelopes

BACKGROUND: T cell receptor (TCR) gene therapy represents an attractive anti-cancer treatment but requires further optimization of its efficacy and safety in clinically relevant models, such as those using a tumor antigen and TCR of human origin. Currently, however, there is no consensus as to what protocol is most optimal for retroviral human TCR gene transfer into primary murine T cells, most notably with respect to virus pseudo-type. METHODS: Primary murine T cells were transduced, expanded and subsequently tested for transgene expression, proliferation and antigen-specific function. To this end, murine leukemia virus (MLV) retroviruses were produced upon transfection of various packaging cells with genes encoding either green fluorescent protein (GFP) or TCRalphabeta specific for human melanoma antigen gp100(280-288) and the helper elements GAG/POL and ENV. Next to viral pseudotyping, the following parameters were studied: T cell densities; T cell activation; the amounts of IL-2 and the source of serum used to supplement medium. RESULTS: The pseudo-type of virus produced by packaging cells critically determines T cell transduction efficiencies. In fact, MLV-A and MLV-E pseudo-typed viruses derived from a co-culture of Phoenix-A and 293T cells resulted in T cell transduction efficiencies that were two-fold higher than those based on retroviruses expressing either VSV-G, GALV, MLV-A or MLV-E envelopes. In addition, T cell densities during transduction were inversely related to transduction efficiencies. Further optimization resulted in transduction efficiencies of over 90% for GFP, and 68% for both a murine and a human (i.e. murinized) TCR. Importantly, TCR-transduced T cells proliferate (i.e. showing a log increase in cell number in a few days) and show antigen-specific function. CONCLUSIONS: We set up a quick and versatile method to genetically modify primary murine T cells based on transient production of TCR-positive retroviruses, and show that retroviral gene transfer of a human TCR into primary murine T cells is critically improved by viral pseudo-typing with both MLV-A and MLV-E envelopes.