Ping-pong amplification of a retroviral vector achieves high-level gene expression: human growth hormone production

Role of retroviral vector-based interventions in combating virus infections

The deployment of viruses as vaccine-vectors has witnessed recent developments owing to a better understanding of viral genomes and mechanism of interaction with the immune system. Vaccine delivery by viral vectors offers various advantages over traditional approaches. Viral vector vaccines are one of the best candidates for activating the cellular arm of the immune system, coupled with the induction of significant humoral responses. Hence, there is a broad scope for the development of effective vaccines against many diseases using viruses as vectors. Further studies are required before an ideal vaccine-vector is developed and licensed for use in humans. In this article, we have outlined the use of retroviral vectors in developing vaccines against various viral diseases.

Movements of HIV-1 genomic RNA-APOBEC3F complexes and PKR reveal cytoplasmic and nuclear PKR defenses and HIV-1 evasion strategies

APOBEC3 cytidine deaminases and viral genomic RNA (gRNA) occur in virions, polysomes, and cytoplasmic granules, but have not been tracked together. Moreover, gRNA traffic is important, but the factors that move it into granules are unknown. Using in situ hybridization of transfected cells and protein synthesis inhibitors that drive mRNAs between locales, we observed APOBEC3F cotrafficking with gRNA without altering its movements. Whereas cells with little cytoplasmic gRNA were translationally active and accumulated Gag, suprathreshold amounts induced autophosphorylation of the cytoplasmic double-stranded RNA (dsRNA)-dependent protein kinase (PKR), causing eIF2α phosphorylation, protein synthesis suppression, and gRNA sequestration in stress granules. Additionally, we confirmed recent evidence that PKR is activated by chromosome-associated cellular dsRNAs after nuclear membranes disperse in prophase. By arresting cells in G2, HIV-1 blocks this mechanism for PKR activation and eIF2α phosphorylation. However, cytopathic membrane damage in CD4- and coreceptor-positive cultures infected with laboratory-adapted fusogenic HIV-1LAI eventually enabled PKR entry and activation in interphase nuclei. These results reveal multiple stages in the PKR-HIV-1 battleground that culminate in cell death. We discuss evidence suggesting that HIV-1s evolve in vivo to prevent or delay PKR activation by all these mechanisms.

Retrovirus-induced spongiform neurodegeneration is mediated by unique central nervous system viral targeting and expression of env alone

Certain murine leukemia viruses (MLVs) can induce progressive noninflammatory spongiform neurodegeneration similar to that caused by prions. The primary MLV determinants responsible have been mapped to within the env gene; however, it has remained unclear how env mediates disease, whether non-Env viral components are required, and what central nervous system (CNS) cells constitute the critical CNS targets. To address these questions, we examined the effect of transplanting engraftable C17.2 neural stem cells engineered to pseudotype, disseminate, and trans-complement neurovirulent (CasBrE, CasE, and CasES) or non-neurovirulent (Friend and SFF-FE) env sequences (SU or SU/TM) within the CNS using either the "non-neurovirulent" amphotropic helper virus, 4070A, or pgag-polgpt (a nonpackaged vector encoding Gag-Pol). These studies revealed that acute MLV-induced spongiosis results from two separable activities of Env. First, Env causes neuropathology through unique viral targeting within the CNS, which was efficiently mediated by ecotropic Envs (CasBrE and Friend), but not 4070A amphotropic Env. Second, Env induces spongiosis through a toxin activity that is MLV-receptor independent and does not require the coexpression of other viral structural proteins. CasBrE and 4070A Envs possess the toxin activity, whereas Friend Env does not. Although the identity of the critical viral target cell(s) remains unresolved, our results appear to exclude microglia and oligodendrocyte lineage cells, while implicating viral entry into susceptible neurons. Thus, MLV-induced disease parallels prionopathies in that a single protein, Env, mediates both the CNS targeting and the toxicity of the infectious agent that manifests itself as progressive vacuolar neurodegeneration.

Misfolding of CasBrE SU is reversed by interactions with 4070A Env: implications for gammaretroviral neuropathogenesis

Background

CasBrE is a neurovirulent murine leukemia virus (MLV) capable of inducing paralytic disease with associated spongiform neurodegeneration. The neurovirulence of this virus has been genetically mapped to the surface expressed subunit (SU) of the env gene. However, CasBrE SU synthesized in the absence of the transmembrane subunit (TM) does not retain ecotropic receptor binding activity, indicating that folding of the receptor binding domain (RBD) requires this domain. Using a neural stem cell (NSC) based viral trans complementation approach to examine whether misfolded CasBrE SU retained neurovirulence, we observed CasBrE SU interaction with the "non-neurovirulent" amphotropic helper virus, 4070A which restored functional activity of CasBrE SU.

Results

Herein, we show that infection of NSCs expressing CasBrE SU with 4070A (CasES+4070A-NSCs) resulted in the redistribution of CasBrE SU from a strictly secreted product to include retention on the plasma membrane. Cell surface cross-linking analysis suggested that CasBrE SU membrane localization was due to interactions with 4070A Env. Viral particles produced from CasES+4070A-NSCS contained both CasBrE and 4070A gp70 Env proteins. These particles displayed ecotropic receptor-mediated infection, but were still 100-fold less efficient than CasE+4070A-NSC virus. Infectious center analysis showed CasBrE SU ecotropic transduction efficiencies approaching those of NSCs expressing full length CasBrE Env (CasE; SU+TM). In addition, CasBrE SU-4070A Env interactions resulted in robust ecotropic superinfection interference indicating near native intracellular SU interaction with its receptor, mCAT-1.

Conclusions

In this report we provided evidence that 4070A Env and CasBrE SU physically interact within NSCs leading to CasBrE SU retention on the plasma membrane, incorporation into viral particles, restoration of mCAT-1 binding, and capacity for initiation of TM-mediated fusion events. Thus, heterotropic Env-SU interactions facilitates CasBrE SU folding events that restore Env activity. These findings are consistent with the idea that one protein conformation acts as a folding scaffold or nucleus for a second protein of similar primary structure, a process reminiscent of prion formation. The implication is that template-based protein folding may represent an inherent feature of neuropathogenic proteins that extends to retroviral Envs.

Development of murine leukemia virus-based retroviral vectors with a minimum possibility of cis-activation

The possibility of insertional mutagenesis in retroviral gene therapy can be reduced by using a vector lacking the enhancer sequence present in the U3 of the long-terminal repeats. However, such vectors suffer from many pitfalls. We attempted to improve a murine leukemia virus-based retroviral vector containing the enhancer-free U3, first by making it easier to construct a producer line and then by introducing the cellular RPL10 promoter as an internal promoter. The reverse orientation of the expression cassette of the transgene was found to give higher transducing titer and higher-level gene expression. The deletion analysis revealed that the 54-bp-long sequence of U3 (34 and 20 bp present at 5' and 3' extreme ends, respectively) was sufficient for the functions of retroviral vectors. The data from the in vitro cell culture assay indicated that the final construct, ROK, containing all these features, had little cis-activation activity, even if it was placed right upstream from the RNA start site of the neighboring gene. Our data suggested that the newly developed vector might provide increased safety, while still producing high viral titer from a stable producer line and high-level gene expression in various target cells including human CD34(+) stem cells.

Genetic fingerprinting of the development and progression of T-cell lymphoma in a murine model of atypical myeloproliferative disorder initiated by the ZNF198-fibroblast growth factor receptor-1 chimeric tyrosine kinase

A mouse model of human ZNF198-fibroblast growth factor receptor-1 (FGFR1) stem cell leukemia lymphoma has been developed to investigate mechanisms of oncogenesis and progression. Using array-based comparative genomic hybridization, we followed disease progression after serial transplantation of ZNF198-FGFR1-transformed stem cells that give rise to a distinct myeloproliferative disorder and T-lymphoblastic leukemia. A consistent, frequently homozygous, chr14:53880459-55011545 deletion, containing the T-cell receptor alpha and delta genes, was identified in the bone marrow, spleen, and lymph nodes in all cases. The absence of cell-surface T-cell receptor alpha in tumor cells precludes CD3 recruitment, resulting in loss of a functional T-cell receptor complex, supporting the idea that prevention of maturation of CD4(+)/CD8(+) double-positive immature T cells is important in ZNF198-FGFR1 disease development. Up-regulation of the B-cell line 2, interleukin 7 receptor alpha and interleuking 2 receptor alpha prosurvival genes in these undifferentiated tumor precursor cells suggests one mechanism that allows them to escape apoptosis in the thymus. Thus, we have defined an important event in the process of ZNF198-FGFR1-induced T-cell leukemia.

Evidence that ecotropic murine leukemia virus contamination in TZM-bl cells does not affect the outcome of neutralizing antibody assays with human immunodeficiency virus type 1

The TZM-bl cell line that is commonly used to assess neutralizing antibodies against human immunodeficiency virus type 1 (HIV-1) was recently reported to be contaminated with an ecotropic murine leukemia virus (MLV) (Y. Takeuchi, M. O. McClure, and M. Pizzato, J. Virol. 82:12585-12588, 2008), raising questions about the validity of results obtained with this cell line. Here we confirm this observation and show that HIV-1 neutralization assays performed with a variety of serologic reagents in a similar cell line that does not harbor MLV yield results that are equivalent to those obtained in TZM-bl cells. We conclude that MLV contamination has no measurable effect on HIV-1 neutralization when TZM-bl cells are used as targets for infection.

Secreted frizzled-related protein-4 reduces sodium-phosphate co-transporter abundance and activity in proximal tubule cells

The phosphatonin, secreted frizzled-related protein-4 (sFRP-4), induces phosphaturia and inhibits 25-hydroxyvitamin D 1alpha-hydroxylase activity normally induced in response to hypophosphatemia. To determine the mechanism by which sFRP-4 alters renal phosphate (P(i)) transport, we examined the effect of sFRP-4 on renal brush border membrane (BBMV) Na(+)-dependent P(i) uptake, and the abundance and localization of the major Na(+)-P(i)-IIa co-transporter in proximal tubules and opossum kidney (OK) cells. Infusion of sFRP-4 increased renal fractional excretion of P(i) and decreased renal beta-catenin concentrations. The increase in renal P(i) excretion with sFRP-4 infusion was associated with a 21.9 +/- 3.4% decrease in BBMV Na(+)-dependent P(i) uptake (P < 0.001) compared with a 39.5 +/- 2.1% inhibition of Na(+)-dependent P(i) transport in renal BBMV induced by PTH (P < 0.001). sFRP-4 infusion was associated with a 30.7 +/- 4.8% decrease in Na(+)-P(i)-IIa co-transporter protein abundance (P < 0.01) assessed by immunoblotting methods compared to a 45.4 +/- 8.8% decrease induced by PTH (P < 0.001). In OK cells, sFRP-4 reduced surface expression of a heterologous Na(+)-P(i)-IIa co-transporter. We conclude that sFRP-4 increases renal P(i) excretion by reducing Na(+)-P(i)-IIa transporter abundance in the brush border of the proximal tubule through enhanced internalization of the protein.

Glycosylation Deficiency at Either One of the Two Glycan Attachment Sites of Cellular Prion Protein Preserves Susceptibility to Bovine Spongiform Encephalopathy and Scrapie Infections

The conversion into abnormally folded prion protein (PrP) plays a key role in prion diseases. PrP(C) carries two N-linked glycan chains at amino acid residues 180 and 196 (mouse). Previous in vitro data indicated that the conversion process may not require glycosylation of PrP. However, it is conceivable that these glycans function as intermolecular binding sites during the de novo infection of cells on susceptible organisms and/or play a role for the interaction of both PrP isoforms. Such receptor-like properties could contribute to the formation of specific prion strains. However, in earlier studies, mutations at the glycosylation sites of PrP led to intracellular trafficking abnormalities, which made it impossible to generate PrP glycosylation-deficient mice that were susceptible to bovine spongiform encephalopathy (BSE) or scrapie. We have now tested more than 25 different mutations at both consensus sites and found one nonglycosylated (T182N/T198A) and two monoglycosylated (T182N and T198A) mutants that rather retained authentic cellular trafficking properties. In vitro all three mutants were converted into PrP(res). PrP mutant T182N/T198A also provoked a strong dominant-negative inhibition on the endogenous wild type PrP conversion reaction. By using the two monoglycosylated mutants, we generated transgenic mice overexpressing PrP(C) in their brains at levels of 2-4 times that of nontransgenic mice. Most interestingly, such mice proved readily susceptible to a challenge with either scrapie (Chandler and Me7) or with BSE. Incubation times were comparable or in some instances even significantly shorter than those of nontransgenic mice. These data indicate that diglycosylation of PrP(C) is not mandatory for prion infection in vivo.

A murine leukemia virus with Cre-LoxP excisible coding sequences allowing superinfection, transgene delivery, and generation of host genomic deletions

BACKGROUND

To generate a replication-competent retrovirus that could be conditionally inactivated, we flanked the viral genes of the Akv murine leukemia virus with LoxP sites. This provirus can delete its envelope gene by LoxP/Cre mediated recombination and thereby allow superinfection of Cre recombinase expressing cells.

RESULTS

In our studies, the virus repeatedly infected the cell and delivered multiple copies of the viral genome to the host genome; the superinfected cells expressed a viral transgene on average twenty times more than non-superinfected cells. The insertion of multiple LoxP sites into the cellular genome also led to genomic deletions, as demonstrated by comparative genome hybridization.

CONCLUSION

We envision that this technology may be particularly valuable for delivering transgenes and/or causing deletions.

Efficient transduction of murine primary T cells requires a combination of high viral titer, preferred tropism, and proper timing of transduction

Retroviral vectors have been used exclusively for genetic modification of primary T cells. Most T cell infection protocols have been developed for human T cells, whereas systematic investigations of the optimal conditions for transduction of murine primary T cells are limited. In this study, ecotropic and 10A1-pseudotyped retroviral vectors were compared for their efficiency in infecting murine primary T cell cells, as well as T cell lines. Various factors that affect transduction efficiency were also explored, including virus titer, times of exposure, timing of infection, low-speed centrifugation, and use of fibronectin fragment. Our results showed that up to 80% of murine primary T cells could be infected after a single exposure. Successful infection required a combination of high virus titer (>10(7) CFU/ml), proper timing of infection (within 24 h after mitogen stimulation), and preferred tropism (ecotropic vectors). These optimization results may help to establish a standard protocol for infection of murine primary T cells and provide some insight into the obstacles to retroviral infection of T cells.

A novel human suspension culture packaging cell line for production of high-titre retroviral vectors

Retroviruses are currently the most widely used vectors in clinical trials for gene therapy. These vectors are, however, limited by low titres partly due to the restrictive nature of monolayer cell culture. We have developed a stable suspension producer cell line derived from human lymphoblastoid cells (WIL-2) by electroporating these cells with the necessary trans components required for production of defective retrovirus particles which encode a nuclear localising beta-galactosidase gene. We show that this anchorage-independent cell line generates viruses at a titre of 7 x 10(5) iu/ml on NIH3T3 indicator cells which remains constant after at least 2 months in culture. The producer cells can be cultured at a density of 6 x 10(6) cells/ml with consistent virus titre production. WIL-2 can also be grown as single cells by rotation culture while maintaining virus production. By treating the cells with the transcriptional activator sodium butyrate titres above 1 x 10(6) i.u./ml are achieved. Concentrating viral supernatants by ultrafiltration can further increase virus titre to 5 x 10(8) i.u./ml. Even at these high titres no replication-competent virus was detected. Virus titre fell only slightly when cells were placed in serum-free media before harvest. The generation of this novel cell line provides proof-of-principle that large-scale production of retroviral vectors in serum-free growth conditions can be safely generated for use in gene therapy.

Role of double-stranded RNA-dependent protein kinase in mediating hypersensitivity of Fanconi anemia complementation group C cells to interferon gamma, tumor necrosis factor-alpha, and double-stranded RNA

Hematopoietic cells bearing inactivating mutations of Fanconi anemia group C (FANCC) are excessively apoptotic and demonstrate hypersensitivity not only to cross-linking agents but also to interferon gamma (IFN-gamma) and tumor necrosis factor-alpha. Seeking essential signaling pathways for this phenotype, this study quantified constitutive and induced RNA-dependent protein kinase (PKR) activation in Fanconi anemia cells of the C complementation group (FA-C). PKR was constitutively phosphorylated and exhibited an increased binding affinity for double-stranded RNA (dsRNA) in FANCC(-/-) cells. FANCC(-/-) cells were hypersensitive to both dsRNA and the combination of dsRNA and IFN-gamma in that these agents induced a higher fraction of apoptosis in FANCC(-/-) cells than in normal cells. Overexpression of wild-type PKR-sensitized FANCC(-/-) cells to apoptosis induced by IFN-gamma and dsRNA. Conversely, inhibition of PKR function by enforced expression of a dominant-negative inhibitory mutant of PKR (PKRDelta6) substantially reduced the IFN and dsRNA hypersensitivity of FANCC(-/-) cells. Two PKR target molecules, IkappaB-alpha and IRF-1, were not differentially activated in FANCC(-/-) cells, but enforced expression of a nonphosphorylatable form of eukaryotic translation initiation factor-2alpha reversed the PKR-mediated block of messenger RNA translation and partially abrogated the PKR-mediated apoptosis in FANCC(-/-) cells. Because no evidence was found of a PKR/FANCC complex in normal cells, it was concluded that an essential function of FANCC is to suppress, indirectly, the activity of PKR and that FANCC inactivation results in IFN hypersensitivity, at least in part, because this function of FANCC is abrogated.

A luciferase-reporter gene-expressing T-cell line facilitates neutralization and drug-sensitivity assays that use either R5 or X4 strains of human immunodeficiency virus type 1

We describe the production and properties of a permanent cell line, CEM.NKR-CCR5-Luc. This line is a derivative of the CEM.NKR-CCR5 line, stably transfected to express the luciferase reporter gene under the transcriptional control of the HIV-2 LTR. Thus the cells respond to Tat expression during HIV-1 infection by producing luciferase, a protein that can be readily and accurately quantitated in a luminometer. The CEM.NKR-CCR5-Luc line expresses both the CCR5 and CXCR4 coreceptors and can therefore be infected with a range of HIV-1 isolates, irrespective of their tropism properties. This is true of HIV-1 isolates from several genetic subtypes and also of a group O isolate. Furthermore, luciferase expression is also activated by infection of the cells with SIVmac239 or SIVmac251. We show that the CEM.NKR-CCR5-Luc cells can be used in assays of HIV-1 neutralization and also for identifying inhibitors of HIV-1 entry targeted at the CCR5 and CXCR4 coreceptors. The luciferase end point simplifies the performance of neutralization and inhibitor-screening assays compared to the use of more conventional end points such as the detection of extracellular p24 antigen.

Cellular and species resistance to murine amphotropic, gibbon ape, and feline subgroup C leukemia viruses is strongly influenced by receptor expression levels and by receptor masking mechanisms

Chinese hamster ovary (CHO) cells are resistant to infections by gibbon ape leukemia virus (GALV) and amphotropic murine leukemia virus (A-MLV) unless they are pretreated with tunicamycin, an inhibitor of N-linked glycosylation. These viruses use the related sodium-phosphate symporters Pit1 and Pit2, respectively, as receptors in nonhamster cells, and evidence has suggested that the corresponding transporters of CHO cells may be masked by tunicamycin-sensitive secreted inhibitors. Although the E36 line of Chinese hamster cells was reported to secrete the putative Pit2 inhibitor and to be sensitive to the inhibitory CHO factors, E36 cells are highly susceptible to both GALV and A-MLV in the absence of tunicamycin. Moreover, expression of E36 Pit2 in CHO cells conferred tunicamycin-independent susceptibilities to both viruses. Based on the latter results, it was suggested that E36 Pit2 must functionally differ from the endogenous Pit2 of CHO cells. To test these ideas, we analyzed the receptor properties of CHO Pit1 and Pit2 in CHO cells. Surprisingly, and counterintuitively, transfection of a CHO Pit2 expression vector into CHO cells conferred strong susceptibility to both GALV and A-MLV, and similar overexpression of CHO Pit1 conferred susceptibility to GALV. Thus, CHO Pit2 is a promiscuous functional receptor for both viruses, and CHO Pit1 is a functional receptor for GALV. Similarly, we found that the natural resistance of Mus dunni tail fibroblasts to subgroup C feline leukemia viruses (FeLV-C) was eliminated simply by overexpression of the endogenous FeLV-C receptor homologue. These results demonstrate a novel and simple method to unmask latent retroviral receptor activities that occur in some cells. Specifically, resistances to retroviruses that are caused by subthreshold levels of receptor expression or by stoichiometrically limited masking or interference mechanisms can be efficiently overcome simply by overexpressing the endogenous receptors in the same cells.

The Fanconi anemia protein FANCC binds to and facilitates the activation of STAT1 by gamma interferon and hematopoietic growth factors

Hematopoietic progenitor cells from Fanconi anemia (FA) group C (FA-C) patients display hypersensitivity to the apoptotic effects of gamma interferon (IFN-gamma) and constitutively express a variety of IFN-dependent genes. Paradoxically, however, STAT1 activation is suppressed in IFN-stimulated FA cells, an abnormality corrected by transduction of normal FANCC cDNA. We therefore sought to define the specific role of FANCC protein in signal transduction through receptors that activate STAT1. Expression and phosphorylation of IFN-gamma receptor alpha chain (IFN-gammaRalpha) and JAK1 and JAK2 tyrosine kinases were equivalent in both normal and FA-C cells. However, in coimmunoprecipitation experiments STAT1 did not dock at the IFN-gammaR of FA-C cells, an abnormality corrected by transduction of the FANCC gene. In addition, glutathione S-transferase fusion genes encoding normal FANCC but not a mutant FANCC bearing an inactivating point mutation (L554P) bound to STAT1 in lysates of IFN-gamma-stimulated B cells and IFN-, granulocyte-macrophage colony-stimulating factor- and stem cell factor-stimulated MO7e cells. Kinetic studies revealed that the initial binding of FANCC was to nonphosphorylated STAT1 but that subsequently the complex moved to the receptor docking site, at which point STAT1 became phosphorylated. The STAT1 phosphorylation defect in FA-C cells was functionally significant in that IFN induction of IFN response factor 1 was suppressed and STAT1-DNA complexes were not detected in nuclear extracts of FA-C cells. We also determined that the IFN-gamma hypersensitivity of FA-C hematopoietic progenitor cells does not derive from STAT1 activation defects because granulocyte-macrophage CFU and erythroid burst-forming units from STAT1(-/-) mice were resistant to IFN-gamma. However, BFU-E responses to SCF and erythropoietin were suppressed in STAT(-/-) mice. Consequently, because the FANCC protein is involved in the activation of STAT1 through receptors for at least three hematopoietic growth and survival factor molecules, we reason that FA-C hematopoietic cells are excessively apoptotic because of an imbalance between survival cues (owing to a failure of STAT1 activation in FA-C cells) and apoptotic and mitogenic inhibitory cues (constitutively activated in FA-C cells in a STAT1-independent fashion).

Critical role of enhanced CD4 affinity in laboratory adaptation of human immunodeficiency virus type 1

Strains of human immunodeficiency virus type 1 (HIV-1) that use the coreceptor CXCR4 (X4 strains) become laboratory adapted (LA) when selected for ability to replicate in leukemic T cell lines such as H9. Compared with patient X4 viruses, the gp120-gp41 complexes of LA viruses have a constellation of common properties including enhanced affinities for CD4, greater sensitivities to inactivations by diverse antibodies and by soluble CD4, increased shedding of gp120, and improved abilities to infect HeLa-CD4 cell clones that contain only trace quantities of CD4. These common characteristics, which may result from a concerted structural rearrangement of the gp120-gp41 complexes, have made it difficult to identify a specific feature that is critical for laboratory adaptation. To test the hypothesis that replication of patient X4 HIV-1 is limited by the low CD4 concentration in H9 cells (7.0 x 10(3) CD4/cell), we constructed H9 derivatives that express at least 10 times more of this receptor. Interestingly, most patient X4 isolates readily grew in these derivative cells, and the resulting virus preparations retained the characteristics of primary viruses throughout multiple passages. In contrast, selection of the same viruses in the parental H9 cells resulted in outgrowth of LA derivatives. We conclude that a weak interaction of patient X4 HIV-1 isolates with CD4 is the primary factor that limits their replication in leukemic T cell lines.

Low-speed centrifugation of retroviral vectors absorbed to a particulate substrate: a highly effective means of enhancing retroviral titre

For many gene therapy applications the effective titre of retroviral vectors is a limiting factor both in vitro and in vivo. Purification and concentration of retrovirus from packaging cell supernatant can overcome this problem. To this end we have investigated a novel procedure which involves complexing retrovirus to a dense and particulate substrate followed by a short low-speed centrifugation. The study reported here uses heat-killed, formaldehyde fixed Staphylococcus aureus (Pansorbin) absorbed to PG13 derived retrovirus. This complex was then used to harvest retrovirus from packaging cell supernatant: centrifugation and washing of this complex allows the retrovirus to be both purified and concentrated. This procedure increases the effective titre of retrovirus by up to 7500-fold after an only 200-fold reduction in volume. The affinity of Pansorbin for retrovirus allows concentration regardless of its encoded genes and makes this protocol applicable to other popular packaging cells and envelope proteins. Possible explanations for the marked increase in titre of concentrated virus and the mechanism governing the complexing of retrovirus to Pansorbin are discussed.

Polymorphisms of the cell surface receptor control mouse susceptibilities to xenotropic and polytropic leukemia viruses

The differential susceptibilities of mouse strains to xenotropic and polytropic murine leukemia viruses (X-MLVs and P-MLVs, respectively) are poorly understood but may involve multiple mechanisms. Recent evidence has demonstrated that these viruses use a common cell surface receptor (the X-receptor) for infection of human cells. We describe the properties of X-receptor cDNAs with distinct sequences cloned from five laboratory and wild strains of mice and from hamsters and minks. Expression of these cDNAs in resistant cells conferred susceptibilities to the same viruses that naturally infect the animals from which the cDNAs were derived. Thus, a laboratory mouse (NIH Swiss) X-receptor conferred susceptibility to P-MLVs but not to X-MLVs, whereas those from humans, minks, and several wild mice (Mus dunni, SC-1 cells, and Mus spretus) mediated infections by both X-MLVs and P-MLVs. In contrast, X-receptors from the resistant mouse strain Mus castaneus and from hamsters were inactive as viral receptors. These results suggest that X-receptor polymorphisms are a primary cause of resistances of mice to members of the X-MLV/P-MLV family of retroviruses and are responsible for the xenotropism of X-MLVs in laboratory mice. By site-directed mutagenesis, we substituted sequences between the X-receptors of M. dunni and NIH Swiss mice. The NIH Swiss protein contains two key differences (K500E in presumptive extracellular loop 3 [ECL 3] and a T582 deletion in ECL 4) that are both required to block X-MLV infections. Accordingly, a single inverse mutation in the NIH Swiss protein conferred X-MLV susceptibility. Furthermore, expression of an X-MLV envelope glycoprotein in Chinese hamster ovary cells interfered efficiently with X-MLV and P-MLV infections mediated by X-receptors that contained K500 and/or T582 but had no effect on P-MLV infections mediated by X-receptors that lacked these amino acids. In contrast, moderate expression of a P-MLV (MCF247) envelope glycoprotein did not cause substantial interference, suggesting that X-MLV and P-MLV glycoproteins interfere nonreciprocally with X-receptor-mediated infections. We conclude that P-MLVs have become adapted to utilize X-receptors that lack K500 and T582. A penalty for this adaptation is a reduced ability to interfere with superinfection. Because failure of interference is a hallmark of several exceptionally pathogenic retroviruses, we propose that it contributes to P-MLV-induced diseases.

A cell line-based neutralization assay for primary human immunodeficiency virus type 1 isolates that use either the CCR5 or the CXCR4 coreceptor

We describe here a cell line-based assay for the evaluation of human immunodeficiency virus type 1 (HIV-1) neutralization. The assay is based on CEM.NKR cells, transfected to express the HIV-1 coreceptor CCR5 to supplement the endogenous expression of CD4 and the CXCR4 coreceptor. The resulting CEM.NKR-CCR5 cells efficiently replicate primary HIV-1 isolates of both R5 and X4 phenotypes. A comparison of the CEM.NKR-CCR5 cells with mitogen-activated peripheral blood mononuclear cells (PBMC) in neutralization assays with sera from HIV-1-infected individuals or specific anti-HIV-1 monoclonal antibodies shows that the sensitivity of HIV-1 neutralization is similar in the two cell types. The CEM.NKR-CCR5 cell assay, however, is more convenient to perform and eliminates the donor-to-donor variation in HIV-1 replication efficiency, which is one of the principal drawbacks of the PBMC-based neutralization assay. We suggest that this new assay is suitable for the general measurement of HIV-1 neutralization by antibodies.

Improved uptake and retention of lipophilic prodrug to improve treatment of HIV

Dideoxynucleosides currently in use for anti-HIV therapy have been found to be inefficient in passing through the blood-brain barrier to enter and maintain therapeutic drug levels in brain, a very significant reservoir of HIV. The low bioavailability of these drugs combined with the bone marrow toxicity of AZT (3'-azido, 3'-deoxythymidine, Zidovudine), resulting in anemia and leukopenia, pancreatitis with ddI (2',3'-dideoxyinosine, Didanosine) and painful peripheral neuropathy in case of ddC (2',3-dideoxycytosine, Zalcitabine) are the limiting factors in their use. In addition, the emergence of strains of HIV resistant to AZT, the most commonly used drug, further restricts its use. Thus the control of AIDS and its complications, needs special therapeutic approaches to combat the disease. In order to overcome these limitations, AZT and ddI have been synthesized as ester-linked ceramide- and phosphatidylcholine-linked prodrugs possessing therapeutic attributes lacking in the parent compounds. There is greater uptake and longer retention of these prodrugs in NIH/3T3 cells in vitro. Pretreatment with our prodrugs blocked infection of these cells by Moloney murine leukemia virus (M-MuLV) for an extended period, which the parent drugs failed to do. When human CD4+ HeLa cells were continuously exposed to the AZT prodrug, subsequent infection of these cells by HIV was blocked. Similar results were obtained with NIH/3T3 cells exposed to M-MuLV. AE(6)C, a prodrug of AZT linked to ceramide via a cleavable ester bond and a six carbon linker, was less toxic to both mouse and human bone marrow progenitor cells than free AZT. Most significantly, the prodrugs concentration was greater and the retention longer, in well known sanctuaries for HIV, such as the brain, testes and thymus.

Antitumor effects of interleukin-2 gene-modified fibroblasts in an orthotopic colon cancer model

We transduced the interleukin-2 (IL-2) gene into murine fibroblasts BALBCL7 or murine colon cancer CT26 using a retroviral vector. BALBCL7 transduced with IL-2 gene secreted 748 pg/ml of IL-2, whereas IL-2 gene-modified CT26 secreted 1,167 pg/ml of IL-2 (48 h incubation, 1x10(6)/ml). Then, we inoculated gene-modified BALBCL7 and/or CT26 cells into BALB/c female mice, and observed the tumor growth. The tumor growth was inhibited in mice inoculated with parental CT26 plus IL-2 gene-modified BALBCL7, compared with that in mice given parental CT26 alone (P<0.01). Moreover, we investigated the cytotoxic activity of spleen cells derived from mice treated with gene-modified cells, and performed phenotypic analysis of the effector cells. The killer cells derived from mice inoculated with IL-2 gene-modified BALBCL7 plus parental CT26 showed higher cytotoxic activity than those from mice inoculated with CT26 alone. The cytotoxic activity was almost completely blocked by anti-CD8 antibody (Ab), and partially blocked by anti-asialo GM1 Ab. Next, we inoculated CT26 tumor tissue into murine cecum orthotopically, and treated the animals with gene-modified BALBCL7 plus parental CT26. The tumor size in the cecum was significantly decreased, compared with parental CT26 alone (P<0.01).

Neural stem cells as engraftable packaging lines can mediate gene delivery to microglia: evidence from studying retroviral env-related neurodegeneration

The induction of spongiform myeloencephalopathy by murine leukemia viruses is mediated primarily by infection of central nervous system (CNS) microglia. In this regard, we have previously shown that CasBrE-induced disease requires late, rather than early, virus replication events in microglial cells (W. P. Lynch et al., J. Virol. 70:8896-8907, 1996). Furthermore, neurodegeneration requires the presence of unique sequences within the viral env gene. Thus, the neurodegeneration-inducing events could result from microglial expression of retroviral envelope protein alone or from the interaction of envelope protein with other viral structural proteins in the virus assembly and maturation process. To distinguish between these possible mechanisms of disease induction, we engineered the engraftable neural stem cell line C17-2 into packaging/producer cells in order to deliver the neurovirulent CasBrE env gene to endogenous CNS cells. This strategy resulted in significant CasBrE env expression within CNS microglia without the appearance of replication competent virus. CasBrE envelope expression within microglia was accompanied by increased expression of activation markers F4/80 and Mac-1 (CD11b) but failed to induce spongiform neurodegenerative changes. These results suggest that envelope expression alone within microglia is not sufficient to induce neurodegeneration. Rather, microglia-mediated disease appears to require neurovirulent Env protein interaction with other viral proteins during assembly or maturation. More broadly, the results presented here prove the efficacy of a novel method by which neural stem cell biology may be harnessed for genetically manipulating the CNS, not only for studying neurodegeneration but also as a paradigm for the disseminated distribution of retroviral vector-transduced genes.

Large-scale manufacturing of safe and efficient retrovirus packaging lines for use in immunotherapy protocols

BACKGROUND

The use of gene modified T lymphocytes for immunotherapy in a cancer or AIDS clinical trial requires an efficient, safe ex vivo method for modification of these cells at manufacturing scale. Since retroviruses have been shown to be a moderately effective means of stably integrating therapeutic genes into T lymphocytes, we wanted to create packaging and producer cell lines that would produce replication competent retrovirus (RCR)-free supernatants, at large scale (> 200 l), and transduce with high efficiency.

METHODS

cDNA expression plasmids containing only coding sequences for gagpol or env were built and sequentially transfected into human 293 cells. Packaging and producer clones were characterized for stability, titer and RCR. A producer clone delivering chimeric immune receptors was scaled-up and supernatants used to transduce patient T lymphocytes for clinical studies. PCR and RT-PCR assays were utilized to evaluate the transmission of HERV-H sequences. Relative infectivity of producer clones pseudotyped with different envelopes was determined by transduction and RT assays.

RESULTS

RCR-free, human 293 split-genome packaging lines, pseudotyped with amphotropic, xenotropic, or 10A1 envelopes, were created. A CC49 zeta producer clone was scaled-up to 5 x 54 l lots and supernatants used to safely and efficiently transduce patient T lymphocytes with minimal ex vivo manipulation. While 293 cells express HERV-H mRNA, the transmission frequency in our packaging clones was less than 1 HERV-H sequence per 5 x 10(5) proviral integrations. Additionally, 10A1 and xenotropic packaging lines had higher infectivities than the amphotropic clone.

CONCLUSION

These packaging lines represent the safest configuration for the large-scale production of retroviral vectors, and are capable of producing high titer, RCR-free retroviral vector for large scale clinical use. While all three clones efficiently transduce human T lymphocytes, the 10A1 clone has the highest infectivity. These packaging cell lines will be valuable for use in human gene therapy protocols.

The Fanconi anemia group C gene product modulates apoptotic responses to tumor necrosis factor-alpha and Fas ligand but does not suppress expression of receptors of the tumor necrosis factor receptor superfamily

Exposure of hematopoietic progenitor cells (HPC) from mice and humans with Fanconi anemia group C (FAC) to interferon-gamma (IFN-gamma) or tumor necrosis factor-alpha (TNF-alpha) at doses too low to inhibit growth of normal HPC induces profound apoptotic responses. Because the IFN-gamma hypersensitivity of cells lacking the FAC protein is mediated, in part, through priming of the Fas pathway, and because several other members of this family are capable of inducing apoptosis either alone or in concert with each other, we tested the hypothesis that IFN-gamma induces increased expression of members of the TNF receptor (TNFR) superfamily in cells nullizygous for the FAC gene. Using isogenic human Epstein-Barr virus-transformed lymphoblast cell lines and c-kit+ bone marrow cells from mice with inactivating mutations of the FAC locus, we quantified mRNA levels by reverse transcriptase polymerase chain reaction and surface expression of the gene products by flow cytometry of TNFR1, TNFR2, Fas, CD30, CD40, and nerve growth factor receptor. We found that neither constitutive nor IFN-gamma-induced expression of these receptors was influenced by the absence of a functional FAC gene product, and expression of these receptors was not suppressed in nullizygous cells complemented with the normal FAC cDNA. We conclude that, although exaggerated apoptotic responses in FAC-deficient cells are at least partially mediated through activation of members of the TNFR superfamily, the normal FAC protein does not function as a direct suppressor of this family of molecules and inactivation of FAC does not augment expression of these proteins.

Involvement of H-ras in erythroid differentiation of TF1 and human umbilical cord blood CD34 cells

To investigate the role of the ras gene in erythroid differentiation, a human erythroleukemic cell line, TF1, was transduced with a selectable retroviral vector carrying a mammalian wild type H-ras gene or a cytoplasmic dominant negative RAS1 gene. Transduction of TF1 cells with the wild type H-ras gene resulted in changes of cell types and up-regulation of erythroid-specific gene expression similar to that seen in differentiating erythroid cells. The number of red blood cell containing colonies derived from TF1 cells transduced with wild type H-ras cDNA was significantly increased and the cells in the colonies were more hemoglobinized as estimated by a deeper red color compared to those colony cells from mock or dominant negative RAS1 gene transduced TF1 cells, suggesting increased erythroid differentiation of TF1 cells after transduction of wild type H-ras in vitro. The mRNA levels of beta- and gamma-, but not alpha-, globin genes were significantly higher in H-ras transduced TF1 cells than those in TF1 cells transduced with mock or dominant negative RAS1 gene. Moreover, a 4kb pre-mRNA of the Erythropoietin receptor (EpoR) was highly expressed only in H-ras transduced TF1 cells. Additionally, human umbilical cord blood (CB) CD34 cells which are highly enriched for hematopoietic stem/progenitor cells were transduced with the same retroviral vectors to evaluate in normal primary cells the activities of H-ras in erythroid differentiation. Increased numbers of erythroid cell containing colonies (BFU-E and CFU-GEMM) were observed in CD34 cells transduced with the H-ras cDNA, compared to that from mock transduced cells. These data suggest a possible role for ras in erythroid differentiation.

Origin and rapid evolution of a novel murine erythroleukemia virus of the spleen focus-forming virus family

The Friend spleen focus-forming virus (SFFV) env gene encodes a glycoprotein with apparent Mr of 55,000 that binds to erythropoietin receptors (EpoR) to stimulate erythroblastosis. A retroviral vector that does not encode any Env glycoprotein was packaged into retroviral particles and was coinjected into mice in the presence of a nonpathogenic helper virus. Although most mice remained healthy, one mouse developed splenomegaly and polycythemia at 67 days; the virus from this mouse reproducibly caused the same symptoms in secondary recipients by 2 to 3 weeks postinfection. This disease, which was characterized by extramedullary erythropoietin-independent erythropoiesis in the spleens and livers, was also reproduced in long-term bone marrow cultures. Viruses from the diseased primary mouse and from secondary recipients converted an erythropoietin-dependent cell line (BaF3/EpoR) into factor-independent derivatives but had no effect on the interleukin-3-dependent parental BaF3 cells. Most of these factor-independent cell clones contained a major Env-related glycoprotein with an Mr of 60,000. During further in vivo passaging, a virus that encodes an Mr-55,000 glycoprotein became predominant. Sequence analysis indicated that the ultimate virus is a new SFFV that encodes a glycoprotein of 410 amino acids with the hallmark features of classical gp55s. Our results suggest that SFFV-related viruses can form in mice by recombination of retroviruses with genomic and helper virus sequences and that these novel viruses then evolve to become increasingly pathogenic.

Effects of CCR5 and CD4 cell surface concentrations on infections by macrophagetropic isolates of human immunodeficiency virus type 1

It has been proposed that changes in cell surface concentrations of coreceptors may control infections by human immunodeficiency virus type 1 (HIV-1), but the mechanisms of coreceptor function and the concentration dependencies of their activities are unknown. To study these issues and to generate stable clones of adherent cells able to efficiently titer diverse isolates of HIV-1, we generated two panels of HeLa-CD4/CCR5 cells in which individual clones express either large or small quantities of CD4 and distinct amounts of CCR5. The panels were made by transducing parental HeLa-CD4 cells with the retroviral vector SFF-CCR5. Derivative clones expressed a wide range of CCR5 quantities which were between 7.0 x 10(2) and 1.3 x 10(5) molecules/cell as measured by binding antibodies specific for CCR5 and the chemokine [125I]MIP1beta. CCR5 was mobile in the membranes, as indicated by antibody-induced patching. In cells with a large amount of CD4, an unexpectedly low trace of CCR5 (between 7 x 10(2) and 2.0 x 10(3) molecules/cell) was sufficient for maximal susceptibility to all tested HIV-1, including primary patient macrophagetropic and T-cell-tropic isolates. Indeed, the titers as indicated by immunoperoxidase staining of infected foci were as high as the tissue culture infectious doses measured in human peripheral blood mononuclear cells. In contrast, cells with a small amount of CD4 required a much larger quantity of CCR5 for maximal infection by macrophagetropic HIV-1 (ca. 1.0 x 10(4) to 2.0 x 10(4) molecules/cell). Cells that expressed low and high amounts of CD4 were infected with equal efficiencies when CCR5 concentrations were above threshold levels for maximal infection. Our results suggest that the concentrations of CD4 and CCR5 required for efficient infections by macrophagetropic HIV-1 are interdependent and that the requirements for each are increased when the other component is present in a limiting amount. We conclude that CD4 and CCR5 directly or indirectly interact in a concentration-dependent manner within a pathway that is essential for infection by macrophagetropic HIV-1. In addition, our results suggest that multivalent virus-receptor bonds and diffusion in the membrane contribute to HIV-1 infections.

Long-term reduction of serum bilirubin levels in Gunn rats by retroviral gene transfer in vivo

Conjugation with glucuronic acid, mediated by bilirubin-uridinediphosphoglucuronate glucuronosyltransferase (bilirubin-UGT), is essential for efficient biliary excretion of bilirubin. Inherited absence of this enzyme activity results in the potentially lethal Crigler-Najjar syndrome type I in humans and lifelong hyperbilirubinemia in Gunn rats. To develop a gene therapy for bilirubin-UGT deficiency, we constructed a high-titer replication-deficient amphotropic recombinant retrovirus (MFG-S hB-UGT1) capable of transferring the gene encoding bilirubin-UGT1, the principal bilirubin-UGT isoform in human liver. To stimulate hepatocyte proliferation, Gunn rats were subjected to 66% hepatectomy. After 24 hours, the portal vein, the hepatic artery, and the inferior vena cava above and below the hepatic vein were clamped, and the portal vein and the isolated segment of the vena cava were cannulated. The liver was perfused with the MFG-S hB-UGT1 preparation through the portal vein at 5 ml/min for 10 minutes, then circulation was restored. Control rat livers were perfused with a recombinant retrovirus expressing Escherichia coli beta-galactosidase. In MFG-S hB-UGT1-perfused rats, but not in controls, expression of human bilirubin-UGT1 was shown by immunotransblotting, bilirubin-UGT assay of liver homogenates, and biliary excretion of bilirubin diglucuronide and monoglucuronide. Mean serum bilirubin levels decreased by 20% to 25% in 3 weeks and remained at that level throughout the study period (18 months). This is the first report of long-term amelioration of inherited jaundice by retrovirus-directed gene therapy in an animal model for Crigler-Najjar syndrome.

DNA Cross-Linker–Induced G2/M Arrest in Group C Fanconi Anemia Lymphoblasts Reflects Normal Checkpoint Function

Cells from individuals with Fanconi anemia (FA) arrest excessively in the G2/M cell cycle compartment after exposure to low doses of DNA cross-linking agents. The relationship of this abnormality to the fundamental genetic defect in such cells is unknown, but many investigators have speculated that the various FA genes directly regulate cell cycle checkpoints. We tested the hypothesis that the protein encoded by the FA group C complementing gene (FAC) functions to control a cell cycle checkpoint and that cells from group C patients (FA[C]) have abnormalities of cell cycle regulation directly related to the genetic mutation. We found that retroviral transduction of FA(C) lymphoblasts with wild-type FAC cDNA resulted in normalization of the cell cycle response to low-dose mitomycin C (MMC). However, when DNA damage was quantified in terms of cytogenetic damage or cellular cytotoxicity, we found similar degrees of G2/M arrest in response to equitoxic amounts of MMC in FA(C) cells as well as in normal lymphoblasts. Similar results were obtained using isogenic pairs of uncorrected, FAC- or mock-corrected (neo only) FA(C) cell lines. To test the function of other checkpoints we examined the effects of hydroxyurea (HU) and ionizing radiation on cell cycle kinetics of FA(C) and normal lymphoblasts as well as with isogenic pairs of uncorrected, FAC-corrected, or mock-corrected FA(C) cell lines. In all cases the cell cycle response of FA(C) and normal lymphoblasts to these two agents were identical. Based on these studies we conclude that the aberrant G2/M arrest that typifies the response of FA(C) cells to low doses of cross-linking agents does not represent an abnormal cell cycle response but instead represents a normal cellular response to the excessive DNA damage that results in FA(C) cells following exposure to low doses of cross-linking agents.

DNA Cross-Linker–Induced G2/M Arrest in Group C Fanconi Anemia Lymphoblasts Reflects Normal Checkpoint Function

Cells from individuals with Fanconi anemia (FA) arrest excessively in the G2/M cell cycle compartment after exposure to low doses of DNA cross-linking agents. The relationship of this abnormality to the fundamental genetic defect in such cells is unknown, but many investigators have speculated that the various FA genes directly regulate cell cycle checkpoints. We tested the hypothesis that the protein encoded by the FA group C complementing gene (FAC) functions to control a cell cycle checkpoint and that cells from group C patients (FA[C]) have abnormalities of cell cycle regulation directly related to the genetic mutation. We found that retroviral transduction of FA(C) lymphoblasts with wild-type FAC cDNA resulted in normalization of the cell cycle response to low-dose mitomycin C (MMC). However, when DNA damage was quantified in terms of cytogenetic damage or cellular cytotoxicity, we found similar degrees of G2/M arrest in response to equitoxic amounts of MMC in FA(C) cells as well as in normal lymphoblasts. Similar results were obtained using isogenic pairs of uncorrected, FAC- or mock-corrected (neo only) FA(C) cell lines. To test the function of other checkpoints we examined the effects of hydroxyurea (HU) and ionizing radiation on cell cycle kinetics of FA(C) and normal lymphoblasts as well as with isogenic pairs of uncorrected, FAC-corrected, or mock-corrected FA(C) cell lines. In all cases the cell cycle response of FA(C) and normal lymphoblasts to these two agents were identical. Based on these studies we conclude that the aberrant G2/M arrest that typifies the response of FA(C) cells to low doses of cross-linking agents does not represent an abnormal cell cycle response but instead represents a normal cellular response to the excessive DNA damage that results in FA(C) cells following exposure to low doses of cross-linking agents.

Inactivation of the Fanconi Anemia Group C Gene Augments Interferon-γ–Induced Apoptotic Responses in Hematopoietic Cells

Hematopoietic progenitor cells (HPC) from mice nullizygous at the Fanconi anemia (FA) group C locus (FAC −/−) are hypersensitive to the mitotic inhibitory effects of interferon (IFN-γ). We tested the hypothesis that HPC from the bone marrow of Fanconi group C children are similarly hypersensitive and that the fas pathway is involved in affecting programmed cell death in response to low doses of IFN-γ. In normal human and murine HPC, IFN-γ primed the fas pathway and induced both fas and interferon response factor-1 (IRF-1) gene expression. These IFN-γ-induced apoptotic responses in HPC from the marrow of a child with FA of the C group (FA-C) and in FAC −/− mice occurred at significantly lower IFN doses (by an order of magnitude) than did the apoptotic responses of normal HPC. Treatment of FA-C CD34+ cells with low doses of recombinant IFN-γ, inhibited growth of colony forming unit granulocyte-macrophage and burst-forming unit erythroid, while treatment with blocking antibodies to fas augmented clonal growth and abrogated the clonal inhibitory effect of IFN-γ. Transfer of the normal FAC gene into FA-C B-cell lines prevented mitomycin C–induced apoptosis, but did not suppress fas expression or inhibit the primed fas pathway. However, the kinetics of Stat1-phosphate decay in IFN-γ–treated cells was prolonged in mutant cells and was normalized by transduction of the normal FAC gene. Therefore, the normal FAC protein serves, in part, to modulate IFN-γ signals. HPC bearing inactivating mutations of FAC fail to normally modulate IFN-γ signals and, as a result, undergo apoptosis executed through the fas pathway.

Inactivation of the Fanconi Anemia Group C Gene Augments Interferon-γ–Induced Apoptotic Responses in Hematopoietic Cells

Hematopoietic progenitor cells (HPC) from mice nullizygous at the Fanconi anemia (FA) group C locus (FAC −/−) are hypersensitive to the mitotic inhibitory effects of interferon (IFN-γ). We tested the hypothesis that HPC from the bone marrow of Fanconi group C children are similarly hypersensitive and that the fas pathway is involved in affecting programmed cell death in response to low doses of IFN-γ. In normal human and murine HPC, IFN-γ primed the fas pathway and induced both fas and interferon response factor-1 (IRF-1) gene expression. These IFN-γ-induced apoptotic responses in HPC from the marrow of a child with FA of the C group (FA-C) and in FAC −/− mice occurred at significantly lower IFN doses (by an order of magnitude) than did the apoptotic responses of normal HPC. Treatment of FA-C CD34+ cells with low doses of recombinant IFN-γ, inhibited growth of colony forming unit granulocyte-macrophage and burst-forming unit erythroid, while treatment with blocking antibodies to fas augmented clonal growth and abrogated the clonal inhibitory effect of IFN-γ. Transfer of the normal FAC gene into FA-C B-cell lines prevented mitomycin C–induced apoptosis, but did not suppress fas expression or inhibit the primed fas pathway. However, the kinetics of Stat1-phosphate decay in IFN-γ–treated cells was prolonged in mutant cells and was normalized by transduction of the normal FAC gene. Therefore, the normal FAC protein serves, in part, to modulate IFN-γ signals. HPC bearing inactivating mutations of FAC fail to normally modulate IFN-γ signals and, as a result, undergo apoptosis executed through the fas pathway.

CD4, CXCR-4, and CCR-5 dependencies for infections by primary patient and laboratory-adapted isolates of human immunodeficiency virus type 1

We have used a focal infectivity method to quantitatively analyze the CD4, CXCR-4, and CCR-5 dependencies for infections by diverse primary patient (PR) and laboratory-adapted (LA) isolates of human immunodeficiency virus type 1 (HIV-1). Infectivities of T-cell-tropic viruses were analyzed in a panel of HeLa-CD4 cell clones that have distinct quantities of CD4 and in human astroglioma U87MG-CD4 cells that express a large quantity of CD4 and become highly susceptible to infection after transfection with a CXCR-4 expression vector. The latter analysis indicated that PR as well as LA T-cell-tropic viruses efficiently employ CXCR-4 as a coreceptor in an optimal human cell line that contains abundant CD4. Previous uncertainties regarding coreceptor usage by PR T-cell-tropic HIV-1 isolates may therefore have derived from the assay conditions. As reported previously, unrelated LA and PR T-cell-tropic HIV-1 isolates differ in infectivities for the HeLa-CD4 clonal panel, with LA viruses infecting all clones equally and PR viruses infecting the clones in proportion to cellular CD4 quantities (D. Kabat, S. L. Kozak, K. Wherly, and B. Chesebro, J. Virol. 68:2570-2577, 1994). To analyze the basis for this difference, we used the HeLa-CD4 panel to compare a molecularly cloned T-cell-tropic PR virus (ELI1) with six of its variants that grow to different extents in CD4-positive leukemic cell lines and that differ only at specific positions in their gp120 and gp41 envelope glycoproteins. All mutations in gp120 or gp41 that contributed to laboratory adaptation preferentially enhanced infectivity for cells that had little CD4 and thereby decreased the CD4 dependencies of the infections. There was a close correlation between abilities of T-cell-tropic ELI viruses to grow in an expanded repertoire of leukemic cell lines, the reduced CD4 dependencies of their infections of the HeLa-CD4 panel, and their sensitivities to inactivation by soluble CD4 (sCD4). Since all of the ELI viruses can efficiently use CXCR-4 as a coreceptor, we conclude that an increase in viral affinity for CD4 rather than a switch in coreceptor specificity is principally responsible for laboratory adaption of T-cell-tropic HIV-1. Syncytium-inducing activities of the ELI viruses, especially when analyzed on cells with low amounts of CD4, were also highly correlated with their laboratory-adapted properties. Results with macrophage-tropic HIV-1 were strikingly different in both coreceptor and CD4 dependencies. When assayed in HeLa-CD4 cells transfected with an expression vector for CCR-5, macrophage-tropic HIV-1 isolates that had been molecularly cloned shortly after removal from patients were equally infectious for cells that had low or high CD4 quantities. Moreover, despite their substantial infectivities for cells that had only a trace of CD4, macrophage-tropic isolates were relatively resistant to inactivation by sCD4. We conclude that T-cell-tropic PR viruses bind weakly to CD4 and preferentially infect cells that coexpress CXCR-4 and large amounts of CD4. Their laboratory adaptation involves corresponding increases in affinities for CD4 and in abilities to infect cells that have relatively little CD4. In contrast, macrophage-tropic HIV-1 appears to interact weakly with CD4 although it can infect cells that coexpress CCR-5 and small quantities of CD4. We propose that cooperative binding of macrophage-tropic HIV-1 onto CCR-5 and CD4 may enhance virus adsorption and infectivity for cells that have only a trace of CD4.

Exceptional fusogenicity of Chinese hamster ovary cells with murine retroviruses suggests roles for cellular factor(s) and receptor clusters in the membrane fusion process

Chinese hamster ovary (CHO) cells are naturally resistant to infection by amphotropic and ecotropic murine retroviruses, but they become susceptible after expressing corresponding receptors rRAM-1 and mCAT-1, respectively, and they then form abundant syncytia when exposed to these viruses. The fusogenic activities of CHO cell clones increase much more strongly with levels of receptor expression than do their susceptibilities to infection, suggesting that the assembly of receptor clusters may limit syncytium formation. However, other cell lines are not fusogenic, even if they express larger amounts of receptors. Our results suggest that a factor that is relatively abundant or active in CHO cells may functionally interact with rRAM-1 and mCAT-1 in a pathway that enables receptor-bearing membranes to fuse with membranes that contain viral envelope glycoproteins. In the case of CHO/rRAM-1 cells, syncytia form at foci of amphotropic 4070A virus infection by fusion-from-within of infected with uninfected cells. This fusogenic propensity is a sole property of the uninfected CHO/rRAM-1 cells, which fuse in cocultures with any cells infected with 4070A virus. With CHO/mCAT-1 cells, fusogenicity is even greater and involves fusion-from-without by ecotropic virion particles. In contrast to infection, which behaves as expected for a process limited by ecotropic virus attachment to single receptors, fusion-from-without increases dramatically for cells that express the highest levels of mCAT-1. We propose that infection and syncytium formation are limited at distinct steps of a common pathway that requires virus binding to a single receptor, assembly of multivalent virus-receptor complexes, structural changes in viral envelope glycoproteins, and membrane fusion. The limiting step in syncytium formation is a cellular process that depends on receptor clustering and is relatively active in CHO cells.

Inhibitory effects of interleukin 12 on retroviral gene transduction into CD34 cord blood myeloid progenitors mediated by induction of tumor necrosis factor-alpha

Interleukin 12 (IL-12), a heterodimeric cytokine with potent biologic activity, was evaluated for effects on retroviral-mediated gene transduction into human myeloid progenitor cells in vitro. Cord blood CD34 cells were prestimulated with Steel factor (SLF), IL-3, GM-CSF, and erythropoietin (Epo) in the presence and absence of 5-80 ng/ml IL-12 for 40 hr in suspension culture prior to gene transduction using viral supernatant collected from a packaging cell line containing the pLNL6 vector encoding Neo sequences. After gene transduction, cells were assayed for colony formation stimulated by Epo, GM-CSF, IL-3, and SLF, and gene transduction efficiency was determined by the percentage of G418 resistant (R) colonies and confirmed by PCR analysis. IL-12 dose-dependently inhibited retroviral-mediated gene transduction into human cord blood CD34 granulocyte-macrophage (CFU-GM) and erythroid (BFU-E) progenitors. These suppressive effects could be neutralized by incubation of IL-12 with polyclonal antihuman IL-12. IL-12 had no inhibitory effects directly on colony formation. To understand the possible mechanisms for this suppression, ELISA assays were used to detect the release of interferon (IFN)-gamma and tumor necrosis factor (TNF)-alpha, which could potentially have been induced by IL-12 from CD34 cells. TNF-alpha protein release was significantly increased in CD34 cells incubated with IL-12. No detectable levels of IFN-gamma were noted. Anti-TNF-alpha, but not anti-IFN-gamma, blocked the inhibitory effects of IL-12 on gene transduction. Moreover, TNF-alpha, but not IFN-gamma, suppressed gene transfer to the same degree as IL-12. No change of amphotropic receptor mRNA expression was noted by Northern blot analysis in cells treated with or without IL-12. The results suggest that the suppressive effects of IL-12 on retroviral gene transduction are, at least in part, mediated by IL-12 induction of the release of TNF-alpha.

Self-contained, tetracycline-regulated retroviral vector system for gene delivery to mammalian cells

Retroviral vectors that contain the tetracycline-inducible (Tet) system were developed. The two components of the Tet system were organized within the vectors in a manner that stringently maintains tetracycline-dependent regulation. Regulated expression of an indicator gene inserted into the retroviral vectors was examined in several different cell types. In infected NIH 3T3 cells, levels of induction in the absence of tetracycline were observed to be as much as 336-fold higher than levels in the presence of tetracycline, which were extremely low. Tetracycline-dependent regulation was observed in all other transduced cell types and ranged from 24- to 127-fold. The generation of retroviral vectors containing regulatory elements that allow for the regulated expression of heterologous genes and that have the ability to infect virtually all dividing target cells should greatly facilitate the biochemical and genetic examination of a broad range of genes. Moreover, these inducible retroviral vectors should prove useful in gene therapy applications.

High-titer packaging cells producing recombinant retroviruses resistant to human serum

Novel retroviral protein expression constructs were designed to retain minimal retroviral sequences and to express dominant selectable markers by reinitiation of translation after expression of the viral genes. HT1080 cells were selected as producer cells for their ability to release high-titer viruses that are resistant to inactivation by human serum. Two HT1080-based packaging cell lines which produce Moloney murine leukemia virus cores with envelope glycoproteins of either amphotropic murine leukemia virus (FLYA13 line) or cat endogenous virus RD114 (FLYRD18 line) are described. Direct comparison with previous retroviral packaging systems indicated that 100-fold-higher titers of helper-free recombinant viruses were released by the FLYA13 and FLYRD18 lines.

The envelope glycoprotein of an amphotropic murine retrovirus binds specifically to the cellular receptor/phosphate transporter of susceptible species

A rat cDNA (rRam-1), which was cloned on the basis that it enables Chinese hamster ovary (CHO) cells to be infected by amphotropic host range murine retroviruses, was recently found to encode a widely expressed Na(+)-phosphate symporter (M. P. Kavanaugh, D. G. Miller, W. Zhang, W. Law, S. L. Kozak, D. Kabat, and A. D. Miller, Proc. Natl. Acad. Sci. USA 91:7071-7075, 1994). CHO cells express the hamster homolog of Ram-1 but are resistant to amphotropic retroviruses. Although the amphotropic envelope glycoprotein gp70 bound weakly onto control CHO cells, CHO/rRam-1 cells had novel high-affinity binding sites, and the resulting strongly adsorbed gp70 was only slowly removed from cell surfaces, with a half-life of greater than 6 h. CHO/rRam-1 cells were also specifically and efficiently killed by exposure to amphotropic gp70 followed by antiserum to gp70 in the presence of complement. Infection with an appropriately pseudotyped form of amphotropic retrovirus 4070A did not perturb control CHO cells or inhibit their phosphate transport. In contrast, 4070A infection of CHO/rRam-1 cells caused major alterations including cell-cell fusions, a specific 40% down-modulation of the rRam-1 component of phosphate transport, and complete interference to super-infection by amphotropic viruses. The 4070A virus-infected CHO/rRam-1 cells retained a substantial cell surface pool of rRam-1 that functioned as a phosphate transporter but not as a viral receptor. We conclude that amphotropic gp70 binds more strongly to rRam-1 than to the homologous hamster protein and that this stable attachment is necessary for infection, interference, membrane fusion, and pathogenesis.

Involvement of SH2-containing phosphotyrosine phosphatase Syp in erythropoietin receptor signal transduction pathways

Erythropoietin (Epo) regulates the proliferation and differentiation of erythroid precursors. The phosphorylation of proteins at tyrosine residues is critical in the growth signaling induced by Epo. This mechanism is regulated by the activities of both protein-tyrosine kinases and protein tyrosine phosphatases. The discovery of phosphotyrosine phosphatases that contain SH2 domains suggests roles for these molecules in growth factor signaling pathways. We found that Syp, a phosphotyrosine phosphatase, widely expressed in all tissues in mammals became phosphorylated on tyrosine after stimulation with Epo in M07ER cells engineered to express high levels of human EpoR. Syp was complexed with Grb2 in Epo-stimulated M07ER cells. Direct binding between Syp and Grb2 was also observed in vitro. Furthermore, Syp appeared to bind directly to tyrosine-phosphorylated EpoR in M07ER cells. Both NH2-terminal and COOH-terminal SH2 domains of Syp, made as glutathione S-transferase fusion proteins, were able to bind to the tyrosine-phosphorylated EpoR in vitro. These results suggest that Syp may be an important signaling component downstream of the EpoR and may regulate the proliferation and differentiation of hematopoietic cells.

Amplified and tissue-directed expression of retroviral vectors using ping-pong techniques

Ping-pong amplification is an efficient process by which helper-free retrovirions replicate in cocultures of cell lines that package retroviruses into distinct host-range envelopes [11]. Transfection of a retroviral vector DNA into these cocultures results in massive virus production, with potentially endless cross-infection between different types of packaging cells. Because the helper-free virus spreads efficiently throughout the coculture, it is unnecessary to use dominant selectable marker genes, and the retroviral vectors can be simplified and optimized for expressing a single gene of interest. The most efficient ping-pong vector, pSFF, derived from the Friend erythroleukemia virus, has been used for high-level expression of several genes that could not be expressed with commonly employed two-gene retroviral vectors. Contrary to previous claims, problems of vector recombination are not inherent to ping-pong methods. Indeed, the pSFF vector has not formed replication-competent recombinants as shown by stringent assays. Here we review these methods, characterize the ping-pong process using the human erythropoietin gene as a model, and describe a new vector (pSFY) designed for enhanced expression in T lymphocytes. Factors that limit tissue-specific expression are reviewed.

Deletions in one domain of the Friend virus-encoded membrane glycoprotein overcome host range restrictions for erythroleukemia

Although the Friend virus-encoded membrane glycoprotein (gp55) activates erythropoietin receptors (EpoR) to cause erythroblastosis only in certain inbred strains of mice but not in other species, mutant viruses can overcome aspects of mouse resistance. Thus, mice homozygous for the resistance allele of the Fv-2 gene are unaffected by gp55 but are susceptible to mutant glycoproteins that have partial deletions in their ecotropic domains. These and other results have suggested that proteins coded for by polymorphic Fv-2 alleles might directly or indirectly interact with EpoR and that changes in gp55 can overcome this defense. A new viral mutant with an exceptionally large deletion in its ecotropic domain is now also shown to overcome Fv-2rr resistance. In all cases, the glycoproteins that activate EpoR are processed to cell surfaces as disulfide-bonded dimers. To initiate analysis of nonmurine resistances, we expressed human EpoR and mouse EpoR in the interleukin 3-dependent mouse cell line BaF3 and compared the abilities of Friend virus-encoded glycoproteins to convert these cells to growth factor independence. Human EpoR was activated in these cells by erythropoietin but was resistant to gp55. However, human EpoR was efficiently activated in these cells by the same viral mutants that overcome Fv-2rr resistance in mice. By construction and analysis of human-mouse EpoR chimeras, we obtained evidence that the cytosolic domain of human EpoR contributes to its resistance to gp55 and that this resistance is mediated by accessory cellular factors. Aspects of host resistance in both murine and nonmurine species are targeted specifically against the ecotropic domain of gp55.

Bystander tumoricidal effect in the treatment of experimental brain tumors

The retrovirus-mediated transfer of the herpes simplex virus-thymidine kinase (HSV-tk) gene into tumor cells renders them sensitive to the cytocidal effect of the antiviral drug ganciclovir. This method has shown promising results as a treatment for experimental brain tumors. These experiments indicate that a major mechanism for the effectiveness of HSV-tk retroviral gene therapy may be the bystander tumoricidal effect. The bystander effect was hypothesized to explain tumor eradication, given that the efficacy of in vivo gene transfer to tumor cells was less than 100%. We demonstrate, in this report, that the bystander tumoricidal effect is a major contributor to the tumoricidal effect of ganciclovir in cell culture experiments using the mouse K1735 C19 cerebral melanoma line, thereby expanding the observation of the bystander phenomenon to a broader range of tumor types. The bystander effect was studied in vitro by coculturing wild-type C19 melanoma cells with HSV-tk-expressing C19 (C19-STK) cells. A maximal tumoricidal effect was seen when only 1 in 10 tumor cells expressed the HSV-tk gene. This suggests that in effect, 1 tumor cell with the HSV-tk gene, when given ganciclovir, will destroy 10 neighboring or bystander cells. The destruction of bystander cells does not appear to be mediated by a soluble factor(s) released into the media but, rather, requires close cell proximity or cell contact. In addition, HSV-tk-expressing C19 cells can exert an antitumoral effect not only on wild-type C19 cells but also on cells from a variety of different tumor cell lines, including a human glioblastoma multiforme cell line, indicating that the bystander effect is not a cell line-specific phenomenon. Finally, we observed that the bystander tumoricidal effect could be harnessed directly without using retrovirus-producing cells to increase survival in the mouse C19 brain tumor model. The potential implications of our findings in treating human brain tumors are discussed.

Effect of heat on viral protein production and budding in cultured mammalian cells

The life cycle of enveloped viruses is intimately associated with, and influenced by, host cell membrane organization, which is altered by hyperthermia. Hyperthermia-modified Moloney murine leukaemia virus (M-MuLV) release, protein production and intracellular protein processing in a chronically infected cultured murine cell line, C9CL98 (C9). Both 44 degrees C/45 min and 42.8 degrees C/135 min substantially decreased cell-free viral env protein 8-48 h postheating, but virus release and cellular viral protein content increased following 42.8 degrees C/25 min. Proteolytic processing of viral Pr65 gag precursor to p30 gag protein, normally observed within unheated C9 cells, was blocked for at least 8 h after 44 degrees C/45 min. Virus released from heated C9 cells was as infectious to NIH/3T3 cells as was virus from control cells. Cells surviving exposure to 42.8 degrees C/135 min became thermotolerant to decreased virus release from a second heating if delivered 10-48 h after the initial heating. The mechanism by which virus release is blocked after hyperthermia remains to be elucidated.

Microglial infection by a neurovirulent murine retrovirus results in defective processing of envelope protein and intracellular budding of virus particles

The observation of murine retrovirus infection of microglial cells in brain regions expressing spongiform neurodegenerative changes suggests that these cells may play an important role in pathogenesis. To evaluate this potential in vitro, murine microglial cells were infected in mixed glial cultures with the highly neurovirulent murine retrovirus, FrCasE. The microglia were then isolated from the mixed cultures on the basis of their differential adherence and shown to be approximately 98% pure. The infected microglia expressed viral envelope protein at the plasma membrane, while viral budding was primarily intracellular. Evaluation of the viral envelope protein by immunoblotting indicated that the immunoreactive species produced was exclusively a 90-kDa precursor protein. Very little of the envelope protein was associated with particles released from these cells, and viral titers in the culture supernatant were low. Interestingly, these cells were still capable of infecting permissive target cells when seeded as infectious centers. This partially defective infection of microglial cells suggests a potential cellular means by which a neurovirulent retrovirus could disrupt normal microglia and in turn central nervous system motor system functioning.

Liver-directed gene therapy: quantitative evaluation of promoter elements by using in vivo retroviral transduction

Liver-directed gene therapy will be applicable to many inherited diseases. Although various protocols have been devised for in vivo delivery of retrovirus, comparison of hepatocyte transduction frequencies has been difficult due to variations in retroviral titer and a paucity of DNA data. We have previously reported an in vivo rat hepatocyte transduction technique which involves 70% hepatectomy followed 24 hr later by portal vein injection of retrovirus during hepatic in-flow occlusion. In this study, we employed this method and concentrated retroviral preparations to achieve transduction of up to 15% of hepatocytes as determined by a quantitative PCR assay. As an initial step toward identifying promoters which lead to high-level long-term expression of retroviral transduced genes, we used our in vivo delivery system to compare the Moloney murine leukemia virus long terminal repeat (LTR) promoter with the promoter for the large subunit of murine RNA polymerase II (Pol-II). Human alpha 1-antitrypsin (hAAT) was used as the reporter gene to facilitate long-term analysis of expression. Serum hAAT levels were higher for the Pol-II promoter (143 ng/ml) than for the LTR promoter (50 ng/ml). This difference was consistent with the higher transduction frequency observed for the Pol-II-hAAT vector. Although serum hAAT expression was sustained for up to 1 year in six of eight Pol-II-hAAT-transduced rats and three of five LTR-hAAT-transduced rats and was proportional to hAAT mRNA level and proviral DNA frequency, in vivo expression was significantly lower than in transduced tissue culture cells. We conclude that a high frequency of in vivo transduction can be achieved by using retroviral vectors and our rapid transduction protocol, but transduced gene expression remains a serious problem. The quantitative assays described herein will facilitate in vivo comparisons of gene regulatory elements.

Screening retroviral packaging cells for highly efficient virus production by using a combined selection procedure

To facilitate the screening for clones of transfected packaging cells producing a high yield of recombinant retrovirus, we present a fast and simple method for the isolation of overexpressing cells. By this method the efficiency of virus production can generally be enhanced 10- to 100-fold by application of high selection pressure. Cell lines which exhibit titers of up to 10(8) CFU/ml were obtained.

Increased viral titer through concentration of viral harvests from retroviral packaging lines

Dependent on the viral vector and the specific assay used, viral titers produced from commonly used retroviral packaging cell lines have an upper limit in the range of 10(5) to 10(7) infectious units/ml. We have developed a generally applicable method, using hollow-fiber filtration technology, which allows for the concentration of infectious virus derived from packaging lines. This method resulted in a reproducible 10- to 30-fold increase in viral titer and can readily be scaled to accommodate larger input volumes. Over 80% of the input virus is recovered in an infectious form in the concentrate. Concentrated virus containing media was seen to produce higher infection frequencies in Jurkat T cells as compared to unconcentrated virus containing media; however, this was not proportional to the differences in viral titer observed by limiting dilution analysis on NIH-3T3 cells. These results are discussed in relation to the importance of factors other than viral titer in determining transduction frequencies.

A receptor for subgroup A Rous sarcoma virus is related to the low density lipoprotein receptor

Cellular receptors are required for efficient entry of retroviruses into cells. We previously cloned a chicken gene responsible for susceptibility to the retrovirus subgroup A Rous sarcoma virus (RSV(A)). Here we have isolated the quail homolog and generated two alternatively spliced processed genes encoding cellular receptors for RSV(A). Predicted products of the processed genes appear to be small membrane-associated proteins with identical 83 amino acid extracellular domains but different membrane anchors. Within the extracellular domain is a region closely related to the ligand-binding repeat of the low density lipoprotein receptor (LDLR). Expression of either processed gene renders mammalian cells specifically susceptible to RSV(A). Antibodies directed against the receptor block subgroup A infection of avian cells via endogenous receptors and have no effect on entry of other RSV subgroups. Thus, small LDLR-related proteins are cellular receptors for RSV(A).