Gene therapy for human immunodeficiency virus infection: genetic antiviral strategies and targets for intervention

CD4(+) T Cells Modified by the Endoribonuclease MazF Are Safe and Can Persist in SHIV-infected Rhesus Macaques

MazF, an endoribonuclease encoded by Escherichia coli, specifically cleaves the ACA (adenine–cytosine–adenine) sequence of single-stranded RNAs. Conditional expression of MazF under the control of the HIV-1 LTR promoter rendered CD4⁺ T cells resistant to HIV-1 replication without affecting cell growth. To investigate the safety, persistence and efficacy of MazF-modified CD4⁺ T cells in a nonhuman primate model in vivo, rhesus macaques were infected with a pathogenic simian/human immunodeficiency virus (SHIV) and transplanted with autologous MazF-modified CD4⁺ T cells. MazF-modified CD4⁺ T cells were clearly detected throughout the experimental period of more than 6 months. The CD4⁺ T cell count values increased in all four rhesus macaques. Moreover, the transplantation of the MazF-modified CD4⁺ T cells was not immunogenic, and did not elicit cellular or humoral immune responses. These data suggest that the autologous transplantation of MazF-modified CD4⁺ T cells in the presence of SHIV is effective, safe and not immunogenic, indicating that this is an attractive strategy for HIV-1 gene therapy.

The next generation of HIV/AIDS drugs: novel and developmental antiHIV drugs and targets

There are presently 42 million people worldwide living with HIV/AIDS, the majority of which have limited access to antiretrovirals. Even if worldwide penetration was possible, our current chemotherapeutic strategies still suffer from issues of cost, patient compliance, deleterious acute and chronic side effects, emerging single and multidrug resistance, and generalized treatment and economic issues. Even our best antiretroviral therapeutic strategy, highly active antiretroviral therapy (HAART), falls short of completely suppressing HIV replication. Therefore, expansion of current therapeutic options by discovering new antiretrovirals and targets will be critical in the coming years. This review addresses the current status of reverse transcriptase and protease inhibitor development, and summarizes the progress in emerging classes of HIV inhibitors, including entry (T-20, T-1249), coreceptor (SCH-C, SCH-D), integrase (beta-Diketos) and p7 nucleocapsid Zn finger inhibitors (thioesters and PATEs). In addition, the processes of virus entry, PIC transport to the nucleus, HIV interaction with nuclear pores, Tat function, Rev function and virus budding (Tsg101 and ubiquitination) are examined, and proof of concept inhibitors and potential antiviral targets discussed.

In vivo safety and persistence of endoribonuclease gene-transduced CD4+ T cells in cynomolgus macaques for HIV-1 gene therapy model

Background

MazF is an endoribonuclease encoded by Escherichia coli that specifically cleaves the ACA sequence of mRNA. In our previous report, conditional expression of MazF in the HIV-1 LTR rendered CD4+ T lymphocytes resistant to HIV-1 replication. In this study, we examined the in vivo safety and persistence of MazF-transduced cynomolgus macaque CD4+ T cells infused into autologous monkeys.

Methodology/Principal Findings

The in vivo persistence of the gene-modified CD4+ T cells in the peripheral blood was monitored for more than half a year using quantitative real-time PCR and flow cytometry, followed by experimental autopsy in order to examine the safety and distribution pattern of the infused cells in several organs. Although the levels of the MazF-transduced CD4+ T cells gradually decreased in the peripheral blood, they were clearly detected throughout the experimental period. Moreover, the infused cells were detected in the distal lymphoid tissues, such as several lymph nodes and the spleen. Histopathological analyses of tissues revealed that there were no lesions related to the infused gene modified cells. Antibodies against MazF were not detected. These data suggest the safety and the low immunogenicity of MazF-transduced CD4+ T cells. Finally, gene modified cells harvested from the monkey more than half a year post-infusion suppressed the replication of SHIV 89.6P.

Conclusions/Significance

The long-term persistence, safety and continuous HIV replication resistance of the mazF gene-modified CD4+ T cells in the non-human primate model suggests that autologous transplantation of mazF gene-modified cells is an attractive strategy for HIV gene therapy.

A historical sketch of the discovery and development of HIV-1 integrase inhibitors

The long process of HIV-1 integrase inhibitor discovery and development can be attributed to both the complexity of HIV-1 integration and poor 'integration' of these researches into mainstream investigations on antiretroviral therapy in the mid-1990s. Of note, some fungal extracts investigated during this period contain the beta-hydroxyketo group, later recognised to be a key structural requirement for keto-enol acids (also referred to as diketo acids) and other integrase inhibitors. This review reconstructs (in the general context of the history of AIDS research) the principal steps that led to the integrase inhibitors currently in clinical trials, and discusses possible future directions.

Novel lentiviral vectors for human gene therapy

This review summarises an emerging viral vector system for use in human gene therapy - lentiviral vectors. Lentiviral vectors have several advantages over existing viral vectors. They can stably express transgenes in non-dividing cells in vivo without provoking a significant immune response. They can be produced to a high titre and can be pseudotyped with heterologous envelope proteins to confer broad tropism. Although not without safety concerns, the properties of lentiviral vectors makes them an attractive choice for human gene therapy.

Cell type-specific targeting with sindbis pseudotyped lentiviral vectors displaying anti-CCR5 single-chain antibodies

Lentiviral vectors are among the most efficient tools for gene delivery into mammalian cells. A major goal of lentiviral gene delivery systems is to develop vectors that can efficiently target specific cell types. In the present work, we attempt to generate viral particles for targeting gene delivery. We have used CCR5-positive cells as the target for our strategy. Therefore, we developed a novel Sindbis pseudotyped lentiviral vector where the Sindbis receptor binding envelope protein was modified to directly encode a single-chain antibody fragment (scFv) against the CCR5 chemokine receptor. We have generated two chimeric scFv-Sindbis envelopes, varying the length of the peptide linker that connects the heavy chain and light chain of anti-CCR5 scFv. The two chimeric scFv-Sindbis envelopes were successfully incorporated into lentiviral-derived vectors, and the resulting pseudotyped viral particles showed specific targeting to CCR5-expressing cells. However, our data demonstrate that the length of the peptide linker significantly affects the efficiency of infection. Pseudotyped viral particles, which display single-chain antibody fragments with longer peptide linkers, allowed higher titers of infection. The present study can be a model strategy for specific gene delivery mediated by lentiviral vectors pseudotyped with Sindbis envelope displaying scFv that recognizes specific cellular surface proteins. Furthermore, this strategy has the potential to become a powerful approach for targeting gene delivery in anti- HIV gene therapy due to the important role of CCR5 expression in disease progression.

Efficient lentiviral vector-mediated control of HIV-1 replication in CD4 lymphocytes from diverse HIV+ infected patients grouped according to CD4 count and viral load

We present preclinical studies that demonstrate in vitro the feasibility and efficacy of lentivirus-based vector antisense gene therapy for control of HIV replication in primary T lymphocytes isolated from HIV-infected patients discordant for clinical status. VRX496 is a VSV-G-pseudotyped HIV-based vector that encodes an antisense payload against the HIV envelope gene. The antisense payload is under the control of the native LTR promoter, which is highly transactivated by tat upon HIV infection in the cell. Transfer of autologous CD4(+) T lymphocytes genetically modified with VRX496 (VRX496T) into HIV-infected patients is intended to provide a reservoir of cells capable of controlling HIV, potentially delaying AIDS onset. To determine the patient population likely to respond to VRX496 for optimal efficacy, we examined the ability of our research vector, VRX494, to modify and suppress HIV in vitro in lymphocytes isolated from 20 study subjects discordant for CD4 count and viral load. VRX494 is analogous to the clinical vector VRX496, except that it contains GFP as a marker gene instead of the 186-tag marker in the clinical vector. To transfer VRX494 to target cells we developed a novel scalable two-step transduction procedure that has been translated to the clinic in an ongoing clinical trial. This procedure achieved unprecedented transduction efficiencies of 94 +/- 5% in HIV(+) study subject cells. In addition the vector inhibited HIV replication >/=93% in culture regardless of the viral load or CD4 count of the subject or tropism of the virus strain with which they were infected. These findings demonstrate that VRX496T therapy is expected to be beneficial to patients that differ in their status in term of CD4 count and viral load. The methods described represent significant technical advances facilitating execution of lentivirus vector-mediated gene therapy for treatment of HIV and are currently being employed in the first trial evaluating lentivirus vector safety in humans.

Ribozymes: a modern tool in medicine

Since the discovery of ribozymes and self-splicing introns, it has been estimated that this biological property of RNA combined with other recombinant DNA technologies would become a tool to combat viral diseases and control oncogenes. These goals seem like a distinct possibility now. However, there is still a lot to be learned about the mobility of RNA inside the cells and the cellular factors that can impede ribozyme action in order to capitalize fully on the targeted RNA inactivation property of ribozymes. The most effective approach to maximize ribozyme function in a complex intracellular environment is to understand as much as possible about the intracellular fate of the RNA that is being targeted. As new techniques in cell biology become available, such understanding will be less problematic. Fundamental studies of ribozyme structure and mechanism of catalysis are flourishing both at the academic and industrial level and it can be expected that many new developments will continue to take place in these areas in the near future. Here, we review the design, stability and therapeutic application of these technologies illustrating relevant gene targets and applications in molecular medicine. Relevant problems in implementation of the technology, group I and II introns and the differences in applications, ribozyme structure and the application of this technology to virus attack and oncogene downregulation are discussed. Also some of the latest RNA-based technologies such as siRNA, RNA/DNA duplexes and RNA decoys have been introduced.

Advanced modular self-inactivating lentiviral expression vectors for multigene interventions in mammalian cells and in vivo transduction

In recent years, lentiviral expression systems have gained an unmatched reputation among the gene therapy community for their ability to deliver therapeutic transgenes into a wide variety of difficult-to-transfect/transduce target tissues (brain, hematopoietic system, liver, lung, retina) without eliciting significant humoral immune responses. We have cloned a construction kit-like self-inactivating lentiviral expression vector family which is compatible to state-of-the-art packaging and pseudotyping technologies and contains, besides essential cis-acting lentiviral sequences, (i) unparalleled polylinkers with up to 29 unique sites for restriction endonucleases, many of which recognize 8 bp motifs, (ii) strong promoters derived from the human cytomegalovirus immediate-early promoter (P(hCMV)) or the human elongation factor 1alpha (P(hEF1)(alpha)), (iii) P(hCMV-) or P(PGK-) (phosphoglycerate kinase promoter) driven G418 resistance markers or fluorescent protein-based expression tracers and (iv) tricistronic expression cassettes for coordinated expression of up to three transgenes. In addition, we have designed a size-optimized series of highly modular lentiviral expression vectors (pLenti Module) which contain, besides the extensive central polylinker, unique restriction sites flanking any of the 5'U3, R-U5-psi+-SD, cPPT-RRE-SA and 3'LTR(DeltaU3) modules or placed within the 5'U3 (-78 bp) and 3'LTR(DeltaU3) (8666 bp). pLentiModule enables straightforward cassette-type module swapping between lentiviral expression vector family members and facilitates the design of Tat-independent (replacement of 5'LTR by heterologous promoter elements), regulated and self-excisable proviruses (insertion of responsive operators or LoxP in the 3'LTR(DeltaU3) element). We have validated our lentiviral expression vectors by transduction of a variety of insect, chicken, murine and human cell lines as well as adult rat cardiomyocytes, rat hippocampal slices and chicken embryos. The novel multi-purpose construction kit-like vector series described here is compatible with itself as well as many other (non-viral) mammalian expression vectors for straightforward exchange of key components (e.g. promoters, LTRs, resistance genes) and will assist the gene therapy and tissue engineering communities in developing lentiviral expression vectors tailored for optimal treatment of prominent human diseases.

Inhibition of HIV-1 replication by an HIV-1 dependent ribozyme expression vector with the Cre/loxP (ON/OFF) system

Antiviral strategies to inhibit HIV-1 replication have included the generation of gene products that provide the intracellular inhibition of an essential viral protein or RNA. When used in conjunction with the HIV-1 long terminal repeat (LTR), an inducible promoter dependent on the virus-encoded trans-activator (tat), relatively high background activity is still observed in the absence of tat (Caruso & Klatzmann, 1992; Dinges et al., 1995). In order to circumvent this problem, we used the Cre/loxP (ON/OFF) recombination system as a tool for our investigation. In the present study, we constructed a loxP-cassette vector with the ribozyme (Rz) expression portion under the control of the tRNAi(Met) promoter between two loxP sequences (plox-Rz-U5). We also constructed an HIV-1 LTR promoter-driven Cre recombinase gene (pLTR-Cre). These vectors were triple-transfected into HeLa CD4 cells with the HIV-1 pseudotype viral expression vector. Basal activity was not detectable before HIV-1 infection. The LTR-dependent Cre protein product in HIV-1 infected HeLa CD4 cells expressed the ribozyme by inducing loxP homologous recombination, which strongly inhibited the HIV-1 gene expression. These results demonstrate the potential of an anti-ribozyme with the Cre/loxP system for controlling HIV-1 infection via gene therapy.

Gene therapy of HIV-1 infection using lentiviral vectors expressing anti-HIV-1 genes

The use of vectors based on primate lentiviruses for gene therapy of human immunodeficiency virus type 1 (HIV-1) infection has many potential advantages over the previous murine retroviral vectors used for delivery of genes that inhibit replication of HIV-1. First, lentiviral vectors have the ability to transduce dividing and nondividing cells that constitute the targets of HIV-1 infection such as resting T cells, dendritic cells, and macrophages. Lentiviral vectors can also transfer genes to hematopoietic stem cells with a superior gene transfer efficiency and without affecting the repopulating capacity of these cells. Second, these vectors could be potentially mobilized in vivo by the wild-type virus to secondary target cells, thus expanding the protection to previously untransduced cells. And finally, lentiviral vector backbones have the ability to block HIV-1 replication by several mechanisms that include sequestration of the regulatory proteins Tat and Rev, competition for packaging into virions, and by inhibition of reverse transcription in heterodimeric virions with possible generation of nonfunctional recombinants between the vector and viral genomes. The inhibitory ability of lentiviral vectors can be further increased by expression of anti-HIV-1 genes. In this case, the lentiviral vector packaging system has to be modified to become resistant to the anti-HIV-1 genes expressed by the vector in order to avoid self-inhibition of the vector packaging system during vector production. This review focuses on the use of lentiviral vectors as the main agents to mediate inhibition of HIV-1 replication and discusses the different genetic intervention strategies for gene therapy of HIV-1 infection.

Intravirion display of a peptide corresponding to the dimer structure of protease attenuates HIV-1 replication

Current treatment of HIV-1-infected individuals involves the administration of several drugs, all of which target either the reverse transcriptase or the protease activity of the virus. Unfortunately, the benefits of such treatments are compromised by the emergence of viruses exhibiting resistance to the drugs. This situation warrants new approaches for interfering with virus replication. Considering the activation of protease in the virus particles, a novel strategy to inhibit HIV-1 replication was tested targeting the dimerization domain of the protease. To test this idea, we have selected four residues from the C terminus of HIV-1 protease that map to the dimer interface region of the enzyme. We have exploited Vpr to display the peptides in the virus particles. The chimeric Vpr exhibited expression and virion incorporation similar to wildtype Vpr. The virus derived from the HIV-1 proviral DNA containing chimeric Vpr sequences registered a reduced level of replication in CEM and CEM X 174 cells in comparison with viruses containing wildtype Vpr. Similar results were observed in a single-round replication assay. These results suggest that the intravirion display of peptides targeting viral proteins is a powerful approach for developing antiviral agents and for dissecting the dynamic interactions between structural proteins during virus assembly and disassembly.

Reduction of target gene expression by a modified U1 snRNA

Although the primary function of U1 snRNA is to define the 5' donor site of an intron, it can also block the accumulation of a specific RNA transcript when it binds to a donor sequence within its terminal exon. This work was initiated to investigate if this property of U1 snRNA could be exploited as an effective method for inactivating any target gene. The initial 10-bp segment of U1 snRNA, which is complementary to the 5' donor sequence, was modified to recognize various target mRNAs (chloramphenicol acetyltransferase [CAT], beta-galactosidase, or green fluorescent protein [GFP]). Transient cotransfection of reporter genes and appropriate U1 antitarget vectors resulted in >90% reduction of transgene expression. Numerous sites within the CAT transcript were suitable for targeting. The inhibitory effect of the U1 antitarget vector is directly related to the hybrid formed between the U1 vector and target transcripts and is dependent on an intact 70,000-molecular-weight binding domain within the U1 gene. The effect is long lasting when the target (CAT or GFP) and U1 antitarget construct are inserted into fibroblasts by stable transfection. Clonal cell lines derived from stable transfection with a pOB4GFP target construct and subsequently stably transfected with the U1 anti-GFP construct were selected. The degree to which GFP fluorescence was inhibited by U1 anti-GFP in the various clonal cell lines was assessed by fluorescence-activated cell sorter analysis. RNA analysis demonstrated reduction of the GFP mRNA in the nuclear and cytoplasmic compartment and proper 3' cleavage of the GFP residual transcript. An RNase protection strategy demonstrated that the transfected U1 antitarget RNA level varied between 1 to 8% of the endogenous U1 snRNA level. U1 antitarget vectors were demonstrated to have potential as effective inhibitors of gene expression in intact cells.

Positive and negative aspects of the human immunodeficiency virus protease: development of inhibitors versus its role in AIDS pathogenesis

In this review we summarize multiple aspects of the human immunodeficiency virus (HIV) protease from both structural and functional viewpoints. After an introductory overview, we provide an up-to-date status report on protease inhibitors (PI). This proceeds from a discussion of PI structural design, to how PI are optimally utilized in highly active antiretroviral triple therapy (one PI along with two reverse transcriptase inhibitors), the emergence of PI resistance, and the natural role of secretory leukocyte PI. Then we switch to another focus: the interaction of HIV protease with other genes in acute and persistent infection, which in turn may have an effect on AIDS pathogenesis. We conclude with a discussion on future directions in HIV treatment, involving multiple-target anti-HIV therapy, vaccine development, and novel reactivation-inhibitory reagents.

Development of multigene and regulated lentivirus vectors

Previously we described safe and efficient three-component human immunodeficiency virus type 1 (HIV-1)-based gene transfer systems for delivery of genes into nondividing cells (H. Mochizuki, J. P. Schwartz, K. Tanaka, R. O. Brady, and J. Reiser, J. Virol. 72:8873-8883, 1998). To apply such vectors in anti-HIV gene therapy strategies and to express multiple proteins in single target cells, we have engineered HIV-1 vectors for the concurrent expression of multiple transgenes. Single-gene vectors, bicistronic vectors, and multigene vectors expressing up to three exogenous genes under the control of two or three different transcriptional units, placed within the viral gag-pol coding region and/or the viral nef and env genes, were designed. The genes encoding the enhanced version of green fluorescent protein (EGFP), mouse heat-stable antigen (HSA), and bacterial neomycin phosphotransferase were used as models whose expression was detected by fluorescence-activated cell sorting, fluorescence microscopy, and G418 selection. Coexpression of these reporter genes in contact-inhibited primary human skin fibroblasts (HSFs) persisted for at least 6 weeks in culture. Coexpression of the HSA and EGFP reporter genes was also achieved following cotransduction of target cells using two separate lentivirus vectors encoding HSA and EGFP, respectively. For the regulated expression of transgenes, tetracycline (Tet)-regulatable lentivirus vectors encoding the reverse Tet transactivator (rtTA) and EGFP controlled by a Tet-responsive element (TRE) were constructed. A binary HIV-1-based vector system consisting of a lentivirus encoding rtTA and a second lentivirus harboring a TRE driving the EGFP reporter gene was also designed. Doxycycline-modulated expression of the EGFP transgene was confirmed in transduced primary HSFs. These versatile vectors can potentially be used in a wide range of gene therapy applications.

Packageable antiviral therapeutics against human immunodeficiency virus type 1: virion-targeted virus inactivation by incorporation of a single-chain antibody against viral integrase into progeny virions

To determine their activities as an antiviral agent packageable within virions and suitable for continued expression in cells, we tested a single-chain antibody (scAb) against human immunodeficiency virus type 1 (HIV-1) integrase and its three fusion proteins: fused to viral protein R (scab-Vpr), a double-cassette of the WXXF motif binding to Vpr (scAb-WXXF), and viral major capsid protein (scAb-CA), respectively. Cotransfection of human 293T cells with expression plasmid for scAb-Vpr or -WXXF along with HIV-1 clone pLAI resulted in the production of a normal amount of progeny virions with infectivity decreased by more than 10(3)-fold. Immunoblot analyses showed that scAb-Vpr or -WXXF was associated with virions, whereas scAb or scAb-CA was not, suggesting that scAb-Vpr or -WXXF was incorporated into virions. The incorporation of scAb-WXXF appeared to be Vpr dependent, because the fusion protein was associated with the wild-type but not with Vpr-truncated HIV-1 virions. Since G418-selected HeLa clones carrying expression plasmid for scAb-WXXF were obtained much more frequently than those for scAb-Vpr, scAb-WXXF was inferred to be less toxic to cells than scAb-Vpr. These results suggest that scAb-WXXF may serve as a novel class of antiviral therapeutic that inactivates progeny HIV virions from within.

Intravirion targeting of a functional anti-human immunodeficiency virus ribozyme directed to pol

Ribozymes are catalytic RNAs that offer several advantages as specific therapeutic genes against human immunodeficiency virus type 1 (HIV-1). Significant challenges in antiviral uses of ribozymes include (1) how best to express and to deliver this agent and (2) what is the best locale to target ribozymes against HIV-1 RNA. To explore the former, we have previously characterized several vector systems for efficient expression/delivery of anti-HIV-1 ribozymes (Dropulic et al., 1992; Dropulic and Jeang, 1994a; Smith et al., 1997). Here, to investigate an optimal locale for ribozyme-targeting, we asked whether it might be advantageous to direct ribozymes into HIV-1 virions as opposed to the more conventional approach of targeting ribozymes into infected cells. Two series of experiments were performed. First, we demonstrated that anti-HIV-1 ribozymes could indeed be packaged specifically and efficiently into virions. Second, we compared the virus suppressing activity of a packageable ribozyme with its counterpart, which cannot be packaged into HIV-1 virions. Our results showed that although both ribozymes cleaved HIV-1 genomic RNA in vitro with equivalent efficiencies, the former ribozyme demonstrated significantly higher virus-suppressing activity than the latter. These findings provide proof-of-principle that to combat productive HIV-1 replication, intravirion targeting is more effective than intracellular targeting of ribozymes.

Inhibition of human immunodeficiency virus replication and growth advantage of CD4+ T cells and monocytes derived from CD34+ cells transduced with an intracellular antibody directed against human immunodeficiency virus type 1 Tat

Current clinical gene therapy protocols for the treatment of human immunodeficiency virus type 1 (HIV-1) infection involve the ex vivo transduction and expansion of CD4+ T cells derived from HIV-positive patients at a late stage in their disease (CD4+ cell count <400 cells/mm3). We examined the efficiency of transduction and transgene expression in adult bone marrow (BM)- and umbilical cord blood (UCB)-derived CD34+ cells induced to differentiate into T cells and monocytes in vitro with an MuLV-based vector encoding the neomycin resistance gene and an intracellular antibody directed against the Tat protein of HIV-1 (sFvtat1-Ckappa). The expression of the marker gene and the effects of antiviral construct on subsequent challenge with monocytotropic and T cell-tropic HIV-1 isolates were monitored in vitro in purified T cells and monocytes generated in culture from the transduced CD34+ cells. Transduction efficiencies of CD34+ cells ranged between 22 and 27%. Differentiation of CD34+ cells into T cells or monocytes was not significantly altered by the transduction process. HIV-1 replication in monocytes and CD4+ T cells derived from CD34+ cells transduced with the intracellular antibody gene was significantly reduced in comparison with the degree of HIV replication seen in monocytes and CD4+ T cells derived from CD34+ cells transduced with the neomycin resistance gene alone. Further, T cells and monocytes derived from CD34+ cells transduced with the intracellular antibody gene were demonstrated to express the sFvtat1-Ckappa transgene by RT-PCR and had a selective growth advantage in cultures that had been challenged with HIV-1. These data demonstrate that sFvtat1-Ckappa inhibits HIV-1 replication in T cells and monocytes developing from CD34+ cells and supports the continuing development of a stem cell gene therapy for the treatment of HIV-1 infection.

Inhibition of HIV-1 by an anti-integrase single-chain variable fragment (SFv): delivery by SV40 provides durable protection against HIV-1 and does not require selection

Human immunodeficiency virus type 1 (HIV-1) encodes several proteins that are packaged into virus particles. Integrase (IN) is an essential retroviral enzyme, which has been a target for developing agents to inhibit virus replication. In previous studies, we showed that intracellular expression of single-chain variable antibody fragments (SFvs) that bind IN, delivered via retroviral expression vectors, provided resistance to productive HIV-1 infection in T-lymphocytic cells. In the current studies, we evaluated simian-virus 40 (SV40) as a delivery vehicle for anti-IN therapy of HIV-1 infection. Prior work suggested that delivery using SV40 might provide a high enough level of transduction that selection of transduced cells might be unnecessary. In these studies, an SV40 expression vector was developed to deliver SFv-IN (SV(Aw)). Expression of the SFv-IN was confirmed by Western blotting and immunofluorescence staining, which showed that > 90% of SupT1 T-lymphocytic cells treated with SV(Aw) expressed the SFv-IN protein without selection. When challenged, HIV-1 replication, as measured by HIV-1 p24 antigen expression and syncytium formation, was potently inhibited in cells expressing SV40-delivered SFv-IN. Levels of inhibition of HIV-1 infection achieved using this approach were comparable to those achieved using murine leukemia virus (MLV) as a transduction vector, the major difference being that transduction using SV40 did not require selection in culture whereas transduction with MLV did require selection. Therefore, the SV40 vector as gene delivery system represents a novel therapeutic strategy for gene therapy to target HIV-1 proteins and interfere with HIV-1 replication.

Isolation of efficient antivirals: genetic suppressor elements against HIV-1

The development of general approaches for the isolation of efficient antivirals and the identification and validation of targets for drug screening are becoming increasingly important, due to the emergence of previously unrecognized viral diseases. The genetic suppressor element (GSE) technology is an approach based on the functional expression selection of efficient genetic inhibitors from random fragment libraries derived from a gene or genome of interest. We have applied this technology to isolate potent genetic inhibitors against HIV-1. Two strategies were used to select for GSEs that interfere with latent virus induction and productive HIV-1 infection based on the expression of intracellular and surface antigens. The selected GSEs clustered in seven narrowly defined regions of the HIV-1 genome and were found to be functionally active. These elements are potential candidates for the gene therapy of AIDS. The developed approaches can be applied to other viral pathogens, as well as for the identification of cellular genes supporting the HIV-1 life cycle.

Gene transfer: a review of methods and applications

Gene transfer is a potentially powerful tool for the treatment of a wide variety of diseases. The transfer of these genes is achieved by utilizing a variety of vectors, including retroviral, adenoviral, adeno-associated virus (AAV) and a number of non-viral mechanisms. Numerous studies have successfully demonstrated transduction of genes into target cells with a variety of vectors, and have provided 'proof-in-principle' that gene transfer can result in prolonged in vivo expression of transduced genes, albeit at low quantities. Furthermore, gene marking studies in acute myeloblastic leukemia (AML), chronic myeloid leukemia (CML) and neuroblastoma have elegantly demonstrated that gene-marked tumor cells contribute to relapse following autologous transplantation. However none of the studies examining the therapeutic benefit of gene therapy has definitively demonstrated a clinically meaningful benefit. Nonetheless, the results of studies involving gene transfer for severe combined immunodeficiency (SCID), chronic granulomatous disease (CGD), melanoma and lung cancer highlight the potential benefit of this strategy. This review will discuss mechanisms of achieving gene transfer into target cells. It will examine some of the pre-clinical and clinical results to date and will discuss some of the potential uses of gene transfer for therapeutic purposes.

Examination of antisense RNA and oligodeoxynucleotides as potential inhibitors of avian leukosis virus replication in RP30 cells

Avian leukosis virus (ALV) is an economically important pathogen of chickens. Both antisense RNA and antisense oligodeoxynucleotides (ODN) have been used to diminish the replication and spread of other retroviruses. The use of antisense RNA and ODN to inhibit ALV replication has been examined in cultured RP30 cells. Using an expression system that constitutively transcribes antisense ALV RNA, one transfected cell clone showed a significant reduction in virus growth. However, this effect was not reproducibly observed in other transfected cell lines or in cells in which the antisense transcript was expressed from a regulatable promoter, even though a substantial amount of antisense transcript was generated. Antisense ODN complementary to several different target sites near the 5' end of the ALV genome were also tested for antiviral activity, by comparison of antisense ODN effects to those of randomized sequence controls. An antisense ODN complementary to the ALV primer binding site demonstrated a reproducible reduction in viral replication. However, when the corresponding region was specifically employed as a target for intracellular antisense RNA expression, there again was no significant inhibition of ALV. These results suggest that in vivo expression of antisense RNA is unlikely to be an effective way to generate transgenic poultry that are resistant to field strains of ALV.

Orphan opioid receptor oligonucleotides inhibit HIV-1 expression in human brain cells

We found that orphan opioid receptor (OR) mRNA was constitutively expressed in human fetal microglia, astrocytes, and cerebral cortical neurons. An OR antisense oligonucleotide markedly inhibited HIV-1 expression in enriched microglial cell and the mixed glial/neuronal cell cultures but not in the chronically infected promonocytic U1 cells. The sense oligonucleotide also inhibited HIV-1 expression in brain cell cultures. These findings indicate that while human brain cells constitutively express mRNA for OR, antisense oligonucleotides to OR non-specifically inhibit acute HIV-1 infection.

Anti-HIV effects of HIV vectors

We have recently developed an HIV-1 packaging cell line, psi 422, as an improved tool for anti-HIV gene therapy. After stable transfection with an HIV-1 or HIV-2 vector, psi 422 has been shown to synthesize virions able to transduce CD4+ T cells and macrophages. We now report that HIV vectors per se, in the absence of antiviral genes, inhibit HIV infection of transduced cells. This antiviral effect was shown to be due, at least in part, to a TAR and RRE decoy effect. These data highlight further advantages of HIV-derived gene delivery systems for HIV therapy, in addition to CD4 cell targeting and the ability to transduce nondividing cells.

The biochemistry of gene therapy for AIDS

Gene therapy has enormous potential and could in the near future involve the clinical biochemist in monitoring its efficacy. The involvement of clinical biochemists in this field could be not only in evaluating the impact of a gene-based strategy on disease progression, but also in measuring the expression of the products of therapeutic genes in treated individuals. Indeed, gene therapy presents new possibilities for the treatment of many diseases and, in particular, merits consideration in the treatment of a fatal disease like AIDS. The aim of this paper is to review the biochemical basis and clinical relevance of the gene therapy approaches directed towards the inhibition of human immunodeficiency virus type-1. We discuss the goals which have been achieved, the problems which have occurred and the efforts that are being made to solve them. In this regard, particular attention is paid to new strategies targeting 'therapeutic' enzymes to human immunodeficiency virus type-1 nucleic acids.

Inhibition of human immunodeficiency virus type 1 replication by nuclear chimeric anti-HIV ribozymes in a human T lymphoblastoid cell line

Human immunodeficiency virus (HIV) infection represents one of the most challenging systems for gene therapy. Thanks to the extended knowledge of the molecular biology of the HIV life cycle, many different strategies have been developed including transdominant modifications of HIV proteins, RNA decoys, antisense RNA, ribozymes, and intracellular antibody fragments. In this paper, we have tested in a human T lymphoblastoid cell line the antiviral activity of ribozymes specifically designed to co-localize inside the nucleus with the Rev pre-mRNA before it is spliced and transported to the cytoplasm. This result was obtained by inserting the ribozyme in the spliceosomal U1 small nuclear RNA (snRNA) and in a derivative that has perfect complementarity with the 5' splice site of the Rev pre-mRNA. These ribozymes were tested in human T cell clones and were shown to be very efficient in inhibiting viral replication. Not only were the p24 levels in the culture medium drastically reduced but so were the intracellular HIV transcripts. Control disabled ribozymes enabled us to show the specificity of the ribozyme activity. Therefore, these constructs have potential utility for gene therapy of HIV-1 infection.

Inhibition of human immunodeficiency virus replication and growth advantage of CD4+ T cells from HIV-infected individuals that express intracellular antibodies against HIV-1 gp120 or Tat

Current clinical gene therapy protocols for the treatment of human immunodeficiency virus type 1 (HIV-1) infection often involve the ex vivo transduction and expansion of CD4+ T cells derived from HIV-positive patients at a late stage in their disease (CD4 count <400). These protocols involve the transduction of T cells by murine leukemia virus (MLV)-based vectors encoding antiviral constructs such as the rev m10 dominant negative mutant or a ribozyme directed against the CAP site of HIV-1 RNA. We examined the efficiency and stability of transduction of CD4+ T cells derived from HIV-infected patients at different stages in the progression of their disease, from seroconversion to AIDS. CD4+ T cells from HIV-positive patients and uninfected donors were transduced with MLV-based vectors encoding beta-galactosidase and an intracellular antibody directed against gp120 (sFv 105) or Tat. (sFvtat1-Ckappa). The expression of marker genes and the effects of the antiviral constructs were monitored in vitro in unselected transduced CD4+ T cells. Efficiency and stability of transduction varied during the course of HIV infection; CD4+ T cells derived from asymptomatic patients were transducible at higher efficiencies and stabilities than CD4+ T cells from patients with acquired immunodeficiency syndrome (AIDS). Expression of the anti-tat intracellular antibody was more effective at stably inhibiting HIV-1 replication in transduced cells from HIV-infected individuals than was sFv 105. The results of this study have important implications for the development of a clinically relevant gene therapy for the treatment of HIV-1 infection.

Strategies for achieving multiple layers of selectivity in gene therapy

Here we review the progress towards the development of targeted vectors for direct in vivo delivery in gene therapy. Currently, there are many separate approaches. These include: simple physical/anatomical localization of administration of the vector at the site where gene transfer is required; exploitation of natural tropisms of plasmid, viral and cellular vectors; and the use of molecular engineering to change the specificity of proteins and nucleic acids so that they specifically recognize target ligands expressed on/in the target cells. Unfortunately, each of these approaches is usually imperfect by itself. However, combinations of these strategies might produce vectors in which several layers of imperfect targeting give an overall level of specificity that can justify systemic delivery of vectors to treat human disease.

Inhibition of recombinant human immunodeficiency virus type 1 replication by a site-specific recombinase

Current molecular genetic strategies to inhibit productive human immunodeficiency virus type 1 (HIV-1) replication have involved the generation of gene products which provide intracellular inhibition of essential virally encoded proteins or RNA structures. A molecular strategy to excise proviral DNA from HIV-1-infected cells and render these cells virus free would provide an attractive direct antiviral strategy, providing a mechanism to remove viral genes from infected cells. The potential of such a molecular genetic intervention was examined by using the Cre-loxP recombination system. A recombinant HIV-1 clone, designated HIV(lox), that contains loxP within a nonessential U3 region of the long terminal repeats was synthesized. The loxP motif was maintained during replication of HIV(lox) in CEM cells, as demonstrated by reverse transcriptase PCR analyses of genomic RNA isolated from virions. Two different types of HIV-1-permissive cells, CEM cells and 293 cells expressing the CD4 glycoprotein, were transformed with a Cre expression vector which was shown to encode Cre DNA binding and recombinase activities. HIV(lox) infection of CEM or CD4+ 293 cells expressing Cre resulted in a substantial reduction in virus replication compared to control cells, and evidence for the presence of the expected excision product was found. Site-specific excision of HIV-1 can therefore be achieved by using this model system with acute infection. These studies represent one step toward the development of a novel antiviral strategy for the treatment of AIDS.

Gene therapy for neurologic disease: benchtop discoveries to bedside applications. 2. The bedside

Important advances in basic research have made it possible to examine the safety, toxicity, and efficacy of gene therapy in humans for over 5 years. The development of sophisticated gene delivery systems has resulted in approval by the Recombinant DNA Advisory Committee (RAC) of 125 gene therapy or gene marking studies. One of the primary applications of current retroviral-mediated gene insertion technology has been for malignant brain tumors. Studies are therefore underway to examine the efficacy of "suicide" gene therapy in children with recurrent brain tumors and adults with newly diagnosed or recurrent gliomas. Since a high proportion of genetic disorders produce neurologic dysfunction, gene therapy is likely to impact the management of neurologic disease in the foreseeable future. Patients with human immunodeficiency virus (HIV), Gaucher's disease, and Hunter syndrome are now enrolled in gene therapy trials. It will be challenging for the child neurologist to stay abreast of rapid developments in the field of gene therapy. By participating in the design and implementation of clinical trials in gene therapy, the neurologist may reduce the intense toll that several neurologic diseases take on children and their families.

Toward highly efficient cell-type-specific gene transfer with retroviral vectors displaying single-chain antibodies

Recently, we constructed retroviral vector particles derived from spleen necrosis virus (SNV) that display a single-chain antibody (scA) on the viral surface. By transient transfection protocols, we showed that such particles are competent for infection and cell type specific. Efficient infection was dependent on the presence of wild-type envelope, although wild-type SNV was not infectious on target cells (T.-H. T. Chu and R. Dornburg, J. Virol. 69:2659-2663, 1995; T.-H. T. Chu, I. Martinez, W. C. Sheay, and R. Dornburg, Gene Ther. 1:292-299, 1994). In this study, stable packaging lines were constructed and detailed biological and biochemical studies were performed. Chimeric scA-envelope fusion proteins were expressed as efficiently as wild-type envelope and were stable over a period of at least 6 h. Only a fully functional wild-type envelope could act as a helper for efficient virus penetration. The ratio of wild-type envelope protein to chimeric envelope protein appears to determine the efficiency of infection. Virus titers of targeting vectors obtained from stable packaging lines were as high as 10(4) CFU/ml. A 25-fold concentration of vector virus stocks resulted in a 200-fold increase in virus titers (up to 10(6) CFU/ml). These data indicate that an inhibitor of infection was (at least partially) removed by the concentration protocol. Our data show that this technology has several variables for further improvements and, therefore, has the potential to become a powerful tool for cell-type-specific in vivo human gene therapy.

Intracellular expression of single-chain variable fragments to inhibit early stages of the viral life cycle by targeting human immunodeficiency virus type 1 integrase

Integration of viral DNA into a chromosome of the infected host cell is required for efficient replication of a retroviral genome, and this reaction is mediated by the virus-encoded enzyme integrase (IN). As IN plays a pivotal role in establishing infection during the early stages of the retroviral life cycle, it is an attractive target for therapeutic intervention. However, the lack of effective antiviral drug therapy against this enzyme has led to the testing of other novel approaches towards its inhibition. In these studies, a panel of anti-human immunodeficiency virus type 1 (anti-HIV-1) IN hybridomas has been used in the construction of single-chain variable antibody fragments (SFvs). The monoclonal antibodies produced by these hybridomas, and derived SFvs, bind to different domains within IN. We now demonstrate that intracellular expression of SFvs which bind to IN catalytic and carboxy-terminal domains results in resistance to productive HIV-1 infection. This inhibition of HIV-1 replication is observed with SFvs localized in either the cytoplasmic or nuclear compartment of the cell. The expression of anti-IN SFvs in human T-lymphocytic cells and peripheral blood mononuclear cells appears to specifically neutralize IN activity prior to integration and, thus, has an effect on the integration process itself. These data support our previous studies with an anti-HIV-1 reverse transcriptase SFv and demonstrate further that intracellularly expressed SFvs can gain access to viral proteins of the HIV-1 preintegration complex. This panel of anti-HIV-1 IN SFvs also provides the tools with which to dissect the molecular mechanism(s) directly involved in integration within HIV-1-infected cells.

Establishment of a therapeutic model for retroviral infection using the genetic resistance mechanism of the host

Resistance to retroviral infection is often regulated by multiple genes that control different aspects of the host-virus interaction. Genetically distinct inbred strains of mice differ in their susceptibility to retrovirus and have allowed the identification of several host-resistant loci that regulate the host defense mechanism to retroviral infection. Using the murine retrovirus infection system, a therapeutic model has been developed of retrovirus infection in association with the resistant mechanism of host genes. The most effective result achieved with the model was when using bone marrow transplantation of retrovirus-resistant cells with receptor interference function, which was genetically defined by the Fv-4 resistant gene. The possible application of these findings to the gene therapy of retrovirus-induced disease of humans is discussed.

A conditionally replicating HIV-1 vector interferes with wild-type HIV-1 replication and spread

Defective-interfering viruses are known to modulate virus pathogenicity. We describe conditionally replicating HIV-1 (crHIV) vectors that interfere with wild-type HIV-1 (wt-HIV) replication and spread. crHIV vectors are defective-interfering HIV genomes that do not encode viral proteins and replicate only in the presence of wt-HIV helper virus. In cells that contain both wt-HIV and crHIV genomes, the latter are shown to have a selective advantage for packaging into progeny virions because they contain ribozymes that cleave wt-HIV RNA but not crHIV RNA. A crHIV vector containing a triple anti-U5 ribozyme significantly interferes with wt-HIV replication and spread. crHIV vectors are also shown to undergo the full viral replicative cycle after complementation with wt-HIV helper-virus. The application of defective interfering crHIV vectors may result in competition with wt-HIVs and decrease pathogenic viral loads in vivo.

Targeting human immunodeficiency virus type 1 reverse transcriptase by intracellular expression of single-chain variable fragments to inhibit early stages of the viral life cycle

Novel molecular approaches to inhibit human immunodeficiency virus type 1 (HIV-1) infection have received increasing attention because of the lack of effective antiviral drug therapies in vivo. We now demonstrate that cells can be intracellularly immunized by cytoplasmic expression of single-chain variable antibody fragments (SFv) which bind to the HIV-1 reverse transcriptase (RT) enzyme. The expression of anti-RT SFv in T-lymphocytic cells specifically neutralizes the RT activity in the preintegration stage and affects the reverse transcription process, an early event of the HIV-1 life cycle. Blocking the virus at these early stages dramatically decreased HIV-1 propagation, as well as the HIV-1-induced cytopathic effects in susceptible human T lymphocytes, by impeding the formation of the proviral DNA. These data also demonstrate that intracellular, complete SFvs may gain access to viral proteins of the HIV-1 preintegration complex. These SFvs will provide a tool with which to better understand the molecular mechanisms involved in restricting viral replication in HIV-1-infected cells.

Reversal of drug sensitivity in MDR subline of P388 leukemia by gene-targeted antisense oligonucleotide

We attempted to reverse multidrug resistance (MDR) by treatment with 25-mer antisense phosphorothioate oligonucleotide. The phosphorothioate analogs, the sequences of which are sense or antisense to the initiation codon of mouse mdr1 mRNA, were tested against murine leukemic P388/S and adriamycin-resistant P388/ADR cell lines. A weak inhibitory effect on the growth of P388/S and P388/ADR cells was observed at a sense and antisense oligonucleotide concentration of 30 microM. Using the monoclonal antibody to P-glycoprotein and a flow cytometry technique, we showed that the level of expression of P-glycoprotein in P388/ADR cells treated with antisense oligonucleotide was lower than when treated with sense oligonucleotide. The antisense oligonucleotide potentiated the growth-inhibitory effect of vinblastine on P388/ADR cells, whereas sense oligonucleotide did not. This was accompanied by an increase in vinblastine retention in the cells. The reversal of the resistance by antisense oligonucleotide was increased by the combination with 1 microM verapamil. These results suggest that the antisense oligonucleotide and low dose verapamil may be useful in circumventing the resistance to anticancer drugs of MDR tumors.

Redirecting the specificity of ubiquitination by modifying ubiquitin-conjugating enzymes

Depletion of specific cellular proteins is a powerful tool in biological research and has many medical and agricultural benefits. In contrast to genetic methods currently available to attenuate protein levels, we describe an alternative approach that redirects the ubiquitin-dependent proteolytic pathway to facilitate specific proteolytic removal. Degradation via the ubiquitin pathway requires the prior attachment of multiple ubiquitins to the target protein. This attachment is accomplished, in part, by a family of enzymes designated E2s (or ubiquitin-conjugating enzymes), some of which use domains near their C termini for target recognition. Here, we demonstrate that E2 target recognition can be redefined by engineering E2s to contain appropriate protein-binding peptides fused to their C termini. In five dissimilar examples, chimeric E2s were created that recognized and ubiquitinated their respective binding partners with high specificity. We also show that ubiquitination of one protein targeted by this method led to its ATP-dependent degradation in vitro. Thus, by exploiting interacting domains derived from natural and synthetic ligands, it may be possible to design E2s capable of directing the selective removal of many intracellular proteins.

Intracellular antibodies: development and therapeutic potential

Single-chain antibodies, synthesized by the cell and targeted to a particular cellular compartment, can be used to interfere in a highly specific manner with cell growth and metabolism. Recent applications of this technology include the phenotypic knockout of growth-factor receptors, the functional inactivation of p21ras and the inhibition of HIV-1 replication. Intracellular antibodies are likely to have a widespread impact in biological research as a simple and effective alternative to other forms of gene inactivation; they demonstrate clear potential as reagents for cancer therapy and for the control of infectious diseases.