A conditionally replicating HIV-based vector that stably expresses an antiviral shRNA against HIV-1 replication

Viral Vectors Applied for RNAi-Based Antiviral Therapy

RNA interference (RNAi) provides the means for alternative antiviral therapy. Delivery of RNAi in the form of short interfering RNA (siRNA), short hairpin RNA (shRNA) and micro-RNA (miRNA) have demonstrated efficacy in gene silencing for therapeutic applications against viral diseases. Bioinformatics has played an important role in the design of efficient RNAi sequences targeting various pathogenic viruses. However, stability and delivery of RNAi molecules have presented serious obstacles for reaching therapeutic efficacy. For this reason, RNA modifications and formulation of nanoparticles have proven useful for non-viral delivery of RNAi molecules. On the other hand, utilization of viral vectors and particularly self-replicating RNA virus vectors can be considered as an attractive alternative. In this review, examples of antiviral therapy applying RNAi-based approaches in various animal models will be described. Due to the current coronavirus pandemic, a special emphasis will be dedicated to targeting Coronavirus Disease-19 (COVID-19).

Bone Marrow Gene Therapy for HIV/AIDS

Bone marrow gene therapy remains an attractive option for treating chronic immunological diseases, including acquired immunodeficiency syndrome (AIDS) caused by human immunodeficiency virus (HIV). This technology combines the differentiation and expansion capacity of hematopoietic stem cells (HSCs) with long-term expression of therapeutic transgenes using integrating vectors. In this review we summarize the potential of bone marrow gene therapy for the treatment of HIV/AIDS. A broad range of antiviral strategies are discussed, with a particular focus on RNA-based therapies. The idea is to develop a durable gene therapy that lasts the life span of the infected individual, thus contrasting with daily drug regimens to suppress the virus. Different approaches have been proposed to target either the virus or cellular genes encoding co-factors that support virus replication. Some of these therapies have been tested in clinical trials, providing proof of principle that gene therapy is a safe option for treating HIV/AIDS. In this review several topics are discussed, ranging from the selection of the antiviral molecule and the viral target to the optimal vector system for gene delivery and the setup of appropriate preclinical test systems. The molecular mechanisms used to formulate a cure for HIV infection are described, including the latest antiviral strategies and their therapeutic applications. Finally, a potent combination of anti-HIV genes based on our own research program is described.

Potential mechanisms for cell-based gene therapy to treat HIV/AIDS

INTRODUCTION

An estimated 35 million people are infected with HIV worldwide. Anti-retroviral therapy (ART) has reduced the morbidity and mortality of HIV-infected patients but efficacy requires strict adherence and the treatment is not curative. Most importantly, the emergence of drug-resistant virus strains and drug toxicity can restrict the long-term therapeutic efficacy in some patients. Therefore, novel treatment strategies that permanently control or eliminate the virus and restore the damaged immune system are required. Gene therapy against HIV infection has been the topic of intense investigations for the last two decades because it can theoretically provide such a durable anti-HIV control.

AREAS COVERED

In this review we discuss two major gene therapy strategies to combat HIV. One approach aims to kill HIV-infected cells and the other is based on the protection of cells from HIV infection. We discuss the underlying molecular mechanisms for candidate approaches to permanently block HIV infection, including the latest strategies and future therapeutic applications.

EXPERT OPINION

Hematopoietic stem cell-based gene therapy for HIV/AIDS may eventually become an alternative for standard ART and should ideally provide a functional cure in which the virus is durably controlled without medication. Recent results from preclinical research and early-stage clinical trials support the feasibility and safety of this novel strategy.

Controllable inhibition of hepatitis B virus replication by a DR1-targeting short hairpin RNA (shRNA) expressed from a DOX-inducible lentiviral vector

As a highly efficient delivery system, lentiviral vectors (LVs) have become a powerful tool to assess the antiviral efficacy of RNA drugs such as short hairpin RNA (shRNA) and decoys. Furthermore, recent advanced systems allow controlled expression of the effector RNA via coexpression of a tetracycline/doxycycline (DOX) responsive repressor (tTR-KRAB). Herein, this system was utilized to assess the antiviral effects of LV-encoded shRNAs targeting three conserved regions on the pregenomic RNA of hepatitis B virus (HBV), namely the region coding for the reverse transcriptase (RT) domain of the viral polymerase (LV-HBV-shRNA1), the core promoter (CP; LV-HBV-shRNA2), and the direct repeat 1 (DR1; LV-HBV-shRNA3). Transduction of just the LV-HBV-shRNA vectors into the stably HBV expressing HepG2.2.15 cell line showed significant reductions in secreted HBsAg and HBeAg, intracellular HBcAg as well as HBV RNA and DNA replicative intermediates for all vectors, however, most pronouncedly for the DR1-targeting shRNA3. The corresponding vector was therefore applied in the DOX-controlled system. Notably, strong interference with HBV replication was found in the presence of the inducer DOX whereas the antiviral effect was essentially ablated in its absence; hence, the silencing effect of the shRNA and consequently HBV replication could be strictly regulated by DOX. This newly established system may therefore provide a valuable platform to study the antiviral efficacy of RNA drugs against HBV in a regulated manner, and even be applicable in vivo.

Quantitation of HIV-1 DNA with a sensitive TaqMan assay that has broad subtype specificity

The increasing diversity of HIV-1 isolates makes virus quantitation challenging, especially when diverse isolates co-circulate in a geographical area. Measuring the HIV-1 DNA levels in cells has become a valuable practical tool for fundamental and clinical research. A quantitative HIV-1 DNA assay was developed based on TaqMan(®) technology. Primers that target the highly conserved LTR region were designed to detect a broad array of HIV-1 variants, including viral isolates from many subtypes, with high sensitivity. Introduction of a pre-amplification step prior to the TaqMan(®) reaction allowed the specific amplification of fully reverse transcribed viral DNA. Execution of the pre-amplification step with a second primer set enables for the exclusive quantitation of the 2-LTR circular HIV-1 DNA form.

RNAi in the regulation of mammalian viral infections

Although RNA interference (RNAi) is known to play an important part in defense against viruses of invertebrates, its contribution to mammalian anti-viral defense has been a matter of dispute. This is surprising because all components of the RNAi machinery necessary for robust RNAi-mediated restriction of viruses are conserved in mammals, and the introduction of synthetic small interfering RNAs (siRNAs) into cells efficiently silences the replication of viruses that contain siRNA complementary sequences in those cells. Here, I discuss the reasons for the dispute, and review the evidence that RNAi is a part of the physiological defense of mammalian cells against viral infections.

MicroRNA-like antivirals

Employing engineered DNA templates to express antiviral microRNA (miRNA) sequences has considerable therapeutic potential. The durable silencing that may be achieved with these RNAi activators is valuable to counter chronic viral infections, such as those caused by HIV-1, hepatitis B, hepatitis C and dengue viruses. Early use of expressed antiviral miRNAs entailed generation of cassettes containing Pol III promoters (e.g. U6 and H1) that transcribe virus-targeting short hairpin RNA mimics of precursor miRNAs. Virus escape from single gene silencing elements prompted later development of combinatorial antiviral miRNA expression cassettes that form multitargeting siRNAs from transcribed long hairpin RNA and polycistronic primary miRNA sequences. Weaker Pol III and Pol II promoters have also been employed to control production of antiviral miRNA mimics, improve dose regulation and address concerns about toxicity caused by saturation of the endogenous miRNA pathway. Efficient delivery of expressed antiviral sequences remains challenging and utilizing viral vectors, which include recombinant adenoviruses, adeno-associated viruses and lentiviruses, has been favored. Investigations using recombinant lentiviruses to transduce CD34+ hematological precursor cells with expressed HIV-1 gene silencers are at advanced stages and show promise in preclinical and clinical trials. Although the use of expressed antiviral miRNA sequences to treat viral infections is encouraging, eventual therapeutic application will be dependent on rigorously proving their safety, efficient delivery to target tissues and uncomplicated large scale preparation of vector formulations. This article is part of a special issue entitled: MicroRNAs in viral gene regulation.

Deriving four functional anti-HIV siRNAs from a single Pol III-generated transcript comprising two adjacent long hairpin RNA precursors

Several different approaches exist to generate expressed RNA interference (RNAi) precursors for multiple target inhibition, a strategy referred to as combinatorial (co)RNAi. One such approach makes use of RNA Pol III-expressed long hairpin RNAs (lhRNAs), which are processed by Dicer to generate multiple unique short interfering siRNA effectors. However, because of inefficient intracellular Dicer processing, lhRNA duplexes have been limited to generating two independent effective siRNA species. In this study, we describe a novel strategy whereby four separate anti-HIV siRNAs were generated from a single RNA Pol III-expressed transcript. Two optimized lhRNAs, each comprising two active anti-HIV siRNAs, were placed in tandem to form a double long hairpin (dlhRNA) expression cassette, which encodes four unique and effective siRNA sequences. Processing of the 3' position lhRNA was more variable but effective multiple processing was possible by manipulating the order of the siRNA-encoding sequences. Importantly, unlike shRNAs, Pol III-expressed dlhRNAs did not compete with endogenous and exogenous microRNAs to disrupt the RNAi pathway. The versatility of expressed lhRNAs is greatly expanded and we provide a mechanism for generating transcripts with modular lhRNAs motifs that contribute to improved coRNAi.

Selective gene silencing by viral delivery of short hairpin RNA

RNA interference (RNAi) technology has not only become a powerful tool for functional genomics, but also allows rapid drug target discovery and in vitro validation of these targets in cell culture. Furthermore, RNAi represents a promising novel therapeutic option for treating human diseases, in particular cancer. Selective gene silencing by RNAi can be achieved essentially by two nucleic acid based methods: i) cytoplasmic delivery of short double-stranded (ds) interfering RNA oligonucleotides (siRNA), where the gene silencing effect is only transient in nature, and possibly not suitable for all applications; or ii) nuclear delivery of gene expression cassettes that express short hairpin RNA (shRNA), which are processed like endogenous interfering RNA and lead to stable gene down-regulation. Both processes involve the use of nucleic acid based drugs, which are highly charged and do not cross cell membranes by free diffusion. Therefore, in vivo delivery of RNAi therapeutics must use technology that enables the RNAi therapeutic to traverse biological membrane barriers in vivo. Viruses and the vectors derived from them carry out precisely this task and have become a major delivery system for shRNA. Here, we summarize and compare different currently used viral delivery systems, give examples of in vivo applications, and indicate trends for new developments, such as replicating viruses for shRNA delivery to cancer cells.

Effectiveness of a 'hunter' virus in controlling human immunodeficiency virus type 1 infection

Engineered therapeutic viruses provide an alternative method for treating infectious diseases, and mathematical models can clarify the system's dynamics underlying this type of therapy. In particular, this study developed models to evaluate the potential to contain human immunodeficiency virus type 1 (HIV-1) infection using a genetically engineered 'hunter' virus that kills HIV-1-infected cells. First, we constructed a novel model for understanding the progression of HIV infection that predicted the loss of the immune system's CD4(+) T cells across time. Subsequently, it determined the effects of introducing hunter viruses in restoring cell population. The model implemented direct and indirect mechanisms by which HIV-1 may cause cell depletion and an immune response. Results suggest that the slow progression of HIV infection may result from a slowly decaying CTL immune response, leading to a limited but constant removal of uninfected CD4 resting cells through apoptosis - and from resting cell proliferation that reduces the rate of cell depletion over time. Importantly, results show that the hunter virus does restrain HIV infection and has the potential to allow major cell recovery to 'functional' levels. Further, the hunter virus persisted at a reduced HIV load and was effective either early or late in the infection. This study indicates that hunter viruses may halt the progression of the HIV infection by restoring and sustaining high CD4(+) T-cell levels.

Possible applications for replicating HIV 1 vectors

Since its discovery some 25 years ago, much has been learned about HIV type 1 and the molecular details of its replication cycle. This insight has been used to develop lentiviral vector systems that have advantages over conventional retroviral vector systems. For safety reasons, the lentiviral vector systems are replication incompetent and the risk of generating a replication competent virus has been minimized. Nevertheless, there may be certain applications for replication competent HIV based vector systems, and we will review our activities in this particular field. This includes the generation of a conditionally replicating HIV 1 variant as a safe live attenuated virus vaccine, the construction of mini HIV variants as cancer selective viruses for virotherapy against leukemia, and the use of a conditionally live anti HIV gene therapy vector. Although safety concerns will undoubtedly remain for the use of replication competent HIV based vector systems, some of the results in cell culture systems are very promising and warrant further testing in appropriate animal models.

RNA interference-based therapeutics for human immunodeficiency virus HIV-1 treatment: synthetic siRNA or vector-based shRNA?

IMPORTANCE OF THE FIELD

Despite the clinical benefits of highly active antiretroviral therapy (HAART), the prospect of life-long antiretroviral treatment poses significant problems, which has spurred interest in developing new drugs and strategies to treat HIV infection and eliminate persistent viral reservoirs. RNAi has emerged as a therapeutic possibility for HIV.

AREAS COVERED IN THIS REVIEW

We discuss progress in overcoming hurdles to translating transient and stable RNAi enabling technologies to clinical application for HIV; covering the past 2 - 3 years.

WHAT THE READER WILL GAIN

HIV inhibition can be achieved by transfection of chemically or enzymatically synthesized siRNAs or by DNA-based vector systems expressing short hairpin RNAs (shRNAs) that are processed intracellularly into siRNA. We compare these approaches, focusing on technical and safety issues that will guide the choice of strategy for clinical use.

TAKE HOME MESSAGE

Introduction of synthetic siRNA into cells or its stable endogenous production using vector-driven shRNA have been shown to suppress HIV replication in vitro and, in some instances, in vivo. Each method has advantages and limitations in terms of ease of delivery, duration of silencing, emergence of escape mutants and potential toxicity. Both appear to have potential as future therapeutics for HIV, once the technical and safety issues of each approach are overcome.

Toward a durable anti-HIV gene therapy based on RNA interference

Basic research in the field of molecular biology led to the discovery of the mechanism of RNA interference (RNAi) in Caenorhabditis elegans in 1998. RNAi is now widely appreciated as an important gene control mechanism in mammals, and several RNAi‐based gene‐silencing applications have already been used in clinical trials. In this review I will discuss RNAi approaches to inhibit the pathogenic human immunodeficiency virus type 1 (HIV‐1), which establishes a chronic infection that would most likely require a durable gene therapy approach. Viruses, such as HIV‐1, are particularly difficult targets for RNAi attack because they mutate frequently, which allows viral escape by mutation of the RNAi target sequence. Combinatorial RNAi strategies are required to prevent viral escape.

Retroviral delivery of RNA interference against Marek's disease virus in vivo

The process of RNA interference (RNAi) has been exploited in cultured chicken cells and in chick embryos to assess the effect of specific gene inhibition on phenotypes related to development and disease. We previously demonstrated that avian leukosis virus-based retroviral vectors are capable of delivering effective RNAi against Marek's disease virus (MDV) in cell culture. In this study, similar RNAi vectors are shown to reduce the replication of MDV in live chickens. Retroviral vectors were introduced into d 0 chick embryos, followed by incubation until hatching. Chicks were challenged with 500 pfu of strain 648A MDV at day of hatch, followed by assays for viremia at 14 d postinfection. Birds were monitored for signs of Marek's disease for 8 wk. A stem-loop PCR assay was developed to measure siRNA expression levels in birds. Delivery of RNAi co-targeting the MDV gB glycoprotein gene and ICP4 transcriptional regulatory gene significantly reduced MDV viremia in vivo, although to lesser extents than were observed in cell culture. Concomitant reductions in disease incidence also were observed, and the extent of this effect depended on the potency of the MDV challenge virus inoculum. Successful modification of phenotypic traits in live birds with retroviral RNAi vectors opens up the possibility that such approaches could be used to alter the expression of candidate genes hypothesized to influence a variety of quantitative traits including disease susceptibility.

Lentiviral delivery of short hairpin RNAs

In less than a decade after discovery, RNA interference-mediated gene silencing is already being tested as potential therapy in clinical trials for a number of diseases. Lentiviral vectors provide a means to express short hairpin RNA (shRNA) to induce stable and long-term gene silencing in both dividing and non-dividing cells and thus, are being intensively investigated for this purpose. However, induction of long-term shRNA expression can also cause toxicities by inducing off-target effects and interference with the endogenous micro-RNA (miRNA) pathway that regulates cellular gene expression. Recently, several advances have been made in the shRNA vector design to mimic cellular miRNA processing and to express multiplex siRNAs in a tightly regulated and reversible manner to overcome toxicities. In this review we describe some of these advances, focusing on the progress made in the development of lentiviral shRNA delivery strategies to combat viral infections.

Conditional RNAi: towards a silent gene therapy

RNA interference (RNAi) has the potential to permit the downregulation of virtually any gene. While transgenic RNAi enables stable propagation of the resulting phenotype to progeny, the dominant nature of RNAi limits its use to applications where the continued suppression of gene expression does not disturb normal cell functioning. This is of particular importance when the target gene product is essential for cell survival, development or differentiation. It is therefore desirable that knockdown be externally regulatable. This review is aimed at providing an overview of the approaches for conditional RNAi in mammalian systems, with a special mention of studies employing these approaches to target therapeutically/biologically relevant molecules, their advantages and disadvantages, and a pointer towards approaches best suited for RNAi-based gene therapy.

Towards a durable RNAi gene therapy for HIV-AIDS

BACKGROUND

RNA interference (RNAi) can be employed as a potent antiviral mechanism.

OBJECTIVE

To discuss RNAi approaches to target pathogenic human viruses causing acute or chronic infections, in particular RNAi gene therapy against HIV-1.

METHODS

A review of relevant literature.

RESULTS/CONCLUSIONS

The future of antiviral RNAi therapeutics is very promising. RNAi was discovered only a decade ago, and although we are still in the early days, the first clinical trials are already ongoing.

Nucleic acids-based therapeutics in the battle against pathogenic viruses

For almost three decades, researchers have studied the possibility to use nucleic acids as antiviral therapeutics. In theory, compounds such as antisense oligonucleotides, ribozymes, DNAzymes, and aptamers can be designed to trigger the sequence-specific inhibition of particular mRNA transcripts, including viral genomes. However, difficulties with their efficiency, off-target effects, toxicity, delivery, and stability halted the development of nucleic acid-based therapeutics that can be used in the clinic. So far, only a single antisense drug, Vitravene for the treatment of CMV-induced retinitis in AIDS patients, has made it to the clinic. Since the discovery of RNA interference (RNAi), there is a renewed interest in the development of nucleic acid-based therapeutics. Antiviral RNAi approaches are highly effective in vitro and in animal models and are currently being tested in clinical trials. Here we give an overview of antiviral nucleic acid-based therapeutics. We focus on antisense and RNAi-based compounds that have been shown to be effective in animal model systems.

HIV-1 latency in actively dividing human T cell lines

Background

Eradication of HIV-1 from an infected individual cannot be achieved by current drug regimens. Viral reservoirs established early during the infection remain unaffected by anti-retroviral therapy and are able to replenish systemic infection upon interruption of the treatment. Therapeutic targeting of viral latency will require a better understanding of the basic mechanisms underlying the establishment and long-term maintenance of HIV-1 in resting memory CD4 T cells, the most prominent reservoir of transcriptional silent provirus. However, the molecular mechanisms that permit long-term transcriptional control of proviral gene expression in these cells are still not well understood. Exploring the molecular details of viral latency will provide new insights for eventual future therapeutics that aim at viral eradication.

Results

We set out to develop a new in vitro HIV-1 latency model system using the doxycycline (dox)-inducible HIV-rtTA variant. Stable cell clones were generated with a silent HIV-1 provirus, which can subsequently be activated by dox-addition. Surprisingly, only a minority of the cells was able to induce viral gene expression and a spreading infection, eventhough these experiments were performed with the actively dividing SupT1 T cell line. These latent proviruses are responsive to TNFα treatment and alteration of the DNA methylation status with 5-Azacytidine or genistein, but not responsive to the regular T cell activators PMA and IL2. Follow-up experiments in several T cell lines and with wild-type HIV-1 support these findings.

Conclusion

We describe the development of a new in vitro model for HIV-1 latency and discuss the advantages of this system. The data suggest that HIV-1 proviral latency is not restricted to resting T cells, but rather an intrinsic property of the virus.

Efficient inhibition of HIV-1 expression by LNA modified antisense oligonucleotides and DNAzymes targeted to functionally selected binding sites

BACKGROUND

A primary concern when targeting HIV-1 RNA by means of antisense related technologies is the accessibility of the targets. Using a library selection approach to define the most accessible sites for 20-mer oligonucleotides annealing within the highly structured 5'-UTR of the HIV-1 genome we have shown that there are at least four optimal targets available.

RESULTS

The biological effect of antisense DNA and LNA oligonucleotides, DNA- and LNAzymes targeted to the four most accessible sites was tested for their abilities to block reverse transcription and dimerization of the HIV-1 RNA template in vitro, and to suppress HIV-1 production in cell culture. The neutralization of HIV-1 expression declined in the following order: antisense LNA > LNAzymes > DNAzymes and antisense DNA. The LNA modifications strongly enhanced the in vivo inhibitory activity of all the antisense constructs and some of the DNAzymes. Notably, two of the LNA modified antisense oligonucleotides inhibited HIV-1 production in cell culture very efficiently at concentration as low as 4 nM.

CONCLUSION

LNAs targeted to experimentally selected binding sites can function as very potent inhibitors of HIV-1 expression in cell culture and may potentially be developed as antiviral drug in patients.

RNAi therapy for HIV infection: principles and practicalities

Inside eukaryotic cells, small RNA duplexes, called small interfering RNAs (siRNAs), activate a conserved RNA interference (RNAi) pathway which leads to specific degradation of complementary target mRNAs through base-pairing recognition. As with other viruses, studies have shown that replication of the HIV-1 in cultured cells can be targeted and inhibited by synthetic siRNAs. The relative ease of siRNA design and the versatility of RNAi to target a broad spectrum of mRNAs have led to the promise that drug discovery in the RNAi pathway could be effective against pathogens.

This review discusses the current experimental principles that guide the application of RNAi against HIV and describes challenges and limitations that need to be surmounted in order for siRNAs to become practical antiviral drugs. The practical use of RNAi therapy for HIV infection will depend on overcoming several challenges, including the ability to establish long-term expression of siRNA without off-target effects and the capacity to counteract mutant escape viruses.

Trans-inhibition of HIV-1 by a long hairpin RNA expressed within the viral genome

Background

Human immunodeficiency virus type 1 (HIV-1) can be inhibited by means of RNA silencing or interference (RNAi) using synthetic short interfering RNAs (siRNAs) or gene constructs encoding short hairpin RNAs (shRNAs) or long hairpin RNAs (lhRNAs). The use of siRNA and shRNA as antiviral therapeutic is limited because of the emergence of viral escape mutants. This problem is theoretically prevented by intracellular expression of lhRNAs generating multiple siRNAs that target the virus simultaneously, thus reducing the chance of viral escape. However, gene constructs encoding lhRNA molecules face problems with delivery to the right cells in an infected individual. In order to solve this problem, we constructed an HIV-1 variant with a 300 bp long hairpin structure in the 3' part of the genome corresponding to the Nef gene (HIV-lhNef).

Results

Intriguingly, HIV-lhNef potently inhibited wild-type HIV-1 production in trans. However, HIV-lhNef demonstrated a severe production and replication defect, which we were able to solve by selecting spontaneous virus variants with truncated hairpin structures. Although these escape variants lost the ability to trans-inhibit HIV-1, they effectively outgrew the wild-type virus in competition experiments in SupT1 cells.

Conclusion

Expression of the lhNef hairpin within the HIV-1 genome results in potent trans-inhibition of wild-type HIV-1. Although the mechanism of trans-inhibition is currently unknown, it remains of interest to study the molecular details because the observed effect is extremely potent. This may have implications for the development of virus strains to be used as live-attenuated virus vaccines.

Construction of doxycyline-dependent mini-HIV-1 variants for the development of a virotherapy against leukemias

T-cell acute lymphoblastic leukemia (T-ALL) is a high-risk type of blood-cell cancer. We describe the improvement of a candidate therapeutic virus for virotherapy of leukemic cells. Virotherapy is based on the exclusive replication of a virus in leukemic cells, leading to the selective removal of these malignant cells. To improve the safety of such a virus, we constructed an HIV-1 variant that replicates exclusively in the presence of the nontoxic effector doxycycline (dox). This was achieved by replacement of the viral TAR-Tat system for transcriptional activation by the Escherichia coli-derived Tet system for inducible gene expression. This HIV-rtTA virus replicates in a strictly dox-dependent manner. In this virus, additional deletions and/or inactivating mutations were introduced in the genes for accessory proteins. These proteins are essential for virus replication in untransformed cells, but dispensable in leukemic T cells. These minimized HIV-rtTA variants contain up to 7 deletions/inactivating mutations (TAR, Tat, vif, vpR, vpU, nef and U3) and replicate efficiently in the leukemic SupT1 T cell line, but do not replicate in normal peripheral blood mononuclear cells. These virus variants are also able to efficiently remove leukemic cells from a mixed culture with untransformed cells. The therapeutic viruses use CD4 and CXCR4 for cell entry and could potentially be used against CXCR4 expressing malignancies such as T-lymphoblastic leukemia/lymphoma, NK leukemia and some myeloid leukemias.