Inhibition of simian immunodeficiency virus by foamy virus vectors expressing siRNAs

Prospects for Foamy Viral Vector Anti-HIV Gene Therapy

Stem cell gene therapy approaches for Human Immunodeficiency Virus (HIV) infection have been explored in clinical trials and several anti-HIV genes delivered by retroviral vectors were shown to block HIV replication. However, gammaretroviral and lentiviral based retroviral vectors have limitations for delivery of anti-HIV genes into hematopoietic stem cells (HSC). Foamy virus vectors have several advantages including efficient delivery of transgenes into HSC in large animal models, and a potentially safer integration profile. This review focuses on novel anti-HIV transgenes and the potential of foamy virus vectors for HSC gene therapy of HIV.

Establishing Restricted Expression of Caveolin-1 in HIV Infected Cells and Inhibition of Virus Replication

Background:

Caveolin-1 (Cav-1) is the major protein of the caveolae and plays a role in multiple cellular functions and implicated to have anti-HIV activity. Regulated expression of Cav-1 is important for safe and effective use in order to exploit Cav-1 for HIV therapeutic applications.

Methods:

A series of Cav-1 and GFP expression vectors were constructed under the control of the HIV LTR for conditional expression or CMV promoter and the expression of Cav-1 was monitored in the presence or absence of Tat or HIV infection in order to establish the restricted expression of Cav-1 to HIV infected cells.

Results:

Cav-1 expression was evident under the control of the HIV LTR in the absence of Tat or HIV infection as demonstrated by immunoblot. Placing two internal ribosomal entry sequences (IRES) and a Rev response element, RRE (5’~ LTR-IRES-GFP-RRE-IRES-Cav-1~3’) resulted in no expression of Cav-1 in the absence of Tat with effective expression in the presence of Tat. Transduction of HIV permissive cells with this construct using a foamy virus vector show that Cav-1 was able to inhibit HIV replication by 82%. Cells that received LTR-IRES-GFP-RRE-IRES-Cav-1 remain healthy in the absence of Tat or HIV infection.

Conclusion:

These results taken together reveal the inclusion of two IRES establishes a significant reduction of leak through expression of Cav-1 in the absence of Tat or HIV infection. Such regulated expression will have therapeutic application of Cav-1 for HIV infection as well as broad applications which can be beneficial for other host-targeted interventions as therapeutics.

Foamy virus vectors for HIV gene therapy

Highly active antiretroviral therapy (HAART) has vastly improved outcomes for patients infected with HIV, yet it is a lifelong regimen that is expensive and has significant side effects. Retroviral gene therapy is a promising alternative treatment for HIV/AIDS; however, inefficient gene delivery to hematopoietic stem cells (HSCs) has so far limited the efficacy of this approach. Foamy virus (FV) vectors are derived from non-pathogenic viruses that are not endemic to the human population. FV vectors have been used to deliver HIV-inhibiting transgenes to human HSCs, and they have several advantages relative to other retroviral vectors. These include an attractive safety profile, broad tropism, a large transgene capacity, and the ability to persist in quiescent cells. In addition, the titers of FV vectors are not reduced by anti-HIV transgenes that affect the production of lentivirus (LV) vectors. Thus FV vectors are very promising for anti-HIV gene therapy. This review covers the advantages of FV vectors and describes their preclinical development for anti-HIV gene therapy.

A foamy virus vector system for stable and efficient RNAi expression in mammalian cells

The promise of the RNA interference (RNAi) technology is equally dependent on the efficiency and stability of gene silencing. The aim of the present study was the development of foamy virus (FV) vectors for stable RNAi, utilizing two potent RNA polymerase III (Pol III) promoters. Using green fluorescent protein as a target gene, we examined the efficiency of mouse U6 (mU6) and human H1 Pol III promoters in different human cell lines and mouse hematopoietic stem cells (HSCs) ex vivo and in vivo, following bone marrow transplantation. Both our mU6 and H1 FV vectors mediated very efficient gene silencing with as low as one vector copy per cell. However, transduction of human cell lines with FV vectors expressing short hairpin RNA from mU6 led to the gradual elimination of cells in culture, as opposed to H1-harboring cells, underscoring the importance of the expression system or cellular context in the evaluation of the overall RNAi effects. The efficiency and stability of the H1 vectors were further shown by the successful silencing of BCR-ABL in K562 cells. Accordingly, mU6 vectors induced efficient and stable gene silencing in mouse HSCs following bone marrow transplantation. Our work is the first in vivo study on the efficiency and stability of RNAi gene silencing in HSCs with FV vectors, currently a safe alternative for viral gene transfer.

Foamy combinatorial anti-HIV vectors with MGMTP140K potently inhibit HIV-1 and SHIV replication and mediate selection in vivo

Highly active antiretroviral therapy has greatly reduced the morbidity and mortality from human immunodeficiency virus (HIV) infection, but AIDS continues to be a serious health problem worldwide. Despite enormous efforts to develop a vaccine, there is still no cure, and alternative approaches including gene therapy should be explored. In this study we developed and compared combinatorial foamy virus (FV) anti-HIV vectors that also express a mutant methylguanine methyltransferase (MGMTP140K) transgene to increase the percentage of gene-modified cells after transplantation. These FV vectors inhibit replication of HIV-1 and also the simian immunodeficiency virus/HIV-1 (SHIV) chimera that can be used in monkey AIDS gene therapy studies. We identified a combinatorial FV vector that expresses 3 anti-HIV transgenes and inhibits viral replication by over 4 logs in a viral challenge assay. This FV anti-HIV vector expresses an HIV fusion inhibitor and two short hairpin RNAs (shRNAs) targeted to HIV-1 tat and rev, and can be produced at high titer (3.8 x 10(7) transducing units ml(-1)) using improved helper plasmids suitable for clinical use. Using a competitive repopulation assay, we show that human CD34(+) cells transduced with this combinatorial FV vector efficiently engraft in a mouse xenotransplantation model, and that the percentage of transduced repopulating cells can be increased after transplantation.

Foamy virus vectors for gene transfer

Foamy virus (FV) vectors are efficient gene delivery vehicles that have shown great promise for gene therapy in preclinical animal models. FVs or spumaretroviruses are not endemic in humans, but are prevalent in nonhuman primates and in other mammals. They have evolved means for efficient horizontal transmission in their host species without pathology. FV vectors have several unique properties that make them well suited for therapeutic gene transfer including a desirable safety profile, a broad tropism, a large transgene capacity, and the ability to persist in quiescent cells. They mediate efficient and stable gene transfer to hematopoietic stem cells (HSCs) in mouse models, and in the canine large animal model. Analysis of FV vector integration sites in vitro and in hematopoietic repopulating cells shows they have a unique integration profile, and suggests they may be safer than gammaretroviruses or lentiviral vectors. Here, properties of FVs relevant to the safety and efficacy of FV vectors are discussed. The development of FV vector systems is described, and studies evaluating their potential in vitro, and in small and large animal models, is reviewed.

Activity of TAR in inducible inhibition of HIV replication by foamy virus vector expressing siRNAs under the control of HIV LTR

In this report we describe foamy virus vectors with conditional expression of short interfering RNAs (siRNAs) in HIV infected cells. Short hairpin RNAs (shRNAs) based on two targets in the 5' end of the untranslated region and one in the rev gene flanked with 5' and 3' microRNA 30 (miR30) sequences were synthesized and placed under the control of an HIV promoter for Tat-mediated expression. HIV permissive cells were transduced with foamy virus vectors containing each hybrid shRNA expression cassette and tested for their efficacy on the inhibition of HIV replication. Effective Tat dependent expression of the shRNAs, as well as GFP placed downstream each shRNA was evident. In addition the results show inhibition of HIV replication by greater than 98%. Interestingly, transduction of cells with a vector lacking an shRNA also revealed GFP expression in the presence of Tat with similar levels of inhibition of virus replication. When the TAR region was removed from this vector there was neither reduction in virus replication nor Tat-induced GFP expression. These results suggest that TAR in the vector, which Tat interacts to promote expression of the shRNA, is a potent inhibitor of virus replication. Previous studies with TAR regulated expression of antiviral genes ignore the contribution of TAR in the repression of virus replication. Interpretation of effective inhibition of HIV replication by antiviral genes located downstream of TAR while neglecting the efficacy of a potent repression by TAR is misleading.

Antiviral gene therapy

This chapter describes the major gene therapeutic approaches for viral infections. The vast majority of published approaches target severe chronic viral infections such as hepatitis B or C and HIV infection. Two basic gene therapy strategies are introduced here. The first involves the expression of a protein or an RNA that inhibits viral replication by targeting crucial steps of the viral life cycle or by interfering with a cellular factor required for virus replication. The major limitation of this approach is that primary levels of gene modification have generally not been sufficient to reduce the availability of target cells permissive for virus replication to a level that significantly decreases overall viral load. Thus, investigators have banked on the expectation that gene-protected cells have a sufficient selective advantage to accumulate and gain prevalence over time, a prediction that so far could not be confirmed in clinical trials. In vivo levels of gene modification can be improved, however, by introducing an additional selectable marker. In addition, a secreted antiviral gene product that exerts a bystander effect could significantly reduce overall virus replication despite relatively low levels of gene modification. In addition to these direct antiviral approaches, several strategies have been developed that employ or aim to enhance host immune responses. The innate immune response has been enhanced, for example, by the in vivo expression of interferons. Alternatively, T cells can be grafted with recombinant receptors to boost adaptive virus-specific immunity. These approaches are especially promising for chronic virus infection, where natural immune responses are evidently not sufficient to effectively control virus replication.

RNA interference and its therapeutic potential against HIV infection

BACKGROUND

HIV-1 infection is the major cause of AIDS. RNA interference (RNAi) has great potential to work as a powerful tool against HIV infection. Therefore, the possibilities of use of siRNA (small-interfering RNA) as a tool to deal with HIV infection are discussed in this article.

OBJECTIVE

Highly active anti retroviral therapy (HAART) has been successful in reducing the rate of progression to AIDS, but HIV utilizes various tricks to escape from the inhibitory effect of HAART. Therefore, new tools are required to delay progression of infection or block the replication cycle of HIV.

METHODS

This article has been written on the basis of informations available in the form of published literature in various journals.

CONCLUSION

RNAi is a very promising strategy that in principle will provide many new targets against HIV infection. The mechanism of sequence complementarity utilized by siRNAs against their targets provides a new approach to fight against HIV infection. However, this technology still needs many fine refinements before its potential for HIV treatment strategies can be utilized. This review discusses the possibilities of using siRNA as a therapeutic tool for HIV treatment.

Foamy virus vectors expressing anti-HIV transgenes efficiently block HIV-1 replication

Gene therapy has the potential to control human immunodeficiency virus (HIV) in patients who do not respond to traditional antiviral therapy. In this study, we tested foamy virus (FV) vectors expressing three anti-HIV transgenes, both individually and in a combination vector. The transgenes tested in this study are RevM10, a dominant negative version of the viral rev protein, Sh1, a short hairpin RNA directed against a conserved overlapping sequence of tat and rev, and membrane-associated C46 (maC46), a membrane-attached peptide that blocks HIV cell entry. FV vectors efficiently transduce hematopoietic stem cells and, unlike lentivirus (LV) vectors, do not share viral proteins with HIV. The titers of the FV vectors described in this study were not affected by anti-HIV transgenes. On a direct comparison of FV vectors expressing the individual transgenes, entry inhibition using the maC46 transgene was found to be the most effective at blocking HIV replication. A clinically relevant FV vector expressing three anti-HIV transgenes effectively blocked HIV infection in primary macrophages derived from transduced, peripheral blood CD34-selected cells and in a cell line used for propagating HIV, CEMx174. These results suggest that there are potential benefits of using FV vectors in HIV gene therapy.

Foamy virus infection in primates

Foamy viruses (FV), the oldest known genus of Retroviridae, are unique among the retroviruses in having no disease association. It is not known why FV are non-pathogenic while infection by their closest relatives can be deadly. This may be related to the estimated 60 million years of coevolution of FV and their primate hosts. We review the current state of knowledge of FV infection, including information about the sites of viral replication and host immune responses, and discuss the role these may play in establishing persistent yet non-pathogenic infections. Whether FV has pathologic consequences in immunosuppressed hosts has not been thoroughly investigated. As most primates in HIV/SIV research are coinfected with FV, investigation into possible interactions between these viruses is of interest. The use of FV as a vector for gene therapy is also discussed.