Stem cell based anti-HIV Gene therapy

Abstract

Human stem cell-based therapeutic intervention strategies for treating HIV infection have recently undergone a renaissance as a major focus of investigation. Unlike most conventional antiviral therapies, genetically engineered hematopoietic stem cells possess the capacity for prolonged self-renewal that would continuously produce protected immune cells to fight against HIV. A successful strategy therefore has the potential to stably control and ultimately eradicate HIV from patients by a single or minimal treatment. Recent progress in the development of new technologies and clinical trials sets the stage for the current generation of gene therapy approaches to combat HIV infection. In this review, we will discuss two major approaches that are currently underway in the development of stem cell-based gene therapy to target HIV: One that focuses on the protection of cells from productive infection with HIV, and the other that focuses on targeting immune cells to directly combat HIV infection.

Introduction

As we enter the fourth decade since the HIV/AIDS pandemic was first recognized in 1981, there still exists a strong and pressing need for the development of novel therapeutic strategies to treat the disease. There is currently no effective, wide-scale vaccination strategy nor is there a practicable therapy that results in the eradication of the virus from infected individuals. However, based on the historically unprecedented research into this infectious disease, the development of antiretroviral drug therapy has radically changed the natural history of the disease throughout the world. These therapies have significant associated problems and ultimately fail to result in a functional “cure”(Volberding and Deeks, 2010). Thus, new approaches are required that can complement or replace existing therapies that enable full control of the virus and the restoration of the damaged immune system that HIV targets. Recent advances in the development of stem-cell based therapeutic approaches as well as the development of technologies that allow the genetic modification of these cells have provided impetus towards recent progress made in developing novel therapeutic strategies that target HIV infection. While many of these approaches are currently in the early stages of investigation, they provide a new avenue that, at the very least, will lend new insights into HIV infection and pathogenesis; and, at the very best, provide a viable therapy that successfully treats HIV infection and has an impact on what is a highly confounding disease.

Current HIV therapy and limitations

The current HIV therapy using combinations of antiretroviral drugs termed highly active antiretroviral therapy (HAART), has decreased the morbidity and mortality of HIV infected patients (Palella et al., 1998). Although, HAART has dramatically improved patient’s quality of life, HAART requires continuous drug administration to suppress virus production from HIV reservoirs (Chun et al., 1999). The life long treatment creates serious complications such as drug toxicities and side effects, adherence difficulties, and drug resistance. In addition, life long treatment costs can be expensive. Even under HAART, ongoing low level viremia is evident in some patients (Dinoso et al., 2009), potentially contributing to chronic inflammation, immune dysfunction and accelerated aging (Deeks, 2010). Long-term HIV control and elimination of latently infected cells have become major challenges in the HAART era (Richman et al., 2009). Despite extensive efforts to purge residential HIV from reservoirs, existing drug therapies do not eliminate HIV reservoirs even by drug intensification (Dinoso et al., 2009). In contrast, a hematopoietic stem/progenitor cell (HSC)-based gene therapy approach would offer continuous, long-term production of genetically engineered HIV resistant or HIV-targeted cells and a potential to provide stable control or eradication of HIV by a one time or minimal treatment.

Hematopoietic stem cell gene therapy approach to achieve long term HIV resistance

Substantial progress has been made in developing a new therapeutic approach using gene therapy through HSCs to attempt to confer long-term resistance against HIV (figure1). HSCs are capable of self-renewal and differentiation into all hematopoietic lineages. In theory, gene therapy approaches that introduce protective genes against HIV via HSCs can continuously produce their anti-HIV genes in all differentiated cells, including HIV target cells such as CD4+ T lymphocytes and macrophages. Successful replacement of a patient’s immune system by gene modified HIV protected cells may have the potential to minimize viral loads as well as reduce reservoirs of infected and latently infected cells. Newly differentiated protected cells may prevent viral production and spread from persistently infected cells and may allow the functional restoration of the damaged immune system. Currently, a significant clinical benefit by HSC-based gene therapy approaches for HIV diseases has not been achieved; however, this approach has the potential to provide long-term control of HIV through a single treatment. If successful, gene therapy through stem cells could free patients from lifelong daily medications and significantly impact their quality of life.

Figure 1.

Schematic illustrating non-myeloablative HSC-based approaches to treat HIV infection. The anti-HIV factor (such as a siRNA to CCR5 or a molecularly cloned TCR) is cloned and characterized (1) and made into lentiviral vector (or other form enabling genetic transduction of target cells) (2). Stem cells are mobilized from the bone marrow of HIV infected individuals and peripheral blood CD34+ HSCs are obtained by apheresis and cell sorting (3). CD34+ stem cells are then genetically transduced with the anti-HIV factor (4), and the cells are then reinfused back into the individual (5). Following infusion, the anti-HIV gene containing stem cells should migrate to the bone marrow where they take up residence as long-term hematopoietic progenitors. Anti-HIV gene containing cells protected from infection should undergo selection by virus in the body and/or cells engineered to target HIV should respond to the virus and proliferate (6). The effects of this are protection of anti-HIV gene containing cells (7a) or directed targeting of HIV or HIV infected cells by anti-HIV gene expressing cells (7b), resulting in the regeneration of antiviral immune responses and targeted eradication of HIV.

Protecting Cells from Infection: Intracellular Immunization

Introduction

Over the past 20 years, researchers have developed numerous gene based reagents capable of inhibiting HIV infection by intracellular immunization. The intracellular immunization approach to HIV treatment intends to make HIV target cells resistant to HIV by introducing anti-HIV genes (Baltimore, 1988). Here, we discuss anti-HIV gene reagents by classifying them as late step inhibitors (post integration) and early step inhibitors (preintegration).

Late step anti-HIV gene development

Initially, anti-HIV genes were designed to inhibit HIV transcription and translation, which occur in the late steps of the viral life cycle. The transdominant negative HIV Rev mutant RevM10 inhibits HIV RNA nuclear cytoplasmic transport by binding to the rev responsive element (RRE) and interfering with Rev function (Bogerd et al., 1995). Intrabodies against HIV Tat and transactivating response region (TAR) decoys inhibit HIV transcription by sequestering the Tat protein (Mhashilkar et al., 1999; Sullenger et al., 1990). Ribozymes were designed to cleave HIV RNA transcripts by enzymatic activities (Amado et al., 2004; Sarver et al., 1990). Antisense RNAs were designed to hybridize with HIV RNA transcripts and inhibit HIV expression and/or replication (Goodchild et al., 1988; Humeau et al., 2004; Levine et al., 2006). Since the discovery of RNA interference (RNAi) (Fire et al., 1998), many small interfering RNAs (siRNA) directed to HIV RNA sequences have been developed. siRNAs directed against various HIV RNA sequences inhibited HIV in various experimental settings (Capodici, Kariko, and Weissman, 2002; Lee et al., 2002; Novina et al., 2002; Park et al., 2003). One weakness of the RNAi approach was recognized when HIV quickly escaped from siRNAs with minimum mutations (Boden et al., 2003; Senserrich et al., 2008). Efforts to make combinations of siRNAs targeting multiple HIV RNA sequences prevented emergence of escape mutations (ter Brake et al., 2006). All of these anti-HIV genes mediated efficient HIV inhibition in various experimental settings. Some of the anti-HIV genes (RevM10, env antisense, ribozymes, RRE decoy and tat/rev siRNA) were tested in clinical trials, as discussed in more detail below.

Early step anti-HIV gene development

Several labs have developed HIV inhibitors that are designed to target HIV at early life cycle steps before genome integration. Theoretically, early step HIV inhibitors have advantages over strategies that inhibit late steps in the viral life cycle. They can protect cells from the establishment of chronic HIV infection and these protected cells may have selective advantage against being killed by HIV-mediated cytotoxicity. Further, inhibitors that work on steps before reverse transcription have a better chance of preventing HIV mutations because they stop the virus prior to the mutagenic effects of the reverse transcription process.

HIV receptor inhibitors

There is ongoing investigation into gene-based reagents that are capable of inhibiting HIV cell surface receptors, HIV fusion, or incoming virions. Gene based reagents to reduce CD4, CCR5 or CXCR4 have been developed using ribozyme, antisense RNAs, intrabodies and siRNAs (Anderson, Banerjea, and Akkina, 2003; Bai, Rossi, and Akkina, 2001; Qureshi et al., 2006; Zhou et al., 2004). Among HIV receptor inhibitors, CCR5 inhibitors provide great promise to protect cells from HIV without adversely affecting human health, because individuals who lack CCR5 expression due to the homozygous Δ32/Δ32 deletion in the CCR5 gene are highly resistant to HIV infection and have an apparently normal health status (Alkhatib et al., 1996; Dragic et al., 1996; Liu et al., 1996; Mummidi et al., 1998; Smith et al., 1997). Heterozygous individuals with a 50% decrease in CCR5 surface expression have lower plasma viral load and a substantially prolonged course of HIV disease (Dean et al., 1996; Rappaport et al., 1997). Recently, the proof of concept for HIV stem cell therapy using CCR5 defective HSC treatment was demonstrated by the “Berlin patient”, who demonstrated long-term control of HIV infection following myeloablation and an allogeneic bone marrow transplant with CCR5 defective cells (Hutter et al., 2009). In this report, Hutter and colleagues intentionally selected a human leukocyte antigen (HLA) matched CCR5 Δ32/Δ32 homozygous bone marrow donor to treat a HIV+ patient with acute myelogenous leukemia. Intensive bone marrow ablation and stem cell transplants resulted in complete replacement of the patient’s peripheral blood system with the CCR5 Δ32/Δ32 homozygous donor cells. Interestingly CCR5 expressing host-derived macrophages remained in the Gut. Though the patient had discontinued HAART at the time of the bone marrow transplant, the patient’s HIV plasma viral load and HIV proviral DNA have remained negative for more than 3.5 years. This case provides the first well documented evidence of a functional cure for HIV infection. A recent follow up paper by the same group provided further evidence of a cure (Kristina Allers, Blood, prepublished online December 8, 2010; DOI 10.1182/blood-2010-09-309591). In this report, the same patient’s blood system remained completely replaced with cells derived from the CCR5 Δ32/Δ32 donor cells and the host-derived CCR5+ macrophages became undetectable after 24 months. HIV DNA and RNA remained undetectable in plasma and peripheral blood mononuclear cells. One of the major limitations of this strategy is the difficulty of identifying HLA matched donors with the CCR5 Δ32/Δ32 homozygous deletion. This is confounded by the fact that the CCR5 Δ32/Δ32 homozygous deletion exists naturally in only one percent of the Caucasian population, and is rarer in other ethnic populations (Lucotte, 1997). In all, this report of a functional cure for HIV, although not practicable for widespread use as a viable therapeutic strategy, is an exciting observation and forms the foundation for strategies that target the CCR5 molecule in treating HIV infection.

Stable CCR5 inhibition by gene reagents

Gene therapy strategies enable the use of genetically modified HSCs for autologous transplant in patients. Several groups have developed gene therapy strategies using ribozyme, intrabodies, shRNA or zinc finger nuclease aiming to make cells resistant to HIV by CCR5 inhibition.

Ribozyme directed to CCR5

Feng and colleagues developed a hairpin ribozyme against CCR5 (Feng et al., 2000). They transduced the CCR5 ribozyme by recombinant adeno associated viral vector into human PM1 cells and demonstrated reduced CCR5 expression and CCR5 tropic HIV replication inhibition in vitro. In independent studies, Cagnon and colleagues developed a CCR5 ribozyme (Cagnon and Rossi, 2000). This ribozyme was combined with a HIV tat/rev shRNA and a TAR decoy in a lentiviral vector and was recently tested in a phase I clinical trial as described further below (DiGiusto et al., 2010).

Intrabodies to inhibit cell surface CCR5 expression

Using a different approach, Steinberger and colleagues developed a CCR5-specific single-chain intrabody that efficiently reduced CCR5 cell surface expression by retaining CCR5 protein in the endoplasmic reticulum (Steinberger et al., 2000). Intrabody expressing PM-1 cell lines, primary CD4+T cells, and human thymocytes derived from thymus/liver implants generated in the NOD-SCID-hu thymus/liver (thy/liv) model were protected from CCR5 tropic HIV infection by CCR5 surface down-regulation (Swan et al., 2006). Further, CCR5 blocked cells were selected by HIV infection during dendritic cell mediated R5 tropic HIV challenge experiments in vitro.

RNA interference to knock down CCR5

Several groups have developed shRNAs to stably knock down CCR5 expression (Anderson and Akkina, 2005; Anderson et al., 2009; Butticaz et al., 2003; Kim et al., 2010; Shimizu et al., 2010). Lentiviral vector transduction of CCR5 directed shRNAs efficiently reduced CCR5 expression and confered resistance against CCR5 tropic HIVs in vitro (Anderson and Akkina, 2005; Anderson et al., 2009; Butticaz et al., 2003; Kim et al., 2010; Shimizu et al., 2010). However, we and others recognized that over expression of shRNA using an U6 RNA polymerase III promoter could induce cytotoxicity in human primary T lymphocytes (An et al., 2006; Kiem et al., 2010; Lo et al., 2007). This cytotoxicity could be caused by saturation of endogenous micro RNA biogenesis (Grimm et al., 2006; Grimm et al., 2010). To avoid the cytotoxicity, we lowered shRNA expression by utilizing a transcriptionally weaker H1 RNA polymerase III promoter. To achieve robust CCR5 knock down with the H1 promoter, we selected a potent CCR5 shRNA from an enzymatically generated shRNA library directed to CCR5 (An et al., 2006). This optimization resulted in a 25 fold CCR5 reduction in human primary T lymphocytes and CD34+ cell-derived macrophages without toxic effects in vitro (An et al., 2007; Liang et al., 2010). CCR5 shRNA transduced CD34+ cells transplanted into myeloablated rhesus macaques resulted in a stable 3–10 fold reduction in CCR5 expression in peripheral blood T lymphocytes in vivo. No apparent adverse effects due to the shRNA were evident in transplanted macaques for 3 years. Importantly, these cells were less susceptible to simian immunodeficiency virus infection ex vivo than were control cells (An et al., 2007). We further examined in vivo human CCR5 knock down in a recently developed humanized NOD/SCID/IL2rγ^null (NSG) bone marrow/liver/thymus (BLT) transplanted mouse model (Shimizu et al., 2010). Lentiviral vector transduction of the CCR5 shRNA into human fetal liver CD34+ cells and subsequent transplant in the BLT humanized mouse resulted in stable CCR5 down regulation in the transplanted thy/liv implant, and in primary and secondary lymphoid organs including the gut-associated lymphoid tissue, which is the major site of HIV replication in humans (Brenchley et al., 2004). CCR5 expression was efficiently reduced more than 5 fold in human CD4+ T cells and CD14+/CD33+ monocytes/macrophage populations. The shRNA-mediated CCR5 knockdown had no apparent adverse effects on T cell development as assessed by polyclonal T cell receptor Vβ family development and naïve/memory T cell differentiation. CCR5 knockdown continued to be observed in mice receiving a secondary transplant from the bone marrow of the mice receiving the first set of CCR5 shRNA modified CD34+ HSCs. This suggests long-term engraftment and self-renewal potential by the shRNA transduced HSCs. Down regulation of CCR5 was sufficient to protect T-cells from HIV challenge ex vivo. These studies demonstrated the feasibility and potential of lentiviral vector-mediated delivery of CCR5 shRNA through HSC transplant as a means of intracellular immunization for the treatment of HIV.

Zinc finger nuclease to disrupt the CCR5 gene

A recent novel approach to disrupt the CCR5 gene was developed by using engineered zinc finger nuclease proteins (ZFNs) (Perez et al., 2008). ZFNs are comprised of custom-made zinc finger DNA binding domains fused to an endonuclease fok I domain to generate a double-strand break at a specific DNA target site. When these double-strand breaks are repaired, deletions and insertions can be introduced at the site of cleavage through a non-homologous end joining (NHEJ) cellular DNA repair mechanism. Holt and colleagues optimized the method to introduce CCR5 gene specific ZFNs into human cord blood and fetal liver derived CD34+ cells (Holt et al., 2010). The group achieved a mean disruption rate of approximately 17% of the total CCR5 alleles in the population of CD34+ cells. They estimated that 5–7% of ZFNs treated cells would be mutated at both alleles. ZFN treated CD34+ cells were transplanted into irradiated neonatal NSG mice. ZFN treated CD34+ cells reconstituted human lymphocytes in systemic lymphoid organs in the mice. Mice were subsequently challenged with highly pathogenic CCR5 tropic HIV-1_BaL. Remarkably, CCR5 negative CD4+ T were rapidly selected in the HIV challenged mice. Genomic DNA PCR analysis of the CCR5 gene revealed accumulations of polyclonal insertion/deletion mutations at the ZFN target sites, suggesting that the enriched CCR5 gene disrupted cells were polyclonal. Viral load decreased over time in the ZFN treated mice. These results were highly remarkable in that the relatively small fraction of CCR5 gene disrupted cells can be selected in vivo in CCR5 tropic HIV-1 infected NSG humanized mice suggesting protection of these cells from infection.

HIV entry inhibitors

In another strategy that targets HIV entry, Egelhofer and colleagues developed a HIV fusion inhibitor, termed C46, which can be stably expressed using retroviral vectors (Egelhofer et al., 2004). C46 is derived from the C-terminal heptad repeat of HIV gp41. It blocks HIV fusion by binding to the N terminal coiled coil domain of HIV gp41 fusion intermediate and prevents the six-helix bundle formation, analogous to the FDA approved soluble peptide drug enfuvirtide (T20) (Lalezari et al., 2003). C46 expression on the cell surface inhibited HIV replication more than 2 logs in cell lines and more than 1 log in primary human T lymphocytes (Perez et al., 2005). A recent report confirmed the robustness of C46 over tat/rev specific shRNAs and a long antisense RNA targeted against HIV envelope (termed VRX496) (Kimpel et al., 2010). C46 expressing cells were effectively selected after HIV challenge in vitro. The safety of C46 has been tested in a phase I clinical trial where autologous T-cells transduced with a retroviral vector expressing C46 were infused into patients, and showed no gene therapy related adverse effects (van Lunzen et al., 2007).

Host restriction factors as anti-HIV reagents for gene therapy

Recent investigations of cross primate HIV permissiveness identified natural host HIV restriction factors (Strebel, Luban, and Jeang, 2009). In Rhesus Macaques, it was found that TRIM5α inhibits HIV infection (Stremlau et al., 2004). Although, the precise inhibitory mechanism is under investigation, TRIM5α inhibits HIV through binding the capsid of incoming virions (Strebel, Luban, and Jeang, 2009). Further, TRIM5α can inhibit HIV infection when it is expressed in human cells as a transgene (Stremlau et al., 2004). However, rhesus TRIM5α is not suitable for gene therapy because of the potential immunogenicity in humans. Human-rhesus chimeric TRIM5α and a single amino acid substituted human TRIM5α (R322P) were created through the investigation of active domains responsible for the HIV restriction (Stremlau et al., 2005; Yap, Nisole, and Stoye, 2005). These minimally modified human TRIM5αs inhibit HIV (Li et al., 2006). Anderson and colleagues demonstrated CD34+ cell transduction of human-rhesus chimeric TRIM5α using a lentiviral vector and thymocyte differentiation in thy/liv tissue in the SCID hu thy/liv mouse model (Anderson and Akkina, 2008). The resultant human thymocytes were protected from ex vivo HIV challenge. In another instance of host restriction, owl monkey cells block HIV infection by an endogenous TRIM5α-cyclophilinA fusion protein (TRIMcyp) (Sayah et al., 2004). The Owl monkey TRIMcyp gene was created by a LINE-1 mediated retrotransposion of cyclophilinA cDNA into TRIM5α gene locus. Interestingly, the TRIMcyp fusion genes were created by independent retrotransposition events in Old World and New World primates, suggesting critical roles of TRIMcyp proteins as host restriction factors in primates (Brennan, Kozyrev, and Hu, 2008; Newman et al., 2008; Virgen et al., 2008; Wilson et al., 2008). Human genome does not carry endogenous TRIMcyp to restrict HIV. Neagu and colleagues recently engineered a human TRIMcyp aiming to use it as a human gene therapy reagent (Neagu et al., 2009). Human TRIMcyp potently inhibited HIV in human CD4+ T cells and macrophages in vitro and in vivo in human TRIMcyp transduced PBMCs trasplanted into Rag2−/−γc−/− mice. Other host HIV restriction factors that could be potentially utilized as gene therapy reagents are APOBEC 3G (Sheehy et al., 2002), APOBEC 3F (Holmes et al., 2007) and Tetherin (Neil, Zang, and Bieniasz, 2008). However, HIV is naturally equipped with the vif and vpu proteins to counteract these host restriction factors. Several researchers developed vif resistant APOBEC 3G by a single amino acid substitution at D128K (Bogerd et al., 2004; Schrofelbauer, Chen, and Landau, 2004; Xu et al., 2004). Gupta and colleagues developed a vpu resistant Tetherin by a single amino acid substitution at T45I (Gupta et al., 2009). These HIV accessory gene resistant HIV restriction factors can be gene therapy candidates because of their resistance to the HIV counter attack. Other cellular genes that restrict HIV infection were identified by cDNA screenings. Mov10 over expression inhibits HIV at multiple steps (Burdick et al., 2010; Furtak et al., 2010). Truncated poly adenylation factor 6 (CPSF6) inhibits HIV nuclear entry by capsid binding (Lee et al., 2010). Recent intensive investigations aimed at better understanding of host factors and HIV interaction may identify novel host restriction factors near future. The ongoing efforts to increase the list of anti-HIV genes may provide new strategies to inhibit HIV by a gene therapy approach.

Combinational anti-HIV strategy to produce highly HIV resistant cells and prevent escape mutations

Effective gene therapy applications against HIV disease will likely require a combination of multiple reagents directed against HIV. Single anti-HIV therapy will likely fail due to the development of escape mutations in the virus that confer resistance to the therapy. Emergence of CXCR4 and dual tropic HIVs will be a concern in the case of CCR5 inhibitors. Utilizing the same rationale as combination antiretroviral therapy, it would be important to develop gene therapy strategies to protect target cells with combinations of anti-HIV genes to inhibit multiple steps in the viral life cycle. With this approach, several groups have incorporated multiple anti-HIV genes into a single lentiviral vector (Anderson and Akkina, 2008; DiGiusto et al., 2010; Kiem et al., 2009; Schopman, ter Brake, and Berkhout, 2010). Combinations of anti-HIV genes were designed to target distinct steps in the viral life cycle to increase the antiviral effect but were also aimed to block the emergence of resistant HIVs. Multiple shRNAs targeting different sites of HIV transcripts were combined within a lentiviral vector (Li et al., 2005; Schopman, ter Brake, and Berkhout, 2010; ter Brake et al., 2006). The combination of three RNA reagents (HIV tat/rev shRNA, TAR decoy and CCR5 ribozyme) was transduced into human CD34+ cells which were then differentiated into monocytes in vitro (Li et al., 2005) and thymocytes in the SCID hu thy/liv mouse model (Anderson et al., 2007). The resultant monocytes and human thymocytes were protected from HIV infection ex vivo (Li et al., 2003; ter Brake et al., 2006). This study provided the preclinical data for a recent phase I clinical trial using the triple anti-RNA combination vector in AIDS lymphoma patients (DiGiusto et al., 2010). Using a different approach, a triple combination of CCR5 shRNA, a human/rhesus chimeric TRIM5α, and a TAR decoy was expressed from a lentiviral vector. This anti-viral RNA combination efficiently inhibited HIV replication in CD34+ cell-derived macrophages in vitro (Anderson et al., 2009). In a separate study, a combination of CCR5 shRNA and a human/rhesus chimeric TRIM5α were induced into induced pluripotent stem cells (iPSc) cells by a lentiviral vector and successfully differentiated through CD133+ cells into HIV resistant macrophages in vitro (Kambal et al., 2010). A membrane anchored fusion inhibitor C46 and multiple tat/rev shRNAs were combined in a lentiviral vector (Kiem et al., 2009). While many of these studies are promising, it is expected that more effective combinations will be developed with newly identified anti-HIV genes in the future.

Clinical trials

Several anti-HIV HSC based gene therapy protocols have been tested in clinical trials. Most clinical trials were phase I studies aimed at evaluating the safety and feasibility of anti-HIV gene transduced autologus hematopoietic stem/progenitor cell transplantation in patients. In early trials, transdominant RevM10 (Kang et al., 2002; Podsakoff et al., 2005), RRE decoy (Kohn et al., 1999), or an anti HIV ribozymeribozyme (Amado et al., 2004) were introduced into patient’s CD34+ cells with Molony murine leukemia virus based gammaretroviral vectors. In summary, all of these phase I clinical studies demonstrated safety and feasibility of the procedures. Gene transfer and stem cell transplantation were well tolerated in all clinical trials and no significant adverse events have been observed. In all of these studies, there were detectable levels of anti-HIV gene expressing cells in patients. However, the gene marking levels were too low to achieve therapeutic benefits.

Mitsuyasu and colleagues recently reported the first phase II clinical trial of an anti-HIV gene therapy. This trial involved 74 subjects enrolled in randomized, double-blind and placebo-controlled groups in a multi-centre trial (Mitsuyasu et al., 2009). A murine gamma retroviral vector was used to transduce a tat/vpr specific ribozyme into granulocyte-colony stimulating factor (G-CSF) mobilized peripheral blood CD34+ cells. Cells were genetically modified ex vivo and re-infused back into the patients without bone marrow conditioning. This procedure did not result in apparent adverse events. Ribozyme DNA and RNA were detectable in 94% of patients but gradually declined to 7% of patients 100 weeks following treatment. Although the levels of ribozyme DNA and RNA were low, lower viral loads and higher CD4+ cell counts were observed in the ribozyme group. These results suggest a marginal effect in the anti-HIV ribozyme group. This study demonstrated a proof of concept that anti- tat/vpr ribozyme HIV transduced autologous HSC transplant in humans is safe and has a capability to produce gene modified cells in a large numbers of human subjects.

In the most recent clinical study, a triple combination of an anti tat/rev shRNA, a TAR decoy and an anti-CCR5 ribozyme was introduced in AIDS lymphoma patients through HSC transplant (DiGiusto et al., 2010). This is the first clinical trial applying a lentiviral vector for anti-HIV gene transduction into patient CD34+ cells. Four AIDS lymphoma patients underwent myeloablative conditioning and anti-HIV gene transduced CD34+ cell infusion. The team infused unmanipulated CD34+ cells into the patients to ensure safety. Gene marking in peripheral blood mononuclear cells was low at 0.02–0.32% of the cells during the follow up period up to 24 months and the RNA transgenes were detectable by PCR. The low levels of vivo gene marking might be caused by the safety transplant procedure with co-infusion of genetically unmodified cells and resultant dilution of the gene modified CD34+ cells. Another possibility is that high expression of shRNA from the transcriptionally strong U6 promoter might have caused cytotoxicity. Their in vitro gene tracking experiment resulted in a sharp decline of vector DNA copies in transduced CD34+ cells within 4 weeks. These results suggest a growth disadvantage of vector transduced CD34+ cells. We and other reported that over expression of shRNA from the U6 promoter can induce cytotoxicity in human T lymphocytes (An et al., 2006; Kiem et al., 2010; Lo et al., 2007). shRNA utilize the endogenous microRNA pathway to mature and therefore may interfere with miRNA biogenesis. Therefore, the level of shRNA expression must be carefully optimized. Switching to a transcriptionally weaker H1 promoter to minimize shRNA expression may provide stable maintenance of shRNA expressing cells. Further optimizations in the vector design and transplant protocol may improve the efficiency of gene modified cells in the future trials. In summary, this clinical trial provided the first time usage of a lentiviral vector with a triple combination of anti-HIV genes through stem cell transplant in AIDS related lymphoma patients.

Summary of intracellular HSC therapeutic protection strategies

Stem cell based gene therapy approaches hold great potential for controlling HIV infection through a single treatment. The challenge of achieving therapeutic levels of genetically modified cells for patient transplants cell in clinical trials remains a major obstacle. Current gene therapy technologies have reached the point where an HSC based gene therapy is able to provide therapeutic benefits to single gene deficiency diseases. Examples of diseases where investigators have observed effective single gene therapies include Adenosine deaminase deficiency severe combined immunodeficiency (ADA SCID) and X-linked SCID, where gene corrected cells have strong in vivo selection in gene- corrected- HSC transplanted patients (Aiuti et al., 2009; Cavazzana-Calvo et al., 2000; Gaspar et al., 2004). In HIV single gene therapyies, in cells expressing anti-HIV genes are predicted to be selected due to resistance to HIV mediated T-cell killing. The recent success of gene therapy clinical trials using lentiviral vectors for the treatment of adrenoleukodystrophy (ALD) patients provided great promise of this vector system to repopulate the human hematopoietic system with genetically modified cells (Cartier et al., 2009). In the clinical trial, 9–14% of the peripheral blood cells express the ALD transgene product and gene corrected microglial cells migrated into the brain and stopped progressive demyelination. However, a recent thalassaemia stem cell based gene therapy using a lentiviral vector provides us a cautionary tale (Cavazzana-Calvo et al., 2010). In one individual, the treatment achieved the criteria for clinical benefit but this was achieved by a single cell clonal out-growth in the erythroid population caused by the over expression of high mobility group AT-hook 2 (HMGA2) via vector insertion into control elements of the gene. The clonal growth was not a malignancy and it is not clear whether this event is a unique case in the erythroid lineage. This thalassaemia example illustrates the notion that we currently do not have enough experience with lentiviral vectors in clinical trials to be certain of their safety. Further investigation will show whether lentiviral vectors can be used as an efficient and safe vector delivery system for HIV gene therapy. A non-viral vector approach using a CCR5 directed zinc finger nuclease showed great promise in a humanized mouse model (Holt et al., 2010), though the fidelity of CCR5 disruption and off target gene disruptions need to be fully investigated with a genome wide analysis. Successful application of stem cell based gene therapy strategies for HIV infection requires further investigation and development of effective anti-HIV genes, efficient gene delivery vehicles, a greater understanding of stem cell biology, and a safe and effective bone marrow transplant procedure. Our hope is to provide a long-term control of HIV infection by a single treatment using stem cell based gene therapy in the near future.

Engineering HIV Immunity

Introduction

In addition to developing HSC gene therapies to protect cells from infection with HIV, there are efforts to “engineer” resultant immune cells to specifically target and kill HIV in the body. As a basis for these approaches, antiviral immune responses towards HIV are naturally generated in most affected individuals; however, in the large majority of cases this response is insufficient at preventing replication and clearing the virus from the body. In the natural course of HIV infection, the levels of virus in the body following the acute stage represent the ongoing struggle between natural immunity and the virus’s ability to replicate and evade these responses. Therapeutic intervention strategies, while largely successful in lowering the levels of virus and prolonging disease progression, are currently incapable of eradicating the virus and repairing the damage done to the immune system by the infection. Pertaining to this, there is a strong demand for new and novel strategies that augment or enhance natural antiviral immune responses and allow reconstitution of other virally perturbed aspects of immunity.

Immune Therapy for HIV Infection

While the potential benefits of immune therapy for HIV infection utilizing a stem cell based gene therapy approach is discussed below, it is important to first discuss the rationale behind therapeutic strategies that attempt to enhance natural immune responses against the virus. The ultimate goal of all of these strategies is to manipulate antiviral immune responses to eradicate the virus from the body. A fundamental problem in HIV infection is that the virus does not possess the immunogenicity to elicit adequate protective responses. This is evident in the natural course of infection and has been a confounding factor in attempts to develop an effective preventative or therapeutic vaccine (McElrath and Haynes, 2010). In addition, HIV attacks the immune system itself and actively subverts it through a variety of mechanisms that further contribute to viral persistence (reviewed by (Trono et al., 2010)).

A relatively new focus has been to develop strategies that are directed at manipulating virus-specific humoral an/or cellular immune responses in hopes of overcoming these barriers to allow immune suppression and clearance. Relatively simple strategies aimed at enhancing antiviral humoral immune responses have demonstrated that passive immunization of rhesus macaques with neutralizing antibodies is protective against simian immunodeficiency virus (SIV) or simian-human immunodeficiency virus (SHIV) challenge and is associated with lower viral loads in infected animals (Baba et al., 2000; Hessell et al., 2009; Kramer, Siddappa, and Ruprecht, 2007; Mascola et al., 2000; Ng et al., 2010; Shibata et al., 1999). In humans, passive administration of HIV-specific neutralizing monoclonal antibodies resulted in delay of viremia following cessation of antiretroviral therapy (Mehandru et al., 2007; Trkola et al., 2005). While recent data strongly suggest that humoral immunity and neutralizing antibodies play an important role in controlling the levels of virus in chronic HIV infection (Huang et al., 2010), enhancing these responses could be therapeutically beneficial.

Efforts have also been made to enhance existing virus antigen-specific cellular immunity in people infected with cytomegalovirus (CMV) and Epstein-Barr Virus (EBV) by expanding virus-antigen specific cytotoxic T lymphocytes (CTLs) ex vivo followed by subsequent infusion. This approach has been demonstrated to be relatively safe and effective at enhancing T cell immunity to these viruses and virally transformed cells in vivo (Bollard et al., 2004; Heslop et al., 2010; Riddell et al., 1992). Ex vivo expansion of autologous HIV-specific CTL from infected individuals has been demonstrated to produce cells that retain their HIV-specific lytic function and migrate to sites of HIV replication in the lymph nodes (Brodie et al., 1999; Brodie et al., 2000; Lieberman et al., 1997). These cells transiently lowered the levels of productively infected CD4+ T cells. However, they did not have a significant effect on lowering viral loads and appear to persist for a relatively short period of time following infusion (Brodie et al., 1999; Brodie et al., 2000). The inability of these cells to have a significant effect on viral loads is likely due to the functional perturbation of these cells prior to ex vivo expansion by an altered immune environment resultant from HIV infection. The lack of persistence of these adoptively transferred cells is likely due to immune exhaustion in the presence of ongoing viral antigenic exposure. These cells likely lack full functional competency and altered effector function and may not represent the numbers necessary to have any effects on enhancing the CTL response when infused into the individual. However, ex vivo expansion and adoptive transfer of antigen-specific cells appears to be more successful when targeting malignancy related antigens where perturbations in the immune environment are not as dramatic as they are in the context of HIV infection (Berry, Moeller, and Darcy, 2009; DiGiusto and Cooper, 2007; June, 2007; Rosenberg, 2004; Rosenberg et al., 2008; Zhou et al., 2005). A methodology to generate large numbers of functionally competent HIV-specific CTL would be a desirable strategy to enhance CTL responses against HIV. These adoptive strategies and the success in the targeting of other viral antigens provide impetus towards the development of strategies that involve genetic enhancement of immune responses towards HIV.

Engineering Antiviral Immunity—Genetic Vaccination for HIV Infection

Gene therapy based approaches that program the immune response to target HIV are undergoing a period of rapid growth and interest. Several approaches that augment or enhance humoral and/or cellular antiviral immune responses are in various phases of development and attempts have been made and are currently under investigation to genetically program cells to target HIV infection in clinical trails. Approaches that rely on “redirecting” peripheral immune cells to target HIV infection are the most developed to this point, with the technology having moved further along and with several clinical trials either completed or in progress (reviewed by (June, Blazar, and Riley, 2009; Rossi, June, and Kohn, 2007)). A primary reason for this is simply that peripheral cells are relatively easy to obtain and manipulate. Stem cell-based approaches that produce immune cells that target viral infection are a newer concept and are more difficult to perform, with most of the preliminary development occurring in mouse systems as proof-of-principle studies (Rossi, June, and Kohn, 2007). However, as will be discussed, there are potentially significant advantages in using stem cell based approaches to engineer antiviral immune responses and their development is rapidly advancing.

Peripheral “redirection” of Antiviral Immunity

Peripheral blood T cells, which are particularly suitable for genetic manipulation, have been the focus of most efforts to date to “redirect” cells to target HIV infection. Studies have involved the utilization of peripheral T cells that have been genetically altered to express a chimeric molecule or a molecularly cloned T cell receptor (TCR) that targets cells to HIV antigens (Clay et al., 1999; Cooper et al., 2000; Deeks et al., 2002; Hofmann et al., 2008; Joseph et al., 2008; Miles, Silins, and Burrows, 2006; Mitsuyasu et al., 2000; Roberts et al., 1994; Varela-Rohena et al., 2008; Walker et al., 2000; Yang et al., 1997). There were early studies and clinical trials based on redirecting T cells using a chimeric receptor that has the gp120 binding domain of the human CD4 molecule that is fused to the zeta chain signaling domain of T cell receptor, termed a universal T cell receptor (UTR) (Deeks et al., 2002; Mitsuyasu et al., 2000; Roberts et al., 1994; Walker et al., 2000; Yang et al., 1997). This chimeric receptor T cell redirection approach avoids problems associated with human leukocyte antigen (HLA) restriction by an antigen-specific TCR and would allow more broad recognition of virally expressing cells. The clinical studies, in sum, showed persistence of cells harboring the chimeric transgene for more that one year. One study demonstrated rises in CD4 T cell levels in people receiving ex vivo expanded autologous T cells and modest decreases in viral reservoirs in HIV infected individuals receiving the UTR versus control (Deeks et al., 2002). Several other approaches that utilize chimeric receptors to redirect T cells to target malignancy related antigens have been performed to varying degrees of success (reviewed in (June, Blazar, and Riley, 2009; Sadelain, Brentjens, and Riviere, 2009)), and thus remain a viable option to target antigen expressing cells.

In addition, studies have been performed to redirect peripheral T cells to express a molecularly cloned TCR specific to HIV (Cooper et al., 2000; Hofmann et al., 2008; Joseph et al., 2008; Miles, Silins, and Burrows, 2006; Varela-Rohena et al., 2008). This approach involves molecularly cloning an HLA-restricted, peptide specific TCR from reactive T cells from an infected individual and subsequent modification of peripheral cells by retroviral vectors (Cooper et al., 2000), lentiviral vectors (Joseph et al., 2008; Varela-Rohena et al., 2008), or RNA electroporation (Hofmann et al., 2008). All of these approaches resulted in functional expression of the transgenic HIV-specific TCR which redirected the transduced peripheral CD8+ T cells in vitro and (in one case) in vivo in humanized SCID mice (Joseph et al., 2008), to respond to HIV. This approach has been used to successfully redirect cells to target malignancy related antigens, in particular the MART-1 melanoma antigen (Clay et al., 1999; Johnson et al., 2006; June, Blazar, and Riley, 2009; Morgan, Dudley, and Rosenberg, 2010; Morgan et al., 2003) and has proven to be safe in patients and resulted in clinical regression of metastatic melanoma lesions (Morgan et al., 2006). In HIV infection, there are efforts to “enhance” this redirection approach by modifying peripheral cells using high-affinity TCRs molecularly cloned, modified, and selected for higher peptide-HLA binding affinity from individuals with robust, HIV-specific CTL responses (Varela-Rohena et al., 2008). These high-affinity TCRs (in this case specific to the relatively conserved, HLA-A*0201-restricted, p17 Gag SLYNTVATL (SL9) viral peptide) appear to have an enhanced functional ability in producing polyfunctional cells that are capable of suppressing viral replication of wild type and naturally occurring SL9 escape variants. A phase 1 clinical trial of this approach is currently underway (see http://clinicaltrials.gov/show/NCT00991224). This or similar approaches that attempt to redirect peripheral T cells to target cells expressing lower levels of antigen or to constrain viral evolution could be beneficial in lowering viral loads or at least provide further insight as to the constraints that TCR interaction with antigen-expressing cells confers on the generation of effective antiviral responses.

However, there are several limitations to the peripheral redirection approach that may hinder its therapeutic development in HIV disease. The ex vivo manipulation of mature T cells inherent in this strategy can functionally alter the cells and result in subsequent and important effects on cellular differentiation in vivo following re-infusion. As recent studies suggest, cellular differentiation phenotypes and methods of in vitro manipulation of peripheral T cells can have dramatic effects in functional outcomes following handling (Hinrichs et al., 2010; Kaneko et al., 2009; Zhang et al., 2007). Further, ex vivo manipulation has a significant impact on the lifespan of the cells once re-infused back in the body (Berry, Moeller, and Darcy, 2009). In addition, a particular problem with the introduction of antigen-specific TCRs involves the fact that the altered cells retain the expression of their endogenous T cell receptors. Modification of peripheral T cells with a cloned TCR could result in cross-receptor paring with the expressed endogenous TCR, producing mixed TCR α and β chain pairs. This could produce self-reactive T cells by overriding peripheral tolerance mechanisms since these cells expressing these cross-paired TCR chains are not subjected to normal thymic negative selection processes. There is evidence to suggest that this mechanism may contribute to graft-versus-host disease (GVHD) in a mouse model that involved the modification of mouse peripheral T cells with a cloned TCR, where mixed TCRs were produced and resulted in autoreactive cells (Bendle et al., 2010). In addition, a study that introduced human TCRs into human peripheral T cells demonstrated the formation of alloreactive cells expressing mixed TCRs in vitro (van Loenen et al., 2010). However, while these studies show that this is a potential consequence of introduced TCR in peripheral T cells expressing an endogenous TCR and that this approach must be performed in humans with careful safeguards, cross-pairing of TCRs may be limited to certain settings as the generation of GVHD does not appear to occur following autologous reinfusion of TCR modified T cells in human clinical trials (Rosenberg, 2010). In addition, methods to optimize transgenic α and β pairing may bypass or reduce the likelihood of this scenario (Govers et al., 2010). As will be discussed below, an approach utilizing human HSCs may overcome many of these problems by allowing natural developmental mechanisms to produce genetically engineered immune cells that target HIV infection that are tolerant of the host and possess prolonged self-renewal capability.

Stem Cell-based Immune Programming

The fact that the immune system is incapable of naturally eradicating HIV infection mandates that a successful immune-based therapeutic strategy would allow long-lived, renewable immune responses capable of generating the quantity of antiviral cells and quality needed to allow complete eradication of HIV from the body. A new focus on this approach is the utilization of human HSCs that would allow the development of human immune cells capable of targeting HIV. The use of a HSC based therapeutic approach would allow multilineage hematopoietic development and the potential of targeting one or more of these arms of the immune response towards HIV. The development of these genetically modified, HIV-targeted cells from HSCs in the body following normal cellular differentiation pathways would allow proper and normal “education” and selection of these cells to produce cells that would, theoretically, be physiologically like unmodified cells. Thus, the activation and expansion of antigen reactive cells in the periphery would allow the differentiation of these cells into long-term memory cells through natural mechanisms. Further, a HSC based approach would allow long-term engraftment of genetically modified cells capable of generating continual lymphopoietic development of antigen-naïve HIV-targeted cells, potentially repairing defects in exhausted HIV-specific cells. They would thus lack the issues of functional impairment, developmental biasing, or exhaustion that other peripheral cell ex vivo modification methods would have. A human stem cell based approach is one of the more recent to gain impetus and one of the more difficult strategies to develop due to the paucity of experimental systems that allow the close examination of human hematopoietic events and cellular function. However, recent advances have been made in the early development of HSC based immune programming as a viable therapeutic strategy.

One initial aim of stem cell based immune therapeutic strategies has focused on engineering the development of B cells that produce neutralizing antibodies or CTL that target and kill HIV infected cells. Recent studies have demonstrated the ability to program B cells resultant from human HSCs to express an anti-HIV neutralizing antibody following differentiation in vitro and differentiation in vivo in humanized mice (Joseph et al., 2010; Luo et al., 2009). Further, expression of the neutralizing antibody following development into B cells in vivo in humanized mice significantly lowered viral loads and levels of infected cells (Joseph et al., 2010). While this approach using a single monoclonal antibody would almost certainly drive viral mutation in vivo, these studies provide the proof-of-principle that can be further explored in the utilization of multiple antibodies directed against HIV. Engineered B cell responses may be useful in enhancing cellular immune responses, particularly in the mucosal and innate immune compartments, and a therapeutic benefit may be obtained alone or in combination with other strategies. Further development in this approach may provide insight in the development of other chimeric receptors or single-chain therapeutic antibodies with recognition domains that target cellular immunity towards HIV infected cells (Roberts et al., 1994; Rossi, June, and Kohn, 2007; Yang et al., 1997).

Due to their importance in controlling HIV infection, engineering HSCs to augment or enhance antiviral T cell immunity is another enticing strategy for investigation as a potential therapeutic approach. Studies performed in mice examining antigen specific T cell responses utilizing transgenic mice that express a mouse TCR specific to one of a variety of antigens provide a basis for exploring this type of genetic modification in human cells (Pircher et al., 1989; Tian et al., 2007). Further, mouse-based studies demonstrated that mouse HSCs modified with cloned mouse TCRs can develop into antigen-specific T cells in vivo (Yang and Baltimore, 2005; Yang et al., 2002). Early studies with human cells demonstrated the ability of human HSCs to develop into T cells following differentiation on murine Delta-like 1 molecule-expressing stromal cell lines (van Lent et al., 2007; Zhao et al., 2007). However, the resultant cells in these studies that expressed the transgenic TCR did not undergo normal positive and negative selection events that a developing T cell would encounter in the human thymic environment. In what we believe is a major step forward in this approach, we recently determined that a molecularly cloned TCR specific for the SL9 peptide allows the development of functional HIV-specific CTL in human thymus tissue in vivo in SCID-hu mice (Kitchen et al., 2009). This TCR recognizes the SL9 epitope in the context of HLA-A*0201, and we demonstrated the necessity for expression of this allele in the thymus tissue to allow proper development of CD8 single positive thymocytes. These studies are the first to demonstrate that a human TCR can allow the development of human stem cells into mature, functional CTLs in human tissue in vivo and further demonstrate the necessity of matching the cloned TCR to the proper HLA type.

A therapeutic strategy involving molecularly cloned, antigen specific TCRs, much like with the modification of peripheral blood T cells with a cloned TCR, would therefore have to be tailored to the HLA type of the individual receiving treatment in order to produce cells that survive T cell selection processes. However, this process of development and thymic positive and negative selection from HSC progenitors would theoretically reduce the possibility of producing cells that are autoreactive, bypassing a major drawback of peripheral T cell modification. A further advantage of using a stem cell based approach is longer engraftment of functionally capable gene marked cells than with peripheral cell based modification. As with the necessity of utilizing multiple molecularly cloned monoclonal antibodies for that approach to be a viable therapeutic strategy, multiple TCRs targeted to different, relatively conserved epitopes of HIV within defined HLA molecules would likely reduce the possibility of viral immune escape and increase the numbers of suppressive antiviral CTLs. Indentifying and further characterizing conserved HIV TCR epitopes is important in the expansion and utilization of an engineered T cell approach. Immune escape to avoid certain CTL epitopes may render a fitness cost to the virus, and therefore lower the capacity of the virus to replicate. There is evidence that viral evolution and immune escape to alter certain conserved epitopes occurs relatively slowly, and immune pressure caused by T cell responses engineered against them may be therapeutically beneficial in forcing the evolution of the virus into a less fit state (Althaus and De Boer, 2008; Troyer et al., 2009).

Due to the technological development of more rapid antigen-specific TCR identification strategies (Balamurugan, Ng, and Yang, 2010), it may become possible to develop multiple “off the shelf” molecularly cloned TCRs to a variety of viral antigens in the context of different HLA types that could be rapidly matched to the individual HLA type and viral genotypes they possess. In addition, chimeric receptors, such as the aforementioned UTR, may provide a way of allowing the generation of antigen-specific cells outside of HLA restriction. Due to the fact that natural antigen specific TCR precursor cell frequencies are relatively low and that a single precursor can produce thousands of antigen-specific progeny cells (Harty and Badovinac, 2008; Wiesel et al., 2009), it would not be necessary to achieve the high levels of genetic transduction efficiencies necessary to protect progeny cells from infection or to redirect peripheral blood cells in other gene therapy models. A recent study, using a highly sensitive, enrichment-based technique, identified the naïve CD8+ T cell precursor frequency for the SL9 epitope in uninfected HLA-A*0201 individuals as approximately one in 3.3 × 10⁶ cells in the peripheral blood (Alanio et al., 2010). This precursor frequency is similar to that of cells with a specificity to a variety of other viral antigens. Increasing this naïve cell, HIV-specific precursor frequency through molecularly cloned HIV-specific TCRs to conserved epitopes of HIV could overcome limits in the magnitude of the CTL response, reconstitute defects that appear in the ability of antigen-specific cells to respond by supplying newly developed, naïve cells, and diversify the breadth of the responses by targeting new epitopes in treated individuals. While the current preclinical development of this approach is ongoing, engineering T cell immunity through the use of molecularly cloned, HIV-specific TCRs and HSCs represents a potential means to increase the quantity and quality of the CTL response in infected individuals. With the ultimate goal of immune control of viral replication, this approach could provide benefit towards delaying or preventing disease progression.

Conclusions

In sum, therapeutic strategies to treat HIV infection utilizing stem cellbased approaches have recently become a major focus of interest. Recently published studies have demonstrated the reproducibility and feasibility of performing genetic manipulation and transplantation of HSCs in an expanded clinical setting. There are currently several therapeutic strategies under investigation that are aimed at treating HIV infection by protecting the cell from infection or by specifically engineering and targeting immune responses towards the virus using adult-derived HSCs. Alternative approaches are also under investigation that use stem cells derived from different sources. For instance, there have been recent reports that human embryonic stem cells (hESC) can differentiate into the major HIV target cells, T cells, dendritic cells, and macrophages (Anderson et al., 2006; Bandi and Akkina, 2008; Galic et al., 2006; Galic et al., 2009; Subramanian et al., 2009; Timmermans et al., 2009). As further investigation into better differentiation protocols for these cells allows this to become more efficient, it is potentially feasible that stem cells sourced from hESC as well as induced pluripotent stem cells (iPSC) (Lowry et al., 2008; Park et al., 2008; Takahashi et al., 2007; Yu et al., 2007) could be used for gene therapeutic strategies to combat multiple diseases, including HIV infection. The coming years will likely see the development of greater numbers of safety and efficacy studies from the next generation of stem cell based therapeutic approaches that will serve as the basis towards the use of these strategies in therapeutic applications for many chronic diseases.