High levels of transduction of human dendritic cells with optimized SIV vectors

Interplay between HIV entry and transportin-SR2 dependency

BACKGROUND

Transportin-SR2 (TRN-SR2, TNPO3, transportin 3) was previously identified as an interaction partner of human immunodeficiency virus type 1 (HIV-1) integrase and functions as a nuclear import factor of HIV-1. A possible role of capsid in transportin-SR2-mediated nuclear import was recently suggested by the findings that a chimeric HIV virus, carrying the murine leukemia virus (MLV) capsid and matrix proteins, displayed a transportin-SR2 independent phenotype, and that the HIV-1 N74D capsid mutant proved insensitive to transportin-SR2 knockdown.

RESULTS

Our present analysis of viral specificity reveals that TRN-SR2 is not used to the same extent by all lentiviruses. The DNA flap does not determine the TRN-SR2 requirement of HIV-1. We corroborate the TRN-SR2 independent phenotype of the chimeric HIV virus carrying the MLV capsid and matrix proteins. We reanalyzed the HIV-1 N74D capsid mutant in cells transiently or stably depleted of transportin-SR2 and confirm that the N74D capsid mutant is independent of TRN-SR2 when pseudotyped with the vesicular stomatitis virus glycoprotein (VSV-G). Remarkably, although somewhat less dependent on TRN-SR2 than wild type virus, the N74D capsid mutant carrying the wild type HIV-1 envelope required TRN-SR2 for efficient replication. By pseudotyping with envelopes that mediate pH-independent viral uptake including HIV-1, measles virus and amphotropic MLV envelopes, we demonstrate that HIV-1 N74D capsid mutant viruses retain partial dependency on TRN-SR2. However, this dependency on TRN-SR2 is lost when the HIV N74D capsid mutant is pseudotyped with envelopes mediating pH-dependent endocytosis, such as the VSV-G and Ebola virus envelopes.

CONCLUSION

Here we discover a link between the viral entry of HIV and its interaction with TRN-SR2. Our data confirm the importance of TRN-SR2 in HIV-1 replication and argue for careful interpretation of experiments performed with VSV-G pseudotyped viruses in studies on early steps of HIV replication including the role of capsid therein.

A cryptic sensor for HIV-1 activates antiviral innate immunity in dendritic cells

Dendritic cells (DC) serve a key function in host defense, linking innate detection of microbes to the activation of pathogen-specific adaptive immune responses(1,2). Whether there is cell-intrinsic recognition of HIV-1 by host innate pattern-recognition receptors and subsequent coupling to antiviral T cell responses is not yet known(3). DC are largely resistant to infection with HIV-1(4), but facilitate infection of co-cultured T-helper cells through a process of trans-enhancement(5,6). We show here that, when DC resistance to infection is circumvented(7,8), HIV-1 induces DC maturation, an antiviral type I interferon response and activation of T cells. This innate response is dependent on the interaction of newly-synthesized HIV-1 capsid (CA) with cellular cyclophilin A (CypA) and the subsequent activation of the transcription factor IRF3. Because the peptidyl-prolyl isomerase CypA also interacts with CA to promote HIV-1 infectivity, our results suggest that CA conformation has evolved under opposing selective pressures for infectivity versus furtiveness. Thus, a cell intrinsic sensor for HIV-1 exists in DC and mediates an antiviral immune response, but it is not typically engaged due to absence of DC infection. The virulence of HIV-1 may be related to evasion of this response, whose manipulation may be necessary to generate an effective HIV-1 vaccine.

Development of a lentiviral vector system to study the role of the Andes virus glycoproteins

To infect target cells, enveloped viruses use their virion surface proteins to direct cell attachment and subsequent entry via virus-cell membrane fusion. How hantaviruses enter cells has been largely unexplored. To study early steps of Andes virus (ANDV) cell infection, a lentiviral vector system was developed based on a Simian immunodeficiency virus (SIV) vector pseudotyped with the ANDV-Gn/Gc envelope glycoproteins. The incorporation of Gn and Gc onto SIV-derived vector particles was assessed using newly generated monoclonal antibodies against ANDV glycoproteins. In addition, sera of ANDV infected humans were able to block cell entry of the SIV vector pseudotyped with ANDV glycoproteins, suggesting that their antigenic conformation is similar to that in the native virus. The use of such SIV vector pseudotyped with ANDV-Gn/Gc glycoproteins should facilitate studies on ANDV cell entry. Along this line, it was found that depletion of cholesterol from target cells strongly diminished cell infection, indicating a possible role of lipid rafts in ANDV cell entry. The Gn/Gc pseudotyped SIV vector has several advantages, notably high titer vector production and easy quantification of cell infection by monitoring GFP reporter gene expression by flow cytometry. Such pseudotyped SIV vectors can be used to identify functional domains in the Gn/Gc glycoproteins and to screen for potential hantavirus cell entry inhibitors.

Characterization of the alpha interferon-induced postentry block to HIV-1 infection in primary human macrophages and T cells

Type I interferon (IFN) inhibits virus replication by activating multiple antiviral mechanisms and pathways. It has long been recognized that alpha interferon (IFN-alpha) can potently block both early and late stages of HIV-1 replication. The mechanistic basis for the early block(s) to infection is unknown, as is the identity of the participating antiviral factor(s). Here, we define the effect(s) of IFN-alpha on HIV-1 infection of primary human macrophages and CD4(+) T cells, as well as several monocytic and T-cell lines. We demonstrate that IFN-alpha treatment of macrophages, THP-1 cells, and, to a lesser extent, primary CD4(+) T cells markedly inhibits infection, whereas the effects are minimal in CD4(+) T-cell lines. Virus entry is essentially unaffected by IFN-alpha, but substantial decreases (sometimes >99%) in nascent cDNA accumulation correlate closely with losses in infectivity. Interestingly, proteasome inhibitors rescue viral cDNA accumulation, revealing a link between the ubiquitin-proteasome system and IFN-alpha-induced viral restriction. We also found that diverse primate and nonprimate retroviruses were susceptible to suppression by IFN-alpha. Importantly, all the primary and immortalized cells used here are proficient at responding to IFN-alpha, as judged by the induced expression of numerous IFN-stimulated genes, including PKR and OAS1, indicating that a general deficiency in IFN-alpha responsiveness does not underlie IFN-alpha's inability to elicit an antiviral state in CD4(+) T-cell lines. Rather, we speculate that IFN-alpha fails to induce antiretroviral factors in these cells and that comparative transcriptional profiling with responsive cells, such as macrophages, invokes a strategy for identifying new host-encoded antiviral effectors.

Limelight on two HIV/SIV accessory proteins in macrophage infection: is Vpx overshadowing Vpr?

HIV viruses encode a set of accessory proteins, which are important determinants of virulence due to their ability to manipulate the host cell physiology for the benefit of the virus. Although these viral proteins are dispensable for viral growth in many in vitro cell culture systems, they influence the efficiency of viral replication in certain cell types. Macrophages are early targets of HIV infection which play a major role in viral dissemination and persistence in the organism. This review focuses on two HIV accessory proteins whose functions might be more specifically related to macrophage infection: Vpr, which is conserved across primate lentiviruses including HIV-1 and HIV-2, and Vpx, a protein genetically related to Vpr, which is unique to HIV-2 and a subset of simian lentiviruses. Recent studies suggest that both Vpr and Vpx exploit the host ubiquitination machinery in order to inactivate specific cellular proteins. We review here why it remains difficult to decipher the role of Vpr in macrophage infection by HIV-1 and how recent data underscore the ability of Vpx to antagonize a restriction factor which counteracts synthesis of viral DNA in monocytic cells.

In vitro and in vivo assays of protein kinase CK2 activity

Protein kinase CK2 (formerly casein kinase 2) is recognized as a central component in the control of the cellular homeostasis; however, much remains unknown regarding its regulation and its implication in cellular transformation and carcinogenesis. Moreover, study of CK2 function and regulation in a cellular context is complicated by the dynamic multisubunit architecture of this protein kinase. Although a number of robust techniques are available to assay CK2 activity in vitro, there is a demand for sensitive and specific assays to evaluate its activity in living cells. We hereby provide a detailed description of several assays for monitoring the CK2 activity and its subunit interaction in living cells. The guidelines presented herein should enable researchers in the field to establish strategies for cellular screenings of CK2 inhibitors.

Potent inhibition of HIV-1 by TRIM5-cyclophilin fusion proteins engineered from human components

New World monkeys of the genus Aotus synthesize a fusion protein (AoT5Cyp) containing tripartite motif-containing 5 (TRIM5) and cyclophilin A (CypA) that potently blocks HIV-1 infection. We attempted to generate a human HIV-1 inhibitor modeled after AoT5Cyp, by fusing human CypA to human TRIM5 (hT5Cyp). Of 13 constructs, 3 showed substantial HIV-1-inhibitory activity when expressed in human cell lines. This activity required capsid binding by CypA and correlated with CypA linkage to the TRIM5a capsid-specificity determinant and the ability to form cytoplasmic bodies. CXCR4- and CCR5-tropic HIV-1 clones and primary isolates were inhibited from infecting multiple human macrophage and T cell lines and primary cells by hT5Cyp, as were HIV-2ROD, SIVAGMtan, FIVPET, and a circulating HIV-1 isolate previously reported to be AoT5Cyp resistant. The anti-HIV-1 activity of hT5Cyp was surprisingly more effective than that of the well-characterized rhesus TRIM5alpha, especially in T cells. hT5Cyp also blocked HIV-1 infection of primary CD4+ T cells and macrophages and conferred a survival advantage to these cells without disrupting their function. Extensive attempts to elicit HIV-1 resistance to hT5Cyp were unsuccessful. Finally, Rag2-/-gammac-/- mice were engrafted with human CD4+ T cells that had been transduced by optimized lentiviral vectors bearing hT5Cyp. Upon challenge with HIV-1, these mice showed decreased viremia and productive infection in lymphoid organs and preserved numbers of human CD4+ T cells. We conclude that hT5Cyp is an extraordinarily robust inhibitor of HIV-1 replication and a promising anti-HIV-1 gene therapy candidate.

Development and use of SIV-based Integrase defective lentiviral vector for immunization

Integrase (IN) defective lentiviral vectors have a high safety profile and might prove useful as immunizing agents especially against HIV-1. However, IN defective SIV-based vectors must be developed in order to test their potential in the non-human primate models (NHP) of AIDS. To this aim we tested a novel SIV-based IN defective lentiviral vector for its ability to induce sustained immune responses in mice. BALB/c mice were immunized once intramuscularly with a SIV-based IN defective lentiviral vector expressing the model antigen enhanced green fluorescence protein (eGFP). Immune responses were evaluated 90 days after the injection and compared with those elicited with the IN competent counterpart. The IN defective vector was able to efficiently elicit specific and long-lasting polyfunctional immune responses as evaluated by enzyme-linked immunospot (ELISPOT) assays for interferon-gamma (IFN-gamma) in spleens, bone marrow (BM) and draining lymph nodes, and by intracellular staining (ICS) for IFN-gamma, Interleukin-2 (IL-2) and tumor necrosis factor (TNF-alpha) in both splenocytes and BM cells without integration of the vector into the host genome. This is the first demonstration that an IN defective SIV-based lentiviral vector provides effective immunization, thus paving the way for the construction of IN defective vectors expressing SIV antigen(s) and test their efficacy against a SIV virus challenge in the NHP model of AIDS.

A cell-based bicistronic lentiviral reporter system for identification of inhibitors of the hepatitis C virus internal ribosome entry site

This report describes the development, optimization and implementation of a persistent cell-based system to test inhibitors of hepatitis C (HCV) translation. The assay is based on a heterologous human immunodeficiency virus-1/simian immunodeficiency virus (HIV-1/SIV) lentiviral vector expressing the bicistronic cassette containing the firefly and renilla luciferase genes, respectively, as reporters, and the HCV internal ribosome entry site (IRES) inserted in between, under the control of the cytomegalovirus (CMV) promoter. The drug target in this assay is the HCV IRES, the activity of which leads to modulation of the renilla luciferase gene expression under its control, which is monitored by luminometry. The system has been validated using interferon (IFN), which is still the only consensual antiviral agent against HCV infection, associated with ribavirin. This bicistronic vector, extended to other viral IRESs and assayed in different cell lines, exhibited weak cell tropism, allowing its broad use in gene therapy, which frequently needs a multicistronic transfer vector to follow the expression of a gene of interest inside the target cells with the aid of a reporter, a drug selection marker, or a suicide gene, expressed from the same transcript.

Lentiviral vector-mediated genetic programming of mouse and human dendritic cells

Dendritic cells (DCs) play a key role in the orchestration of immune reactions. Manipulation of DC function through genetic manipulation for vaccine development provides a multitude of applications for active immunotherapy of cancer and chronic infections. Several laboratories have shown that lentiviral vectors (LVs) are efficient and consistent tools for ex vivo gene manipulation of DCs and their precursors. LVs integrate in the genome of target cells resulting in persistent and stable transgene expression, and gene delivery does not result in cytostatic or nonspecific adverse immunomodulatory reactions. Mouse, macaque, and human DCs are efficiently transduced with LVs, allowing preclinical vaccination studies to be gradually implemented into clinical trials. This chapter describes HIV-1-derived LV transduction used for ex vivo gene delivery of marking genes, antigens, and immunomodulatory molecules into mouse and human hematopoietic precursors and DCs. With the perspective of bioengineering DCs from the inside-out, we also describe a one-hit LV transduction method for constitutive expression of GM-CSF and IL-4 genes, which allows self-differentiation of mouse and human hematopoietic precursor cells into highly viable and potent DCs.

Characterization of simian immunodeficiency virus SIVSM/human immunodeficiency virus type 2 Vpx function in human myeloid cells

Human immunodeficiency virus type 2 (HIV-2)/simian immunodeficiency virus SIV(SM) Vpx is incorporated into virion particles and is thus present during the early steps of infection, when it has been reported to influence the nuclear import of viral DNA. We recently reported that Vpx promoted the accumulation of full-length viral DNA following the infection of human monocyte-derived dendritic cells (DCs). This positive effect was exerted following the infection of DCs with cognate viruses and with retroviruses as divergent as HIV-1, feline immunodeficiency virus, and even murine leukemia virus, leading us to suggest that Vpx counteracted an antiviral restriction present in DCs. Here, we show that Vpx is required, albeit to a different extent, for the infection of all myeloid but not of lymphoid cells, including monocytes, macrophages, and monocytoid THP-1 cells that had been induced to differentiate with phorbol esters. The intracellular localization of Vpx was highly heterogeneous and cell type dependent, since Vpx localized differently in HeLa cells and DCs. Despite these differences, no clear correlation between the functionality of Vpx and its intracellular localization could be drawn. As a first insight into its function, we determined that SIV(SM)/HIV-2 and SIV(RCM) Vpx proteins interact with the DCAF1 adaptor of the Cul4-based E3 ubiquitin ligase complex recently described to associate with HIV-1 Vpr and HIV-2 Vpx. However, the functionality of Vpx proteins in the infection of DCs did not strictly correlate with DCAF1 binding, and knockdown experiments failed to reveal a functional role for this association in differentiated THP-1 cells. Lastly, when transferred in the context of a replication-competent viral clone, Vpx was required for replication in DCs.

Reconstitution of the myeloid and lymphoid compartments after the transplantation of autologous and genetically modified CD34+ bone marrow cells, following gamma irradiation in cynomolgus macaques

Background

Prolonged, altered hematopoietic reconstitution is commonly observed in patients undergoing myeloablative conditioning and bone marrow and/or mobilized peripheral blood-derived stem cell transplantation. We studied the reconstitution of myeloid and lymphoid compartments after the transplantation of autologous CD34⁺ bone marrow cells following gamma irradiation in cynomolgus macaques.

Results

The bone marrow cells were first transduced ex vivo with a lentiviral vector encoding eGFP, with a mean efficiency of 72% ± 4%. The vector used was derived from the simian immunodeficiency lentivirus SIVmac251, VSV-g pseudotyped and encoded eGFP under the control of the phosphoglycerate kinase promoter. After myeloid differentiation, GFP was detected in colony-forming cells (37% ± 10%). A previous study showed that transduction rates did not differ significantly between colony-forming cells and immature cells capable of initiating long-term cultures, indicating that progenitor cells and highly immature hematopoietic cells were transduced with similar efficiency. Blood cells producingeGFP were detected as early as three days after transplantation, and eGFP-producing granulocyte and mononuclear cells persisted for more than one year in the periphery.

Conclusion

The transplantation of CD34⁺ bone marrow cells had beneficial effects for the ex vivo proliferation and differentiation of hematopoietic progenitors, favoring reconstitution of the T- and B-lymphocyte, thrombocyte and red blood cell compartments.

Lentiviral transduction of the murine lung provides efficient pseudotype and developmental stage-dependent cell-specific transgene expression

Gene transfer for cystic fibrosis (CF) airway disease has been hampered by the lung's innate refractivity to pathogen infection. We hypothesized that early intervention with an integrating gene transfer vector capable of transducing the lung via the lumen may be a successful therapeutic approach. An HIV-based lentiviral vector pseudotyped with the baculovirus gp64 envelope was applied to the fetal, neonatal or adult airways. Fetal intra-amniotic administration resulted in transduction of approximately 14% of airway epithelial cells, including both ciliated and non-ciliated epithelia of the upper, mid and lower airways; there was negligible alveolar or nasal transduction. Following neonatal intra-nasal administration we observed significant transduction of the airway epithelium (approximately 11%), although mainly in the distal lung, and substantial alveolar transduction. This expression was still detectable at 1 year after application. In the adult, the majority of transduction was restricted to the alveoli. In contrast, vesicular stomatitis virus glycoprotein pseudotyped virus transduced only alveoli after adult and neonatal application and no transduction was observed after fetal administration. Repeat administration did not increase transduction levels of the conducting airway epithelia. These data demonstrate that application at early developmental stages in conjunction with an appropriately pseudotyped virus provides efficient, high-level transgene expression in the murine lung. This may provide a modality for treatment for lung disease in CF.

Viral-mediated coexpression of Pdx1 and p48 regulates exocrine pancreatic differentiation in mouse ES cells

Embryonic stem cells (ES) can spontaneously activate a pancreatic differentiation program in vitro, although with low efficiency. The aim was to improve such process by using viral mediated gene transduction. In this study, we have examined the suitability of using viral vectors to express key transcriptional factors involved in pancreatic development. ES cell lines that constitutively express Pdx1, a homeodomain protein involved in both exocrine and endocrine pancreatic development and differentiation, were established using a lentiviral vector. These cells were additionally infected with an adenovirus expressing p48, a bHLH factor that is also crucial for pancreatic development and acinar differentiation. Quantitative RT-PCR analysis demonstrated an increase in the expression of exocrine genes, including those coding for both digestive enzymes and transcription factors. Immunocytochemical staining also revealed an increase in the number of amylase-expressing cell clusters. However, other important genes involved in acinar cell maturation (i.e., Mist1) were not modulated under these conditions, suggesting that the cells display features of immature exocrine cells or because of an uncoupled gene expression of the exocrine differentiation program. Importantly, this effect was selective for the acinar lineage as the expression of a large set of endocrine markers remained unchanged. Therefore, combined expression of key genes involved in pancreatic development may be a promising approach to generate mature pancreatic exocrine cells.

Stable reduction of CCR5 by RNAi through hematopoietic stem cell transplant in non-human primates

RNAi is a powerful method for suppressing gene expression that has tremendous potential for therapeutic applications. However, because endogenous RNAi plays a role in normal cellular functions, delivery and expression of siRNAs must be balanced with safety. Here we report successful stable expression in primates of siRNAs directed to chemokine (c-c motif) receptor 5 (CCR5) introduced through CD34+ hematopoietic stem/progenitor cell transplant. After hematopoietic reconstitution, to date 14 months after transplant, we observe stably marked lymphocytes expressing siRNAs and consistent down-regulation of chemokine (c-c motif) receptor 5 expression. The marked cells are less susceptible to simian immunodeficiency virus infection ex vivo. These studies provide a successful demonstration that siRNAs can be used together with hematopoietic stem cell transplant to stably modulate gene expression in primates and potentially treat blood diseases such as HIV-1.

Isolation and characterization of human cells resistant to retrovirus infection

Background

Identification of host cell proteins required for HIV-1 infection will add to our knowledge of the life cycle of HIV-1 and in the development of therapeutics to combat viral infection. We and other investigators have mutagenized rodent cells and isolated mutant cell lines resistant to retrovirus infection. Since there are differences in the efficiency of single round infection with VSVG pseudotyped HIV-1 on cells of different species, we conducted a genetic screen to isolate human cells resistant to HIV-1 infection. We chemically mutagenized human HeLa cells and validated our ability to isolate mutants at test diploid loci. We then executed a screen to isolate HeLa cell mutants resistant to infection by an HIV-1 vector coding for a toxic gene product.

Results

We isolated two mutant cell lines that exhibit up to 10-fold resistance to infection by HIV-1 vectors. We have verified that the cells are resistant to infection and not defective in gene expression. We have confirmed that the resistance phenotype is not due to an entry defect. Fusion experiments between mutant and wild-type cells have established that the mutations conferring resistance in the two clones are recessive. We have also determined the nature of the block in the two mutants. One clone exhibits a block at or before reverse transcription of viral RNA and the second clone has a retarded kinetic of viral DNA synthesis and a block at nuclear import of the preintegration complex.

Conclusion

Human cell mutants can be isolated that are resistant to infection by HIV-1. The mutants are genetically recessive and identify two points where host cell factors can be targeted to block HIV-1 infection.

Impact of viral accessory proteins of SIVsmmPBj on early steps of infection of quiescent cells

Although lentiviruses like HIV-1 are able to infect non-dividing cells, particular resting cells such as non-stimulated primary peripheral blood mononuclear cells (PBMC) are resistant to infection. In contrast to other lentiviruses, SIVsmmPBj can replicate in non-stimulated PBMC. Moreover, SIVsmmPBj-derived, but not HIV-1-derived, replication-incompetent vectors enable gene transfer into G(0)-arrested human cell lines and primary human monocytes. Here, we demonstrate that transduction of G(0)-arrested cell lines by SIVsmmPBj-derived vectors is independent of the viral accessory proteins Vif, Vpx, Vpr, or Nef. In contrast, for the transduction of primary human monocytes, the Vpx protein proved to be essential. However, trans-complementation of HIV-1 vectors with SIVsmmPBj Vpx did not provide the property of gene transfer into monocytes. Taken together, these data indicate that Vpx is essential for the infection of primary monocytes by SIVsmmPBj. Additionally, further genome functions besides the accessory proteins are required for the particular capacity of SIVsmmPBj in transduction or infection events.

An Antiproliferative Genetic Screening Identifies a Peptide Aptamer That Targets Calcineurin and Up-regulates Its Activity

Peptide aptamers are combinatorial recognition molecules that consist of a constant scaffold protein displaying a doubly constrained variable peptide loop. They bind specifically target proteins and interfere with their function. We have built a peptide aptamer library in a lentiviral expression system to isolate aptamers that inhibit cell proliferation in vitro. Using one of the isolated aptamers (R5G42) as a bait protein, we have performed yeast two-hybrid screening of cDNA libraries and identified calcineurin A as a target protein candidate. R5G42 bound calcineurin A in vitro and stimulated its phosphatase activity. When expressed transiently in human cells, R5G42 induced the dephosphorylation of BAD. We have identified an antiproliferative peptide aptamer that binds calcineurin and stimulates its activity. The use of this ligand may help elucidate the still elusive structural mechanisms of activation and inhibition of calcineurin. Our work illustrates the power of phenotypic screening of combinatorial protein libraries to interrogate the proteome and chart molecular regulatory networks.

SIVSM/HIV-2 Vpx proteins promote retroviral escape from a proteasome-dependent restriction pathway present in human dendritic cells

Background

Vpx is a non-structural protein coded by members of the SIV_SM/HIV-2 lineage that is believed to have originated by duplication of the common vpr gene present in primate lentiviruses. Vpx is incorporated into virion particles and is thus present during the early steps of viral infection, where it is thought to drive nuclear import of viral nucleoprotein complexes. We have previously shown that Vpx is required for SIV_MAC-derived lentiviral vectors (LVs) infection of human monocyte-derived dendritic cells (DCs). However, since the requirement for Vpx is specific for DCs and not for other non-dividing cell types, this suggests that Vpx may play a role other than nuclear import.

Results

Here, we show that the function of Vpx in the infection of DCs is conserved exclusively within the SIV_SM/HIV-2 lineage. At a molecular level, Vpx acts by promoting the accumulation of full length viral DNA. Furthermore, when supplied in target cells prior to infection, Vpx exerts a similar effect following infection of DCs with retroviruses as divergent as primate and feline lentiviruses and gammaretroviruses. Lastly, the effect of Vpx overlaps with that of the proteasome inhibitor MG132 in DCs.

Conclusion

Overall, our results support the notion that Vpx modifies the intracellular milieu of target DCs to facilitate lentiviral infection. The data suggest that this is achieved by promoting viral escape from a proteasome-dependent pathway especially detrimental to viral infection in DCs.

Recent advances in globin gene transfer for the treatment of beta-thalassemia and sickle cell anemia

PURPOSE OF REVIEW

The beta-thalassemias and sickle cell anemia are severe congenital anemias for which there is presently no curative therapy other than allogeneic hematopoietic stem cell transplantation. This therapeutic option, however, is not available to most patients due to the lack of an HLA-matched bone marrow donor. The transfer of a regulated globin gene in autologous hematopoietic stem cells is therefore a highly attractive alternative treatment. This strategy, simple in principle, raises major challenges in terms of controlling transgene expression, which ideally should be erythroid specific, differentiation and stage restricted, elevated, position independent, and sustained over time.

RECENT FINDINGS

Using lentiviral vectors, May et al. demonstrated that an optimized combination of proximal and distal transcriptional control elements permits lineage-specific and elevated beta-globin expression in vivo, resulting in therapeutic hemoglobin production and correction of anemia in beta-thalassemic mice. Several groups have extended these findings to various models of beta-thalassemia and sickle cell disease. While the addition of the wild-type beta-globin gene is naturally suited for treating beta-thalassemia, several alternatives have been proposed for the treatment of sickle cell disease, using either gamma or mutant beta-globin gene addition, trans-splicing or RNA interference.

SUMMARY

These recent advances bode well for the clinical investigation of stem cell-based gene therapy in the severe hemoglobinopathies.

Gene transfer system derived from the caprine arthritis-encephalitis lentivirus

Lentiviruses are attractive candidates for therapeutic vectors, because of their ability to infect non-dividing target cells. Vectors based on HIV-1 efficiently transfer gene expression to a variety of dividing or quiescent cells, but are subject to reservations on safety grounds. Caprine arthritis encephalitis virus (CAEV) is a lentivirus inducing only minor pathology in its natural host and in related species after cross-species transmission. To test the CAEV potential as vector for gene transfer, a cassette expressing the green fluorescent protein (GFP) under control of a CMV promoter was inserted into the CAEV genome, producing the pK2EGFPH vector. When pseudotyped with vesicular stomatitis virus (VSV)-G envelope protein, this vector allowed efficient transfer of GFP expression in human cells (up to 86% of GFP-expressing cells into the TE671 cell line). Three vectors carrying different parts of the viral gag, pol and env genes were then developed, together with a CAEV packaging system. These vectors allowed delimitation of the minimal CAEV sequences necessary for an improvement of vector production compared to the previously described CAEV-based vectors [Mselli-Lakhal et al., 1998. Defect in RNA transport and packaging are responsible for low transduction efficiency of CAEV-based vectors. Arc. Virol. 143, 681-695]. While our previous vectors were produced in a helper/vector system, the present vectors are produced in a helper/free system. However, these vector titers remain lower than those obtained with other lentiviral vectors carrying equivalent packaging sequences. We discuss on possible reasons of such differences and possible improvements.

Calpain product of WT-CRMP2 reduces the amount of surface NR2B NMDA receptor subunit

The brain is particularly vulnerable to ischaemia; however, neurons can become tolerant to ischaemic insult. This tolerance has been shown to involve activation of NMDA receptors, but its mechanisms have not yet been fully elucidated. Using a preconditioning protocol, we show that neurons surviving to a transient NMDA exposure become resistant to the glutamatergic agonist. Using a proteomic approach, we found that alterations of the protein pattern of NMDA-resistant neurons are restricted mainly to the five collapsin response mediator proteins (CRMPs). A sustained increase in calpain activity following NMDA treatment is responsible for the production of cleaved CRMPs. Finally, we provide evidence for the involvement of the cleaved form of WT-CRMP2 in the down-regulation of NR2B. Our data suggests that, beside their role in neuronal morphogenesis, CRMPs may contribute to neuronal plasticity.

From pathogen to medicine: HIV-1-derived lentiviral vectors as vehicles for dendritic cell based cancer immunotherapy

Over the years, the unique capacity of dendritic cells (DC) for efficient activation of naive T cells has led to their extensive use in cancer immunotherapy protocols. In order to be able to fulfil their role as antigen-presenting cells, the antigen of interest needs to be efficiently introduced and subsequently correctly processed and presented by the DC. For this purpose, a variety of both viral and non-viral antigen-delivery systems have been evaluated. Amongst those, HIV-1-derived lentiviral vectors have been used successfully to transduce DC. This review considers the use of HIV-1-derived lentiviral vectors to transduce human and murine DC for cancer immunotherapy. Lentivirally transduced DC have been shown to present antigenic peptides, prime transgene-specific T cells in vitro and elicit a protective cytotoxic T-lymphocyte (CTL) response in animal models. Different parameters determining the efficacy of transduction are considered. The influence of lentiviral transduction on the DC phenotype and function is described and the induction of immune responses by lentivirally transduced DC in vitro and in vivo is discussed in detail. In addition, direct in vivo administration of lentiviral vectors aiming at the induction of antigen-specific immunity is reviewed. This strategy might overcome the need for ex vivo generation and antigen loading of DC. Finally, future perspectives towards the use of lentiviral vectors in cancer immunotherapy are presented.

With a little help from a friend: increasing HIV transduction of monocyte-derived dendritic cells with virion-like particles of SIVMAC

Modification of dendritic cells (DCs) is a promising avenue for gene therapy purposes, given the versatility and the multiplicity of functions of these cells. In this study, we show that preincubation of monocyte-derived DCs with low amounts of non-infectious virion-like particles derived from the simian immunodeficiency virus (SIV(MAC) VLPs) increases up to 10-fold the efficiency of transduction by HIV-1 lentiviral vectors at low multiplicity of infections yielding up to 90% of transduced cells, in the absence of alterations of DCs behavior. This effect is restricted to DCs and specified by the viral accessory protein Vpx. Thus, preincubation with empty VLPs of SIV(MAC) can be used in transduction protocols to increase the efficacy of HIV-1-mediated modification of DCs.

The impact of electrical charge on the viability and physiology of dendritic cells

The use of electrical charge for electroporation or electrofusion is widely applied to customize dendritic cells (DC) and their immunological properties as anticancer vaccines. The aim of this study was to evaluate the influence of various electrical field strengths on the recovery, viability and physiology of DC. Immature DC were transferred into low-conductive medium and electrically charged within a range of 0-1500 V/cm. Viability was assessed by Trypan Blue dye exclusion or staining with impermeant nucleic acid stains and fluorescence-activated cell sorter analysis. Additionally, apoptosis was determined by flow cytometry after staining with Annexin-V, endocytosis by uptake of fluorescein isothiocyanate-dextran and metabolic activity by a standardized fluorescent live/dead assay. There was a strong correlation between the electrical field strength and the viability and physiology of DC. Field strengths > or =1000 V/cm significantly impaired viability, metabolism and endocytotic activity. Dual fluorescence with 7-7-amino-actinomycin D and Annexin-V demonstrated that loss of viability was predominantly due to necrosis rather than apoptosis. Field strengths < or =500 V/cm allowed to maintain good cell viability and recovery of DC and did not cause alterations of metabolism and endocytosis. Therefore, the frequently used amplification of field strengths to improve the efficacy of electroporation and electrofusion requires critical re-evaluation.

Characterization and gene transfer in mesenchymal stem cells derived from human umbilical-cord blood

It has been shown that the stromal-cell population found in bone marrow can be expanded and differentiated into cells with the phenotypes of bone, cartilage, muscle, neural, and fat cells. However, whether mesenchymal stem cells (MSCs) are present in human umbilical-cord blood (UCB) has been the subject of ongoing debate. In this study, we report on a population of fibroblastlike cells derived from the mononuclear fraction of human UCB with osteogenic and adipogenic potential, as well as the presence of a subset of cells that have been maintained in continuous culture for more than 6 months. These cells were found to express CD29, CD44, CD90, CD95, CD105, CD166, and MHC class, but not CD14, CD34, CD40, CD45, CD80, CD86, CD117, CD152, or MHC class II. We also compared gene expression after gene transfer using lenti- and adenoviral vectors carrying the green fluorescence protein to the MSCs derived from UCB because a reliable gene-delivery system is required to transfer target genes into MSCs, which have attracted attention as potential platforms for the systemic delivery of therapeutic genes. The lentiviral vectors can transduce these cells more efficiently than can adenoviral vectors, and we maintained transgene expression for at least 5 weeks. This is the first report showing that UCB-derived MSCs can express exogenous genes by way of a lentivirus vector. These results demonstrate that human UCB is a source of mesenchymal progenitors and may be used in cell transplantation and a wide range of gene-therapy treatments.

Progress Toward the Genetic Treatment of the β-Thalassemias

The beta-thalassemias are congenital anemias that are caused by mutations that reduce or abolish expression of the beta-globin gene. They can be cured by allogeneic hematopoietic stem cell (HSC) transplantation, but this therapeutic option is not available to most patients. The transfer of a regulated beta-globin gene in autologous HSCs is a highly attractive alternative treatment. This strategy, which is simple in principle, raises major challenges in terms of controlling expression of the globin transgene, which ideally should be erythroid specific, differentiation- and stage-restricted, elevated, position independent, and sustained over time. Using lentiviral vectors, May et al. demonstrated in 2000 that an optimized combination of proximal and distal transcriptional control elements permits lineage-specific and elevated beta-globin expression, resulting in therapeutic hemoglobin production and correction of anemia in beta-thalassemic mice. Several groups have by now replicated and extended these findings to various mouse models of severe hemoglobinopathies, thus fueling enthusiasm for a potential treatment of beta-thalassemia based on globin gene transfer. Current investigation focuses on safety issues and the need for improved vector production methodologies. The safe implementation of stem cell-based gene therapy requires the prevention of the formation of replication-competent viral genomes and minimization of the risk of insertional oncogenesis. Importantly, globin vectors, in which transcriptional activity is highly restricted, have a lesser risk of activating oncogenes in hematopoietic progenitors than non-tissue-specific vectors, by virtue of their late-stage erythroid specificity. As such, they provide a general paradigm for improving vector safety in stem cell-based gene therapy.

Cell cycle features of primate embryonic stem cells

Using flow cytometry measurements combined with quantitative analysis of cell cycle kinetics, we show that rhesus monkey embryonic stem cells (ESCs) are characterized by an extremely rapid transit through the G1 phase, which accounts for 15% of the total cell cycle duration. Monkey ESCs exhibit a non-phasic expression of cyclin E, which is detected during all phases of the cell cycle, and do not growth-arrest in G1 after gamma-irradiation, reflecting the absence of a G1 checkpoint. Serum deprivation or pharmacological inhibition of mitogen-activated protein kinase kinase (MEK) did not result in any alteration in the cell cycle distribution, indicating that ESC growth does not rely on mitogenic signals transduced by the Ras/Raf/MEK pathway. Taken together, these data indicate that rhesus monkey ESCs, like their murine counterparts, exhibit unusual cell cycle features in which cell cycle control mechanisms operating during the G1 phase are reduced or absent.

Optimization of equine infectious anemia derived vectors for hematopoietic cell lineage gene transfer

Gene transfer into hematopoietic cells may allow correction of a variety of hematopoietic and metabolic disorders. Optimized HIV-1 based lentiviral vectors have been developed for improved gene transfer and transgene expression into hematopoietic cells. However, the use of HIV-1 based vectors for human gene therapy may be limited due to ethical and biosafety issues. We report that vectors based on the non-primate equine infectious anemia virus (EIAV) transduce a variety of human hematopoietic cell lines and primary blood cells. To investigate optimization of gene expression in hematopoietic cells, we compared a variety of post-transcriptional elements and promoters in the context of EIAV vectors. We observed cell specific increase in the number of transgene expressing cells with the different post-transcriptional elements, whereas the use of elongation factor alpha 1 (EFalpha1) promoter resulted in significant increases in both the number of transgene expressing cells and the level of transgene protein in all cell types tested. We then demonstrate increased transduction of hematopoietic cells using a second-generation EIAV vector containing a self-inactivating EIAV LTR and the EIAV central polypurine tract (cppt). These data suggest that optimized EIAV vectors may be a suitable alternative to HIV-1 vectors for use in hematopoietic gene therapy.

Lentiviral transduction of human hematopoietic cells by HIV-1- and SIV-based vectors containing a bicistronic cassette driven by various internal promoters

BACKGROUND

Lentiviral gene transfer into hematopoietic cells has been mostly optimized with vectors carrying a single reporter gene. For many clinical applications, lentiviral vectors should contain more than one gene because transduced cells should be enriched by a selectable marker or killed for safety reasons after use. Thus, we compared various vectors containing a bicistronic cassette driven by different ubiquitous promoters for their ability to transduce human T-lymphocytes, CD34+-cells, and dendritic cells (DCs) derived from CD34+-cells or monocytes.

METHODS

We designed HIV or SIV constructs containing a bicistronic cassette composed of two reporter genes (thy1/GFP) linked by an internal ribosome entry site sequence and driven by the cytomegalovirus (CMV) or elongation factor 1alpha (EF1alpha) promoters. The woodchuck hepatitis virus post-transcriptional regulatory element (WPRE) was or not inserted within the constructs, the Vpx accessory protein was or not used for SIV vectors. Target cells were infected at the same multiplicity of infection, transduction efficiency was analyzed both by flow cytometry and vector integration.

RESULTS

For T-cells, HIV-based vectors/WPRE+ in which the thy1/GFP cassette was driven by the EF1alpha promoter were more efficient than SIV-based vectors. For CD34+-cells and CD34+-derived DCs, better thy1/GFP expression was achieved when the CMV promoter drove the cassette inserted into HIV-based vectors/WPRE+. Conversely, for monocyte-derived DCs, the cassette yielded better thy1/GFP expression when inserted into SIV-based vectors/WPRE+ and driven by the CMV or EF1alpha promoters, the use of Vpx significantly improving the expression levels.

CONCLUSIONS

Our results provide guidelines for improving the transduction of T-cells, CD34+-cells or DCs with lentiviral bicistronic vectors designed for clinical applications.

Inhibition of PrPSc formation by lentiviral gene transfer of PrP containing dominant negative mutations

Currently, there is no treatment to cure transmissible spongiform encephalopathies. By taking advantage of the 'prion-resistant' polymorphisms Q171R and E219K that naturally exist in sheep and humans, respectively, we have evaluated a therapeutic approach of lentiviral gene transfer. Here, we show that VSV-G (vesicular stomatitis virus G glycoprotein) pseudotyped FIV-(feline immunodeficiency virus) derived vectors carrying the mouse Prnp gene in which these mutations have been inserted, are able to inhibit prion replication in chronically prion-infected cells. Because lentiviral tools are able to transduce post-mitotic cells such as neurons or cells of the lymphoreticular system, this result might help the development of gene- or cell-therapy approaches to prion disease.

A universal transgene silencing method based on RNA interference

Inducible gene expression systems have contributed significantly to the understanding of molecular regulatory networks. Here we describe a simple and powerful RNA interference-based method that can silence the expression of any transgene. We first used an IRES bicistronic lentiviral vector and showed that targeting the second cistron with a specific siRNA resulted in silencing of both transgenes. We then inserted a siRNA minimal target sequence in the 3'-untranslated region (3'-UTR) of a transgene and showed that the cognate siRNA delivered by a lentiviral vector led to the partial silencing of the transgene. The multimerization of this siRNA target sequence led to the highly efficient silencing of four different transgenes. This new method to silence transgene expression is more versatile than existing methods of conditional inactivation of gene expression, such as transcriptional switches or site-specific recombination. It is applicable to a wide variety of models including primary cells, terminally differentiated cells and transgenic animals.

Lentiviral vectors

Vectors based on lentiviruses have reached a state of development such that clinical studies using these agents as gene delivery vehicles have now begun. They have particular advantages for certain in vitro and in vivo applications especially the unique capability of integrating genetic material into the genome of non-dividing cells. Their rapid progress into clinical use reflects in part the huge body of knowledge which has accumulated about HIV in the last 20 years. Despite this, many aspects of viral assembly on which the success of these vectors depends are rather poorly understood. Sufficient is known however to be able to produce a safe and reproducible high titre vector preparation for effective transduction of growth-arrested tissues such as neural tissue, muscle and liver.

Characterization of a mobilization-competent simian immunodeficiency virus (SIV) vector containing a ribozyme against SIV polymerase

Exploitation of the intracellular virus machinery within infected cells to drive an anti-viral gene therapy vector may prove to be a feasible alternative to reducing viral loads or overall virus infectivity while propagating the spread of a therapeutic vector. Using a simian immunodeficiency virus (SIV)-based system, it was shown that the pre-existing retroviral biological machinery within SIV-infected cells can drive the expression of an anti-SIV pol ribozyme and mobilize the vector to transduce neighbouring cells. The anti-SIV pol ribozyme vector was derived from the SIV backbone and contained the 5'- and 3'LTR including transactivation-response, Psi and Rev-responsive elements, thus requiring Tat and Rev and therefore limiting expression to SIV-infected cells. The data presented here show an early reduction in SIV p27 levels in the presence of the anti-SIV pol ribozyme, as well as successful mobilization (vector RNA constituted approximately 17 % of the total virus pool) and spread of the vector containing this ribozyme. These findings provide direct evidence that mobilization of an anti-retroviral SIV gene therapy vector is feasible in the SIV/macaque model.

Efficient gene transfer into rhesus repopulating hematopoietic stem cells using a simian immunodeficiency virus-based lentiviral vector system

High-titer, HIV-1-based lentiviral vector particles were found to transduce cytokine-mobilized rhesus macaque CD34(+) cells and clonogenic progenitors very poorly (< 1%), reflecting the postentry restriction in rhesus cells to HIV infection. To overcome this barrier, we developed a simian immunodeficiency virus (SIV)-based vector system. A single exposure to a low concentration of amphotropic pseudotyped SIV vector particles encoding the green fluorescent protein (GFP) resulted in gene transfer into 68% +/- 1% of rhesus bulk CD34(+) cells and 75% +/- 1% of clonogenic progenitors. Polymerase chain reaction (PCR) analysis of DNA from individual hematopoietic colonies confirmed these relative transduction efficiencies. To evaluate SIV vector-mediated stem cell gene transfer in vivo, 3 rhesus macaques underwent transplantation with transduced, autologous cytokine-mobilized peripheral blood CD34(+) cells following myeloablative conditioning. Hematopoietic reconstitution was rapid, and an average of 18% +/- 8% and 15% +/- 7% GFP-positive granulocytes and monocytes, respectively, were observed 4 to 6 months after transplantation, consistent with the average vector copy number of 0.19 +/- 0.05 in peripheral blood leukocytes as determined by real-time PCR. Vector insertion site analysis demonstrated polyclonal reconstitution with vector-containing cells. SIV vectors appear promising for evaluating gene therapy approaches in nonhuman primate models.

Morphological Analysis and Lentiviral Transduction of Fetal Monkey Bone Marrow-Derived Mesenchymal Stem Cells

We explored the transduction kinetics of HIV-1-derived lentiviral vectors containing the CMV, EF1alpha, or PGK promoter expressing EGFP in fetal rhesus monkey bone marrow-derived mesenchymal stem cells (rhMSC). Studies included the effects of transduction (MOI 0-100) on growth, cell cycle, and differentiation toward an osteogenic lineage. Flow cytometric analysis indicated an approximate 8- to 10-fold greater quantity of EGFP-expressing rhMSC when cells were transduced with the CMV or EF1alpha promoter compared to PGK, although quantitative PCR revealed no differences at the DNA level. The CMV promoter initially expressed 10- to 100-fold higher levels of EGFP compared to EF1alpha or PGK, respectively, at increasing MOI, although a significant decline in transgene expression was observed posttransduction and with advancing passage (P < 0.01), whereas a significant increase in the level of expression was observed over time with the EF1alpha promoter. At an MOI of 100, a transient arrest at the S phase of the cell cycle was observed for both vector constructs. Transduced rhMSC differentiated toward an osteogenic lineage comparable to untransduced rhMSC and showed equivalent levels of alkaline phosphatase activity. These findings suggest that the SIN HIV-1-derived lentiviral vectors used in these studies can efficiently transduce rhMSC in vitro (CMV > EF1alpha > PGK) without inhibiting differentiation potential, although the cell cycle was transiently altered at high MOI

Heterologous human immunodeficiency virus type 1 lentiviral vectors packaging a simian immunodeficiency virus-derived genome display a specific postentry transduction defect in dendritic cells

Heterologous lentiviral vectors (LVs) represent a way to address safety concerns in the field of gene therapy by decreasing the possibility of genetic recombination between vector and packaging constructs and the generation of replication-competent viruses. Using described LVs based on human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus MAC251 (SIV(MAC251)), we asked whether heterologous virion particles in which trans-acting factors belonged to HIV-1 and cis elements belonged to SIV(MAC251) (HIV-siv) would behave as parental homologous vectors in all cell types. To our surprise, we found that although the heterologous HIV-siv vector was as infectious as its homologous counterpart in most human cells, it was defective in the transduction of dendritic cells (DCs) and, to a lesser extent, macrophages. In DCs, the main postentry defect was observed in the formation of two-long-terminal-repeat circles, despite the fact that full-length proviral DNA was being synthesized and was associated with the nucleus. Taken together, our data suggest that heterologous HIV-siv vectors display a cell-dependent infectivity defect, most probably at a post-nuclear entry migration step. As homologous HIV and SIV vectors do transduce DCs, we believe that these results underscore the importance of a conserved interaction between cis elements and trans-acting viral factors that is lost or suboptimal in heterologous vectors and essential only in the transduction of certain cell types. For gene therapy purposes, these findings indicate that the cellular tropism of LVs can be modulated not only through the use of distinct envelope proteins or tissue-specific promoters but also through the specific combinatorial use of packaging and transfer vector constructs.

Viral vectors for dendritic cell-based immunotherapy

Dendritic cells (DCs) constitute a specialised system of antigen-presenting cells with a high capacity to induce and to modulate the immune response against microbial, tumour and self-antigens. New techniques to generate large amounts of DCs together with the molecular identification of human tumour-associated antigens (TAA) have opened new ways for antigen-specific cancer immunotherapies. DCs loaded either with TAA-derived MHC class I-specific synthetic peptides or with whole tumour cell preparations have been used in numerous clinical trials evaluating the efficacy of DCs in patients with cancer. However, the disadvantages of DCs pulsed with synthetic peptides from TAA include the uncertainty regarding the longevity of antigen presentation, the restriction by the patient's haplotype and the relatively low number of known MHC class I and in particular of MHC class II helper cell-related epitopes. Whole tumour cell preparations are difficult to standardise, and they depend on the availability of tumour cells. Thus the utilisation of viral vectors genetically modified to express TAA for the ex vivo transduction of DCs is an attractive alternative to achieve a MHC I- and MHC II-restricted presentation of tumoural antigens. To induce protective anti-tumoural immune response an increasing number of modified viral vectors have been used to transduce DCs. Although high transduction efficacies were reported for several viruses, analysis of the interaction of viral vectors with DCs has revealed several viral mechanisms that interfere with main functions of DCs, dampening somewhat the initial optimism in the field of DC transduction. However, promising results with different vectors have been achieved. In this review we summarise available data and discuss advantages and drawbacks of currently available vectors.

Lentiviral vectors with two independent internal promoters transfer high-level expression of multiple transgenes to human hematopoietic stem-progenitor cells

Lentiviral vectors (LVs) offer several advantages over traditional oncoretroviral vectors. LVs efficiently transduce slowly dividing cells, including hematopoietic stem-progenitor cells (HSCs), resulting in stable gene transfer and expression. Additionally, recently developed self-inactivating (SIN) LVs allow promoter-specific transgene expression. For many gene transfer applications, transduction of more than one gene is needed. We obtained inconsistent results in our attempts to coexpress two transgenes linked by an internal ribosomal entry site (IRES) element in a single bicistronic LV transcript. In more than six bicistronic LVs we constructed containing a gene of interest followed by an IRES and the GFP reporter gene, GFP fluorescence was undetectable in transduced cells. We therefore investigated how to achieve consistent and efficient coexpression of two transgenes by LVs. In a SIN LV containing the elongation factor 1alpha promoter, we included a second promoter from cytomegalovirus, the phosphoglycerate kinase gene, or the HLA-DRalpha gene. Using a single LV containing two constitutive promoters, we achieved strong and sustained expression of both transgenes in transduced engrafting CD34(+) HSCs and their progeny, as well as in other human cell types. Thus, such dual-promoter LVs can coexpress multiple transgenes efficiently in a single target cell and will enable many gene transfer applications.

Vectors derived from simian immunodeficiency virus (SIV)

In contrast to other retroviruses, lentiviruses have the unique property of infecting non-proliferating cells. Thus vectors derived from lentiviruses are promising tools for in vivo gene delivery applications. Vectors derived from human primate and non-primate lentiviruses have recently been described and, unlike retroviral vectors derived from murine leukemia viruses, lead to stable integration of the transgene into quiescent cells in various organs. Despite all the safety safeguards that have been progressively introduced in lentiviral vectors, the clinical acceptance of vectors derived from pathogenic lentiviruses is subject to debate. It is therefore essential to design vectors derived from a wide range of lentivirus types and to comparatively examine their properties in terms of transduction efficiency and bio-safety. Here, we review the properties of lentiviral vectors derived from simian immunodeficiency virus (SIV).