A packaging cell line generating CD4-specific retroviral vectors for efficient gene transfer into primary human T-helper lymphocytes

Selective gene transfer to T lymphocytes using coreceptor-specific [MLV(HIV)] pseudotype vectors in a transgenic mouse model

The coreceptor usage of HIV-1 envelope proteins (Env) is mainly dependent on a defined variable region within the V3-loop of Env. Thus, retroviral vectors derived from murine leukemia virus (MLV), which have been pseudotyped with HIV-1 envelope proteins holding different V3-loops, enable selective gene delivery into either CXCR4 or CCR5 positive cultured cells. Here, we tested the distribution of CD4/CCR5-tropic [MLV(HIV)]-pseudotype vectors in transgenic mice expressing CD4 and either CXCR4 or CCR5 of human origin. The specificity of gene transfer was analyzed by ex vivo transduction of spleen cells as well as after i.v. or i.p. injection of transgenic mice. Expression of the transferred marker gene EGFP and vector sequences could be detected exclusively in lymphocytes expressing (hu)CD4 and (hu)CCR5, whereas MLV vectors pseudotyped with the VSV-G envelope glycoprotein mediated gene transfer in mice of all genotypes investigated. These data demonstrated that cell-specific gene delivery via [MLV(HIV)]-pseudotyped vectors, as previously shown for cultured cells, is also achievable in vivo.

Assessment of HIV-1 entry inhibitors by MLV/HIV-1 pseudotyped vectors

Background

Murine leukemia virus (MLV) vector particles can be pseudotyped with a truncated variant of the human immunodeficiency virus type 1 (HIV-1) envelope protein (Env) and selectively target gene transfer to human cells expressing both CD4 and an appropriate co-receptor. Vector transduction mimics the HIV-1 entry process and is therefore a safe tool to study HIV-1 entry.

Results

Using FLY cells, which express the MLV gag and pol genes, we generated stable producer cell lines that express the HIV-1 envelope gene and a retroviral vector genome encoding the green fluorescent protein (GFP). The BH10 or 89.6 P HIV-1 Env was expressed from a bicistronic vector which allowed the rapid selection of stable cell lines. A codon-usage-optimized synthetic env gene permitted high, Rev-independent Env expression. Vectors generated by these producer cells displayed different sensitivity to entry inhibitors.

Conclusion

These data illustrate that MLV/HIV-1 vectors are a valuable screening system for entry inhibitors or neutralizing antisera generated by vaccines.

Targeting retroviral and lentiviral vectors

Retroviral vectors capable of efficient in vivo gene delivery to specific target cell types or to specific locations of disease pathology would greatly facilitate many gene therapy applications. The surface glycoproteins of membrane-enveloped viruses stand among the choice candidates to control the target cell receptor recognition and host range of retroviral vectors onto which they are incorporated. This can be achieved in many ways, such as the exchange of glycoprotein by pseudotyping, their biochemical modifications, their conjugation with virus-cell bridging agents or their structural modifications. Understanding the fundamental properties of the viral glycoproteins and the molecular mechanism of virus entry into cells has been instrumental in the functional alteration of their tropism. Here we briefly review the current state of our understanding of the structure and function of viral envelope glycoproteins and we discuss the emerging targeting strategies based on retroviral and lentiviral vector systems.

Replacement of the murine leukemia virus (MLV) envelope gene with a truncated HIV envelope gene in MLV generates a virus with impaired replication capacity

Murine leukemia virus (MLV) capsid particles can be efficiently pseudotyped with a variant of the HIV-1 envelope protein (Env) containing the surface glycoprotein gp120-SU and a carboxyl-terminally truncated transmembrane (TM) protein, with only seven cytoplasmic amino acids. MLV/HIV pseudotyped vector particles acquire the natural host tropism of HIV-1 and their entry is dependent on the presence of CD4 and an appropriate co-receptor on the surface of the target cell. We describe here the construction of chimeric MLV/HIV proviruses containing the truncated HIV envelope gene. The MLV/HIV provirus was generated by direct replacement of the MLV envelope gene with HIV Env coding sequences either with or without the additional inclusion of the woodchuck hepatitis virus posttranscriptional regulatory element (WPRE). Chimeric MLV/HIV particles could be generated from transfected 293T cells and were able to infect CD4/CXCR4-positive target cells. However, the second round of infection of target cells was severely impaired, despite the fact that the WPRE element enhanced the amount of viral mRNA detected. Viral particles released from infected cells showed reduced HIV Env incorporation, indicating that additional factors required for efficient replication of MLV/HIV pseudotyped viruses are missing.

MLV/HIV-pseudotyped vectors: a new treatment option for cutaneous t cell lymphomas

Cutaneous T cell lymphomas (CTCLs) are lymphoproliferative disorders involving the skin. Malignant cells have a CD4+ T-helper phenotype and are found in early stages of the disease in plaques and cutaneous tumors. MLV/HIV-pseudotyped retroviral vectors target gene transfer to CD4-positive T cells and are therefore well suited to be specific delivery vehicles to treat CTCLs. We established a mouse xenograft model for CTCL and generated MLV/HIV-pseudotyped vectors encoding the herpes simplex virus thymidine kinase (HSV-TK), a well-known suicide gene, to prove the efficacy of MLV/HIV vectors in CTCL treatment. Vector particles were intratumorally injected into CTCL nude mouse xenografts. Mice were systemically treated with ganciclovir (GCV) and the tumor tissue was analyzed. A significant delay in tumor growth was observed for HSV-TK-transduced and GCV-treated tumors. GFP expression could be detected exclusively in CD4+ cells of tumors after transduction with GFP-encoding control vectors. The data demonstrate a cell-specific in vivo gene delivery via MLV/HIV-pseudotyped vectors and open new avenues for the treatment of CTCL in humans.

Generation of a replication-competent, propagation-deficient virus vector based on the transmissible gastroenteritis coronavirus genome

Replication-competent propagation-deficient virus vectors based on the transmissible gastroenteritis coronavirus (TGEV) genome that are deficient in the essential E gene have been developed by complementation within E(+) packaging cell lines. Cell lines expressing the TGEV E protein were established using the noncytopathic Sindbis virus replicon pSINrep21. In addition, cell lines stably expressing the E gene under the CMV promoter have been developed. The Sindbis replicon vector and the ectopic TGEV E protein did not interfere with the rescue of infectious TGEV from full-length cDNA. Recombinant TGEV deficient in the nonessential 3a and 3b genes and the essential E gene (rTGEV-Delta3abDeltaE) was successfully rescued in these cell lines. rTGEV-Delta3abDeltaE reached high titers (10(7) PFU/ml) in baby hamster kidney cells expressing porcine aminopeptidase N (BHK-pAPN), the cellular receptor for TGEV, using Sindbis replicon and reached titers up to 5 x 10(5) PFU/ml in cells stably expressing E protein under the control of the CMV promoter. The virus titers were proportional to the E protein expression level. The rTGEV-Delta3abDeltaE virions produced in the packaging cell line showed the same morphology and stability under different pHs and temperatures as virus derived from the full-length rTGEV genome, although a delay in virus assembly was observed by electron microscopy and virus titration in the complementation system in relation to the wild-type virus. These viruses were stably grown for >10 passages in the E(+) packaging cell lines. The availability of packaging cell lines will significantly facilitate the production of safe TGEV-derived vectors for vaccination and possibly gene therapy.

Expression of the human CD4 receptor is sufficient for the transduction of murine T-cells with MLV/HIV pseudotyped vectors

Murine leukemia virus (MLV) can be pseudotyped with a variant of the HIV envelope gene encoding the surface glycoprotein gp120-SU and a carboxyl-terminally truncated transmembrane (TM) protein, with only seven cytoplasmic amino acids. MLV/HIV pseudotyped retroviral vectors selectively target human CD4+ cells and can be used as tools to study entry of HIV into cells. Mouse T-cells are immune to HIV infection, which is primarily caused by the weak binding affinity of HIV gp120 to the murine CD4 receptor. Here we show that expression of the human CD4 receptor in murine T-cells is sufficient for syncytia formation with HIV-1 envelope expressing cells and entry of MLV/HIV pseudotyped retroviral vectors. This implies that the murine CXCR4 receptor is a functional coreceptor for MLV/HIV pseudotyped vectors and confirms previous data that the inability of HIV to replicate in murine T-cells is due to a post entry block.

Specific transduction of HIV-1 envelope expressing cells by retroviral vectors pseudotyped with hybrid CD4/CXCR4 receptors

Infection of a target cell by HIV is initiated by the interaction of the envelope glycoprotein with the CD4 receptor molecule on the surface of the target cell. This is followed by binding of a coreceptor of the chemokine receptor family and subsequently fusion of viral and cellular membranes. Membrane fusion is independent of whether the viral envelope protein is on the viral or on the cellular membrane. Accordingly, targeting of HIV infected cells by retroviral vectors has been previously achieved both by coincorporation of CD4 and coreceptors into murine leukemia virus (MLV) and lentivirus based vector particles. It was, therefore, tested whether hybrid genes of CD4 and CXCR4 are also able to yield 'receptor' vectors. A construct containing the four extracellular loops of CD4 fused to CXCR4 (CD4-D4-X4) allowed gene transfer into HIV-1 envelope expressing cells by vectors based on either MLV or lentiviruses. The CD4-D2-X4 hybrid receptor, containing the first two extracellular CD4 domains, allowed gene transfer only by lentiviral vectors. Attempts to increase vector titres by deletion of the intracellular part of CXCR4 failed. Vector titres obtained by hybrid receptors were slightly lower than published titres obtained by separate expression of CD4 and CXCR4. Thus, CD4-D4-CXCR4 hybrids are useful for the generation of retroviral and lentiviral vectors with specificity for HIV-1 envelope expressing cells.