Transduction efficiency of MLV but not of HIV-1 vectors is pseudotype dependent on human primary T lymphocytes

Development of HIV-1 vectors pseudotyped with envelope proteins of other retroviruses

In the past three decades, human immunodeficiency virus type 1 (HIV-1)-derived vectors were evolved and became indispensable to transduce therapeutic genes into a range of different target cell types to facilitate a variety of gene therapeutic strategies. To achieve this, i) the biosafety profile of the vectors was incrementally enhanced and ii) the CD4-restricted tropism mediated by the envelope proteins (Env) of the parental virus needed to be directed towards recruitment of other receptors expressed on the desired target cells. Here, a closer look is first taken at the development of vector components and the mechanisms of Env incorporation into particles. While envelope proteins originating from a broad range of very diverse virus species were successfully utilized, members of the Retroviridae family most frequently provided Env or further engineered variants thereof to form transduction-competent HIV-1 pseudotype vector particles. The development of these vectors is reviewed and anticipated to further contribute to the future progression of somatic gene therapy.

Designed Ankyrin Repeat Protein (DARPin) to target chimeric antigen receptor (CAR)-redirected T cells towards CD4+ T cells to reduce the latent HIV+ cell reservoir

Chimeric Antigen Receptor (CAR)-redirected T cells show great efficacy in the patient-specific therapy of hematologic malignancies. Here, we demonstrate that a DARPin with specificity for CD4 specifically redirects and triggers the activation of CAR engineered T cells resulting in the depletion of CD4+ target cells aiming for elimination of the human immunodeficiency virus (HIV) reservoir.

Improved GaLV-TR Glycoproteins to Pseudotype Lentiviral Vectors: Impact of Viral Protease Activity in the Production of LV Pseudotypes

Lentiviral vectors (LVs) are excellent tools for gene transfer into mammalian cells. It is noteworthy that the first gene therapy treatment using LVs was approved for commercialization in 2017. The G glycoprotein from rhabdovirus vesicular stomatitis virus (VSV-G) is the glycoprotein most used to pseudotype LVs, due to its high efficiency in transducing several cell types and its resistance to viral vector purification and storage conditions. However, VSV-G expression induces cytotoxicity, which limits LV production to short periods. As alternative to VSV-G, γ-retrovirus glycoproteins (4070A derived, GaLV derived, and RD114 derived) have been used to pseudotype both γ-retroviral vectors (RVs) and LVs. These glycoproteins do not induce cytotoxicity, allowing the development of stable LV producer cells. Additionally, these LV pseudotypes present higher transduction efficiencies of hematopoietic stem cells when compared to VSV-G. Here, new 4070A-, RD114-TR-, and GaLV-TR-derived glycoproteins were developed with the aim of improving its cytoplasmic tail R-peptide cleavage and thus increase LV infectious titers. The new glycoproteins were tested in transient LV production using the wild-type or the less active T26S HIV-1 protease. The GaLV-TR-derived glycoproteins were able to overcome titer differences observed between LV production using wild-type and T26S protease. Additionally, these glycoproteins were even able to increase LV titers, evidencing its potential as an alternative glycoprotein to pseudotype LVs.

Efficient lentiviral transduction and transgene expression in primary human B cells

Primary human B cells are an attractive target for gene-therapeutic applications, but have been found to be relatively resistant toward transduction with lentiviral vectors (LVVs), even though a number of different envelope pseudotypes were tested. Moreover, low transgene expression in primary human B cells has impeded the use of LVVs for this target cell. We investigated the transduction potential of gibbon-ape leukemia virus (GALV) Env-pseudotyped LVVs for primary human B cells. By establishing optimized transduction kinetics and multiplicities of infection, we were able to regularly obtain transduction efficiencies of more than 50% in CD40L-activated B cells. Noteworthy, with the use of GALV-pseudotyped LVVs we could achieve a more than 10-fold higher yield of transduced activated B cells in direct comparison with LVVs pseudotyped with measles virus glycoproteins. Phenotyping of transduced primary B cells revealed a majority of memory B cells, a long-lived phenotype, presumed to be well suited for enduring therapeutic interventions. Finally, by combining the enhancer (Eμ) and the matrix/scaffold-attachment regions (MARs) of the human immunoglobulin heavy chain with the promoter of spleen focus-forming virus (SFFV) we aimed at generating a novel LVV particularly suitable for B cell transgenesis. We show that the optimized vector facilitated significantly higher transgene expression in various B cell lines and, more importantly, primary human B cells (mean factor of three). In summary, we have established a novel protocol for the efficient lentiviral transduction of primary human B cells and have improved transgene expression in B cells by a specific vector modification.

Chemoprotection by transfer of resistance genes

Dose-limiting toxicity of chemotherapeutic agents, i.e., myelosuppression, can limit their effectiveness. The transfer and expression of drug-resistance genes might decrease the risks associated with acute hematopoietic toxicity. Protection of hematopoietic stem/progenitor cells by transfer of drug-resistance genes provides the possibility of intensification or escalation of antitumor drug doses and consequently an improved therapeutic index. This chapter reviews drug-resistance gene transfer strategies for either myeloprotection or therapeutic gene selection. Selecting candidate drug-resistance gene(s), gene transfer methodology, evaluating the safety and the efficiency of the treatment strategy, relevant in vivo models, and oncoretroviral transduction of human hematopoietic stem/progenitor cells under clinically applicable conditions are described.

Replication-competent Vectors and Empty Virus-like Particles: New Retroviral Vector Designs for Cancer Gene Therapy or Vaccines

Replication-defective vectors based on murine oncoretroviruses were the first gene transfer vectors to be used in successful gene therapies. Despite this achievement, they have two major drawbacks: insufficient efficacy for in vivo gene transfer and insertional mutagenesis. Attempts to overcome these problems have led to two retroviral vector designs of principally opposite character: replication-competent vectors transducing largely intact genomes and genome-free vectors. Replication-competent retroviral vectors have achieved dramatically improved efficacy for in vivo cancer gene therapy and genome-free retroviral vectors expressing different kinds of antigens have proven excellent as immunogens. Current developments aim to improve the safety of the replication-competent vectors and to augment the production efficiency of the genome-free vectors by expression from heterologous viral or non-viral vectors. Together with the continuous advances of classical defective retroviral vectors for ex vivo gene therapy, these developments illustrate that, due to their tremendous design versatility, retroviral vectors remain important vectors for gene therapy applications.

Redirecting T lymphocyte specificity by T cell receptor gene transfer – A new era for immunotherapy

The therapeutic efficacy of adoptively transferred cytotoxic T lymphocytes (CTL) has been demonstrated in clinical trials for the treatment of chronic myelogenous leukemia, cytomegalovirus-mediated disease, and Epstein-Barr virus-positive B cell lymphomas. It is however limited by the difficulty of generating sufficient amounts of CTLs in vitro, especially for the treatment of solid tumors. Recent gene therapy approaches, including two clinical trials, successfully apply genetic engineering of T cell specificity by T cell receptor (TCR) gene transfer. In this review we want to elucidate several principles of the redirection of T cell specificity. We cover basic aspects of retroviral gene transfer, regarding transduction efficacy and transgene expression levels. It was demonstrated that the number of TCR molecules on a T cell is important for its function. Therefore, an efficient transfer system that yields high transduction efficiency and strong and stable transgene expression is a prerequisite to achieve effector function by redirected T cells. Furthermore, we consider more recent aspects of T cell specificity engineering. These include the possibility of co-transferring coreceptors to create for example functional T helper cells by engrafting CD4(+) T cells with a MHC class I restricted TCR and the CD8 coreceptor and vice versa. Also, risks related to the adoptive transfer of TCR gene-modified T cells and possible safety mechanisms are discussed. Finally, we summarize recent findings describing transferred TCRs capable of displacing endogenous TCRs from the cell surface.

Retroviral vectors for vaccine development: induction of HIV-1-specific humoral and cellular immune responses in rhesus macaques using a novel MLV(HIV-1) pseudotype vector

Retroviral vectors have yet not been tested for their potential as vaccines despite their frequent utilization in gene therapy allowing for highly efficient gene transfer into a number of cell types and their suitability for large-scale production in biotechnology. To investigate MLV-based vectors suitability for inducing immune response against HIV-1-antigens, we generated a MLV(HIV-1) pseudotype vector enabling CD4-specific transduction of HIV-1 genes env, vpu, tat and rev originating from the pathogenic SHIV-89.6P. Functional expression of the lentiviral genes in packaging cells, human and rhesus CD4+ target cells was demonstrated by various assays. Following highly efficient ex vivo transduction, up to 3.4x10(7) autologous, transfer vector-positive rhesus peripheral blood mononuclear cells (rhPBMCs) were re-inoculated into a rhesus macaque. Five weeks after the initial inoculation HIV-1 Env-specific antibodies were detected using ELISA. ELIspot-assay revealed the induction of a HIV-1 Rev and Env-specific CTL-response 7.5 weeks after immunization. Thus, these novel MLV(HIV-1) vectors facilitate efficient transduction and subsequent expression of HIV-1-genes in CD4-positive host cells. Induction of both humoral and cellular HIV-1-specific immune responses in vivo confirmed their potential as an effective HIV-1 vaccine to be further studied in SHIV/rhesus macaque model of lentivirus infection.

Envelope proteins of spleen necrosis virus form infectious human immunodeficiency virus type 1 pseudotype vector particles, but fail to incorporate upon substitution of the cytoplasmic domain with that of Gibbon ape leukemia virus

The wild-type (wt) envelope (Env) proteins of spleen necrosis virus (SNV), together with the transmembrane (TM) protein fused to antibody domains (scFv), have been used for the generation of stable packaging cell lines releasing pseudotyped cell targeting vectors derived from SNV and Murine leukemia virus (MLV). As a first step towards assessing whether HIV-1(SNV/TM-scFv) packaging cells could be established for the production of lentiviral cell targeting vectors, it is reported here that infectious HIV-1-derived particles pseudotyped with wt SNV Env proteins could be generated. Using novel chimeric SNV-derived Env proteins encompassing wt and engineered cytoplasmic domains (C-tail) of the Gibbon ape leukemia virus (GaLV) TM protein, it was further shown that the wt C-tail not only excludes the GaLV TM protein from incorporation into HIV-1 particles, but confers this phenotype to other retroviral envelopes upon C-terminal fusion.

Transduction of nondividing human macrophages with gammaretrovirus-derived vectors

It is commonly accepted that infection of nondividing cells by gammaretroviruses such as the murine leukemia viruses is inefficient due to their inability to cross the nuclear envelope barrier. Challenging this notion, we now show that human nondividing macrophages display a specific window of susceptibility to transduction with a Friend murine leukemia virus (F-MLV)-derived vector during their differentiation from monocytes. This finding suggests that factors other than the nuclear membrane govern permissiveness to gammaretroviral infection and raises the possibility of using the macrophage tropism of F-MLV in gene therapy.

Stable Transduction of Primary Human Monocytes by Simian Lentiviral Vector PBj

Despite the ability to infect nonproliferating cells, current lentiviral vectors are inefficient at mediating gene transfer into quiescent primary human cells such as monocytes. Here, a replication-incompetent vector based on a molecular clone of simian immunodeficiency virus strain PBj (SIVsmmPBj1.9) was generated that, in contrast to lenti- and gamma-retroviral control vectors, enabled transfer of heterologous genes into human diploid fibroblasts and cell lines blocked in the G(0) phase of the cell cycle. Moreover, freshly isolated human monocytes refractory to HIV-1-derived vectors were efficiently transduced by the PBj vector independent of the viral Nef protein. Stable chromosomal integration of PBj-derived viral expression vectors was verified in transduced cells. The capability of the PBj vector to transduce quiescent cells such as unstimulated primary human monocytes is an important extension of human gene therapy perspectives.

Altering the tropism of lentiviral vectors through pseudotyping

The host range of retroviral vectors including lentiviral vectors can be expanded or altered by a process known as pseudotyping. Pseudotyped lentiviral vectors consist of vector particles bearing glycoproteins (GPs) derived from other enveloped viruses. Such particles possess the tropism of the virus from which the GP was derived. For example, to exploit the natural neural tropism of rabies virus, vectors designed to target the central nervous system have been pseudotyped using rabies virus-derived GPs. Among the first and still most widely used GPs for pseudotyping lentiviral vectors is the vesicular stomatitis virus GP (VSV-G), due to the very broad tropism and stability of the resulting pseudotypes. Pseudotypes involving VSV-G have become effectively the standard for evaluating the efficiency of other pseudotypes. This review samples a few of the more prominent examples from the ever-expanding list of published lentiviral pseudotypes, noting comparisons made with pseudotypes involving VSV-G in terms of titer, viral particle stability, toxicity, and host-cell specificity. Particular attention is paid to publications of successfully targeting a specific organ or cell types.

Gene therapy progress and prospects: novel gene therapy approaches for AIDS

Acquired immunodeficiency syndrome (AIDS), caused by human immunodeficiency virus (HIV), kills millions worldwide every year. Vaccines against HIV still seem a distant promise. Pharmaceutical treatments exist, but these are not always effective, and there is increasing prevalence of viral strains with multidrug resistance. Highly active antiretroviral therapy (HAART) consists of inhibitors of viral enzymes (reverse transcriptase (RT) and protease). Gene therapy, first introduced as intracellular immunization, may offer hopes for new treatments to be used alone, or in conjunction with, conventional small molecule drugs. Gene therapy approaches against HIV-1, including suicide genes, RNA-based technology, dominant negative viral proteins, intracellular antibodies, intrakines, and peptides, are the subject of this review.