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

Construction and Applications of Mammalian Cell-Based DNA-Encoded Peptide/Protein Libraries

DNA-encoded peptide/protein libraries are the starting point for protein evolutionary modification and functional peptide/antibody selection. Different display technologies, protein directed evolution, and deep mutational scanning (DMS) experiments employ DNA-encoded libraries to provide sequence variations for downstream affinity- or function-based selections. Mammalian cells promise the inherent post-translational modification and near-to-natural conformation of exogenously expressed mammalian proteins and thus are the best platform for studying transmembrane proteins or human disease-related proteins. However, due to the current technical bottlenecks of constructing mammalian cell-based large size DNA-encoded libraries, the advantages of mammalian cells as screening platforms have not been fully exploited. In this review, we summarize the current efforts in constructing DNA-encoded libraries in mammalian cells and the existing applications of these libraries in different fields.

Virus-Like Particles as Nanocarriers for Intracellular Delivery of Biomolecules and Compounds

Virus-like particles (VLPs) are nanostructures assemble from viral proteins. Besides widely used for vaccine development, VLPs have also been explored as nanocarriers for cargo delivery as they combine the key advantages of viral and non-viral vectors. While it protects cargo molecules from degradation, the VLP has good cell penetrating property to mediate cargo passing the cell membrane and released into cells, making the VLP an ideal tool for intracellular delivery of biomolecules and drugs. Great progresses have been achieved and multiple challenges are still on the way for broad applications of VLP as delivery vectors. Here we summarize current advances and applications in VLP as a delivery vector. Progresses on delivery of different types of biomolecules as well as drugs by VLPs are introduced, and the strategies for cargo packaging are highlighted which is one of the key steps for VLP mediated intracellular delivery. Production and applications of VLPs are also briefly reviewed, with a discussion on future challenges in this rapidly developing field.

A Thermostable Oral SARS-CoV-2 Vaccine Induces Mucosal and Protective Immunity

An ideal protective vaccine against SARS-CoV-2 should not only be effective in preventing disease, but also in preventing virus transmission. It should also be well accepted by the population and have a simple logistic chain. To fulfill these criteria, we developed a thermostable, orally administered vaccine that can induce a robust mucosal neutralizing immune response. We used our platform based on retrovirus-derived enveloped virus-like particles (eVLPs) harnessed with variable surface proteins (VSPs) from the intestinal parasite Giardia lamblia, affording them resistance to degradation and the triggering of robust mucosal cellular and antibody immune responses after oral administration. We made eVLPs expressing various forms of the SARS-CoV-2 Spike protein (S), with or without membrane protein (M) expression. We found that prime-boost administration of VSP-decorated eVLPs expressing a pre-fusion stabilized form of S and M triggers robust mucosal responses against SARS-CoV-2 in mice and hamsters, which translate into complete protection from a viral challenge. Moreover, they dramatically boosted the IgA mucosal response of intramuscularly injected vaccines. We conclude that our thermostable orally administered eVLP vaccine could be a valuable addition to the current arsenal against SARS-CoV-2, in a stand-alone prime-boost vaccination strategy or as a boost for existing vaccines.

Chimeric Antigen Receptor T Cell Therapy Management and Safety: A Practical Tool From a Multidisciplinary Team Perspective

Purpose

The use process for chimeric antigen receptor T (CAR-T) cell drugs is complex and has been associated with a number of potentially severe complications, which requires management by a multidisciplinary team. Pharmacists are a key element in the team and have roles and responsibilities. Our objective was to develop a structured and practical guide that supports hospital pharmacist responsibilities and defines specific activities in a CAR-T cell therapy program, specifically in Europe.

Methods

A literature review was performed, and the recommendations related to pharmacy practice in CAR-T therapy programs were analyzed. A multidisciplinary team was assembled, and meetings were held to address the key tasks in the CAR-T cells’ management process and to create the guide, based on national and international recommendations and in expert’s opinions.

Results

The multidisciplinary team defined the following key tasks and issued recommendations to improve patient safety, treatment efficacy, and quality: patient selection and evaluation, CAR-T cell drug order to manufacturer, apheresis and material shipment, reception of CAR-T cell drug and storing, CAR-T cell drug prescription and pharmacy verification, CAR-T cell drug thawing and dispensing, CAR-T cell drug administration, patient education, pharmacovigilance and monitoring and outcomes’ record and evaluation. In each task the pharmacist’s role and how it can improve patient care are defined. A checklist was created to guarantee the compliance of standard operating procedures approved in the institution to manage CAR-T cell therapy and as a tool to collect required data for outcomes’ record and evaluation.

Conclusion

This article provides a consensus set of safety recommendations regarding CAR-T therapy management in clinical practice, easily implementable by other institutions in the European setting. The guide identifies key steps where the involvement of hospital pharmacists would improve the safety and quality of the process and is a support guide to standardize hospital pharmacists’ responsibilities within the multidisciplinary team.

Novel Semi-Replicative Retroviral Vector Mediated Double Suicide Gene Transfer Enhances Antitumor Effects in Patient-Derived Glioblastoma Models

As glioblastomas are mostly localized infiltrative lesions, gene therapy based on the retroviral replicating vector (RRV) system is considered an attractive strategy. Combinations of multiple suicide genes can circumvent the limitations associated with each gene, achieving direct and synergistic cytotoxic effects, along with bystander cell killing. In this study, we constructed a semi-and pseudotyped-RRV (sp-RRV) system harboring two suicide genes—herpes simplex virus type 1 thymidine kinase (TK) and yeast cytosine deaminase (CD)—to verify the dissemination and antitumor efficacy of our sp-RRV system (spRRVe-sEF1α-TK/sRRVgp-sEF1α-CD) in seven patient-derived glioblastoma stem-like cells (GSCs). Flow cytometry and high-content analysis revealed a wide range of transduction efficiency and good correlation between the delivery of therapeutic genes and susceptibility to the prodrugs ganciclovir and 5-fluorocytosine in patient-derived GSCs in vitro. Intra-tumoral delivery of spRRVe-sEF1α-TK/sRRVgp-sEF1α-CD, combined with prodrug treatment, synergistically inhibited cell proliferation and angiogenesis while increasing apoptosis and the depletion of tumor-associated macrophages in orthotopic glioblastoma xenografts. Genomic profiling of patient-derived GSCs revealed that the key genes preventing sp-RRV infection and transmission were associated with cell adhesion, migration, development, differentiation, and proliferation. This is the first report demonstrating that a novel sp-RRV-mediated TK/CD double suicide gene transfer system has high oncolytic power against extremely heterogeneous and treatment-refractory glioblastomas.

Management guidelines for paediatric patients receiving chimeric antigen receptor T cell therapy

In 2017, an autologous chimeric antigen receptor (CAR) T cell therapy indicated for children and young adults with relapsed and/or refractory CD19+ acute lymphoblastic leukaemia became the first gene therapy to be approved in the USA. This innovative form of cellular immunotherapy has been associated with remarkable response rates but is also associated with unique and often severe toxicities, which can lead to rapid cardiorespiratory and/or neurological deterioration. Multidisciplinary medical vigilance and the requisite health-care infrastructure are imperative to ensuring optimal patient outcomes, especially as these therapies transition from research protocols to standard care. Herein, authors representing the Pediatric Acute Lung Injury and Sepsis Investigators (PALISI) Network Hematopoietic Stem Cell Transplantation (HSCT) Subgroup and the MD Anderson Cancer Center CAR T Cell Therapy-Associated Toxicity (CARTOX) Program have collaborated to provide comprehensive consensus guidelines on the care of children receiving CAR T cell therapy.

Viral and Nonviral Engineering of Natural Killer Cells as Emerging Adoptive Cancer Immunotherapies

Natural killer (NK) cells are powerful immune effectors whose antitumor activity is regulated through a sophisticated network of activating and inhibitory receptors. As effectors of cancer immunotherapy, NK cells are attractive as they do not attack healthy self-tissues nor do they induce T cell-driven inflammatory cytokine storm, enabling their use as allogeneic adoptive cellular therapies. Clinical responses to adoptive NK-based immunotherapy have been thwarted, however, by the profound immunosuppression induced by the tumor microenvironment, particularly severe in the context of solid tumors. In addition, the short postinfusion persistence of NK cells in vivo has limited their clinical efficacy. Enhancing the antitumor immunity of NK cells through genetic engineering has been fueled by the promise that impaired cytotoxic functionality can be restored or augmented with the use of synthetic genetic approaches. Alongside expressing chimeric antigen receptors to overcome immune escape by cancer cells, enhance their recognition, and mediate their killing, NK cells have been genetically modified to enhance their persistence in vivo by the expression of cytokines such as IL-15, avoid functional and metabolic tumor microenvironment suppression, or improve their homing ability, enabling enhanced targeting of solid tumors. However, NK cells are notoriously adverse to endogenous gene uptake, resulting in low gene uptake and transgene expression with many vector systems. Though viral vectors have achieved the highest gene transfer efficiencies with NK cells, nonviral vectors and gene transfer approaches—electroporation, lipofection, nanoparticles, and trogocytosis—are emerging. And while the use of NK cell lines has achieved improved gene transfer efficiencies particularly with viral vectors, challenges with primary NK cells remain. Here, we discuss the genetic engineering of NK cells as they relate to NK immunobiology within the context of cancer immunotherapy, highlighting the most recent breakthroughs in viral vectors and nonviral approaches aimed at genetic reprogramming of NK cells for improved adoptive immunotherapy of cancer, and, finally, address their clinical status.

Viral and Synthetic RNA Vector Technologies and Applications

Use of RNA is an increasingly popular method to transiently deliver genetic information for cell manipulation in basic research and clinical therapy. In these settings, viral and nonviral RNA platforms are employed for delivery of small interfering RNA and protein-coding mRNA. Technological advances allowing RNA modification for increased stability, improved translation and reduced immunogenicity have led to increased use of nonviral synthetic RNA, which is delivered in naked form or upon formulation. Alternatively, highly efficient viral entry pathways are exploited to transfer genes of interest as RNA incorporated into viral particles. Current viral RNA transfer technologies are derived from Retroviruses, nonsegmented negative-strand RNA viruses or positive-stranded Alpha- and Flaviviruses. In retroviral particles, the genes of interest can either be incorporated directly into the viral RNA genome or as nonviral RNA. Nonsegmented negative-strand virus-, Alpha- and Flavivirus-derived vectors support prolonged expression windows through replication of viral RNA encoding genes of interest. Mixed technologies combining viral and nonviral components are also available. RNA transfer is ideal for all settings that do not require permanent transgene expression and excludes potentially detrimental DNA integration into the target cell genome. Thus, RNA-based technologies are successfully applied for reprogramming, transdifferentiation, gene editing, vaccination, tumor therapy, and gene therapy.

Adjuvants: Classification, Modus Operandi, and Licensing

Vaccination is one of the most efficient strategies for the prevention of infectious diseases. Although safer, subunit vaccines are poorly immunogenic and for this reason the use of adjuvants is strongly recommended. Since their discovery in the beginning of the 20th century, adjuvants have been used to improve immune responses that ultimately lead to protection against disease. The choice of the adjuvant is of utmost importance as it can stimulate protective immunity. Their mechanisms of action have now been revealed. Our increasing understanding of the immune system, and of correlates of protection, is helping in the development of new vaccine formulations for global infections. Nevertheless, few adjuvants are licensed for human vaccines and several formulations are now being evaluated in clinical trials. In this review, we briefly describe the most well known adjuvants used in experimental and clinical settings based on their main mechanisms of action and also highlight the requirements for licensing new vaccine formulations.

Replication-Competent Foamy Virus Vaccine Vectors as Novel Epitope Scaffolds for Immunotherapy

The use of whole viruses as antigen scaffolds is a recent development in vaccination that improves immunogenicity without the need for additional adjuvants. Previous studies highlighted the potential of foamy viruses (FVs) in prophylactic vaccination and gene therapy. Replication-competent FVs can trigger immune signaling and integrate into the host genome, resulting in persistent antigen expression and a robust immune response. Here, we explored feline foamy virus (FFV) proteins as scaffolds for therapeutic B and T cell epitope delivery in vitro. Infection- and cancer-related B and T cell epitopes were grafted into FFV Gag, Env, or Bet by residue replacement, either at sites of high local sequence homology between the epitope and the host protein or in regions known to tolerate sequence alterations. Modified proviruses were evaluated in vitro for protein steady state levels, particle release, and virus titer in permissive cells. Modification of Gag and Env was mostly detrimental to their function. As anticipated, modification of Bet had no impact on virion release and affected virus titers of only some recombinants. Further evaluation of Bet as an epitope carrier was performed using T cell epitopes from the model antigen chicken ovalbumin (OVA), human tyrosinase-related protein 2 (TRP-2), and oncoprotein E7 of human papillomavirus type 16 (HPV16E7). Transfection of murine cells with constructs encoding Bet-epitope chimeric proteins led to efficient MHC-I-restricted epitope presentation as confirmed by interferon-gamma enzyme-linked immunospot assays using epitope-specific cytotoxic T lymphocyte (CTL) lines. FFV infection-mediated transduction of cells with epitope-carrying Bet also induced T-cell responses, albeit with reduced efficacy, in a process independent from the presence of free peptides. We show that primate FV Bet is also a promising T cell epitope carrier for clinical translation. The data demonstrate the utility of replication-competent and -attenuated FVs as antigen carriers in immunotherapy.

Enveloped virus-like particle platforms: vaccines of the future?

The techniques to produce effective vaccines have evolved, and the early vaccines (live, inactivated, subunit...) are no longer considered as the most appropriate for new vaccine development. We question here what will be the future vaccines, and argue that virus-like particle (VLP)-based vaccines are promising candidates. In addition to being effective vaccines against analogous viruses from which they are derived, VLPs can also be used to present foreign epitopes to the immune system. The achievement of this strategy can be illustrated by the recent development of malaria candidate vaccine. We point out recent VLP-based vaccine developments and discuss future perspectives.

Nanotechnology-based approaches for the development of diagnostics, therapeutics, and vaccines

The architecture of nanoparticles of biological origin, generally also known as bionanoparticles, presents several features that are ideal for their use in developing diagnostics, therapeutics, and vaccines. In this regard, particles formed by viral proteins using recombinant DNA technology resemble authentic virus particles. However, they lack infectivity due to the absence of genetic components such as DNA or RNA. Hence, they are designated as virus-like particles (VLP). VLPs possess the following characteristics: (1) they can be generated by either a single or a few viral proteins; (2) their size, formed by viral proteins, is in the range of 20 to100 nm; (3) the number of protein molecules required for particle assembly is from hundreds to thousands, depending on the VLP; (4) the protein(s) responsible for their assembly are amenable for manipulation; and (5) multiple proteins/peptides can be incorporated into a VLP. The potential advantages of VLPs directed by retroviral proteins are discussed in this review.

Efficacy of DNA vaccines forming e7 recombinant retroviral virus-like particles for the treatment of human papillomavirus-induced cancers

Human papillomavirus (HPV) is involved in the development of anogenital tumors and also in the development of oropharyngeal head and neck carcinomas, where HPV-16, expressing the E6 and E7 oncoproteins, is the most frequent serotype. Although vaccines encoding L1 and L2 capsid HPV proteins are efficient for the prevention of HPV infection, they are inadequate for treating established tumors. Hence, development of innovative vaccine therapies targeting E6/E7 is important for controlling HPV-induced cancers. We have engineered a nononcogenic mutated E7-specific plasmo-retroVLP vaccine (pVLP-E7), consisting of plasmid DNA, that is able to form recombinant retrovirus-based virus-like particles (VLPs) that display E7 antigen into murine leukemia virus Gag proteins pseudotyped with vesicular stomatitis virus envelope glycoprotein (VSV-G). pVLP-E7 vaccinations were studied for their ability to generate specific immune responses and for induction of protective immunity against tumor cell challenge in preventive and therapeutic models. The produced VLPs induce the maturation of human dendritic cells in vitro and mount specific E7 T cell responses. Intradermic vaccinations of mice with pVLP-E7 show their efficacy to generate antigen-specific T cell responses, to prevent and protect animals from early TC-1 tumor development compared with standard DNA or VLP immunizations. The vaccine efficacy was also evaluated for advanced tumors in mice vaccinated at various time after the injection of TC-1 cells. Data show that pVLP-E7 vaccination can cure mice with already established tumors only when combined with Toll-like receptor-7 (TLR7) and TLR9 agonists. Our findings provide evidence that pVLPs, combining the advantages of DNA and VLP vaccines, appear to be a promising strategy for the treatment of HPV-induced cancers.

Virus-like particle vaccine induces cross-protection against human metapneumovirus infections in mice

Human metapneumovirus (HMPV) is a paramyxovirus that causes acute respiratory-tract infections in children and adults worldwide. A safe and effective vaccine could decrease the burden of disease associated with this novel pathogen. We engineered HMPV viral-like particles (HMPV-VLPs) derived from retroviral core particles that mimic the properties of the viral surface of two HMPV viruses of either lineage A or B. These VLPs functionally display F and G HMPV surface glycoproteins. When injected in mice, HMPV-VLPs induce strong humoral immune response against both homologous and heterologous strains. Moreover, the induced neutralizing antibodies prevented mortality upon subsequent infection of the lungs with both homologous and heterologous viruses. Upon challenge, viral titers in the lungs of immunized animals were significantly reduced as compared to those of control animals. In conclusion, a HMPV-VLP vaccine that induces cross-protective immunity in mice is a promising approach to prevent HMPV infections.

Trans-packaging of human immunodeficiency virus type 1 genome into Gag virus-like particles in Saccharomyces cerevisiae

BACKGROUND

Yeast is recognized as a generally safe microorganism and is utilized for the production of pharmaceutical products, including vaccines. We previously showed that expression of human immunodeficiency virus type 1 (HIV-1) Gag protein in Saccharomyces cerevisiae spheroplasts released Gag virus-like particles (VLPs) extracellularly, suggesting that the production system could be used in vaccine development. In this study, we further establish HIV-1 genome packaging into Gag VLPs in a yeast cell system.

RESULTS

The nearly full-length HIV-1 genome containing the entire 5' long terminal repeat, U3-R-U5, did not transcribe gag mRNA in yeast. Co-expression of HIV-1 Tat, a transcription activator, did not support the transcription. When the HIV-1 promoter U3 was replaced with the promoter for the yeast glyceraldehyde-3-phosphate dehydrogenase gene, gag mRNA transcription was restored, but no Gag protein expression was observed. Co-expression of HIV-1 Rev, a factor that facilitates nuclear export of gag mRNA, did not support the protein synthesis. Progressive deletions of R-U5 and its downstream stem-loop-rich region (SL) to the gag start ATG codon restored Gag protein expression, suggesting that a highly structured noncoding RNA generated from the R-U5-SL region had an inhibitory effect on gag mRNA translation. When a plasmid containing the HIV-1 genome with the R-U5-SL region was coexpressed with an expression plasmid for Gag protein, the HIV-1 genomic RNA was transcribed and incorporated into Gag VLPs formed by Gag protein assembly, indicative of the trans-packaging of HIV-1 genomic RNA into Gag VLPs in a yeast cell system. The concentration of HIV-1 genomic RNA in Gag VLPs released from yeast was approximately 500-fold higher than that in yeast cytoplasm. The deletion of R-U5 to the gag gene resulted in the failure of HIV-1 RNA packaging into Gag VLPs, indicating that the packaging signal of HIV-1 genomic RNA present in the R-U5 to gag region functions similarly in yeast cells.

CONCLUSIONS

Our data indicate that selective trans-packaging of HIV-1 genomic RNA into Gag VLPs occurs in a yeast cell system, analogous to a mammalian cell system, suggesting that yeast may provide an alternative packaging system for lentiviral RNA.

Recombinant retrovirus-derived virus-like particle-based vaccines induce hepatitis C virus-specific cellular and neutralizing immune responses in mice

While the immunological correlates of hepatitis C virus (HCV)-specific immunity are not well understood, it is now admitted that an effective vaccine against HCV will need to induce both cellular and humoral immune responses and address viral heterogeneity to prevent immune escape. We developed a vaccine platform specifically aimed at inducing such responses against HCV antigens displayed by recombinant retrovirus-based virus-like particles (VLPs) made of Gag of murine leukemia virus. Both ex vivo produced VLPs and plasmid DNA encoding VLPs can be used as vaccines. Here, we report that immunizations with plasmid DNA forming VLPs pseudotyped with HCV E1 and E2 envelope glycoproteins (HCV-specific plasmo-retroVLPs) induce strong T-cell-mediated immune responses that can be optimized by using proper DNA delivery methods and/or genetic adjuvants. Additionally, multigenotype or multi-specific T-cell responses were observed after immunization with plasmids that encode VLPs pseudotyped with E1E2 derived from numerous viral genotypes and/or displaying NS3 antigen in capsid proteins. While homologous prime-boost immunizations with HCV-specific plasmo-retroVLPs or ex vivo produced VLPs induce a low level of specific antibody responses, optimal combination of plasmo-retroVLPs and VLPs was identified for inducing HCV-specific T-cell and B-cell responses as well as neutralizing antibodies. Altogether, these results have important meanings for the development of anti-HCV preventive vaccines and exemplify the flexibility and potential of our retrovirus-based platform in inducing broad cellular and humoral immune responses.

Induction of ebolavirus cross-species immunity using retrovirus-like particles bearing the Ebola virus glycoprotein lacking the mucin-like domain

Background

The genus Ebolavirus includes five distinct viruses. Four of these viruses cause hemorrhagic fever in humans. Currently there are no licensed vaccines for any of them; however, several vaccines are under development. Ebola virus envelope glycoprotein (GP_1,2) is highly immunogenic, but antibodies frequently arise against its least conserved mucin-like domain (MLD). We hypothesized that immunization with MLD-deleted GP_1,2 (GPΔMLD) would induce cross-species immunity by making more conserved regions accessible to the immune system.

Methods

To test this hypothesis, mice were immunized with retrovirus-like particles (retroVLPs) bearing Ebola virus GPΔMLD, DNA plasmids (plasmo-retroVLP) that can produce such retroVLPs in vivo, or plasmo-retroVLP followed by retroVLPs.

Results

Cross-species neutralizing antibody and GP_1,2-specific cellular immune responses were successfully induced.

Conclusion

Our findings suggest that GPΔMLD presented through retroVLPs may provide a strategy for development of a vaccine against multiple ebolaviruses. Similar vaccination strategies may be adopted for other viruses whose envelope proteins contain highly variable regions that may mask more conserved domains from the immune system.

Pseudotype-independent nonspecific uptake of gammaretroviral and lentiviral particles in human cells

The effective entry of retroviruses into target cells depends on the presence of viral envelope (Env) proteins and cognate cellular receptors, such as the murine cationic amino acid transporter-1 (mCAT-1) for the ecotropic murine leukemia virus (MLV-E). Here, we examined whether human cells internalize MLV-E or other retroviral pseudotypes irrespective of the presence of a specific receptor. Using fluorescently tagged Gag to monitor viral internalization, and treating cells with chloroquine or bafilomycin A1, we show that endocytosis is the main pathway for productive transduction with ecotropic particles, but endocytosis of retroviral particles itself does not depend on a suitable receptor or Env. Nonspecific endosomal uptake and lysosomal degradation occurred with all "illegitimate" envelope-receptor combinations tested: MLV particles pseudotyped with the ecotropic envelope or measles virus H and F proteins as well as "ecotropic" or "bald" HIV-1 particles. Kinetic studies in cell lines and primary human T lymphocytes showed the persistence of Gag-GFP signals for more than 10 days after exposure to retroviral vector particles, even in the absence of a suitable receptor. Further studies testing the Gag-mediated transfer of protein or retroviral mRNA revealed that nonspecific endocytosis prevented the release of functional particle-associated proteins and nucleic acids into the cytosol. We conclude that receptor-targeted retroviral particles are unlikely to escape nonspecific cellular uptake unless appropriate protective principles are discovered. Conversely, as lysosomal degradation was found to inactivate mRNA and proteins embedded into retroviral particles, receptor targeting is a useful strategy for both transient and permanent cell modification by retrovirus-like particles.

A prime-boost strategy using virus-like particles pseudotyped for HCV proteins triggers broadly neutralizing antibodies in macaques

Chronic hepatitis C virus (HCV) infection, with its cohort of life-threatening complications, affects more than 200 million persons worldwide and has a prevalence of more than 10% in certain countries. Preventive and therapeutic vaccines against HCV are thus much needed. Neutralizing antibodies (NAbs) are the foundation for successful disease prevention for most established vaccines. However, for viruses that cause chronic infection such as HIV or HCV, induction of broad NAbs from recombinant vaccines has remained elusive. We developed a vaccine platform specifically aimed at inducing NAbs based on pseudotyped virus-like particles (VLPs) made with retroviral Gag. We report that VLPs pseudotyped with E2 and/or E1 HCV envelope glycoproteins induced high-titer anti-E2 and/or anti-E1 antibodies, as well as NAbs, in both mouse and macaque. The NAbs, which were raised against HCV 1a, cross-neutralized the five other genotypes tested (1b, 2a, 2b, 4, and 5). Thus, the described VLP platform, which can be pseudotyped with a vast array of virus envelope glycoproteins, represents a new approach to viral vaccine development.

Down-regulation of CD81 tetraspanin in human cells producing retroviral-based particles: tailoring vector composition

Retroviral-derived biopharmaceuticals (RV) target numerous therapeutic applications, from gene therapy to virus-like particle (rVLP)-based vaccines. During particle formation, beside the pseudotyped envelope proteins, RV can incorporate proteins derived from the virus producer cells (VPC). This may be detrimental by reducing the amounts of the pseudotyped envelope and/or by incorporating protein capable of inducing immune responses when non-human VPC are used. Manipulating the repertoire of VPC proteins integrated onto the vector structure is an underexplored territory and should provide valuable insights on potential targets to improve vector pharmacokinetic and pharmacodynamic properties. In this work, human HEK 293 cells producing retrovirus-like particles (rVLPs) and infectious RV vectors were used to prove the concept of customizing RV composition by manipulating cellular protein content. The tetraspanin CD81 was chosen since it is significantly incorporated in the RV membrane, conferring to the vector significant immunogenicity when used in mice. RNA interference-mediated by shRNA lentiviral vector transduction was efficiently used to silence CD81 expression (up to 99%) and the rVLPs produced by knocked-down cells lack CD81. Silenced clones were analyzed for cell proliferation, morphological changes, susceptibility to oxidative stress conditions, and rVLP productivities. The results showed that the down-regulation of VPC proteins requires close monitoring for possible side effects on cellular production performance. Yet, they confirm that it is possible to change the composition of host-derived immunogens in RV by altering cellular protein content with no detriment for vector productivity and titers. This constitutes an important manipulation tool in vaccinology--by exploiting the potential adjuvant effect of VPC proteins or using them as fusion agents to other proteins of interest to be exposed on the vector membrane--and in gene therapy, by reducing the immunogenicity of RV-based vector and enhancing in vivo half-life. Such tools can also be applied to lentiviral or other enveloped viral vectors.

Adenovirus-retrovirus hybrid vectors achieve highly enhanced tumor transduction and antitumor efficacy in vivo

Murine leukemia virus (MLV)-based replication-competent retrovirus (RCR) vectors have been shown to mediate efficient, selective, and persistent tumor transduction, thereby achieving significant therapeutic benefit in a wide variety of cancer models. To further augment the efficiency of this strategy, we have developed a delivery method employing a gutted adenovirus encoding an RCR vector (AdRCR); thus, tumor cells transduced with the adenoviral vector transiently become RCR vector producer cells in situ. As expected, high-titer AdRCR achieved significantly higher initial transduction levels in human cancer cells both in vitro and in vivo, as compared to the original RCR vector itself. Notably, even at equivalent initial transduction levels, more secondary RCR progeny were produced from AdRCR-transduced cells as compared to RCR-transduced cells, resulting in further acceleration of subsequent RCR replication kinetics. In pre-established tumor models in vivo, prodrug activator gene therapy with high-titer AdRCR could achieve enhanced efficacy compared to RCR alone, in a dose-dependent manner. Thus, AdRCR hybrid vectors offer the advantages of high production titers characteristic of adenovirus and secondary production of RCR in situ, which not only accelerates subsequent vector spread and progressive tumor transduction, but can also significantly enhance the therapeutic efficacy of RCR-mediated prodrug activator gene therapy.

Comparative evaluation of preclinical in vivo models for the assessment of replicating retroviral vectors for the treatment of glioblastoma

Despite impressive improvements in neurosurgical techniques, radiation and chemotherapy during the past few years, little progress has been made in the treatment of malignant gliomas. Recently, the efficacy of suicide gene therapy based on replication-competent retroviral (RCR) vectors as delivery vehicles for the therapeutic gene has been described in the treatment of experimental cancer, including gliomas. In this study, we have thus critically evaluated a panel of human and rodent glioma/glioblastoma cell lines (U-87MG, U-118MG, LN-18, LN-229, 8-MG-BA, 42-MG-BA, A-172, T-98G, UVW, C6, 9L, G-26, GL-261, Tu-2449, Tu-9648) with respect to RCR virus vector spread, sensitivity towards the cytosine deaminase (CD)/5-flurocytosine (5-FC)/5-flurouracil (5-FU) suicide system, and orthotopic growth characteristics in mice to identify suitable preclinical animal models for the development of a glioblastoma gene therapy. Rapid virus spread was observed in eight out of nine human cell lines tested in vitro. As expected, only CD-expressing cells became sensitive to 5-FC, due to their ability to convert the prodrug in its toxic form, 5-FU. All LD(50) values were within the range of concentrations obtained in human body fluids after conventional antifungal 5-FC administration. In addition, a significant bystander effect was observed in all human glioma cell lines tested. Injection of the RCR vector into pre-established orthotopic mouse tumor xenografts revealed substantial infection and virus spread of tumor tissue from most cell types.

Protein transduction from retroviral Gag precursors

Retroviral particles assemble a few thousand units of the Gag polyproteins. Proteolytic cleavage mediated by the retroviral protease forms the bioactive retroviral protein subunits before cell entry. We hypothesized that this process could be exploited for targeted, transient, and dose-controlled transduction of nonretroviral proteins into cultured cells. We demonstrate that gammaretroviral particles tolerate the incorporation of foreign protein at several positions of their Gag or Gag-Pol precursors. Receptor-mediated and thus potentially cell-specific uptake of engineered particles occurred within minutes after cell contact. Dose and kinetics of nonretroviral protein delivery were dependent upon the location within the polyprotein precursor. Proteins containing nuclear localization signals were incorporated into retroviral particles, and the proteins of interest were released from the precursor by the retroviral protease, recognizing engineered target sites. In contrast to integration-defective lentiviral vectors, protein transduction by retroviral polyprotein precursors was completely transient, as protein transducing retrovirus-like particles could be produced that did not transduce genes into target cells. Alternatively, bifunctional protein-delivering particle preparations were generated that maintained their ability to serve as vectors for retroviral transgenes. We show the potential of this approach for targeted genome engineering of induced pluripotent stem cells by delivering the site-specific DNA recombinase, Flp. Protein transduction of Flp after proteolytic release from the matrix position of Gag allowed excision of a lentivirally transduced cassette that concomitantly expresses the canonical reprogramming transcription factors (Oct4, Klf4, Sox2, c-Myc) and a fluorescent marker gene, thus generating induced pluripotent stem cells that are free of lentivirally transduced reprogramming genes.