An engineered HIV-1 Gag-based VLP displaying high antigen density induces strong antibody-dependent functional immune responses.

Antigen display on the surface of Virus-Like Particles (VLPs) improves immunogenicity compared to soluble proteins. We hypothesised that immune responses can be further improved by increasing the antigen density on the surface of VLPs. In this work, we report an HIV-1 Gag-based VLP platform engineered to maximise the presence of antigen on the VLP surface. An HIV-1 gp41-derived protein (Min), including the C-terminal part of gp41 and the transmembrane domain, was fused to HIV-1 Gag. This resulted in high-density MinGag-VLPs. These VLPs demonstrated to be highly immunogenic in animal models using either a homologous (VLP) or heterologous (DNA/VLP) vaccination regimen, with the latter yielding 10-fold higher anti-Gag and anti-Min antibody titres. Despite these strong humoral responses, immunisation with MinGag-VLPs did not induce neutralising antibodies. Nevertheless, antibodies were predominantly of an IgG2b/IgG2c profile and could efficiently bind CD16-2. Furthermore, we demonstrated that MinGag-VLP vaccination could mediate a functional effect and halt the progression of a Min-expressing tumour cell line in an in vivo mouse model.

Production of HIV-1-based virus-like particles for vaccination: achievements and limits.

Over the past years, much knowledge has been gained about the HIV-1 virus structure and infection cycle. This knowledge has been used to conceive different types of potential vaccines and vaccination strategies. This review focuses on the characteristics of the virus and the vaccines that have been developed, particularly on those using virus-like particles, as well as on the developments for their production and purification. The production of HIV-1 VLPs has been investigated in different platforms such as, yeast, plants, insect and mammalian cells. Their purification follows the same rational as for viral vectors: clarification, nuclease treatment, concentration/capture, polishing, formulation and viral clearance. Analytical techniques to characterise the obtained productions will be of paramount relevance for their final application, considering that the raw production obtained in bioreactors comprises not only the VLPs of interest but also many other extracellular vesicles. Finally, it should also be considered that VLPs are prone to carry host cell proteins and DNA.

A statistical approach to improve compound screening in cell culture media.

The Chinese hamster ovary (CHO) cell line is widely used for the production of recombinant proteins due to its high growing capacity and productivity, as well as other cell lines derived later than CHO. Adapting cell culture media for each specific cell line is a key to exploit these features for cost effective and fast product generation. Media supplementation is generally addressed by means of one-factor-at-a-time or classical design of experiments approaches but these techniques may not be efficient enough in preliminary screening phases. In this study, a novel strategy consisting in folding over the Plackett-Burman design was used to increase cell growth and trastuzumab production of different CHO cell lines through supplementation with nonanimal recombinant compounds. Synergies between compounds could be detected with a reduced number of experiments by using this methodology in comparison to more conventional fractional factorial designs. In the particular case reported here, the sequential use of this modified Plackett-Burman in combination with a Box-Behnken design led to a 1.5-fold increase in cell growth (10 × 106 cells/mL) and a two-fold in trastuzumab titer (122 mg/L) in suspension batch culture.

Advancements in mammalian cell transient gene expression (TGE) technology for accelerated production of biologics.

Transient gene expression (TGE) in animal cell cultures has been used for almost 30 years to produce milligrams and grams of recombinant proteins, virus-like particles and viral vectors, mainly for research purposes. The need to increase the amount of product has led to a scale-up of TGE protocols. Moreover, product quality and process reproducibility are also of major importance, especially when TGE is employed for the preparation of clinical lots. This work gives an overview of the different technologies that are available for TGE and how they can be combined, depending on each application. Then, a critical assessment of the challenges of large-scale transient transfection follows, focusing on suspension cell cultures transfected with polyethylenimine (PEI), which is the most widely used methodology for transfection. Finally, emerging opportunities for transient transfection arising from gene therapy, personalized medicine and vaccine development are reviewed.

Transient gene expression optimization and expression vector comparison to improve HIV-1 VLP production in HEK293 cell lines.

Transient gene expression (TGE) has been used at small and medium scale for the production of biologicals in sufficient quantities to perform pre-clinical and characterization studies. Polyethyleneimine (PEI)-mediated transfection offers a low toxicity and non-expensive method for cell transfection. DNA and PEI concentration for transient gene expression has been extensively optimized in order to increase product titers. However, the possibility to extrapolate the optimal concentrations found for a specific bioprocess when expression vectors or cell lines need to be changed has not been investigated.In this work, the combination of three different HEK293 cell lines with three different vectors was studied for the production of HIV-1 virus-like particles (VLPs). The concentration of DNA and PEI was optimized for the nine combinations. The obtained results were very similar in all cases (DNA = 2.34 ± 0.18 µg/mL and PEI = 5.81 ± 0.18 µg/mL), revealing that transfection efficiency is not dependent on the cell line or vector type, but on DNA and PEI quantities. Furthermore, two of the cell lines tested stably expressed a protein able to recognize specific origins of replication: HEK293T/SV40 and HEK293E/oriP. Origins of replication were included in the vector sequences in order to test their capacity to increase production titers. HEK293T/SV40 resulted in a decrease of cell density and productivity of 2.3-fold compared to a control plasmid. On the other hand, HEK293E/OriP platform enabled a threefold improvement in HIV-1 VLP production keeping the same cell densities and viabilities compared to a control plasmid.

Production of HIV virus-like particles by transient transfection of CAP-T cells at bioreactor scale avoiding medium replacement.

Human-derived CAP-T cell line has been demonstrated to be a powerful platform for high-titer production of HIV virus-like particles (VLPs) by PEI-mediated transient transfection. Scale-up of transfection processes is key to ensure the necessary quantities for pre-clinical and clinical testing. One of the major operational challenges of large-scale transient transfection is the medium replacement step that is often required before transfection. In this work, CAP-T cells were cultured in 1L bioreactor with addition of sodium bicarbonate and surface aeration, which were observed to improve cell state for transfection. Remarkably, the medium replacement step was avoided by culturing the cells in a combination of media (FreeStyleF17+1% of PEM) compatible with cell growth and PEI-mediated transient transfection. In the conditions developed in this work, 0.5×106cells/mL were seeded in 1L bioreactor. Two days later, ∼2×106cells/mL were transfected without medium exchange, using 0.5pg of DNA/cell and 3pg of PEI/cell. Transfection efficiency and VLP production comparable to shake flasks were obtained with a production of 4×1010VLPs/mL. This novel strategy significantly simplifies large-scale transient transfection, while suitable cell growth, transfection efficiency, and high quality VLP production are achieved.

Identification of HIV-1-Based Virus-like Particles by Multifrequency Atomic Force Microscopy.

Virus-like particles (VLPs) have become a promising platform for vaccine production. VLPs are formed by structural viral proteins that inherently self-assemble when expressed in a host cell. They represent a highly immunogenic and safe vaccine platform, due to the absence of the viral genome and its high protein density. One of the most important parameters in vaccine production is the quality of the product. A related bottleneck in VLP-based products is the presence of cellular vesicles as a major contaminant in the preparations, which will require the set up of techniques allowing for specific discrimination of VLPs from host vesicular bodies. In this work novel, to our knowledge, multifrequency (MF) atomic force microscopy (AFM) has permitted full structural nanophysical characterization by its access to the virus capsid of the HIV-based VLPs. The assessment of these particles by advanced amplitude modulation-frequency modulation (AM-FM) viscoelastic mapping mode has enhanced the imaging resolution of their nanomechanical properties, opening a new window for the study of the biophysical attributes of VLPs. Finally, the identification and differentiation of HIV-based VLPs from cellular vesicles has been performed under ambient conditions, providing, to our knowledge, novel methodology for the monitoring and quality control of VLPs.

Optimized production of HIV-1 virus-like particles by transient transfection in CAP-T cells.

HIV-1 virus-like particles (VLPs) have great potential as new-generation vaccines. The novel CAP-T cell line is used for the first time to produce Gag-GFP HIV-1 VLPs by means of polyethylenimine (PEI)-mediated transient transfection. CAP-T cells are adapted to grow to high cell densities in serum-free medium, and are able to express complex recombinant proteins with human post-translational modifications. Furthermore, this cell line is easily transfected with PEI, which offers the flexibility to rapidly generate and screen a number of candidates in preclinical studies. Transient transfection optimization of CAP-T cells has been performed systematically in this work. It is determined that for optimal production, cells need to be growing at mid-exponential phase, Protein Expression Medium (PEM) medium has to be added post-transfection, and cells can be transfected by independent addition of DNA and PEI with no prior complexation. A Box-Behnken experimental design is used to optimize cell density at time of transfection, DNA/cell and PEI/cell ratios. The optimal conditions determined are transfection at a density of 3.3E + 06 cells/mL with 0.5 pg of DNA/cell and 3 pg of PEI/cell. Using the optimized protocol, 6 × 10(10) VLP/mL are obtained, demonstrating that CAP-T is a highly efficient cell line for the production of HIV-1 VLPs and potentially other complex viral-based biotherapeutics.

Selection and optimization of transfection enhancer additives for increased virus-like particle production in HEK293 suspension cell cultures.

The manufacturing of biopharmaceuticals in mammalian cells typically relies on the use of stable producer cell lines. However, in recent years, transient gene expression has emerged as a suitable technology for rapid production of biopharmaceuticals. Transient gene expression is particularly well suited for early developmental phases, where several potential therapeutic targets need to be produced and tested in vivo. As a relatively new bioprocessing modality, a number of opportunities exist for improving cell culture productivity upon transient transfection. For instance, several compounds have shown positive effects on transient gene expression. These transfection enhancers either facilitate entry of PEI/DNA transfection complexes into the cell or nucleus or increase levels of gene expression. In this work, the potential of combining transfection enhancers to increase Gag-based virus-like particle production levels upon transfection of suspension-growing HEK 293 cells is evaluated. Using Plackett-Burman design of experiments, it is first tested the effect of eight transfection enhancers: trichostatin A, valproic acid, sodium butyrate, dimethyl sulfoxide (DMSO), lithium acetate, caffeine, hydroxyurea, and nocodazole. An optimal combination of compounds exhibiting the highest effect on gene expression levels was subsequently identified using a surface response experimental design. The optimal consisted on the addition of 20 mM lithium acetate, 3.36 mM valproic acid, and 5.04 mM caffeine which increased VLP production levels 3.8-fold, while maintaining cell culture viability at 94%.

Extended gene expression by medium exchange and repeated transient transfection for recombinant protein production enhancement.

Production of recombinant products in mammalian cell cultures can be achieved by stable gene expression (SGE) or transient gene expression (TGE). The former is based on the integration of a plasmid DNA into the host cell genome allowing continuous gene expression. The latter is based on episomal plasmid DNA expression. Conventional TGE is limited to a short production period of usually about 96 h, therefore limiting productivity. A novel gene expression approach termed extended gene expression (EGE) is explored in this study. The aim of EGE is to prolong the production period by the combination of medium exchange and repeated transfection of cell cultures with plasmid DNA to improve overall protein production. The benefit of this methodology was evaluated for the production of three model recombinant products: intracellular GFP, secreted GFP, and a Gag-GFP virus-like particles (VLPs). Productions were carried out in HEK 293 cell suspension cultures grown in animal-derived component free media using polyethylenimine (PEI) as transfection reagent. Transfections were repeated throughout the production process using different plasmid DNA concentrations, intervals of time, and culture feeding conditions in order to identify the best approach to achieve sustained high-level gene expression. Using this novel EGE strategy, the production period was prolonged between 192 and 240 h with a 4-12-fold increase in production levels, depending on the product type considered.

Development and validation of a quantitation assay for fluorescently tagged HIV-1 virus-like particles.

Upon expression, the Gag polyprotein of HIV-1 assembles spontaneously in the vicinity of the plasma membrane giving rise to enveloped virus-like particles (VLPs). These particulate immunogens offer great promise as HIV-1 vaccines. Robust VLP production and purification processes are required to generate VLPs of sufficient quality and quantity for both pre-clinical and clinical evaluation. The availability of simple, fast and reliable quantitation tools is critical to develop, optimize and monitor such processes. Traditionally, enzyme-linked immunosorbent assays (ELISA) are used to quantify p24 antigen concentrations, which reflect tightly virus particle concentrations. However, this quantitation technique is not only time-consuming, laborious and costly but it is also prone to methodological variability. As an alternative, the development and validation of a fluorescence-based quantitation assay for Gag VLPs is presented here. A Gag polyprotein fused to the enhanced green fluorescent protein was used for generation of fluorescent VLPs. A purified standard reference Gag-GFP VLP material was prepared and characterized in house. The method was validated according to ICH guidelines. The validation characteristics evaluated included accuracy, precision, specificity, linearity, range and limit of detection. The method showed to be specific for Gag-GFP. The fluorescence signal correlated well with p24 concentrations measured using a reference p24 ELISA assay. The method showed little variability compared to ELISA and was linear over a 3-log range. The limit of detection was ~10 ng of p24/mL. Finally, fluorescence-based titers were in good agreement with those obtained using transmission electron microscopy and nanoparticle tracking analysis. This simple, rapid and cost-effective quantitation assay should facilitate development and optimization of bioprocessing strategies for Gag-based VLPs.

Generation of HIV-1 Gag VLPs by transient transfection of HEK 293 suspension cell cultures using an optimized animal-derived component free medium.

Virus-like particles (VLPs) offer great promise as candidates for new vaccine strategies. Large-scale approaches for the manufacturing of HIV-1 Gag VLPs have mainly focused on the use of the baculovirus expression system. In this work, the development and optimization of an HIV-1 Gag VLP production protocol by transient gene expression in mammalian cell suspension cultures is reported. To facilitate process optimization, a Gag-GFP fusion construct enabling the generation of fluorescent VLPs was used. The great majority of Gag-GFP present in cell culture supernatants was shown to be correctly assembled into virus-like particles of the expected size and morphology consistent with immature HIV-1 particles. Medium optimization was performed using design of experiments (DoE). Culture medium supplementation with non-animal derived components including recombinant proteins and lipids of synthetic or non-animal-derived origin resulted in improved HEK 293 cell growth and VLP production. The maximum cell density attained using the optimized Freestyle culture medium was 5.4×10(6)cells/mL in batch mode, almost double of that observed using the unsupplemented medium (2.9×10(6)cells/mL). Best production performance was attained when cells were transfected at mid-log phase (2-3×10(6)cells/mL) with medium exchange at the time of transfection using standard amounts of plasmid DNA and polyethylenimine. By using an optimized production protocol, VLP titers were increased 2.4-fold obtaining 2.8µg of Gag-GFP/mL or 2.7×10(9)VLPs/mL according to ELISA and nanoparticle tracking quantification analyses, respectively.