Efficient single tobamoviral vector-based bioproduction of broadly neutralizing anti-HIV-1 monoclonal antibody VRC01 in Nicotiana benthamiana plants and utility of VRC01 in combination microbicides

Therapeutic Potential of Marine Bioactive Peptides against Human Immunodeficiency Virus: Recent Evidence, Challenges, and Future Trends

Acquired immunodeficiency syndrome (AIDS) is a chronic and potentially fatal ailment caused by the human immunodeficiency virus (HIV) and remains a major health problem worldwide. In recent years, the research focus has shifted to a greater emphasis on complementing treatment regimens involving conventional antiretroviral (ARV) drug therapies with novel lead structures isolated from various marine organisms that have the potential to be utilized as therapeutics for the management of HIV-AIDS. The present review summarizes the recent developments regarding bioactive peptides sourced from various marine organisms. This includes a discussion encompassing the potential of these novel marine bioactive peptides with regard to antiretroviral activities against HIV, preparation, purification, and processing techniques, in addition to insight into the future trends with an emphasis on the potential of exploration and evaluation of novel peptides to be developed into effective antiretroviral drugs.

Polycistronic Expression of Multi-Subunit Complexes in the Eukaryotic Environment: A Narrative Review

Protein complexes are involved in many vital biological processes. Therefore, researchers need these protein complexes for biochemical and biophysical studies. Several methods exist for expressing multi-subunit proteins in eukaryotic cells, such as 2A sequences, IRES, or intein. Nevertheless, each of these elements has several disadvantages that limit their usage. In this article, we suggest a new system for expressing multi-subunit proteins, which have several advantages over existing methods meanwhile it, lacks most of their disadvantages. Leishmania is a unicellular eukaryote and member of the Trypanosomatidae family. In the expression system of Leishmania, pre-long RNAs that contain several protein sequences transcribe. Then these long RNAs separate into mature mRNAs in the process named trans splicing. For producing multi-subunit protein, Leishmania transformed with a vector containing the sequences of all subunits. Therefore, those subunits translate and form the complex under eukaryotic cell conditions. The sequence of each protein must separate by the spatial sequence needed for trans splicing. Based on a Leishmania expression pattern, not only is it possible to produce the complexes with the correct structures and post-translational modifications, but also it is possible to overcome previous method problems.

Reproducibility and flexibility of monoclonal antibody production with Nicotiana benthamiana

The ongoing SARS-CoV-2 coronavirus pandemic of 2020-2021 underscores the need for manufacturing platforms that can rapidly produce monoclonal antibody (mAb) therapies. As reported here, a platform based on Nicotiana benthamiana produced mAb therapeutics with high batch-to-batch reproducibility and flexibility, enabling production of 19 different mAbs of sufficient purity and safety for clinical application(s). With a single manufacturing run, impurities were effectively removed for a representative mAb product (the ZMapp component c4G7). Our results show for the first time the reproducibility of the platform for production of multiple batches of clinical-grade mAb, manufactured under current Good Manufacturing Practices, from Nicotiana benthamiana. The flexibility of the system was confirmed by the results of release testing of 19 different mAbs generated with the platform. The process from plant infection to product can be completed within 10 days. Therefore, with a constant supply of plants, response to the outbreak of an infectious disease could be initiated within a matter of weeks. Thus, these data demonstrated that this platform represents a reproducible, flexible system for rapid production of mAb therapeutics to support clinical development.

Manufacturing plant-made monoclonal antibodies for research or therapeutic applications

Monoclonal antibodies (mAbs) hold great promise for treating diseases ranging from cancer to infectious disease. Manufacture of mAbs is challenging, expensive, and time-consuming using mammalian systems. We describe detailed methods used by Kentucky BioProcessing (KBP), a subsidiary of British American Tobacco, for producing high quality mAbs in a Nicotiana benthamiana host. Using this process, mAbs that meet GMP standards can be produced in as little as 10 days. Guidance for using individual plants, as well as detailed methods for large-scale production, are described. These procedures enable flexible, robust, and consistent production of research and therapeutic mAbs.

Safety and Efficacy of Avaren-Fc Lectibody Targeting HCV High-Mannose Glycans in a Human Liver Chimeric Mouse Model

BACKGROUND & AIMS

Infection with hepatitis C virus (HCV) remains a major cause of morbidity and mortality worldwide despite the recent advent of highly effective direct-acting antivirals. The envelope glycoproteins of HCV are heavily glycosylated with a high proportion of high-mannose glycans (HMGs), which serve as a shield against neutralizing antibodies and assist in the interaction with cell-entry receptors. However, there is no approved therapeutic targeting this potentially druggable biomarker.

METHODS

The anti-HCV activity of a fusion protein consisting of Avaren lectin and the fragment crystallizable (Fc) region of a human immunoglobulin G1 antibody, Avaren-Fc (AvFc) was evaluated through the use of in vitro neutralization assays as well as an in vivo challenge in a chimeric human liver (PXB) mouse model. Drug toxicity was assessed by histopathology, serum alanine aminotransferase, and mouse body weights.

RESULTS

AvFc was capable of neutralizing cell culture-derived HCV in a genotype-independent manner, with 50% inhibitory concentration values in the low nanomolar range. Systemic administration of AvFc in a histidine-based buffer was well tolerated; after 11 doses every other day at 25 mg/kg there were no significant changes in body or liver weights or in blood human albumin or serum alanine aminotransferase activity. Gross necropsy and liver pathology confirmed the lack of toxicity. This regimen successfully prevented genotype 1a HCV infection in all animals, although an AvFc mutant lacking HMG binding activity failed.

CONCLUSIONS

These results suggest that targeting envelope HMGs is a promising therapeutic approach against HCV infection, and AvFc may provide a safe and efficacious means to prevent recurrent infection upon liver transplantation in HCV-related end-stage liver disease patients.

Virus-based pharmaceutical production in plants: an opportunity to reduce health problems in Africa

BACKGROUND

Developing African countries face health problems that they struggle to solve. The major causes of this situation are high therapeutic and logistical costs. Plant-made therapeutics are easy to produce due to the lack of the safety considerations associated with traditional fermenter-based expression platforms, such as mammalian cells. Plant biosystems are easy to scale up and inexpensive, and they do not require refrigeration or a sophisticated medical infrastructure. These advantages provide an opportunity for plant-made pharmaceuticals to counteract diseases for which medicines were previously inaccessible to people in countries with few resources.

MAIN BODY

The techniques needed for plant-based therapeutic production are currently available. Viral expression vectors based on plant viruses have greatly enhanced plant-made therapeutic production and have been exploited to produce a variety of proteins of industrial, pharmaceutical and agribusiness interest. Some neglected tropical diseases occurring exclusively in the developing world have found solutions through plant bioreactor technology. Plant viral expression vectors have been reported in the production of therapeutics against these diseases occurring exclusively in the third world, and some virus-derived antigens produced in plants exhibit appropriate antigenicity and immunogenicity. However, all advances in the use of plants as bioreactors have been made by companies in Europe and America. The developing world is still far from acquiring this technology, although plant viral expression vectors may provide crucial help to overcome neglected diseases.

CONCLUSION

Today, interest in these tools is rising, and viral amplicons made in and for Africa are in progress. This review describes the biotechnological advances in the field of plant bioreactors, highlights factors restricting access to this technology by those who need it most and proposes a solution to overcome these limitations.

Griffithsin Retains Anti-HIV-1 Potency with Changes in gp120 Glycosylation and Complements Broadly Neutralizing Antibodies PGT121 and PGT126

Griffithsin (Grft) is an antiviral lectin that has been shown to potently inhibit HIV-1 by binding high-mannose N-linked glycosylation sites on HIV-1 gp120. A key factor for Grft potency is glycosylation at N295 of gp120, which is directly adjacent to N332, a target glycan for an entire class of broadly neutralizing antibodies (bNAbs). Here, we unify previous work on the importance of other glycans to Grft potency against HIV-1 and Grft's role in mediating the conformational change of gp120 by mutating nearly every glycosylation site in gp120. In addition to a significant loss of Grft activity by the removal of glycosylation at N295, glycan absence at N332 or N448 was found to have moderate effects on Grft potency. Interestingly, in the absence of N295, Grft effectiveness could be improved by a mutation that results in the glycan at N448 shifting to N446, indicating that the importance of individual glycans may be related to their effect on glycosylation density. Grft's ability to alter the structure of gp120, exposing the CD4 binding site, correlated with the presence of glycosylation at N295 only in clade B strains, not clade C strains. We further demonstrate that Grft can rescue the activity of the bNAbs PGT121 and PGT126 in the event of a loss or a shift of glycosylation at N332, where the bNAbs suffer a drastic loss of potency. Despite targeting the same region, Grft in combination with PGT121 and PGT126 produced additive effects. This indicates that Grft could be an important combinational therapeutic.

Engineering of a Lectibody Targeting High-Mannose-Type Glycans of the HIV Envelope

High-mannose-type glycans (HMGs) are aberrantly enriched on HIV envelope glycoproteins. However, there is currently no drug selectively targeting HIV-associated HMGs. Here, we describe a novel HMG-targeting "lectibody," a recombinant Fc-fusion protein comprising human IgG1 Fc and a novel actinohivin lectin variant (Avaren) obtained by structure-guided modifications for improved overall surface charge properties (AvFc). AvFc was engineered and produced using a rapid and scalable plant-based transient overexpression system. The lectibody exhibited potent antiviral activity against HIV-1 groups M and O primary viruses, as well as HIV-2 and simian immunodeficiency virus (SIV) strains, without affecting normal human blood cells. Furthermore, the lectibody induced Fc-mediated cell killing activity against HIV-1-infected cells and selectively recognized SIVmac239-infected macaque mesenteric lymph node cells in vitro. AvFc showed an extended serum half-life in rats and rhesus macaques, while no discernible toxicity was observed upon repeated systemic dosing in mice. These results highlight AvFc's potential as a biotherapeutic targeting HIV-associated HMGs of cell-free virions, as well as productively infected cells, providing a foundation for new anti-HIV strategies. Efficient and cost-effective bioproduction in greenhouse facilities may open unique possibilities for further development of AvFc.

The potential of plant systems to break the HIV-TB link

Tuberculosis (TB) and human immunodeficiency virus (HIV) can place a major burden on healthcare systems and constitute the main challenges of diagnostic and therapeutic programmes. Infection with HIV is the most common cause of Mycobacterium tuberculosis (Mtb), which can accelerate the risk of latent TB reactivation by 20-fold. Similarly, TB is considered the most relevant factor predisposing individuals to HIV infection. Thus, both pathogens can augment one another in a synergetic manner, accelerating the failure of immunological functions and resulting in subsequent death in the absence of treatment. Synergistic approaches involving the treatment of HIV as a tool to combat TB and vice versa are thus required in regions with a high burden of HIV and TB infection. In this context, plant systems are considered a promising approach for combatting HIV and TB in a resource-limited setting because plant-made drugs can be produced efficiently and inexpensively in developing countries and could be shared by the available agricultural infrastructure without the expensive requirement needed for cold chain storage and transportation. Moreover, the use of natural products from medicinal plants can eliminate the concerns associated with antiretroviral therapy (ART) and anti-TB therapy (ATT), including drug interactions, drug-related toxicity and multidrug resistance. In this review, we highlight the potential of plant system as a promising approach for the production of relevant pharmaceuticals for HIV and TB treatment. However, in the cases of HIV and TB, none of the plant-made pharmaceuticals have been approved for clinical use. Limitations in reaching these goals are discussed.

A novel anti-HIV-1 bispecific bNAb-lectin fusion protein engineered in a plant-based transient expression system

The discovery of broadly neutralizing antibodies (bNAbs) has been a major step towards better prophylactic and therapeutic agents against human immunodeficiency virus type 1 (HIV-1). However, effective therapy will likely require a combination of anti-HIV agents to avoid viral evasion. One possible solution to this problem is the creation of bispecific molecules that can concurrently target two vulnerable sites providing synergistic inhibitory effects. Here, we describe the production in plants and anti-HIV activity of a novel bispecific fusion protein consisting of the antigen-binding fragment (Fab) of the CD4 binding site-specific bNAb VRC01 and the antiviral lectin Avaren, which targets the glycan shield of the HIV-1 envelope (VRC01Fab -Avaren). This combination was justified by a preliminary experiment demonstrating the synergistic HIV-1 neutralization activity of VRC01 and Fc-fused Avaren dimer (Avaren-Fc). Using the GENEWARE® tobacco mosaic virus vector, VRC01Fab -Avaren was expressed in Nicotiana benthamiana and purified using a three-step chromatography procedure. Surface plasmon resonance and ELISA demonstrated that both the Avaren and VRC01Fab moieties retain their individual binding specificities. VRC01Fab -Avaren demonstrated enhanced neutralizing activity against representative HIV-1 strains from A, B and C clades, compared to equimolar combinations of VRC01Fab and Avaren. Notably, VRC01Fab -Avaren showed significantly stronger neutralizing effects than the bivalent parent molecules VRC01 IgG and Avaren-Fc, with IC50 values ranging from 48 to 310 pm. These results support the continued development of bispecific anti-HIV proteins based on Avaren and bNAbs, to which plant-based transient overexpression systems will provide an efficient protein engineering and production platform.

Plant-Made Antibodies: Properties and Therapeutic Applications

BACKGROUND

A cost-effective plant platform for therapeutic monoclonal antibody production is both flexible and scalable. Plant cells have mechanisms for protein synthesis and posttranslational modification, including glycosylation, similar to those in animal cells. However, plants produce less complex and diverse Asn-attached glycans compared to animal cells and contain plant-specific residues. Nevertheless, plant-made antibodies (PMAbs) could be advantageous compared to those produced in animal cells due to the absence of a risk of contamination from nucleic acids or proteins of animal origin.

OBJECTIVE

In this review, the various platforms of PMAbs production are described, and the widely used transient expression system based on Agrobacterium-mediated delivery of genetic material into plant cells is discussed in detail.

RESULTS

We examined the features of and approaches to humanizing the Asn-linked glycan of PMAbs. The prospects for PMAbs in the prevention and treatment of human infectious diseases have been illustrated by promising results with PMAbs against human immunodeficiency virus, rotavirus infection, human respiratory syncytial virus, rabies, anthrax and Ebola virus. The pre-clinical and clinical trials of PMAbs against different types of cancer, including lymphoma and breast cancer, are addressed.

CONCLUSION

PMAb biosafety assessments in patients suggest that it has no side effects, although this does not completely remove concerns about the potential immunogenicity of some plant glycans in humans. Several PMAbs at various developmental stages have been proposed. Promise for the clinical use of PMAbs is aimed at the treatment of viral and bacterial infections as well as in anti-cancer treatment.

Hydroponic Treatment of Nicotiana benthamiana with Kifunensine Modifies the N-glycans of Recombinant Glycoprotein Antigens to Predominantly Man9 High-Mannose Type upon Transient Overexpression

Nicotiana benthamiana transient overexpression systems offer unique advantages for rapid and scalable biopharmaceuticals production, including high scalability and eukaryotic post-translational modifications such as N-glycosylation. High-mannose-type glycans (HMGs) of glycoprotein antigens have been implicated in the effectiveness of some subunit vaccines. In particular, Man₉GlcNAc₂ (Man9) has high binding affinity to mannose-specific C-type lectin receptors such as the mannose receptor and dendritic cell-specific intracellular adhesion molecule 3-grabbing non-integrin (DC-SIGN). Here, we investigated the effect of kifunensine, an α-mannosidase I inhibitor, supplemented in a hydroponic culture of N. benthamiana for the production of Man9-rich HMG glycoproteins, using N-glycosylated cholera toxin B subunit (gCTB) and human immunodeficiency virus gp120 that are tagged with a H/KDEL endoplasmic reticulum retention signal as model vaccine antigens. Biochemical analysis using anti-fucose and anti-xylose antibodies as well as Endo H and PNGase F digestion showed that kifunensine treatment effectively reduced plant-specific glycoforms while increasing HMGs in the N-glycan compositions of gCTB. Detailed glycan profiling revealed that plant-produced gp120 had a glycan profile bearing mostly HMGs regardless of kifunensine treatment. However, the gp120 produced under kifunensine-treatment conditions showed Man9 being the most prominent glycoform (64.5%), while the protein produced without kifunensine had a substantially lower Man9 composition (20.3%). Our results open up possibilities for efficient production of highly mannosylated recombinant vaccine antigens in plants.

Effect of temperature post viral vector inoculation on the amount of hemagglutinin transiently expressed in Nicotiana benthamiana leaves

Transient gene expression in whole plants by using viral vectors is promising as a rapid, mass production system for biopharmaceutical proteins. Recent studies have indicated that plant growth conditions such as air temperature markedly influence the accumulation levels of target proteins. Here, we investigated time course of the amount of recombinant hemagglutinin (HA), a vaccine antigen of influenza virus, in leaves of Nicotiana benthamiana plants grown at 20°C or 25°C post viral vector inoculation. The HA content per unit of leaf biomass increased and decreased from 4 to 6 days post inoculation at 20°C and 25°C, respectively, irrespective of the subcellular localization of HA. The overall HA contents were higher when HA was targeted to the endoplasmic reticulum (ER) rather than the apoplast. Necrosis of leaf tissues was specifically observed in plants inoculated with the ER-targeting vector and grown at 25°C. With the ER-targeting vector, the maximum HA contents at 20°C and 25°C were recorded at 6 and 4 days post inoculation, respectively, and were comparable to each other. HA contents thereafter decreased at both temperatures; the rate of reduction appeared faster at 25°C than at 20°C. From a practical point of view, our results indicate that the strategy of targeting HA to the ER, growing plants at a lower temperature of 20°C, and harvesting leaves at around a week after vector inoculation should be implemented to obtain a high HA yield stably and efficiently.

Pharmacokinetics and Preliminary Safety of Pod-Intravaginal Rings Delivering the Monoclonal Antibody VRC01-N for HIV Prophylaxis in a Macaque Model

The broadly neutralizing antibody (bNAb) VRC01, capable of neutralizing 91% of known human immunodeficiency virus type 1 (HIV-1) isolates in vitro, is a promising candidate microbicide for preventing sexual HIV infection when administered topically to the vagina; however, accessibility to antibody-based prophylactic treatment by target populations in sub-Saharan Africa and other underdeveloped regions may be limited by the high cost of conventionally produced antibodies and the limited capacity to manufacture such antibodies. Intravaginal rings of the pod design (pod-IVRs) delivering Nicotiana-manufactured VRC01 (VRC01-N) over a range of release rates have been developed. The pharmacokinetics and preliminary safety of VRC01-N pod-IVRs were evaluated in a rhesus macaque model. The devices sustained VRC01-N release for up to 21 days at controlled rates, with mean steady-state VRC01-N levels in vaginal fluids in the range of 10² to 10³ μg g^-1 being correlated with in vitro release rates. No adverse safety indications were observed. These findings indicate that pod-IVRs are promising devices for the delivery of the candidate topical microbicide VRC01-N against HIV-1 infection and merit further preclinical evaluation.

Griffithsin-Modified Electrospun Fibers as a Delivery Scaffold To Prevent HIV Infection

Despite current prophylactic strategies, sexually transmitted infections (STIs) remain significant contributors to global health challenges, spurring the development of new multipurpose delivery technologies to protect individuals from and treat virus infections. However, there are few methods currently available to prevent and no method to date that cures human immunodeficiency virus (HIV) infection or combinations of STIs. While current oral and topical preexposure prophylaxes have protected against HIV infection, they have primarily relied on antiretrovirals (ARVs) to inhibit infection. Yet continued challenges with ARVs include user adherence to daily treatment regimens and the potential toxicity and antiviral resistance associated with chronic use. The integration of new biological agents may avert some of these adverse effects while also providing new mechanisms to prevent infection. Of the biologic-based antivirals, griffithsin (GRFT) has demonstrated potent inhibition of HIV-1 (and a multitude of other viruses) by adhering to and inactivating HIV-1 immediately upon contact. In parallel with the development of GRFT, electrospun fibers (EFs) have emerged as a promising platform for the delivery of agents active against HIV infection. In the study described here, our goal was to extend the mechanistic diversity of active agents and electrospun fibers by incorporating the biologic GRFT on the EF surface rather than within the EFs to inactivate HIV prior to cellular entry. We fabricated and characterized GRFT-modified EFs (GRFT-EFs) with different surface modification densities of GRFT and demonstrated their safety and efficacy against HIV-1 infection in vitro We believe that EFs are a unique platform that may be enhanced by incorporation of additional antiviral agents to prevent STIs via multiple mechanisms.

Griffithsin: An Antiviral Lectin with Outstanding Therapeutic Potential

Griffithsin (GRFT), an algae-derived lectin, is one of the most potent viral entry inhibitors discovered to date. It is currently being developed as a microbicide with broad-spectrum activity against several enveloped viruses. GRFT can inhibit human immunodeficiency virus (HIV) infection at picomolar concentrations, surpassing the ability of most anti-HIV agents. The potential to inhibit other viruses as well as parasites has also been demonstrated. Griffithsin's antiviral activity stems from its ability to bind terminal mannoses present in high-mannose oligosaccharides and crosslink these glycans on the surface of the viral envelope glycoproteins. Here, we review structural and biochemical studies that established mode of action and facilitated construction of GRFT analogs, mechanisms that may lead to resistance, and in vitro and pre-clinical results that support the therapeutic potential of this lectin.

When plant virology met Agrobacterium: the rise of the deconstructed clones

In the early days of molecular farming, Agrobacterium-mediated stable genetic transformation and the use of plant virus-based vectors were considered separate and competing technologies with complementary strengths and weaknesses. The demonstration that 'agroinfection' was the most efficient way of delivering virus-based vectors to their target plants blurred the distinction between the two technologies and permitted the development of 'deconstructed' vectors based on a number of plant viruses. The tobamoviruses, potexviruses, tobraviruses, geminiviruses and comoviruses have all been shown to be particularly well suited to the development of such vectors in dicotyledonous plants, while the development of equivalent vectors for use in monocotyledonous plants has lagged behind. Deconstructed viral vectors have proved extremely effective at the rapid, high-level production of a number of pharmaceutical proteins, some of which are currently undergoing clinical evaluation.

HRA2pl peptide: a fusion inhibitor for human metapneumovirus produced in tobacco plants by transient transformation

MAIN CONCLUSION

The HRA2pl peptide expressed by transient transformation in N. tabacum plants is capable of inhibiting the binding of the human metapneumovirus to HEp-2 cells at the fusion stage. Human metapneumovirus (hMPV) is an agent responsible for acute respiratory infections that mainly affects children under 3 years, the elderly and immunocompromised patients. In children younger than 5 years, respiratory tract infections account for 20 % of deaths worldwide. However, there is currently no treatment or vaccine available against hMPV. The production of a safe, efficient and low cost treatment against this virus is a current challenge. Plants provide a system for recombinant protein production that is cost effective and is easier to scale up to an industrial level than other platforms; in addition, the plant tissue may be used as raw food, dried or, alternatively, proteins may be partially or fully purified and administered in aerosol or capsules as dry powder. In this study, we designed a gene expressing an antiviral peptide against hMPV based on the heptad repeat A domain of the F protein of the virus. We produced the recombinant peptide by a viral transient expression system (Magnifection(®)) in Nicotiana tabacum plants. The efficacy of this antiviral peptide was confirmed by in vitro assays in HEp-2 cell line. This is a promising result that can offer a prophylactic approach against hMPV.

A dual-intein autoprocessing domain that directs synchronized protein co-expression in both prokaryotes and eukaryotes

Being able to coordinate co-expression of multiple proteins is necessary for a variety of important applications such as assembly of protein complexes, trait stacking, and metabolic engineering. Currently only few options are available for multiple recombinant protein co-expression, and most of them are not applicable to both prokaryotic and eukaryotic hosts. Here, we report a new polyprotein vector system that is based on a pair of self-excising mini-inteins fused in tandem, termed the dual-intein (DI) domain, to achieve synchronized co-expression of multiple proteins. The DI domain comprises an Ssp DnaE mini-intein N159A mutant and an Ssp DnaB mini-intein C1A mutant connected in tandem by a peptide linker to mediate efficient release of the flanking proteins via autocatalytic cleavage. Essentially complete release of constituent proteins, GFP and RFP (mCherry), from a polyprotein precursor, in bacterial, mammalian, and plant hosts was demonstrated. In addition, successful co-expression of GFP with chloramphenicol acetyltransferase, and thioredoxin with RFP, respectively, further substantiates the general applicability of the DI polyprotein system. Collectively, our results demonstrate the DI-based polyprotein technology as a highly valuable addition to the molecular toolbox for multi-protein co-expression which finds vast applications in biotechnology, biosciences, and biomedicine.

Protein and oligonucleotide delivery systems for vaginal microbicides against viral STIs

Intravaginal delivery offers an effective option for localized, targeted, and potent microbicide delivery. However, an understanding of the physiological factors that impact intravaginal delivery must be considered to develop the next generation of microbicides. In this review, a comprehensive discussion of the opportunities and challenges of intravaginal delivery are highlighted, in the context of the intravaginal environment and currently utilized dosage forms. After a subsequent discussion of the stages of microbicide development, the intravaginal delivery of proteins and oligonucleotides is addressed, with specific application to HSV and HIV. Future directions may include the integration of more targeted delivery modalities to virus and host cells, in addition to the use of biological agents to affect specific genes and proteins involved in infection. More versatile and multipurpose solutions are envisioned that integrate new biologicals and materials into potentially synergistic combinations to achieve these goals.

Griffithsin tandemers: flexible and potent lectin inhibitors of the human immunodeficiency virus

BACKGROUND

The lectin griffithsin (GRFT) is a potent antiviral agent capable of prevention and treatment of infections caused by a number of enveloped viruses and is currently under development as an anti-HIV microbicide. In addition to its broad antiviral activity, GRFT is stable at high temperature and at a broad pH range, displays little toxicity and immunogenicity, and is amenable to large-scale manufacturing. Native GRFT is a domain-swapped homodimer that binds to viral envelope glycoproteins and has displayed mid-picomolar activity in cell-based anti-HIV assays. Previously, we have engineered and analyzed several monomeric forms of this lectin (mGRFT) with anti-HIV EC50 values ranging up to 323 nM. Based on our previous analysis of mGRFT, we hypothesized that the orientation and spacing of the carbohydrate binding domains GRFT were key to its antiviral activity.

RESULTS

Here we present data on engineered tandem repeats of mGRFT (mGRFT tandemers) with antiviral activity at concentrations as low as one picomolar in whole-cell anti-HIV assays. mGRFT tandemers were analyzed thermodynamically, both individually and in complex with HIV-1 gp120. We also demonstrate by dynamic light scattering and cryo-electron microscopy that mGRFT tandemers do not aggregate HIV virions. This establishes that, although the intra-virion crosslinking of HIV envelope glycoproteins is likely integral to their activity, the antiviral activity of these lectins is not due to virus aggregation caused by inter-virion crosslinking.

CONCLUSIONS

The engineered tandemer constructs of mGRFT may provide novel and powerful agents for prevention of infection by HIV and other enveloped viruses.

Characterization of VRC01, a potent and broadly neutralizing anti-HIV mAb, produced in transiently and stably transformed tobacco

The proposed clinical trial in Africa of VRC01, a potent broadly neutralizing antibody (bNAb) capable of neutralizing 91% of known HIV-1 isolates, raises concerns about testing a treatment which will be too expensive to be accessible by the most important target population, the poor in under-developed regions such as sub-Saharan Africa. Here, we report the expression of VRC01 in plants as an economic alternative to conventional mammalian-cell-based production platforms. The heavy and light chain genes of VRC01 were cloned onto a single vector, pTRAk.2, which was transformed into Nicotiana benthamiana or Nicotiana tabacum using transient and stable expression production systems respectively. VRC01 has been successfully expressed transiently in plants with expression level of approximately 80 mg antibody/kg; stable transgenic lines expressing up to 100 mg antibody/kg were also obtained. Plant-produced VRC01 from both systems showed a largely homogeneous N-glycosylation profile with a single dominant glycoform. The binding kinetics to gp120 IIIB (approximately 1 nM), neutralization of HIV-1 BaL or a panel of 10 VRC01-sensitive HIV-1 Env pseudoviruses of VRC01 produced in transient and stable plants were also consistent with VRC01 from HEK cells.

Plant virus expression vector development: new perspectives

Plant made biologics have elicited much attention over recent years for their potential in assisting those in developing countries who have poor access to modern medicine. Additional applications such as the stockpiling of vaccines against pandemic infectious diseases or potential biological warfare agents are also under investigation. Plant virus expression vectors represent a technology that enables high levels of pharmaceutical proteins to be produced in a very short period of time. Recent advances in research and development have brought about the generation of superior virus expression systems which can be readily delivered to the host plant in a manner that is both efficient and cost effective. This review presents recent innovations in plant virus expression systems and their uses for producing biologics from plants.

Monoclonal Antibody Purification (Nicotiana benthamiana Plants)

Plant-based expression systems provide an alternative biomanufacturing platform for recombinant proteins (Matoba et al., 2011). In particular, plant virus-based vectors can overexpress proteins within days in the leaf tissue of Nicotiana benthamiana (N. benthamiana). To overcome the issues of genetic instability and limited infectivity of recombinant viruses, Agrobacterium-mediated delivery of "deconstructed" virus vectors has become the mainstay for the production of large and/or multicomponent proteins, such as immunoglobulin (Ig)G monoclonal antibodies (mAbs). Here, we describe a method of producing human IgG mAbs in N. benthamiana using the tobamoviral replicon vector magnICON^®. The vector can express up to a few hundred mg of a mAb per kg of leaf material in 7 days. A representative case for the broadly neutralizing anti-HIV and anti-influenza mAbs, VRC01 and CR6261 respectively, is shown (Hamorsky et al., 2013). Leaf tissue is homogenized and the extract is clarified by filtration and centrifugation. The mAb is purified by fast protein liquid chromatography (FPLC) using Protein A affinity and Phenyl HP hydrophobic interection resins.

Plant viral vectors for delivery by Agrobacterium

Plant viral vectors delivered by Agrobacterium are the basis of several manufacturing processes that are currently in use for producing a wide range of proteins for multiple applications, including vaccine antigens, antibodies, protein nanoparticles such as virus-like particles (VLPs), and other protein and protein-RNA scaffolds. Viral vectors delivered by agrobacterial T-DNA transfer (magnifection) have also become important tools in research. In recent years, essential advances have been made both in the development of second-generation vectors designed using the 'deconstructed virus' approach, as well as in the development of upstream manufacturing processes that are robust and fully scalable. The strategy relies on Agrobacterium as a vector to deliver DNA copies of one or more viral RNA/DNA replicons; the bacteria are delivered into leaves by vacuum infiltration, and the viral machinery takes over from the point of T-DNA transfer to the plant cell nucleus, driving massive RNA and protein production and, if required, cell-to-cell spread of the replicons. Among the most often used viral backbones are those of the RNA viruses Tobacco mosaic virus (TMV), Potato virus X (PVX) and Cowpea mosaic virus (CPMV), and the DNA geminivirus Bean yellow dwarf virus. Prototypes of industrial processes that provide for high yield, rapid scale up and fast manufacturing cycles have been designed, and several GMP-compliant and GMP-certified manufacturing facilities are in place. These efforts have been successful as evidenced by the fact that several antibodies and vaccine antigens produced by magnifection are currently in clinical development.