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.

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.

Oral administration of a recombinant cholera toxin B subunit promotes mucosal healing in the colon

Cholera toxin B subunit (CTB) is a component of a licensed oral cholera vaccine. However, CTB has pleiotropic immunomodulatory effects whose impacts on the gut are not fully understood. Here, we found that oral administration in mice of a plant-made recombinant CTB (CTBp) significantly increased several immune cell populations in the colon lamina propria. Global gene expression analysis revealed that CTBp had more pronounced impacts on the colon than the small intestine, with significant activation of TGFβ-mediated pathways in the colon epithelium. The clinical relevance of CTBp-induced impacts on colonic mucosa was examined. In a human colon epithelial model using Caco2 cells, CTBp, but not the non-GM1-binding mutant G33D-CTBp, induced TGFβ-mediated wound healing. In a dextran sodium sulfate (DSS) acute colitis mouse model, oral administration of CTBp protected against colon mucosal damage as manifested by mitigated body weight loss, decreased histopathological scores, and blunted escalation of inflammatory cytokine levels while inducing wound healing-related genes. Furthermore, biweekly oral administration of CTBp significantly reduced disease severity and tumorigenesis in the azoxymethane/DSS model of ulcerative colitis and colon cancer. Altogether, these results demonstrate CTBp's ability to enhance mucosal healing in the colon, highlighting its potential application in ulcerative colitis therapy besides cholera vaccination.

Cholera toxin B: one subunit with many pharmaceutical applications

Cholera, a waterborne acute diarrheal disease caused by Vibrio cholerae, remains prevalent in underdeveloped countries and is a serious health threat to those living in unsanitary conditions. The major virulence factor is cholera toxin (CT), which consists of two subunits: the A subunit (CTA) and the B subunit (CTB). CTB is a 55 kD homopentameric, non-toxic protein binding to the GM1 ganglioside on mammalian cells with high affinity. Currently, recombinantly produced CTB is used as a component of an internationally licensed oral cholera vaccine, as the protein induces potent humoral immunity that can neutralize CT in the gut. Additionally, recent studies have revealed that CTB administration leads to the induction of anti-inflammatory mechanisms in vivo. This review will cover the potential of CTB as an immunomodulatory and anti-inflammatory agent. We will also summarize various recombinant expression systems available for recombinant CTB bioproduction.

N-glycosylation of cholera toxin B subunit in Nicotiana benthamiana: impacts on host stress response, production yield and vaccine potential

Plant-based transient overexpression systems enable rapid and scalable production of subunit vaccines. Previously, we have shown that cholera toxin B subunit (CTB), an oral cholera vaccine antigen, is N-glycosylated upon expression in transgenic Nicotiana benthamiana. Here, we found that overexpression of aglycosylated CTB by agroinfiltration of a tobamoviral vector causes massive tissue necrosis and poor accumulation unless retained in the endoplasmic reticulum (ER). However, the re-introduction of N-glycosylation to its original or an alternative site significantly relieved the necrosis and provided a high CTB yield without ER retention. Quantitative gene expression analysis of PDI, BiP, bZIP60, SKP1, 26Sα proteasome and PR1a, and the detection of ubiquitinated CTB polypeptides revealed that N-glycosylation significantly relieved ER stress and hypersensitive response, and facilitated the folding/assembly of CTB. The glycosylated CTB (gCTB) was characterized for potential vaccine use. Glycan profiling revealed that gCTB contained approximately 38% plant-specific glycans. gCTB retained nanomolar affinity to GM1-ganglioside with only marginal reduction of physicochemical stability and induced an anti-cholera holotoxin antibody response comparable to native CTB in a mouse oral immunization study. These findings demonstrated gCTB's potential as an oral immunogen and point to a potential role of N-glycosylation in increasing recombinant protein yields in 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.

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

Broadly neutralizing monoclonal antibodies (bnMAbs) may offer powerful tools for HIV-1 preexposure prophylaxis, such as topical microbicides. However, this option is hampered due to expensive MAb biomanufacturing based on mammalian cell culture. To address this issue, we developed a new production system for bnMAb VRC01 in Nicotiana benthamiana plants using a tobamovirus replicon vector. Unlike conventional two-vector-based expression, this system was designed to overexpress full-length IgG1 from a single polypeptide by means of kex2p-like enzyme recognition sites introduced between the heavy and light chains. An enzyme-linked immunosorbent assay (ELISA) revealed that gp120-binding VRC01 IgG1 was maximally accumulated on 5 to 7 days following vector inoculation, yielding ~150 mg of the bnMAb per kg of fresh leaf material. The plant-made VRC01 (VRC01p) was efficiently purified by protein A affinity followed by hydrophobic-interaction chromatography. ELISA, surface plasmon resonance, and an HIV-1 neutralization assay demonstrated that VRC01p has gp120-binding affinity and HIV-1-neutralization capacity virtually identical to the human-cell-produced counterpart. To advance VRC01p's use in topical microbicides, we analyzed combinations of the bnMAb with other microbicide candidates holding distinct antiviral mechanisms in an HIV-1 neutralization assay. VRC01p exhibited clear synergy with the antiviral lectin griffithsin, the CCR5 antagonist maraviroc, and the reverse transcriptase inhibitor tenofovir in multiple CCR5-tropic HIV-1 strains from clades A, B, and C. In summary, VRC01p is amenable to robust, rapid, and large-scale production and may be developed as an active component in combination microbicides with other anti-HIV agents such as antiviral lectins, CCR5 antagonists, and reverse transcriptase inhibitors.

Rapid and scalable plant-based production of a cholera toxin B subunit variant to aid in mass vaccination against cholera outbreaks

INTRODUCTION

Cholera toxin B subunit (CTB) is a component of an internationally licensed oral cholera vaccine. The protein induces neutralizing antibodies against the holotoxin, the virulence factor responsible for severe diarrhea. A field clinical trial has suggested that the addition of CTB to killed whole-cell bacteria provides superior short-term protection to whole-cell-only vaccines; however, challenges in CTB biomanufacturing (i.e., cost and scale) hamper its implementation to mass vaccination in developing countries. To provide a potential solution to this issue, we developed a rapid, robust, and scalable CTB production system in plants.

METHODOLOGY/PRINCIPAL FINDINGS

In a preliminary study of expressing original CTB in transgenic Nicotiana benthamiana, the protein was N-glycosylated with plant-specific glycans. Thus, an aglycosylated CTB variant (pCTB) was created and overexpressed via a plant virus vector. Upon additional transgene engineering for retention in the endoplasmic reticulum and optimization of a secretory signal, the yield of pCTB was dramatically improved, reaching >1 g per kg of fresh leaf material. The protein was efficiently purified by simple two-step chromatography. The GM1-ganglioside binding capacity and conformational stability of pCTB were virtually identical to the bacteria-derived original B subunit, as demonstrated in competitive enzyme-linked immunosorbent assay, surface plasmon resonance, and fluorescence-based thermal shift assay. Mammalian cell surface-binding was corroborated by immunofluorescence and flow cytometry. pCTB exhibited strong oral immunogenicity in mice, inducing significant levels of CTB-specific intestinal antibodies that persisted over 6 months. Moreover, these antibodies effectively neutralized the cholera holotoxin in vitro.

CONCLUSIONS/SIGNIFICANCE

Taken together, these results demonstrated that pCTB has robust producibility in Nicotiana plants and retains most, if not all, of major biological activities of the original protein. This rapid and easily scalable system may enable the implementation of pCTB to mass vaccination against outbreaks, thereby providing better protection of high-risk populations in developing countries.

Bioluminescence inhibition assay for the detection of hydroxylated polychlorinated biphenyls

Hydroxylated polychlorinated biphenyls (OH-PCBs) are an important class of contaminants that mainly originate from polychlorinated biphenyl metabolism. They may conceivably be as dangerous and persistent as the parent compounds; most prominently, OH-PCBs are endocrine disruptors. Due to increasing evidence of the presence of OH-PCBs in the environment and in living organisms, including humans, and of their toxicity, methods of detection for OH-PCBs are needed in the environmental and medical fields. Herein, we describe the development and optimization of a protein-based inhibition assay for the quantification of OH-PCBs. Specifically, the photoprotein aequorin was utilized for the detection of OH-PCBs. We hypothesized that OH-PCBs interact with aequorin, and we established that OH-PCBs actually inhibit the bioluminescence of aequorin in a dose-dependent manner. We took advantage of this phenomenon to develop an assay that is capable of detecting a wide variety of OH-PCBs with a range of detection limits, the best detection limit being 11 nM for the compound 2-hydroxy-2',3,4',5',6-pentachorobiphenyl. The viability of this system for the screening of OH-PCBs in spiked biological and environmental samples was also established. We envision the implementation of this novel bioluminescence inhibition assay as a rapid, sensitive, and cost-effective method for monitoring OH-PCBs. Furthermore, to the best of our knowledge, this is the first time aequorin has been employed to detect an analyte by the inhibition of its bioluminescence reaction. Hence, this strategy may prove to be a general approach for the development of a new generation of protein-based inhibition assays.

A protein switch sensing system for the quantification of sulfate

Protein engineering has generated versatile methods and technologies that have been instrumental in advancements in the fields of sensing, therapeutics, and diagnostics. Herein, we demonstrate the employment of rational design to engineer a unique bioluminescence-based protein switch. A fusion protein switch combines two totally unrelated proteins, with distinct characteristics, in a manner such that the function of one protein is dependent on another. Herein we report a protein switch sensing system by insertion of the sulfate-binding protein (SBP) into the structure of the photoprotein aequorin (AEQ). In the presence of sulfate, SBP undergoes a conformational change bringing the two segments of AEQ together, "turning on" bioluminescence in a dose-dependent fashion, thus allowing quantitative detection of sulfate. A calibration plot was obtained by correlating the amount of bioluminescence generated with the concentration of sulfate present. The switch demonstrated selectivity and reproducibility, and a detection limit of 1.6×10(-4)M for sulfate. Moreover, the sensing system was validated by performing sulfate detection in clinical and environmental samples, such as, serum, urine, and tap water. The detection limits and working ranges in all three samples fall within the average normal/recommended sulfate levels in the respective matrices.

Cyclic AMP receptor protein-aequorin molecular switch for cyclic AMP

Molecular switches are designer molecules that combine the functionality of two individual proteins into one, capable of manifesting an "on/off" signal in response to a stimulus. These switches have unique properties and functionalities and thus, can be employed as nanosensors in a variety of applications. To that end, we have developed a bioluminescent molecular switch for cyclic AMP. Bioluminescence offers many advantages over fluorescence and other detection methods including the fact that there is essentially zero background signal in physiological fluids, allowing for more sensitive detection and monitoring. The switch was created by combining the properties of the cyclic AMP receptor protein (CRP), a transcriptional regulatory protein from E. Coli that binds selectively to cAMP with those of aequorin, a bioluminescent photoprotein native of the jellyfish Aequorea victoria . Genetic manipulation to split the genetic coding sequence of aequorin in two and genetically attach the fragments to the N and C termini of CRP resulted in a hybrid protein molecular switch. The conformational change experienced by CRP upon the binding of cyclic AMP is suspected to result in the observed loss of the bioluminescent signal from aequorin. The "on/off" bioluminescence can be modulated by cyclic AMP over a range of several orders of magnitude in a linear fashion in addition to the capacity to detect changes in cellular cyclic AMP of intact cells exposed to different external stimuli without the need to lyse the cells. We envision that the molecular switch could find applications in vitro as well as In Vivo cyclic AMP detection and/or imaging.