Merozoite surface protein-1 of Plasmodium yoelii fused via an oligosaccharide moiety of cholera toxin B subunit glycoprotein expressed in yeast induced protective immunity against lethal malaria infection in mice

Exploiting bacterial-origin immunostimulants for improved vaccination and immunotherapy: current insights and future directions

Vaccination is a valid strategy to prevent and control newly emerging and reemerging infectious diseases in humans and animals. However, synthetic and recombinant antigens are poor immunogenic to stimulate efficient and protective host immune response. Immunostimulants are indispensable factors of vaccines, which can promote to trigger fast, robust, and long-lasting immune responses. Importantly, immunotherapy with immunostimulants is increasing proved to be an effective and promising treatment of cancer, which could enhance the function of the immune system against tumor cells. Pattern recognition receptors (PRRs) play vital roles in inflammation and are central to innate and adaptive immune responses. Toll-like receptors (TLRs)-targeting immunostimulants have become one of the hotspots in adjuvant research and cancer therapy. Bacterial-origin immunoreactive molecules are usually the ligands of PRRs, which could be fast recognized by PRRs and activate immune response to eliminate pathogens. Varieties of bacterial immunoreactive molecules and bacterial component-mimicking molecules have been successfully used in vaccines and clinical therapy so far. This work provides a comprehensive review of the development, current state, mechanisms, and applications of bacterial-origin immunostimulants. The exploration of bacterial immunoreactive molecules, along with their corresponding mechanisms, holds immense significance in deepening our understanding of bacterial pathogenicity and in the development of promising immunostimulants.

Th1-biased immunoadjuvant effect of the recombinant B subunit of an Escherichia coli heat-labile enterotoxin on an inactivated porcine reproductive and respiratory syndrome virus antigen via intranasal immunization in mice

Porcine reproductive and respiratory syndrome (PRRS) is one of the major swine diseases responsible for a significant challenge in the global swine industry. The current PRRS inactivated vaccine only confers limited protection against PRRSV. Thus, using an appropriate adjuvant via a suitable administration route may help improve vaccine efficacy. In this study, the recombinant B subunit of the Escherichia coli heat-labile enterotoxin rLTB, was highly expressed in Pichia pastoris, through high-density fermentation. rLTB intranasal adjuvant properties were evaluated on an inactivated PRRS antigen in mice. Compared to the group immunized with solely PRRS antigen, a dose of 50 µg rLTB remarkably raised antigen-specific IgA antibodies at mucosal sites, and increased serum IgG antibodies, preferentially the IgG2a and IgG2b subclasses. Further, rLTB induced increases in Th1- (IFN-γ and IL-12) and Th17 (IL-6) cytokine profiles, but had little effect on Th2 cytokine profiles (IL-4 and IL-10). Moreover, there were no overt toxicities associated with intranasal rLTB administration. Our data provide evidence that the rLTB produced by P. pastoris fermentation portrays low toxicity, and its intranasal adjuvant effect involves immune system modulation to a Th1 profile.

The Corn Smut ('Huitlacoche') as a New Platform for Oral Vaccines

The development of new alternative platforms for subunit vaccine production is a priority in the biomedical field. In this study, Ustilago maydis, the causal agent of common corn smut or ‘huitlacoche’has been genetically engineered to assess expression and immunogenicity of the B subunit of the cholera toxin (CTB), a relevant immunomodulatory agent in vaccinology. An oligomeric CTB recombinant protein was expressed in corn smut galls at levels of up to 1.3 mg g-1 dry weight (0.8% of the total soluble protein). Mice orally immunized with ‘huitlacoche’-derived CTB showed significant humoral responses that were well-correlated with protection against challenge with the cholera toxin (CT). These findings demonstrate the feasibility of using edible corn smut as a safe, effective, and low-cost platform for production and delivery of a subunit oral vaccine. The implications of this platform in the area of molecular pharming are discussed.

Cholera Toxin Subunit B as Adjuvant--An Accelerator in Protective Immunity and a Break in Autoimmunity

Cholera toxin subunit B (CTB) is the nontoxic portion of cholera toxin. Its affinity to the monosialotetrahexosylganglioside (GM1) that is broadly distributed in a variety of cell types including epithelial cells of the gut and antigen presenting cells, macrophages, dendritic cells, and B cells, allows its optimal access to the immune system. CTB can easily be expressed on its own in a variety of organisms, and several approaches can be used to couple it to antigens, either by genetic fusion or by chemical manipulation, leading to strongly enhanced immune responses to the antigens. In autoimmune diseases, CTB has the capacity to evoke regulatory responses and to thereby dampen autoimmune responses, in several but not all animal models. It remains to be seen whether the latter approach translates to success in the clinic, however, the versatility of CTB to manipulate immune responses in either direction makes this protein a promising adjuvant for vaccine development.

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.

Cholera toxin B subunit-five-stranded α-helical coiled-coil fusion protein: "five-to-five" molecular chimera displays robust physicochemical stability

To create a physicochemically stable cholera toxin (CT) B subunit (CTB), it was fused to the five-stranded α-helical coiled-coil domain of cartilage oligomeric matrix protein (COMP). The chimeric fusion protein (CTB-COMP) was expressed in Pichia pastoris, predominantly as a pentamer, and retained its affinity for the monosialoganglioside GM1, a natural receptor of CT. The fusion protein displayed thermostability, tolerating the boiling temperature of water for 10min, whereas unfused CTB readily dissociated to its monomers and lost its affinity for GM1. The fusion protein also displayed resistance to strong acid at pHs as low as 0.1, and to the protein denaturant sodium dodecyl sulfate at concentrations up to 10%. Intranasal administration of the fusion protein to mice induced anti-B subunit serum IgG, even after the protein was boiled, whereas unfused CTB showed no thermostable mucosal immunogenicity. This study demonstrates that CTB fused to a pentameric α-helical coiled coil has a novel physicochemical phenotype, which may provide important insight into the molecular design of enterotoxin-B-subunit-based vaccines and vaccine delivery molecules.

Effect of pressure on refolding of recombinant pentameric cholera toxin B

The production of recombinant proteins is an essential tool for the expansion of modern biological research and biotechnology. The expression of heterologous proteins in Escherichia coli often results in an incomplete folding process that leads to the accumulation of inclusion bodies (IB), aggregates that hold a certain degree of native-like secondary structure. High hydrostatic pressure (HHP) impairs intermolecular hydrophobic and electrostatic interactions, leading to dissociation of aggregates under non-denaturing conditions and is therefore a useful tool to solubilize proteins for posterior refolding. Cholera toxin (CT) is composed of a non-toxic pentamer of B subunits (CTB), a useful adjuvant in vaccines, and a toxic subunit A (CTA). We studied the process of refolding of CTB using HHP. HHP was shown to be effective for dissociation of CTB monomers from IB. Posterior incubation at atmospheric pressure of concentrated CTB (1mg/ml) is necessary for the association of the monomers. Pentameric CTB was obtained when suspensions of CTB IB were compressed at 2.4kbar for 16h in the presence of Tween 20 and incubated at 1bar for 120h. Soluble and biologically active pentameric CTB was obtained, with a yield of 213mg CTB/liter of culture. The experience gained in this study can be important to improve the refolding of proteins with quaternary structure.

Expression of Neospora caninum NcSRS2 surface protein in Pichia pastoris and its application for serodiagnosis of Neospora infection

Neospora caninum is considerd a major cause of abortion in cattle worldwide. The antigenic domain of NcSRS2 in N. caninum is an important surface antigen present in the membrane of this parasite. In the present study, the Pichia pastoris expression system proved to be a useful tool for the production of recombinant protein. The truncated NcSRS2 gene (by removal of the N-terminal hydrophobic sequence), was cloned in the vector pPICZalphaB, and integrated on the genome of the methylotrophic yeast P. pastoris. Subsequently, the NcSRS2 protein was expressed, purified, and characterized using naturally infected cattle sera and Mab 6xhistag. The recombinant protein NcSRS2 was present in the supernatant of the culture, where later it was concentrated and purified using ammonium sulfate (∼100 mg/ml). An indirect immunoenzymatic assay (ELISA) was performed using cattle sera from endemic N. caninum area.

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.

N-Glycosylation enhances functional and structural stability of recombinant β-glucuronidase expressed in Pichia pastoris

Recombinant β-glucuronidase (GUS) expressed in Pichia pastoris GS115 is an important glycoprotein, encoded by a gene with four potential N-glycosylation sites. To investigate the impact of N-linked carbohydrate moieties on the stability of recombinant GUS, it was deglycosylated by peptide-N-glycosidase F (PNGase-F) under native conditions. The enzymatic activities of the glycosylated and deglycosylated GUS were compared under various conditions such as temperature, pH, organic solvents, detergents and chaotropic agent. The results demonstrated that the glycosylated GUS retained greater fraction of maximum enzymatic activity against various types of denaturants compared with the deglycosylated. The conformational stabilities of both GUS were analyzed by monitoring the unfolding equilibrium by using the denaturant guanidinium chloride (dn-HCl). The glycosylated GUS displayed a significant increase in its conformational stability than the deglycosylated counterpart. These results affirmed the key role of N-glycosylation on the structural and functional stability of β-glucuronidase and could have potential applications in the functional enhancement of industrial enzymes.