The Bacterial Spore as a Mucosal Vaccine Delivery System

The development of efficient mucosal vaccines is strongly dependent on the use of appropriate vectors. Various biological systems or synthetic nanoparticles have been proposed to display and deliver antigens to mucosal surfaces. The Bacillus spore, a metabolically quiescent and extremely resistant cell, has also been proposed as a mucosal vaccine delivery system and shown able to conjugate the advantages of live and synthetic systems. Several antigens have been displayed on the spore by either recombinant or non-recombinant approaches, and antigen-specific immune responses have been observed in animals immunized by the oral or nasal route. Here we review the use of the bacterial spore as a mucosal vaccine vehicle focusing on the advantages and drawbacks of using the spore and of the recombinant vs. non-recombinant approach to display antigens on the spore surface. An overview of the immune responses induced by antigen-displaying spores so far tested in animals is presented and discussed.

First Display of Haloalkane Dehalogenase LinB on the Surface of Bacillus subtilis Spore

BACKGROUND

LinB, as a Haloalkane dehalogenase, has good catalytic activity for many highly toxic and recalcitrant compounds, and can realize the elimination of chemical weapons HD in a green non-toxic mode.

OBJECTIVES

In order to display Haloalkane dehalogenase LinB on the surface of Bacillus subtilis spore.

METHODS

We have constituted the B. subtilis spore surface display system of halogenated alkanes dehalogenase LinB by gene recombination.

RESULTS

Data revealed that LinB can display on spore surface successfully. The hydrolyzing HD analogue 2-chloroethyl ethylsulfide (2-CEES) activity of displayed LinB spores was 4.30±0.09 U/mL, and its specific activity was 0.78±0.03U/mg. Meanwhile, LinB spores showed a stronger stress resistance activity on 2-CEES than free LinB. This study obtained B. subtilis spores of LinB (phingobium japonicum UT26) with enzyme activity that was not reported before.

CONCLUSION

Spore surface display technology uses resistance spore as the carrier to guarantee LinB activity, enhances its stability, and reduces the production cost, thus expanding the range of its application.

Transcriptome Analysis Reveals Immunomodulatory Effect of Spore-Displayed p75 on Human Intestinal Epithelial Caco-2 Cells

Lacticaseibacillus rhamnosus GG (LGG) can promote intestinal health by modulating the immune responses of the gastrointestinal tract. However, knowledge about the immunomodulatory action of LGG-derived soluble factors is limited. In our previous study, we have displayed LGG-derived p75 protein on the spore surface of Bacillus subtilis. The objective of this study was to determine the effect of spore-displayed p75 (CotG-p75) on immune system by investigating transcriptional response of Caco-2 cells stimulated by CotG-p75 through RNA-sequencing (RNA-seq). RNA-seq results showed that CotG-p75 mainly stimulated genes involved in biological processes, such as response to stimulus, immune regulation, and chemotaxis. KEGG pathway analysis suggested that many genes activated by CotG-p75 were involved in NF-ĸB signaling and chemokine signaling pathways. CotG-p75 increased cytokines and chemokines such as CXCL1, CXCL2, CXCL3, CXCL8, CXCL10, CCL20, CCL22, and IL1B essential for the immune system. In particular, CotG-p75 increased the expression levels of NF-ĸB-related genes such as NFKBIA, TNFAIP3, BIRC3, NFKB2, and RELB involved in immune and inflammatory responses. This study provides genes and pathways involved in immune responses influenced by CotG-p75. These comprehensive transcriptome profiling could be used to elucidate the immunomodulatory action of CotG-p75.

Induction of a Specific Humoral Immune Response by Nasal Delivery of Bcla2ctd of Clostridioides difficile

Clostridioides difficile, formerly known as Clostridium difficile, is a spore-forming bacterium considered as the most common cause of nosocomial infections in developed countries. The spore of C. difficile is involved in the transmission of the pathogen and in its first interaction with the host; therefore, a therapeutic approach able to control C. difficile spores would improve the clearance of the infection. The C-terminal (CTD) end of BclA2, a spore surface protein of C. difficile responsible of the interaction with the host intestinal cells, was selected as a putative mucosal antigen. The BclA2 fragment, BclA2_CTD, was purified and used to nasally immunize mice both as a free protein and after adsorption to the spore of Bacillus subtilis, a well-established mucosal delivery vehicle. While the adsorption to spores increased the in vitro stability of BclA2_CTD, in vivo both free and spore-adsorbed BclA2_CTD were able to induce a similar, specific humoral immune response in a murine model. Although in the experimental conditions utilized the immune response was not protective, the induction of specific IgG indicates that free or spore-bound BclA2_CTD could act as a putative mucosal antigen targeting C. difficile spores.

Surface Display of Antigen Protein VP8* of Porcine Rotavirus on Bacillus Subtilis Spores Using CotB as a Fusion Partner

Porcine rotavirus is a major cause of acute viral gastroenteritis in suckling piglets, and vaccination is considered to be an effective measure to control these infections. The development of a live mucosal vaccine using Bacillus subtilis spores as an antigen delivery vehicle is a convenient and attractive vaccination strategy against porcine rotavirus. In this study, a shuttle vector was constructed for the spore surface display of the spike protein VP8* from porcine rotavirus (the genotype was G5P[7]). A successful display of the CotB-VP8* fusion protein on the spore surface was confirmed by Western blot and immunofluorescence microscopy analysis. The capacity for immune response generated after immunization with the recombinant strain was evaluated in a mouse model. The intestinal fecal IgA and serum IgG were detected by enzyme-linked-immunosorbent serologic assay (ELISA). Importantly, recombinant strain spores could elicit strong specific mucosal and humoral immune responses. These encouraging results suggest that recombinant B. subtilis BV could provide a strategy for a potential novel application approach to the development of a new and safe mucosal subunit vaccine against porcine rotavirus.

Highly active spore biocatalyst by self-assembly of co-expressed anchoring scaffoldin and multimeric enzyme

We report a spore-based biocatalysis platform capable of producing and self-assembling active multimeric enzymes on a spore surface with a high loading density. This was achieved by co-expressing both a spore surface-anchoring scaffoldin protein containing multiple cohesin domains and a dockerin-tagged enzyme of interest in the mother cell compartment during Bacillus subtilis sporulation. Using this method, tetrameric β-galactosidase was successfully displayed on the spore surface with a loading density of 1.4 × 10⁴ active enzymes per spore particle. The resulting spore biocatalysts exhibited high conversion rates of transgalactosylation in water/organic emulsions. With easy manufacture, enhanced thermostability, excellent reusability, and long-term storage stability at ambient temperature, this approach holds a great potential in a wide range of biocatalysis applications especially involving organic phases.

Spore-displayed enzyme cascade with tunable stoichiometry

Taking the advantages of inert and stable nature of endospores, we developed a biocatalysis platform for multiple enzyme immobilization on Bacillus subtilis spore surface. Among B. subtilis outer coat proteins, CotG mediated a high expression level of Clostridium thermocellum cohesin (CtCoh) with a functional display capability of ∼10⁴ molecules per spore of xylose reductase-C. thermocellum dockerin fusion protein (XR-CtDoc). By co-immobilization of phosphite dehydrogenase (PTDH) on spore surface via Ruminococcus flavefaciens cohesin-dockerin modules, regeneration of NADPH was achieved. Both xylose reductase (XR) and PTDH exhibited enhanced stability upon spore surface display. More importantly, by altering the copy numbers of CtCoh and RfCoh fused with CotG, the molar ratio between immobilized enzymes was adjusted in a controllable manner. Optimization of spore-displayed XR/PTDH stoichiometry resulted in increased yields of xylitol. In conclusion, endospore surface display presents a novel approach for enzyme cascade immobilization with improved stability and tunable stoichiometry. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 33:383-389, 2017.

Production of d-Allulose with d-Psicose 3-Epimerase Expressed and Displayed on the Surface of Bacillus subtilis Spores

The production of d-allulose is usually conducted via isolated-enzyme or whole-cell biocatalysis reactions. In the present study, an innovative biocatalyst, d-psicose 3-epimerase (DPEase) from Clostridium scindens ATCC 35704, presented on the surface of Bacillus subtilis spores, was applied for d-allulose production. DPEase was fused at the C-terminus of the anchoring protein, CotZ, via a peptide linker, and trophic genes were used as selection markers during the chromosomal integration. The optimal temperature and pH of the fusion protein CotZ-DPEase were 55 °C and pH 7.5-8.0, respectively, and the anchored DPEase exhibited high thermostability. Under optimal conditions, 30 g/L of recombinant spores can produce 85 g/L d-allulose from 500 g/L d-fructose after 12 h, and 60% of the yield was maintained after five cycles of utilization. Therefore, this biocatalyst system, capable of expressing and immobilizing DPEase on the spore surface of B. subtilis, was an appropriate alternative for d-allulose production.