A novel in vivo inducible expression system in Edwardsiella tarda for potential application in bacterial polyvalence vaccine

Identification and application of T3SS translocation signal in Edwardsiella piscicida attenuated carrier as a bivalent vaccine

Type III secretion system (T3SS)-dependent translocation has been used to deliver heterologous antigens by vaccine carriers into host cells. In this research, we identified the translocation signal of Edwardsiella piscicida T3SS effector EseG and constructed an antibiotic resistance-free balanced-lethal system as attenuated vaccine carrier to present antigens by T3SS. Edwardsiella piscicida LSE40 asd gene deletion mutant was constructed and complemented with pYA3342 harbouring the asd (aspartate β-semialdehyde dehydrogenase) gene from Salmonella. Fusion proteins composed of EseG N-terminal 1-108 amino acids and the TEM1-β-lactamase reporter were inserted in plasmid pYA3342. The fusion protein could secrete into the cell culture, translocate into HeLa cells, and localize in the membrane fraction. Then, the double gene deletion mutant LSE40ΔasdΔpurA was constructed as an attenuated vaccine carrier, and Aeromonas hydrophila GapA (glyceraldehyde-3-phosphate dehydrogenase) was fused with the translocation signal, instead of the TEM1-β-lactamase reporter. The bivalent vaccine could protect blue gourami (Trichogaster trichopterus) against E. piscicida and A. hydrophila, with the relative per cent survival of 80.77% and 63.83%, respectively. These results indicated that EseG N-terminal 1-108 amino acid peptide was the translocation signal of E. piscicida T3SS, which could be used to construct bivalent vaccines based on an attenuated E. piscicida carrier.

Recombinant baculovirus BacCarassius-D4ORFs has potential as a live vector vaccine against CyHV-2

Cyprinid herpesvirus II (CyHV-2) is highly contagious and pathogenic to Carassius auratus gibelio (gibel carp), causing enormous economic losses in aquaculture in Yancheng city, Jiangsu province, China; however, to date, there is no effective way to protect C. auratus gibelio from CyHV-2 infection. In this study, a recombinant baculovirus vector vaccine, BacCarassius-D4ORFs, containing a fused codon-optimized sequence D4ORFs comprising the ORF72 (region 1-186 nt), ORF66 (region 993-1197 nt), ORF81 (region 603-783 nt) and ORF82 (region 85-186 nt) genes of CyHV-2, driven by a Megalobrama amblycephala β-actin promoter, was constructed. Then, qPCR, Western blotting and immunofluorescence assays showed that the fused gene D4ORFs was successfully delivered and expressed in fish cells or tissues by transduction with BacCarassius-D4ORFs. The fused gene D4ORFs could not be detected by PCR in the C. auratus gibelio injected with BacCarassius-D4ORFs after 7 weeks. Specific antibody against ORF72 could be detected in the serum of vaccinated C. auratus gibelio by injection with BacCarassius-D4ORFs. Furthermore, when C. auratus gibelio were vaccinated with BacCarassius-D4ORFs via the oral or injection route, followed by challenge with CyHV-2, the relative survival rate of immunized C. auratus gibelio reached 59.3% and 80.01%, respectively. These results suggested that BacCarassius-D4ORFs has the potential to be used as a vector-based vaccine for the prevention and treatment of disease caused by CyHV-2 infection.

Cyprinid viral diseases and vaccine development

In the past decades, global freshwater fish production has been rapidly growing, while cyprinid takes the largest portion. Along with the rapid rise of novel forms of intensive aquaculture, increased global aquatic animal movement and various anthropogenic stress to aquatic ecosystems during the past century, freshwater fish farming industry encounter the emergence and breakout of many diseases, especially viral diseases. Because of the ability to safely and effectively prevent aquaculture diseases, vaccines have become the mainstream technology for prevention and control of aquatic diseases in the world. In this review, authors summarized six major cyprinid viral diseases, including koi herpesvirus disease (KHVD), spring viraemia of carp (SVC), grass carp hemorrhagic disease (GCHD), koi sleepy disease (KSD), carp pox disease (CPD) and herpesviral haematopoietic necrosis (HPHN). The present review described the characteristics of these diseases from epidemiology, pathology, etiology and diagnostics. Furthermore, the development of specific vaccines respective to these diseases is stated according to preparation methods and immunization approaches. It is hoped that the review could contribute to aquaculture in prevention and controlling of cyprinid viral diseases, and serve the healthy and sustainable development of aquaculture industry.

A controllable bacterial lysis system to enhance biological safety of live attenuated Vibrio anguillarum vaccine

Bacterial strains used as backbone for the generation of vaccine prototypes should exhibit an adequate and stable safety profile. Given the fact that live attenuated vaccines often contain some potential risks in virulence recovery and spread infections, new approaches are greatly needed to improve their biological safety. Here, a critically iron-regulated promoter PviuA was screened from Vibrio anguillarum, which was demonstrated to respond to iron-limitation signal both in vitro and in vivo. By using PviuA as a regulatory switch to control the expression of phage P22 lysis cassette 13-19-15, a novel in vivo inducible bacterial lysis system was established in V. anguillarum. This system was proved to be activated by iron-limitation signals and then effectively lyse V. anguillarum both in vitro and in vivo. Further, this controllable bacterial lysis system, after being transformed into a live attenuated V. anguillarum vaccine strain MVAV6203, was confirmed to significantly improve biological safety of the live attenuated vaccine without impairing its immune protection efficacy.

In Vivo Programmed Gene Expression Based on Artificial Quorum Networks

The quorum sensing (QS) system, as a well-functioning population-dependent gene switch, has been widely applied in many gene circuits in synthetic biology. In our work, an efficient cell density-controlled expression system (QS) was established via engineering of the Vibrio fischeri luxI-luxR quorum sensing system. In order to achieve in vivo programmed gene expression, a synthetic binary regulation circuit (araQS) was constructed by assembling multiple genetic components, including the quorum quenching protein AiiA and the arabinose promoter ParaBAD, into the QS system. In vitro expression assays verified that the araQS system was initiated only in the absence of arabinose in the medium at a high cell density. In vivo expression assays confirmed that the araQS system presented an in vivo-triggered and cell density-dependent expression pattern. Furthermore, the araQS system was demonstrated to function well in different bacteria, indicating a wide range of bacterial hosts for use. To explore its potential applications in vivo, the araQS system was used to control the production of a heterologous protective antigen in an attenuated Edwardsiella tarda strain, which successfully evoked efficient immune protection in a fish model. This work suggested that the araQS system could program bacterial expression in vivo and might have potential uses, including, but not limited to, bacterial vector vaccines.

A quorum sensing-based in vivo expression system and its application in multivalent bacterial vaccine

BACKGROUND

Delivery of antigens by live bacterial carriers can elicit effective humoral and cellular responses and may be an attractive strategy for live bacterial vaccine production through introduction of a vector that expresses an exogenous protective antigen. To overcome the instability and metabolic burden associated with plasmid introduction, alternative strategies, such as the use of in vivo-inducible promoters, have been proposed. However, screening an ideal in vivo-activated promoter with high efficiency and low leak expression in a particular strain poses great challenges to many researchers.

RESULTS

In this work, we constructed an in vivo antigen-expressing vector suitable for Edwardsiella tarda, an enteric Gram-negative invasive intracellular pathogen of both animals and humans. By combining quorum sensing genes from Vibrio fischeri with iron uptake regulons, a synthetic binary regulation system (ironQS) for E. tarda was designed. In vitro expression assay demonstrated that the ironQS system is only initiated in the absence of Fe2+ in the medium when the cell density reaches its threshold. The ironQS system was further confirmed in vivo to present an in vivo-triggered and cell density-dependent expression pattern in larvae and adult zebrafish. A recombinant E. tarda vector vaccine candidate WED(ironQS-G) was established by introducing gapA34, which encodes the protective antigen glyceraldehyde-3-phosphate dehydrogenase (GAPDH) from the fish pathogen Aeromonas hydrophila LSA34 into ironQS system, and the immune protection afforded by this vaccine was assessed in turbot (Scophtalmus maximus). Most of the vaccinated fish survived under the challenge with A. hydrophila LSA34 (RPS=67.0%) or E. tarda EIB202 (RPS=72.3%).

CONCLUSIONS

Quorum sensing system has been extensively used in various gene structures in synthetic biology as a well-functioning and population-dependent gene circuit. In this work, the in vivo expression system, ironQS, maintained the high expression efficiency of the quorum sensing circuit and achieved excellent expression regulation of the Fur box. The ironQS system has great potential in applications requiring in vivo protein expression, such as vector vaccines. Considering its high compatibility, ironQS system could function as a universal expression platform for a variety of bacterial hosts.

Antigen Delivery System II

This chapter reviews papers mostly written since 2005 that report results using live attenuated bacterial vectors to deliver after administration through mucosal surfaces, protective antigens, and DNA vaccines, encoding protective antigens to induce immune responses and/or protective immunity to pathogens that colonize on or invade through mucosal surfaces. Papers that report use of such vaccine vector systems for parenteral vaccination or to deal with nonmucosal pathogens or do not address induction of mucosal antibody and/or cellular immune responses are not reviewed.

Cell-penetrating peptides mediated protein cross-membrane delivery and its use in bacterial vector vaccine

It is an attractive strategy to develop a recombinant bacterial vector vaccine by expressing exogenous protective antigen to induce the immune response, and the main concern is how to enhance the cellular internalization of antigen produced by bacterial vector. Cell-penetrating peptides (CPPs) are short cationic/amphipathic peptides which facilitate cellular uptake of various molecular cargoes and therefore have great potentials in vector vaccine design. In this work, eleven different CPPs were fused to the C-terminus of EGFP respectively, and the resultant EGFP-CPP fusion proteins were expressed and purified to assay their cross-membrane transport in macrophage J774 A.1 cells. Among the tested CPPs, TAT showed an excellent capability to deliver the cargo protein EGFP into cytoplasm. In order to establish an efficient antigen delivery system in Escherichia coli, the EGFP-TAT synthesis circuit was combined with an in vivo inducible lysis circuit PviuA-E in E. coli to form an integrated antigen delivery system, the resultant E. coli was proved to be able to lyse upon the induction of a mimic in vivo signal and thus release intracellular EGFP-TAT intensively, which were assumed to undergo a more efficient intracellular delivery by CPP to evoke protective immune responses. Based on the established antigen delivery system, the protective antigen gene flgD from an invasive intracellular fish pathogen Edwardsiella tarda EIB202, was applied to establish an E. coli recombinant vector vaccine. This E. coli vector vaccine presented superior immune protection (RPS = 63%) under the challenge with E. tarda EIB202, suggesting that the novel antigen delivery system had great potential in bacterial vector vaccine applications.

Asd-based balanced-lethal system in attenuated Edwardsiella tarda to express a heterologous antigen for a multivalent bacterial vaccine

Edwardsiella tarda is an enteric Gram-negative invasive intracellular pathogen, which causes enteric septicemia in fish. It could be potentially used to develop a recombinant attenuated E. tarda vaccine for the aquaculture industry. Because live vaccine strains can potentially be released into the environment upon vaccination, medical and environmental safety issues must be considered. Deletion of the asdB gene in E. tarda resulted in a diaminopimelic acid (DAP)-dependent mutant. The wild type asdB gene was inserted in place of the antibiotic-resistance gene in the plasmid, and the resultant non-antibiotic resistant vector was transformed into the attenuated and DAP-dependent E. tarda vaccine strain (WEDΔasdB) to obtain a balanced-lethal system for heterologous antigen expression. The balanced-lethal expression system was further optimized by comparing plasmid replicons with different Shine-Dalgarno sequences and start codons for the asdB gene. Utilizing the optimized balanced-lethal expression system, the protective antigen gene gapA34 from the fish pathogen Aeromonas hydrophila LSA34 was expressed in the attenuated E. tarda to generate the multivalent vaccine candidate WEDΔasdB/pUTta4DGap. This vaccine was shown to evoke an effective immune response against both E. tarda and A. hydrophila LSA34 by vaccinating turbot via a simple immersion route. This multivalent E. tarda vector vaccine has great potential for broad applications in aquaculture.

Development of a Fur-dependent and tightly regulated expression system in Escherichia coli for toxic protein synthesis

Background

There is a continuous demanding for tightly regulated prokaryotic expression systems, which allow functional synthesis of toxic proteins in Escherichia coli for bioscience or biotechnology application. However, most of the current promoter options either are tightly repressed only with low protein production levels, or produce substantial protein but lacking of the necessary repression to avoid mutations initiated by leaky expression in the absence of inducer. The aim of this study was to develop a tightly regulated, relatively high-efficient expression vector in E. coli based on the principle of iron uptake system.

Results

By using GFP as reporter, P_fhuA with the highest relative fluorescence units, but leaky expression, was screened from 23 iron-regulated promoter candidates. P_fhuA was repressed by ferric uptake regulator (Fur)-Fe²⁺ complex binding to Fur box locating at the promoter sequence. Otherwise, P_fhuA was activated without Fur-Fe²⁺ binding in the absence of iron. In order to improve the tightness of P_fhuA regulation for toxic gene expression, Fur box in promoter sequence and fur expression were refined through five different approaches. Eventually, through substituting E. coli consensus Fur box for original one of P_fhuA, the induction ratio of modified P_fhuA (named P_fhuA1) was improved from 3 to 101. Under the control of P_fhuA1, strong toxic gene E was successfully expressed in high, middle, low copy-number vectors, and other two toxic proteins, Gef and MazF were functionally synthesized without E. coli death before induction.

Conclusions

The features of easy control, tight regulation and relatively high efficiency were combined in the newly engineered P_fhuA1. Under this promoter, the toxic genes E, gef and mazF were functionally expressed in E. coli induced by iron chelator in a tightly controllable way. This study provides a tightly regulated expression system that might enable the stable cloning, and functional synthesis of toxic proteins for their function study, bacterial programmed cell death in biological containment system and bacterial vector vaccine development.