Expression of foreign proteins on gram-positive commensal bacteria for mucosal vaccine delivery

Neutralizing-antibody-mediated protection of chickens against infectious bursal disease via one-time vaccination with inactivated recombinant Lactococcus lactis expressing a fusion protein constructed from the RCK protein of Salmonella enterica and VP2 of infectious bursal disease virus

Background

Infectious bursal disease (IBD) is an acute contagious immunosuppressive disease which lead to acute bursal injury and immune dysfunction in poultry. It has caused heavy economic losses in the commercial poultry industry for many years in worldwide. Attenuated live vaccine has widely used in poultry showing some promising signs against IBDV infection. But it has defects such as generating enhanced virulence and immunosuppression prohibits. Therefore, the development of mucosal vaccines using the food-grade lactic acid bacterium is necessary. Here, we construct a recombinant Lactococcus co-expressing the major IBDV antigens VP2 and RCK protein of Salmonella enterica to prevent IBD.

Results

The recombinant fusion protein VP2-RCK was expressed in a soluble and stable form in the cytoplasm of the recombinant Lactococcus lactis. Animal experiments showed that: (1) the survival rates of the injected immunization inactivated recombinant LAB group and oral immunization live recombinant LAB group were 100% and 80%, respectively; (2) ELISA titers of all serum samples from all experimental groups were negative, but high amounts of specific neutralizing antibodies were detected (1:2¹⁰ to 1:2¹²); and (3) the bursas of the injected immunization inactivated recombinant LAB group did not suffer damage, as confirmed by clinical observation and bursal histopathological examination. Our results indicate that r-L. lactis-OptiVP2-RCK induces a specific neutralizing-antibody-mediated immune response that confers full protection against very-virulent IBDV (vvIBDV) challenge.

Conclusion

Lactococcus lactis NZ3900 strain and its matching plasmid pNZ8149 could express the recombinant fusion protein VP2-RCK in a soluble form in the cytoplasm. The protective efficacy of r-L. lactis-OptiVP2-RCK (100%) was better than r-L. lactis-OptiVP2 (0%) which prove RCK protein played its unique role. The neutralizing antibodies titers against infectious bursal disease virus via one-time vaccination with inactivated r-L. lactis-OptiVP2-RCK could reach 1:2¹⁰ to 1:2¹², but ELISA titers of all serum samples were negative. For this phenomenon, perhaps because of the change of delivery pathway or the spatial structure of fusion protein. We need further study to test these hypotheses.

Intranasal bacteria induce Th1 but not Treg or Th2

Commensal microorganisms colonize the nasal mucosa without inducing inflammation. Pathogens perturbing the commensal flora often invade evading immune defenses. The different types of adaptive responses that drive the distinct behaviors of commensals and pathogens, allowing one to persist at mucosal surfaces and the other to survive within tissues, are not yet clear. In the present work we demonstrate that although crossing epithelial barriers, the commensal Lactobacillus murinus stimulates epitope-specific CD4(+) T cells in nasal-associated lymphoid tissue (NALT) less efficiently than the pathogen Streptococcus pyogenes. In NALT antigen-presenting cells other than CCR6(+) CD11c(+) dendritic cells process and present the microbial antigens. Effector/memory CD4(+) T cells generated after intranasal priming with L. murinus and S. pyogenes surprisingly express similar proinflammatory cytokines and are not CD25+/FoxP3+ T-regulatory cells when recirculating in the spleen. These findings suggest that when a commensal crosses the nasal epithelial barrier it induces a proinflammatory response similar to a pathogen but without causing disease.

Induction of Partial Protection in Mice after Oral Administration ofLactococcus lactisProducingBrucella abortusL7/L12 Antigen

The Brucella abortus ribosomal protein L7/L12 is an immunodominant antigen and an interesting candidate for the development of oral live vaccines against brucellosis. Here, a recombinant Lactococcus lactis strain producing L7/L12 under the control of nisin inducible promoter was orally administered to BALB/c mice. Significant levels of anti-L7/L12 specific IgA detected in feces revealed an induced local humoral immune response. However, serum analysis did not reveal any anti-L7/L12 antibodies suggesting the absence of a systemic response. Nevertheless, the vaccinated mice showed a partial protective immunity against B. abortus virulent strain (S2308) challenged by intraperitoneal inoculation.

Oral immunization of BALB/c mice by intragastric delivery of Streptococcus gordonii-expressing Giardia cyst wall protein 2 decreases cyst shedding in challenged mice

Giardia lamblia (Giardia duodenalis or Giardia intestinalis) is a protozoan parasite of vertebrates with broad host specificity. Specific antibodies directed against cyst antigens can interfere with the cyst wall-building process. In this study, we engineered Streptococcus gordonii to express a 26 kDa fragment of cyst wall protein 2 (CWP2), containing a relevant B cell epitope, on the cell surface. This is the first report of S. gordonii expressing a protein of parasite origin. As S. gordonii was intended for intestinal delivery of CWP2, it was determined that this oral commensal bacterium is able to persist in the murine intestine for 30 days. Immunization with recombinant streptococci expressing the 26 kDa fragment resulted in higher antibody levels. Specific anti-CWP2 IgA antibodies were detected in fecal samples and anti-CWP2 IgG antibodies were detected in serum demonstrating the efficacy of S. gordonii for intragastric antigen delivery. In a pilot challenge experiment, immunized mice demonstrated a significant 70% reduction in cyst output.

Identification and characterization of novel surface proteins in Lactobacillus johnsonii and Lactobacillus gasseri

We have identified and sequenced the genes encoding the aggregation-promoting factor (APF) protein from six different strains of Lactobacillus johnsonii and Lactobacillus gasseri. Both species harbor two apf genes, apf1 and apf2, which are in the same orientation and encode proteins of 257 to 326 amino acids. Multiple alignments of the deduced amino acid sequences of these apf genes demonstrate a very strong sequence conservation of all of the genes with the exception of their central regions. Northern blot analysis showed that both genes are transcribed, reaching their maximum expression during the exponential phase. Primer extension analysis revealed that apf1 and apf2 harbor a putative promoter sequence that is conserved in all of the genes. Western blot analysis of the LiCl cell extracts showed that APF proteins are located on the cell surface. Intact cells of L. johnsonii revealed the typical cell wall architecture of S-layer-carrying gram-positive eubacteria, which could be selectively removed with LiCl treatment. In addition, the amino acid composition, physical properties, and genetic organization were found to be quite similar to those of S-layer proteins. These results suggest that APF is a novel surface protein of the Lactobacillus acidophilus B-homology group which might belong to an S-layer-like family.

Analysis of factors affecting surface expression and immunogenicity of recombinant proteins expressed by gram-positive commensal vectors

Several key protein structural attributes were altered in an effort to optimize expression and immunogenicity of a foreign protein (M protein from Streptococcus pyogenes) exposed on the surface of Streptococcus gordonii commensal bacterial vectors: (i) a shorter N-terminal region, (ii) the addition of a 94-amino-acid spacer, and (iii) the addition of extra C-repeat regions (CRR) from the M6 protein. A decrease in the amount of cell surface M6 was observed upon deletion of 10 or more amino acid residues at the N terminus. On the other hand, reactivity of monoclonal antibody to surface M6 increased with the addition of the spacer adjacent to the proline- and glycine-rich region, and an increase in epitope dosage was obtained by adding another CRR immediately downstream of the original CRR. The results obtained should facilitate the design of improved vaccine candidates using this antigen delivery technology.

Expression of active monomeric and dimeric nuclease A from the gram-positive Streptococcus gordonii surface protein expression system

We used the surface protein expression (SPEX) system to express an anchored and a secreted form of staphylococcal nuclease A (NucA) from gram-positive bacteria. NucA is a small ( approximately 18 kDa), extracellular, monomeric enzyme from Staphylococcus aureus. A deletion of amino acids 114-119 causes monomeric NucA to form homodimers. The DNA sequence encoding either wild-type or deletion mutant NucA was cloned via homologous recombination into Streptococcus gordonii. S. gordonii strains expressing either anchored or secreted, monomeric or dimeric NucA were isolated and tested for enzymatic activity using a novel fluorescence enzyme assay. We show that active monomeric and dimeric NucA enzyme can be expressed either anchored on the cell surface or secreted into the culture medium. The activity of the dimer NucA was approximately 100-fold less than the monomer. Secreted and anchored, monomeric NucA migrated on SDS-polyacrylamide gels at approximately 18 or approximately 30 kDa, respectively. In addition, similar to S. aureus NucA, the S. gordonii recombinant NucA enzyme was dependent on CaCl(2) and was heat stable. In contrast, however, the recombinant NucA activity was maximal at pH 7.0-7.5 whereas S. aureus NucA was maximal at pH 9.0. These results show, for the first time, expression of active enzyme and polymeric protein in secreted and anchored forms using SPEX. This further demonstrates the utility of this gram-positive surface protein expression system as a potential commensal bacterial delivery system for active, therapeutic enzymes, biopharmaceuticals, or vaccines.

Streptococcus gordonii strains resistant to fluorodeoxyuridine contain mutations in the thymidine kinase gene and are deficient in thymidine kinase activity

Mutants of Streptococcus gordonii resistant to 5-fluorodeoxyuridine (FUdR(r)) were isolated. Each strain contained a point mutation resulting in the premature termination of the thymidine kinase (TK) open reading frame (tdk). In vitro translation of the mutant tdk coding regions resulted in synthesis of truncated TK polypeptides deficient in TK activity.

Production and cell surface anchoring of functional fusions between the SLH motifs of the Bacillus anthracis S-layer proteins and the Bacillus subtilis levansucrase

Many surface proteins of Gram-positive bacteria contain motifs, about 50 amino acids long, called S-layer homology (SLH) motifs. Bacillus anthracis, the causal agent of anthrax, synthesizes two S-layer proteins, each with three SLH motifs towards the amino-terminus. We used biochemical and genetic approaches to investigate the involvement of these motifs in cell surface anchoring. Proteinase K digestion produced polypeptides lacking these motifs, and stable three-motif polypeptides were produced in Escherichia coli that were able to bind the B. anthracis cell walls in vitro, demonstrating that the three SLH motifs were organized into a cell surface anchoring domain. We also determined the function of these SLH domains by constructing chimeric genes encoding the SLH domains fused to the normally secreted levansucrase of Bacillus subtilis. Cell fractionation and electron microscopy studies showed that each three-motif domain was sufficient for the efficient anchoring of levansucrase onto the cell surface. Proteins consisting of truncated SLH domains fused to levansucrase were unstable and associated poorly with the cell surface. Surface-exposed levansucrase retained its enzymatic and antigenic properties.

Use of luciferase genes as biosensors to study bacterial physiology in the digestive tract

A method based on the use of the bacterial luciferase genes was developed in order to investigate Lactococcus lactis gene expression in the mouse digestive tract. Germfree mice were monoassociated with different strains containing transcriptional fusions of promoters with the luciferase genes. Our results demonstrate that this method is readily applicable to the study of promoter strength and physiology of bacteria in the digestive tract.

Surface display of Zymomonas mobilis levansucrase by using the ice-nucleation protein of Pseudomonas syringae

The ice-nucleation protein (Inp) is a glycosyl phosphatidylinositol-anchored outer membrane protein found in some Gram-negative bacteria. Using Pseudomonas syringae inp as an anchoring motif, we investigated the functional display of a foreign protein, Zymomonas mobilis levansucrase (LevU), on the surface of Escherichia coli. The cells expressing Inp-LevU were found to retain both the ice-nucleation and whole-cell levansucrase enzyme activities, indicating the functional expression of Inp-LevU hybrid protein on the cell surface. The surface localization was further verified by immunofluorescence microscopy, fluorescence-activated cell sorting flow cytometry and immunogold electron microscopical examination. No growth inhibition or changes in the outer membrane integrity were observed upon the induction of fusion protein synthesis. Viability of the cells was also maintained over 48 hours in the stationary phase. Surface-displayed levansucrases were found to be resistant to the externally added proteases unless the cells were treated with EDTA. When the levansucrase-displayed cells were used as the enzyme source, levan (44 g/L) was efficiently synthesized from sucrose (130 g/L) with 34% (wt/wt) conversion yield, generating glucose (65 g/L) as a by-product.

Efficient secretion of the model antigen M6-gp41E in Lactobacillus plantarum NCIMB 8826

Four Lactobacillus strains (Lb. plantarum NCIMB 8826, Lb. paracasei LbTGS1.4, Lb. casei ATCC 393 and Lb. fermentum KLD) were tested for their ability to produce and secrete heterologous proteins. These strains were first screened with an alpha-amylase reporter under the control of a set of expression or expression/secretion signals from various lactic acid bacteria. With most of the constructions tested, the level of extracellular production was highest in Lb. plantarum NCIMB 8826, and lowest in Lb. paracasei LbTGS1.4. These two strains were next assayed using a model antigen consisting of the N-terminal part of the M6 protein from Streptococcus pyogenes fused to the linear epitope ELDKWAS from human immunodeficiency virus gp41 protein. Secretion of this heterologous protein was inefficient in Lb. paracasei LbTGS1.4, which accumulated a large intracellular pool of the unprocessed precursor, whereas Lb. plantarum NCIMB 8826 was able to secrete the antigen to a level as high as 10 mg l-1.

Commensal bacteria as vectors for mucosal vaccines against sexually transmitted diseases: vaginal colonization with recombinant streptococci induces local and systemic antibodies in mice

There is a need to develop vaccines to control the spread of sexually transmitted diseases (STDs). Novel immunization strategies that elicit a mucosal immune response in the genital tract, may show improved protection by preventing or at least limiting entry of the pathogenic micro-organism. However, it has proven difficult to obtain a local immune response in the vaginal mucosa. Our approach is based on the use of recombinant bacteria capable of colonizing mucosal surfaces as live vaccine vectors. The human commensal Streptococcus gordonii, engineered to express the E7 protein of human papillomavirus type 16, was used for intravaginal immunization of mice. A single inoculum of recombinant bacteria was sufficient to establish colonization of the murine vagina and therefore induce papillomavirus-specific vaginal IgA and serum IgG. Evidence that mucosal colonization with recombinant commensal bacteria can induce a local immune response in the female genital tract represents a significant step toward the development of new vaccines against STDs.

Bacterial surface display: trends and progress

Heterologous surface display on Gram-negative bacterial was first described a decade ago and is now an active research area. More recently, strategies for surface display on Gram-positive bacterial have also been devised and these carry some inherent advantages. Bacterial surface display has found a range of applications in the expression of various antigenic determinants, heterologous enzymes, single-chain antibodies, polyhistidyl tags and even entire peptide libraries. This article explains the basis of bacterial surface display and discusses current uses and possible future trends of this emerging technology.

Cell wall anchoring of the Streptococcus pyogenes M6 protein in various lactic acid bacteria

The M6 protein from Streptococcus pyogenes is the best-characterized member of a family of cell envelope-associated proteins. Based on the observation that the C-terminal sorting signals of these proteins can drive cell wall anchoring of heterologous unanchored proteins, we have cloned and expressed the emm6 structural gene for the M6 protein in various lactic acid bacteria (LAB). The emm6 gene was successfully expressed from lactococcal promoters in several Lactococcus lactis strains, an animal-colonizing Lactobacillus fermentum strain, Lactobacillus sake, and Streptococcus salivarius subsp. thermophilus. The M6 protein was efficiently anchored to the cell wall in all strains tested. In lactobacilli, essentially all detectable M6 protein was cell wall associated. These results suggest the feasibility of using the C-terminal anchor moiety of M6 for protein surface display in LAB.

Protection against tetanus toxin in mice nasally immunized with recombinant Lactococcus lactis expressing tetanus toxin fragment C

Mice inoculated intranasally (i.n.) with a recombinant strain of live Lactococcus lactis expressing tetanus toxin fragment C (TTFC), produced both serum and secretory antibodies to TTFC. Killed bacteria which had accumulated TTFC intracellularly in vitro also elicited protective serum antibody responses. There was no requirement for either colonization or invasion of the mucosa. In addition secretory antibody responses in the lung and nasal tissues were elicited after i.n. inoculation in the presence of an adjuvant.

Surface display on staphylococci: a comparative study

Two different host-vector expression systems, designed for cell surface display of heterologous receptors on Staphylococcus xylosus and Staphylococcus carnosus, respectively, were compared for the surface display of four variants of a 101 amino acid region derived from the G glycoprotein of human respiratory syncytial virus (RSV). Surface localization of the different chimeric receptors was evaluated by a colorimetric assay and by fluorescence-activated cell sorting. It was concluded that the S. carnosus system was better both in the ability to translocate inefficiently secreted peptides and in the number of exposed hybrid receptors. The potential use of the described staphylococci as live bacterial vaccine vehicles or alternatives to filamentous phages for surface display of protein libraries is discussed.

Differential effects of the streptococcal fibronectin-binding protein, FBP54, on adhesion of group A streptococci to human buccal cells and HEp-2 tissue culture cells

We have previously demonstrated that fibronectin mediates streptococcal adhesion to host cells and that streptococci interact primarily with the N-terminal domain of fibronectin. FBP54 is a 54-kDa protein from group A streptococci that binds fibronectin. In this report, we show that the N-terminal domain of fibronectin reacts with FBP54 and preferentially blocks streptococcal adhesion to buccal epithelial cells. FBP54 blocked adhesion to human buccal epithelial cells by 80% in a dose-related fashion. In contrast, FBP54 had little effect on adhesion of group A streptococci to HEp-2 tissue culture cells. The fibronectin-binding domain of FBP54 has been localized to the first 89 N-terminal residues of the protein. Experiments using affinity-purified antibodies to this region indicated that the N terminus of FBP54 is exposed on the surface of streptococci in a manner that can interact with immobilized receptors. Analysis of sera from patients with post-streptococcal glomerulonephritis and acute rheumatic fever indicated that FBP54 is expressed in vivo and is immunogenic in the human host. These data indicate that FBP54 is a streptococcal adhesin that is expressed in the human host and that preferentially mediates adhesion to certain types of human cells.

Surface display of a functional single-chain Fv antibody on staphylococci

Two different host-vector expression systems designed for cell surface display of chimeric receptors on Staphylococcus xylosus and Staphylococcus carnosus have been evaluated for surface display of a mouse immunoglobulin G1(kappa) [IgG1(kappa)] anti-human IgE single-chain Fv (scFv) antibody fragment. To achieve surface anchoring of the chimeric receptors containing the scFv, the cell surface attachment regions from Staphylococcus aureus protein A were used in both expression systems. The different chimeric receptors could be recovered from cell wall extracts of both S. xylosus and S. carnosus, and surface localization was demonstrated by taking advantage of a serum albumin-binding reporter region present within the two types of receptors. In addition, the two different recombinant staphylococci carrying hybrid receptors containing the scFv were demonstrated to react with the antigen, which was human IgE, in whole-cell enzyme-linked immunosorbent assays. This is the first report of an antibody fragment expressed in a functional form anchored to the surface of gram-positive bacteria. The potential use of recombinant gram-positive bacteria as whole-cell diagnostic devices or alternatives to filamentous phages for surface display of scFv libraries is discussed.

Cell surface display of recombinant proteins on Staphylococcus carnosus

A novel expression system for surface display of heterologous proteins on Staphylococcus carnosus cells has been developed. Taking advantage of the promoter and secretion signals, including a propeptide region, from the lipase gene of Staphylococcus hyicus and the cell wall-spanning and membrane-binding region of protein A from Staphylococcus aureus, efficient surface display of an 80-amino-acid peptide from a malaria blood stage antigen could be achieved. A serum albumin binding protein from streptococcal protein G was used both as a general reporter molecule and to increase the accessibility of the surface-displayed proteins. Immunoblotting, immunogold staining, and immunofluorescence on intact recombinant S. carnosus cells verified the presence of the propeptide, the malaria antigen, and the albumin-binding reporter protein on the bacterial surface. For the first time, fluorescence-activated cell sorting was used to analyze the presence of surface-displayed hybrid receptors on gram-positive bacteria.