Immunogenicity of the B monomer of Escherichia coli heat-labile toxin expressed on the surface of Streptococcus gordonii

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.

Protective Immunity Against Enterotoxigenic Escherichia coli by Oral Vaccination of Engineered Lactococcus lactis

Enterotoxigenic Escherichia coli (ETEC) is one of the leading causes of diarrhea in children globally, and thus suitable vaccines are desired. Antigen display on lactic acid bacteria is a reliable approach for efficient oral vaccination and preventing bowel diseases. To develop an oral vaccine against ETEC, the gene of the binding domain from heat-labile toxin (LTB), a key ETEC virulence factor, was codon-optimized and cloned into a construct containing a signal peptide and an anchor for display on L. lactis. Bioinformatics analysis showed a codon adaptation index of 0.95 for the codon-optimized gene. Cell surface expression of LTB was confirmed by transmission electron microscopy and blotting. White New Zealand rabbits were immunized per os (PO) with the recombinant L. lactis, and the antibody titers were assayed with ELISA. In vitro neutralization assay was performed using mouse adrenal tumor cells and rabbit ileal loop test was performed as the in vivo assay. ELISA results indicated that oral administration of the engineered L. lactis elicited a significant production of IgA in the intestine. In vitro neutralization assay showed that the effect of the toxin could be neutralized with 500 µg/ml of IgG isolated from the oral vaccine group. Furthermore, the dose of ETEC causing fluid accumulation in the ileal loop test showed a tenfold increase in rabbits immunized with either recombinant L. lactis or LTB protein compared to other groups. Our results imply that recombinant L. lactis could potentially be an effective live oral vaccine against ETEC toxicity.

Spore-adsorption: Mechanism and applications of a non-recombinant display system

Surface display systems have been developed to express target molecules on almost all types of biological entities from viruses to mammalian cells and on a variety of synthetic particles. Various approaches have been developed to achieve the display of many different target molecules, aiming at several technological and biomedical applications. Screening of libraries, delivery of drugs or antigens, bio-catalysis, sensing of pollutants and bioremediation are commonly considered as fields of potential application for surface display systems. In this review, the non-recombinant approach to display antigens and enzymes on the surface of bacterial spores is discussed. Examples of molecules displayed on the spore surface and their potential applications are summarized and a mechanism of display is proposed.

Lactic acid bacteria - promising vaccine vectors: possibilities, limitations, doubts

Gram‐positive, nonpathogenic lactic acid bacteria (LAB) are considered to be promising candidates for the development of novel, safe production and delivery systems of heterologous proteins. Recombinant LAB strains were shown to elicit specific systemic and mucosal immune responses against selected antigens. For this reason, this group of bacteria is considered as a potential replacement of classical, often pathogenic, attenuated microbial carriers. Mucosal administration of recombinant LAB, especially via the best explored and universal oral route, offers many advantages in comparison to systemic inoculation, and is attractive from the immunological and practical point of view. Research aimed at designing efficient, mucosally applied vaccines in combination with improved immunization efficiency, monitoring of in vivo antigen production, determination of optimal dose for vaccination, strain selection and characterization is a priority in modern vaccinology. This paper summarizes and organizes the available knowledge on the application of LAB as live oral vaccine vectors. It constitutes a valuable source of general information for researchers interested in mucosal vaccine development and constructing LAB strains with vaccine potential.

Mucosal vaccine delivery by non-recombinant spores of Bacillus subtilis

Development of mucosal vaccines strongly relies on an efficient delivery system and, over the years, a variety of approaches based on phages, bacteria or synthetic nanoparticles have been proposed to display and deliver antigens. The spore of Bacillus subtilis displaying heterologous antigens has also been considered as a mucosal vaccine vehicle, and shown able to conjugate some advantages of live microrganisms with some of synthetic nanoparticles. Here we review the use of non-recombinant spores of B. subtilis as a delivery system for mucosal immunizations. The non-recombinant display is based on the adsorption of heterologous molecules on the spore surface without the need of genetic manipulations, thus avoiding all concerns about the use and environmental release of genetically modified microorganisms. In addition, adsorbed molecules are stabilized and protected by the interaction with the spore, suggesting that this system could reduce the rapid degradation of the antigen, often observed with other delivery systems and identified as a major drawback of mucosal vaccines.

Non-recombinant display of the B subunit of the heat labile toxin of Escherichia coli on wild type and mutant spores of Bacillus subtilis

BACKGROUND

Mucosal infections are a major global health problem and it is generally accepted that mucosal vaccination strategies, able to block infection at their entry site, would be preferable with respect to other prevention approaches. However, there are still relatively few mucosal vaccines available, mainly because of the lack of efficient delivery systems and of mucosal adjuvants. Recombinant bacterial spores displaying a heterologous antigen have been shown to induce protective immune responses and, therefore, proposed as a mucosal delivery system. A non-recombinant approach has been recently developed and tested to display antigens and enzymes.

RESULTS

We report that the binding subunit of the heat-labile toxin (LTB) of Escherichia coli efficiently adsorbed on the surface of Bacillus subtilis spores. When nasally administered to groups of mice, spore-adsorbed LTB was able to induce a specific immune response with the production of serum IgG, fecal sIgA and of IFN-γ in spleen and mesenteric lymph nodes (MLN) of the immunized animals. Dot blotting experiments showed that the non-recombinant approach was more efficient than the recombinant system in displaying LTB and that the efficiency of display could be further increased by using mutant spores with an altered surface. In addition, immunofluorescence microscopy experiments showed that only when displayed on the spore surface by the non-recombinant approach LTB was found in its native, pentameric form.

CONCLUSION

Our results indicate that non-recombinant spores displaying LTB pentamers can be administered by the nasal route to induce a Th1-biased, specific immune response. Mutant spores with an altered coat are more efficient than wild type spores in adsorbing the antigen, allowing the use of a reduced number of spores in immunization procedures. Efficiency of display, ability to display the native form of the antigen and to induce a specific immune response propose this non-recombinant delivery system as a powerful mucosal vaccine delivery approach.

Prime-boost strategies in mucosal immunization affect local IgA production and the type of th response

Combinations of different delivery routes for priming and boosting represent vaccination strategies that can modulate magnitude, quality, and localization of the immune response. A murine model was used to study T cell clonal expansion following intranasal (IN) or subcutaneous (SC) priming, and secondary immune responses after boosting by either homologous or heterologous routes. T cell primary activation was studied by using the adoptive transfer model of ovalbumin-specific transgenic CD4(+) T cells. Both IN and SC immunization efficiently elicited, in the respective draining lymph nodes, primary clonal expansion of antigen-specific CD4(+) T cells that disseminated toward distal lymph nodes (mesenteric and iliac) and the spleen. After boosting, a significant serum IgG response was induced in all groups independent of the combination of immunization routes used, while significant levels of local IgA were detected only in mice boosted by the IN route. Mucosal priming drove a stronger Th1 polarization than the systemic route, as shown by serum IgG subclass analysis. IFN-gamma production was observed in splenocytes of all groups, while prime-boost vaccine combinations that included the mucosal route, yielded higher levels of IL-17. Memory lymphocytes were identified in both spleen and draining lymph nodes in all immunized mice, with the highest number of IL-2 producing cells detected in mice primed and boosted by the nasal route. This work shows the critical role of immunization routes in modulating quality and localization of immune responses in prime-boost vaccine strategies.

Immunization with the conjugate vaccine Vi-CRM₁₉₇ against Salmonella typhi induces Vi-specific mucosal and systemic immune responses in mice

Typhoid fever is a public health problem, especially among young children in developing countries. To address this need, a glycoconjugate vaccine Vi-CRM₁₉₇, composed of the polysaccharide antigen Vi covalently conjugated to the non-toxic mutant of diphtheria toxin CRM₁₉₇, is under development. Here, we assessed the antibody and cellular responses, both local and systemic, following subcutaneous injection of Vi-CRM₁₉₇. The glycoconjugate elicited Vi-specific serum IgG titers significantly higher than unconjugated Vi, with prevalence of IgG1 that persisted for at least 60 days after immunization. Vi-specific IgG, but not IgA, were present in intestinal washes. Lymphocytes proliferation after restimulation with Vi-CRM₁₉₇ was observed in spleen and mesenteric lymph nodes. These data confirm the immunogenicity of Vi-CRM₁₉₇ and demonstrate that the vaccine-specific antibody and cellular immune responses are present also in the intestinal tract, thus strengthening the suitability of Vi-CRM₁₉₇ as a promising candidate vaccine against Salmonella Typhi.

The factor H-binding fragment of PspC as a vaccine antigen for the induction of protective humoral immunity against experimental pneumococcal sepsis

Pneumococcal surface protein C (PspC) is a major virulence factor of Streptococcus pneumoniae and interferes with complement activity by binding complement factor H (fH). In this study, protection against experimental sepsis caused by pneumococci carrying different PspC variants was evaluated by immunisation with the fH-binding fragment of PspC. The mechanisms of protection mediated by antibodies to PspC were also studied. Mice were immunised with a PspC fragment (PspC(39-261)) from the type 3 strain HB565 and infected intravenously with either strain HB565 (homologous challenge), or strains D39 and TIGR4 (heterologous challenge). Immunisation with PspC(39-261) elicited high titers (>300,000) of PspC-specific serum IgG and conferred protection from challenge with HB565. In contrast, cross-protection was either limited or absent in vaccinated animals infected with D39 and TIGR4, respectively. To correlate protection with reactivity and function of PspC antibodies, pooled sera from vaccinated mice were tested in IgG binding and complement deposition experiments. IgG antibodies efficiently bound to HB565, while binding was lower with D39 and absent with TIGR4. In the presence of mouse post-immune sera, C3 deposition was increased onto HB565, while no effect was observed with D39 and TIGR4. Antibody cross-reactivity and complement deposition progressively declined with reduced amino acid identity between PspC variants. Antibodies to PspC were also found to interfere with fH binding to HB565. Finally, in vitro and ex vivo phagocytosis assays demonstrated that PspC-specific antibodies promoted opsonophagocytic killing of bacteria.

Surface display of recombinant protein on the cell surface of Bacillus subtilis by the CotB anchor protein

We developed a novel surface display system based on the CotB anchoring motif in order to express foreign protein on the surface of vegetative Bacillus subtilis cells. CotB is a protein in the B. subtilis spore coat. In this system, three repeats of the immunodominant ovalbumin T-cell epitope (OVA_323–339) were linked with the cholera toxin B subunit (CTB) to construct a fusion protein, CTB-OVA epi, which was then fused to the C-terminal of the CotB protein so that CTB-OVA epi was expressed in vegetatively-growing B. subtilis. The expression and localization of the CTB-OVA epi protein was confirmed by western blotting, immunofluorescence microscopy, and flow cytometry. The results indicated that a CotB-based surface display system was successfully used to express the CTB-OVA epi protein on the surface of vegetative B. subtilis cells.

Synergistic BM-DC activation and immune induction by the oral vaccine vector Streptococcus gordonii and exogenous tumor necrosis factor

Streptococcus gordonii, a potential mucosal vaccine delivery vector, is proficient at colonizing murine oral mucosa; however, it often fails to elicit significant antibody titers against its vaccine antigen payloads. The poor response may be due to an inability of S. gordonii to elicit cytokines needed to suppress mucosal tolerance; exogenously supplied cytokines, such as TNF, could overcome this effect. To test this, murine bone marrow-derived dendritic cells (BM-DCs) were stimulated with UV-killed S. gordonii PM14, that surface expresses a fragment of the immunodominant S1 subunit of pertussis toxin. Peptidoglycan (PGN), lipoteichoic acid (LTA), lipoprotein (LP), and DNA were also isolated from the bacteria, and used to stimulate BM-DCs. Stimulation with TNF, S. gordonii, PGN, LTA, or LP all resulted in increased surface expression of MHCII, CD80, and CD86, compared to unstimulated BM-DCs. Stimulation with S. gordonii elicited IL-6, IL-10, and IL-12p70 production from the BM-DCs, while stimulation with the bacterial components induced some or all of the three cytokines. When BM-DCs were simultaneously stimulated with S. gordonii and TNF, a marginal increase in surface marker upregulation was observed, and the two stimuli synergized to elicit substantially greater quantities of IL-6, IL-10, and IL-12p70. Synergy between TNF and the purified bacterial components was also observed. The effect of TNF was abolished when BM-DCs were obtained from mice deficient for either TNFR1 or TNFR2, and cytokine induction by S. gordonii was entirely dependent on functional MyD88. Synergistic IL-10 induction by S. gordonii and TNF was not observed in TLR-2(-/-) BM-DCs, and TNF was found to cause TLR-2 upregulation, providing at least a partial mechanism for the observed synergy. When S. gordonii and TNF were used to immunize mice, a more robust anti-S. gordonii IgG response was elicited as compared to immunization with S. gordonii alone. However, the addition of TNF did not result in stronger responses against the antigenic insert (S1 fragment) in immunized mice. These findings collectively demonstrate that TNF is able to prime BM-DCs to better respond to S. gordonii, through a mechanism at least partially involving TLR-2 upregulation.

Primary activation of antigen-specific naive CD4+ and CD8+ T cells following intranasal vaccination with recombinant bacteria

The primary activation of T-helper and T-cytotoxic cells following mucosal immunization with recombinant Streptococcus gordonii was studied in vivo by adoptive transfer of ovalbumin (OVA)-specific transgenic CD8(+) (OT-I) and CD4(+) (OT-II) T cells. A recombinant strain, expressing on the surface the vaccine antigen Ag85B-ESAT-6 from Mycobacterium tuberculosis fused to OVA T-helper and T-cytotoxic epitopes (peptides 323 to 339 and 257 to 264), was constructed and used to immunize C57BL/6 mice by the intranasal route. Recombinant, but not wild-type, bacteria induced OVA-specific CD4(+) and CD8(+) T-cell clonal expansion in cervical lymph nodes, lung, and spleen. OVA-specific CD4(+) and CD8(+) T-cell proliferation appeared first in cervical lymph nodes and later in the spleen, suggesting a possible migration of activated cells from the inductive site to the systemic district. A significant correlation between the percentages of CD4(+) and CD8(+) proliferating T cells was observed for each animal. The expression of CD69, CD44, and CD45RB on proliferating T lymphocytes changed as a function of the cell division number, confirming T-cell activation following the antigen encounter. These data indicate that intranasal immunization with recombinant S. gordonii is capable of inducing primary activation of naive antigen-specific CD4(+) and CD8(+) T cells, both locally and systemically.

Intranasal immunization of mice with recombinant Streptococcus gordonii expressing NadA of Neisseria meningitidis induces systemic bactericidal antibodies and local IgA

NadA and NhhA, two surface proteins of serogroup B Neisseria meningitidis identified as candidate vaccine antigens, were expressed on the surface of the human oral commensal bacterium Streptococcus gordonii. Recombinant strains were used to immunize BALB/c mice by the intranasal route and the local and systemic immune response was assessed. Mice were inoculated with recombinant bacteria administered alone or with LTR72, a partially inactivated mutant of Escherichia coli heat-labile enterotoxin, as a mucosal adjuvant. Intranasal immunization with live bacteria expressing NadA induced a significant serum antibody response, with a prevalence of the IgG2a subclass, bactericidal activity in the sera of 71% of animals, and a NadA-specific IgA response in nasal and bronchoalveolar lavages. A formalin-inactivated recombinant strain of S. gordonii expressing NadA was also administered intranasally, inducing a systemic and mucosal humoral response comparable to that of live bacteria. The administration of recombinant bacteria with the mucosal adjuvant LTR72 stimulated a stronger systemic antibody response, protective in 85% of sera, while did not increase the local IgA response. Recombinant S. gordonii expressing NhhA induced a systemic but not mucosal antibody response. These data support the role of NadA as vaccine candidate against serogroup B meningococci, and the use of S. gordonii as vector for intranasal vaccination.

Expression of a functional single-chain variable-fragment antibody against complement receptor 1 in Streptococcus gordonii

Streptococcus gordonii, an oral commensal organism, is a candidate vector for oral-vaccine development. Previous studies have shown that recombinant S. gordonii expressing heterologous antigens was weakly immunogenic when delivered intranasally. In this study, antigen was specifically targeted to antigen-presenting cells (APC) in order to potentiate antigen-APC interactions and increase the humoral immune response to the antigen. To achieve this goal, a single-chain variable-fragment (scFv) antibody against complement receptor 1 (CR1) was constructed. Anti-CR1 scFv purified from Escherichia coli was able to bind to mouse mixed lymphocytes and bone marrow-derived dendritic cells. The in vivo function of the anti-CR1 scFv protein was assessed by immunizing mice intranasally with soluble scFv and determining the immune response against the hemagglutinin (HA) peptide located on the carboxy terminus of the scFv. The serum anti-HA immunoglobulin G (IgG) immune response was dose dependent; as little as 100 ng of anti-CR1 scFv induced a significant IgG immune response, while such a response was minimal when the animals were given an unrelated scFv. The anti-CR1 scFv was expressed in S. gordonii as a secreted protein, which was functional, as it bound to dendritic cells. Mice orally colonized by the anti-CR1-secreting S. gordonii produced an anti-HA IgG immune response, indicating that such an approach can be used to increase the immune response to antigens produced by this bacterium.

Stimulation of human monocytes with the gram-positive vaccine vector Streptococcus gordonii

Streptococcus gordonii is a bacterial vaccine vector which has previously been shown to activate dendritic cells in vitro and to induce local and systemic immune responses in vivo. In the present study, human monocytes (THP-1 cell line and peripheral blood monocytes) were characterized following interaction with S. gordonii. Treatment of human monocytes with S. gordonii but not latex beads induced a clear up-regulation of CD83, CD40, CD80, and CD54 and the down-regulation of CD14. Furthermore, bacterial treatment stimulated an increased expression of Toll-like receptor 5 (TLR5), TLR6, and TLR7, production of the proinflammatory cytokines tumor necrosis factor alpha and interleukin 1 beta, and reduction of the phagocytic activity. This work shows that the immunostimulatory activity of S. gordonii is not restricted to induction of dendritic-cell maturation but also affects the differentiation process of human monocytes.

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.

In vivo activation of naive CD4+ T cells in nasal mucosa-associated lymphoid tissue following intranasal immunization with recombinant Streptococcus gordonii

The antigen-specific primary activation of CD4+ T cells was studied in vivo by adoptive transfer of ovalbumin-specific transgenic T cells (KJ1-26+ CD4+) following intranasal immunization with recombinant Streptococcus gordonii. A strain of S. gordonii expressing on its surface a model vaccine antigen fused to the ovalbumin (OVA) peptide from position 323 to 339 was constructed and used to study the OVA-specific T-cell activation in nasal mucosa-associated lymphoid tissue (NALT), lymph nodes, and spleens of mice immunized by the intranasal route. The recombinant strain, but not the wild type, activated the OVA-specific CD4+ T-cell population in the NALT (89% of KJ1-26+ CD4+ T cells) just 3 days following immunization. In the cervical lymph nodes and in the spleen, the percentage of proliferating cells was initially low, but it reached the peak of activation at day 5 (90%). This antigen-specific clonal expansion of KJ1-26+ CD4+ T cells after intranasal immunization was obtained with live and inactivated recombinant bacteria, and it indicates that the NALT is the site of antigen-specific T-cell priming.

Clinical and microbiological responses of volunteers to combined intranasal and oral inoculation with a Streptococcus gordonii carrier strain intended for future use as a group A streptococcus vaccine

Streptococcus gordonii shows promise as a live mucosal vaccine vector for immunization against respiratory pathogens. In preparation for clinical trials to evaluate S. gordonii engineered to express group A streptococcal M protein antigens, we characterized the responses of 150 healthy volunteers to combined nasal and oral inoculation with approximately 1.5 x 10(9) CFU of SP204(1-1), an S. gordonii strain not bearing vaccine antigens. SP204(1-1) was selected for resistance to streptomycin and 5-fluoro-2-deoxyuridine to distinguish it from indigenous flora. In two antibiotic treatment studies, we performed serial culturing of nose, mouth, and saliva samples from 120 subjects treated with azithromycin beginning 5 days after inoculation to determine whether SP204(1-1) could be rapidly eliminated should safety concerns arise. A natural history study was performed to assess the time until spontaneous eradication in the remaining 30 subjects, who did not receive the antibiotic and who were monitored with repeated culturing for 14 weeks after inoculation. SP204(1-1) was generally well tolerated. Symptoms reported most often within 5 days of inoculation were nasal congestion (36%), headache (30%), and sore throat (19%). The strain was detected by culturing in 98% of subjects. A single dose of azithromycin eliminated colonization in 95% of subjects; all subjects receiving a 5-day course of an antibiotic showed clearance by day 11. Without the antibiotic, 82% of subjects showed spontaneous eradication of the implanted strain within 7 days, and all showed clearance by 35 days. The results of these clinical trials provide encouragement that the use of S. gordonii as a live mucosal vaccine vector is a feasible strategy.

Selection and characterization of a promoter for expression of single-copy recombinant genes in Gram-positive bacteria

BACKGROUND

In the past ten years there has been a growing interest in engineering Gram-positive bacteria for biotechnological applications, including vaccine delivery and production of recombinant proteins. Usually, bacteria are manipulated using plasmid expression vectors. The major limitation of this approach is due to the fact that recombinant plasmids are often lost from the bacterial culture upon removal of antibiotic selection. We have developed a genetic system based on suicide vectors on conjugative transposons allowing stable integration of recombinant DNA into the chromosome of transformable and non-transformable Gram-positive bacteria.

RESULTS

The aim of this work was to select a strong chromosomal promoter from Streptococcus gordonii to improve this genetic system making it suitable for expression of single-copy recombinant genes. To achieve this task, a promoterless gene encoding a chloramphenicol acetyltransferase (cat), was randomly integrated into the S. gordonii chromosome and transformants were selected for chloramphenicol resistance. Three out of eighteen chloramphenicol resistant transformants selected exhibited 100% stability of the phenotype and only one of them, GP215, carried the cat gene integrated as a single copy. A DNA fragment of 600 base pairs exhibiting promoter activity was isolated from GP215 and sequenced. The 5' end of its corresponding mRNA was determined by primer extention analysis and the putative -10 and a -35 regions were identified. To study the possibility of using this promoter (PP) for single copy heterologous gene expression, we created transcriptional fusions of PP with genes encoding surface recombinant proteins in a vector capable of integrating into the conjugative transposon Tn916. Surface recombinant proteins whose expression was controlled by the PP promoter were detected in Tn916-containing strains of S. gordonii and Bacillus subtilis after single copy chromosomal integration of the recombinant insertion vectors into the resident Tn916. The surface recombinant protein synthesized under the control of PP was also detected in Enterococcus faecalis after conjugal transfer of a recombinant Tn916 containing the transcriptional fusion.

CONCLUSION

We isolated and characterized a S. gordonii chromosomal promoter. We demonstrated that this promoter can be used to direct expression of heterologous genes in different Gram-positive bacteria, when integrated in a single copy into the chromosome.

Oral priming of mice by recombinant spores of Bacillus subtilis

Recombinant Bacillus subtilis spores were employed as a vaccine delivery system in a heterologous mucosal priming-parenteral boosting vaccination strategy in the mouse model. BALB/c and C57BL/6 mice were orally immunised with recombinant spores expressing tetanus toxin fragment C (TTFC) fused to the spore outer coat protein CotB, and then subcutaneously boosted with soluble TTFC (without adjuvant). Two weeks after boosting, a significantly higher serum TTFC-specific IgG response was stimulated in mice primed with recombinant spores (antibody concentration of 2600 +/- 915 in C57BL/6 and 1200 +/- 370 ng/ml in BALB/c) compared to mice inoculated with wild type spores (650 +/- 250 and 250 +/- 130 ng/ml, respectively). IgG subclass analysis showed a prevalence of IgG1 and IgG2b, indicative of a Th2 type of immune response. Oral administration of recombinant spores stimulated also a significant local TTFC-specific IgA response. These data show that recombinant spores of B. subtilis are able to prime the immune system by the oral route, and that a combined mucosal/parenteral strategy can stimulate both local and systemic antigen-specific immune responses.

Display of heterologous antigens on the Bacillus subtilis spore coat using CotC as a fusion partner

We report the use of CotC, a major component of the Bacillus subtilis spore coat, as a fusion partner for the expression of two heterologous antigens on the spore coat. Recombinant spores expressing tetanus toxin fragment C (TTFC) of Clostridium tetani or the B subunit of the heat-labile toxin of Escherichia coli (LTB) were used for oral dosing and shown to generate specific systemic and mucosal immune responses in a murine model. This report, expanding the previously described expression of TTFC on the spore surface by fusion to CotB [J Bacteriol 183 (2001) 6294] and its use for oral vaccination [Infect Immun 71 (2003) 2810] shows that different antigens can be successfully presented on the spore coat and supports the use of the spore as an efficient vehicle for mucosal immunisation.

In vivo mucosal delivery of bioactive human interleukin 1 receptor antagonist produced by Streptococcus gordonii

BACKGROUND

Interleukin-1 (IL-1) is a cytokine involved in the initiation and amplification of the defence response in infectious and inflammatory diseases. IL-1 receptor antagonist (IL-1ra) is an inactive member of the IL-1 family and represents one of the most potent mechanisms for controlling IL-1-dependent inflammation. IL-1ra has proven effective in the therapy of acute and chronic inflammatory diseases in experimental animal models and also in preliminary clinical trials. However, optimisation of therapeutic schedules is still needed. For instance, the use of drug delivery systems targeting specific mucosal sites may be useful to improve topical bioavailability and avoid side effects associated with systemic administration.

RESULTS

In order to develop systems for the delivery of IL-1ra to mucosal target sites, a Streptococcus gordonii strain secreting human IL-1ra was constructed. The recombinant IL-1ra produced by S. gordonii was composed of the four amino acid residues RVFP of the fusion partner at the N-terminus, followed by the mature human IL-1ra protein. RFVP/IL-1ra displayed full biological activity in vitro in assays of inhibition of IL-1beta-induced lymphocyte proliferation and was released by recombinant S. gordonii in vivo both at the vaginal and the gastrointestinal mucosa of mice. RFVP/IL-1ra appeared beneficial in the model of ulcerative colitis represented by IL-2-/- mice (knock-out for the interleukin-2 gene), as shown by the body weight increase of IL-2-/- mice locally treated with S. gordonii producing RFVP/IL-1ra.

CONCLUSIONS

These results indicate that recombinant S. gordonii can be successfully used as a delivery system for the selective targeting of mucosal surfaces with therapeutic proteins.

Oral colonization and immune responses to Streptococcus gordonii: Potential use as a vector to induce antibodies against respiratory pathogens

PURPOSE OF REVIEW

Mucosal immunization should be an excellent method of preventing respiratory infections because the local immunoglobulin A antibodies can neutralize the invading pathogens at the site of entry. Because Streptococcus gordonii, a normal inhabitant of the human oral cavity, can naturally elicit a mucosal immune response, it has been a prime candidate for investigations as a live oral vaccine vector for immunization against respiratory infections.

RECENT FINDINGS

Antigens from a number of respiratory bacteria, such as Bordetella pertussis, and one virus have been expressed extracellularly or on the cell surface of S. gordonii. The antigens expressed were single or multiple proteins from one or more pathogens. The recombinant S. gordonii expressing surface-localized heterologous antigens could colonize and persist in the oral cavity of mice and rats. Oral colonization induced a mucosal immunoglobulin A response and, in some instances, also a systemic immunoglobulin G response to the heterologous antigens. When given parenterally, the heterologous antigens generated a systemic immunoglobulin G response. These findings indicate that antigens expressed by S. gordonii are immunogenic. A new approach to the use of S. gordonii as a vaccine vector is to modulate immune responses by co-expressing cytokines with the antigen.

SUMMARY

The ability to express antigens from respiratory pathogens and induce immune responses during oral colonization suggests that S. gordonii may be developed into a live vector for oral immunization against respiratory infections. The major challenge ahead is to find ways to achieve a high level of immune response following oral colonization.

The microbicide cyanovirin-N expressed on the surface of commensal bacterium Streptococcus gordonii captures HIV-1

OBJECTIVE

To explore the feasibility of expressing the potent HIV-inactivating protein, cyanovirin-N (CV-N), in the human commensal bacterium Streptococcus gordonii, as a possible approach for local delivery of CV-N to prevent sexual transmission of HIV-1.

DESIGN AND METHODS

To express CV-N in S. gordonii, we used the host-vector system we had previously developed. CV-N was expressed as a fusion protein both attached to the bacterial surface and secreted in soluble form in the supernatant of liquid cultures. The soluble form of recombinant CV-N was tested for gp120-binding activity in an enzyme-linked immunosorbent assay, whereas S. gordonii strain expressing CV-N on the surface was analyzed in an in vitro HIV capturing assay.

RESULTS

Two recombinant S. gordonii strains secreting or displaying CV-N on the bacterial surface were constructed and the expression of CV-N was confirmed by immunoblot and flow-cytometric analysis. The secreted form of recombinant CV-N exhibited a concentration-dependent binding to the envelope glycoprotein gp120 of HIV-1, whereas CV-N displayed on the bacterial surface was able to capture HIV virions efficiently.

CONCLUSION

The anti-HIV protein CV-N in S. gordonii was expressed in a biologically active form. This represents a first step in the development of a system to deliver and maintain an effective concentration of a microbicide in the vaginal mucosa.

Recombinant Streptococcus gordonii for mucosal delivery of a scFv microbicidal antibody

The gram-positive bacterium Streptococcus gordonii was engineered to express the microbicidal molecule H6, which is an antiidiotypic single chain antibody mimicking a yeast killer toxin. S. gordonii is a human commensal which we developed as a model system for mucosal delivery of heterologous proteins. The in vivo candidacidal activity of both H6-secreting and H6-surface-displaying streptococcal strains were assayed in a well-established rat model of vaginal candidiasis. At day 21 full clearance of Candida albicans infection was observed in 75% of animals treated with the H6-secreting strain, and in 37.5% of animals treated with the strain expressing H6 on the surface, while all animals treated with the control strain were still infected. The observed candidacidal effect was comparable with that observed with the antimycotic drug fluconazole. These data confirm the potential of H6 as a candidacidal agent and show how promising is the approach of using recombinant bacteria for mucosal delivery of biologically active molecules.

Genetic engineering of Streptococcus gordonii for the simultaneous display of two heterologous proteins at the bacterial surface

The Gram-positive bacterium Streptococcus gordonii has been genetically engineered to allow the simultaneous expression of two heterologous proteins at the cell surface. A family of recombinant streptococci displaying two different antigens was constructed. All the strains were genetically stable and expressed both proteins at the surface of the same bacterial cell. S. gordonii co-expressing the immunomodulating molecule LTB (B monomer of Escherichia coli heat-labile toxin) and the V3 domain of HIV-1 gp120 were inoculated subcutaneously to BALB/c mice. Animals were capable of responding to both antigens, producing LTB- and V3-specific serum IgG. The V3-specific IgG titer was four-fold higher in mice immunised with the double protein-expressing bacteria, as compared to control animals inoculated either with S. gordonii expressing the V3 domain alone or with a mixture of the two strains expressing LTB and V3, separately. Therefore, LTB was able to potentiate the antibody response towards the V3 domain, and this effect was observed only when LTB was co-expressed on the same bacterial cell.

Bacterial toxins as tools for mucosal vaccination

Several studies have demonstrated that the biological properties of secreted bacterial toxins could be harnessed for the induction of mucosal and systemic immunity following application at epithelial surfaces. Although the properties and potential application of several of these toxins will be discussed in this review, special focus will be placed on Pseudomonas aeruginosa exotoxin A (PE). A non-toxic form of PE (ntPE) into which antigenic epitopes can be integrated appears to be a particularly promising vaccination tool, which is able to cross the polarized epithelia of the gastrointestinal, respiratory and reproductive tracts and selectively target macrophages and dendritic cells.

Surface display of recombinant proteins on Bacillus subtilis spores

We developed a novel surface display system based on the use of bacterial spores. A protein of the Bacillus subtilis spore coat, CotB, was found to be located on the spore surface and used as fusion partner to express the 459-amino-acid C-terminal fragment of the tetanus toxin (TTFC). Western, dot blot and fluorescent-activated cell sorting analyses were used to monitor TTFC surface expression on purified spores. We estimated that more than 1.5 x 10(3) TTFC molecules were exposed on the surface of each spore and recognized by TTFC-specific antibodies. The efficient surface presentation of the heterologous protein, together with the simple purification procedure and the high stability and safety record of B. subtilis spores, makes this spore-based display system a potentially powerful approach for surface expression of bioactive molecules.

Edible genetically modified microorganisms and plants for improved health

The development of new strategies for the delivery of vaccine antigens or immune modulators to the mucosal tissue includes innovative approaches such as the use of genetically modified food microorganisms and plants. Even though the 'proof-of-concept' has recently been established for these two systems, key questions mainly related to efficacy and risk of breaking oral tolerance remain to be critically addressed in the immediate future.

Oral immunization with recombinant Streptococcus gordonii expressing porphyromonas gingivalis FimA domains

Porphyromonas gingivalis, a gram-negative anaerobe, is implicated in the etiology of adult periodontitis. P. gingivalis fimbriae are one of several critical surface virulence factors involved in both bacterial adherence and inflammation. P. gingivalis fimbrillin (FimA), the major subunit protein of fimbriae, is considered an important antigen for vaccine development against P. gingivalis-associated periodontitis. We have previously shown that biologically active domains of P. gingivalis fimbrillin can be expressed on the surface of the human commensal bacterium Streptococcus gordonii. In this study, we examined the effects of oral coimmunization of germfree rats with two S. gordonii recombinants expressing N (residues 55 to 145)- and C (residues 226 to 337)-terminal epitopes of P. gingivalis FimA to elicit FimA-specific immune responses. The effectiveness of immunization in protecting against alveolar bone loss following P. gingivalis infection was also evaluated. The results of this study show that the oral delivery of P. gingivalis FimA epitopes via S. gordonii vectors resulted in the induction of FimA-specific serum (immunoglobulin G [IgG] and IgA) and salivary (IgA) antibody responses and that the immune responses were protective against subsequent P. gingivalis-induced alveolar bone loss. These results support the potential usefulness of the S. gordonii vectors expressing P. gingivalis fimbrillin as a mucosal vaccine against adult periodontitis.

Immunoglobulin-binding domains of peptostreptococcal protein L enhance vaginal colonization of mice by Streptococcus gordonii

Protein L, an immunoglobulin-binding protein of some strains of the anaerobic bacterium Peptostreptococcus magnus, has been hypothesized to be a virulence determinant in bacterial vaginosis. In order to investigate the role of protein L in peptostreptococcal virulence, the Ig-binding domains of protein L were expressed at the surface of the human oral commensal Streptococcus gordonii. Recombinant streptococci were used in vaginal colonization experiments, and protein L-expressing S. gordonii demonstrated enhanced ability to colonize the vaginal mucosa. Compared to the control strain, they also persisted for a longer period in the murine vagina.

Immunization with recombinant Streptococcus gordonii expressing tetanus toxin fragment C confers protection from lethal challenge in mice

Tetanus toxin fragment C (TTFC) was expressed on the surface of the vaccine vector Streptococcus gordonii, a Gram-positive commensal bacterium of the human oral cavity. The immunogenicity of recombinant S. gordonii expressing TTFC was assayed in mice immunized by the parenteral and mucosal routes. High serum TTFC-specific IgG responses were induced in both BALB/c and C57BL/6 mice immunized subcutaneously. A total of 82% of vaccinated BALB/c mice were protected from the lethal challenge with 50 LD(50) of tetanus toxin (TT) and a direct correlation between the serum TTFC-specific IgG concentration and survival time of unprotected animals was observed. Intranasal immunization of BALB/c mice was also effective in inducing TTFC-specific serum IgG and local IgA in lung washes. Furthermore, 38% of animals immunized intranasally were protected from the lethal challenge with 10 LD(50) of TT while all control animals died within 24 h. Analysis of the serum IgG subclasses showed that the IgG1 subclass was predominant after parenteral immunization in BALB/c mice (IgG1/IgG2a ratio congruent with6) while following mucosal immunization a mixed IgG1 and IgG2a pattern (IgG1/IgG2a ratio congruent with1) was observed. These data show that TTFC expressed on the surface of S. gordonii is immunogenic by the subcutaneous and mucosal routes and the immune response induced is capable of conferring protection from the lethal challenge with TT.

Therapy of mucosal candidiasis by expression of an anti-idiotype in human commensal bacteria

Two recombinant strains of Streptococcus gordonii, secreting or displaying a microbicidal single-chain antibody (H6), and stably colonizing rat vagina, were used to treat an experimental vaginitis caused by Candida albicans. A post-challenge intravaginal delivery of the H6-secreting strain was as efficacious as fluconazole in rapidly abating the fungal burden. Three weeks after challenge, 75% and 37.5% of the rats treated with the H6-secreting or displaying bacteria, respectively, were cured of the infection, which persisted in 100% of the animals treated with a S. gordonii strain expressing an irrelevant single-chain antibody. Thus, a human commensal bacterium can be suitably engineered to locally release a therapeutic antibody fragment.

Expression and purification of histidine-tagged proteins from the gram-positive Streptococcus gordonii SPEX system

Streptococcus gordonii (S. gordonii) has been used as a gram-positive bacterial expression vector for secreted or surface-anchored recombinant proteins. Fusion of the gram-positive bacterial N-terminal signal sequence to the target protein is all that is required for efficient export. This system is termed SPEX for Surface Protein EXpression and has been used to express proteins for a variety of uses. In this study, the SPEX system has been further developed by the construction of vectors that express polyhistidine-tagged fusion proteins. SPEX vectors were constructed with an N-terminal or C-terminal histidine tag. The C-repeat region (CRR) from Streptococcus pyogenes M6 protein and the Staphylococcus aureus nuclease A (NucA) enzyme were tested for expression. The fusion proteins were purified using metal affinity chromatography (MAC). Results show that the fusion proteins were expressed and secreted from S. gordonii with the His tag at either the N- or C-terminal position and could be purified using MAC. The M6 fusions retained immunoreactivity after expression and purification as determined by immunoblots and ELISA analyses. In addition, NucA fusions retained functional activity after MAC purification. The M6-His and NucA-His fusions were purified approximately 15- and 10-fold respectively with approximately 30% recovery of protein using MAC. This study shows that the polyhistidine tag in either the N- or C-terminal position is a viable way to purify secreted heterologous proteins from the supernatant of recombinant S. gordonii cultures. This study further illustrates the value of the SPEX system for secreted expression and purification of proteins.