In vivo expression and immunoadjuvancy of a mutant of heat-labile enterotoxin of Escherichia coli in vaccine and vector strains of Vibrio cholerae

Live-Attenuated Bacterial Vectors for Delivery of Mucosal Vaccines, DNA Vaccines, and Cancer Immunotherapy

Vaccines save millions of lives each year from various life-threatening infectious diseases, and there are more than 20 vaccines currently licensed for human use worldwide. Moreover, in recent decades immunotherapy has become the mainstream therapy, which highlights the tremendous potential of immune response mediators, including vaccines for prevention and treatment of various forms of cancer. However, despite the tremendous advances in microbiology and immunology, there are several vaccine preventable diseases which still lack effective vaccines. Classically, weakened forms (attenuated) of pathogenic microbes were used as vaccines. Although the attenuated microbes induce effective immune response, a significant risk of reversion to pathogenic forms remains. While in the twenty-first century, with the advent of genetic engineering, microbes can be tailored with desired properties.

In this review, I have focused on the use of genetically modified bacteria for the delivery of vaccine antigens. More specifically, the live-attenuated bacteria, derived from pathogenic bacteria, possess many features that make them highly suitable vectors for the delivery of vaccine antigens. Bacteria can theoretically express any heterologous gene or can deliver mammalian expression vectors harboring vaccine antigens (DNA vaccines). These properties of live-attenuated microbes are being harnessed to make vaccines against several infectious and noninfectious diseases. In this regard, I have described the desired features of live-attenuated bacterial vectors and the mechanisms of immune responses manifested by live-attenuated bacterial vectors. Interestingly anaerobic bacteria are naturally attracted to tumors, which make them suitable vehicles to deliver tumor-associated antigens thus I have discussed important studies investigating the role of bacterial vectors in immunotherapy. Finally, I have provided important discussion on novel approaches for improvement and tailoring of live-attenuated bacterial vectors for the generation of desired immune responses.

Transcutaneous immunization with a Vibrio cholerae O1 Ogawa synthetic hexasaccharide conjugate following oral whole-cell cholera vaccination boosts vibriocidal responses and induces protective immunity in mice

A shortcoming of currently available oral cholera vaccines is their induction of relatively short-term protection against cholera compared to that afforded by wild-type disease. We were interested in whether transcutaneous or subcutaneous boosting using a neoglycoconjugate vaccine made from a synthetic terminal hexasaccharide of the O-specific polysaccharide of Vibrio cholerae O1 (Ogawa) coupled to bovine serum albumin as a carrier (CHO-BSA) could boost lipopolysaccharide (LPS)-specific and vibriocidal antibody responses and result in protective immunity following oral priming immunization with whole-cell cholera vaccine. We found that boosting with CHO-BSA with immunoadjuvantative cholera toxin (CT) or Escherichia coli heat-labile toxin (LT) following oral priming with attenuated V. cholerae O1 vaccine strain O395-NT resulted in significant increases in serum anti-V. cholerae LPS IgG, IgM, and IgA (P < 0.01) responses as well as in anti-Ogawa (P < 0.01) and anti-Inaba (P < 0.05) vibriocidal titers in mice. The LPS-specific IgA responses in stool were induced by transcutaneous (P < 0.01) but not subcutaneous immunization. Immune responses following use of CT or LT as an adjuvant were comparable. In a neonatal mouse challenge assay, immune serum from boosted mice was associated with 79% protective efficacy against death. Our results suggest that transcutaneous and subcutaneous boosting with a neoglycoconjugate following oral cholera vaccination may be an effective strategy to prolong protective immune responses against V. cholerae.

Exploiting cholera vaccines as a versatile antigen delivery platform

The development of safe, immunogenic and protective cholera vaccine candidates makes possible their use as a versatile antigen delivery platform. Foreign antigens can be delivered to the immune system with cholera vaccines by expressing heterologous antigens in live attenuated vectors, as fusion proteins with cholera toxin subunits combined with inactivated Vibrio cholerae whole cells or by exposing them on the surface of V. cholerae ghosts. Progress in our understanding of the genes expressed by V. cholerae during infection creates unprecedented opportunities to develop an improved generation of vaccine vectors to induce immune protection against a broad range of pathogenic organisms.

Oral vaccination: where we are?

As early as 900 years ago, the Bedouins of the Negev desert were reported to kill a rabid dog, roast its liver and feed it to a dog-bitten person for three to five days according to the size and number of bites [1] . In sixteenth century China, physicians routinely prescribed pills made from the fleas collected from sick cows, which purportedly prevented smallpox. One may dismiss the wisdom of the Bedouins or Chinese but the Nobel laureate, Charles Richet, demonstrated in 1900 that feeding raw meat can cure tuberculous dogs - an approach he termed zomotherapy. Despite historical clues indicating the feasibility of oral vaccination, this particular field is notoriously infamous for the abundance of dead-end leads. Today, most commercial vaccines are delivered by injection, which has the principal limitation that recipients do not like needles. In the last few years, there has been a sharp increase in interest in needle-free vaccine delivery; new data emerges almost daily in the literature. So far, there are very few licensed oral vaccines, but many more vaccine candidates are in development. Vaccines delivered orally have the potential to take immunization to a fundamentally new level. In this review, the authors summarize the recent progress in the area of oral vaccines.

Transcutaneous immunization with Clostridium difficile toxoid A induces systemic and mucosal immune responses and toxin A-neutralizing antibodies in mice

Clostridium difficile is the leading cause of nosocomial infectious diarrhea. C. difficile produces two toxins (A and B), and systemic and mucosal anti-toxin A antibodies prevent or limit C. difficile-associated diarrhea. To evaluate whether transcutaneous immunization with formalin-treated C. difficile toxin A (CDA) induces systemic and mucosal anti-CDA immune responses, we transcutaneously immunized three cohorts of mice with CDA with or without immunoadjuvantative cholera toxin (CT) on days 0, 14, 28, and 42. Mice transcutaneously immunized with CDA and CT developed prominent anti-CDA and anti-CT immunoglobulin G (IgG) and IgA responses in serum and anti-CDA and anti-CT IgA responses in stool. Sera from immunized mice were able to neutralize C. difficile toxin A activity in an in vitro cell culture assay. CDA itself demonstrated adjuvant activity and enhanced both serum and stool anti-CT IgA responses. Our results suggest that transcutaneous immunization with CDA toxoid may be a feasible immunization strategy against C. difficile, an important cause of morbidity and mortality against which current preventative strategies are failing.

Production of Escherichia coli heat labile toxin (LT) B subunit in soybean seed and analysis of its immunogenicity as an oral vaccine

The B subunit of the heat labile toxin of enterotoxigenic Escherichia coli (LTB) was used as a model immunogen for production in soybean seed. LTB expression was directed to the endoplasmic reticulum (ER) of seed storage parenchyma cells for sequestration in de novo synthesized inert protein accretions derived from the ER. Pentameric LTB accumulated to 2.4% of the total seed protein at maturity and was stable in desiccated seed. LTB-soybean extracts administered orally to mice induced both systemic IgG and IgA, and mucosal IgA antibody responses, and was particularly efficacious when used in a parenteral prime-oral gavage boost immunization strategy. Sera from immunized mice blocked ligand binding in vitro and immunized mice exhibited partial protection against LT challenge. Moreover, soybean-expressed LTB stimulated the antibody response against a co-administered antigen by 500-fold. These results demonstrate the utility of soybean as an efficient production platform for vaccines that can be used for oral delivery.

Transcutaneous immunization with toxin-coregulated pilin A induces protective immunity against Vibrio cholerae O1 El Tor challenge in mice

Toxin-coregulated pilin A (TcpA) is the main structural subunit of a type IV bundle-forming pilus of Vibrio cholerae, the cause of cholera. Toxin-coregulated pilus is involved in formation of microcolonies of V. cholerae at the intestinal surface, and strains of V. cholerae deficient in TcpA are attenuated and unable to colonize intestinal surfaces. Anti-TcpA immunity is common in humans recovering from cholera in Bangladesh, and immunization against TcpA is protective in murine V. cholerae models. To evaluate whether transcutaneously applied TcpA is immunogenic, we transcutaneously immunized mice with 100 mug of TcpA or TcpA with an immunoadjuvant (cholera toxin [CT], 50 mug) on days 0, 19, and 40. Mice immunized with TcpA alone did not develop anti-TcpA responses. Mice that received transcutaneously applied TcpA and CT developed prominent anti-TcpA immunoglobulin G (IgG) serum responses but minimal anti-TcpA IgA. Transcutaneous immunization with CT induced prominent IgG and IgA anti-CT serum responses. In an infant mouse model, offspring born to dams transcutaneously immunized either with TcpA and CT or with CT alone were challenged with 10(6) CFU (one 50% lethal dose) wild-type V. cholerae O1 El Tor strain N16961. At 48 h, mice born to females transcutaneously immunized with CT alone had 36% +/- 10% (mean +/- standard error of the mean) survival, while mice born to females transcutaneously immunized with TcpA and CT had 69% +/- 6% survival (P < 0.001). Our results suggest that transcutaneous immunization with TcpA and an immunoadjuvant induces protective anti-TcpA immune responses. Anti-TcpA responses may contribute to an optimal cholera vaccine.

Octaplex PCR and fluorescence-based capillary electrophoresis for identification of human diarrheagenic Escherichia coli and Shigella spp

A multiplex PCR assay, amplifying seven specific virulence genes and one internal control gene in a single reaction, was developed to identify the five main pathotypes of diarrheagenic Escherichia coli and Shigella spp. The virulence genes selected for each category were Stx1, Stx2, and eaeA for enterohemorrhagic E. coli (EHEC), eaeA for enteropathogenic E. coli (EPEC), STIb and LTI for enterotoxigenic E. coli (ETEC), ipaH for enteroinvasive E. coli (EIEC) and Shigella spp., and aggR for enteroaggregative E. coli (EAEC). Each forward primer was labelled with a fluorochrome and the PCR products were separated by multicolour capillary electrophoresis on an ABI PRISM310 Genetic Analyzer (Applied Biosystems). If present, several gene variants of each virulence gene were identified. The internal control gene rrs, encoding 16S rRNA, was amplified in all 110 clinical strains analyzed. Virulence genes were demonstrated in 103 (94%) of these strains. In the majority of the cases (98/103, 95%), classification obtained by the novel multiplex PCR assay was in agreement with that previously determined by phenotypic assays combined with other molecular genetic approaches. Numerous multiplex PCR assays have been published, but only a few of them detect all five E. coli pathotypes within a single reaction, and none of them has used multicolour capillary electrophoresis to separate the PCR products. The octaplex PCR assay followed by capillary electrophoresis presented in the present paper provides a simple, reliable, and rapid procedure that in a single reaction identifies the five main pathotypes of E. coli, and Shigella spp. This assay will replace the previous molecular genetic methods used in our laboratory and work as an important supplement to the more time-consuming phenotypic assays.

Live attenuated oral cholera vaccines

Live, orally administered, attenuated vaccine strains of Vibrio cholerae have many theoretical advantages over killed vaccines. A single oral inoculation could result in intestinal colonization and rapid immune responses, obviating the need for repetitive dosing. Live V. cholerae organisms can also respond to the intestinal environment and immunological exposure to in vivo expressed bacterial products, which could result in improved immunological protection against wild-type V. cholerae infection. The concern remains that live oral cholera vaccines may be less effective among partially immune individuals in cholera endemic areas as pre-existing antibodies can inhibit live organisms and decrease colonization of the gut. A number of live oral cholera vaccines have been developed to protect against cholera caused by the classical and El Tor serotypes of V. cholerae O1, including CVD 103-HgR, Peru-15 and V. cholerae 638. A number of live oral cholera vaccines have also been similarly developed to protect against cholera caused by V. cholerae O139, including CVD 112 and Bengal-15. Live, orally administered, attenuated cholera vaccines are in various stages of development and evaluation.

Induction and recall of immune memory by mucosal immunization with a non-toxic recombinant enterotoxin-based chimeric protein

Previous reports have suggested that peroral delivery of antigens chemically coupled to non-toxic recombinant enterotoxin B subunits, such as the cholera toxin B subunit (CTB), induces tolerance to the antigen that may be abrogated by the toxic enzyme activity of intact enterotoxins, such as cholera toxin (CT). The aim of this study was to examine the immunogenicity of a genetically coupled protein composed of the saliva-binding region (SBR) of the Streptococcus mutans surface antigen AgI/II and the non-toxic A2 and B subunits of CT (SBR-CTA2/B) compared with that of recombinant SBR admixed with CT (SBR + CT) and SBR chemically coupled to recombinant CTB (SBR-CTB) following peroral delivery by intragastric (i.g.) immunization. The results showed that i.g. immunization with SBR-CTA2/B, like SBR + CT, induced antigen-specific serum immunoglobulin G (IgG) and salivary IgA antibodies, and sensitized splenic T cells. Comparison studies with SBR-CTB produced serum IgG but not salivary IgA titres and failed to sensitize splenic cells. Immunization with SBR-CTA2/B via the intranasal route also primed for the recall of antigen-specific memory antibody responses 6 months later. These findings show that SBR-CTA2/B is an immunogenic, not tolerogenic, chimeric protein that can induce and recall antigen-specific memory responses upon mucosal immunization.

Protection against aerosolized Yersinia pestis challenge following homologous and heterologous prime-boost with recombinant plague antigens

A Yersinia pestis-derived fusion protein (F1-V) has shown great promise as a protective antigen against aerosol challenge with Y. pestis in murine studies. In the current study, we examined different prime-boost regimens with F1-V and demonstrate that (i) boosting by a route other than the route used for the priming dose (heterologous boosting) protects mice as well as homologous boosting against aerosol challenge with Y. pestis, (ii) parenteral immunization is not required to protect mice against aerosolized plague challenge, (iii) the route of immunization and choice of adjuvant influence the magnitude of the antibody response as well as the immunoglobulin G1 (IgG1)/IgG2a ratio, and (iv) inclusion of an appropriate adjuvant is critical for nonparenteral immunization.

Mucosal adjuvants

Induction of immune responses following oral immunization is frequently dependent upon the co-administration of appropriate adjuvants that can initiate and support the transition from innate to adaptive immunity. The three bacterial products with the greatest potential to function as mucosal adjuvants are the ADP-ribosylating enterotoxins (cholera toxin and the heat-labile enterotoxin of Escherichia coli), synthetic oligodeoxynucleotides containing unmethylated CpG dinucleotides (CpG ODN), and monophosphoryl lipid A (MPL). The mechanism of adjuvanticity of the ADP-ribosylating enterotoxins is the subject of considerable debate. Our own view is that adjuvanticity is an outcome and not an event. It is likely that these molecules exert their adjuvant function by interacting with a variety of cell types, including epithelial cells, dendritic cells, macrophages, and possibly B- and T-lymphocytes. The adjuvant activities of CpG and MPL are due to several different effects they have on innate and adaptive immune responses and both MPL and CpG act through MyD88-dependent and -independent pathways. This presentation will summarize the probable mechanisms of action of these diverse mucosal adjuvants and discuss potential synergy between these molecules for use in conjunction with plant-derived vaccines.

Improved immune responses to influenza vaccination in the elderly using an immunostimulant patch

The elderly have greater morbidity and mortality due to influenza, and respond poorly to influenza vaccination compared to younger adults. This study was designed to determine if the adjuvant heat-labile enterotoxin from Escherichia coli (LT), administered as an immunostimulant (IS) patch on the skin with influenza vaccination, improves influenza immune responses in the elderly. Three weeks following vaccination, hemagglutination inhibition (HAI) responses in LT IS patch recipients showed improvement over those of elderly receiving vaccine alone, as demonstrated by significance or trends in fold rise [A/Panama (P = 0.004), A/New Caledonia (P = 0.09)], seroconversion [A/New Caledonia (63% versus 40%, P = 0.01), A/Panama (54% versus 36%, P = 0.08)] and seroprotection [26%, 20% and 16% greater for the patch group for A/New Caledonia, A/Panama and B/Shandong strains, respectively]. The data suggest that an LT IS patch may further enhance influenza vaccine immune responses in the elderly.

Effect of homologous and heterologous prime–boost on the immune response to recombinant plague antigens

Among the pathogens that have been identified as potential agents of biological warfare or bioterrorism, Yersinia pestis is one of the main concerns due to the severity and potential transmissibility of the pneumonic form of the disease in humans. There are no approved vaccines for protection against pneumonic plague, but a Y. pestis-derived fusion protein (F1-V) has shown great promise as a protective antigen in murine studies. In the current study, we examine different prime-boost regimens, including parenteral, mucosal, and transcutaneous delivery, in order to explore the effect of changing the route of prime and boost on the ability of recombinant F1-V to promote the development of long-lasting, high-titer antibodies. The most significant findings of the study reported here are that (1) intranasal and subcutaneous immunizations are both effective and essentially equivalent for induction of serum and bronchioalveolar anti-F1-V IgG1 responses when a single booster dose is administered by the same (homologous) route, (2) heterologous boosting can be as or more effective than homologous boosting for induction of either serum or bronchioalveolar anti-F1-V IgG1 responses, and (3) anti-F1 and anti-V total IgG responses were highest in animals primed intranasally and boosted by any route when compared to animals primed transcutaneously or subcutaneously. As with previously published studies, there were still significant levels of circulating anti-F1-V antibodies 1 year post-primary immunization. These studies provide important insights into the development of new-generation biodefense vaccines.

Mucosal immunity: overcoming the barrier for induction of proximal responses

Vaccination represents one of the most efficacious and cost-effective medical interventions. It is the only medical intervention proven to eliminate disease at a global level. Many of the pathogens against which we most require adequate vaccines infect via the highly exposed mucosal surfaces. For this reason the mucosa is often considered the first, and sometimes only, line of defense. Therefore, responses that protect the local mucosa are vital. In this review, we first explore the immunological mechanisms that protect the mucosa. We then review the literature of mucosal vaccines within the principles of antigenic composition, dose, and danger, highlighting the need and niche for the next generation of mucosal vaccines.

New generation of mucosal adjuvants for the induction of protective immunity

Invasion of infectious agents through mucosal surfaces can be prevented by use of the common mucosal immune system (CMIS), which interconnects inductive tissues, including Peyer's patches (PPs) and nasopharyngeal-associated lymphoreticular tissue (NALT), and effector tissues of the intestinal and respiratory tracts. In order for the CMIS to induce maximal protective mucosal immunity, co-administration of mucosal adjuvant has been shown to be essential. When vaccine antigen is administered together with mucosal adjuvant, antigen-specific T-helper (Th) 1 and Th2 cells, cytotoxic T lymphocytes (CTLs) and IgA B cell responses are effectively induced by oral or nasal routes via the CMIS. In the early stages of induction of mucosal immune response, the uptake of orally or nasally administered antigens is achieved through a unique set of antigen-sampling cells, M cells located in follicle-associated epithelium (FAE) of inductive sites. After successful uptake, the antigens are immediately processed and presented by the underlying dendritic cells (DCs). Elucidation of the molecular/cellular characteristics of M cells and mucosal DCs will greatly facilitate the design of a new generation of effective mucosal adjuvants and of a vaccine delivery vehicle that maximises the use of the CMIS. Our recent efforts at mucosal vaccine development have focused on nasal administration of vaccine antigen together with nontoxic mutant-based or cytokine-/chemokine-based adjuvant for the induction of the protective immunity. To this end, a chimeric form of a nontoxic adjuvant combining the merits of mutant cholera toxin A subunit (mCT-A) and heat labile toxin B subunit (LT-B) was created as the second generation of detoxified toxin-based mucosal adjuvant. When a vaccine antigen was coexpressed together with an immune stimulatory/delivery molecule in crop seed, this edible vaccine is not only effective but also extremely practical in that it can be produced in huge quantities and preserved and shipped over long distances at room temperature without altering the quality of the vaccine. Because such qualities would greatly facilitate global vaccination, this new generation edible vaccines with a built-in adjuvant and/or M cell-targeted edible vaccine promises to be a powerful weapon for combating infectious diseases and bioterrorism.

Oral immunization with ATP-dependent protease-deficient mutants protects mice against subsequent oral challenge with virulent Salmonella enterica serovar typhimurium

We evaluated the efficacy of mutants with a deletion of the stress response protease gene as candidates for live oral vaccine strains against Salmonella infection through infection studies with mice by using a Salmonella enterica serovar Typhimurium mutant with a disruption of the ClpXP or Lon protease. In vitro, the ClpXP protease regulates flagellum synthesis and the ClpXP-deficient mutant strain exhibits hyperflagellated bacterial cells (T. Tomoyasu et al., J. Bacteriol. 184:645-653, 2002). On the other hand, the Lon protease negatively regulates the efficacy of invading epithelial cells and the expression of invasion genes (A. Takaya et al., J. Bacteriol. 184:224-232, 2002). When 5-week-old BALB/c mice were orally administered 5 x 10(8) CFU of the ClpXP- or Lon-deficient strain, bacteria were detected with 10(3) to 10(4) CFU in the spleen, mesenteric lymph nodes, Peyer's patches, and cecum 1 week after inoculation and the bacteria then decreased gradually in each tissue. Significant increases of lipopolysaccharide-specific immunoglobulin G (IgG) and secretory IgA were detected at week 4 and maintained until at least week 12 after inoculation in serum and bile, respectively. Immunization with the ClpXP- or Lon-deficient strain protected mice against oral challenge with the serovar Typhimurium virulent strain. Both the challenged virulent and immunized avirulent salmonellae were completely cleared from the spleen, mesenteric lymph nodes, Peyer's patches, and even cecum 5 days after the challenge. These data indicate that Salmonella with a disruption of the ATP-dependent protease ClpXP or Lon can be useful in developing a live vaccine strain.

Comparison of mucosal and systemic humoral immune responses after transcutaneous and oral immunization strategies

In order to compare the ability of transcutaneous and oral immunization strategies to induce mucosal and systemic immune responses, we inoculated mice transcutaneously with cholera toxin (CT) or the non-toxic B subunit of cholera toxin (CtxB), or orally with Peru2(pETR1), an attenuated vaccine strain of Vibrio cholerae expressing CtxB. In addition, we also evaluated dual immunization regimens (oral inoculation with transcutaneous boosting, and transcutaneous immunization with oral boosting) in an attempt to optimize induction of both mucosal and systemic immune responses. We found that transcutaneous immunization with purified CtxB or CT induces much more prominent systemic IgG anti-CtxB responses than does oral inoculation with a vaccine vector strain of V. cholerae expressing CtxB. In comparison, anti-CtxB IgA in serum, stool and bile were comparable in mice either transcutaneously or orally immunized. Overall, the most prominent systemic and mucosal anti-CtxB responses occurred in mice that were orally primed with Peru2(pETR1) and transcutaneously boosted with CT. Our results suggest that combination oral and transcutaneous immunization strategies may most prominently induce both mucosal and systemic humoral responses.

Plasmid vectors encoding cholera toxin or the heat-labile enterotoxin from Escherichia coli are strong adjuvants for DNA vaccines

Two plasmid vectors encoding the A and B subunits of cholera toxin (CT) and two additional vectors encoding the A and B subunits of the Escherichia coli heat-labile enterotoxin (LT) were evaluated for their ability to serve as genetic adjuvants for particle-mediated DNA vaccines administered to the epidermis of laboratory animals. Both the CT and the LT vectors strongly augmented Th1 cytokine responses (gamma interferon [IFN-gamma]) to multiple viral antigens when codelivered with DNA vaccines. In addition, Th2 cytokine responses (interleukin 4 [IL-4]) were also augmented by both sets of vectors, with the effects of the LT vectors on IL-4 responses being more antigen dependent. The activities of both sets of vectors on antibody responses were antigen dependent and ranged from no effect to sharp reductions in the immunoglobulin G1 (IgG1)-to-IgG2a ratios. Overall, the LT vectors exhibited stronger adjuvant effects in terms of T-cell responses than did the CT vectors, and this was correlated with the induction of greater levels of cyclic AMP by the LT vectors following vector transfection into cultured cells. The adjuvant effects observed in vivo were due to the biological effects of the encoded proteins and not due to CpG motifs in the bacterial genes. Interestingly, the individual LT A and B subunit vectors exhibited partial adjuvant activity that was strongly influenced by the presence or absence of signal peptide coding sequences directing the encoded subunit to either intracellular or extracellular locations. Particle-mediated delivery of either the CT or LT adjuvant vectors in rodents and domestic pigs was well tolerated, suggesting that bacterial toxin-based genetic adjuvants may be a safe and effective strategy to enhance the potency of both prophylactic and therapeutic DNA vaccines for the induction of strong cellular immunity.

Immunomodulators and delivery systems for vaccination by mucosal routes

Current paediatric immunization programmes include too many injections in the first months of life. Oral or nasal vaccine delivery eliminates the requirement for needles and can induce immunity at the site of infection. However, protein antigens are poorly immunogenic when so delivered and can induce tolerance. Novel ways to enhance immune responses to protein or polysaccharide antigens have opened up new possibilities for the design of effective mucosal vaccines. Here, we discuss the immunological principles underlying mucosal vaccine development and review the application of immunomodulatory molecules and delivery systems to the selective enhancement of protective immune responses at mucosal surfaces.

Facilitated intranasal induction of mucosal and systemic immunity to mutans streptococcal glucosyltransferase peptide vaccines

Synthetic peptide vaccines which are derived from functional domains of Streptococcus mutans glucosyltransferases (GTF) have been shown to induce protective immunity in Sprague-Dawley rats after subcutaneous injection in the salivary gland region. Since mucosal induction of salivary immunity would be preferable in humans, we explored methods to induce mucosal antibody in the rat to the GTF peptide vaccines HDS and HDS-GLU after intranasal administration. Several methods of facilitation of the immune response were studied: the incorporation of peptides in bioadhesive poly(D,L-lactide-coglycolide) (PLGA) microparticles, the use of monoepitopic (HDS) or diepitopic (HDS-GLU) peptide constructs, or the use of mucosal adjuvants. Salivary immunoglobulin A (IgA) responses were not detected after intranasal administration of diepitopic HDS-GLU peptide constructs in alum or after incorporation into PLGA microparticles. However, significant primary and secondary salivary IgA and serum IgG antibody responses to HDS were induced in all rats when cholera holotoxin (CT) or a detoxified mutant Escherichia coli heat-labile enterotoxin (R192G LT) were intranasally administered with HDS peptide constructs in PLGA. Coadministration of LT with HDS resulted in predominantly IgG2a responses in the serum, while coadministration with CT resulted in significant IgG1 and IgG2a responses to HDS. Serum IgG antibody, which was induced to the HDS peptide construct by coadministration with these adjuvants, also bound intact mutans streptococcal GTF in an enzyme-linked immunosorbent assay and inhibited its enzymatic activity. Thus, immune responses which are potentially protective for dental caries can be induced to peptide-based GTF vaccines after mucosal administration if combined with the CT or LT R192G mucosal adjuvant.

Attenuated Shigella flexneri 2a vaccine strain CVD 1204 expressing colonization factor antigen I and mutant heat-labile enterotoxin of enterotoxigenic Escherichia coli

A multivalent live oral vaccine against both Shigella spp. and enterotoxigenic Escherichia coli (ETEC) is being developed based on the hypothesis that protection can be achieved if attenuated shigellae express ETEC fimbrial colonization factors and genetically detoxified heat-labile toxin from a human ETEC isolate (LTh). Two detoxified derivatives of LTh, LThK63 and LThR72, were engineered by substitution-serine to lysine at residue 63, or lysine to arginine at residue 72. The genes encoding these two derivatives were cloned separately on expression plasmids downstream from the CFA/I operon. Following electroporation into S. flexneri 2a vaccine strain CVD 1204, coexpression of CFA/I and LThK63 or LThR72 was demonstrated by Western blot analysis, GM(1) binding assays, and agglutination with anti-CFA/I antiserum. Hemagglutination and electron microscopy confirmed surface expression of CFA/I. Guinea pigs immunized intranasally on days 0 and 15 with CVD 1204 expressing CFA/I and LThK63 or LThR72 exhibited high titers of both serum immunoglobulin G (IgG) and mucosal secretory IgA anti-CFA/I; 40% of the animals produced antibodies directed against LTh. All immunized guinea pigs also produced mucosal IgA (in tears) and serum IgG anti-S. flexneri 2a O antibodies. Furthermore, all immunized animals were protected from challenge with wild-type S. flexneri 2a. This prototype Shigella-ETEC hybrid vaccine demonstrates the feasibility of expressing multiple ETEC antigens on a single plasmid in an attenuated Shigella vaccine strain and engendering immune responses against both the heterologous antigens and vector strain.

Cholera vaccines

Cholera causes significant morbidity and mortality worldwide. For travelers, the risk of developing cholera per month of stay in a developing country is approximately 0.001%-0.01%, and cholera may present as traveler's diarrhea. In the United States, only a poorly tolerated, marginally effective, parenterally administered, phenol-inactivated vaccine is available. Outside the United States, 2 additional vaccines are commercially available: an oral killed whole cell-cholera toxin recombinant B subunit vaccine (WC-rBS) and an oral live attenuated Vibrio cholerae vaccine (CVD 103-HgR). These oral vaccines are well tolerated. In field trials, WC-rBS provides 80%-85% protection from cholera caused by V. cholerae serogroup O1 for at least 6 months. In volunteer studies, CVD 103-HgR provides 62%-100% protection against cholera caused by V. cholerae for at least 6 months. No commercially available cholera vaccine protects against disease caused by V. cholerae serogroup O139. New cholera vaccines are being developed.

Recent advances in mucosal vaccine development

Proper stimulation of the mucosal immune system is critical for the effective protection of mucosal surfaces against colonization and invasion of infectious agents. This requires administration of vaccine antigens directly to various mucosal sites. Due to the low absorption efficiency of mucosally delivered vaccines, however, almost all of the currently marketed vaccines are administered parentally. In addition, sub-optimal immune responses are frequently induced by mucosal immunization and the use of mucosal adjuvants is commonly required. As a result, development of successful mucosal vaccines depends largely on the improvement of mucosal antigen delivery and on the discovery of new and effective mucosal adjuvants. In this review, recent advances in both areas are briefly discussed.

Adjuvant activity of a nontoxic mutant of Escherichia coli heat-labile enterotoxin on systemic and mucosal immune responses elicited against a heterologous antigen carried by a live Salmonella enterica serovar Typhimurium vaccine strain

Systemic and mucosal antibody responses against both the major subunit of colonization factor antigen I (CFA/I) of enterotoxigenic Escherichia coli (ETEC) and the somatic lipopolysaccharide expressed by recombinant bivalent Salmonella vaccine strains were significantly enhanced by coadministration of a detoxified derivative with preserved adjuvant effects of the ETEC heat-labile toxin, LT((R192G)). The results further support the adjuvant effects of LT((R192G)) and represent a simple alternative to improve responses against passenger antigens expressed by orally delivered Salmonella vaccine strains.

In vitro and in vivo analyses of constitutive and in vivo-induced promoters in attenuated vaccine and vector strains of Vibrio cholerae

The optimal promoter for in vivo expression of heterologous antigens by live, attenuated vaccine vector strains of Vibrio cholerae is unclear; in vitro analyses of promoter activity may not accurately predict expression of antigens in vivo. We therefore introduced plasmids expressing the B subunit of cholera toxin (CtxB) under the control of a number of promoters into V. cholerae vaccine strain Peru2. We evaluated the tac promoter, which is constitutively expressed in V. cholerae, as well as the in vivo-induced V. cholerae heat shock htpG promoter and the in vivo-induced V. cholerae iron-regulated irgA promoter. The functionality of all promoters was confirmed in vitro. In vitro antigenic expression was highest in vaccine strains expressing CtxB under the control of the tac promoter (2 to 5 microgram/ml/unit of optical density at 600 nm [OD(600)]) and, under low-iron conditions, in strains containing the irgA promoter (5 microgram/ml/OD(600)). We orally inoculated mice with the various vaccine strains and used anti-CtxB immune responses as a marker for in vivo expression of CtxB. The vaccine strain expressing CtxB under the control of the tac promoter elicited the most prominent specific anti-CtxB responses in vivo (serum immunoglobulin G [IgG], P </= 0.05; serum IgA, P </= 0.05; stool IgA, P </= 0.05; bile IgA, P </= 0.05), despite the finding that the tac and irgA promoters expressed equivalent amounts of CtxB in vitro. Vibriocidal antibody titers were equivalent in all groups of animals. Our results indicate that in vitro assessment of antigen expression by vaccine and vector strains of V. cholerae may correlate poorly with immune responses in vivo and that of the promoters examined, the tac promoter may be best suited for expression from plasmids of at least certain heterologous antigens in such strains.

Optimizing the germfree mouse model for in vivo evaluation of oral Vibrio cholerae vaccine and vector strains

The germfree mouse model of Vibrio cholerae infection can be used to judge immune responses to V. cholerae vaccine and vector strains. In the original model, a single oral inoculation was administered on day 0, a booster oral inoculation was administered on day 14, and immune responses were analyzed with samples collected on day 28. Unfortunately, immune responses in this model frequently were low level, and interanimal variability occurred. In order to improve this model, we evaluated various primary and booster V. cholerae inoculation schedules. The most prominent systemic and mucosal antibody responses were measured in mice that received a multiple primary inoculation series on days 0, 2, 4, and 6 and booster inoculations on days 28 and 42. These modifications result in improved preliminary evaluation of V. cholerae vaccine and vector strains in mice.

Development of a DeltaglnA balanced lethal plasmid system for expression of heterologous antigens by attenuated vaccine vector strains of Vibrio cholerae

We have previously shown that more prominent immune responses are induced to antigens expressed from multicopy plasmids in live attenuated vaccine vector strains of Vibrio cholerae than to antigens expressed from single-copy genes on the V. cholerae chromosome. Here, we report the construction of a DeltaglnA derivative of V. cholerae vaccine strain Peru2. This mutant strain, Peru2DeltaglnA, is unable to grow on medium that does not contain glutamine; this growth deficiency is complemented by pKEK71-NotI, a plasmid containing a complete copy of the Salmonella typhimurium glnA gene, or by pTIC5, a derivative of pKEK71-NotI containing a 1. 8-kbp fragment that directs expression of CtxB with a 12-amino-acid epitope of the serine-rich Entamoeba histolytica protein fused to the amino terminus. Strain Peru2DeltaglnA(pTIC5) produced 10-fold more SREHP-12-CtxB in supernatants than did ETR3, a Peru2-derivative strain containing the same fragment inserted on the chromosome. To assess immune responses to antigens expressed by this balanced lethal system in vivo, we inoculated germfree mice on days 0, 14, 28, and 42 with Peru2DeltaglnA, Peru2DeltaglnA(pKEK71-NotI), Peru2(pTIC5), Peru2DeltaglnA(pTIC5), or ETR3. All V. cholerae strains were recoverable from stool for 8 to 12 days after primary inoculation, including Peru2DeltaglnA; strains containing plasmids continued to harbor pKEK71-NotI or pTIC5 for 8 to 10 days after primary inoculation. Animals were sacrificed on day 56, and serum, stool and biliary samples were analyzed for immune responses. Vibriocidal antibody responses, reflective of in vivo colonization, were equivalent in all groups of animals. However, specific anti-CtxB immune responses in serum (P </= 0.05) and bile (P </= 0. 001) were significantly higher in animals that received Peru2DeltaglnA(pTIC5) than in those that received ETR3, confirming the advantage of higher-level antigen expression in vivo. The development of this balanced lethal system thus permits construction and maintenance of vaccine and vector strains of V. cholerae that express high levels of immunogenic antigens from plasmid vectors without the need for antibiotic selection pressure.