Native and mutant forms of cholera toxin and heat-labile enterotoxin effectively enhance protective efficacy of live attenuated and heat-killed Shigella vaccines

Live attenuated enterotoxigenic Escherichia coli (ETEC) vaccine with dmLT adjuvant protects human volunteers against virulent experimental ETEC challenge

Background

There is no licensed vaccine against enterotoxigenic Escherichia coli (ETEC), a major cause of diarrhea-associated morbidity and mortality among infants and children in low-income countries and travelers. The results of this vaccination/challenge study demonstrate strong protection by an attenuated ETEC vaccine candidate, ACE527, when co-administered with a mucosal adjuvant, the double-mutant heat-labile toxin (dmLT) of ETEC.

Methods

Sixty healthy adults participated in a randomized, placebo-controlled, double-blind study with three doses of lyophilized ACE527 (∼3 × 10⁹ of each strain per dose) administered orally with or without dmLT adjuvant (25 µg/dose). Six months later, 36 of these volunteers and a control group of 21 unvaccinated volunteers were challenged with virulent ETEC strain H10407. The primary outcome was severe diarrhea, defined as passing >800 g of unformed stools during the inpatient period following challenge.

Findings

The vaccine was well tolerated and induced robust immune responses to key antigens. The protective efficacy (PE) against the primary outcome of severe diarrhea was 65.9% (95% confidence interval [CI] 5.4–87.7, p = 0.003). Among subjects receiving the adjuvanted vaccine, the attack rate of severe diarrhea was 23.1, while in unimmunized controls it was 67.7%. The PE against diarrhea of any severity was 58.5% (95% CI 3.8– 82.1, p = 0.016). There was a strong inverse correlation between shedding of the vaccine strain after either of the first two doses and absence of severe diarrhea upon challenge (RR = 0.29, 95% CI 0.08–1.05, p = 0.041). Challenge strain shedding was 10-fold lower in those receiving the adjuvant than in those receiving vaccine alone. The unadjuvanted vaccine was not protective (PE = 23.1%).

Interpretation

The results of this study support further development of ACE527 + dmLT as a vaccine for children in endemic countries and travelers. This is the first clinical demonstration that dmLT can contribute significantly to vaccine efficacy and may warrant testing with other oral vaccines.

(ClinicalTrials.gov registration: NCT01739231).

Sublingual Adjuvant Delivery by a Live Attenuated Vibrio cholerae-Based Antigen Presentation Platform

Diarrheal disease is the most common infectious disease of children in the developing world. Our goal is to develop a diarrheal antigen presentation platform based on whole Vibrio cholerae cells that does not depend on protein purification. We have previously shown the feasibility of genetically fusing antigens to the V. cholerae biofilm matrix protein RbmA for presentation on the cell surface. A mucosal adjuvant could improve immunogenicity of such a vaccine at the mucosal surface. Here we engineer a live attenuated V. cholerae vaccine to constitutively synthesize mmCT, a nontoxic form of cholera toxin. When this vaccine is delivered sublingually, in vivo-synthesized mmCT acts as both an adjuvant and antigen. This could greatly increase the magnitude and duration of the immune response elicited by codelivered heterologous antigens.

A sublingually delivered heterologous antigen presentation platform that does not depend on antigen or adjuvant purification would be of great benefit in protection against diarrheal disease. In proof-of-concept studies, we previously showed that when a fusion protein comprised of the Vibrio cholerae biofilm matrix protein RbmA and the B subunit of cholera toxin (R-CTB) is expressed from a plasmid within V. cholerae, R-CTB is sequestered in the biofilm matrix, leading to decoration of the cell surface. Sublingual delivery of live attenuated R-CTB-decorated cells results in a mucosal immune response to CTB. To improve the immune response to diarrheal antigens presented by this platform, we have engineered our live attenuated vaccine to express the mucosal adjuvant mmCT (i.e., multiply mutated CT). Here we report that delivery of this adjuvant via sublingual administration of our vaccine enhances the mucosal immune response to V. cholerae LPS and elicits a systemic and mucosal immune response to CTB. However, provision of R-CTB with mmCT selectively blunts the mucosal immune response to CTB. We propose that mmCT delivered by this live attenuated Vibrio cholerae vaccine platform may serve as a mucosal adjuvant for heterologous antigens, provided they are not too similar to mmCT.

IMPORTANCE Diarrheal disease is the most common infectious disease of children in the developing world. Our goal is to develop a diarrheal antigen presentation platform based on whole Vibrio cholerae cells that does not depend on protein purification. We have previously shown the feasibility of genetically fusing antigens to the V. cholerae biofilm matrix protein RbmA for presentation on the cell surface. A mucosal adjuvant could improve immunogenicity of such a vaccine at the mucosal surface. Here we engineer a live attenuated V. cholerae vaccine to constitutively synthesize mmCT, a nontoxic form of cholera toxin. When this vaccine is delivered sublingually, in vivo-synthesized mmCT acts as both an adjuvant and antigen. This could greatly increase the magnitude and duration of the immune response elicited by codelivered heterologous antigens.

A mutated cholera toxin without the ADP-ribosyltransferase activity induces cytokine production and inhibits apoptosis of splenocytes in mice possibly via toll-like receptor-4 signaling

Native cholera toxin (CT) and its mutated form (CT-2*) without ADP-ribosyltransferase activity differ in their immunomodulatory effects on host cells, and the mechanisms of these differences are poorly understood. In this study, we demonstrated that CT-2* induced higher levels of cytokine production and down-regulated ex-vivo apoptosis of splenocytes from C57BL/6 mice. After exposure of the splenocytes ex-vivo to CT or CT-2* (2μg/ml) for 48h, CT-2* stimulated expression of the toll-like receptor (TLR-4) gene was much higher and the cells produced increased levels of interleukin (IL)-12, interferon (IFN)-γ, and tumor necrosis factor (TNF)-α, compared to splenocytes of mice exposed to native CT. We confirmed these findings by observing that CT-2*, induced much lower levels of IL-12, IFN-γ, and TNF-α in a TLR-4 knockout macrophage cell line derived from C57BL/6 mice. In addition, while CT is known to stimulate apoptosis in splenocytes, we observed that CT-2* significantly down-regulated apoptosis (4.2%), compared to splenocytes exposed to CT (18.7%) or PBS (negative control, 8.5%). On the contrary, we noted both native CT and CT-2* to exhibit similar levels of apoptosis in TLR-4(-/-) cell line. Overall, the evidence supports the conclusion that CT-2* modulated cytokine production and apoptosis in splenocytes of mice possibly through the TLR-4 signaling pathway.

An environmental Escherichia albertii strain, DM104, induces protective immunity to Shigella dysenteriae in guinea pig eye model

The environmental Escherichia albertii strain DM104, which cross-reacts serologically with Shigella dysenteriae was assessed for pathogenic properties, immunogenicity, and protective efficacy in different animal models to evaluate it as a vaccine candidate against S. dysenteriae, which causes the severe disease, shigellosis. The DM104 isolate was found to be non-invasive and did not produce any entero- or cyto-toxins. The strain also showed negative results in the mouse lethal activity assay. The non-pathogenic DM104 strain gave, however, a high protective efficacy as an ocularly administered vaccine in the guinea pig eye model against S. dysenteriae type 4 challenge. It also induced a high titer of serum IgG against S. dysenteriae type 4 whole cell lysate and lipopolysaccharide. Taken together, all these results indicate a good potential for the use of the DM104 as a live vaccine candidate against shigellosis.

Development and preclinical evaluation of a trivalent, formalin-inactivated Shigella whole-cell vaccine

Studies were undertaken to manufacture a multivalent Shigella inactivated whole-cell vaccine that is safe, effective, and inexpensive. By using several formalin concentrations, temperatures, and incubation periods, an optimized set of inactivation conditions was established for Shigella flexneri 2a, S. sonnei, and S. flexneri 3a to produce inactivated whole cells expressing a full repertoire of Ipa proteins and lipopolysaccharide (LPS). The inactivation conditions selected were treatment with 0.2% formalin (S. flexneri 2a and 3a) or 0.6% formalin (S. sonnei) for 48 h at 25°C. Vaccine formulations prepared under different inactivation conditions, in different doses (10E5, 10E7, and 10E9 cells), and with or without the inclusion of double-mutant heat-labile toxin (dmLT) were evaluated in mice. Two intranasal immunizations with ≥10E7 inactivated whole cells resulted in high levels of anti-Invaplex and moderate levels of LPS-specific IgG and IgA in serum and in lung and intestinal wash samples. Addition of dmLT to the vaccine formulations did not significantly enhance humoral immunogenicity. Minimal humoral responses for IpaB, IpaC, or IpaD were detected after immunization with inactivated whole Shigella cells regardless of the vaccine inactivation conditions. In guinea pigs, monovalent formulations of S. flexneri 2a of 3a or S. sonnei consisting of 10E8, 10E9, or 10E10 cells were protective in a keratoconjunctivitis assay. A trivalent formulation provided protection against all three serotypes (S. flexneri 2a, P = 0.018; S. flexneri 3a, P = 0.04; S. sonnei, P < 0.0001). The inactivated Shigella whole-cell vaccine approach incorporates an uncomplicated manufacturing process that is compatible with multivalency and the future development of a broadly protective Shigella vaccine.

Recent progress towards development of a Shigella vaccine

The burden of dysentery due to shigellosis among children in the developing world is still a major concern. A safe and efficacious vaccine against this disease is a priority, since no licensed vaccine is available. This review provides an update of vaccine achievements focusing on subunit vaccine strategies and the forthcoming strategies surrounding this approach. In particular, this review explores several aspects of the pathogenesis of shigellosis and the elicited immune response as being the basis of vaccine requirements. The use of appropriate Shigella antigens, together with the right adjuvants, may offer safety, efficacy and more convenient delivery methods for massive worldwide vaccination campaigns.

Inactivated and subunit vaccines to prevent shigellosis

Shigellosis remains a formidable disease globally, with children of the developing world bearing the greatest number of infections. The need for an affordable, safe and efficacious vaccine has persisted for decades. Vaccines to prevent shigellosis can be divided into living and nonliving approaches. Several nonliving Shigella vaccines are currently at different stages of development and show substantial promise. Outlined here is an overview of multiple nonliving vaccine technologies, highlighting their current status and recent advances in testing. In addition, gaps in the knowledge base regarding immune mechanisms of protection are explored.

Vaccine potential for inactivated shigellae

We used human monocyte-derived dendritic cells (DC) and Balb/c mice as models to establish the immunogenic and protective potential of formalin-inactivated Shigella spp. Incubation of DC with inactivated or live bacteria induced DC maturation and cytokine release. Mice immunized orally or intranasally with killed S. flexneri, S. sonnei, or S. dysenteriae developed IgG and fecal IgA titers to the homologous LPS. Following respiratory challenge with the live homologous organisms, 80-100% survival was seen in all vaccinated groups compared to negligible survival in mice given PBS. Oral or intranasal immunization with an inactivated S. flexneri 2a strain (CVD1203) expressing the CFA/I and CS3 antigens of enterotoxigenic Escherichia coli induced IgG responses to both heterologous antigens. These in vivo and in vitro data indicate that inactivated shigellae retain the ability to interact effectively with key antigen presenting cells and induce protective immune responses in mice.

Safety and immunogenicity of an oral, inactivated, whole-cell vaccine for Shigella sonnei: preclinical studies and a Phase I trial

Orally delivered, inactivated whole-cell vaccines are safe methods of inducing local and systemic immunity. To increase surface proteins associated with adherence and invasion, Shigella sonnei were grown in BHI broth containing deoxycholate. A whole-cell vaccine (SsWC) was then produced by formalin inactivation. In pre-clinical studies, the SsWC vaccine was immunogenic and protected against S. sonnei-induced keratoconjunctivitis in the guinea pig model. In a randomized, double-blind, placebo-controlled, Phase I study, 10 evaluable subjects received either three doses of SsWC on Days 0, 14, and 28 (N = 3); five doses of SsWC on Days 0, 2, 4, 6, and 28 (N = 4); or placebo (N = 3). Each dose contained 2.0 x 10(10) inactivated cells. Serum and fecal antibodies against SsWC, LPS, and IpaC were measured by ELISA. A > or = 4-fold increase in titer was considered significant. Both SsWC dosing regimens were well tolerated. No fever or severe gastrointestinal symptoms were noted by any of the vaccinated subjects. Antibody responses were similar in the two dosing groups. Serum IgG or IgA responses to SsWC were seen in six of seven vaccinees (86%), to LPS in four of seven (57%), and to IpaC in five of seven (61%). Fecal IgA responses to these three antigens developed in five of five, three of five, and three of five subjects, respectively. Among the seven vaccinees, geometric mean rises in serum IgA levels to all three immunogens were significant; IgG increases trended toward significance (paired one-tailed t-test). We conclude that SsWC was immunogenic and protective in animal studies and well tolerated and immunogenic in a Phase I trial.

Oral spore vaccine based on live attenuated nontoxinogenic Bacillus anthracis expressing recombinant mutant protective antigen

An attenuated nontoxinogenic nonencapsulated Bacillus anthracis spore vaccine expressing high levels of recombinant mutant protective antigen (PA), which upon subcutaneous immunization provided protection against a lethal B. anthracis challenge, was found to have the potential to serve also as an oral vaccine. Guinea pigs immunized per os with the recombinant spore vaccine were primed to B. anthracis vegetative antigens as well as to PA, yet only a fraction of the animals (30% to 50%) mounted a humoral response to all of these antigens. Protective immunity provided by per os immunization correlated with a threshold level of PA neutralizing antibody titers and was long-lasting. Protection conferred by per os immunization was attained when the vaccine was administered in the sporogenic form, which, unlike the vegetative cells, survived passage through the gastrointestinal tract. A comparison of immunization of unirradiated spores with immunization of gamma-irradiated spores demonstrated that germination and de novo synthesis of PA were prerequisites for mounting an immune protective response. Oral immunization of guinea pigs with attenuated B. anthracis spores resulted in a characteristic anti-PA immunoglobulin isotype profile (immunoglobulin [G1 IgG1] versus IgG2), as well as induction of specific anti-PA secretory IgA, indicating development of mucosal immunity.

Considerations for development of whole cell bacterial vaccines to prevent diarrheal diseases in children in developing countries

Enteric pathogens constitute a major pediatric threat in the developing world through their impact on morbidity and mortality, physical and cognitive development and cause and effect relationship with malnutrition. Although many bacterial pathogens can cause diarrheal diseases, a group of less than 10 including Shigella spp., enterotoxigenic Escherichia coli (ETEC), Vibrio cholerae, and possibly, Campylobacter jejuni account for a significant percentage of these diseases in developing countries. Rotavirus is also a major cause of diarrheal diseases. Vaccines against these agents offer a potentially effective control measure against these diseases, but safe, practical, and effective vaccines for many of these agents have yet to be realized. Many vaccine development approaches are under investigation, but the one that is currently most advanced and that has been most widely applied to enteric pathogens is the use of orally administered live or killed whole pathogen preparations. If inactivated, these vaccines will probably be administered as multiple doses with approximately 10(10) to 10(11) total particles per dose, but they are relatively safe for oral administration. Further, they may not require a buffer for delivery and can be stored in liquid formulations. Fewer doses may be required for some live attenuated pathogen vaccines, but a buffer will most likely be required for oral delivery and the product must be stored in a dried formulation. Also, safety becomes more of a concern with live pathogens depending on the degree of attenuation, host immunocompetence, and the total number and kinds of attenuated pathogens which may be present in a combined agent vaccine. Both live and killed whole pathogen vaccines can be immunogenic and have the possibility to serve as vectors for other antigens. Although many organisms and serotypes are clinically important, by exploiting antigenic cross reactivity and using some pathogen components as vectors for cloned antigens of other pathogens, it could be possible to induce immunity against major enteric pathogens/serotypes with <10 whole pathogen components in a multi-agent vaccine. Safe and effective mucosal adjuvants may in the future be useful in whole pathogen vaccines, but they do not seem to be essential for immunization. Further, dietary supplements such as zinc, mixed routes of delivery and new regimens are under study which may in the future enhance further the effectiveness of the whole pathogen vaccines which now seem realizable in the near term. For this to happen, however, a coordinated and committed effort is necessary now to address the immunologic, regulatory, manufacturing, testing and implementation issues which will be involved in the realization of this important product to benefit children's health worldwide.

Intranasal immunization of guinea pigs with an immunodominant foot-and-mouth disease virus peptide conjugate induces mucosal and humoral antibodies and protection against challenge

Guinea pigs immunized intranasally with a keyhole limpet hemocyanin-linked peptide, corresponding to the prominent G-H loop of the VP1 protein of foot-and-mouth disease virus, raised substantial levels of antipeptide and virus-neutralizing antibodies in sera and of peptide-specific secretory immunoglobulin A in nasal secretions. In groups of animals immunized intranasally without adjuvant, 86 percent were fully protected upon challenge with homotypic virus. Surprisingly, animals given the peptide conjugates plus the mucosal adjuvant cholera toxin were afforded only partial protection in that primary lesions were observed in most animals, although spread to other feet was prevented. These results indicate that intranasal inoculation with the peptide offers a potential route of vaccination against foot-and-mouth disease and may be useful for eliciting protection in the upper respiratory tracts of susceptible animals.

Differences in immunogenicity and protection in mice and guinea pigs following intranasal immunization with Helicobacter pylori outer membrane antigens

Mice and guinea pigs were intranasally immunized with either recombinant lipoprotein 20 or Helicobacter pylori outer membrane vesicles (OMV). Cholera toxin was used as mucosal adjuvant. In mice, both vaccines elicited systemic and local IgG responses, which correlated with significantly lower levels of H. pylori colonization. In contrast, only OMV proved immunogenic in guinea pigs, with the development of both systemic and local immune responses. These antibodies did not, however, correlate with protection in these animals, which suggests that vaccine formulation is as important as choice of antigen in the development of an H. pylori vaccine.

Orogastric vaccination of guinea pigs with Helicobacter pylori sonicate and a high dose of cholera toxin lowers the burden of infection

Guinea pigs were vaccinated orogastrically with Helicobacter pylori cell sonicate (CS) and 10 microg or 100 microg cholera toxin (CT) or CT only. Nai;ve animals were used as a control. In both experiments, vaccination primed the local IgG and IgA response, irrespective of the CT dose. After challenge, only the group of animals immunised with CS and 100 microg CT had a significantly lower number of H. pylori in the antral region of the stomach, but vaccination did not prevent H. pylori infection. This protective effect was not associated with a switch in IgG subclass, which remained predominantly IgG2. The levels of specific antibodies in serum and the gastric mucosa which were similar to naive unprotected animals. In conclusion, the ability of mucosal adjuvants such as CT to induce a protective immune response may be host dependent and findings in the Helicobacter-mouse model should be interpreted with caution.

Role of B7 costimulatory molecules in the adjuvant activity of the heat-labile enterotoxin of Escherichia coli

Much interest has been directed at understanding the adjuvant properties of the heat-labile enterotoxin of Escherichia coli (LT). In this study, we have assessed how LT compared with the nonenzymatic mutant LT (E112K) affect the level of B7-1 and B7-2 expression on APCs, and we determined how these costimulatory molecules influence their adjuvant properties. Analysis of B7-1 and B7-2 expression on B cells revealed that LT enhanced B7-2 but not B7-1, while LT (E112K) had no effect on the expression of either costimulatory molecule. Treatment of macrophage or dendritic cells with LT resulted in a predominant enhancement of B7-2, while LT (E112K) induced mainly B7-1 expression. Analysis of LT- and LT (E112K)-treated B cells, macrophage, and dendritic cells also revealed significant differences in their ability to enhance anti-CD3-stimulated CD4(+) T cell proliferative responses via B7-1 and B7-2. Furthermore, the ability of LT to enhance both Ab and CD4(+) T cell responses to a coadministered Ag was severely abrogated in B7-2- but not B7-1-deficient mice. In contrast, the in vivo adjuvant properties of LT (E112K) appeared to be mediated by both B7-1 and B7-2 for optimal CD4(+) T cell responses, while B7-1 appeared to be the predominant B7 molecule involved in the ability of LT (E112K) to augment Ab responses to a coadministered Ag. These findings demonstrate distinct differences in the ability of LT and LT (E112K) to enhance B7-1 and B7-2 on APC, as well as a dependence upon these costimulatory molecules for their adjuvant properties.

The conjunctival lymphoid follicle in mucosal immunology

The conjunctiva forms a continuous mucosal surface from the eyelid margin to the cornea, and makes contact with airborne antigens and those on the adjacent eyelid skin and preocular tear film. Conjunctival lymphoid follicles (CLF) undergo hyperplasia upon conjunctival infection by a specific array of pathogens; infection-associated enlargement of draining preauricular lymph nodes suggests that CLF participate in the afferent limb of acquired immune responses for the ocular surface. In this review, we examine the evidence for classification of CLF as part of the common mucosal immune system, and explore the possible therapeutic implications.

Transcutaneous immunization using colonization factor and heat-labile enterotoxin induces correlates of protective immunity for enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) diarrheal disease is a worldwide problem that may be addressed by transcutaneous delivery of a vaccine. In several human settings, protective immunity has been associated with immune responses to E. coli colonization factors and to the heat-labile toxin that induces the diarrhea. In this set of animal studies, transcutaneous immunization (TCI) using recombinant colonization factor CS6 and cholera toxin (CT) or heat-labile enterotoxin (LT) as the adjuvant induced immunoglobulin G (IgG) and IgA anti-CS6 responses in sera and stools and antibody responses that recognized CS6 antigen in its native configuration. The antitoxin immunity induced by TCI was also shown to protect against enteric toxin challenge. Although immunization with LT via the skin induced mucosal secretory IgA responses to LT, protection could also be achieved by intravenous injection of the immune sera. Finally, a malaria vaccine antigen, merzoite surface protein 1(42) administered with CT as the adjuvant, induced both merzoite surface protein antibodies and T-cell responses while conferring protective antitoxin immunity, suggesting that both antiparasitic activity and antidiarrheal activity can be obtained with a single vaccine formulation. Overall, our results demonstrate that relevant colonization factor and antitoxin immunity can be induced by TCI and suggest that an ETEC traveler's diarrhea vaccine could be delivered by using a patch.

Nasal mucosal immunogenicity for the horse of a SeM peptide of Streptococcus equi genetically coupled to cholera toxin

The intranasal immunogenicity of cholera toxin (CT) genetically coupled to peptide sequence aa236-334 (F3) of the SeM protein of Streptococcus equi was studied in five young adult Welsh ponies. All ponies made rapid CTB- and SeMF3-specific serum antibody responses following the first immunization. Specific nasal IgA responses were detected in two ponies 14 days after the first immunization, in another two 14 days after a second immunization on day 14, and in all ponies 28 days after a third immunization on day 42. SeMF3-specific antibody responses in sera and nasal washes were dominated by IgGb and IgA, respectively, and remained elevated for at least 140 days. Strong serum IgGa and IgG(T) responses were also observed. These antibody responses were qualitatively similar to those induced during recovery from equine strangles. Antibody responses in mucosal secretions were boosted in some ponies by immunizations subsequent to the first immunization, but antibodies in serum were never boosted. In vitro survival of S. equi was significantly reduced by SeMF3-specific antibodies in sera obtained 14 days after the second immunization but survival increased in sera collected following subsequent immunizations, possibly due to absence of synthesis of high affinity antibodies. Finally, the susceptibility of all immunized ponies to commingling challenge by S. equi indicated either that SeMF3 lacks protective epitopes or that the antibodies induced by the chimera were not at effective levels.

Genetically manipulated bacterial toxin as a new generation mucosal adjuvant

Cholera toxin (CT) and heat-labile toxin (LT) of Escherichia coli act as adjuvants for the enhancement of mucosal and serum antibody (Ab) responses to mucosally co-administered protein antigen (Ag). Both LT and CT induce B7-2 expression on antigen-presenting cells (APCs) for subsequent co-stimulatory signalling to CD4+ T cells. CT directly affects CD4+ T cells activated via the TCR-CD3 complex with selective inhibition of Th1 responses whereas LT maintains Th1 cytokine responses with inhibition of interleukin (IL)-4 production. Interestingly, while CT failed to induce mucosal adjuvant activity in the absence of IL-4, LT did so. Nontoxic mutant (m)CTs (S61F and E112K) retain adjuvant properties by inducing CD4+ Th2 cells, which provided effective help for the Ag-specific mucosal immunoglobulin (Ig)A, as well as serum IgG1, IgE and IgA Ab responses. The mCT E112K has been shown to exhibit two distinct mechanisms for its adjuvanticity. Firstly, mCT enhanced the B7-2 expression of APCs. Secondly, this nontoxic CT derivative directly affected CD4+ T cells and selectively inhibited Th1 cytokine responses. Thus, several lines of evidence indicate that enzyme activity can be separated from adjuvant properties of CT and this offers promise for the development of safe delivery of vaccines for mucosal IgA responses.

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.

Transcutaneous immunization with bacterial ADP-ribosylating exotoxins, subunits, and unrelated adjuvants

We have recently described a needle-free method of vaccination, transcutaneous immunization, consisting of the topical application of vaccine antigens to intact skin. While most proteins themselves are poor immunogens on the skin, we have shown that the addition of cholera toxin (CT), a mucosal adjuvant, results in cellular and humoral immune responses to the adjuvant and coadministered antigens. The present study explores the breadth of adjuvants that have activity on the skin, using diphtheria toxoid (DTx) and tetanus toxoid as model antigens. Heat-labile enterotoxin (LT) displayed adjuvant properties similar to those of CT when used on the skin and induced protective immune responses against tetanus toxin challenge when applied topically at doses as low as 1 microg. Interestingly, enterotoxin derivatives LTR192G, LTK63, and LTR72 and the recombinant CT B subunit also exhibited adjuvant properties on the skin. Consistent with the latter finding, non-ADP-ribosylating exotoxins, including an oligonucleotide DNA sequence, as well as several cytokines (interleukin-1beta [IL-1beta] fragment, IL-2, IL-12, and tumor necrosis factor alpha) and lipopolysaccharide also elicited detectable anti-DTx immunoglobulin G titers in the immunized mice. These results indicate that enhancement of the immune response to topical immunization is not restricted to CT or the ADP-ribosylating exotoxins as adjuvants. This study also reinforces earlier findings that addition of an adjuvant is important for the induction of robust immune responses to vaccine antigens delivered by topical application.

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.

The induction of systemic and mucosal immune responses to antigen-adjuvant compositions administered into the skin: alterations in the migratory properties of dendritic cells appears to be important for stimulating mucosal immunity

The properties of various vaccine-adjuvant formulations that are capable of inducing both systemic and common mucosal immunity subsequent to their intradermal administration are described. Effective mucosal adjuvants, including bacterial toxins, chemical enhancers of cyclic AMP, and the active form of vitamin D3, all shared the ability to promote dendritic cell migration from the skin to Peyer's patches subsequent to antigen induced maturation. Our data suggests that skin dendritic cells may function as effective antigen presenting cells for the induction of mucosal immune responses, if microenvironmental conditions are appropriately manipulated subsequent to their stimulation by antigen.