Mucosal immunogenicity of the Escherichia coli heat-labile enterotoxin: role of the A subunit

Immunogenicity of IMS 1113 plus soluble subunit and chimeric proteins containing Mycoplasma hyopneumoniae P97 C-terminal repeat regions

The surface adhesin P97 mediates the adherence of Mycoplasma hyopneumoniae to swine cilia. Two reiterated repeats R1 and R2 are located at the C-terminus of P97. The purpose of this study was to evaluate the immunogenicity of Montanide adjuvant IMS 1113 plus soluble subunit proteins rR1, rR1R2 and their chimeric forms coupled with B subunit of the heat-labile enterotoxin of Escherichia coli (LTB). Each recombinant protein in this study was capable of eliciting anti-R1 specific humoral antibodies (IgG), mucosal antibodies (IgG and IgA) and IFN-γ production. The chimeric protein rLTBR1R2 elicited the quickest humoral antibody response among the recombinant proteins. Serum and bronchoalveolar lavage analysis revealed that each recombinant protein was capable of inducing both Th1 and Th2 responses. Importantly, all of the proteins induced an anti-R1-specific Th2-biased response in both humoral and mucosal compartments, similar to the response observed in a natural infection or vaccination process. These observations indicate that rR1, rR1R2, rLTBR1 and rLTBR1R2 with IMS 1113 might represent a promising subunit vaccine strategy against porcine enzootic pneumonia in pigs.

Cholera toxin, LT-I, LT-IIa and LT-IIb: the critical role of ganglioside binding in immunomodulation by type I and type II heat-labile enterotoxins

The heat-labile enterotoxins expressed by Vibrio cholerae (cholera toxin) and Escherichia coli (LT-I, LT-IIa and LT-IIb) are potent systemic and mucosal adjuvants. Coadministration of the enterotoxins with a foreign antigen produces an augmented immune response to that antigen. Although each enterotoxin has potent adjuvant properties, the means by which the enterotoxins induce various immune responses are distinctive for each adjuvant. Various mutants have been engineered to dissect the functions of the enterotoxins required for their adjuvanticity. The capacity to strongly bind to one or more specific ganglioside receptors appears to drive the distinctive immunomodulatory properties associated with each enterotoxin. Mutant enterotoxins with ablated or altered ganglioside-binding affinities have been employed to investigate the role of gangliosides in enterotoxin-dependent immunomodulation.

Towards an oral influenza vaccine: comparison between intragastric and intracolonic delivery of influenza subunit vaccine in a murine model

In this paper we investigated to which part of the gastro-intestinal (GI) tract, the upper or lower part, an oral influenza vaccine should be targeted to result in an effective immune response in mice. Our study demonstrates that without adjuvant substantial systemic but low respiratory mucosal immune responses were induced in mice after delivery of influenza subunit vaccine to the upper GI-tract (intragastric) as well as the lower GI-tract (intracolonically). When the vaccine was adjuvanted with Escherichia coli heat-labile enterotoxin (LT) these responses were significantly enhanced. Interestingly, intracolonic administration of vaccine with adjuvant also resulted in enhanced cellular immune responses and the desired Th1-skewing of these responses. Intragastric administration of the adjuvanted vaccine also increased T-helper responses. However, Th1-skewing was absent. In conclusion, the right combination of strong mucosal adjuvant (e.g. LT) and antigen delivery site (e.g. the lower part of the gastro-intestinal tract) might result in effective vaccination via the oral route.

Improved uptake of plant-derived LTB-linked proteins in carp gut and induction of specific humoral immune responses upon infeed delivery

Oral vaccination of fish is an effortless and stress free immunisation method which can be used for almost any age. However, vaccination via the mucosal route does have disadvantages. For example, the vaccine may induce tolerance and has to be protected to escape digestion. Also the vaccine should be efficiently delivered to immune-competent cells in the gut or other lymphoid organs. In addition, it should be cost effective. Here we present a novel fish vaccination model using potato tubers as vaccine production and delivery system. The model vaccines discussed here include fusion proteins consisting of a gut adhesion molecule (LTB) and a viral peptide or green fluorescent protein (GFP) expressed in potato tubers. The adhesion molecule mediates binding to and uptake from the gut, whereas the viral peptide or GFP functions as model vaccine antigen provoking the induction of an immune response. We demonstrate that fusion to LTB facilitates an elevated uptake of the model vaccines in carp gut mucosa. The plant-derived fusion proteins also elicit a specific systemic humoral immune response upon oral application of crude tuber material incorporated into a standard dietary feed pellet. The data presented here show the promising potentials of the plant as a production system for oral vaccines in aquaculture and feed mediated immunisation of fish.

A recombinant chimera composed of R1 repeat region of Mycoplasma hyopneumoniae P97 adhesin with Escherichia coli heat-labile enterotoxin B subunit elicits immune response in mice

Swine mycoplasmal pneumonia (SMP), caused by fastidious bacterium Mycoplasma hyopneumoniae, is the most important respiratory disease in swine breeding. The commonly used vaccines to control this disease consist of inactivated whole cells (bacterins), whose production cost is high and the efficiency is limited. The objective of this study was to develop and to evaluate in BALB/c mice a recombinant subunit vaccine (rLTBR1) containing the R1 region of P97 adhesin of M. hyopneumoniae (R1) fused to the B subunit of the heat-labile enterotoxin of Escherichia coli (LTB). rLTBR1 formed functional oligomers that presented high affinity to GM1 ganglioside. Mice inoculated with rLTBR1 by intranasal (IN) or intramuscular (IM) route produced high levels of anti-R1 systemic and mucosal antibodies (IgA), which recognized the native P97. On the other hand, mice inoculated with the inactivated whole cell vaccine did not produce anti-R1 antibodies. The administration route influenced the modulation of the immune response by LTB, showing that IM rLTBR1 induced Th2-biased immune responses and IN rLTBR1 induced Th1-biased immune responses. rLTBR1 administrated by IN route also induced IFN-gamma secretion by lymphocytes. rLTBR1 may constitute a new strategy for preventing infection by M. hyopneumoniae and may have potential for developing vaccines against other infectious diseases as well.

Analysis of immune response in young and aged mice vaccinated with corn-derived antigen against Escherichia coli heat-labile enterotoxin

Enterotoxigenic strains of Escherichia coli produce a heat-labile holotoxin (LT), which causes diarrhea. We engineered corn seeds to produce LT-B, the nontoxic subunit of LT, to serve as a plant-derived vaccine to traveler's diarrhea and as an adjuvant for co-administered proteins. We previously demonstrated that a strong mucosal and systemic antibody response is elicited in young mice with oral administration of corn-derived LT-B. The present study examined systemic and mucosal antibody responses to LT-B in young and aged mice, and recall responses to oral administration and injection of LT-B in aged mice. Specific IgA and IgG antibodies were detectable during an 11-mo period, although the concentration of antigen-specific antibodies declined gradually. Booster by feeding or injection dramatically increased the concentration of specific IgA from that seen in young mice. Specific IgG levels were boosted to concentrations similar to those in young mice. This effect may be age-dependent and related to prior immunization exposure. Analysis of the antibody response of naïve aged mice against corn-derived LT-B demonstrated an age-related suppression in specific IgG production, but not specific IgA. These results may provide important information for edible vaccine strategies for young and aged individuals.

Construction of prokaryotic expression system of ltB-ureB fusion gene and identification of the recombinant protein immunity and adjuvanticity

AIM: To construct ltB-ureB fusion gene and its prokaryotic expression system and identify immunity and adjuvanticity of the expressed recombinant protein.

METHODS: The ureB gene from a clinical Helicobacter pylori (H pylori) strain Y06 and the ltB gene from Escherichia coli (E. coli) strain 44851 were linked into ltB-ureB fusion gene by PCR. The fusion gene sequence was analyzed after T-A cloning. A prokaryotic recombinant expression vector pET32a inserted with ltB-ureB fusion gene (pET32a-ltB-ureB) was constructed. Expression of the recombinant LTB-UreB protein (rLTB-UreB) in E. coli BL21DE3 induced by isopropylthio-β-D-galactoside (IPTG) at different concentrations was detected by SDS-PAGE. Western blot assays were used to examine the immunoreaction of rLTB-UreB by a commercial antibody against whole cell of H pylori and a self-prepared rabbit anti-rUreB serum, respectively, and determine the antigenicity of the recombinant protein on inducing specific antibody in rabbits. GM₁-ELISA was used to demonstrate the adjuvanticity of rLTB-UreB. Immunoreaction of rLTB-UreB to the UreB antibody positive sera from 125 gastric patients was determined by using ELISA.

RESULTS: In comparison with the corresponding sequences of original genes, the nucleotide sequence homologies of the cloned ltB-ureB fusion gene were 100%. IPTG with different dosages of 0.1-1.0 mmol/L could efficiently induce pET32a-ltB-ureB-E. coli BL21DE3 to express the rLTB-UreB. The output of the target recombinant protein expressed by pET32a-ureB-E. coli BL21DE3 was approximately 35% of the total bacterial proteins. rLTB-UreB mainly presented in the form of inclusion body. Western blotting results demonstrated that rLTB-UreB could combine with the commercial antibody against whole cell of H pylori and anti-rUreB serum as well as induce rabbit to produce specific antibody. The strong ability of rLTB-UreB binding bovine GM₁ indicated the existence of adjuvanticity of the recombinant protein. All the UreB antibody positive sera from the patients (125/125) were positive for rLTB-UreB.

CONCLUSION: A recombinant prokaryotic expression system with high expression efficiency of the target fusion gene ltB-ureB was successfully established. The expressed rLTB-UreB showed qualified immunogenicity, antigenicity and adjuvanticity. All the results mentioned above laid a firm foundation for further development of H pylori genetically engineered vaccine.

Mucosal and systemic antibody responses against an acellular pertussis vaccine in mice after intranasal co-administration with recombinant cholera toxin B subunit as an adjuvant

To investigate the possibility of intranasal immunization with an acellular pertussis vaccine, groups of mice were administered intranasally with aluminium-non-adsorbed pertussis toxoid (PTd; 0.5 or 5 microg) and formalin-treated filamentous hemagglutinin (fFHA; 5 microg) with and without recombinant cholera toxin B subunit (rCTB; 10 microg) as a mucosal adjuvant. At a low concentration of PTd, the following things became clear: (1) earlier and higher elevation of serum anti-PTd and anti-FHA IgG antibody titres in the presence of rCTB than in its absence, (2) higher serum anti-PTd and anti-FHA IgG antibody titres than 200 and 100 ELISA units ml(-1) (EU ml(-1)) in all mice, respectively, in the presence of rCTB, which were obtained by calibration against a reference anti-pertussis mouse serum, and (3) in an intranasal challenge experiment with Bordetella pertussis, slightly more rapid elimination of the bacteria from the lungs of mice intranasally immunized in the presence of rCTB, suggesting the effectiveness of rCTB as a mucosal adjuvant. However, irrespective of rCTB and dose of PTd, mice which were immunized four times and sacrificed on day 35 developed high levels of anti-PTd serum IgG antibodies, high or moderate levels of anti-FHA serum IgG antibodies and mucosal anti-PTd IgA antibodies in the lungs; only a slight or no increase of anti-FHA mucosal IgA antibodies was observed in the lung. These facts suggested the immunogenicity and mucosal adjuvanticity of PTd, and therefore, the mucosal adjuvanticity of rCTB seemed to be inconspicuous. Moreover, the addition of rCTB induced higher anti-PTd serum IgE antibody responses than no addition of it depending on dose of PTd. These results show that dose of PTd included in an acellular pertussis vaccine had better be low as possible and the addition of rCTB may not be always necessary in case of this nasal vaccine alone unlike tetanus and diphtheria toxoids and hepatitis B virus vaccine reported before.

Inflammatory cytokines and antigen presenting cell activation

Immune responses are stimulated in response to threats against health. In animals, defense against infectious agents, particularly rapidly growing viruses and bacteria, requires an immediate response to limit growth and dissemination, and then stimulation of a more prolonged, specific immunity to prevent re-infection. The process by which animals meet the dual needs of an immediate response to danger and initiation of long-term protection is substantially influenced by inflammatory cytokines produced primarily by macrophages and professional antigen presenting cells (APCs). Inflammatory cytokines mobilize the immune system in response to danger and increase the efficiency of an immune response as effectors of APC function. Here we review the evidence for the involvement of inflammatory cytokines in immune induction and as mediators of APC activity, with a particular emphasis on swine and on the induction of immunity at mucosal surfaces. The vast majority of infections occur at mucosal surfaces of the enteric, respiratory and reproductive tracts, and induction of protective immunity at these sites is particularly challenging. Induction of immunity at mucosal surfaces of the small intestine is greatly facilitated by the oral adjuvant, cholera toxin (CT). CT potentiates inflammatory cytokine and costimulatory molecule expression in macrophages, and stimulates humoral and cell-mediated immune responses both locally and systemically. These observations are consistent with the hypothesis that activation of APCs is a key step in the induction of antigen-specific immunity, and that inflammatory cytokine expression is a hallmark of activated APC function. The efficacy of vaccine adjuvants, particularly in the context of mucosal immunity, may be determined by their ability to induce a controlled inflammatory response in gut-associated lymphoid tissue, characterized by the expression of various costimulatory molecules and inflammatory cytokines. Thus, elucidation of the patterns of inflammatory cytokine expression and features of APC activation will help to facilitate the rational development of more efficacious vaccines.

Nasal or intramuscular immunization of mice with influenza subunit antigen and the B subunit of Escherichia coli heat-labile toxin induces IgA- or IgG-mediated protective mucosal immunity

Local mucosal IgA antibodies play a central role in protection of the respiratory tract against influenza virus infection. Therefore, new-generation influenza vaccines should aim at stimulating not only systemic, but also local antibody responses. Previously, we demonstrated that the recombinant B subunit of the Escherichia coli heat-labile toxin (LTB) is a potent adjuvant towards nasally administered influenza subunit antigen. Here, we investigated the protection conferred by LTB-supplemented influenza subunit antigen given intranasally (i.n.) or intramuscularly (i.m.) to mice. Both i.n. and i.m. immunization with subunit antigen and LTB completely protected the animals against viral infection. Protection upon i.n. immunization was associated with the induction of antigen-specific serum IgG and mucosal IgA, whereas protection upon i.m. immunization correlated with strong serum and mucosal IgG, but not IgA responses. We conclude that LTB-supplemented influenza subunit antigen, given either i.n. or i.m, induces protective antibody-mediated mucosal immunity and thus represents a promising novel flu vaccine candidate.

Oral immunisation of naive and primed animals with transgenic potato tubers expressing LT-B

The efficacy of edible vaccines produced in potato tubers was examined in mice. Transgenic plants were developed by Agrobacterium tumefaciens-mediated transformation. The antigen selected was the non-toxic B subunit of the Escherichia coli enterotoxin (recLT-B). A synthetic gene coding for recLT-B was made and optimised for expression in potato tubers and accumulation in the endoplasmic reticulum. Introduction of this gene under control of the tuber-specific patatin promoter in potato plants resulted in the production of functional, i.e. Gm1-binding, recLT-B pentamers in tubers. Selected tubers containing about 13 microg of recLT-B per gram fresh weight were used for immunisation. Subcutaneous immunisation with an extract of recLT-B tubers yielded high antibody titres in serum that were similar to those obtained with bacterial recLT-B. The efficacy of oral administration of recLT-B tubers was determined by measuring mucosal and systemic immune responses in naive and primed mice. Animals were primed by subcutaneous injection of an extract of recLT-B tuber plus adjuvant. Naive and primed mice were fed 5 g of tubers ( approximately 65 microg of recLT-B) or were intubated intragastrically with 0.4 ml of tuber extract ( approximately 2 microg of recLT-B). In naive mice, feeding recLT-B tubers or intubation of tuber extract did not induce detectable anti-LT antibody titres. In primed animals, however, oral immunisation resulted in significant anti-LT IgA antibody responses in serum and faeces. Intragastric intubation of tuber extract revealed higher responses than feeding of tubers. These results indicate clearly that functional recLT-B can be produced in potato tubers, that this recombinant protein is immunogenic and that oral administration thereof elicits both systemic and local IgA responses in parentally primed, but not naive, animals.

Mucosal immunization against hepatitis B virus by intranasal co-administration of recombinant hepatitis B surface antigen and recombinant cholera toxin B subunit as an adjuvant

Recombinant cholera toxin B subunit (rCTB) produced by Bacillus brevis carrying pNU212-CTB has been previously found to be a potent mucosal adjuvant to aluminium-non-adsorbed tetanus toxoid (nTT) and diphtheria toxoid (nDT) co-administered intranasally, and the possibility of needle-free inoculation of these vaccines with rCTB has been suggested. In this paper we examined the potentiality of rCTB as a mucosal adjuvant to aluminium-non-adsorbed yeast-derived recombinant hepatitis B surface antigen (rHBs) being a particulate antigen when administered intranasally with rCTB. In-house ELISA showed that a mixture of rHBs (1 or 5 microg) and rCTB (10 microg) elevated not only systemic responses but also mucosal immune responses at the nasal cavity, the lung, the saliva, the small intestine and the vagina against rHBs, and these could be further increased with higher doses of antigen. With antibody isotypes of IgG, there were equally high levels of serum HBs-specific IgG1, IgG2a and IgG2b antibodies and induction of mixed Th1- and Th2-type responses was considered to occur in combination of rHBs and rCTB. Serum anti-HBs titres in almost all mice obtained from sandwich EIA using a commercial kit were higher than 1000 milli-international units ml(-1) (mIU ml(-1)). These results show that rCTB is also very effective as a mucosal adjuvant for a particulate antigen like rHBs, as well as soluble antigens like nTT and nDT reported previously, suggesting the possibility of intranasal immunization with rHBs plus rCTB in humans.

Intranasal immunization with mumps virus DNA vaccine delivered by influenza virosomes elicits mucosal and systemic immunity

To improve the efficiency of liposome-mediated DNA transfer as a tool for gene therapy or vaccinology, we have further developed a new delivery system based on the modified immunopotentiating reconstituted influenza virus (IRIV). In this study, we engineered a plasmid DNA vector expressing the mumps virus hemagglutinin or the fusion protein. The administration of this DNA vaccine delivered by influenza virosomes, in combination with the mucosal adjuvant Escheriagen via the intranasal route, was efficient for inducing an immune response, both mucosally and systemically, in mice. The production of IgG2a mumps virus-specific antibodies and the secretion of interleukin 10 (IL-10) by antigen-specific T cells indicated that not only Th1 but also Th2 responses were induced by this DNA vaccine formulation. These results suggest that cationic virosomes in combination with Escheriagen may have great potential as an efficient delivery system for intranasal DNA immunization and provide an immune barrier at the mucosal sites.

Improving vaccine performance with adjuvants

New vaccines are presently under development and in testing for the control of infectious diseases, including human immunodeficiency virus (HIV) and tuberculosis. Several of these vaccines are composed of synthetic, recombinant, or highly purified subunit antigens. Subunit vaccines are designed to include only the antigens required for protective immunization and to be safer than whole-inactivated or live-attenuated vaccines. However, the purity of the subunit antigens and the absence of the self-adjuvanting immunomodulatory components associated with attenuated or killed vaccines often result in weaker immunogenicity. Immunologic adjuvants are agents that enhance specific immune responses to vaccines. Formulation of vaccines with potent adjuvants is an attractive approach for improving the performance of vaccines composed of subunit antigens. Adjuvants have diverse mechanisms of action and should be selected for use on the basis of the route of administration and the type of immune response (antibody, cell-mediated, or mucosal immunity) that is desired for a particular vaccine.

Mechanisms of vaccine adjuvanticity at mucosal surfaces

The vast majority of pathogens invade via mucosal surfaces, including those of the intestine. Vaccination directly on these surfaces may induce local protective immunity and prevent infection and disease. Although vaccine delivery to the gut mucosa is fraught with obstacles, immunization can be enhanced using adjuvants with properties specific to intestinal immunity. In this review, we present three general mechanisms of vaccine adjuvant function as originally described by Freund, and we discuss these principles with respect to intestinal adjuvants in general and to the prototypical mucosal adjuvant, cholera toxin. The key property of intestinal adjuvants is to induce an immunogenic context for the presentation of the vaccine antigen. The success of oral vaccine adjuvants is determined by their ability to induce a controlled inflammatory response in the gut-associated lymphoid tissues, characterized by the expression of various costimulatory molecules and cytokines. An understanding of the specific molecular mechanisms of adjuvanticity in the gut will allow the rational development of safe and effective oral vaccines.

Efficient extracellular production of recombinant Escherichia coli heat-labile enterotoxin B subunit by using the expression/secretion system of Bacillus brevis and its mucosal immunoadjuvanticity

A gene encoding the mature Escherichia coli heat-labile enterotoxin B subunit (LTB) was introduced in a vector pNU212 and expressed at high levels in Bacillus brevis HPD31. The maximum amount of recombinant LTB (rLTB) secreted into the modified 5PY medium containing erythromycin was about 350 mg l(-1) when cultivated at 30 degrees C for 8 days. The rLTB purified directly from the culture supernatant by using D-galactose immobilized agarose was identical to the native LTB with respect to the molecular weight determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), and the amino terminal amino acid sequence. Western blot analysis with antiserum to cholera toxin B subunit (CTB) indicated that rLTB had cross-reactivity to native CTB and its GM1 binding ability was almost the same as that of the CTB. The rLTB predominantly showed the pentameric form when non-boiled samples were applied to SDS-PAGE. When rLTB was administered intranasally to mice with diphtheria toxoid (D(T)), it resulted in the substantial stimulation of D(T)-specific serum IgG antibody, and in the induction of moderate levels of D(T)-specific mucosal IgA antibody responses in the nasal cavities and in the lung, suggesting that purified rLTB acts as a promising immunoadjuvant on mucosal immunizations.

Induction of apoptosis in lymphoid tissues of mice after intramuscular injection of enterotoxigenic Escherichia coli enterotoxin

Although it has been reported that intravenous injection of Escherichia coli enterotoxin induces atrophies of the thymus and spleen by necrosis, the toxin injected intramuscularly to mice induced atrophies of both tissues, which were associated with apoptosis of lymphocytes. Apoptosis predominantly occurred in the thymus and increased in a time-dependent manner up to 26 h and faint ladder band patterns of DNA were observed at 36 h. Although the high dose of toxin also induced in vitro apoptosis in cultured thymocytes, the toxin was not detected in the serum at levels sufficient to cause in vitro apoptosis after intramuscular administration. By flow cytometric analysis, CD4+ CD8+ double-positive T cell and CD45+ positive B cell numbers were found to be mainly decreased in thymus and spleen, respectively, of mice. These results suggest that the atrophies of thymus and spleen by intramuscular administration of the toxin to mice are due to apoptosis of CD4+ CD8+ double-positive T and CD45+ positive B cells, respectively, but the toxin does not reach these cells via the circulation. A different mechanism from that in vitro in cultured cells might be involved in the induction of apoptosis in vivo.

Mutants of Escherichia coli heat-labile toxin act as effective mucosal adjuvants for nasal delivery of an acellular pertussis vaccine: differential effects of the nontoxic AB complex and enzyme activity on Th1 and Th2 cells

Mucosal delivery of vaccines is dependent on the identification of safe and effective adjuvants that can enhance the immunogenicity of protein antigens administered by nasal or oral routes. In this study we demonstrate that two mutants of Escherichia coli heat-labile toxin (LT), LTK63, which lacks ADP-ribosylating activity, and LTR72, which has partial enzyme activity, act as potent mucosal adjuvants for the nasal delivery of an acellular pertussis (Pa) vaccine. Both LTK63 and LTR72 enhanced antigen-specific serum immunoglobulin G (IgG), secretory IgA, and local and systemic T-cell responses. Furthermore, using the murine respiratory challenge model for infection with Bordetella pertussis, we demonstrated that a nasally delivered diphtheria, tetanus, and acellular pertussis (DTPa) combination vaccine formulated with LTK63 as an adjuvant conferred a high level of protection, equivalent to that generated with a parenterally delivered DTPa vaccine formulated with alum. This study also provides significant new information on the roles of the binding and enzyme components of LT in the modulation of Th1 and Th2 responses. LTK63, which lacks enzyme activity, promoted T-cell responses with a mixed Th1-Th2 profile, but LTR72, which retains partial enzyme activity, and the wild-type toxin, especially at low dose, induced a more polarized Th2-type response and very high IgA and IgG antibody titers. Our findings suggest that the nontoxic AB complex has broad adjuvant activity for T-cell responses and that the ADP-ribosyltransferase activity of the A subunit also appears to modulate cytokine production, but its effect on T-cell subtypes, as well as enhancing, may be selectively suppressive.

Outer membrane vesicles from group B meningococci are strongly immunogenic when given intranasally to mice

Outer membrane vesicles (OMVs) from group B meningococci induced both serum and mucosal antibodies when given as a nasal and rectal vaccine to mice. Cholera toxin (CT) enhanced the antibody responses in serum both after nasal and rectal immunizations, and the mucosal responses after rectal immunizations only. Nasal immunizations, however, were most effective, with mucosal responses which were not dependent on the use of CT. The serum bactericidal activity was similarly not enhanced by CT, indicating that the positive effect of CT on the serum IgG level was not including bactericidal activity. A small nasal booster dose induced antibody responses in serum as far as eight months after intranasal and subcutaneous immunizations, and in saliva after intranasal immunizations. Nasal vaccines may thus be favorably combined with parenteral vaccines.

Construction and murine immunogenicity of recombinant Bacille Calmette Guérin vaccines expressing the B subunit of Escherichia coli heat labile enterotoxin

Three recombinant strains of Mycobacterium bovis Bacille Calmette Guerin (rBCG) were prepared in which the immunogenic B subunit of human Escherichia coli heat labile enterotoxin (LT-Bh) was expressed either as a cytoplasm protein, a cell wall associated lipoprotein or a secreted protein. Intraperitoneal immunisation of mice with these rBCG induced IgG and IgA antibodies to LT-Bh and shifted the serum IgG subclass response to subsequent challenge with purified LT-Bh from IgG1 to an IgG2a. Oral administration of recombinant BCG induced mucosal and serum IgA antibodies to LT-Bh which peaked four months after immunisation. Antibody responses were greater when LT-Bh was expressed as a secreted protein or lipoprotein rather than in the cytoplasm. Oral vaccination with recombinant BCG may be an effective approach, particularly to induce mucosal IgA and prime for a serum TH1 recall response.

Role of macrophage cytokines in mucosal adjuvanticity

Delivery of protein antigens to the GALT can result in immunity or oral tolerance depending on the circumstances of the encounter. One mechanism by which mucosal adjuvants can affect these circumstances is by the induction of macrophage cytokines, including IL-1 and IL-12. These cytokines can directly affect the immune response by their effects on antigen-specific T cells and by the induction of IFN-gamma by T cells or NK cells. This IFN-gamma also activates macrophages to up-regulate MHC or costimulatory molecules and by further inducing IL-1 and IL-12. In effect, mucosal adjuvants function both directly and indirectly as activators of antigen presenting cells, resulting in stimulation of the immune response to coincidental antigens. Our studies in swine have shown CT is a potent mucosal adjuvant for CT-B. CT also increased IL-1 and IL-12 mRNA in cultured macrophages, especially after activation with IFN-gamma. The effect of CT on the secretion of bioactive IL-12 protein is currently being investigated. While the mucosal adjuvanticity of CT involves a variety of mechanisms, these findings suggest a role for the induction of the macrophage cytokines IL-1 and IL-12.

Mucosal immunoadjuvant activity of recombinant Escherichia coli heat-labile enterotoxin and its B subunit: induction of systemic IgG and secretory IgA responses in mice by intranasal immunization with influenza virus surface antigen

The Escherichia coli heat-labile enterotoxin (LT) is a very potent mucosal immunogen. LT also has strong adjuvant activity towards coadministered unrelated antigens and is therefore of potential interest for development of mucosal vaccines. However, despite the great demand for such mucosal vaccines, the use of LT holotoxin as an adjuvant is essentially precluded by its toxicity. LT is composed of an A subunit, carrying the toxic ADP-ribosylation activity, and a pentamer of identical B subunits, which mediates binding to ganglioside GM1, the cellular receptor for the toxin. In this paper, we demonstrate that recombinant enzymatically inactive variants of LT, including the LTB pentamer by itself, retain the immunoadjuvant activity of LT holotoxin in a murine influenza model. Mice were immunized intranasally (i.n.) with influenza virus subunit antigen, consisting mostly of the isolated surface glycoprotein hemagglutinin (HA), supplemented with either recombinant LTB (rLTB), a nontoxic LT mutant (E112K, with a Glu112-->Lys substitution in the A subunit), or LT holotoxin, and the induction of systemic IgG and local S-IgA responses was evaluated by direct enzyme-linked immunosorbent assay (ELISA). Immunization with subunit antigen alone resulted in a poor systemic IgG response and no detectable S-IgA. However, supplementation of the antigen with E112K or rLTB resulted in a substantial stimulation of the serum IgG level and in induction of a strong S-IgA response in the nasal cavity. The adjuvant activity of E112K or rLTB under these conditions was essentially the same as that of the LT holotoxin. The present results demonstrate that nontoxic variants of LT, rLTB in particular, represent promising immunoadjuvants for potential application in an i.n. influenza virus subunit vaccine. Nontoxic LT variants may also be used in i.n. vaccine formulations directed against other mucosal pathogens. In this respect, it is of interest that LT(B)-stimulated antibody responses after i.n. immunization were also observed at distant mucosal sites, including the urogenital system. This, in principle, opens the possibility to develop i.n. vaccines against sexually transmitted infectious diseases.

Role of GM1 binding in the mucosal immunogenicity and adjuvant activity of the Escherichia coli heat-labile enterotoxin and its B subunit

Escherichia coli (E. coli) heat-labile toxin (LT) is a potent mucosal immunogen and immunoadjuvant towards co-administered antigens. LT is composed of one copy of the A subunit, which has ADP-ribosylation activity, and a homopentamer of B subunits, which has affinity for the toxin receptor, the ganglioside GM1. Both the ADP-ribosylation activity of LTA and GM1 binding of LTB have been proposed to be involved in immune stimulation. We investigated the roles of these activities in the immunogenicity of recombinant LT or LTB upon intranasal immunization of mice using LT/LTB mutants, lacking either ADP-ribosylation activity, GM1-binding affinity, or both. Likewise, the adjuvant properties of these LT/LTB variants towards influenza virus subunit antigen were investigated. With respect to the immunogenicity of LT and LTB, we found that GM1-binding activity is essential for effective induction of anti-LTB antibodies. On the other hand, an LT mutant lacking ADP-ribosylation activity retained the immunogenic properties of the native toxin, indicating that ADP ribosylation is not critically involved. Whereas adjuvanticity of LTB was found to be directly related to GM1-binding activity, adjuvanticity of LT was found to be independent of GM1-binding affinity. Moreover, a mutant lacking both GM1-binding and ADP-ribosylation activity, also retained adjuvanticity. These results demonstrate that neither ADP-ribosylation activity nor GM1 binding are essential for adjuvanticity of LT, and suggest an ADP-ribosylation-independent adjuvant effect of the A subunit.

The role of ADP-ribosylation and G(M1)-binding activity in the mucosal immunogenicity and adjuvanticity of the Escherichia coli heat-labile enterotoxin and Vibrio cholerae cholera toxin

The mucosal route of vaccination has attracted a great deal of attention recently. Not only is mucosal application of vaccines, for example, orally or intranasally, particularly convenient, it also offers the possibility to induce locally produced and secreted S-IgA antibodies in addition to systemic IgG antibodies. These IgA antibodies are known to play a key role in protection against pathogens that invade the host through mucosal surfaces. Induction of such responses is not readily achieved by currently used vaccination strategies, which generally involve intramuscular or subcutaneous injection with inactivated pathogens or antigens thereof. For the induction of a mucosal immune response, the vaccine needs to be applied locally. However, local vaccination with non-replicating antigens is usually ineffective and may result in tolerance unless a mucosal immunoadjuvant is included. The most potent mucosal immunoadjuvants known to date are probably cholera toxin (CT) and the closely related Escherichia coli heat-labile enterotoxin (LT). Although CT and LT have become standard adjuvants for experimental mucosal vaccines, the intrinsic toxicity has thus far precluded their use as adjuvants for human vaccine formulations. In the present review, the mucosal immunogenic and adjuvant properties of LT and CT are described, with special emphasis on the functional role of the individual subunits on their immune-stimulatory properties.