Cholera toxin and its subunits as potential oral adjuvants

Biochemical and immunological characterization of an ETEC CFA/I adhesin cholera toxin B subunit chimera

Surface-expressed colonization factors and their subunits are promising candidates for inclusion into a multivalent vaccine targeting enterotoxigenic Escherichia coli (ETEC), a leading cause of acute bacterial diarrhea in developing regions. However, soluble antigens are often poorly immunogenic in the absence of an adjuvant. We show here that the serum immune response to CfaE, the adhesin of the ETEC colonization factor CFA/I, can be enhanced in BALB/c mice by immunization with a chimeric antigen containing CfaE and pentameric cholera toxin B subunit (CTB) of cholera toxin from Vibrio cholerae. We constructed this antigen by replacing the coding sequence for the A1 domain of the cholera toxin A subunit (CTA) with the sequence of donor strand complemented CfaE (dscCfaE) within the cholera toxin operon, resulting in a dscCfaE-CTA2 fusion. After expression, via non-covalent interactions between CTA2 and CTB, the fusion and CTB polypeptides assemble into a complex containing a single dscCfaE-CTA2 protein bound to pentameric CTB (dscCfaE-CTA2/CTB). This holotoxin-like chimera retained the GM1 ganglioside binding activity of CTB, as well as the ability of CfaE to mediate the agglutination of bovine red blood cells when adsorbed to polystyrene beads. When administered intranasally to mice, the presence of CTB in the chimera significantly increased the serum immune response to CfaE compared to dscCfaE alone, stimulating a response similar to that obtained with a matched admixture of dscCfaE and CTB. However, by the orogastric route, immunization with the chimera elicited a superior functional immune response compared to an equivalent admixture of dscCfaE and CTB, supporting further investigation of the chimera as an ETEC vaccine candidate.

Preformulation studies with the Escherichia coli double mutant heat-labile toxin adjuvant for use in an oral vaccine

Double mutant heat-labile toxin (dmLT) is a promising adjuvant for oral vaccine administration. The aims of our study were to develop sensitive methods to detect low concentrations of dmLT and to use the assays in preformulation studies to determine whether dmLT remains stable under conditions encountered by an oral vaccine. We developed a sandwich ELISA specific for intact dmLT and a sensitive SDS-PAGE densitometry method, and tested stability of dmLT in glass and plastic containers, in saliva, at the pH of stomach fluid, and in high-osmolarity buffers. The developed ELISA has a quantification range of 62.5 to 0.9 ng/mL and lower limit of detection of 0.3 ng/mL; the limit of quantification of the SDS-PAGE is 10 μg/mL. This work demonstrates the application of dmLT assays in preformulation studies to development of an oral vaccine containing dmLT. Assays reported here will facilitate the understanding and use of dmLT as an adjuvant.

NKp46+ Innate Lymphoid Cells Dampen Vaginal CD8 T Cell Responses following Local Immunization with a Cholera Toxin-Based Vaccine

Innate and adaptive immune cells work in concert to generate efficient protection at mucosal surface. Vaginal mucosa is an epithelial tissue that contains innate and adaptive immune effector cells. Our previous studies demonstrated that vaginal administration of Cholera toxin -based vaccines generate antigen-specific CD8 T cells through the stimulation of local dendritic cells (DC). Innate lymphoid cells (ILC) are a group of lymphocytes localized in epithelial tissues that have important immune functions against pathogens and in tissue homeostasis. Their contribution to vaccine-induced mucosal T cell responses is an important issue for the design of protective vaccines. We report here that the vaginal mucosa contains a heterogeneous population of NKp46⁺ ILC that includes conventional NK cells and ILC1-like cells. We show that vaginal NKp46⁺ ILC dampen vaccine-induced CD8 T cell responses generated after local immunization. Indeed, in vivo depletion of NKp46⁺ ILC with anti-NK1.1 antibody or NKG2D blockade increases the magnitude of vaginal OVA-specific CD8 T cells. Furthermore, such treatments also increase the number of DC in the vagina. NKG2D ligands being expressed by vaginal DC but not by CD8 T cells, these results support that NKp46⁺ ILC limit mucosal CD8 T cell responses indirectly through the NKG2D-dependent elimination of vaginal DC. Our data reveal an unappreciated role of NKp46⁺ ILC in the regulation of mucosal CD8 T cell responses.

Recent advances and safety issues of transgenic plant-derived vaccines

Transgenic plant-derived vaccines comprise a new type of bioreactor that combines plant genetic engineering technology with an organism's immunological response. This combination can be considered as a bioreactor that is produced by introducing foreign genes into plants that elicit special immunogenicity when introduced into animals or human beings. In comparison with traditional vaccines, plant vaccines have some significant advantages, such as low cost, greater safety, and greater effectiveness. In a number of recent studies, antigen-specific proteins have been successfully expressed in various plant tissues and have even been tested in animals and human beings. Therefore, edible vaccines of transgenic plants have a bright future. This review begins with a discussion of the immune mechanism and expression systems for transgenic plant vaccines. Then, current advances in different transgenic plant vaccines will be analyzed, including vaccines against pathogenic viruses, bacteria, and eukaryotic parasites. In view of the low expression levels for antigens in plants, high-level expression strategies of foreign protein in transgenic plants are recommended. Finally, the existing safety problems in transgenic plant vaccines were put forward will be discussed along with a number of appropriate solutions that will hopefully lead to future clinical application of edible plant vaccines.

Characterization of a mutant Escherichia coli heat-labile toxin, LT(R192G/L211A), as a safe and effective oral adjuvant

Despite the fact that the adjuvant properties of the heat-labile enterotoxins of Escherichia coli (LT) and Vibrio cholerae (CT) have been known for more than 20 years, there are no available oral vaccines containing these molecules as adjuvants, primarily because they are both very potent enterotoxins. A number of attempts with various degrees of success have been made to reduce or eliminate the enterotoxicity of LT and CT so they can safely be used as oral adjuvants or immunogens. In this report we characterize the structural, enzymatic, enterotoxic, and adjuvant properties of a novel mutant of LT, designated LT(R192G/L211A), or dmLT. dmLT was not sensitive to trypsin activation, had reduced enzymatic activity for induction of cyclic AMP in Caco-2 cells, and exhibited no enterotoxicity in the patent mouse assay. Importantly, dmLT retained the ability to function as an oral adjuvant for a coadministered antigen (tetanus toxoid) and to elicit anti-LT antibodies. In vitro and in vivo data suggest that the reduced enterotoxicity of this molecule compared to native LT or the single mutant, LT(R192G), is a consequence of increased sensitivity to proteolysis and rapid intracellular degradation in mammalian cells. In conclusion, dmLT is a safe and powerful detoxified enterotoxin with the potential to function as a mucosal adjuvant for coadministered antigens and to elicit anti-LT antibodies without undesirable side effects.

Oral vaccine formulations stimulate mucosal and systemic antibody responses against staphylococcal enterotoxin B in a piglet model

Despite the potential for its use as an agent of biowarfare or bioterrorism, no approved vaccine against staphylococcal enterotoxin B (SEB) exists. Nontoxic, mutant forms of SEB have been developed; however, it has been difficult to determine the efficacy of such subunit vaccine candidates due to the lack of superantigen activity of native SEB in rodents and due to the limitations of primate models. Since pigs respond to SEB in a manner similar to that of human subjects, we utilized this relevant animal model to investigate the safety and immunogenicity of a triple mutant of SEB carrying the amino acid changes L45R, Y89A, and Y94A. This recombinant mutant SEB (rmSEB) did not possess superantigen activity in pig lymphocyte cultures. Furthermore, rmSEB was unable to compete with native SEB for binding to pig leukocytes. These in vitro studies suggested that rmSEB could be a safe subunit vaccine. To test this possibility, piglets immunized orally with rmSEB formulations experienced no significant decrease in food consumption and no weight loss during the vaccination regimen. Oral vaccination with 1-mg doses of rmSEB on days 0, 7, 14, and 24 resulted in serum IgG and fecal IgA levels by day 36 that cross-reacted with native SEB. Surprisingly, the inclusion of cholera toxin adjuvant in vaccine formulations containing rmSEB did not result in increased antibody responses compared to formulations using the immunogen alone. Taken together, these studies provide additional evidence for the potential use of nontoxic forms of SEB as vaccines.

Vaccine adjuvants: current challenges and future approaches

For humans, companion animals, and food producing animals, vaccination has been touted as the most successful medical intervention for the prevention of disease in the twentieth century. However, vaccination is not without problems. With the development of new and less reactogenic vaccine antigens, which take advantage of molecular recombinant technologies, also comes the need for more effective adjuvants that will facilitate the induction of adaptive immune responses. Furthermore, current vaccine adjuvants are successful at generating humoral or antibody mediated protection but many diseases currently plaguing humans and animals, such as tuberculosis and malaria, require cell mediated immunity for adequate protection. A comprehensive discussion is presented of current vaccine adjuvants, their effects on the induction of immune responses, and vaccine adjuvants that have shown promise in recent literature.

Effect of adjuvants and route of immunizations on the immune response to recombinant plague antigens

In this study, we compare four different adjuvants, LT(R192G), CpG ODN, MPL((R))TDM, and alum, for their ability to affect the magnitude, distribution, and duration of antibody responses against F1-V, the lead-candidate antigen for the next generation vaccine against plague, in a murine model. In addition, three different routes of immunization-intranasal (IN), transcutaneous (TC), and subcutaneous (SC) were compared with each adjuvant. Since aerosol exposure to biological warfare agents is of primary concern, both serum and bronchioalveolar lavage (BAL) were analyzed for antigen-specific antibody responses. The most significant findings of the study reported here are that (1) the adjuvant influences the Type 1/Type 2 balance of the antibody response in both the serum and BAL, (2) mucosal immunization is not necessary to obtain F1-V-specific BAL responses, (3) non-traditional adjuvants such as LT(R192G) work when delivered subcutaneously, (4) the route of immunization affects the magnitude of the immune response, and (5) F1-V is highly immunogenic by some routes even in the absence of an exogenously applied adjuvant. These studies provide important insights into the influence of different classes of adjuvants on the immune outcome in biodefense vaccines and for development of new-generation vaccines against other pathogens as well.

Expression of a synthetic cholera toxin B subunit in tobacco using ubiquitin promoter and bar gene as a selectable marker

A protocol has been developed to produce a cholera toxin B subunit (CTB) in tobacco tolerant to the herbicide phosphinothricin (PPT) by means of in vitro selection. The synthetic CTB subunit gene was altered to modify the codon usage to that of tobacco plant genes. The gene was then cloned into a plant expression vector and was under the control of the ubiquitin promoter and transformed into tobacco plants by Agrobacterium-mediated transformation. Transgenic plantlets were selected in a medium supplemented with 5 mg/L PPT. Polymerase chain reaction analysis confirmed stable integration of the synthetic CTB gene into a chromosomal DNA. A high level of CTB (1.8% of total soluble protein) was expressed in transgenic plants, which was 18-fold higher than that under the control of the expressed CaMV 35S promoter with native gene. The transgenic plants when transferred to a greenhouse proved to be resistant to 2% PPT.

Mucosal adjuvant properties of mutant LT-IIa and LT-IIb enterotoxins that exhibit altered ganglioside-binding activities

LT-IIa and LT-IIb, the type II heat-labile enterotoxins of Escherichia coli, are closely related in structure and function to cholera toxin and LT-I, the type I heat-labile enterotoxins of Vibrio cholerae and E. coli, respectively. Recent studies from our group demonstrated that LT-IIa and LT-IIb are potent systemic and mucosal adjuvants. To determine whether binding of LT-IIa and LT-IIb to their specific ganglioside receptors is essential for adjuvant activity, LT-IIa and LT-IIb enterotoxins were compared with their respective single-point substitution mutants which have no detectable binding activity for their major ganglioside receptors [e.g., LT-IIa(T34I) and LT-IIb(T13I)]. Both mutant enterotoxins exhibited an extremely low capacity for intoxicating mouse Y1 adrenal cells and for inducing production of cyclic AMP in a macrophage cell line. BALB/c female mice were immunized by the intranasal route with the surface adhesin protein AgI/II of Streptococcus mutans alone or in combination with LT-IIa, LT-IIa(T34I), LT-IIb, or LT-IIb(T13I). Both LT-IIa and LT-IIb potentiated strong mucosal and systemic immune responses against AgI/II. Of the two mutant enterotoxins, only LT-IIb(T13I) had the capacity to strongly potentiate mucosal anti-AgI/II and systemic anti-AgI/II antibody responses. Upon boosting with AgI/II, however, both LT-IIa(T34I) and LT-IIb(T13I) enhanced humoral memory responses to AgI/II. Flow cytometry demonstrated that LT-IIa(T34I) had no affinity for cervical lymph node lymphocytes. In contrast, LT-IIb(T13I) retained binding activity for T cells, B cells, and macrophages, indicating that this immunostimulatory mutant enterotoxin interacts with one or more unknown lymphoid cell receptors.

Cholera toxin B-subunit gene enhances mucosal immunoglobulin A, Th1-type, and CD8+ cytotoxic responses when coadministered intradermally with a DNA vaccine

A plasmid vector encoding the cholera toxin B subunit (pCtB) was evaluated as an intradermal genetic adjuvant for a model DNA vaccine expressing the human papillomavirus type 16 L1 capsid gene (p16L1) in mice. p16L1 was coadministered with plasmid pCtB or commercial polypeptide CtB as a positive control. Coadministration of pCtB induced a significant increment of specific anti-L1 immunoglobulin A (IgA) antibodies in cervical secretions (P < 0.05) and fecal extracts (P < 0.005). Additionally, coadministration of pCtB enhanced the production of interleukin-2 and gamma interferon by spleen cells but did not affect the production of interleukin-4, suggesting a Th1-type helper response. Furthermore, improved CD8+ T-cell-mediated cytotoxic activity was observed in mice vaccinated with the DNA vaccine with pCtB as an adjuvant. This adjuvant effect was comparable to that induced by the CtB polypeptide. These results indicate that intradermal coadministration of pCtB is an adequate means to enhance the mucosa-, Th1-, and CD8(+)-mediated cytotoxic responses induced by a DNA vaccine.

Intravaginal and intranasal immunizations are equally effective in inducing vaginal antibodies and conferring protection against vaginal candidiasis

Oophorectomized, estrogen-treated rats were immunized by the intravaginal or intranasal route with a mannoprotein extract (MP) or secreted aspartyl proteinases (Sap) of Candida albicans, with or without cholera toxin as a mucosal adjuvant. Both routes of immunization were equally effective in (i) inducing anti-MP and anti-Sap vaginal antibodies and (ii) conferring a high degree of protection against the vaginal infection by the fungus. These data suggest that appropriate fungal antigens and adjuvant can be used to protect against candidal vaginitis, by either route.

Differential biological and adjuvant activities of cholera toxin and Escherichia coli heat-labile enterotoxin hybrids

Two bacterial products that have been demonstrated to function as mucosal adjuvants are cholera toxin (CT), produced by various strains of Vibrio cholerae, and the heat-labile enterotoxin (LT) produced by some enterotoxigenic strains of Escherichia coli. Although LT and CT have many features in common, they are clearly distinct molecules with biochemical and immunologic differences which make them unique. The goal of this study was to determine the basis for these biological differences by constructing and characterizing chimeric CT-LT molecules. Toxin gene fragments were subcloned to create two constructs, each expressing the enzymatically active A subunit of one toxin and the receptor binding B subunit of the other toxin. These hybrid toxins were purified, and the composition and assembly of CT A subunit (CT-A)-LT B subunit (LT-B) and LT A subunit (LT-A)-CT B subunit (CT-B) were confirmed. Hybrids were evaluated for enzymatic activity, as measured by the accumulation of cyclic AMP in Caco-2 cells, and the enterotoxicity of each toxin was assessed in a patent-mouse assay. The results demonstrated that LT-A-CT-B induces the accumulation of lower levels of cyclic AMP and has less enterotoxicity than either wild-type toxin or the other hybrid. Nonetheless, this hybrid retains adjuvant activity equivalent to or greater than that of either wild-type toxin or the other hybrid when used in conjunction with tetanus toxoid for intranasal immunization of BALB/c mice. Importantly, the ability of LT to induce a type 1 cytokine response was found to be a function of LT-A. Specifically, LT-A-CT-B was able to augment the levels of antigen-specific gamma interferon (IFN-gamma) and interleukin 5 to levels comparable to those achieved with native LT, while CT-A-LT-B and native CT both produced lower levels of antigen-specific IFN-gamma. Thus, these toxin hybrids possess unique biological characteristics and provide information about the basis for differences in the biological activities observed for CT and LT.

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.

Host-pathogen interaction in amebiasis and progress in vaccine development

Entamoeba histolytica, the causative organism of invasive intestinal and extraintestinal amebiasis, infects approximately 50 million people each year, causing an estimated 40 to 100 thousand deaths annually. Because amebae only infect humans and some higher non-human primates, an anti-amebic vaccine could theoretically eradicate the organism. Uncontrolled epidemiologic studies indicate that acquired immunity to amebic infection probably occurs and that such a vaccine might be feasible. Application of molecular biologic techniques has led to rapid progress towards understanding how Entamoeba histolytica causes disease, and to the identification of several amebic proteins associated with virulence. These proteins are now being evaluated as potential vaccine components. Parenteral and oral vaccine preparations containing recombinant amebic proteins have been effective in preventing disease in a gerbil model of amebic liver abscess. Although systemic and mucosal cellular and humoral immunity both appear to play a role in protection against Entamoeba histolytica, the relative importance of each in the human immune response remains unknown. No animal model of intestinal amebiasis currently exists, moreover, so it has been impossible to evaluate protection against colonization and colitis. Further investigation of the fundamental mechanisms by which Entamoeba histolytica causes disease and of the human immune response to amebic infection is necessary to assess the true feasibility of an anti-amebic vaccine.

HIV-1 Neutralizing Antibodies in the Genital and Respiratory Tracts of Mice Intranasally Immunized with Oligomeric gp160

Because mucosal surfaces are a primary route of HIV-1 infection, we evaluated the mucosal immunogenicity of a candidate HIV-1 vaccine, oligomeric gp160 (o-gp160). In prior studies, parenteral immunization of rabbits with o-gp160 elicited broad neutralizing serum Ab responses against both T cell line-adapted HIV-1 and some primary HIV-1 isolates. In this study, nasal immunization of mice with o-gp160, formulated with liposomes containing monophosphoryl lipid A (MPL), MPL-AF, proteosomes, emulsomes, or proteosomes with emulsomes elicited strong gp160-specific IgG and IgA responses in serum as well as vaginal, lung, and intestinal washes and fecal pellets. The genital, respiratory, and intestinal Abs were determined to be locally produced. No mucosal immune responses were measurable when the immunogen was given s.c. Abs from sera and from vaginal and lung washes preferentially recognized native forms of monomeric gp120, suggesting no substantial loss in protein tertiary conformation after vaccine formulation and mucosal administration. Inhibition of HIV-1MN infection of H9 cells was found in sera from mice immunized intranasally with o-gp160 formulated with liposomes plus MPL, proteosomes, and proteosomes plus emulsomes. Formulations of o-gp160 with MPL-AF, proteosomes, emulsomes, or proteosomes plus emulsomes elicited HIV-1MN-neutralizing Ab in lung wash, and formulations with proteosomes, emulsomes, or proteosomes plus emulsomes elicited HIV-1MN-neutralizing Ab in vaginal wash. These data demonstrate the feasibility of inducing both systemic and mucosal HIV-1-neutralizing Ab by intranasal immunization with an oligomeric gp160 protein.

Role of receptor binding in toxicity, immunogenicity, and adjuvanticity of Escherichia coli heat-labile enterotoxin

The role of receptor binding in the toxicity, immunogenicity, and adjuvanticity of the heat-labile enterotoxin of Escherichia coli (LT) was examined by comparing native LT and LT(G33D), a B-subunit receptor binding mutant, with respect to the ability to bind to galactose and to GM1, toxicity on mouse Y-1 adrenal tumor cells, the ability to stimulate adenylate cyclase in Caco-2 cells, enterotoxicity in the patent mouse model, and oral immunogenicity and adjuvanticity. In contrast to native LT, LT(G33D) was unable to bind to the galactosyl moiety of Sepharose 4B or GM1 but did retain the lectin-like ability to bind to immobilized galactose on 6% agarose beads. LT(G33D) had no enterotoxicity in the patent mouse model but exhibited residual toxicity on mouse Y-1 adrenal tumor cells and had an ability equivalent to that of native LT to stimulate adenylate cyclase in Caco-2 cells (5,000 versus 6,900 pmol per mg of protein). In addition, LT(G33D) was unable to serve as an effective oral adjuvant for induction of immunoglobulin G or A directed against a coadministered antigen. Furthermore, LT(G33D) elicited negligible serum and mucosal antibody responses against itself. These data indicate that the toxicity, immunogenicity, and oral adjuvanticity of LT are dependent upon binding of the B subunit to ganglioside GM1.

Development of antibody-secreting cells and antigen-specific T cells in cervical lymph nodes after intranasal immunization

Intranasal (i.n.) immunization with bacterial protein antigens coupled to cholera toxin B subunit (CTB) effectively induces mucosal, especially salivary immunoglobulin A (IgA), and nonmucosal antibody responses in mice. To examine the regional distribution of antigen-specific B and T cells after i.n. immunization, antibody-secreting cells and antigen-responsive T cells in cervical lymph nodes (CLN) were compared with those found after intraoral or subcutaneous (in the neck) administration of the same antigen and with T cells found in mesenteric lymph nodes (MLN) and spleen after intragastric immunization. The i.n. immunization induced predominantly IgA antibody-secreting cells in salivary glands and IgA and IgG antibody-secreting cells in the superficial and central CLN; these responses were quantitatively enhanced if the antigen was coupled to CTB. Intraoral immunization also induced IgA and IgG antibody-secreting cells in the superficial and central CLN, but only if intact cholera toxin was included as an adjuvant. In contrast, subcutaneous (neck) immunization induced IgG antibody-secreting cells mainly in the draining facial lymph nodes. CLN cell populations resembled those of MLN, except that CLN lymphocytes had higher proportions of T cells and lower proportions of B cells and a slightly higher CD4+/CD8+ ratio among T cells than the MLN lymphocytes did. T cells that proliferated in response to antigen in vitro were found especially in central CLN 2 days after i.n. immunization and persisted for up to 6 months, whereas after intragastric immunization, responsive T cells were not found in the MLN for up to 14 days. After culture with antigen in vitro, T cells from the superficial CLN of i.n. immunized mice secreted both gamma interferon and interleukin-4. Therefore, after i.n. immunization, superficial and central CLN represent sites of regional lymphocyte development, and the central CLN in particular appear to be sites where memory T cells persist.

Salivary, nasal, genital, and systemic antibody responses in monkeys immunized intranasally with a bacterial protein antigen and the Cholera toxin B subunit

Previous attempts to induce mucosal antibodies in rhesus monkeys by enteric immunization have resulted in only modest and short-lived responses, dominated by immunoglobulin M (IgM) antibodies in the plasma. In this study, two groups of rhesus monkeys were immunized intranasally three times at 2-week intervals with a bacterial protein antigen (AgI/II) either chemically coupled to or mixed with the B subunit of cholera toxin (CT), a known potent mucosal immunogen and carrier for other immunogens. Cells secreting antibodies, predominantly of the IgA isotype, to AgI/II and to CT were detected in the peripheral blood 1 week after each immunization, indicating the dissemination of IgA-secreting precursor cells through the mucosal immune system. IgG and, to a lesser extent, IgA antibodies to both proteins were induced in the plasma commencing after the second immunization. Plasma IgE concentrations and IgE antibody levels were not consistently raised during the immunization period. IgA antibodies were found in nasal and vaginal washes. Nasal IgG but not IgA antibodies showed a significant positive correlation with plasma IgG antibody levels, suggesting that they were largely derived by transudation from the circulation. Analysis of the molecular form of vaginal IgA indicated that both secretory and monomeric forms of IgA were present in various proportions. Furthermore, neither IgG nor IgA antibodies in vaginal washes were correlated with plasma antibody responses, suggesting the contribution of locally synthesized antibodies of both isotypes. Comparison of the responses between the two groups of animals showed only sporadic significant differences, indicating that intranasal immunization with AgI/II either coupled to or mixed with the B subunit of CT was equally effective at inducing generalized IgA antibody responses in the mucosal immune system and predominantly IgG antibodies in the plasma.

Immunisation against gastric infection with Helicobacter species: first step in the prophylaxis of gastric cancer?

The discovery of the gastric bacterium, Helicobacter pylori and the demonstration of its role in the pathogenesis of gastroduodenal disease, has been one of the major microbiological advances in the last decade. Recent demonstration of long term infection with this bacterium as a risk factor in gastric carcinoma suggests that intervention in a disease of major morbidity and mortality is possible. Using a model of Helicobacter infection in mice it has been shown that oral immunisation with a sonicate of Helicobacter felis plus the adjuvant cholera toxin results in protection against an oral challenge with large numbers of viable bacteria. The success of the immunising regimen has been shown to correlate with the development of local immunity. Formulation of equivalent safe vaccines of H. pylori will make possible the immunisation of children in countries such as China, Japan and Columbia and so prevent the establishment of long term inflammation and thus significantly reduce the incidence of gastric cancer in those societies. This animal model is proposed as a major tool in the development of effective oral immunisation.

Immune response of the female rat genital tract after oral and local immunization with keyhole limpet hemocyanin conjugated to the cholera toxin B subunit

The immune response of the female rat genital tract was evaluated with Lewis rats given primary and secondary immunizations with keyhole limpet hemocyanin (KLH) alone or coupled to the cholera toxin (CT) B subunit (CTB) by the oral or intravaginal-uterine route or a combination of routes. CT (2 to 5 micrograms) was administered as an adjuvant with the KLH-CTB conjugate. While a significant mucosal immunoglobulin A (IgA) response was induced by KLH, there were no significant differences among the immunized groups in the levels of IgA antibodies in salivary gland, gut, vaginal, and uterine secretions, with the exception that rats immunized only orally with the KLH-CTB conjugate lacked a detectable vaginal response. Levels of IgA antibodies to CT, however, were significantly increased in genital tract secretions of rats immunized locally versus orally with the KLH-CTB conjugate. Antibody activity of the IgG isotype against both KLH and CT was significantly elevated in genital tract secretions of rats immunized with KLH-CTB by the oral or intravaginal-uterine route and given genital tract boosters, in comparison with the results for the other groups. IgM antibody titers were generally negligible in the different secretions. An enzyme-linked spot-forming assay revealed IgA and IgG antibody-secreting cells in salivary gland and uterine tissues. A highly significant correlation between the numbers of antibody-secreting cells and antibody titers existed for uterine IgG but not IgA responses to KLH among the different groups of rats. In conclusion, a vigorous local immune response was induced after immunization of the female rat reproductive tract alone or in combination with peroral challenge with the KLH-CTB conjugate.

Prospects for human mucosal vaccines

The selective induction of antibodies in external secretions and mucosal T cell-mediated immunity are desirable for the prevention of various systemic as well as predominantly mucosa-restricted infections. An enormous surface area of mucosal membranes is protected primarily by antibodies that belong, in many species, to the IgA isotype. Such antibodies are produced locally by large numbers of IgA-containing plasma cells distributed in subepithelial spaces of mucosal membranes and in the stroma of secretory glands. In humans and in some animal species, plasma-derived IgA antibodies do not enter external secretions in significant quantities and systemically administered preformed IgA antibodies would be of little use for passive immunization. Systemic administration of microbial antigens may boost an effective S-IgA immune response only in a situation whereby an immunized individual had previously encountered the same antigen by the mucosal route. Immunization routes that involve ingestion or possibly inhalation of antigens lead to the induction of not only local but also generalized immune responses, manifested by the parallel appearance of S-IgA antibodies to ingested or inhaled antigens in secretions of glands distant from the site of immunization. Convincing evidence is available that antigen-sensitized and IgA-committed precursors of plasma cells and T cells from IgA inductive sites (e.g., BALT, GALT, and tonsils) are disseminated to the gut, other mucosa-associated tissues, and exocrine glands. However, due to the limited absorption of desired antigens from the gut lumen of orally immunized individuals, repeated large doses of antigens are required for an effective S-IgA response. Novel antigen delivery systems for the stimulation of such responses has been briefly reviewed here. These, of course, include genetically engineered bacteria and viruses, CT/CFB, liposomes and microspheres. Live attenuated or genetically manipulated bacteria expressing other microbial antigens have been used for selective colonization of GALT. Unique antigen packaging and the use of adjuvants suitable for oral administration hold promise for an efficient antigen delivery to critical tissues in the intestine and deserve extensive exploration. The oral immunization route appears to have many advantages over systemic immunization; however, one must consider alternate IgA inductive sites and compartmentalization within the Common Mucosal Immune System. In addition to providing immunity on mucosal surfaces, which are the most common sites of entry of infectious agents, the mucosal routes of administration are more acceptable and do not require stringent criteria applicable for injectable vaccines, storage problems may be simplified, and large populations of individuals can be immunized simultaneously without the assistance of highly trained health personnel.

Distribution, persistence, and recall of serum and salivary antibody responses to peroral immunization with protein antigen I/II of Streptococcus mutans coupled to the cholera toxin B subunit

After peroral immunization of mice with surface protein antigen (Ag) I/II of Streptococcus mutans conjugated to the cholera toxin B (CTB) subunit, cells actively secreting immunoglobulin A (IgA) antibodies specific for Ag I/II, but not for CT, were induced in the salivary glands; salivary IgA anti-Ag I/II antibodies and total salivary IgA were also elevated. The development of large numbers of IgA and IgG antibody-secreting cells in the mesenteric lymph nodes and spleen and high levels of serum IgA and IgG antibodies to Ag I/II and CT demonstrated that a response to both antigens occurred. At least two to three intragastric doses of 15 micrograms or more of Ag I/II-CTB conjugate, plus free CT as an adjuvant, were needed to induce the salivary IgA anti-Ag I/II response, which peaked at about 35 days and persisted at lower levels for 5 to 6 months. A single booster intragastric immunization did not induce enhanced salivary IgA anti-Ag I/II antibodies relative to the primary response, but serum IgA and IgG antibodies to both Ag I/II and CT showed evidence of marked anamnestic responses. The results indicated that relatively long-term mucosal IgA antibody responses could be induced by peroral immunization with small quantities of a CTB-conjugated protein. However, additional factors governed the distribution of cells secreting antibodies of different specificities, or capable of mounting anamnestic responses, between different compartments of the mucosal and circulatory immune systems.

Oral administration of SSG, a beta-glucan obtained from Sclerotinia sclerotiorum, affects the function of Peyer's patch cells

The effect of orally administered SSG, a beta-1,3-glucan obtained from the culture filtrate of a fungus, Sclerotinia sclerotiorum IFO 9395, on the function of Peyer's patch (PP) cells was investigated in comparison with that on spleen cells in mice. Oral administration of SSG enhanced the proliferative response of PP cells to a T-cell mitogen, concanavalin A (Con A), and a B-cell mitogen, lipopolysaccharide (LPS), although the response of spleen cells was not affected. Peyer's patch cells taken from mice which had received oral administration of SSG two days before, showed enhanced plaque-forming cell (PFC) response to sheep red blood cells (SRBC) after antigen (SRBC) stimulation for 5 days in vitro. These results suggest that oral administration of SSG can modulate the mucosal immune response.