Mutants of Escherichia coli heat-labile enterotoxin as an adjuvant for nasal influenza vaccine

Progress towards a needle-free hepatitis B vaccine

Hepatitis B virus (HBV) infection is a worldwide public health problem. Vaccination is the most efficient way to prevent hepatitis B. Despite the success of the currently available vaccine, there is a clear need for the development of new generation of HBV vaccines. Needle-free immunization is an attractive approach for mass immunization campaigns, since avoiding the use of needles reduces the risk of needle-borne diseases and prevents needle-stick injuries and pain, thus augmenting patient compliance and eliminating the need for trained medical personnel. Moreover, this kind of immunization was shown to induce good systemic as well as mucosal immunological responses, which is important for the creation of both a prophylactic and therapeutic vaccine. In order to produce a better, safer, more efficient and more suitable vaccine, adjuvants have been used. In this article, several adjuvants tested over the years for their potential to help create a needle-free vaccine against HBV are reviewed.

The encapsulation of enterotoxigenic Escherichia coli colonization factor CS3 in biodegradable microspheres enhances the murine antibody response following intranasal administration

The aim of this study was to measure serum and mucosal antibody responses following intranasal administration of biodegradable poly(dl-lactide-co-glycolide) (PLGA) microspheres loaded with the CS3 colonization factor isolated from enterotoxigenic Escherichia coli (ETEC). The response was compared against that measured in mice similarly administered the native CS3 antigen and in mice co-administered, along with the CS3 antigen, a known mucosal adjuvant, the R192G mutant heat-labile enterotoxin (mLT). The integrity of the CS3 antigen released from the microspheres was maintained as determined by SDS-PAGE and immunoblotting. Native CS3 induced serum and mucosal (bronchoalveolar, small intestinal and faecal) IgG and IgA responses. The co-administration of the mLT mucosal adjuvant significantly enhanced (P<0·001) serum and mucosal antibody responses to the CS3 protein. Likewise, the CS3-loaded PLGA microspheres induced significantly greater (P<0·001) serum and mucosal antibody responses than native CS3, as well as inducing antibody responses superior to those of the CS3 plus mLT formulation. Following administration of CS3 plus mLT, the mice became distressed (loss of activity, increased huddling, ruffled fur), a situation not seen following administration of the CS3-loaded PLGA microspheres. The results in this trial show that the CS3-loaded PLGA microspheres when administered intranasally to mice caused no observable distress to the mice and significantly (P<0·001) enhanced the immunogenicity of the CS3 protein.

Protection against influenza virus infection by intranasal vaccine with surf clam microparticles (SMP) as an adjuvant

A safe and effective adjuvant is necessary to enhance mucosal immune responses for the development of an inactivated intranasal influenza vaccine. The present study demonstrated the effectiveness of surf clam microparticles (SMP) derived from natural surf clams as an adjuvant for an intranasal influenza vaccine. The adjuvant effect of SMP was examined when co-administered intranasally with inactivated A/PR8 (H1N1) influenza virus hemagglutinin vaccine in BALB/c mice. Administration of the vaccine with SMP induced a high anti-PR8 haemagglutinin (HA)-specific immunoglobulin A (IgA) response in the nasal wash and immunoglobulin G (IgG) response in the serum, resulting in protection against both nasal-restricted infection and lethal lung infection by A/PR8 virus. In addition, administration of SMP with A/Yamagata (H1N1), A/Beijing (H1N1), or A/Guizhou (H3N2) vaccine conferred complete protection against A/PR8 virus challenge in the nasal infection model, suggesting that SMP adjuvanted vaccine can confer cross-protection against variant influenza viruses. The use of SMP is suggested as a new safe and effective mucosal adjuvant for nasal vaccination against influenza virus infection.

Enterotoxin-based mucosal adjuvants alter antigen trafficking and induce inflammatory responses in the nasal tract

The safety of nasal vaccines containing enterotoxin-based mucosal adjuvants has not been studied in detail. Previous studies have indicated that native cholera toxin (nCT) can alter antigen trafficking when applied nasally. In this study, we determined the enterotoxin-based variables that alter antigen trafficking. To measure the influence of enterotoxin-based mucosal adjuvants on antigen trafficking in the nasal tract, native and mutant enterotoxins were coadministered with radiolabeled tetanus toxoid (TT). The nCT and heat-labile enterotoxin type 1 (LTh-1) redirected TT into the olfactory neuroepithelium (ON/E). Antigen redirection occurred mainly across the nasal epithelium without subsequent transport along olfactory neurons into the olfactory bulbs (OB). Thus, no significant accumulation of the vaccine antigen TT was observed in the OB when coadministered with nCT. In contrast, neither mutant CT nor mutant LTh-1, which lack ADP-ribosyltransferase activity, redirected TT antigen into the ON/E. Thus, ADP-ribosyltransferase activity was essential for antigen trafficking across the olfactory epithelium. Accumulation of TT in the ON/E was also due to B-subunit binding to GM1 gangliosides, as was demonstrated (i) by redirection of TT by LTh-1 in a dose-dependent manner, (ii) by ganglioside inhibition of the antigen redirection by LTh-1 and nCT, and (iii) by the use of LT-IIb, a toxin that binds to gangliosides other than GM1. Redirection of TT into the ON/E coincided with elevated production of interleukin 6 (IL-6) but not IL-1beta or tumor necrosis factor alpha in the nasal mucosa. Thus, redirection of TT is dependent on ADP-ribosyltransferase activity and GM1 binding and is associated with production of the inflammatory cytokine IL-6.

Effective nasal influenza vaccine delivery using chitosan

Nasal influenza vaccination may prove to be a good alternative to parenteral injection because of the enhancement of the mucosal immune response and the ease of vaccine administration. This study investigated the use of chitosan, a bioadhesive polymer, as a nasal delivery system with inactivated, subunit influenza vaccine. Subjects received nasally 15 or 7.5 microg of the standard inactivated trivalent influenza vaccine with chitosan or 15 microg of the same vaccine intramuscularly. Serum haemagglutination inhibition (HI) titres for all three vaccine components were measured prior to, and at time points up to 14 weeks after dosing. Serum HI titres following intranasal vaccination with the nasal chitosan-influenza vaccine met the criteria set by the Committee for Proprietary Medicinal Products in terms of seroprotection rate, seroconversion rate and mean fold increase of HI titre for at least one of the three antigens in the vaccination schedules used. These data show that nasal immunisation with chitosan plus trivalent inactivated influenza is a potentially effective, easily-administered form of vaccination.

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.

Protection against influenza virus infection by intranasal administration of hemagglutinin vaccine with chitin microparticles as an adjuvant

Chitin in the form of microparticles (chitin microparticles, CMP) has been demonstrated to be a potent stimulator of macrophages, promoting T-helper-1 (Th1) activation and cytokine response. In order to examine the mucosal adjuvant effect of CMP co-administered with influenza hemagglutinin (HA) vaccine against influenza infection, CMP were intranasally co-administered with influenza HA vaccine prepared from PR8 (H1N1) virus. Inoculation of the vaccine with CMP induced primary and secondary anti-HA IgA responses in the nasal wash and anti-HA IgG responses in the serum, which were significantly higher than those of nasal vaccination without CMP, and provided a complete protection against a homologous influenza virus challenge in the nasal infection influenza model. In addition, CMP-based immunization using A/Yamagata (H1N1) and A/Guizhou (H3N2) induced PR8 HA-reactive IgA in the nasal washes and specific-IgG in the serum. The immunization with A/Yamagata and CMP resulted in complete protection against a PR8 (H1N1) challenge in A/Yamagata (H1N1)-vaccinated mice, while that with A/Guizhou (H3N2) and CMP exhibited a 100-fold reduction of nasal virus titer, demonstrating the cross-protective effect of CMP and influenza vaccine. It is suggested that CMP provide a safe and effective adjuvant for nasal vaccination with inactivated influenza vaccine.

Protection against influenza virus infection by intranasal administration of C3d-fused hemagglutinin

For the induction of mucosal immune responses by intranasal vaccination, cholera toxin B subunits (CTB) and Escherichia coli heat-labile toxin (LT) are often administered as mucosal adjuvants in order to enhance immune responses to mucosally co-administered bystander antigens. However, these toxin also are the causative agents of diarrhea. There is a demand for the establishment of an effective and safer adjuvant or vaccine that elicits mucosal immunity, but does not require the use of CTB or LT adjuvants. In order to induce protective mucosal immune responses in the nasal area against influenza virus infection, we have examined the recombinant protein composed of the complement component, C3d, which is fused to the secreted form of hemagglutinin (sHA-mC3d3) in the influenza-BALB/c mouse model. The fusion protein sHA-mC3d3, the secretory form of hemagglutinin, and the transmembrane form of HA (tmHA) from the influenza virus were intranasally administered to the mice with or without CTB containing a trace amount of holotoxin (CTB*) as an adjuvant. After intranasal administration of these proteins with CTB*, all mice produced nasal IgA and serum IgG antibodies (Abs) against the viral HA. In addition, viral infection was completely inhibited in these mice. In contrast, in the absence of the adjuvant, only sHA-mC3d3-induced locally secreted IgA and serum IgG Abs and provided complete protection against the influenza virus challenge. Thus, C3d fused to the influenza HA antigen is an effective and safe tool for mucosal vaccination.

Characterization of protective immune responses induced by nasal influenza vaccine containing mutant cholera toxin as a safe adjuvant (CT112K)

Immune responses induced by a nasal influenza vaccine with a mutant cholera toxin (CT112K), known to be a safe adjuvant, were characterized in BALB/c mice to confirm the most suitable regimen of this vaccine for humans. Mice received a primary intranasal administration of the adjuvant (0.1 micro g)-combined PR8 vaccine (0.1 micro g) and a secondary administration of the PR8 vaccine alone (0.1 micro g) 4 weeks later. Two weeks after the secondary immunization, the mice were infected with a nonlethal or a lethal dose of PR8 viruses. Nasal and lung wash virus titers 1 or 3 days after infection indicated that complete protection could be provided by secondary immune responses, which had an immediate effect of preventing infection 2 weeks after the secondary immunization. In this two-dose regimen, high levels of secondary IgA, IgG and IgM antibody-forming cell (AFC) responses were induced in the nasal-associated lymphoid tissue and the spleen. In parallel with the AFC responses, high levels of nasal wash anti-PR8 HA IgA, and lung and serum IgG antibody (Ab) responses were induced 2 weeks after the secondary immunization. The two-dose regimen also induced accelerated delayed-type hypersensitivity responses, which exhibited almost the same peak height as that in the case of the primary response. In addition, the two-dose regimen induced a low memory cell activity of cytotoxic T lymphocytes, detected by in vitro culture of spleen cells. Thus, the immediate effect of preventing infection was mainly provided by the secondary Ab responses. Moreover, the levels of nasal wash IgA Abs correlated well with cross-protection against infection with variant viruses in the upper respiratory tract (RT). These results suggest that the major protective factors among Ab and T cell-mediated immune responses, which are induced by the two-dose regimen using CT112K-combined vaccines, are the cross-reactive IgA Abs in the upper RT and the less cross-reactive IgG Abs in the lower RT, and that the two-dose regimen is a suitable vaccination condition for humans.

Intranasal immunization against influenza

Nasalflu is a novel influenza subunit vaccine, which is administered by the intranasal route using a spray device. Nasalflu is based on the virosomal concept which is registered in the EU as Epaxal Berna, a vaccine against Hepatitis A, and Inflexal Berna V, a subunit influenza vaccine. The virosome is a carrier system which delivers antigens to cells and is able to induce both B- and T-cell immunity. When virosomal vaccines are given parenterally, an immune response is elicited fast and sufficiently.

A dilemma for mucosal vaccination: efficacy versus toxicity using enterotoxin-based adjuvants

In the development of mucosal vaccines, cholera toxin (CT) has been shown to be an effective adjuvant and to induce both mucosal and systemic immune responses via a Th2 cell-dependent pathway. However, a major concern for use of mucosal adjuvants such as CT is that this molecule is not suitable for use in humans because of its innate toxicity. Recent vaccine development efforts have emphasized nasal application of antigen and CT for the induction of mucosal IgA responses. When we examined potential toxicity of CT for the central nervous system (CNS), both CT and CT-B accumulated in the olfactory nerves/epithelium and olfactory bulbs of mice when given by the nasal route. The development of effective mucosal vaccines for the elderly is also an important issue; however, only limited information is available. When mucosal adjuvanticity of CT was evaluated in aged mice, an early immune dysregulation was evident in the mucosal immune system. The present review discusses these potential problems for effective mucosal vaccine development.

Bacterial toxins as tools for mucosal vaccination

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

Mutant Escherichia coli heat-labile enterotoxin [LT(R192G)] enhances protective humoral and cellular immune responses to orally administered inactivated influenza vaccine

Influenza vaccines capable of inducing both systemic and mucosal antibody responses are highly desirable. Optimal induction of mucosal IgA is accomplished by mucosal delivery of vaccine. Mucosal adjuvants may improve the immunogenicity and efficacy of vaccines delivered by this route. Here, we compare the adjuvant activities of a mutant of heat-labile enterotoxin from Escherichia coli [LT(R192G)] with those of the wildtype LT (wtLT) for oral vaccination with inactivated influenza vaccine in BALB/c mice. Compared with administration of oral influenza vaccine alone, co-administration of vaccine with LT(R192G) provided enhanced protection from infection in the upper and lower respiratory tract equivalent to and at similar doses as that obtained with wtLT. Likewise, LT(R192G) augmented virus-specific IgG and IgA responses in serum, lung and nasal washes and the numbers of virus-specific antibody-forming cells in spleen, lung and Peyer's patches in a manner comparable to wtLT. Virus-specific splenic CD4(+) cells from mice administered oral vaccine with either adjuvant produced a mixed Th1- and Th2-type cytokine response pattern. Taken together, these results indicate that LT(R192G), like wtLT, is a potent adjuvant for oral vaccination of mice with influenza vaccine.

A revisit of mucosal IgA immunity and oral tolerance

Induction of mucosal immunity by oral immunization with protein antigen alone is difficult: potent mucosal adjuvants, vectors, or other special delivery systems are required. Cholera toxin (CT) has been shown to be an effective adjuvant for the development of mucosal vaccines and, when given with vaccine, induces both mucosal and systemic immune responses via a Th2 cell-dependent pathway. However, and in addition to potential type-I hypersensitivity, a major concern for use of mucosal adjuvants such as CT is that this molecule is not suitable for use in humans because of its inherent toxicity. When we examined the potential toxicity of CT for the central nervous system, both CT and CT-B accumulated in the olfactory nerves/epithelium and olfactory bulbs of mice when given by the nasal route. The development of effective mucosal vaccines for the elderly is also an important issue; however, only limited information is available. When mucosal adjuvanticity of CT was evaluated in aged mice, an early immune dysregulation was evident in the mucosal immune system. The present review discusses these potential problems for effective mucosal vaccine development. Tolerance represents the most common and important response of the host to environmental antigens, including food and commensal bacterial components, for the maintenance of an appropriate immunological homeostasis. We have examined whether Peyer patches could play a more important role for the maintenance of oral tolerance. Using Peyer patch-null mice, we found that mice lacking this gut-associated lymphoid tissue retained their capability to produce secretory IgA antibodies but did not develop normal oral tolerance to protein antigens.

Onjisaponins, from the root of Polygala tenuifolia Willdenow, as effective adjuvants for nasal influenza and diphtheria-pertussis-tetanus vaccines

Active substances from hot water extracts from 267 different Chinese and Japanese medicinal herbs were screened for mucosal adjuvant activity with influenza HA vaccine in mice. The extract from the root of Polygala tenuifolia was found to contain potent mucosal adjuvant activity. The active substances were purified and identified as onjisaponins A, E, F, and G. When each onjisaponin (10 microg) was intranasally (i.n.) inoculated with influenza vaccine (10 microg) in mice, serum hemagglutination-inhibiting (HI) antibody titers increased 3-14 times over control mice administered vaccine alone after 4 weeks. When each onjisaponin (10 microg) was i.n. inoculated with the vaccine (10 microg) followed by i.n. vaccination of the vaccine alone after 3 weeks, serum HI antibody titers increased 27-50 fold over those mice given i.n. vaccinations without onjisaponins. These same conditions also significantly increased nasal anti-influenza virus IgA antibody titers. Two inoculations with onjisaponin F (1 microg) and influenza HA vaccine (1 microg) at 3 weeks intervals, significantly increased serum HI antibody and nasal anti-influenza virus IgA and IgG antibody titers after only 1 week over mice given HA vaccine alone after the secondary vaccination. Intranasal vaccination with onjisaponin F inhibited proliferation of mouse adapted influenza virus A/PR/8/34 in bronchoalveolar lavages of infected mice. Separate intranasal vaccinations with onjisaponins A, E, F, and G (10 microg) each and diphtheria-pertussis-tetanus (DPT) vaccine (10 microg) of mice followed by i.n. vaccination with DPT vaccine alone after 4 weeks showed significant increases in serum IgG and nasal IgA antibody titers after 2 weeks following secondary vaccination over mice vaccinated with DPT vaccine alone. All onjisaponins showed little hemolytic activity at concentrations up to 100 microg/ml. The results of this study suggest that onjisaponins may provide safe and potent adjuvants for intranasal inoculation of influenza HA and DPT vaccines.

Safety and immunogenicity of oral inactivated whole-cell Helicobacter pylori vaccine with adjuvant among volunteers with or without subclinical infection

Helicobacter pylori infection of the gastric mucosa can be found in approximately 50% of the world's population and is associated with a range of pathology, including peptic ulcer, atrophic gastritis, and gastric cancer. To explore immunization as a strategy for preventing and treating H. pylori-associated disease, we assessed the safety and immunogenicity in healthy adults of a formalin-inactivated, oral H. pylori whole-cell (HWC) vaccine, administered with or without mutant Escherichia coli heat-labile toxin (LT(R192G)) as a mucosal adjuvant. In a dose-response study, 23 subjects with or without H. pylori infection were vaccinated with either 2.5 x 10(6) HWC, 2.5 x 10(8) HWC, or 2.5 x 10(10) HWC, plus 25 microg of LT(R192G). Thereafter, a randomized study was conducted in which 18 H. pylori-infected subjects were assigned, in a double-blind fashion, to receive either 2.5 x 10(10) HWC plus placebo-adjuvant, placebo-vaccine plus 25 microg of LT(R192G), placebo-vaccine plus placebo-adjuvant, or 2.5 x 10(10) HWC plus 25 microg of LT(R192G). Diarrhea (six subjects), low-grade fever (five subjects), and vomiting (two subjects) were observed, usually after the first dose. Significant rises in geometric mean mucosal (fecal and salivary) anti-HWC immunoglobulin A antibodies occurred among H. pylori-infected and uninfected subjects following inoculation with 2.5 x 10(10) HWC plus 25 microg of LT(R192G). Moreover, among H. pylori-negative volunteers, this regimen induced significant lymphoproliferative responses in 5 of 10 subjects and gamma interferon production responses to H. pylori sonicate in 7 of 10 subjects. There was no evidence that vaccination eradicated H. pylori in infected volunteers. These results suggest that it is possible to stimulate mucosal and systemic immune responses in humans to H. pylori antigens by using an HWC vaccine.

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.

Effectiveness and safety of mutant Escherichia coli heat-labile enterotoxin (LT H44A) as an adjuvant for nasal influenza vaccine

The effectiveness and safety of mutant Escherichia coli heat-labile enterotoxin, LT H44A (His to Arg substitution at position 44 from the N-terminus of the A1 fragment of the A subunit) as an adjuvant for nasal influenza vaccine were examined. (1) When 0.2 microg of LT H44A, together with 0.2 microg of influenza A/PR/8/34 virus (PR8, H1N1) vaccine, was administered intranasally into BALB/c mice (twice, 4 weeks apart), anti-PR8 hemagglutinin (HA) IgA and IgG antibody (Ab) responses were induced at levels that were sufficient to provide either complete protection against infection with a small volume of PR8 virus suspension or partial protection against infection with a lethal dose of the suspension. The dose of the mutant LT and vaccine used here (0.2 microg/ 20 g doses mouse) corresponded to the estimated dose per person, i.e. 0.1 mg/10 kg body weight. (2) Using these vaccination conditions, no additional total IgE Ab responses were induced. (3) The mutant was confirmed to be less toxic than the native LT when the toxicity was analyzed either using Y1 adrenal cells in vitro (1/483 EC(50)) or by an ileal loop test. (4) One hundred micrograms of the mutant, administered intranasally or intraperitoneally into guinea-pigs (Heartley strain, 0.3-0.4 kg), caused no body-weight changes 7 days after administration, although 100 microg of the native LT administered intraperitoneally caused death in all guinea-pigs due to diarrhea within 2 days. The intranasal administration of 100 microg of the mutant resulted in almost no pathological changes in the nasal mucosa 3 days after administration. These results suggest that LT H44A, which can be produced in high yields in an E. coli culture (about 5 mg/l), could be used as one of the effective and safe adjuvants for nasal influenza vaccine in humans.

Effects of intranasal administration of cholera toxin (or Escherichia coli heat-labile enterotoxin) B subunits supplemented with a trace amount of the holotoxin on the brain

Effects of intranasal administration of cholera toxin (CT) [or Escherichia coli heat-labile enterotoxin (LT)] B subunits supplemented with a trace amount of the holotoxin, CTB* or LTB*, on the brain were examined in BALB/c mice by comparing with those of the intracerebral injection. Intracerebral injection of CTB* at doses more than 10 microg/mouse caused significant body weight loss and dose-dependent death within 7 days, with localization of conjugates of horseradish peroxidase with CTB (HRP-CTB) in the ventricular system and in the perineural space of olfactory nerves of the nasal mucosa 3 h after injection. Intracerebral injection of CTB* at doses less than 3 microg/mouse (or LTB* at doses less than 22.7 microg/mouse) did not cause any significant body weight loss for 7 days, with localization of HRP-CTB in the brain but not in the nasal mucosa. On the other hand, intranasal administration of 10 microg of CTB* caused localization of HRP-CTB in the nasal mucosa but not in the brain 3 h after administration and caused body weight loss even after 30 administrations. Neither any histological changes of brain tissues nor marked changes in serum biochemical parameters were found in mice after the 30 administrations of CTB* or LTB*. These results suggest that 0.1 microg of CTB* or LTB*, which is known to be close to the minimal effective dose as an adjuvant for nasal influenza vaccine in mice and corresponds to 100 microg per person, can be used as a safe nasal adjuvant without adversely affecting the brain.

Carbohydrate biopolymers enhance antibody responses to mucosally delivered vaccine antigens

We have evaluated the ability of two carbohydrate biopolymers, chitosan and gellan, to enhance antibody responses to subunit influenza virus vaccines delivered to the respiratory tracts of mice. Groups of mice were vaccinated three times intranasally (i.n.) with 10 microg of purified influenza B/Panama virus surface antigens (PSAs), which consist of hemagglutinin (HA) and neuraminidase (NA), either alone or admixed with chitosan or gellan solutions. Separate groups were vaccinated subcutaneously (s.c.) with PSAs adsorbed to Alhydrogel or chitosan or gellan alone i.n. Serum antibody responses were determined by enzyme-linked immunosorbent assay (ELISA) for influenza virus-specific immunoglobulin G (IgG) and by HA inhibition (HAI) and NA inhibition (NAI) assays. The local respiratory immune response was measured by assaying for influenza virus-specific IgA antibody in nasal secretions and by enumerating nasal and pulmonary lymphocytes secreting IgA, IgG, and IgM anti-influenza virus-specific antibodies by enzyme-linked immunospotting (ELISPOT). When administered alone i.n., B/Panama PSA was poorly immunogenic. Parenteral immunization with B/Panama PSA with Alhydrogel elicited high titers of anti-B/Panama antibodies in serum but a very poor respiratory anti-B/Panama IgA response. In contrast, i.n. immunization with PSA plus chitosan stimulated very strong local and systemic anti-B/Panama responses. Gellan also enhanced the local and serum antibody responses to i.n. PSA but not to the same extent as chitosan. The ability of chitosan to augment the immunogenicity of influenza vaccines given i.n. was confirmed using PSA prepared from an influenza A virus (A/Texas H1N1).

Use of CpG DNA for enhancing specific immune responses

CpG ODN, owing to its wide range of immunostimulatory effects has been found to be a potent Th1-type adjuvant that is effective with virtually any type of antigen, although responses are less impressive with PS than protein antigens. The use of CpG ODN as an adjuvant may allow the development of vaccines against a wider range of diseases, which could include therapeutic vaccines for chronic infections or cancer, effective pediatric vaccines for newborns, and easily delivered mucosal vaccines.

New approaches to mucosal immunization

Every year more than 17 million deaths worldwide are caused by infectious diseases. The great majority of these deaths occur in underdeveloped countries and are attributed to diseases preventable by existing vaccines, or diseases that could potentially be prevented with new vaccines. The fact that most human and veterinary pathogens establish infection in the host by initiating contact at a mucosal surface, provide the rationale for the development of mucosal vaccines. An increasing number of strategies have been proposed to facilitate mucosal immunization. Among the most widely investigated strategies are the use of attenuated microorganisms; the inclusion of immunizing antigens in lipid-based carriers, the genetic creation of transgenic plants and the use of mucosal adjuvants derived from bacterial toxins. This review provides a brief summary of the most recent advances in the field of mucosal immunization with an special emphasis on a promising genetically detoxified mucosal adjuvant, LT(R192G), derived from the heat-labile toxin of enterotoxigenic E. coli. We present evidence regarding the safety, immunogenicity, and efficacy of LT(R192G) for the development of a new generation of mucosal vaccines.

Effects of adjuvants on the immune response of staphylococcal alpha toxin and capsular polysaccharide (CPS) in rabbit

This study was performed to isolate a vaccine strain of S. aureus from clinical or subclinical mastitis and to choose the most optimal adjuvant for immune response of alpha toxin and capsular polysaccharide (CPS) of field strain. Of thirty strains of S. aureus isolated from milk of clinical or subclinical mastitis, V112 strain isolated from milk of gangrenous mastitis was used in this vaccine. Twenty one of rabbits were allocated into 5 groups based on adjuvants and immunized twice every 2 weeks for 8 weeks. This vaccine was composed of alpha toxin (10 hemolytic units) and formalinized whole cells (1 x 10(11) cells/ml. Five rabbits received PBS solution as a control group. The highest antibody titers against alpha toxin and CPS were observed in dextran sulfate- and aluminium hydroxide-adjuvant group at 8 weeks after immunization, respectively. These results of the study showed that one adjuvant could not induce strong and long-term immune response of alpha toxin and CPS antigens. Therefore, the use of combined adjuvants in subunit vaccine may be useful and feasible.

Intranasal immunization with inactivated influenza vaccine

The development of improved vaccines against epidemic and pandemic influenza virus infection remains a priority in vaccine research. Killed vaccines given by injection are both cost-effective and induce immunity; however, their limitations are well known. Live vaccines have been in development for many years, but difficulties and safety concerns have prohibited their licensing in Western countries. However, the newer technologies of vaccine development, including DNA vaccines and attenuated virus vaccines produced by reverse genetics, remain a hope for the future. With these problems in mind, emphasis has been given to the development of inactivated vaccines that are administered intranasally, either as repeated doses of saline vaccine or in conjunction with suitable carriers or adjuvants. This review describes these latter developments and concludes that this approach offers advantages and should be vigorously researched.

Principles of transcutaneous immunization using cholera toxin as an adjuvant

Transcutaneous immunization is a novel strategy for immunization employing topical application of antigen and adjuvant to the skin surface and resulting in detectable antigen/adjuvant specific IgG in plasma and mucosal secretions. In this study we show that transcutaneous immunization with cholera toxin (CT) as an adjuvant can be used in several inbred mouse strains with varying H-2 major histocompatibility complex genes (C57BL/6 (H-2(b)), BALB/c (H-2(d)), and C3H (H-2(k))). Although the primary anti-CT antibody responses reflected previously described MHC restriction patterns for this protein, the differences were overcome after two booster immunizations. Potent antibody responses against hen egg lysozyme and/or diphtheria toxoid were observed using CT as adjuvant. We also demonstrate that the unshaved dorsal or ventral surface of the ear can be effectively used for transcutaneous immunization and that gentle swabbing with alcohol increases the magnitude of the host immune response. Together these data further our understanding of the principles governing this new platform technology and support its integration into novel and existing human vaccine strategies.

The role of cAMP in mucosal adjuvanticity of Escherichia coli heat-labile enterotoxin (LT)

Heat-labile enterotoxin (LT) produced by enterotoxigenic Escherichia coli (ETEC) and cholera toxin (CT) produced by Vibrio cholerae have been shown to function as potent mucosal adjuvants. A number of studies have examined the effects of different mutations at either the active site or the protease site of LT and CT and the influence of those mutations on toxicity and adjuvanticity. However, different observations reported by various groups using a variety of animal models with different antigens or different routes of immunization have provided contradictory findings and evoked many questions regarding the underlying mechanisms of mucosal adjuvanticity of LT and CT. In this study, the role of cAMP in mucosal adjuvanticity was examined by comparing three LT active site mutants (S61F, A69G, E112K), a protease site mutant (R192G) and recombinant LT-B for toxicity, cAMP activity and mucosal adjuvanticity using tetanus toxoid (TT) as a model antigen. While all mutants examined showed reduced toxicity, the effects of each mutation on its ability to function as an adjuvant varied. Following intranasal immunization, native LT as well as protease and active site mutants of LT induced serum anti-TT IgG and their responses were virtually indistinguishable from one another. In addition, LT-B was also able to enhance production of serum anti-TT IgG, though at a level significantly lower than that achieved by native LT and mutants. Following oral immunization, the best serum anti-TT IgG responses were obtained with native LT and mutants that retained the ability to induce accumulation of cAMP. Despite the nearly identical serum anti-TT IgG responses following intranasal immunization, there was a strong correlation between the ability to induce accumulation of cAMP in cultured Caco-2 cells and the ability to elicit production of antigen-specific Th1 or Th2 cytokines.

Mucosal immunization with DNA vaccines

DNA vaccines can induce potent humoral and cellular immune responses in numerous animal models. Most DNA vaccines have been administered parenterally; however, more effective protection against mucosal pathogens could be achieved with mucosal immunization. This review concentrates on the use of DNA vaccines for the induction of mucosal immunity.