Escherichia coli heat-labile enterotoxin B subunits supplemented with a trace amount of the holotoxin as an adjuvant for nasal influenza vaccine

Harnessing Nasal Immunity with IgA to Prevent Respiratory Infections

The nasal cavity is a primary checkpoint for the invasion of respiratory pathogens. Numerous pathogens, including SARS-CoV-2, S. pneumoniae, S. aureus, etc., can adhere/colonize nasal lining to trigger an infection. Secretory IgA (sIgA) serves as the first line of immune defense against foreign pathogens. sIgA facilitates clearance of pathogenic microbes by intercepting their access to epithelial receptors and mucus entrapment through immune exclusion. Elevated levels of neutralizing IgA at the mucosal surfaces are associated with a high level of protection following intranasal immunizations. This review summarizes recent advances in intranasal vaccination technology and challenges in maintaining nominal IgA levels at the mucosal surface. Overall, the review emphasizes the significance of IgA-mediated nasal immunity, which holds a tremendous potential to mount protection against respiratory pathogens.

Recombinant adenovirus carrying a core neutralizing epitope of porcine epidemic diarrhea virus and heat-labile enterotoxin B of Escherichia coli as a mucosal vaccine

Porcine epidemic diarrhea virus (PEDV) targets the intestinal mucosa in pigs. To protect against PEDV invasion, a mucosal vaccine is utilized effectively. In this study, we generated a recombinant adenovirus vaccine encoding the heat-labile enterotoxin B (LTB) and the core neutralizing epitope (COE) of PEDV (rAd-LTB-COE). The fusion protein LTB-COE was successfully expressed by the recombinant adenovirus in HEK293 cells, and the immunogenicity of the vaccine candidate was assessed in BALB/c mice and piglets. Three intramuscular or oral vaccinations with rAd-LTB-COE at two-week intervals induced robust humoral and mucosal immune responses. Moreover, a cell-mediated immune response was promoted in immunized mice, and the neutralizing antibody inhibited both the vaccine strain and the emerging PEDV isolate. Immunization experiments in piglets revealed that rAd-LTB-COE was immunogenic and induced good immune responses in piglets. Further studies are required to evaluate the efficacy of rAd-LTB-COE against a highly virulent PEDV challenge.

The Mucosal Vaccine Adjuvant LT(R192G/L211A) or dmLT

Perhaps the best-studied mucosal adjuvants are the bacterially derived ADP-ribosylating enterotoxins. This adjuvant family includes heat-labile enterotoxin of Escherichia coli (LT), cholera toxin (CT), and mutants or subunits of LT and CT. These proteins promote a multifaceted antigen-specific response, including inflammatory Th1, Th2, Th17, cytotoxic T lymphocytes (CTLs), and antibodies. However, more uniquely among adjuvant classes, they induce antigen-specific IgA antibodies and long-lasting memory to coadministered antigens when delivered mucosally or even parenterally. The purpose of this minireview is to describe the general properties, history and creation, preclinical studies, clinical studies, mechanisms of action, and considerations for use of the most promising enterotoxin-based adjuvant to date, LT(R192G/L211A) or dmLT. This review is timely due to completed, ongoing, and planned clinical investigations of dmLT in multiple vaccine formulations by government, nonprofit, and industry groups in the United States and abroad.

Mucosal Immune Response in Nasal-Associated Lymphoid Tissue upon Intranasal Administration by Adjuvants

The nasal administration of vaccines directed against diseases caused by upper respiratory tract infections of pathogens, such as the influenza virus, mimics the natural infection of pathogens and induces immunoglobulin A (IgA) production in the nasal cavity to effectively protect viral entry. Therefore, the development of a nasally administered vaccine is a research objective. Because the antigenicity of influenza split vaccines is low, nasal inoculation with the vaccine alone does not induce strong IgA production in the nasal cavity. However, the addition of adjuvants activates the innate immune response, enhancing antigen-specific IgA production and the T-cell response. Although the development of suitable adjuvants for nasal vaccinations is in progress, the mechanism by which adjuvants promote the immune response is still unclear. In this review, we discuss the mucosal immune response, especially in the nasal-associated lymphoid tissue, induced in response to the intranasal inoculation of an influenza vaccine and adjuvants in animal models.

Immunogenicity of a fusion protein comprising coli surface antigen 3 and labile B subunit of enterotoxigenic Escherichia coli

Background: Enterotoxigenic Escherichia coli (ETEC) strains are the major causes of diarrheal disease in humans and animals. Colonization factors and enterotoxins are the major virulence factors in ETEC pathogenesis. For the broad-spectrum protection against ETEC, one could focus on colonization factors and non-toxic heat labile as a vaccine candidate. Methods: A fusion protein is composed of a major fimbrial subunit of coli surface antigen 3, and the heat-labile B subunit (LTB) was constructed as a chimeric immunogen. For optimum level expression of protein, the gene was synthesized with codon bias of E. coli. Also, recombinant protein was expressed in E. coli BL21DE3. ELISA and Western tests were carried out for determination of antigen and specificity of antibody raised against recombinant protein in animals. The anti-toxicity and anti-adherence properties of the immune sera against ETEC were also evaluated. Results: Immunological analyses showed the production of high titer of specific antibody in immunized mice. The built-in LTB retains native toxin properties which were approved by GM₁ binding assay. Pre-treatment of the ETEC cells with anti-sera significantly decreased their adhesion to Caco-2 cells. Conclusion: The results indicated the efficacy of the recombinant chimeric protein as an effective immunogen inducing strong humoral response. The designated chimer would be an interesting prototype for a vaccine and worthy of further investigation.

Contrasting effects of type I interferon as a mucosal adjuvant for influenza vaccine in mice and humans

To identify an adjuvant that enhances antibody responses in respiratory secretions to inactivated influenza virus vaccine (IVV), a comparison was made of responses to intranasal vaccinations of mice with IVV containing monophosphoryl lipid A (MPL), type I interferon (IFN) or cholera toxin B (CTB). Antibody in nasal secretions and lung wash fluids from mice was increased after vaccination and lung virus was significantly reduced after challenge to a similar level in each adjuvant group. Interferon was selected for a trial in humans. Trivalent inactivated influenza vaccine was given intranasally to healthy adult volunteers alone or with 1 million units (Mu) or 10 Mu of alpha interferon. Vaccinations were well tolerated but neither serum hemagglutination-inhibiting nor neutralizing antibody responses among the vaccine groups were significantly different. Similarly, neither neutralizing nor IgA antibody responses in nasal secretions were significantly different. Thus, despite exhibiting a significant adjuvant effect in mice, interferon did not exhibit an adjuvant effect for induction of antibody in respiratory secretions of humans to inactivated influenza virus vaccine given intranasally.

Poly(gamma-glutamic acid) nano-particles combined with mucosal influenza virus hemagglutinin vaccine protects against influenza virus infection in mice

Adding poly(gamma-glutamic acid) nano-particles (gamma-PGA-NPs), a safe, natural material, to subcutaneous immunization with influenza virus hemagglutinin (HA) vaccine increases the protective immune responses against influenza virus in mice. Here, we examined whether intranasal administration of the HA vaccine with gamma-PGA-NPs would induce protection from influenza virus challenge in mice. Intranasal immunization with the mixture of gamma-PGA-NPs and HA vaccine from an influenza virus strain A/PR/8/34 (H1N1) or A/New Caledonia/20/99 (H1N1) enhanced protection of mice from A/PR/8/34 infection. Intranasal immunization with A/New Caledonia/20/99 HA vaccine and gamma-PGA-NPs induced cell-mediated immune responses and neutralizing antibody production for both A/New Caledonia/20/99 and A/PR/8/34. These data suggest that gamma-PGA-NPs may have potential for clinical applications as a mucosal adjuvant.

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.

Modified Pulmonary Surfactant Is a Potent Adjuvant That Stimulates the Mucosal IgA Production in Response to the Influenza Virus Antigen

The intranasal administration of influenza hemagglutinin (HA) vaccine with Surfacten, a modified pulmonary surfactant free of antigenic c-type lectins, as a mucosal adjuvant induced the highest protective mucosal immunity in the airway. The intranasal immunization of mice with HA vaccine (0.2 microg)-Surfacten (0.2 microg) selectively induced the neutralizing anti-HA IgA, but not IgG, and conferred nearly maximal protection in the airway, without inducing a systemic response. In contrast, intranasal inoculation of vaccine with 0.2 microg of the potent mucosal adjuvant cholera toxin B* (CT-B*), prepared by adding 0.2% native CT to the B subunit of CT, induced both anti-HA IgA and IgG in the airway and in the serum. The intranasal administration of HA vaccine alone induced a limited amount of mucosal IgA against influenza virus. Although the s.c. administration of HA vaccine prominently induced serum IgG and IgA, Surfacten and CT-B* did not enhance their induction, and the concentrations of Abs leaking into the airways were insufficient to prevent viral multiplication. The intranasal administration of HA-Surfacten stimulated the expression of MHC class II, CD40, and CD86 molecules in the CD11c-positive cells isolated from the nasal mucosa, but not the expression of cells from the lungs or spleens. Lymphocytes isolated from the airway mucosa after intranasal HA-Surfacten immunization prominently induced TGF-beta1 which, compared with inoculation without Surfacten, promoted an Ag-specific mucosal IgA response. Surfacten alone, however, did not induce TGF-beta1. Our observations suggest that Surfacten, by mimicking the natural surfactant, is an effective mucosal adjuvant in the process of airway immunization.

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.

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.

Construction and characterization of bivalent Shigella flexneri 2a vaccine strains SC608(pCFAI) and SC608(pCFAI/LTB) that express antigens from enterotoxigenic Escherichia coli

An invasive strain of Shigella flexneri 2a (SC608) has been developed as a vector for the expression and delivery of heterologous antigens. SC608 is an aspartate semialdehyde dehydrogenase (asd) derivative of SC602 (icsA iuc), a well-characterized live attenuated vaccine strain which has undergone several clinical trials in human volunteers. When administered orally at a single 10(4) (CFU) dose, SC602 is both immunogenic and efficacious against shigellosis. Using asd-based plasmid vectors, we designed SC608 to express the enterotoxigenic Escherichia coli (ETEC) fimbrial subunit CfaB (CFA/I structural subunit) alone or in combination with the E. coli B subunit of heat-labile enterotoxin (LTB). The expression of each heterologous protein in SC608 was verified by immunoblot analysis. Each strain was comparable to the parent strain, SC602, in a HeLa cell invasion assay. After intranasal immunizations of guinea pigs, serum and mucosal immune responses were detected against both Shigella lipopolysaccharide and heterologous ETEC antigens by enzyme-linked immunosorbent assay and ELISPOT analysis. All immunized animals were subsequently protected against a challenge with wild-type S. flexneri 2a in a keratoconjunctivitis Sereny test. Serum antibodies generated against LTB and CfaB demonstrated antitoxin and agglutination activities, respectively. These results suggest that CfaB and LTB expressed in SC608 retain important conformational epitopes that are required for the generation of antibodies that have functional activities. These initial experiments demonstrate that a fully invasive Shigella vaccine strain can be engineered to deliver antigens from other diarrheal pathogens.

Glycol chitosan improves the efficacy of intranasally administrated replication defective human adenovirus type 5 expressing glycoprotein D of bovine herpesvirus 1

The ability of two soluble formulations, namely chitosan and glycol chitosan, when used as an intranasal adjuvant, to improve the immunogenicity of an intranasal human adenovirus type 5 replication defective expressing bovine herpesvirus 1 (BoHV-1) glycoprotein D based vaccine, was investigated in cattle. Their adjuvant effects on immune response by increasing clinical and especially virological protection against an intranasal BoHV-1 challenge were then evaluated. The best virological protection was obtained in calves immunized with the vaccine vector adjuvanted with glycol chitosan which decreased the challenge BoHV-1 virus excretion titres by 0.5-1.5 log when compared to those obtained in calves immunized with the vaccine vector alone or adjuvanted with chitosan. A slight difference in clinical scores was observed in calves immunized with the adjuvanted vaccine vector compared to calves immunized with the vaccine vector alone. The obtained data suggest that the tested soluble formulation of glycol chitosan has promising potential use as an intranasal adjuvant for recombinant viral vector vaccines in cattle.

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.

Expression of the B subunit of the heat-labile enterotoxin of Escherichia coli in tobacco mosaic virus-infected Nicotiana benthamiana plants and its characterization as mucosal immunogen and adjuvant

We have produced biologically active recombinant (r) LTB, the nontoxic B subunit of heat-labile toxin (LT) of Escherichia coli in tobacco mosaic virus (TMV)-infected Nicotiana benthamiana plants. We amplified the LTB encoding sequence with its leader and introduced a hexahistidyl tag and an endoplasmic reticulum retention signal. The resulting product was ligated into a TMV-based plant viral expression vector that was used for the generation of recombinant viral RNA. Eighty-nine percent of N. benthamiana plants inoculated with the recombinant viral RNA were systemically infected as determined by anti-TMV enzyme-linked immunosorbent assay (ELISA) experiments. The rLTB monomer was identified by LT-specific as well as by histidyl-tag-specific immunoblots. rLTB from plant extracts of TMV-infected N. benthamiana leaves was purified to give 75 microg rLTB pentamers per gram fresh plant material and was capable of binding G(M)1 ganglioside. The immunogenicity of the plant-produced rLTB was tested in mice and showed that intranasal application of rLTB (15 microg per mouse) induced LTB-specific IgG1 antibodies. To prove its adjuvanticity, rLTB was intranasally co-administered with the Hevea latex allergen Hev b 3, leading to allergen-specific IgG1 and IgG2a antibody production. The fact that intranasal application of rLTB and Hev b 3 prior to systemic challenge with the allergen enhanced the Th2 responses at the humoral and cellular level indicated that rLTB promoted immune responses that were naturally induced by the antigen/allergen. In conclusion, these results indicate that the plant viral expression system is suitable for the rapid large-scale production of biologically active LTB with strong mucosal adjuvant capacity.

Mucosal immunity: overcoming the barrier for induction of proximal responses

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

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.

Pinellic acid from the tuber of Pinellia ternata Breitenbach as an effective oral adjuvant for nasal influenza vaccine

This study describes the isolation, purification, characterization, and adjuvant activity of an orally active adjuvant substance from the tuber of Pinellia ternata, as an active herbal component of the traditional Japanese herbal (Kampo) medicine, Sho-seiryu-to (SST, Chinese name: Xiao-Qing-Long-Tang), which has been reported to show oral adjuvant activity for nasally administered influenza HA vaccine [Int. J. Immunopharmacol. 16 (1994) 605]. The active compound was identified as 9S, 12S, 13S-trihydroxy-10E-octadecenoic acid using infrared spectra, proton magnetic resonance, mass spectrometry, and circular dichroism, and named pinellic acid. Oral administration of pinellic acid (1 microg) to BALB/c mice given primary and secondary intranasal inoculations of influenza HA vaccine (1 microg) enhanced antiviral IgA antibody (Ab) titers 5.2- and 2.5-fold in nasal and bronchoalveolar washes, respectively, and antiviral IgG Ab titers 3-fold in bronchoalveolar wash and serum. Intranasal administration of pinellic acid (1 microg) with influenza HA vaccine (1 microg) slightly enhanced antiviral IgG Ab titers in bronchoalveolar wash and serum but not antiviral IgA Ab titers in nasal and bronchoalveolar washes. Pinellic acid showed no hemolytic activity. The results of this study suggest that pinellic acid may provide a safe and potent oral adjuvant for nasal influenza HA vaccine.

ProJuvant (Pluronic F127/chitosan) enhances the immune response to intranasally administered tetanus toxoid

The potential to generate both a systemic and local immune response makes the mucosal system an attractive site for immunization. However, mucosal administration of protein and peptide antigens generally results in a poor immune response. Successful mucosal vaccination is therefore largely dependent on the development of effective mucosal adjuvants. In this study we have examined the effect of mucosal administration of tetanus toxoid (TT) in the presence of a non-ionic block copolymer, Pluronic F127 (F127), with chitosan or lysophosphatidylcholine (LPC) on the systemic and mucosal immune response. Balb/c mice, immunized intraperitoneally (i.p.) with TT and boosted intranasally (i.n.) with TT in F127/chitosan, demonstrated a significant enhancement in the systemic anti-TT antibody response compared to mice boosted i.n. with TT in PBS or mice boosted i.n. with TT in F127/LPC. We determined the antigen specific IgA response in the nasal and lung washes of these animals and found a significant increase in anti-TT mucosal IgA response in the group boosted with TT in F127/chitosan. Similarly, mice immunized and boosted i.n. with TT in F127/chitosan had a significant enhancement of their systemic anti-TT IgG and mucosal IgA antibody responses compared to the animals immunized and boosted i.n. with TT in PBS or TT in F127/LPC. The results of these studies suggest that F127/chitosan represents a novel mucosal vaccine delivery system, consisting of two components, that appear to exert an additive or synergistic effect on the immune response.

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.

Mucosal immunization of mice using CpG DNA and/or mutants of the heat-labile enterotoxin of Escherichia coli as adjuvants

Cholera toxin (CT) and the Escherichia coli heat-labile enterotoxin (LT) are potent mucosal adjuvants in animals associated, at least in part, with their ability to induce cAMP. While toxicity generally precludes their use in humans, a number of different subunit or genetically detoxified mutants of CT and LT have been developed. Another type of adjuvant that has been shown to be effective at mucosal surfaces comprises synthetic oligodeoxynucleotides (ODN) containing immunostimulatory CpG motifs (CpG ODN). We have previously demonstrated a synergy between CpG ODN and native toxins after intranasal (IN) administration to mice, and herein have examined whether this synergy is linked to the cAMP activity. The adjuvanticity of CpG ODN was evaluated with IN and oral delivery of tetanus toxoid or the hepatitis B surface antigen, relative to and in combination with native LT holotoxin (LTh), three active site mutants (LTS61F, LTA69G, LTE112K), a protease site mutant (LTR192G), and the B subunit of LT (LTB). At an equivalent dose, the adjuvants could generally be divided into two groups: one that included CpG ODN, LTh, LTR192G, and LTA69G which acted as strong adjuvants; and the second which comprised LTB, LTS61F, and LTE112K, which produced significantly weaker immune responses. When CpG ODN was co-administered with bacterial toxin-derivatives, in most cases, no synergy between CpG and the LT derivatives was found for strength of the humoral response. Nevertheless, for both routes and antigens, CpG ODN combined with any LT derivative induced a more Type 1-like response than LT derivative alone. These results suggest that while the synergy seen previously with native toxins may have been due in part to inherent cAMP activity, it may have also depended on the particular antigen used and the route of immunization.

Oral administration of influenza vaccine in combination with the adjuvants LT-K63 and LT-R72 induces potent immune responses comparable to or stronger than traditional intramuscular immunization

Mucosal immunization strategies are actively being pursued in the hopes of improving the efficacy of vaccines against the influenza virus. Our group investigated the oral immunization of mice via intragastric gavage with influenza hemagglutinin (HA) combined with mutant Escherichia coli heat-labile enterotoxins K63 (LT-K63) and R72 (LT-R72). These oral immunizations resulted in potent serum antibody and HA inhibition titers, in some cases stronger than those obtained with traditional intramuscular administration, in addition to HA-specific immunoglobulin A in the saliva and nasal secretions. This study demonstrates that it may be possible to develop effective oral influenza 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.

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.

A novel bioadhesive intranasal delivery system for inactivated influenza vaccines

The aim of the current studies was to evaluate a bioadhesive delivery system for intranasal administration of a flu vaccine, in combination with a mucosal adjuvant (LTK63). A commercially available influenza vaccine, containing hemagglutinin (HA) from influenza/A Johannesberg H1N1 1996, and LTK63 or LTR72 adjuvants, which are genetically detoxified derivatives of heat labile enterotoxin from Escherichia coli, were administered IN in a bioadhesive delivery system, which comprised esterified hyaluronic acid (HYAFF) microspheres, to mice, rabbits and micro-pigs at days 0 and 28. For comparison, additional groups of animals were immunized intranasally with the HA vaccine alone, with soluble HA+LTK63, or IM with HA. In all three species, the groups of animals receiving IN immunization with the bioadhesive microsphere formulations, including LT mutants, showed significantly enhanced serum IgG responses (P<0.05) and higher hemagglutination inhibition (HI) titers in comparison to the other groups. In addition, the bioadhesive formulation also showed a significantly enhanced nasal wash IgA response (P<0.05). Most encouragingly, in pigs, the bioadhesive microsphere vaccine delivery system induced serum immune responses following IN immunization, which were significantly more potent than those induced by traditional IM immunization at the same vaccine dose (P<0.05).

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).

Parenteral adjuvant activities of Escherichia coli heat-labile toxin and its B subunit for immunization of mice against gastric Helicobacter pylori infection

The heat-labile toxin (LT) of Escherichia coli is a potent mucosal adjuvant that has been used to induce protective immunity against Helicobacter felis and Helicobacter pylori infection in mice. We studied whether recombinant LT or its B subunit (LTB) has adjuvant activity in mice when delivered with H. pylori urease antigen via the parenteral route. Mice were immunized subcutaneously or intradermally with urease plus LT, recombinant LTB, or a combination of LT and LTB prior to intragastric challenge with H. pylori. Control mice were immunized orally with urease plus LT, a regimen shown previously to protect against H. pylori gastric infection. Parenteral immunization using either LT or LTB as adjuvant protected mice against H. pylori challenge as effectively as oral immunization and enhanced urease-specific immunoglobulin G (IgG) responses in serum as effectively as aluminum hydroxide adjuvant. LT and LTB had adjuvant activity at subtoxic doses and induced more consistent antibody responses than those observed with oral immunization. A mixture of a low dose of LT and a high dose of LTB stimulated the highest levels of protection and specific IgG in serum. Urease-specific IgG1 and IgG2a antibody subclass responses were stimulated by all immunization regimens tested, but relative levels were dependent on the adjuvant used. Compared to parenteral immunization with urease alone, LT preferentially enhanced IgG1, while LTB or the LT-LTB mixture preferentially enhanced IgG2a. Parenteral immunization using LT or LTB as adjuvant also induced IgA to urease in the saliva of some mice. These results show that LT and LTB stimulate qualitatively different humoral immune responses to urease but are both effective parenteral adjuvants for immunization of mice against H. pylori infection.

Humoral immunoresponse to varicella-zoster virus pernasally coadministered with Escherichia coli enterotoxin in mice

It is evaluated whether Escherichia coli enterotoxin is useful for induction of immunity to varicella-zoster virus (VZV) as a mucosal adjuvant in mice. When a commercially available live varicella vaccine (Oka strain) and toxin were administered simultaneously via a nasal route three times at 2 or 6 month intervals, an antibody neutralizing VZV was detected in half or all of the mice vaccinated, respectively. The antibody specific to the vaccine strain of VZV reacted to five proteins, molecular weights of which were 110 K, 100 K, 62 K, 54 K and 46 K. These proteins were composed of glycosylated products of all kinds of glycoproteins. These results suggest that although a nasal administration of the vaccine without the adjuvant has little immunogenicity in mice, the simultaneous administration of the live vaccine and the toxin over a long period induces a specific humoral immunity to VZV.

Mucosal and systemic antibody responses after peroral or intranasal immunization: effects of conjugation to enterotoxin B subunits and/or of co-administration with free toxin as adjuvant

The mucosa-binding molecules cholera toxin (CT) from Vibrio cholerae and heat-labile enterotoxin (LT) from Escherichia coli have previously been used as mucosal adjuvants and carriers for many types of antigen. However, since these molecules are toxic and cannot be used in human vaccines, it is important to study whether their non-toxic mucosa-binding B subunits, CTB and LTB, can be used as alternative safe mucosal adjuvants and/or carrier molecules. We have as a model protein antigen used human gammaglobulin (HGG) for admixture with or chemical conjugation to recombinantly produced CTB and LTB, respectively, and measured antigen-specific local secretory IgA antibodies in saponin extracts from intestine and lung tissue by ELISA following intra-nasal (i.n.) or per-oral (p.o.) immunization. The results show that local antibody formation against HGG was increased after immunization with conjugated as compared to free HGG. However, while the conjugates alone gave rise to significant immune responses in the lung and also, to a lesser degree, in the intestine after i.n. immunization, co-administration of a small amount of free CT/LT as adjuvant was needed to induce a significant immune response in the intestine after p.o. immunization. We also found that following i.n. immunization, the addition of CTB to HGG, without coupling, increased the mucosal immune response to some extent, indicating that CTB by itself can work as an adjuvant by the i.n. route of immunization. A striking finding was that, as a carrier, CTB was superior to LTB when the conjugates were used by the oral but not by the i.n. route of immunization. In conclusion, conjugation of an antigen to mucosa-binding molecules such as CTB and/or LTB can dramatically increase their mucosal immunogenicity. This approach may thus be useful in the preparation of mucosal vaccines.

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.

Intranasal immunization of mice with influenza vaccine in combination with the adjuvant LT-R72 induces potent mucosal and serum immunity which is stronger than that with traditional intramuscular immunization

Immunization of mice by the intranasal route with influenza virus hemagglutinin in combination with the mutant Escherichia coli heat-labile enterotoxin R72 (LT-R72) induced significantly enhanced serum and mucosal antibodies, surpassing, in most cases, responses achieved by traditional intramuscular immunization using inactivated split influenza vaccine. Furthermore, intranasal immunization with LT-R72 induced a potent serum immunoglobulin G2a response, indicating that this adjuvant has Th1 character.

Mucosal Immunity to Influenza Without IgA: An IgA Knockout Mouse Model

IgA knockout mice (IgA−/−) were generated by gene targeting and were used to determine the role of IgA in protection against mucosal infection by influenza and the value of immunization for preferential induction of secretory IgA. Aerosol challenge of naive IgA−/− mice and their wild-type IgA+/+ littermates with sublethal and lethal doses of influenza virus resulted in similar levels of pulmonary virus infection and mortality. Intranasal and i.p. immunization with influenza vaccine plus cholera toxin/cholera toxin B induced significant mucosal and serum influenza hemagglutinin-specific IgA Abs in IgA+/+ (but not IgA−/−) mice as well as IgG and IgM Abs in both IgA−/− and IgA+/+ mice; both exhibited similar levels of pulmonary and nasal virus replication and mortality following a lethal influenza virus challenge. Monoclonal anti-hemagglutinin IgG1, IgG2a, IgM, and polymeric IgA Abs were equally effective in preventing influenza virus infection in IgA−/− mice. These results indicate that IgA is not required for prevention of influenza virus infection and disease. Indeed, while mucosal immunization for selective induction of IgA against influenza may constitute a useful approach for control of influenza and other respiratory viral infections, strategies that stimulate other Igs in addition may be more desirable.

Immune modulation by the cholera-like enterotoxins: from adjuvant to therapeutic

Cholera toxin and its close relative, Escherichia coli heat-labile enterotoxin, are potent immunogens and mucosal adjuvants. The recent findings that their B subunits can promote tolerance highlights the complexity of their interactions with the immune system. Here, Neil Williams and colleagues review the mechanisms by which these molecules modulate leukocyte populations and seek to explain the paradox.

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.

Mutational analysis of the role of ADP-ribosylation activity and GM1-binding activity in the adjuvant properties of the Escherichia coli heat-labile enterotoxin towards intranasally administered keyhole limpet hemocyanin

The Escherichia coli heat-labile enterotoxin (LT) is known for its potent mucosal immunoadjuvant activity towards co-administered antigens. LT is composed of one A subunit, which has ADP-ribosylation activity, and a homopentameric B subunit, which has high affinity for the toxin receptor, ganglioside GM1. In previous studies, we have investigated the role of the LTA and LTB subunits in the adjuvanticity of LT towards influenza virus hemagglutinin (HA), administered intranasally to mice. We now studied the adjuvant properties of LT and LT variants towards keyhole limpet hemocyanin (KLH), which, in contrast to HA, does not bind specifically to mucosal surfaces. It is demonstrated that LT mutants without ADP-ribosylation activity, as well as LTB, retain mucosal immunoadjuvant activity when administered intranasally to mice in conjunction with KLH. As with influenza HA, adjuvanticity of LTB required GM1-binding activity, whereas GM1-binding was not essential for adjuvant activity of LT. Furthermore, we found that also recombinant LTA alone acts as a potent mucosal adjuvant, and that this adjuvanticity is independent of ADP-ribosylation activity. It is concluded that binding of the antigen to mucosal surfaces does not play an essential role in the immunostimulation by LT and LT variants, and that both recombinant LTA and LTB represent powerful nontoxic mucosal adjuvants.

An experimental prime-boost regimen leading to HIV type 1-specific mucosal and systemic immunity in BALB/c mice

Induction of mucosal as well as systemic immunity to HIV-1 is considered to have high priority in current concepts of future AIDS vaccines. Here we show that the desired immune responses can be elicited by an experimental prime-boost regimen consisting of mucosal (intragastric) application of a recombinant vaccinia virus carrying the HIV-1 env gene (vSC25), followed by parenteral (intradermal) immunization with the recombinant HIV-1 glycoprotein 160 (rgp160). Following intragastric immunization of mice with vSC25 in combination with the mucosal adjuvant cholera toxin (CT), HIV-1 env-specific IgA was secreted by B cells of Peyer's patches and the lamina propria. Moreover, mucosal (intragastric and intranasal) application of vSC25 (both in presence or absence of CT) induced a long-lasting, HIV-1 env-specific systemic cytotoxic T cell response. Subsequent intradermal boosters with rgp160 led to HIV-1-specific T cell memory and serum antibodies.

Protection against measles virus-induced encephalitis by antibodies from mice immunized intranasally with a synthetic peptide immunogen

Balb/c mice were immunized intranasally (i.n.) with a chimeric synthetic peptide containing two copies of a T- and one copy of a B-cell epitope (TTB) from measles virus (MV) fusion protein, plus cholera toxin B (CTB) adjuvant. The antibodies induced cross-reacted with, and neutralized MV and on passive transfer, protected mice against encephalitis induced by neuroadapated MV. Immunization with TTB alone induced antibodies which increased survival but not significantly compared to controls. Furthermore, i.n. immunization with TTB plus CTB induced TTB-specific IgA antibodies in saliva and nasal washes. Co-administration of CTB increased the affinity of antibodies to the B-cell epitope of TTB and caused a relative increase in the level of anti-peptide antibodies of the IgG2a subclass and the overall titre of IgG antibodies. These results indicate the potential of the i.n. route for immunization with synthetic peptide immunogens for induction of both local and systemic anti-peptide antibody responses.

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

The effectiveness and safety of known mutants of Escherichia coli heat-labile enterotoxin (LT) as an adjuvant for nasal influenza vaccine were examined. Six mutants, called LT7K (Arg to Lys), LT61F (Ser to Phe), LT112K (Glu to Lys), LT118E (Gly to Glu), LT146E (Arg to Glu) and LT192G (Arg to Gly) were constructed by the replacement of one amino acid at one position of the A1 subunit to another using site-directed mutagenesis. All mutants were confirmed to be less toxic than wild-type LT when analyzed using Y-1 adrenal cells in vitro. When influenza vaccine was administered intranasally with LT7K and LT192G, BALB/c mice developed high levels of serum and local antibodies to the HA molecules. The adjuvant activity of these mutant LTs corresponded to that of wild-type LT when 1 microgram of these mutant LTs (or wild-type LT) was coadministered with the vaccine. From the point of view of safety, LT7K was considered to be the most potent mucosal adjuvant and was examined in more detail. The adjuvant activity of the mutant was lowered more rapidly with a decrease in dose than was that of wild-type LT. The low level of adjuvant of a relatively small amount of LT7K was heightened by adding LTB to the mutant LT. These results suggest that LT7K supplemented with LTB can be used as a less toxic, effective adjuvant for nasal influenza vaccine.

Effects of frequent intranasal administration of adjuvant-combined influenza vaccine on the protection against virus infection

In previous papers, we have shown that Escherichia coli heat-labile enterotoxin B subunit, supplemented with a trace amount of the holotoxin (LTB*) could be used as a potent adjuvant for a nasal influenza HA (haemagglutinin) vaccine in humans. The present study was designed to determine whether the effectiveness of a combined LTB*-HA vaccine could be limited by preexisting immunity to LTB and how many times the adjuvant-combined vaccine could be administered intranasally without reducing its protective efficacy in BALB/c, C3H and B10 mice. The magnitude of both nasal and serum Ab responses to HA vaccine was correlated with the degree of protection against virus infection. Higher doses of LTB*-combined vaccine were required for inducing high enough levels of anti-HA Ab responses to provide complete protection in low responder mice. Repeated pretreatments with LTB* alone (more than six times), which provided high levels of preexisting Abs to LTB, inhibited the induction of anti-HA Ab responses and reduced the protective efficacy of the adjuvant-combined vaccine. However, the LTB*-combined vaccine could be given repeatedly (about ten times) to mice without reducing the effectiveness of the adjuvant-combined vaccine. These results suggest that the LTB*-combined nasal influenza vaccine can be given to humans once every few years when an epidemic of influenza may occur.

Anticarrier immunity suppresses the antibody response to polysaccharide antigens after intranasal immunization with the polysaccharide-protein conjugate

We have conjugated cholera toxin (CT) B subunit (CTB) to dextran and studied the effect in mice of previous immunization with CT and CTB on the response to dextran after intranasal immunizations with conjugate. Preexisting immunity to CTB was found to inhibit both the lung mucosal response and serum antibody response to dextran, but this effect could be overcome by using a higher dose of conjugate and delaying the conjugate immunization until the CTB antibody titers had declined. The role of anti-CTB antibodies on the mucosal surface was probably to prevent uptake of the conjugate through a mechanism of immune exclusion. Passively transferred serum antibodies against CTB, on the other hand, suppressed both the serum response and the local antibody response against CTB but did not affect the response to dextran after intranasal immunization with conjugate.