Antibody responses in volunteers induced by nasal influenza vaccine combined with Escherichia coli heat-labile enterotoxin B subunit containing a trace amount of the holotoxin

Oral immunization with urease and Escherichia coli heat-labile enterotoxin is safe and immunogenic in Helicobacter pylori-infected adults

BACKGROUND & AIMS

Oral immunization with Helicobacter pylori urease can cure Helicobacter infection in animals. As a step toward therapeutic immunization in humans, the safety and immunogenicity of oral immunization with recombinant H. pylori urease were tested in H. pylori-infected adults.

METHODS

Twenty-six H. pylori-infected volunteers were randomized in a double-blind study to four weekly oral doses of 180, 60, or 20 mg of urease with 5 microg heat-labile enterotoxin of Escherichia coli (LT), LT alone, or placebo. Side effects and immune responses were evaluated weekly after immunization, and gastric biopsy specimens were obtained after 1 month and 6 months for histology and quantitative cultures.

RESULTS

Diarrhea was noted in 16 of 24 (66%) of the volunteers who completed the study. Antiurease serum immunoglobulin A titers increased 1. 58-fold +/- 0.37-fold and 3.66-fold +/- 1.5-fold (mean +/- SEM) after immunization with 60 and 180 mg urease, respectively, whereas no change occurred in the placebo +/- LT groups (P = 0.005). Circulating antiurease immunoglobulin A-producing cells increased in volunteers exposed to urease compared with placebo (38.9 +/- 13. 6/10(6) vs. 5.4 +/- 3.1; P = 0.018). Eradication of H. pylori infection was not observed, but urease immunization induced a significant decrease in gastric H. pylori density.

CONCLUSIONS

H. pylori urease with LT is well tolerated and immunogenic in H. pylori-infected individuals. An improved vaccine formulation may induce curative immunity.

Host response and vaccine development to Helicobacter pylori infection

Studies in both humans and animals demonstrate that H. pylori is capable of illiciting an innate response that in part is regulated by the genetic makeup of the host. These innate responses includes stimulating immune effector mechanisms at the cellular and biochemical level resulting in the influx of neutrophils into the lamina propria and have even been shown to modify gastric acid secretion. The availability of good animal models of chronic Helicobacter infection has also allowed investigators to begin to examine how the adaptive host immune response prevents and/or exacerbates Helicobacter-induced gastroduodenal disease. The experimental H. felis/mouse model has been utilized by a number of laboratories to investigate mechanisms of host defense against chronic Helicobacter infection. This model and the more recently developed H. pylori rodent model has not only allowed investigators to confirm the feasibility of immunotherapy to prevent and/or cure Helicobacter infection but also to begin to examine how the host immune response prevents and/or exacerbates Helicobacter-induced gastroduodenal disease. Based on these studies a hypothesis is emerging that suggests that protection and/or cure from Helicobacter infection is mediated primarily by an upregulated cellular immune response which may act via an antibody independent mechanism. Paradoxically, following natural infection with H. pylori, a component of the cellular immune response also promotes chronic gastric inflammation without clearance of the organism. The recent development of reliable and reproducible H. pylori/rodent models of disease and the availability of numerous inbred strains, transgenic and knockout animals, will allow investigators to continue to explore the role the host cellular and humoral immune response plays in promoting or preventing this infection.

The adjuvants MF59 and LT-K63 enhance the mucosal and systemic immunogenicity of subunit influenza vaccine administered intranasally in mice

Commercial influenza vaccines generate serum antibody, but not local IgA. Influenza vaccines that induce both serum and secretory antibody are more likely to protect against infection and disease progression. The adjuvants MF59 and LT-K63 were tested intramuscularly and intranasally with subunit HA. In naive mice, intranasal adjuvant effect was more apparent when included with the first than second immunization. In previously infected mice, intranasal adjuvants had little effect on serum antibodies and were most effective for nasal antibodies after the second immunization. Overall, both adjuvants enhanced anti-HA IgA and IgG by intranasal vaccination whereas, by intramuscular vaccination, they only enhanced serum IgG.

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.

Effectiveness of a vaccine composed of heat-killed Candida albicans and a novel mucosal adjuvant, LT(R192G), against systemic candidiasis

The incidence of fungal infections caused by the opportunistic yeast Candida albicans has increased significantly in recent years. The ability to vaccinate selected patients against the organism would be advantageous. In this paper we describe a potential anti-C. albicans vaccine consisting of heat-killed C. albicans (HK-CA) in combination with the novel mucosal adjuvant LT(R192G), a genetically detoxified form of the heat-labile toxin of enterotoxigenic Escherichia coli. Groups of male CBA/J mice were immunized intranasally on three occasions at weekly intervals with 2 x 10(7) HK-CA per dose, alone or in conjunction with 10 micrograms of LT(R192G) per dose. Two weeks following the last application of antigen, some animals were challenged intravenously (i.v.) with 10(4), 10(5), or 10(6) viable C. albicans to assess protection as measured by survival and/or culture. Some groups of animals were footpad tested with C. albicans mannan to assess delayed-type hypersensitivity (DTH), and all the animals were bled for antibody assays. In two independent studies, all the animals immunized with HK-CA plus LT(R192G) were able to eradicate 10(4) C. albicans completely, as determined by kidney culture 4 weeks after challenge. Animals immunized with HK-CA only had reduced levels of C. albicans compared to the adjuvant or saline-only control. Greatly enhanced survival was observed when mice immunized with HK-CA plus LT(R192G) were challenged with 10(5) live C. albicans as well. Animals immunized with HK-CA plus LT(R192G) developed a significant DH response, while those given HK-CA alone developed only marginal DH responses. High immunoglobulin G (IgG) levels to cytoplasmic antigens developed in mice immunized with HK-CA plus LT(R192G), but they were found only after i.v. challenge. Addition of adjuvant shifted the antibody isotype production in i.v.-challenged animals to a response dominated by IgG2a. Clearly, intranasal immunization with killed C. albicans in conjunction with LT(R192G) afforded significant levels of protection. This novel approach offers new possibilities for the development of an effective vaccine against candidiasis for use in humans.

Bacterial toxins as mucosal adjuvants

The use of mucosally administered killed bacteria or viruses as vaccines has a number of attractive features over the use of viable attenuated organisms, including safety, cost, storage and ease of delivery. Unfortunately, mucosally administered killed organisms are not usually effective as vaccines. The use of LT(R192G), a genetically detoxified derivative of LT, as a mucosal adjuvant enables the use of killed bacteria or viruses as vaccines by enhancing the overall humoral and cellular host immune response to these organisms, especially the Th1 arm of the immune response. With this adjuvant, protective responses equivalent to those elicited by live attenuated organisms can be achieved with killed organisms without the potential side effects. These findings have significant implications for vaccine development and further support the potential of LT(R192G) to function as a safe, effective adjuvant for mucosally administered vaccines. There are a number of unresolved issues regarding the use of LT and CT mutants as mucosal adjuvants. Both active-site and protease-site mutants of LT and CT have been constructed and adjuvanticity reported for these molecules in various animal models and with different antigens. There needs to be a side-by-side comparison of CT, LT, active-site mutants, protease-site mutants and recombinant B subunits regarding the ability to induce specific, targeted immunological outcomes as a function of route of immunization and nature of the co-administered antigen. Those side-by-side comparisons have not been carried out and there is a substantial body of evidence indicating that the outcomes may very well be different. With that information, vaccine strategies could be designed employing the optimum adjuvant/antigen formulation and route of administration for a variety of bacterial and viral pathogens. Also lacking is an understanding of the underlying cellular and intracellular signaling pathways activated by these different molecules and an understanding of the mechanisms of adjuvanticity at the cellular level. These are important issues because they take us beyond the phenomenological observations of "enhanced immunity" to a more clear understanding of the mechanisms of adjuvant activity.

Recombinant cholera toxin B subunit is not an effective mucosal adjuvant for oral immunization of mice against Helicobacter felis

Cholera toxin is a potent oral mucosal adjuvant for enteric immunization. Several studies suggest that commercial cholera toxin B subunit (cCTB; purified from holotoxin) may be an effective non-toxic alternative for oral immunization. The present study was performed, using an infectious disease model, to determine if the oral mucosal adjuvanticity of CTB is dependent on contaminating holotoxin. Mice were orally immunized with Helicobacter felis sonicate and either cholera holotoxin, cCTB or recombinant cholera toxin B subunit (rCTB). Serum immunoglobulin G (IgG) and intestinal immunoglobulin A (IgA) antibody responses were determined and the mice were challenged with live H. felis to determine the degree of protective immunity induced. All orally immunized mice responded with serum IgG antibody titres regardless of the adjuvant used. However, only mice immunized with either holotoxin or the cCTB responded with an intestinal mucosal IgA response. Consistent with the production of mucosal antibodies, mice immunized with either holotoxin or cCTB as adjuvants were protected from challenge while mice receiving H. felis sonicate and rCTB all became infected. cCTB induced the accumulation of cAMP in mouse thymocytes at a level equal to 0.1% of that induced by holotoxin, whereas rCTB was devoid of any activity. These results indicate that CTB possesses no intrinsic mucosal adjuvant activity when administered orally. Therefore, when used as an oral adjuvant, CTB should also include small, non-toxic doses of cholera toxin.

Intranasal immunization of mice with herpes simplex virus type 2 recombinant gD2: the effect of adjuvants on mucosal and serum antibody responses

Mucosal immunization offers the potential for inducing IgA antibody responses in the vagina, the site of infection for many viruses, including herpes simplex type 2 (HSV-2). To investigate this possibility, mice were immunized intranasally with 10 micrograms glycoprotein D2 (gD2) from HSV combined with a series of adjuvants of proven efficacy; the oil in water emulsion MF59, poly(D,L-lactide-co-glycolide) microparticles (PLG) (encapsulated or co-administered), immune-stimulating complexes (iscoms) (incorporated or co-administered with iscomatrix) and the genetically detoxified enterotoxin from Escherichia coli, LT-K63. Encapsulation of gD2 into PLG microparticles, incorporation of gD2 into iscoms and co-administration of gD2 with LT-K63 induced mucosal IgA antibody responses (nasal wash, saliva and vaginal wash) which were greater than those induced by intramuscular administration of gD2 with MF59. Intranasal immunization with these formulations also induced substantial levels of serum IgG and neutralizing antibodies. These studies demonstrated that intranasal immunization with potent adjuvants is an effective means to induce mucosal antibody responses, even in the lower genital tract.

LT(R192G), a non-toxic mutant of the heat-labile enterotoxin of Escherichia coli, elicits enhanced humoral and cellular immune responses associated with protection against lethal oral challenge with Salmonella spp

In the current study we examined the ability of a novel mucosal adjuvant, LT(R192G), to enhance the humoral and cellular immune responses against killed Salmonella spp. and to affect protection against lethal oral challenge with wild-type organisms. Mice orally immunized with killed S. dublin in conjunction with LT(R192G) were protected against lethal oral challenge and had higher IFN-gamma, IL-2 and IgG responses than did mice orally immunized with killed S. dublin alone which were not protected. This study demonstrates that the function of LT(R192G) in protection against typhoid-like disease is to upregulate/enhance the Th1 arm of the immune response against killed organisms. When used as a mucosal adjuvant, LT(R192G) enables the use of killed bacteria or viruses as vaccines by enhancing the overall humoral and cellular host immune response to these organisms, especially the Th1 arm of the immune response. These findings have significant implications for vaccine development and further support the potential of LT(R192G) to function as a safe, effective adjuvant for mucosally administered vaccines.

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.

Intranasal immunization is superior to vaginal, gastric, or rectal immunization for the induction of systemic and mucosal anti-HIV antibody responses

Vaginal anti-HIV antibody responses may be beneficial, and possibly required, for vaccine-induced protection against HIV infection acquired through receptive vaginal intercourse. We have previously determined that intranasal immunization with a hybrid HIV peptide and cholera toxin induced vaginal anti-HIV IgA responses in BALB/c and C57BL/6 mice. To determine if vaginal, gastric, or rectal boosting would enhance the induction of vaginal anti-HIV IgA responses over those observed with intranasal immunization only, C57BL/6 mice were intranasally immunized with the hybrid HIV peptide T1SP10MN(A) and cholera toxin (days 0 and 14) and boosted via the vaginal, gastric, or rectal route (days 7 and 28). Four intranasal immunizations was superior to all other immunizations evaluated for the induction of plasma anti-peptide IgG, vaginal anti-peptide IgG and IgA, and peptide-specific delayed-type hypersensitivity. In addition, intranasal priming with gastric boosting was associated with greatly elevated total serum IgE concentrations whereas intranasal immunization only was associated with only a modest increase in total serum IgE. These results suggest that intranasal immunization is a viable route of immunization for the induction of systemic and mucosal anti-HIV immune responses.

Intranasal vaccination of humans with recombinant cholera toxin B subunit induces systemic and local antibody responses in the upper respiratory tract and the vagina

Forty-five volunteers were vaccinated twice intranasally with 10, 100, or 1,000 microg of cholera toxin B subunit (CTB). Blood and nasal and vaginal secretions were collected before and 1 week after the second vaccination from all volunteers, and the specific and total immunoglobulin A (IgA) and IgG titers were determined by enzyme-linked immunosorbent assay. Samples were also taken 6 months (n = 16) and 1 year (n = 14) after the vaccination. The 10- and 100-microg doses were well tolerated by the volunteers, but the 1,000-microg dose induced increased secretions from the nose and repetitive sneezings for several hours. The CTB-specific serum IgA and IgG increased 21- and 7-fold, respectively, 1 week after vaccination with the medium dose and increased 61- and 37-fold, respectively, after the high dose. In nasal secretions the specific IgA and IgG increased 2- and 6-fold after the medium dose and 2- and 20-fold after the high dose, respectively. In vaginal secretions the specific IgA and IgG increased 3- and 5-fold after the medium dose and 56- and 74-fold after the high dose, respectively. The lowest dose did not induce any significant antibody titer increases in serum or in secretions. The specific IgA and IgG levels in secretions were still elevated after 6 months but were decreasing 1 year after the vaccination. These results show that intranasal vaccination of humans with CTB induces strong systemic and mucosal antibody responses and suggest that CTB may be used as a carrier for antigens that induce protective immunity against systemic as well as respiratory and genital infections.