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

Mucosal Immunization with DTaP Confers Protection against Bordetella pertussis Infection and Cough in Sprague-Dawley Rats

Pertussis is a respiratory disease caused by the Gram-negative pathogen, Bordetella pertussis. The transition from a whole-cell pertussis vaccine (wP and DTP) to an acellular pertussis vaccine (aP, DTaP, and Tdap) correlates with an increase in pertussis cases, despite widespread vaccine implementation and coverage, and it is now appreciated that the protection provided by aP rapidly wanes. To recapitulate the localized immunity observed from natural infection, mucosal vaccination with aP was explored using the coughing rat model of pertussis. Overall, our goal was to evaluate the route of vaccination in the coughing rat model of pertussis. Immunity induced by both oral gavage and intranasal vaccination of aP in B. pertussis challenged rats over a 9-day infection was compared to intramuscular wP (IM-wP)- and IM-aP-immunized rats that were used as positive controls. Our data demonstrate that mucosal immunization of aP resulted in the production of anti-B. pertussis IgG antibody titers similar to IM-wP- and IM-aP-vaccinated controls postchallenge. IN-aP also induced anti-B. pertussis IgA antibodies in the nasal cavity. Immunization with IM-wP, IM-aP, IN-aP, and OG-aP immunization protected against B. pertussis-induced cough, whereas OG-aP immunization did not protect against respiratory distress. Mucosal immunization by both intranasal and oral gavage administration protected against acute inflammation and decreased bacterial burden in the lung compared to mock-vaccinated challenge rats. The data presented in this study suggest that mucosal vaccination with aP can induce a mucosal immune response and provide protection against B. pertussis challenge. This study highlights the potential benefits and uses of the coughing rat model of pertussis; however, further questions regarding waning immunity still require additional investigation.

Vaccine-Induced Cellular Immunity against Bordetella pertussis: Harnessing Lessons from Animal and Human Studies to Improve Design and Testing of Novel Pertussis Vaccines

Pertussis ('whooping cough') is a severe respiratory tract infection that primarily affects young children and unimmunised infants. Despite widespread vaccine coverage, it remains one of the least well-controlled vaccine-preventable diseases, with a recent resurgence even in highly vaccinated populations. Although the exact underlying reasons are still not clear, emerging evidence suggests that a key factor is the replacement of the whole-cell (wP) by the acellular pertussis (aP) vaccine, which is less reactogenic but may induce suboptimal and waning immunity. Differences between vaccines are hypothesised to be cell-mediated, with polarisation of Th1/Th2/Th17 responses determined by the composition of the pertussis vaccine given in infancy. Moreover, aP vaccines elicit strong antibody responses but fail to protect against nasal colonisation and/or transmission, in animal models, thereby potentially leading to inadequate herd immunity. Our review summarises current knowledge on vaccine-induced cellular immune responses, based on mucosal and systemic data collected within experimental animal and human vaccine studies. In addition, we describe key factors that may influence cell-mediated immunity and how antigen-specific responses are measured quantitatively and qualitatively, at both cellular and molecular levels. Finally, we discuss how we can harness this emerging knowledge and novel tools to inform the design and testing of the next generation of improved infant pertussis vaccines.

Mucosal Immunization Against Pertussis: Lessons From the Past and Perspectives

Background

Current vaccination strategies against pertussis are sub-optimal. Optimal protection against Bordetella pertussis, the causative agent of pertussis, likely requires mucosal immunity. Current pertussis vaccines consist of inactivated whole B. pertussis cells or purified antigens thereof, combined with diphtheria and tetanus toxoids. Although they are highly protective against severe pertussis disease, they fail to elicit mucosal immunity. Compared to natural infection, immune responses following immunization are short-lived and fail to prevent bacterial colonization of the upper respiratory tract. To overcome these shortcomings, efforts have been made for decades, and continue to be made, toward the development of mucosal vaccines against pertussis.

Objectives

In this review we systematically analyzed published literature on protection conferred by mucosal immunization against pertussis. Immune responses mounted by these vaccines are summarized.

Method

The PubMed Library database was searched for published studies on mucosal pertussis vaccines. Eligibility criteria included mucosal administration and the evaluation of at least one outcome related to efficacy, immunogenicity and safety.

Results

While over 349 publications were identified by the search, only 63 studies met the eligibility criteria. All eligible studies are included here. Initial attempts of mucosal whole-cell vaccine administration in humans provided promising results, but were not followed up. More recently, diverse vaccination strategies have been tested, including non-replicating and replicating vaccine candidates given by three different mucosal routes: orally, nasally or rectally. Several adjuvants and particulate formulations were tested to enhance the efficacy of non-replicating vaccines administered mucosally. Most novel vaccine candidates were only tested in animal models, mainly mice. Only one novel mucosal vaccine candidate was tested in baboons and in human trials.

Conclusion

Three vaccination strategies drew our attention, as they provided protective and durable immunity in the respiratory tract, including the upper respiratory tract: acellular vaccines adjuvanted with lipopeptide LP1569 and c-di-GMP, outer membrane vesicles and the live attenuated BPZE1 vaccine. Among all experimental vaccines, BPZE1 is the only one that has advanced into clinical development.

The Path to New Pediatric Vaccines against Pertussis

Whooping cough, or pertussis, mostly caused by Bordetella pertussis, is a respiratory disease that affects all age groups, but severe and fatal pertussis occurs almost exclusively in young children. The widespread use of whole-cell and, more recently, of acellular vaccines has substantially reduced the disease incidence. However, it has not been eliminated in any part of the world and has made a worrisome rebound in several areas. Cocoon and maternal immunization have been implemented in several countries but have their intrinsic limitations. To effectively control pertussis, novel vaccines are needed that protect against disease and prevent B. pertussis infection and transmission, which is not the case for current vaccines. Several approaches are contemplated, including alternative administration routes, such as nasal immunization, improvement of acellular vaccines by adding more antigens and T-cell-promoting adjuvants, and the development of novel vaccines, such as outer membrane vesicles and live attenuated vaccines. Among them, only a live attenuated vaccine has so far been assessed for safety and immunogenicity in preclinical models other than mice and is in clinical development. Before any of these vaccines can be used in neonates, extensive safety and immunogenicity assessment in pre-clinical neonatal models and in carefully designed clinical trials is necessary. The aim of this review is to discuss the current pertussis problem, implemented strategies to resolve it, the value of animal models and novel vaccine approaches.

Investigation of the Cellular Immune Response to Recombinant Fragments of Filamentous Hemagglutinin and Pertactin of Bordetella pertussis in BALB/c Mice

Vaccination with whole-cell or acellular (Ac) vaccines has been very effective for the control of pertussis. The immune response to Ac vaccines has been generally associated with a shift toward the Th2 profile. In the present study, overlapping recombinant fragments of filamentous hemagglutinin (FHA) and pertactin (PRN) were produced in Escherichia coli. BALB/c mice were immunized with recombinant FHA and PRN together with the native pertussis toxin and alum or CpG as adjuvant. Immunized mice were subsequently aerosol challenged with Bordetella pertussis. Bacterial growth was assessed in bronchoalveolar lavage samples and the levels of cytokines were quantitated in supernatants of stimulated splenocytes by enzyme-linked immunosorbent assay. Our results demonstrated that both PRN and FHA antigens were able to induce IFN-γ, IL-4, and to some extent IL-17 cytokines in challenged mice. The level of IFN-γ was higher in response to CpG formulated antigens. These findings indicate immunoprotective efficacy of our recombinant FHA and PRN antigens in mice.

The Role of Mucosal Immunity in Pertussis

Pertussis or whooping cough, mainly caused by Bordetella pertussis, is a severe respiratory disease that can affect all age groups but is most severe and can be life-threatening in young children. Vaccines against this disease are widely available since the 1950s. Despite high global vaccination coverage, the disease is not under control in any country, and its incidence is even increasing in several parts of the world. Epidemiological and experimental evidence has shown that the vaccines fail to prevent B. pertussis infection and transmission, although they are very effective in preventing disease. Given the high infection rate of B. pertussis, effective control of the disease likely requires prevention of infection and transmission in addition to protection against disease. With rare exceptions B. pertussis infections are restricted to the airways and do not usually disseminate beyond the respiratory epithelium. Therefore, protection at the level of the respiratory mucosa may be helpful for an improved control of pertussis. Yet, compared to systemic responses, mucosal immune responses have attracted relatively little attention in the context of pertussis vaccine development. In this review we summarize the available literature on the role of mucosal immunity in the prevention of B. pertussis infection. In contrast to vaccination, natural infection in humans and experimental infections in animals induce strong secretory IgA responses in the naso-pharynx and in the lungs. Several studies have shown that secretory IgA may be instrumental in the control of B. pertussis infection. Furthermore, studies in mouse models have revealed that B. pertussis infection, but not immunization with current acellular pertussis vaccines induces resident memory T cells, which may also contribute to protection against colonization by B. pertussis. As these resident memory T cells are long lived, vaccines that are able to induce them should provide long-lasting immunity. As of today, only one vaccine designed to induce potent mucosal immunity is in clinical development. This vaccine is a live attenuated B. pertussis strain delivered nasally in order to mimic the natural route of infection. Due to its ability to induce mucosal immunity it is expected that this approach will contribute to improved control of pertussis.

Intradermal vaccination with a Pseudomonas aeruginosa vaccine adjuvanted with a mutant bacterial ADP-ribosylating enterotoxin protects against acute pneumonia

Respiratory infections are a leading cause of morbidity and mortality globally. This is partially due to a lack of effective vaccines and a clear understanding of how vaccination route and formulation influence protective immunity in mucosal tissues such as the lung. Pseudomonas aeruginosa is an opportunistic pathogen capable of causing acute pulmonary infections and is a leading cause of hospital-acquired and ventilator-associated pneumonia. With multidrug-resistant P. aeruginosa infections on the rise, the need for a vaccine against this pathogen is critical. Growing evidence suggests that a successful P. aeruginosa vaccine may require mucosal antibody and Th1- and Th17-type CD4⁺ T cells to prevent pulmonary infection. Intradermal immunization with adjuvants, such as the bacterial ADP-Ribosylating Enterotoxin Adjuvant (BARE) double mutant of E. coli heat-labile toxin (dmLT), can direct protective immune responses to mucosal tissues, including the lungs. We reasoned that intradermal immunization with P. aeruginosa outer membrane proteins (OMPs) adjuvanted with dmLT could drive neutralizing antibodies and migration of CD4⁺ T cells to the lungs and protect against P. aeruginosa pneumonia in a murine model. Here we show that mice immunized with OMPs and dmLT had significantly more antigen-specific IgG and Th1- and Th17-type CD4⁺ memory T cells in the pulmonary environment compared to control groups of mice. Furthermore, OMPs and dmLT immunized mice were significantly protected against an otherwise lethal lung infection. Protection was associated with early IFN-γ and IL-17 production in the lungs of immunized mice. These results indicate that intradermal immunization with dmLT can drive protective immunity to the lung mucosa and may be a viable vaccination strategy for a multitude of respiratory pathogens.

Novel intranasal pertussis vaccine based on bacterium-like particles as a mucosal adjuvant

Pertussis, or whooping cough, has recently reemerged as a major public health threat despite high levels of vaccination. The development of a novel pertussis vaccine, especially an intranasal (i.n.) vaccine is undoubtedly necessary, and mucosal adjuvants have been explored to enhance the immune response. In the present study, bacterium-like particles (BLPs) were adopted as a mucosal adjuvant for an i.n. pertussis vaccine and evaluated on the ability to induce serum and mucosal antibodies as well as potency against i.n. challenge in mice. Groups with or without aluminum adjuvant were also evaluated through both i.n. and intraperitoneal inoculations. Vaccination with BLPs via the i.n. route led to rapid IgG and IgA production and provided strong protection against inflammation induced by infection. The results support an i.n. pertussis vaccine with BLPs adjuvant as a promising candidate to elicit protective immunity against whooping cough.

Lipid-Based Particles: Versatile Delivery Systems for Mucosal Vaccination against Infection

Vaccination is the process of administering immunogenic formulations in order to induce or harness antigen (Ag)-specific antibody and T cell responses in order to protect against infections. Important successes have been obtained in protecting individuals against many deleterious pathological situations after parenteral vaccination. However, one of the major limitations of the current vaccination strategies is the administration route that may not be optimal for the induction of immunity at the site of pathogen entry, i.e., mucosal surfaces. It is now well documented that immune responses along the genital, respiratory, or gastrointestinal tracts have to be elicited locally to ensure efficient trafficking of effector and memory B and T cells to mucosal tissues. Moreover, needle-free mucosal delivery of vaccines is advantageous in terms of safety, compliance, and ease of administration. However, the quest for mucosal vaccines is challenging due to (1) the fact that Ag sampling has to be performed across the epithelium through a relatively limited number of portals of entry; (2) the deleterious acidic and proteolytic environment of the mucosae that affect the stability, integrity, and retention time of the applied Ags; and (3) the tolerogenic environment of mucosae, which requires the addition of adjuvants to elicit efficient effector immune responses. Until now, only few mucosally applicable vaccine formulations have been developed and successfully tested. In animal models and clinical trials, the use of lipidic structures such as liposomes, virosomes, immune stimulating complexes, gas-filled microbubbles and emulsions has proven efficient for the mucosal delivery of associated Ags and the induction of local and systemic immune reponses. Such particles are suitable for mucosal delivery because they protect the associated payload from degradation and deliver concentrated amounts of Ags via specialized sampling cells (microfold cells) within the mucosal epithelium to underlying antigen-presenting cells. The review aims at summarizing recent development in the field of mucosal vaccination using lipid-based particles. The modularity ensured by tailoring the lipidic design and content of particles, and their known safety as already established in humans, make the continuing appraisal of these vaccine candidates a promising development in the field of targeted mucosal vaccination.

Adjuvants: Classification, Modus Operandi, and Licensing

Vaccination is one of the most efficient strategies for the prevention of infectious diseases. Although safer, subunit vaccines are poorly immunogenic and for this reason the use of adjuvants is strongly recommended. Since their discovery in the beginning of the 20th century, adjuvants have been used to improve immune responses that ultimately lead to protection against disease. The choice of the adjuvant is of utmost importance as it can stimulate protective immunity. Their mechanisms of action have now been revealed. Our increasing understanding of the immune system, and of correlates of protection, is helping in the development of new vaccine formulations for global infections. Nevertheless, few adjuvants are licensed for human vaccines and several formulations are now being evaluated in clinical trials. In this review, we briefly describe the most well known adjuvants used in experimental and clinical settings based on their main mechanisms of action and also highlight the requirements for licensing new vaccine formulations.

Delivery strategies to enhance oral vaccination against enteric infections

While the majority of human pathogens infect the body through mucosal sites, most licensed vaccines are injectable. In fact the only mucosal vaccine that has been widely used globally for infant and childhood vaccination programs is the oral polio vaccine (OPV) developed by Albert Sabin in the 1950s. While oral vaccines against Cholera, rotavirus and Salmonella typhi have also been licensed, the development of additional non-living oral vaccines against these and other enteric pathogens has been slow and challenging. Mucosal vaccines can elicit protective immunity at the gut mucosa, in part via antigen-specific secretory immunoglobulin A (SIgA). However, despite their advantages over the injectable route, oral vaccines face many hurdles. A key challenge lies in design of delivery strategies that can protect antigens from degradation in the stomach and intestine, incorporate appropriate immune-stimulatory adjuvants and control release at the appropriate gastrointestinal site. A number of systems including micro and nanoparticles, lipid-based strategies and enteric capsules have significant potential either alone or in advanced combined formulations to enhance intestinal immune responses. In this review we will outline the opportunities, challenges and potential delivery solutions to facilitate the development of improved oral vaccines for infectious enteric diseases.

Genetically detoxified pertussis toxin (PT-9K/129G): implications for immunization and vaccines

Pertussis toxin (PT) is one of the major virulence factors of Bordetella pertussis and the primary component of all pertussis vaccines available to date. Because of its various noxious effects the toxin needs to be detoxified. In all currently available vaccines, detoxification is achieved by treatment with high quantity of chemical agents such as formaldehyde, glutaraldehyde or hydrogen peroxide. Although effective in detoxification, this chemical treatment alters dramatically the immunological properties of the toxin. In contrast, PT genetically detoxified through the substitution of two residues necessary for its enzymatic activity maintains all functional and immunological properties. This review describes in detail the characteristics of this PT-9K/129G mutant and shows that it is non-toxic and a superior immunogen compared with chemically detoxified PT. Importantly, data from an efficacy trial show that the PT-9K/129G-based vaccine induces earlier and longer-lasting protection, further supporting the hypothesis that PT-9K/129G represents an ideal candidate for future pertussis vaccine formulations.

Improved pertussis vaccines based on adjuvants that induce cell-mediated immunity

Bordetella pertussis is a Gram-negative bacterium that causes the severe and sometimes lethal respiratory disease whooping cough in infants and children. There has been a recent resurgence in the number of cases of pertussis in several countries with high vaccine coverage. This has been linked with waning or ineffective immunity induced by current acellular pertussis vaccines. These acellular pertussis vaccines are formulated with alum as the adjuvant, which promotes strong antibody responses but is less effective at inducing Th1-type responses crucial for effective bacterial clearance. Studies in animal models have demonstrated that replacing alum with alternative adjuvants, such as toll-like receptor agonists, can promote more robust cell-mediated immunity and confer a high level of protection against infection following respiratory challenge.

Intranasal DNA Vaccine for Protection against Respiratory Infectious Diseases: The Delivery Perspectives

Intranasal delivery of DNA vaccines has become a popular research area recently. It offers some distinguished advantages over parenteral and other routes of vaccine administration. Nasal mucosa as site of vaccine administration can stimulate respiratory mucosal immunity by interacting with the nasopharyngeal-associated lymphoid tissues (NALT). Different kinds of DNA vaccines are investigated to provide protection against respiratory infectious diseases including tuberculosis, coronavirus, influenza and respiratory syncytial virus (RSV) etc. DNA vaccines have several attractive development potential, such as producing cross-protection towards different virus subtypes, enabling the possibility of mass manufacture in a relatively short time and a better safety profile. The biggest obstacle to DNA vaccines is low immunogenicity. One of the approaches to enhance the efficacy of DNA vaccine is to improve DNA delivery efficiency. This review provides insight on the development of intranasal DNA vaccine for respiratory infections, with special attention paid to the strategies to improve the delivery of DNA vaccines using non-viral delivery agents.

Mechanisms of environmental influence on human autoimmunity: a National Institute of Environmental Health Sciences expert panel workshop

The mechanisms leading to autoimmune diseases remain largely unknown despite numerous lines of experimental inquiry and epidemiological evidence. The growing number of genome-wide association studies and the largely incomplete concordance for autoimmune diseases in monozygotic twins support the role of the environment (including infectious agents and chemicals) in the breakdown of tolerance leading to autoimmunity via numerous mechanisms. The present article reviews the major theories on the mechanisms of the environmental influence on autoimmunity by addressing the different degrees of confidence that characterize our knowledge. The theories discussed herein include (i) the role of innate immunity mediated by toll-like receptors in triggering the autoimmune adaptive response characterizing the observed pathology; (ii) changes in spleen marginal zone B cells in autoantibody production with particular focus on the B10 subpopulation; (iii) Th17 cell differentiation and T regulatory cells in the aryl hydrocarbon receptor model; (iv) self antigen changes induced by chemical and infectious agents which could break tolerance by post-translational modifications and molecular mimicry; and finally (v) epigenetic changes, particularly DNA methylation, that are induced by environmental stimuli and may contribute to autoimmunity initiation. We are convinced that these working hypotheses, in most cases supported by solid evidence, should be viewed in parallel with animal models and epidemiological observations to provide a comprehensive picture of the environmental causes of autoimmune diseases.

Mucosal vaccine design and delivery

Mucosal surfaces are a major portal of entry for many human pathogens that are the cause of infectious diseases worldwide. Vaccines capable of eliciting mucosal immune responses can fortify defenses at mucosal front lines and protect against infection. However, most licensed vaccines are administered parenterally and fail to elicit protective mucosal immunity. Immunization by mucosal routes may be more effective at inducing protective immunity against mucosal pathogens at their sites of entry. Recent advances in our understanding of mucosal immunity and identification of correlates of protective immunity against specific mucosal pathogens have renewed interest in the development of mucosal vaccines. Efforts have focused on efficient delivery of vaccine antigens to mucosal sites that facilitate uptake by local antigen-presenting cells to generate protective mucosal immune responses. Discovery of safe and effective mucosal adjuvants are also being sought to enhance the magnitude and quality of the protective immune response.

Transcutaneous immunization with a Vibrio cholerae O1 Ogawa synthetic hexasaccharide conjugate following oral whole-cell cholera vaccination boosts vibriocidal responses and induces protective immunity in mice

A shortcoming of currently available oral cholera vaccines is their induction of relatively short-term protection against cholera compared to that afforded by wild-type disease. We were interested in whether transcutaneous or subcutaneous boosting using a neoglycoconjugate vaccine made from a synthetic terminal hexasaccharide of the O-specific polysaccharide of Vibrio cholerae O1 (Ogawa) coupled to bovine serum albumin as a carrier (CHO-BSA) could boost lipopolysaccharide (LPS)-specific and vibriocidal antibody responses and result in protective immunity following oral priming immunization with whole-cell cholera vaccine. We found that boosting with CHO-BSA with immunoadjuvantative cholera toxin (CT) or Escherichia coli heat-labile toxin (LT) following oral priming with attenuated V. cholerae O1 vaccine strain O395-NT resulted in significant increases in serum anti-V. cholerae LPS IgG, IgM, and IgA (P < 0.01) responses as well as in anti-Ogawa (P < 0.01) and anti-Inaba (P < 0.05) vibriocidal titers in mice. The LPS-specific IgA responses in stool were induced by transcutaneous (P < 0.01) but not subcutaneous immunization. Immune responses following use of CT or LT as an adjuvant were comparable. In a neonatal mouse challenge assay, immune serum from boosted mice was associated with 79% protective efficacy against death. Our results suggest that transcutaneous and subcutaneous boosting with a neoglycoconjugate following oral cholera vaccination may be an effective strategy to prolong protective immune responses against V. cholerae.

Modified heat-stable toxins (hSTa) of enterotoxigenic Escherichia coli lose toxicity but display antigenicity after being genetically fused to heat-labile toxoid LT(R192G)

Enterotoxigenic Escherichia coli (ETEC) strains are a major cause of diarrhea in humans and animals. Heat-stable (STa) and heat-labile (LT) enterotoxins produced by ETEC disrupt fluid homeostasis in host small intestinal epithelial cells and cause fluid and electrolyte hyper-secretion that leads to diarrhea. ETEC strains producing STa or LT are sufficiently virulent to cause diarrhea, therefore STa and LT antigens must be included in ETEC vaccines. However, potent toxicity and poor immunogenicity (of STa) prevent them from being directly applied as vaccine components. While LT toxoids, especially LT(R192G), being used in vaccine development, STa toxoids have not been included. A recent study (IAI, 78:316-325) demonstrated porcine-type STa toxoids [pSTa(P12F) and pSTa(A13Q)] elicited protective anti-STa antibodies after being fused to a porcine-type LT toxoid [pLT(R192G)]. In this study, we substituted the 8th, 9th, 16th, or the 17th amino acid of a human-type STa (hSTa) and generated 28 modified STa peptides. We tested each STa peptide for toxicity and structure integrity, and found nearly all modified STa proteins showed structure alteration and toxicity reduction. Based on structure similarity and toxic activity, three modified STa peptides: STa(E8A), STa(T16Q) and STa(G17S), were selected to construct LT₁₉₂-STa_-toxoid fusions. Constructed fusions were used to immunize mice, and immunized mice developed anti-STa antibodies. Results from this study provide useful information in developing toxoid vaccines against ETEC diarrhea.

Striking activation of NALT and nasal passages lymphocytes induced by intranasal immunization with Cry1Ac protoxin

Cry1Ac protoxin from Bacillus thuringiensis is a potent mucosal immunogen and adjuvant. When delivered intranasally (i.n.) Cry1Ac elicits significant antibody response and is able to improve vaccination against Naegleria fowleri infection, but the functional effects occurring in nasal lymphocytes when this protein is administered alone have not been determined. Here, we investigated the effects of i.n. immunization with Cry1Ac on antibody production, lymphocyte activation and cytokine production in lymphocytes from nasal-associated lymphoid tissue (NALT) and nasal passages (NP). Our results show that i.n. immunization with Cry1Ac induced significant specific IgA and IgG cell responses, especially in NP. Besides, it increased the proportion of lymphocytes expressing the activation markers CD25 and CD69 in both nasal tissues, but differently. CD25 was increased in B cells along with CD4 and CD8 T cells from NALT and NP, while CD69 was increased in B cells from both tissues but only in CD4 T cells from NP. Finally, we found that Cry1Ac augmented especially a Th2 profile of cytokines, as the proportion of T cells that spontaneously produced IL-4, IL-5 and IL-10 was increased and this effect was higher in NP than in NALT. These data contribute to explain the potent immunogenicity of Cry1Ac via i.n. route.

Determining the activity of mucosal adjuvants

Mucosal vaccination offers the advantage of blocking pathogens at the portal of entry, improving patient's compliance, facilitating vaccine delivery, and decreasing the risk of unwanted spread of infectious agents via contaminated syringes.Recent advances in vaccinology have created an array of vaccine constructs that can be delivered to mucosal surfaces of the respiratory, gastrointestinal, and genitourinary tracts using intranasal, oral, and vaginal routes. Due to the different characteristics of mucosal immune response, as compared with systemic response, mucosal immunization requires particular methods of antigen presentation. Well-tolerated adjuvants that enhance the efficacy of such vaccines will play an important role in mucosal immunization. Among promising mucosal adjuvants, mutants of cholera toxin and the closely related heat-labile enterotoxin (LT) of enterotoxigenic Escherichia coli present powerful tools, augmenting the local and systemic serum antibody response to co-administered antigens.In this chapter, we describe the formulation and application of vaccines using the genetically modified LTK63 mutant as a prototype of the family of these mucosal adjuvants and the tools to determine its activity in the mouse model.

Prolonged protection against Intranasal challenge with influenza virus following systemic immunization or combinations of mucosal and systemic immunizations with a heat-labile toxin mutant

Seasonal influenza virus infections cause considerable morbidity and mortality in the world, and there is a serious threat of a pandemic influenza with the potential to cause millions of deaths. Therefore, practical influenza vaccines and vaccination strategies that can confer protection against intranasal infection with influenza viruses are needed. In this study, we demonstrate that using LTK63, a nontoxic mutant of the heat-labile toxin from Escherichia coli, as an adjuvant for both mucosal and systemic immunizations, systemic (intramuscular) immunization or combinations of mucosal (intranasal) and intramuscular immunizations protected mice against intranasal challenge with a lethal dose of live influenza virus at 3.5 months after the second immunization.

New horizons in adjuvants for vaccine development

Over the last decade, there has been a flurry of research on adjuvants for vaccines, and several novel adjuvants are now in licensed products or in late stage clinical development. The success of adjuvants in enhancing the immune response to recombinant antigens has led many researchers to re-focus their vaccine development programs. Successful vaccine development requires knowing which adjuvants to use and knowing how to formulate adjuvants and antigens to achieve stable, safe and immunogenic vaccines. For the majority of vaccine researchers this information is not readily available, nor is access to well-characterized adjuvants. In this review, we outline the current state of adjuvant research and development and how formulation parameters can influence the effectiveness of adjuvants.

Topical vaccination: the skin as a unique portal to adaptive immune responses

Skin is an ideal tissue for vaccine administration, as it is comprised of immunocompetent cells such as keratinocytes and Langerhans cells and elicits both innate and adaptive immune responses. In this paper, we summarize the immune responses induced by topical vaccination of the skin and review the effects of adjuvants on skin vaccination. We also summarize the existing techniques for skin vaccination. New techniques such as the use of lasers to enhance skin permeability are also discussed, as well as the role of the stratum corneum in skin vaccination. A recent study demonstrating enhanced skin vaccination by using surfactants to extract partial lamellar lipids of the stratum corneum will also be introduced in this review.

Mucosal administration of Ag85B-ESAT-6 protects against infection with Mycobacterium tuberculosis and boosts prior bacillus Calmette-Guerin immunity

We have examined the intranasal administration of a vaccine against Mycobacterium tuberculosis (M.tb) consisting of the mucosal adjuvant LTK63 and the Ag Ag85B-ESAT-6. Vaccination with LTK63/Ag85B-ESAT-6 gave a strong and sustained Th1 response mediated by IFN-gamma-secreting CD4 cells, which led to long-lasting protection against tuberculosis, equivalent to that observed with bacillus Calmette-Guérin (BCG) or Ag85B-ESAT-6 in dimethyldioctadecylammonium bromide/monophosphoryl lipid A. Because a crucial element of novel vaccine strategies is the ability to boost BCG-derived immunity, we also tested whether LTK63/Ag85B-ESAT-6 could act as a BCG booster vaccine in BCG-vaccinated mice. We found that vaccinating with LTK63/Ag85B-ESAT-6 strongly boosted prior BCG-stimulated immunity. Compared with BCG-vaccinated nonboosted mice, we observed that infection with M.tb led to a significant increase in anti-M.tb-specific CD4 T cells in the lungs of LTK63/Ag85B-ESAT-6-boosted animals. This correlated with a significant increase in the protection against M.tb in LTK63/Ag85B-ESAT-6-boosted mice, compared with BCG-vaccinated animals. Thus, LTK63/Ag85B-ESAT-6 represents an efficient preventive vaccine against tuberculosis with a strong ability to boost prior BCG immunity.

Evaluation of the immune response induced by a nasal anthrax vaccine based on the protective antigen protein in anaesthetized and non-anaesthetized mice

To better protect against inhalational anthrax infection, a nasal anthrax vaccine based on the protective antigen (PA) protein of Bacillus anthracis could be an attractive alternative to the current Anthrax-Vaccine-Adsorbed (AVA), which was licensed for cutaneous anthrax prevention. Previously, we have demonstrated that an anti-PA immune response comparable with that in mice subcutaneously immunized with PA protein adjuvanted with aluminium hydroxide was induced in both the systemic compartment and the mucosal secretions of the nose and lung of anaesthetized mice when they were nasally immunized with PA protein incorporated into previously reported LPD (Liposome-Protamine-DNA) particles. In this study, we evaluated the anti-PA immune response induced by the nasal PA/LPD particles in non-anaesthetized mice and compared it with that in anaesthetized mice. Our data showed that the anti-PA antibody response and the anthrax lethal toxin-neutralization activity induced by the nasal PA/LPD in non-anaesthetized mice was relatively weaker than that in anaesthetized mice. However, the splenocytes isolated from the nasally immunized mice, anaesthetized and non-anaesthetized, proliferated comparably after in-vitro re-stimulation. By evaluating the uptake of fluorescence-labelled LPD particles by phagocytes in the nasal and broncho-alveolar lavages of mice after the nasal administration, we concluded that the relatively weaker anti-PA immune response in the non-anaesthetized mice might be partially attributed to the reduced retention of the PA/LPD particles in the nasal cavity of the non-anaesthetized mice. Data collected in this study are expected to be useful for future anthrax nasal vaccine studies when mice are used as a model.

Adjuvant effects of adenylate cyclase toxin of Bordetella pertussis after intranasal immunisation of mice

This study examined the ability of the adenylate cyclase toxin (CyaA) of Bordetella pertussis to act as a mucosal adjuvant for other antigens when co-administered by the intranasal route in mice. Two forms of CyaA were used: the cell-invasive, enzymically active form and a cell-invasive toxin lacking adenylate cyclase enzymic activity (CyaA*). Co-administration intranasally (i/n) of CyaA or CyaA* with ovalbumin (Ova) significantly enhanced (P<0.05) anti-Ova IgG and IgA antibody responses in the serum and anti-Ova IgA responses in lung and nasal secretions compared to those generated by immunisation i/n with Ova alone. The effects were greater with CyaA*. Administration of CyaA* with Ova induced priming of Ova-specific T cells in vivo to a greater extent than that obtained after immunisation with Ova alone. Co-administration of CyaA or CyaA* with pertactin (Prn) significantly enhanced (P<0.05) the serum anti-Prn IgG responses and immunisation with Prn and CyaA* significantly increased the anti-Prn IgA responses in the lungs compared with responses after immunisation with Prn alone. Immunisation i/n with Prn alone partially protected mice (P<0.05) against challenge i/n with B. pertussis. Co-administration of CyaA or CyaA* with pertactin (Prn) significantly increased protection (P<0.05) against challenge compared to that obtained with Prn alone. These effects were particularly apparent with CyaA* as the adjuvant.

Enhancement of intranasal vaccination in mice with deglycosylated chain A ricin by LTR72, a novel mucosal adjuvant

Intranasal (i.n.) vaccination with two suboptimal doses of 8 microg of deglycosylated chain A ricin (DGCA) stimulated low anti-ricin ELISA IgG and neutralizing antibody responses and the vaccine was only marginally protective against a lethal ricin toxin aerosol challenge. However, in the presence of 4, 2, or 1 microg of the mucosal adjuvant LTR72, a mutant of the heat-labile enterotoxin of Escherichia coli, the low antibody response and protection were substantially enhanced. In comparison to the vaccination with DGCA alone, vaccination with DGCA in the presence of three dose levels of LTR72, the anti-ricin ELISA serum IgG geometric mean titer (GMT) was increased, respectively, 191-, 572-, and 51-fold for IgG; 91-, 93-, and 60-fold for IgG1; nine-, six-, and two-fold for IgG2a; zero-, two-, and zero-fold for IgA. The three dose levels of the adjuvant enhanced the anti-ricin ELISA immunoglobulin GMTs in the lung lavage 4-, 14-, and 7-fold for IgG; two-, five-, and six-fold for IgG1; two-, six-, and two-fold IgG2a; and zero-, three-, and zero-fold for IgA, respectively. Compared to GMT obtained with the aqueous vaccine (1:2), the 10% serum neutralizing antibody GMT for the three dose levels was enhanced 25-, 60-, and 62-fold, respectively while the 50% neutralizing antibody GMT was enhanced more than 3-, 19- and 10-fold. Only 20% of the mice immunized with DGCA survived the lethal whole body aerosol challenge with 5-10 LD50 ricin toxin, while in the presence of 4, 2, and 1 microg LTR72, 100, 100 and 90% of the vaccinated mice survived, respectively. Safety of administration of two doses of LTR72 is indicated by the absence of histopathological changes in every organ including the lung and the CNS of the mice during the vaccination and during 57 days of the study. In the nasal passages of the mice in the absence of DGCA, LTR72 caused a transient inflammation for less than 7 weeks without permanent epithelial changes. Administration of the adjuvant in the presence of DGCA did not cause additional changes. Compared to the surviving mice vaccinated with DGCA alone, administration of the mucosal adjuvant with DGCA in spite of the better efficacy did not attenuate the lung injury at a single time point (16 days post-challenge). In mice treated with high(er) dose of vaccine, histological examinations during longer observation period rather than at one time point could reveal a different pattern.

Mucosal adjuvants

Induction of immune responses following oral immunization is frequently dependent upon the co-administration of appropriate adjuvants that can initiate and support the transition from innate to adaptive immunity. The three bacterial products with the greatest potential to function as mucosal adjuvants are the ADP-ribosylating enterotoxins (cholera toxin and the heat-labile enterotoxin of Escherichia coli), synthetic oligodeoxynucleotides containing unmethylated CpG dinucleotides (CpG ODN), and monophosphoryl lipid A (MPL). The mechanism of adjuvanticity of the ADP-ribosylating enterotoxins is the subject of considerable debate. Our own view is that adjuvanticity is an outcome and not an event. It is likely that these molecules exert their adjuvant function by interacting with a variety of cell types, including epithelial cells, dendritic cells, macrophages, and possibly B- and T-lymphocytes. The adjuvant activities of CpG and MPL are due to several different effects they have on innate and adaptive immune responses and both MPL and CpG act through MyD88-dependent and -independent pathways. This presentation will summarize the probable mechanisms of action of these diverse mucosal adjuvants and discuss potential synergy between these molecules for use in conjunction with plant-derived vaccines.

Innate imprinting by the modified heat-labile toxin of Escherichia coli (LTK63) provides generic protection against lung infectious disease

In a healthy individual, the lung contains few lymphoid cells. However, amplified immune responses, as exemplified during lung infection, can cause extensive tissue damage. We have previously demonstrated that one lung infection modulates the immunopathological outcome to a subsequent unrelated pathogen. Mimicking heterologous immunity may provide a means of enhancing both innate and acquired immunity. We now show that prior lung administration of a modified heat-labile toxin from Escherichia coli (LTK63) enhances immunity to respiratory syncytial virus, influenza virus, and the fungus Cryptococcus neoformans. Treatment with LTK63 decreased lung inflammation and tissue damage and improved the ability to resolve the infection. APCs expressing the activation markers MHC class II, CD80, and CD40 increased in number in the lung. LTK63 treatment increased the pathogen-specific IgA response in the nasal mucosa and simultaneously decreased inflammatory cytokine production (IFN-gamma and TNF-alpha) after infection. The number of activated CD8(+)CD44(+) T cells and the respiratory syncytial virus- or influenza-specific CD8-proliferative responses increased, although the total inflammatory infiltrate was reduced. LTK63 treatment matured lung APCs (LTK63 prevented efficient presentation of whole OVA to DO11.10 cells, whereas OVA peptide presentation was unaffected), enhanced immunity in both a Th1 and Th2 environment, was long lasting, and was not pathogen or host strain specific; the protective effects were partially independent of T and B cells. Innate imprinting by toxin-based immunotherapeutics may provide generic protection against infectious disease in the lung, without the need for coadministered pathogen-specific Ag.

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.

Adjuvanticity of native and detoxified adenylate cyclase toxin of Bordetella pertussis towards co-administered antigens

The cell-invasive adenylate cyclase toxin (CyaA) of Bordetella pertussis was shown to be highly antigenic in mice, stimulating serum anti-CyaA IgG antibody responses which were able to neutralise the cytotoxic effect of CyaA on J774.2 macrophage-like cells. The effect of co-administration to mice of the fully functional CyaA toxin or a toxin lacking adenylate cyclase enzymic activity (CyaA*) with other antigens from B. pertussis, namely pertussis toxin (PT) or pertussis toxoid (PTd), filamentous haemagglutinin (FHA) and pertactin (PRN), was investigated. CyaA* enhanced the serum IgG antibody responses to each of these antigens whereas, with CyaA, only anti-PRN antibody titres showed a modest increase. Peritoneal macrophages and spleen cells, collected at 2 weeks post-immunisation, were cultured and tested for nitric oxide (NO) and IFNgamma production, respectively, after stimulation in vitro with heat-killed B. pertussis cells or CyaA proteins. NO and IFNgamma production were higher in cells collected from mice immunised with CyaA or CyaA* in combination with a PT, FHA and PRN antigen mixture than from those taken from mice injected with antigen mixture alone, again with CyaA* acting as a better adjuvant than CyaA. The apparent enhancement of immune responses to the antigen mixture by CyaA* in particular was not paralleled by increased protection of mice against aerosol challenge with B. pertussis, but a statistically significant increase in protection was seen after intranasal challenge with B. parapertussis.

Role of T cells in the adjuvant effect of Bacillus firmus on the immune system of mice: intranasal and intratracheal immunization study with ovalbumin

Functions of T cells were determined after intranasal and intratracheal immunization of mice with ovalbumin (Ova) and Bacillus firmus (Bf), a Gram-positive nonpathogenic bacterium of the external environment, or delipidated Bf (dBf) as adjuvants, with the aim to elucidate the mechanism of support of Ova-specific antibody production caused by Bf that had been observed in an identical experiment. Neither Bf nor dBf in a mixture with Ova stimulated Ova-specific T-cell response tested as antigen-specific blast transformation. By contrast, a mild polyclonal stimulation was observed in splenocytes from mice given dBf. In vitro incubation of splenocytes with 100 micrograms (but not 10 micrograms) of Bf or dBf led to a highly significant inhibition of proliferation below the control level in all groups of animals. Supernatants of splenocyte cultures were further tested for cytokine production. IL-10 and IFN-gamma were released after in vitro challenge with dBf and in some cases also with Bf. Analysis of sera demonstrated that administration of Ova + adjuvant brought about an increase in anti-Ova IgG1, IgG2a and IgG2b whereas treatment with Ova alone caused a rise in IgG1 only. The role of Bf or dBf in the enhancement of antigen-specific antibody production could be in influencing macrophages and inducing cytokine milieu composed of IL-10, IFN-gamma and other factors that leads to a bystander stimulation of specifically activated Ova-B cell receptor (Ova-BCR)-bearing cells.

New approaches in vaccine development

In the last century, vaccines have been one of the most powerful tools for preventing infectious diseases. Smallpox has been eradicated and other diseases such as poliomyelitis or measles have been reduced to very low levels in many regions of the world. However, infectious diseases remain the leading cause of death worldwide. Thus, the development of vaccines to prevent diseases for which no vaccine currently exists such as AIDS or malaria as well as the improvement of efficacy and safety of existing vaccines remains a high priority. Achieving such ambitious goals in a near future will certainly require a strong modification of the methods that have been used so far to identify vaccine candidates. In particular, modern vaccinology could strongly benefit of the latest developments of molecular biology and immunology. Here, we will discuss some potential applications of the increasing knowledge of pathogen genomes as well as the immune system for the discovery of new antigenic targets and the development of new strategies of vaccination.

Mucosal delivery of the human immunodeficiency virus-1 Tat protein in mice elicits systemic neutralizing antibodies, cytotoxic T lymphocytes and mucosal IgA

Human immunodeficiency virus (HIV)-1 Tat protein induces protection in non-human primates upon systemic vaccination. In view of the design of mucosal vaccines against HIV-1 we studied the immune response to native Tat (aa 1-86) in mice following intranasal delivery of the protein with two mucosal adjuvants, Escherichia coli heat-labile enterotoxin (LT) and LT-R72, a non-toxic mutant of LT. Immunization with Tat and the two adjuvants induced in BALB/c but not in C57BL/6 mice high and persistent levels of serum IgG and secretory IgA in vaginal and intestinal fluids. Mice sera neutralized Tat and recognized two epitopes mapping in the regions 1-20 and 46-60. Furthermore, their splenocytes proliferated and secreted IFN-gamma and IL-6 in response to Tat. Finally, CTLs were also elicited and they recognized an epitope localized within aa 11-40 of Tat.

Enhanced mucosal and systemic immune responses to Helicobacter pylori antigens through mucosal priming followed by systemic boosting immunizations

It is estimated that Helicobacter pylori infects the stomachs of over 50% of the world's population and if not treated may cause chronic gastritis, peptic ulcer disease, gastric adenocarcinoma and gastric B-cell lymphoma. The aim of this study was to enhance the mucosal and systemic immune responses against the H. pylori antigens cytotoxin-associated gene A (CagA) and neutrophil-activating protein (NAP), through combinations of mucosal and systemic immunizations in female BALB/c mice. We found that oral or intranasal (i.n.) followed by i.m. immunizations induced significantly higher serum titres against NAP and CagA compared to i.n. alone, oral alone, i.m. alone, i.m. followed by i.n. or i.m. followed by oral immunizations. However, only oral followed by i.m. immunizations induced anti-NAP antibody-secreting cells in the stomach. Moreover, mucosal immunizations alone or in combination with i.m., but not i.m. immunizations alone, induced mucosal immunoglobulin A (IgA) responses in faeces. Any single route or combination of immunization routes with NAP and CagA preferentially induced antigen-specific splenic interleukin-4-secreting cells and far fewer interferon-gamma-secreting cells in the spleen. Moreover, i.n. immunizations alone or in combination with i.m. immunizations induced predominantly serum IgG1 and far less serum IgG2a. Importantly, we found that while both i.n. and i.m. recall immunizations induced similar levels of serum antibody responses, mucosal IgA responses in faeces were only achieved through i.n. recall immunization. Collectively, our data show that mucosal followed by systemic immunization significantly enhanced local and systemic immune responses and that i.n. recall immunization is required to induce both mucosal and systemic memory type responses.

Recent advances in veterinary vaccine adjuvants

Next generation veterinary vaccines are going to mainly comprise of either subunit or inactivated bacteria/viruses. These vaccines would require optimal adjuvants and delivery systems to accord long-term protection from infectious diseases in animals. There is an urgent need for the development of new and improved veterinary and human vaccine adjuvants. Adjuvants can be broadly divided into two classes, based on their principal mechanisms of action: vaccine delivery systems and 'immunostimulatory adjuvants'. Vaccine delivery systems are generally particulate e.g. emulsions, microparticles, ISCOMS and liposomes, and mainly function to target associated antigens into antigen presenting cells (APC). In contrast, immunostimulatory adjuvants are predominantly derived from pathogens and often represent pathogen associated molecular patterns, e.g. LPS, MPL and CpG DNA, which activate cells of the innate immune system. Recent progress in innate immunity is beginning to yield insight into the initiation of immune responses and the ways in which immunostimulatory adjuvants might enhance this process in animals and humans alike.