Role of antigen form in development of mucosal immunoglobin A response to Shigella flexneri Antigens

Intranasal formulations: promising strategy to deliver vaccines

INTRODUCTION

The emergence of new diseases and the lack of efficient vaccines against numerous non-treatable pathogens require the development of novel vaccination strategies. To date, only a few mucosal vaccines have been approved for humans. This was in part due to i) the use of live attenuated vaccines, which are not suitable for certain groups of individuals, ii) safety concerns derived from implementation in humans of some mucosal vaccines, iii) the poor stability, absorption and immunogenicity of antigens delivered by the mucosal route and iv) the limited number of available technologies to overcome the bottlenecks associated with mucosal antigen delivery. Recent advances make feasible the development of efficacious mucosal vaccines with adequate safety profile. Thus, currently intranasal vaccines represent an attractive and valid alternative to conventional vaccines.

AREAS COVERED

The present review is focused on the potentials and limitations of market-approved intranasal vaccines and promising candidates undergoing clinical investigations. Furthermore, emerging strategies to overcome main bottlenecks including efficient breaching of the mucosal barrier and safety concerns by implementation of new adjuvants and delivery systems are discussed.

EXPERT OPINION

The rational design of intranasal vaccines requires an in-depth understanding of the anatomic, physicochemical and barrier properties of the nasal mucosa, as well as the molecular mechanisms governing the activation of the local innate and adaptive immune system. This would provide the critical knowledge to establish effective approaches to deliver vaccine antigens across the mucosal barrier, supporting the stimulation of a long-lasting protective response at both mucosal and systemic levels. Current developments in the area of adjuvants, nanotechnologies and mucosal immunology, together with the identification of surface receptors that can be exploited for cell targeting and manipulating their physiological properties, will become instrumental for developing a new generation of more effective intranasal vaccines.

Evaluation of virulent and live Shigella sonnei vaccine candidates in a gnotobiotic piglet model

Newborn gnotobiotic (GB) piglets given virulent Shigella orally develop many of the clinical symptoms and gastrointestinal (GI) manifestations that mimic human shigellosis. Shigella sonnei virulent strain Moseley, a mutant ShET2-1,2, lacking enterotoxin SenA and its paralog SenB, and vaccine candidates WRSS1 and WRSs3 were evaluated in this model for rates of diarrhea, colonization and other GI symptoms and pathology. Moseley-infected piglets developed diarrhea from 1 to 7 days, with the highest rates seen on days 2-4 after inoculation. In contrast, WRSs3-infected piglets did not have diarrhea over the entire experimental period. Compared to the Moseley group, lower diarrheal rates were observed in the double enterotoxin mutant and significantly lower in the WRSS1 group. Moseley infection also caused marked mucosal damage in the GI tissues at PID1 to PID8, and induced predominantly proinflammatory cytokine secretion. IL-8 and to a lesser extent IL-6 and IL-1β were observed early after inoculation and IL-12 secretion could be measured till late in infection. The ShET2-1,2 mutant, WRSS1 and WRSs3 also colonized the GI tract in a manner similar to Moseley; however, both vaccine candidates developed milder histopathological indices and cytokine responses. WRSs3-infected animals showed the least pathology. Furthermore, unlike the other strains, WRSs3 was rarely detected in organs outside the gastrointestinal tract. These results support the development of the GB piglet model as a sensitive in vivo oral model for the evaluation of virulence of different Shigella strains which could be applied to other oral vaccine candidates.

Considerations for development of whole cell bacterial vaccines to prevent diarrheal diseases in children in developing countries

Enteric pathogens constitute a major pediatric threat in the developing world through their impact on morbidity and mortality, physical and cognitive development and cause and effect relationship with malnutrition. Although many bacterial pathogens can cause diarrheal diseases, a group of less than 10 including Shigella spp., enterotoxigenic Escherichia coli (ETEC), Vibrio cholerae, and possibly, Campylobacter jejuni account for a significant percentage of these diseases in developing countries. Rotavirus is also a major cause of diarrheal diseases. Vaccines against these agents offer a potentially effective control measure against these diseases, but safe, practical, and effective vaccines for many of these agents have yet to be realized. Many vaccine development approaches are under investigation, but the one that is currently most advanced and that has been most widely applied to enteric pathogens is the use of orally administered live or killed whole pathogen preparations. If inactivated, these vaccines will probably be administered as multiple doses with approximately 10(10) to 10(11) total particles per dose, but they are relatively safe for oral administration. Further, they may not require a buffer for delivery and can be stored in liquid formulations. Fewer doses may be required for some live attenuated pathogen vaccines, but a buffer will most likely be required for oral delivery and the product must be stored in a dried formulation. Also, safety becomes more of a concern with live pathogens depending on the degree of attenuation, host immunocompetence, and the total number and kinds of attenuated pathogens which may be present in a combined agent vaccine. Both live and killed whole pathogen vaccines can be immunogenic and have the possibility to serve as vectors for other antigens. Although many organisms and serotypes are clinically important, by exploiting antigenic cross reactivity and using some pathogen components as vectors for cloned antigens of other pathogens, it could be possible to induce immunity against major enteric pathogens/serotypes with <10 whole pathogen components in a multi-agent vaccine. Safe and effective mucosal adjuvants may in the future be useful in whole pathogen vaccines, but they do not seem to be essential for immunization. Further, dietary supplements such as zinc, mixed routes of delivery and new regimens are under study which may in the future enhance further the effectiveness of the whole pathogen vaccines which now seem realizable in the near term. For this to happen, however, a coordinated and committed effort is necessary now to address the immunologic, regulatory, manufacturing, testing and implementation issues which will be involved in the realization of this important product to benefit children's health worldwide.

Live Shigella flexneri 2a and Shigella sonnei I vaccine candidate strains with two attenuating markers. I. Construction of vaccine candidate strains with retained invasiveness but reduced intracellular multiplication

Live Shigella flexneri 2a and Shigella sonnei Phase I vaccine candidate strains with two virulence-reducing markers were constructed through stepwise incorporation of weakly attenuating purine auxotrophy with subsequent rifampicin-resistance (RNA polymerase) mutation to yield optimal attenuation. These vaccine candidate strains showed an unaltered plasmid profile; did not cause keratoconjunctivitis in the Sereny test, while being excreted for a short but still marked period and providing partial protection from wild-strain infection; exhibited for guinea-pig conjunctival epithelia, HeLa cells and rat enterocytes a maintained invasiveness with reduced intracellular multiplication with little, if any, reversible cell damage; and produced, just as their ultrasonic lysates, no exudative reaction in the rabbit gut loop test.

Rabbit model for mucosal immunity in the bowel: I. Establishment of virus-infected ileal loops

An animal model was designed for use in studies of initial cellular immune responses to virus infection of the intestinal mucosa. The animal chosen was the New Zealand White rabbit and the mucosal site the subterminal ileum, isolated in a Thirty-Vella loop. The antigen used was parainfluenzavirus type 3, which would normally be destroyed by bile salts if ingested. Loops approximately 20 cm in length, each containing at least one Peyer's patch, were exteriorised through left paramedian stomata. Atrophic changes began to appear in the loops by 7 days, but no observable diminution in their associated lymphoid tissues was evident. The genesis of parainfluenzavirus type 3 infection in the loops was monitored by assay of sequential loop washings for infectious virus and in fluorescent antibody studies of cells from infected loop epithelia. Infectious virus was recovered for up to 13 days after inoculation and specific intracytoplasmic immunofluorescence was detected in loop epithelial cells. There was little serological evidence of systemic spread of the virus. A localised cellular immune response against parainfluenzavirus type 3 was mounted in the lymphoid tissues associated with the infected loops by day 14, but was not detected in systemic lymphoid tissues. No reactivity was detected in rabbits given inactivated virus via their loops or in those receiving infectious virus intravenously. This model appears to be capable of generating mucosal cellular responses to infection and may therefore be suitable for further studies in this field.

Mucosal and systemic immunity to Campylobacter jejuni in rabbits after gastric inoculation

The mucosal and systemic immune responses to Campylobacter jejuni were studied in rabbits receiving gastric inoculation with live organisms. A lavage procedure was used to facilitate repeated monitoring of the intestinal immune response to C. jejuni. Immunity to C. jejuni was determined by secondary challenge by using the removable intestinal tie adult rabbit diarrhea (RITARD) model and monitoring for resistance to colonization and bacteremia. Oral-gastric inoculation of normal rabbits produced a transient intestinal colonization without diarrhea. C. jejuni serotypes differed in their ability to colonize the intestines of rabbits and to stimulate primary intestinal and serum antibody responses. Animals previously colonized were resistant to recolonization and the development of bacteremia after homologous challenge by the RITARD procedure but were not resistant to heterologous challenges. Anticampylobacter intestinal and serum IgA titers before this secondary infection were the most reliable predictors of resistance to colonization and bacteremia.

Effect of antigen form on local immunoglobulin A memory response of intestinal secretions to Shigella flexneri

An enhanced memory response, as shown by increased titers of specific immunoglobulin A (IgA), was seen in intestinal secretions from isolated Thiry-Vella loops in rabbits primed orally with live, locally invasive Shigella sp. X16 and challenged 60 days later with a single oral dose of the same antigen. Heat-killed shigella preparations, when used as either the priming or challenge antigen, did not elicit such a memory response in this system. In the present study, the role of antigen form and dosage in eliciting the enhanced local IgA response was investigated. A noninvasive strain, Shigella flexneri 2457-0, was capable of significantly enhancing the mucosal IgA memory response, whereas heat-killed Shigella sp. X16 was unable to augment the local IgA response, even when the priming dose was increased 100-fold. A proposed mucosal adjuvant, DEAE-dextran, given orally with live Shigella sp. X16, did not enhance the local IgA response. Viable, noninvasive shigellae were effective priming agents in enhancing the local IgA memory response. The poor mucosal response to heat-killed shigella preparations is thought to be related to an ineffective delivery of nonviable bacterial antigens into gut-associated lymphoid tissues. The ability of the live, noninvasive strain to elicit a vigorous local IgA memory response when given orally to rabbits was consistent with previous findings that live preparations elicit the best mucosal IgA response.

Effect of parenteral immunization on the local immunoglobulin A response of the intestine to Shigella flexneri antigens

Most traditional methods of immunization involve parenteral vaccines. Using a chronically isolated ileal loop model as the probe, we examined the effect of a primary parenteral immunization on the local immune response of the intestine. Secretions from isolated ileal loops of rabbits given a primary parenteral immunization with Shigella flexneri without adjuvant showed a small, but definite, specific immunoglobulin A (IgA) response locally, whereas a vigorous serum antigen-specific IgG response was elicited. Also, stronger antigen-specific IgG activity was detected in secretions of parenterally immunized animals than from animals given shigella only by oral immunization. No local IgA or IgG memory response could be elicited by prior parenteral priming with S. flexneri antigens. Lastly, no increase in the primary local IgA response was found in secretions from animals given an oral dosage regimen previously shown to prime for mucosal memory and then challenged with a single parenteral dose of the same antigen. These studies demonstrate that without adjuvant, parenteral doses of S. flexneri antigens elicit both specific serum and local immune responses, but they are not able to prime for a mucosal memory response upon subsequent mucosal challenge. The implications of these findings for programs that use parenteral vaccination to protect against mucosal diseases are discussed.

T- and B-cell memory on mucosal surfaces

The secretory immune system exhibits memory responses in both B- and T-cell systems. Booster immunization mediates rapid activation of immune lymphocytes that may be important for host protection. Local memory responses may be generated following application of the antigen systemically or at other distant mucosal sites. The dichotomy between the systemic and local immune systems could be overcome, at least partially, in memory responses, depending upon the route and amount of antigen employed for the booster immunization.

Intestinal immunoglobulin A responses in rabbits to a Salmonella typhi strain harboring a Shigella sonnei plasmid

Salmonella typhi 5076-IC, which contains a plasmid that encodes the form I antigen of Shigella sonnei and which expresses S. typhi 9 and 12 and S. sonnei form I antigens, was used to immunize rabbits via chronically isolated ileal loops. Intestinal immunoglobulin A activity was detected against S. typhi, S. sonnei form I, and S. typhi strain 5076-IC. Thus S. typhi 5076-IC can effectively elicit mucosal immunoglobulin A to both S. typhi and S. sonnei.