Oral Vaccine Delivery: Can it Protect Against Non-mucosal Pathogens?

Lina Wang; Ross L Coppel

Disclosures

Expert Rev Vaccines. 2008;7(6):729-738. 

In This Article

Oral Vaccine-delivery Systems

Most oral vaccines available today are composed of attenuated live microorganisms that can replicate in the mucosa and elicit a sustained immune response.[1] However, many important pathogens cannot be attenuated to facilitate the development of vaccines. Some are difficult or impossible to culture and manipulate using existing techniques or may require human tissues or blood for growth. Consequently, effort has focused on subunit vaccines delivered orally by a wide range of means.

The capacity of some bacteria to colonize and infect the intestinal mucosa has led to their development as effective vectors for the oral delivery of vaccine antigens.[7] Heterologous recombinant antigens expressed by such bacteria, either through insertion into a plasmid or integration into the host chromosome, can be delivered directly to the antigen-presenting cells in the GALT and effectively stimulate serum and mucosal antibodies, as well as cell-mediated immunity. Both commensal and attenuated bacteria, of which Salmonella is the most extensively studied example, have been used to deliver various viral, bacterial and parasitic antigens. Salmonella spp. can be attenuated by defined genetic means so that they become avirulent, yet preserve invasiveness. Several reviews have detailed examples of the use of Salmonella strains as carriers of heterologous antigens.[8,9,10] In various model systems, many of these recombinant Salmonella have been shown to afford protection against the pathogen from which the heterologous antigen is derived. However, this promise has yet to be substantiated in human clinical trials.[11] Strategies are being developed to improve the immunogenicity of heterologous antigens delivered by Salmonella, including transgene stabilization, antigen-expression control and optimization of antigen localization.[11,12] Efforts are also being directed toward developing recombinant Salmonella strains that do not contain antibiotic-resistance markers, owing to the concerns that resistance genes may spread to pathogenic organisms in the environment, rendering them refractory to antibiotic treatment.[10]

Particulate vaccine formulations can be designed to protect entrapped antigens against degradation in the gut and/or to target antigens for uptake into the GALT. A number of particulate systems have been developed for oral delivery of vaccines, including microparticles, liposomes, cochleates, immune-stimulating complexes (ISCOMs) and virus-like particles (VLPs).[2] Vaccine antigens can be encapsulated into microparticles using a range of biodegradable polymers, such as poly(lactide-co-glycolide acid) (PLGA), and encapsulated antigens are able to induce stronger immune responses than soluble antigens.[13] Similarly, antigens can be encased in liposomes (layered membranous vesicles consisting primarily of phospholipids and lipopolyaccharides.[14] or cochleates (an alternative lipid-based membrane system consisting of phospholipids and calcium that form a multilayered spiral structure.[15] Other formulations that have been used include ISCOMs (cage-like particles, in which antigens can be incorporated, formed spontaneously when cholesterol is mixed with QuilA [an adjuvant.[16]) or VLPs (self-assembling nonreplicating viral-core structures, often from nonenveloped viruses that are produced by recombinant means.[17]. A combination of antigens, adjuvants and targeting molecules can be incorporated into these particulate formulations to allow them to bind specifically to particular cells or to the extracellular matrix in tissues. In general, all of the particulate formulations act to protect vaccine antigens against degradation and enhance their uptake by the immune systems. Their potential for oral vaccine delivery has been demonstrated in laboratory animals and, in limited cases, clinical trials.[13] For example, oral immunization of human volunteers with microencapsulated enterotoxigenic Escherichia coli CS6 antigen elicited specific immune responses, although the antibody levels were modest and no protection data were available.[18] Clearly, further work is needed to evaluate the utility of particular particulate antigen formulations, as well as their toxicity and allergenicity.

The most potent and widely used adjuvants for oral vaccines are cholera toxin (CT) and the closely related E. coli heat-labile enterotoxin (LT). Both are unsafe for human use in their native form and have been modified to generate toxicologically acceptable derivatives that retain adjuvant activity.[19] For example, a genetically detoxified derivative of LT, LT (R192G), exhibits similar adjuvant activity to the native toxin.[20,21,22] The CT subunit B (CTB), which is involved in crossing cell membranes, has been produced as a recombinant protein that shows little or no toxicity in animals, and its membrane-crossing ability has been utilized to introduce antigens into cells by chemically or genetically conjugating the antigen of interest to the toxin subunit.[23] Bacterial DNA or synthetic oligodeoxynucleotides containing cytosine-phosphate-guanosine (CpG) motifs are also being investigated as adjuvants. As a ligand for the Toll-like receptor 9, CpG has been shown to be a potent adjuvant for mucosal vaccination with various antigens.[24,25] Finally, crude Quillaja saponin extract, which is routinely used by the food and beverage industry and, as such, has a better safety profile than bacterial enterotoxin, has also been demonstrated to have mucosal adjuvant properties.[26] These nontoxic mucosal adjuvant and mucosal carrier systems will contribute to the development of oral vaccines for human use. As is the case for parenteral adjuvants, we are still lacking a systematic and full understanding of the various immune effector mechanisms that each adjuvant can induce and how these should be combined with a particular antigen for the most effective protective response to control any particular infection.

A recent development with considerable potential for oral vaccine production and delivery is the generation of plants expressing antigenic proteins. Plant-based expression systems offer an oral delivery alternative with low production cost in which the antigen is encapsulated within the plant cell. Orally administered plant-expressed vaccines are currently being developed for a number of human and animal diseases, several of which have been tested in end-point species, including humans.[27,28] A major challenge of such an approach is the accumulation of a sufficient amount of vaccine antigens in plants so that a required dose can be consumed easily. Progress has been made toward the improvement of protein yield in plants, including optimization of codon usage, development of novel promoters and improvement of protein stability by targeting to specific intracellular compartments.[28] Technology for the stable transformation of chloroplast genomes is maturing and offers the promise of high-yield recombinant protein expression.[29] The next generation of viral vectors is also generating exciting results.[30] In addition, downstream processing technologies have advanced, and plant materials can be freeze-dried to increase the antigen dose on a per-gram basis. Issues still to be addressed before an orally delivered plant-based vaccine becomes possible include the uniformity and quality control of products, ecological risk assessment and public acceptance of genetically modified plants.

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