CommentaryOral vaccination in man using antigens in particles: current status
Introduction
Intuitively, oral vaccination using antigens in particles appears to have a sound scientific rationale based on the protection of an antigen from exposure to gastric acid, bile and pancreatic secretions. At the same time, advantage is taken of the inherent inclination of intestinal Peyer’s patch (PP) M cells to take up particulates as part of its duty as a sentinel in the triggering of mucosal immunity against enteric pathogens (reviewed in Krahenbuhl and Neutra, 2000). Yet much controversy exists in respect of how stable particles really need to be, of how long the antigen needs to be encapsulated in the gastrointestinal tract, and as to whether M cells really take up more than just a few particles in vivo. Importantly, due to the lack of particle data on human tissue there is difficulty in relating particle uptake data in mice to man. Even if we accept the assertion that mouse oral immune data with antigens in particles has predictive merit, we are still faced with difficulties since even in the majority of successful mouse oral studies to date, the doses of antigen are still too high and the immune outcomes too low. It is possible that adjuvants or even targeting ligands (reviewed in Clark et al., 2000) might address the latter, but to date, results using adjuvants in various formats with particles have been quite poor. PP targeting work to discover new human M cell ligands is at an early research stage but appears promising.
Section snippets
Particle uptake by the intestinal Peyer’s patches
Estimates of particle uptake ability and capacity by PP M cells have been plagued by lack of standardisation of methodology. These include experimental variations in the intestinal loop model used in different species, in the numbers of particles used, and in the calculations of binding to PP and of uptake into the draining lymphatics (reviewed in Delie, 1998). Particle absorption values as high as 30% were detected in the early to mid-1990s using mainly polystyrene polymer. Recent use of lower
Of mice and men
Despite numerous papers over the past 15 years suggesting that antigens in particles can give immune responses in mice when given orally, it should be pointed out that much of this data was achieved with model stable proteins such as ovalbumen or bovine serum albumen. These antigens are atypical and have limited predictive value. Studies with real antigens require complicated neutralising antibody or challenge assays in order to show that any immunity generated is functional. A review by
Adjuvants for oral vaccination
A common and very reproducible experiment is to give a solution of tetanus toxoid (TTx) with a potent mucosal adjuvant, cholera toxin (CT), to bicarbonate-pretreated mice by the oral route (Jackson et al., 1993). Invariably, antibody titres are seen to both agents, albeit at high doses of TTx and at toxic doses of CT. The responses to TTx given orally with CT do not compare well, however, with those seen to TTx solution by the s.c. route at 1/100 the dose of TTx. In man, when the urease antigen
Human M cell targeting
M cell targeting using a ligand–particle construct has many technical hurdles to overcome to prove the scientific concept even in mice. Firstly, demonstration that the mouse M cell specific lectin, UEA-1, can confer a genuine enhanced immune outcome from a targeted particle containing a fluorescently-tagged antigen would be helpful. Secondly, it needs to be shown if the immune response is directly related to the amount of particles seen in M cells from selected mice from the same immunisation
Commercialisation reality
In the event of a targeted vaccine particle working in man, a major technical advance would have been demonstrated, combining the best of pharmaceutical technology, molecular biology, M cell knowledge and immunology. However, this is not enough to carry a product to commercialisation. There are additional process steps required for a reproducible and safe formulation. Since the product would be multicomponent, containing at least one antigen and a targeting agent, the assays for quality control
Conclusions
Many hard lessons have been learned over the past 15 years by those working with non-living vaccines administered by the oral route, and especially with particle formulations. Overall, there is not much evidence that antigens in particles can induce adequate protective immune responses compared to antigens in solution, bearing in mind some of the pitfalls described. Recent confocal measurements in single cells suggest that particle uptake by M cells may be a less frequent event than previously
References (36)
- et al.
Novel oral drug delivery gateways for biotechnology products: polypeptides and vaccines
Pharm. Sci. Tech. Today
(1998) - et al.
Lectin-mediated mucosal delivery of drugs and nanoparticles
Adv. Drug Del. Rev.
(2000) - et al.
Development of a single shot subunit vaccine for HIV-1. 5. Programmable in vivo autoboost and long lasting neutralizing neutralising response
J. Pharm. Sci.
(1998) - et al.
Protection against Bordetella pertussis infection following parenteral or oral immunization with antigens entrapped in biodegradable particles: effect of formulation and route of immunization on induction of Th1 and Th2 cells
Vaccine
(2001) - et al.
PLG microparticles stabilised using enteric coating polymers as oral vaccine delivery systems
Vaccine
(1999) Evaluation of nano- and microparticle uptake by the gastrointestinal tract
Adv. Drug Del. Rev.
(1998)- et al.
M cells in the Peyer’s patches of the intestine
Int. Rev. Cytol.
(1996) - et al.
Expression of specific markers and particle transport in a new human intestinal M cell model
Biochem. Biophys. Res. Commun.
(2000) - et al.
Towards clinical testing of a single-administration tetanus vaccine based on PLA/PLGA microspheres
Vaccine
(2000) - et al.
Oral immunisation with urease and Escherichia coli heat-labile enterotoxin is safe and immunogenic in Helicobacter pylori-infected adults
Gastroenterology
(1999)