Bioactivities of Chitosan and Its Derivatives - Chitosan Based Hydrogels: Functions and Applications - page 109

to transport the vaccine to the dome of PPs [2-7]. After transport of the microparticles to
the dome of PPs, the microparticles are degraded and the vaccine is released into the lym-
phoid tissue. Following stimulation by an antigen in PPs and its presentation to B- and
T-cells, the antigen induces B- and T-cell proliferation and these cells subsequently leave
PPs via efferent lymphatics and reach the systemic circulation through the thoracic duct.
The uptake efficiency by PPs is mainly dependent on the size of microparticles: particles
larger than 10 μm are not internalized by M-cells [8].
In the nasal-associated lymphoid tissue, morphologically and functionally similar cells
have been described [9]. Although vaccine degradation is not as drastic as in the gut, for
this route also a carrier system is needed [10]. Since the half-time of clearance in the human
nasal cavity is only about 15 min and protein-like agents are hardly transported over the
epithelial barrier, vaccine encapsulation in microparticles may enhance the uptake by
M-cells or coadministering the antigen with an absorption enhancer may increase the
induction of immune responses after nasal application.
3.1.1.1 Vaccine Encapsulation Matrix
Chitosan, the deacetylated form of chitin, is able to open the tight junctions and in this way
allows paracellular transport across the epithelium. Both nasal and oral drug delivery
research has demonstrated that significantly higher amounts of macromolecular drugs can
be transported after coadministration with chitosan. Besides its ability to facilitate paracel-
lular transport, chitosan can also be used to prepare microparticles or nanoparticles. In
contrast to chitosan formulations, which are able to open the tight junctions, particulate
vaccine delivery systems are taken up by M-cells and subsequently biodegraded. Only then
the lymphoid tissue will be targeted efficiently. Particulate systems for macromolecular and
hydrophilic drug delivery need to be smaller than 200 nm to be taken up by epithelial cells
or to release the drug upon arrival at the mucosae [11].
Molecular weight (MW) and degree of acetylation determine the properties of chitosan.
Because of its biocompatibility, biodegradability, low cost, and ability to open intercellular
tight junctions, this polymer is a valuable excipient for oral drug delivery systems. Recently,
numerous studies on chitosan as a drug absorption enhancer have been published.
Chitosan formulations are used for ocular, oral, parenteral, and nasal delivery. Furthermore,
chitosan can easily form microparticles. Advances in microparticulate drug delivery
researches have opened up the way to apply these techniques in oral vaccination. Due to
the high protein-binding properties of some types of chitosan microparticles, they are also
potential candidates for oral delivery of antigens [12]. Mild preparation can protect the
proteins when they are incorporated during preparation of the microparticles [12]. In
order to circumvent protein denaturation conditions, chitosan microparticles can be loaded
passively. Such a mild loading procedure has been described by Jameela et al. [13].
Chitosan microparticles, 4.3 ± 07 μm in size and positively charged (20 ± 1 mV), have a
suitable size and zeta potential to be taken up by the M-cells of PPs. These microparticles
show a high loading capacity and loading efficacy for the model antigen ovalbumin.
About 90% of the ovalbumin remained in the microparticles after release studies for 4 h in
phosphate-buffered saline (PBS). Because the chitosan microparticles are biodegradable,
this entrapped ovalbumin will be released after intracellular digestion in PPs. Initial stud-
ies in vivo demonstrated that fluorescently labeled chitosan microparticles can be taken up
by the epithelium of murine PPs. Since uptake by PPs is an essential step in oral vaccina-
tion, these results show that the presently developed porous chitosan microparticles are a
very promising vaccine delivery system [14].