(603c) Pollen Grains - a Novel Biomaterial for Oral Vaccination

BACKGROUND: Oral vaccination is one of the ‘holy grails’ in the field of vaccine delivery. We have recently shown that pollen grains, the bane of allergies for some, can be repurposed and engineered to function as microcapsules and microcontainers for transport of vaccines across the harsh environment of the stomach. The unique feature of pollens is that their shell is made up of a resistant material that can withstand the acidic conditions of the stomach without degradation. Here we studied different aspects of pollens as a novel biomaterial for oral vaccination.

METHODS: Ragweed pollen were chemically treated to strip them off their cellular material and biomolecules. Pollens were then characterized using elemental analysis, SDS-PAGE, and scanning electron microscopy (SEM). Biocompatibility and safety was evaluated on Caco-2 cell monolayers, which served as a model intestinal epithelial cell line. Immunomodulatory properties of ragweed pollens were evaluated by culturing bone-marrow derived macrophage and dendritic cells in the presence of ragweed pollens and quantifying secreted pro-inflammatory cytokines and cell surface expression markers. The ability of ragweed pollens to generate systemic and mucosal antibody responses was evaluated in mice by orally vaccinating Balb/c mice with ovalbumin as a model antigen formulated with ragweed pollens.

RESULTS: SEM micrographs showed that chemical treatment of pollens helped to produce pollens that were intact and clean. Nitrogen analysis coupled with SDS-PAGE further confirmed that no protein was detectable in the pollens after treatment. Biocompatibility and safety studies on Caco-2 cells showed that the electrical resistance across the epithelial cells did not reduce upon addition of different concentrations of pollens. Furthermore, as compared to poly-lactic-co-glycolic acid particles, no significant increase in cytotoxicity was observed. Macrophages and dendritic cells released pro-inflammatory cytokines and also upregulated surface markers of activation. This shows that pollen grains have an immunomodulatory property. Indeed, when pollens were used in vivo for oral vaccination, a significant increase in systemic IgG and mucosal IgA antibodies against ovalbumin (model antigen) were seen as compared to ovalbumin alone cohorts. Examination of IgE antibody in the mouse serum against ragweed extract did not show any increase indicating that ragweed pollens, which were stripped and cleaned off their natural ragweed allergens, do not induce an allergic response. Lastly, ex vivo examination of mouse intestinal tissue via confocal microscopy showed that ragweed pollens can penetrate the intestinal epithelial barrier and reach the sub-mucosal space.

CONCLUSIONS: The data of this study provides proof-of-concept that pollen grains can be repurposed for use as a natural biomaterial for oral vaccination. This data also sheds light on the immunomodulatory property of pollens and offers insight into the potential mechanism of how pollen grains help to generate an immune response. The data thus supports future development of pollens for oral vaccine delivery.