Allergen-free pollen shell as a novel biomaterial for oral vaccine delivery: Development, characterization, in vivo efficacy, and mechanistic insight

Oral vaccination is painless, easy to administer, and can elicit immunity in the gastrointestinal tract where many microbes invade the body. The success of delivering vaccines via oral route critically relies upon the ability to protect the vaccines from proteolytic degradation in the stomach and to overcome inherent immunological tolerance exist in the gut-associated lymphoid tissue. Meeting these challenges is still elusive and oral vaccination thus remains an unmet goal. To face oral vaccination challenges, allergen-free pollen shell was developed as a novel microcapsule to deliver vaccines orally. Pollen shells were extracted from natural pollen grains via a facile chemical treatment method to remove naturally existing lipids, proteins, nucleic acids, and polysaccharides. Structurally intact, hollow, porous, and allergen-free pollen shells along with their pristine ultrastructure were obtained from multiple species of pollen. An array of comprehensive characterization techniques including scanning electron microscopy, transmission electron microscopy, elemental analysis, gel electrophoresis, laser scanning confocal microscopy, Fourier transform infrared spectroscopy, and thermogravimetric analysis were used to study the role of the chemical treatment and to benchmark pollen treatment method. Together these analyses showed that the treated pollen shells are lipid and protein free and are enriched with carboxylic acid and hydroxyl functional groups.

Using a model protein ovalbumin (OVA) this study showed that pollen shells can carry vaccine in both encapsulated and adsorbed form. Oral immunization with pollen shell formulation stimulated significantly higher anti-OVA antibody response in mice serum and bone marrow compared to OVA formulated without pollen shells. Pollen shells also induced higher mucosal anti-OVA IgA response in local and distal mucosal compartments. Immune responses remained high after 17 months post-immunization. No significant allergic response to pollen protein or OVA was observed throughout the study. Pollens with echinate surfaces (i.e., ragweed and sunflower) induced significantly higher immune response than pollens with smooth surfaces (black alder and lamb’s quarters). Overall, this study confirms the effectiveness of pollen shell-based oral vaccine delivery.

To follow the underlying immune activation mechanism, the interaction between pollen shells and various innate immune cells were studied using allergen-free ragweed pollen. Ragweed pollen shells induced human colon epithelial cells (Caco-2), mouse bone-marrow-derived dendritic cells (BMDCs), and macrophages to release proinflammatory cytokines and chemokines including IL-6, IL-8, MCP-1, TNF-α, and IL-1β. Ragweed shells also activated BMDCs (high CD40) and upregulated BMDC maturation markers (CD80, CD86, and MHC-II). These results suggest a dual role of pollen shell both as a vaccine carrier and as a mucosal adjuvant.

This dissertation won 1st Place in the Texas Tech University Outstanding Thesis and Dissertation Award, Mathematics, Physical Sciences & Engineering, 2020.

Embargo status: Restricted until January 2024. To request an access exception from the author, click on the PDF link to the left.