(9d) A Mechanistic Insight into Pollen Grains As an Oral Vaccine Delivery System

Mucosal surfaces are the major portal of entry for pathogens. While mucosal immune system acts as the first line of defense against these pathogens, the conventional parenteral vaccines are not effective at inducing adaptive mucosal immunity. To induce adaptive mucosal immunity, the vaccines must be delivered across the mucosal surfaces. We have recently developed pollen grains (PGs) as unique method to deliver antigens across the gastrointestinal mucosa. Chemical processing of PGs enables removals of native proteins and other biomolecules that may cause allergic reactions, thus rendering them safe for oral delivery. We have found that using ovalbumin as a model antigen, processed PGs can help induce both systemic and mucosal antibody response against ovalbumin.

In this study we wanted to better understand how processed PGs activate the innate immune system. Macrophage and dendritic cells (DCs) are the major immune cells that bridge innate and adaptive immunity. In this work mouse bone marrow derived dendritic cells (BMDCs) and macrophages were used to investigate how they respond in the presence of PGs. Particular focus was given to the specific pro-inflammatory cytokines (IL-1β, TNF-α, IL-6, IL-12 and IL-18) that macrophages and DCs produce in response to particles. Scanning electron microscopy (SEM) imaging revealed that macrophages attempt to phagocytose PGs, and PGs of different size and morphology interact uniquely with macrophages. Caco-2 cells were used to model the epithelium of the gastrointestinal tract to evaluate cytotoxicity of PGs. A concentration dependent effect of PGs on cytotoxicity was observed. Flow cytometry analysis showed an upregulation of MHC II in DCs that were cultured in the presence of PGs. Together these in vitro experiments suggest that processed PGs can activate macrophages and DCs, and thus provide critical information about the mechanism of the potential adjuvant property of PGs for oral vaccine delivery.