Abstract
To define the bottlenecks that restrict antigen expression after oral administration of viral-vectored vaccines, we tracked vectors derived from the human adenovirus type 5 at whole body, tissue, and cellular scales throughout the digestive tract in a murine model of oral delivery. After intragastric administration of vectors encoding firefly luciferase or a model antigen, detectable levels of transgene-encoded protein or mRNA were confined to the intestine, and restricted to delimited anatomical zones. Expression of luciferase in the form of multiple small bioluminescent foci in the distal ileum, cecum, and proximal colon suggested multiple crossing points. Many foci were unassociated with visible Peyer's patches, implying that transduced cells lay in proximity to villous rather than follicle-associated epithelium, as supported by detection of transgene-encoded antigen in villous epithelial cells. Transgene-encoded mRNA but not protein was readily detected in Peyer's patches, suggesting that post-transcriptional regulation of viral gene expression might limit expression of transgene-encoded antigen in this tissue. To characterize the pathways by which the vector crossed the intestinal epithelium and encountered sentinel cells, a fluorescent-labeled vector was administered to mice by the intragastric route or inoculated into ligated intestinal loops comprising a Peyer's patch. The vector adhered selectively to microfold cells in the follicle-associated epithelium, and, after translocation to the subepithelial dome region, was captured by phagocytes that expressed CD11c and lysozyme. In conclusion, although a large number of crossing events took place throughout the intestine within and without Peyer's patches, multiple firewalls prevented systemic dissemination of vector and suppressed production of transgene-encoded protein in Peyer's patches.
Highlights
Among the non-replicative viral vectors that have been explored as vaccine carriers, those derived from human adenovirus type 5 (HAdV-5) have proven effective in eliciting immune responses against transgene-encoded antigen (Shiver et al, 2002)
At the whole body level, transgene expression—whether evaluated by RT-qPCR or bioluminescence imaging—was not observed in extra-intestinal tissues, such as liver and spleen, these two organs are highly transduced after parenteral administration of HAdV-5-based vectors
Transgene expression—as visualized by bioluminescence imaging—took the form of multiple small bioluminescent foci found primarily in the distal small intestine, cecum and colon. This suggested that the vector crossed the epithelium at multiple sites in different zones of the intestine, whence it was able to transduce cells and produce transgeneencoded protein
Summary
Among the non-replicative viral vectors that have been explored as vaccine carriers, those derived from human adenovirus type 5 (HAdV-5) have proven effective in eliciting immune responses against transgene-encoded antigen (Shiver et al, 2002). Whereas this observation strictly concerns parenteral delivery routes, orally-delivered vaccines are sought, owing to their ease of delivery in humans and other species and their potential to elicit mucosal immunity. In this capacity, nonreplicative HAdV5-based vaccines have proven less effective. When different delivery routes have been compared in the same trial, higher doses have generally been required to afford protection by the oral route (Ertl, 2005)
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