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Date of Award

4-2002

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Medical Sciences

Supervisor

Dr. Mary H. Perdue

Abstract

Enhanced transepithelial antigen transport in intestine of sensitized rats was previously demonstrated using horseradish peroxidase (HRP) as a model antigen. In senitized rats, transepithelial antigen transport was rapid (< 2 min) and of greater magnitude (three-fold) compared to controls. In my project, the essential components involved in the enhanced antigen transcytosis were investigated using mice with genetic deletions for IL-4 and CD23/FcεII. Increased transepithelial antigen transport was demonstrated in actively sensitized IL-4⁺/⁺ mice, but not IL-4⁻/⁻ mice, in which the phenomenon paralleled the expression of CD23 protein on enterocytes. Passively sensitized mice (both IL-4⁺/⁺ and IL-4⁻/⁻ mice) displayed greater antigen transport after transfer of immune serum unless the serum was first depleted of IgE or IL-4. IL-4 added to cultures of epithelial cells upregulated expression of CD23 mRNA. Finally, this augmented antigen uptake system was inhibited by luminal anti-CD23 and was absent in sensitized CD23⁻/⁻ mice. RT-PCR showed that cultured cells expressed only the b isoform of CD23. Sequencing revealed classical and alternative CD23b transcripts lacking exon 5 (b∆5) or 6 (b∆6), in which all forms were translated into functional IgE receptors. Endocytosis of the b∆5 and b∆6, but not the classical CD23b protein, was observed after binding with saturating anti-CD23, suggesting continuous endocytosis of the alternative forms that agrees with their intracellular localization at steady state. Classical CD23b proteins were expressed on the cell surface and only endocytosed upon antigen-induced IgE cross-linking of the receptor. Taken together, the results demonstrated that IgE/CD23 mediates enhanced transepithelial transport of antigen in sensitized mouse intestine, and that IL-4 plays a major regulatory role. We identified the expression of classical and alternative CD23b transcripts in intestinal epithelial cells, and demonstrated taht the translated proteins display distinct endocytic functions. We concluded that antigen binding to epithelial CD23/IgE facilitates its entry into the body resulting in intestinal anaphylaxis.

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