Astragalus polysaccharide RAP enters Peyer’s patches to initiate immune response through M cell transcytosis via TLR2/4 mediated pathway

This project will explore how the Astragalus polysaccharide RAP enters Peyer’s patches via M cells to initiate immune cell responses.

Astragali Radix (黃芪), the mostly used Qi tonic in Chinese Medicine, has been popularly prescribed to enhance the immune functions of cancer patients in China. The orally dosed Astragalus polysaccharide RAP has proved to have tumor suppressive effects, which are associated with its immunomodulatory activities. While the effects have been verified, little is known about the underlying mechanisms.

Peyer’s patches, the main immune organ in the small intestine, might be the major target organ when RAP initiates the immune responses after administered orally. We have found that RAP enters the small intestine immediately after gavage, and is digested three hours later when it arrives in the large intestine. Early in the first hour, RAP has already induced the MAPK/NFkB signaling pathways in Peyer’s patches. These pathways involve several groups of immune cells such as T cells, dendritic cells, and macrophages. And RAP can increase the ratio of cytotoxic T cells and induce IL-6 production in Peyer’s patches. As we observed in vitro, the IL-6 production could be attributed to RAP’s inducing effects on macrophages and dendritic cells. All these evidences focus on Peyer’s patches; however, how RAP enters Peyer’s patches remains unknown.

On the surface of Peyer’s patches, uptake by M cells is the main transport route for macromolecules/antigens to enter the host because the transcytosis occurs quickly, within 10 minutes. Our preliminary data obtained from the in vitro M cell model indicates that M cells could transport RAP to cross the epithelial cell monolayer and induce the IL-6 production of RAW 264.7 cells. Therefore, we hypothesize that transcytosis through M cells is the route by which RAP enters Peyer’s patches. As both TLR2 and TLR4 are found to be RAP’s receptors, we further hypothesize that TLR2/4 on the surface of M cells mediates the transcytosis.

To test this hypothesis, this project aims: 1) to confirm the uptake of RAP by M cells, especially by using immune staining and deconvolution microscopy; and 2) to determine the molecular mechanism by which this occurs by using TLR2-/-, TLR4-/- mice, and TLR2/4 inhibitors.

The success of this project will help explain the mechanism by which one herb-derived polysaccharide initiates physiological responses. In a larger context, it will contribute to greater understanding of how and why Chinese medicinal decoctions, of which polysaccharides are the major components, are so effective.