Peyer's patch dendritic cells sample antigens by extending dendrites through M cell-specific transcellular pores

Peyer's patches (PPs) of the small intestine are antigen sampling and inductive sites that help establish mucosal immunity. Luminal antigens are transported from the mucosal surface of PPs to the subepithelial dome (SED), through the specialized epithelial M cells of the follicle-associated epithelium. Among the SED resident dendritic cells (DCs), which are situated ideally for taking up these antigens, some express high levels of lysozyme (LysoDC) and have strong phagocytic activity. We investigated the mechanisms by which LysoDCs capture luminal antigens in vivo.

We performed 2-photon microscopy on explants of PPs from mice in which the enhanced green fluorescent protein gene was inserted into the lysozyme M locus (lys-EGFP mice), allowing fluorescence detection of LysoDC.

LysoDC extended dendrites through M-cell-specific transcellular pores to the gut lumen. The M-cell adhesion molecules junctional adhesion molecule-A and epithelial cell adhesion molecule were recruited to sites of transcellular migration. Transcellular dendrites scanned the M-cell apical surface and the gut luminal content; they were able to take pathogenic bacteria and inert particles in the lumen before retracting back to the SED.

We describe an antigen sampling mechanism that occurs in PPs and involves cooperation between M cells of the follicle-associated epithelium and DCs of the subepithelial dome. This process might be developed to target vaccines to the mucosa.