Teleost Glycoprotein 2: A Marker for Intestinal Microfold Cell Subset and a Transcytotic Receptor for the Antigen Uptaking Through a Mannose-Dependent Manner in Flounder (Paralichthys olivaceus).

Mammalian microfold (M) cells are specialized epithelial cells that sample luminal antigens, and glycoprotein 2 (GP2) is a marker of mature M cells and functions as a bacterial receptor. However, its presence in teleosts M cells remains unknown. Previously, we identified two M cell subsets, UEA-1+WGA- and UEA-1+WGA+, in intestinal epithelium of teleost flounder (Paralichthys olivaceus). Here, we investigated whether GP2 is present and functional in these M cells. Using fluorescence in situ hybridization combined with lectin histochemistry, gp2 mRNA was detected in a subset of intestinal UEA-1+ M cells. With the developed mouse anti-GP2 antibodies, we identified two M cell subsets: GP2+UEA-1+WGA- and GP2-UEA-1+WGA+. Flow cytometry showed the highest proportion of GP2+UEA-1+ M cells in foregut, followed by hindgut and midgut, consistent with gp2 expression. Functionally, GP2+UEA-1+ M cells captured microspheres and bacterial antigens more rapidly and efficiently than GP2-UEA-1+ M cells, preferentially taking up Vibrio anguillarum over Lactococcus lactis. Anti-GP2 antibody preincubation of the intestinal epithelium in vivo and the isolated intestinal cells in vitro, as well as mannose pretreatment of V. anguillarum, inhibited V. anguillarum invasion. In GP2-transfected HEK293T cells, mannose-dependent GP2-V. anguillarum binding and internalization, and GP2- and mannose/fucose-independent L. lactis uptake, were further demonstrated. These findings for the first time identified teleost GP2 as a marker of a M cell subset and as a mannose-dependent transcytotic receptor, providing a new target for M cell-directed mucosal vaccines in teleosts .