Teleost intestinal microfold cells: identification, characterization, and mucosal antigen sampling in olive flounder (Paralichthys olivaceus)

Mammalian microfold cells (MCs) are specialized epithelial cells (ECs) that initiate mucosal immune responses through the uptake and transcytosis of luminal antigens. However, such type of antigen-sampling cells remains largely unknown in teleost fish. In this study, the lectins Ulex europaeus agglutinin-1 (UEA-1) and wheat germ agglutinin (WGA), as well as the monoclonal antibody (mAb) NKM-16-2-4 raised against mouse MCs, were used to identify the MCs in the intestines of olive flounder (Paralichthys olivaceus). These cells exhibit binding characteristics with UEA-1 and mAb NKM-16-2-4, similar to mammalian MCs, and were negative for acid and alkaline phosphatase staining. In addition, unique NKM-16-2-4⁺/WGA⁺ or UEA-1⁺/WGA⁺ cells were also identified. The UEA-1⁺ (including UEA-1⁺/WGA⁺) cells of the anterior, mid, and posterior intestines could take up inactivated Vibrio anguillarum and fluorescent microspheres. These were transported across the epithelium to the underlying lamina propria. Consequently, we renamed these UEA-1⁺ cells as MCs. The MCs of flounder showed short and irregular microvilli, without a mammalian-type basal pocket-like structure, but macrophages and lymphocytes at their basal areas. Co-staining confirmed that the flounder MCs lacked MHC II expression, and that MHC II⁺, IgM⁺ B, and CD83⁺ dendritic cells were underneath or adjacent to the MCs. After oral administration, inactivated V. anguillarum were detected in the MHC II⁺ cells at 6 h. These results indicated that the MCs in teleost intestines act as entry points within the epithelial barrier for particulate antigens. They also provide data valuable for developing novel mucosal vaccines targeting the MCs in teleosts.