Clusterin in human gut-associated lymphoid tissue, tonsils, and adenoids: localization to M cells and follicular dendritic cells.

The follicle-associated epithelium (FAE) overlying the follicles of mucosa-associated lymphoid tissue is a key player in the initiation of mucosal immune responses. We recently reported strong clusterin expression in the FAE of murine Peyer's patches. In this study, we examined the expression of clusterin in the human gut-associated lymphoid tissue (GALT) and Waldeyer's ring. Immunohistochemistry for clusterin in human Peyer's patches, appendix and colon lymphoid follicles revealed expression in M cells and in follicular dendritic cells (FDCs). Using cryo-immunogold electron microscopy in Peyer's patches, we observed cytosolic immunoreactivity in M cells and labeling in the ER/Golgi biosynthetic pathway in FDCs. In palatine tonsils and adenoids, we demonstrated clusterin expression in germinal centers and in the lymphoepithelium in the crypts where M cells are localized. In conclusion, clusterin is expressed in M cells and follicular dendritic cells at inductive sites of human mucosa-associated lymphoid tissue suggesting a role for this protein in innate immune responses. Moreover, the use of clusterin as a human M cell marker could prove to be a valuable tool in future M cell research.