Lymphocyte compartments in antigen-sampling regions of rabbit mucosal lymphoid organs

Uptake and delivery of antigens to immunocompetent cells in the gut are critical factors for the development of oral vaccines. Particulate antigens are transported within minutes by M cells to intraepithelial lymphocytes and into the follicle dome. The dome contains B cells, CD4+ T cells, and macrophages, indicating that the cells involved in antigen presentation are located below the dome's epithelium. The high number of M cells in rabbits and the development of monoclonal antibodies against rabbit lymphocytes have enabled the detailed study of lymphocytes associated with M cells. The follicle epithelium of rabbit Peyer's patches contains B cells and a population of CD4-/CD8-, major histocompatibility complex class II+ mononuclear cells of unknown function. These cells are phenotypically distinct from T cells in follicle domes, in T cell-dependent areas, in villus epithelium, or in villus lamina propria. In addition, lymphocytes in M-cell pockets express an activation antigen (3B6) not found on CD4+ or CD8+ cells in T cell-dependent areas. These results indicate that M-cell pocket lymphocytes in follicle epithelium form a phenotypically distinct compartment situated at the interface between M-cell-driven antigen uptake and the mucosal immune system.