Microfold cells (commonly abbreviated as M cells) are a distinct type of epithelial cell that line the follicle-associated epithelium (FAE) of mucosa-associated lymphoid tissues (MALT), such as Peyer’s patches, isolated lymphoid follicles, and the nasopharynx‑associated lymphoid tissue (NALT). Their primary role is to transport luminal antigens, microorganisms, and particulate matter from the mucosal surface to underlying immune cells, thereby facilitating the initiation of mucosal immune responses.
Morphology and Structure
- The apical surface of an M cell lacks the dense microvilli typical of absorptive enterocytes; instead, it exhibits a sparse, irregular “microfold” architecture that gives the cell its name.
- Beneath the apical membrane, M cells have a thin glycocalyx and are characterized by a reduced number of lysosomes compared with neighboring enterocytes.
- The basolateral side forms tight junctions with adjacent epithelial cells, maintaining barrier integrity while allowing transcytosis.
- Cytoskeletal rearrangements and a specialized endocytic machinery enable efficient uptake of particles ranging from bacteria to viruses and inert beads.
Location and Distribution
- In the small intestine, M cells are concentrated over the domes of Peyer’s patches and isolated lymphoid follicles.
- Similar cells are found in the tonsils, adenoids, and bronchial-associated lymphoid tissue, where they are referred to as follicle‑associated epithelium cells.
Physiological Function
- Antigen Sampling: M cells internalize luminal antigens via endocytosis, macropinocytosis, and receptor‑mediated uptake, then transport them across the epithelium to dendritic cells, macrophages, and B cells in the subepithelial dome.
- Immune Initiation: By delivering antigens to antigen‑presenting cells, M cells trigger the production of secretory IgA and the development of mucosal tolerance or protective immunity.
- Pathogen Entry: Certain pathogens (e.g., Salmonella, Shigella, Yersinia, and some viruses) exploit M cells as portals of entry, taking advantage of their transcytotic capacity.
Molecular Markers and Regulation
- Surface markers: Glycoprotein 2 (GP2) has been identified as a specific receptor for Salmonella on mouse M cells. Other markers include annexin V, Fc receptors, and the transcription factor Spi‑B, which is crucial for M‑cell differentiation.
- Differentiation cues: Signals from underlying stromal cells, lymphotoxin β receptor (LTβR) activation, and RANKL (receptor activator of nuclear factor κB ligand) are central to the development of M cells from intestinal stem cells.
Research and Clinical Relevance
- Vaccinology: Exploiting M‑cell targeting enhances oral vaccine delivery; formulations conjugated to GP2 ligands or other M‑cell–specific molecules increase antigen uptake and immunogenicity.
- Therapeutics: Modulating M‑cell differentiation (e.g., via RANKL agonists) is under investigation to boost mucosal immunity in immunocompromised patients.
- Disease associations: Dysregulation of M‑cell function may contribute to inflammatory bowel disease (IBD) pathogenesis by altering antigen sampling and immune activation.
Key Points
- Microfold cells are specialized epithelial cells in MALT that mediate transcytosis of antigens from the lumen to immune compartments.
- Their unique microfolded apical surface and distinct molecular profile differentiate them from surrounding enterocytes.
- They play a dual role in immune surveillance and as potential entry points for pathogens, making them a focal point for vaccine and therapeutic development.
