Abstract
Intestinal M (microfold or membranous) cells are an enigmatic lineage of intestinal epithelial cells that initiate mucosal immune responses through the uptake and transcytosis of luminal antigens. Due to their rarity, the mechanisms of M-cell function and differentiation are poorly understood. To overcome this problem, experimental strategies to enrich for M-cells have been established. Transcriptome analyses have provided valuable insight, especially on the receptors for antigen uptake, and such studies have broadened our knowledge of M-cell function. In another line of investigation, we and others have begun to dissect the molecular pathways of M-cell differentiation. Among them, receptor activator of NF-κB ligand (RANKL) has been identified as an essential factor for M-cell differentiation. We have focused on the M-cell inducible activity of RANKL and have been able to observe temporal transitions during M-cell differentiation by using in vivo ectopic M-cell differentiation induced by exogenous RANKL treatment. We have found that the ets-family transcription factor Spi-B is essential for functional maturation of M cells. In the absence of Spi-B, the immune response to Salmonella Typhimurium is severely impaired, suggesting that M cells are important for maintaining intestinal homeostasis.