Paneth cells: leukocyte-like mediators of innate immunity in the intestine

Paneth cells are secretory intestinal epithelial cells located at the base of the crypts of Lieberkühn in the small intestine. They display prominent cytoplasmic granules, containing antibacterial proteins such as lysozyme, secretory phospholipase A2 type IIA, and alpha-defensins, which are released into the intestinal lumen in response to a range of stimuli. In this, they resemble circulating leukocytes, which also elaborate and secrete lysozyme and alpha-defensins as part of an antibacterial defense function, and the resemblance is sustained at other levels. The cells also strongly and specifically express the NOD2 gene product, one of an emerging family of critical, intracellular mediators of innate immune responses, which is also highly expressed in peripheral blood mononuclear cells, and they express RNA for tumor necrosis factor alpha, a major myelomonocytic cell-derived cytokine, which has a crucial role in the pathogenesis of diseases such as rheumatoid arthritis and Crohn's disease (CD). Thus, these cells, which are derived from the pluripotent intestinal epithelial stem-cell lineage, are sessile, resident host-defense cells, which may share with leukocytes the beneficial function of secreting antimicrobial peptides, as well as the potentially harmful capacity for promoting inflammation and tissue damage. Paneth cells are most abundant in the distal small intestine, which is the region most frequently affected by CD, and there is great interest in the potential role of these cells in this condition. This brief review summarizes current knowledge and speculates on how the study of these fascinating cells might be advanced.