Potential role of NK cells in the pathogenesis of inflammatory bowel disease.

NK cells are a major component of the innate immune system and play an important role in the tissue inflammation associated with autoimmune diseases such as inflammatory bowel disease (IBD). NK cells are unique in bearing both stimulatory and inhibitory receptors specific for MHC class I molecules, and their function is regulated by a series of inhibiting or activating signals. The delicate balance between activation and inhibition that decides NK cell final action provides an opportunity for their possible modulatory effect on specific therapeutic settings. Intestinal NK cells are phenotypically distinct from their counterparts in the blood and resemble "helper" NK cells, which have potentially important functions both in promoting antipathogen responses and in the maintenance of intestinal epithelial homeostasis. NK cell activities have been found to be significantly below normal levels in both remissive and active stages of IBD patients. However, some proinflammatory cytokines (e.g., IL-15, IL-21, and IL-23) could potently induce NK cell activation to secret high levels of proinflammatory cytokines (e.g., IFN-γ and TNF) and promote the cytolytic activities against the target cells. This paper provides the characteristics of intestinal NK cells and their potential role in the pathogenesis of IBD.