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Crohn's disease and ulcerative colitis represent two distinct forms of inflammatory bowel diseases (IBD). In this paper, we discuss how immunological mechanisms contribute to the pathogenesis of IBD. Intestinal homeostasis is sustained by various kinds of cells, such as epithelial cells, lymphocytes, antigen presenting cells, and other innate immune cells. We pay special attention to intestinal CD14+ macrophages. Intestinal macrophages play a central role in the regulation of immune responses against commensal bacteria. In the physiological condition, intestinal macrophages lack the expression of innate-immune receptor CD14 and do not produce proinflammatory cytokines. We identified a unique macrophage subset of IBD in the human intestine, which expressed both macrophage (CD14, CD33, CD68) and dendritic cell (DC) markers (CD205, CD209) and produced larger amounts of proinflammatory cytokines, such as interleukin (IL)-23 and tumor necrosis factor (TNF)-α. In addition, the CD14+ macrophages contributed to interferon (IFN)-γ production rather than IL-17 production by lamina propria mononuclear cells dependent on IL-23. We discuss herein this IL-23/IFN-γ-positive feedback loop in IBD patients. We also discuss IFN-γ and IL-17 production from mucosal T cells and natural killer (NK) cells. Here, we show our recent findings about the plasticity of T helper cells in colitis. Th17 cells express T-bet, and finally lose the expression of retinoic acid-related orphan receptor (ROR) γt, the master regulator of Th17 cells, and are differentiated 'alternative Th1 cells.' In addition to Th1 cells, mucosal NK cells are also important sources of IFN-γ. Some of our ideas may be provocative, but we hope this review paper will provide new and firm understanding of the pathogenesis of IBD. [ABSTRACT FROM AUTHOR]

Understanding of the pathophysiology of inflammatory bowel disease (IBD) is constantly evolving and, recently, a number of biologic agents that selectively target specific molecules or pathways to correct the imbalance of the gut immune system have been developed. Among them, an antibody to tumor necrosis factor (anti-TNF) is the first developed drug which has dramatically improved the management of patients with IBD. However, more than one-third of IBD patients do not respond to medications, and there is the problem of antibody formation. Therefore, enormous efforts have been made into the development of novel anti-cytokines and stem cell injection as an alternative to has been made. However, the efficacy and safety of stem cell treatment are under investigation. Some studies have reported very promising data; however, others have shown conflicting results. In addition, most trials involved a very small number of subjects and did not compare stem cell treatment with anti- TNF. The present paper reviews the function and therapeutic mechanism of stem cells for the treatment of IBD. [ABSTRACT FROM AUTHOR]