Role of Monocytes and Intestinal Macrophages in Crohnʼs Disease and Ulcerative Colitis

Monocytes and macrophages are part of the body's first line of defence, eliminating pathogens by phagocytosis or by releasing a broad array of inflammatory mediators, such as cytokines, chemokines, and proteases. In humans, 3 subsets of monocytes are described in blood with seemingly different functions, the classical (CD14++CD16−) monocytes, the intermediate (CD14++CD16+) monocytes, and the nonclassical (CD14+CD16++) monocytes. In the intestine, macrophages can be divided into resident and inflammatory macrophages that are distinguished by low and high expression of CD14, respectively. However, the roles and function of the 3 monocyte subsets in health and disease are not fully understood. In this review, we describe what is known about the origin of human intestinal macrophages and their blood monocytic counterparts and many of their numerous distinct mechanisms influencing the intestinal immune system.