Abstract
Macrophages are part of the innate immunity and are key players for the maintenance of intestinal homeostasis. They belong to the group of mononuclear phagocytes, which exert bactericidal functions and help to clear apoptotic cells. Moreover, they play essential roles for the maintenance of epithelial integrity and tissue remodeling during wound healing processes and might be implicated in intestinal tumor development. Macrophages are antigen-presenting cells and secrete immune-modulatory factors, like chemokines and cytokines, which are necessary to activate other intestinal immune cells and therefore to shape immune responses in the gut. However, overwhelming activation or increased secretion of pro-inflammatory cytokines might also contribute to the pathogenesis of inflammatory bowel disease. Presently, intestinal macrophages are in the center of intense studies, which might help to develop new therapeutic strategies to counteract the development or treat already existing inflammatory diseases in the gut. In this review, we focus on the origin of intestinal macrophages and, based on current knowledge, discuss their role in the gut during homeostasis and inflammation, as well as during intestinal wound healing and tumor development.
Highlights
For a long time, the digestive tract was described to have an immense surface with 260–300 m2 .More recently, the surface area was re-calculated to be “only” 32 m2 [1]
We aimed to discuss the different roles of intestinal macrophages for the maintenance of intestinal homeostasis, and for the development of intestinal inflammation, their influence on intestinal wound healing and their role during intestinal tumorigenesis
Most of the macrophages present in the adult gut are constantly replenished by Ly6Chigh progenitor cells, which originally develop in the bone marrow
Summary
The digestive tract was described to have an immense surface with 260–300 m2. The gastrointestinal mucosa harbors the largest number of macrophages in the body [16] These intestinal macrophages play key roles for the maintenance of intestinal homeostasis, and during inflammation, wound healing, infection, and tumorigenesis. They are implicated in the development of IBD. A new concept of macrophage classification based on their function was proposed by Mosser and colleagues in 2008 According to this model, macrophages can be divided into three different groups based on their homeostatic activities: host defense, wound healing, and immune regulation [21]. Since specific macrophage populations might contribute to the development of IBD, they potentially serve as a therapeutic target, which will be addressed in this review
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