Establishment and maintenance of the macrophage niche

Self-maintaining resident macrophages populate all mammalian organs. In addition to their role as immune sentinels, macrophages perform day-to-day functions essential to tissue homeostasis. The homeostatic functions of macrophages are regulated by so-called tissular "niches'' that control the size of the macrophage population and imprint their tissue-specific identity. Here, we review the mechanisms underlying self-maintenance of distinct macrophage populations and outline the organizing principles of the macrophage niche. We examine recent studies that uncovered mutually beneficial cell-cell circuits established between macrophages and their niche and propose a modular view of tissues that integrates the resident macrophage as an essential component of each individual module. Manipulating macrophage niche cells to control the function of resident macrophages in vivo might have therapeutic value in various disease settings