Macrophage-stem cell crosstalk: regulation of the stem cell niche.

The cells of the innate immune system are the sentinels of tissue homeostasis, acting as 'first responders' to cellular damage and infection. Although the complex interplay of different immune cells during the initial inflammatory phases of infection and repair has been documented over many decades, recent studies have begun to define a more direct role for specific immune cells in the modulation of tissue repair. One particular cell of the innate immune system, the macrophage, has emerged as a central integrator of the complex molecular processes that drive tissue repair and, in some cases, the development of specific cell types. Although macrophages display directed orchestration of stem cell activities, bidirectional cellular crosstalk mechanisms allow stem cells to regulate macrophage behaviour within their niche, thus increasing the complexity of niche regulation and control. In this Review, we characterize the roles of macrophage subtypes in individual regenerative and developmental processes and illustrate the surprisingly direct role for immune cells in coordinating stem cell formation and activation.