Barrier Tissue Macrophages: Functional Adaptation to Environmental Challenges

Macrophages are found throughout the body, where they play crucial roles in tissue development, homeostasis and remodelling, as well as being sentinels of the innate immune system that can contribute to protective immunity and inflammation. Barrier tissues such as the intestine, lung, skin and liver are exposed constantly to the outside world, placing special demands on resident cell populations such as macrophages. Here we review the mounting evidence that although macrophages in different barrier tissues may be derived from distinct progenitors, their highly specific properties are shaped by the local
environment, allowing them to adapt precisely to the needs of their anatomical niche. We discuss the properties of macrophages in steady state barrier tissues, outline the factors that shape their differentiation and behaviour and describe how macrophages change during protective immunity and inflammation.