Common principles of macrophage biology in blood–tissue barriers

Blood–tissue barriers play crucial roles in specialized tissues such as the central nervous system (CNS), eye, testis, and placenta. Tissue-resident macrophages in these tissues are indispensable for maintaining tissue homeostasis and responding to pathological conditions. Recent advances in high-throughput and high-dimensional single-cell analysis techniques, coupled with fate-mapping tools, have revealed a remarkable diversity of tissue-resident macrophages at the blood–tissue barrier. However, while comprehensive expression profiling has revealed the heterogeneity of macrophages within individual tissues, the commonalities of macrophages across anatomically similar structures like blood–tissue barriers remain poorly understood. This review focuses on the diversity and functional specialization of macrophages in tissues with blood–tissue barriers, highlighting recent insights into their anatomical distribution, developmental origins, phenotypic characteristics, and roles in maintaining tissue homeostasis. These findings may deepen our understanding of macrophage adaptation mechanisms in tissues with blood–tissue barriers, potentially leading to improved therapies for related disorders. Furthermore, examining the similarities and differences of macrophages across tissues may elucidate the molecular underpinnings of tissue-specific adaptation mechanisms and functional specialization.