Border-associated macrophages: From physiology to therapeutic targets in Alzheimer's disease

Border-associated macrophages (BAMs) constitute a highly heterogeneous group of central nervous system-resident macrophages at the brain boundaries. Despite their significance, BAMs have mainly been overlooked compared to microglia, resulting in a limited understanding of their functions. However, recent advancements in single-cell immunophenotyping and transcriptomic analyses of BAMs have revealed a previously unrecognized complexity in these cells, in addition to their critical roles under non-pathological conditions and diseases like Alzheimer's disease (AD), Parkinson's disease, glioma, and ischemic stroke. In this review, we discuss the origins, self-renewal capabilities, and extensive heterogeneity of BAMs, and clarify their important physiological functions such as immune monitoring, waste removal and vascular permeability regulation. We also summarize experimental evidence linking BAMs to the progression of AD. Finally, we review therapeutic strategies targeting brain innate immune cells mainly focusing on strategies aimed at modulating BAMs to treat AD and evaluate their potential in clinical applications.