Border-Associated Macrophages in CNS Health and Disease: A Comprehensive Review of Ontogeny, Heterogeneity, and Functional Plasticity at Neural Interfaces.

Border-associated macrophages (BAMs) represent a specialized population of tissue-resident immune cells strategically positioned at the critical interfaces between the central nervous system (CNS) and peripheral circulation, including the meninges, choroid plexus, and perivascular spaces. As frontline sentinels of the neuroimmune system, BAMs perform essential functions in immune surveillance, barrier integrity maintenance, and homeostatic regulation, yet their unique biology and disease-associated roles remain incompletely characterized compared to parenchymal microglia. This review aims to synthesize current knowledge on BAM ontogenetic origins, compartment-specific heterogeneity, transcriptional programs, and functional outputs in both health and neurological disorders. We conducted a comprehensive literature analysis integrating findings from lineage tracing studies, single-cell RNA sequencing, spatial transcriptomics, and functional interrogation in animal models of disease. The results reveal that BAMs exhibit remarkable cellular diversity shaped by distinct ontogenetic origins-primarily yolk sac-derived erythro-myeloid progenitors with variable contributions from fetal liver and postnatal monocytes depending on anatomical compartment. Compartment-specific marker combinations (CD206, LYVE1, CD163, MHCII) define functionally distinct subsets, and core transcriptional regulators including PU.1 and IRF8 maintain BAM identity while CSF-1/IL-34-CSF1R signaling governs survival and renewal. In neurological disorders including ischemic stroke, Alzheimer's disease, multiple sclerosis, and brain tumors, BAMs display pronounced double-edged roles, transitioning from protective homeostatic guardians to pathogenic drivers depending on disease stage and microenvironmental context. This comprehensive analysis establishes a unified framework for understanding BAM biology and identifies critical opportunities for developing subset-specific therapeutic strategies targeting these interface macrophages in neurological diseases.