Hematopoietic stem cell requirement for macrophage regeneration is tissue-specific

Tissue-resident macrophages (TRM{Phi}) are important immune sentinels responsible for maintaining tissue and immune homeostasis within their specific niche. Recently, the origins of TRM{Phi} have undergone intense scrutiny where now most TRM{Phi} are thought to originate early during embryonic development independent of hematopoietic stem cells (HSCs). We previously characterized two distinct subsets of mouse peritoneal cavity macrophages (Large and Small Peritoneal Macrophages; LPM and SPM, respectively) whose origins and relationship to both fetal and adult long-term (LT)-HSCs have not been fully investigated. Here we employ highly purified LT-HSC transplantation and in vivo lineage tracing to show a dual ontogeny for LPM and SPM, where the initial wave of peritoneal macrophages is seeded from yolk sac-derived precursors, which later require LT-HSCs for regeneration. In contrast, transplanted fetal and adult LT-HSCs are not able to regenerate brain-resident microglia. Thus, we demonstrate that LT-HSCs retain the potential to develop into TRM{Phi}, but their requirement is tissue-specific.