Heterogeneity and Ontogeny of Mouse Thymic Macrophages Reveal a Requirement for Csf1r-Expressing Myeloid Cells During Early T Cell Development

Thymic macrophages (TMs) maintain tissue homeostasis by clearing the large numbers of apoptotic cells generated during T cell development, but how TM heterogeneity relates to their developmental origin and role in thymocyte maturation remains incompletely understood. Using complementary flow-cytometric, single-cell transcriptomic, and genetic approaches, we resolved two major TM populations corresponding to TIMD4+ cortical and CX3CR1+ medullary/cortico-medullary macrophages. TIMD4+ VCAM1+ TMs displayed a prominent efferocytosis and apoptotic-cell-clearance program, whereas TIMD4- VCAM1+ TMs were enriched for antigen-presentation and interferon-response pathways. Fate mapping revealed unequal progenitor contributions to these populations, and CCR2 deficiency selectively reduced TIMD4- VCAM1+ TMs and thymic monocytes, supporting ongoing input from circulating precursors. Exploratory pseudotime analysis further identified a transcriptional continuum from Ly6c2+ Ccr2+ monocytes toward macrophage states. Using MaFIA fetal thymic organ cultures, AP20187-mediated depletion of Csf1r-expressing myeloid cells reduced CD4+CD8+ thymocyte differentiation and produced a coordinated accumulation of DN3 cells, loss of DN4 cells, and reduction in CD27 expression. These convergent changes identify the DN3-to-DN4 transition as a developmental stage that requires an intact Csf1r-expressing myeloid compartment, and establish a functional connection between the thymic myeloid niche and early {beta} T cell development. Together, our study refines the phenotypic and developmental organization of mouse TMs and reveals a previously under-appreciated requirement for myeloid-cell support during progression through the {beta}-selection checkpoint.