Single-cell transcriptomic profiling reveals sex-specific microglial responses to simulated galactic cosmic radiation without classical inflammatory activation

Long‑duration deep space missions expose astronauts to galactic cosmic radiation (GCR), a complex mixture of high‑ and low‑LET ions that can compromise central nervous system (CNS) health. Microglia, the resident immune cells of the brain, are highly sensitive to radiation, yet their cell‑state adaptations to space‑relevant mixed‑field exposure remain poorly defined. Here, we performed single‑cell RNA sequencing (scRNA‑seq) of hippocampal microglia isolated from male and female C57BL/6J mice 53 days after acute 100 cGy 5‑ion GCR simulation. Behavioral testing showed no overt deficits, but scRNA‑seq revealed subtle, sex‑specific transcriptional remodeling. Male microglia engaged a stress‑adaptive program characterized by upregulation of Fkbp5, Rtp4, Creb5, and metabolic regulators, with concurrent downregulation of ER chaperone and ribosomal genes. Female microglia displayed a transcriptionally blunted phenotype with selective induction of Fkbp5, Ccnd3, and Tnfaip3, and broad downregulation of inflammatory and NOD/TNF pathways. Canonical inflammatory or disease‑associated microglia signatures were absent and homeostatic markers (P2ry12, Cx3cr1) were preserved. These findings define a non‑inflammatory, sex‑specific microglial response to space‑relevant GCR and provide a cellular framework for sex‑informed CNS risk assessment and countermeasure development for deep space travel.