Microglial state transitions dominate the brain immune response in the subacute phase after cardiac arrest

Cardiac arrest (CA) is a life-threatening medical emergency, and most victims are elderly. Despite advances in resuscitation, post-CA morbidity and mortality remain high, and this is thought to result largely from brain injury in which neuroinflammation plays a key role. Yet, how individual immune cell populations contribute to the immune response in the post-CA brain is still poorly understood. Here, using single-cell RNA-sequencing (scRNA-seq), we revealed the first immune landscape of the young and aged brain on day 3 after CA. Our data demonstrate that transitions in microglial states constituted a dominant immune change in the post-CA brain. We identified 5 CA-associated microglial states that included 3 major clusters defined by pro-inflammatory signatures, a proliferative phenotype, and high Spp1 expression. These 3 states exhibited age-dependent differences: the inflammatory subset was markedly expanded in aged mice, whereas the proliferative and Spp1⁺ clusters were more prominent in young mice. Such divergent responses likely underpin age-related disparities in neurologic outcome after CA. Notably, Spp1⁺ microglia displayed unique spatiotemporal dynamics. These cells were robustly induced by CA, and were enriched in selective brain regions including the basal ganglia where they were associated with a microinfarct-like injury pattern. Lastly, we found that microglial depletion worsened functional outcome after CA, suggesting an overall protective role for microglia. Together, these findings provide novel insights into microglial heterogeneity and age-dependent immune responses in the brain after CA, and highlight the central role of microglia in shaping post-CA recovery.