Neonatal Microglia and Their Secretome as Mediators of Brain Repair

Microglia are essential regulators of myelin integrity and repair, yet their regenerative capacity declines with ageing and in neurodegenerative diseases such as multiple sclerosis (MS). Neonatal microglia retain a uniquely reparative program that may offer insight into restoring lost functions in the adult CNS. Here we show that transplantation of neonatal microglia ameliorates disability, reduces leukocyte infiltration, and promotes remyelination in both inflammatory (EAE) and non-inflammatory (cuprizone) models, and reverses cognitive decline in aged mice. These benefits persisted even when transplanted cells remained confined to the meninges and were reproduced by the neonatal microglia secretome, indicating a paracrine mechanism. Multi-omic profiling revealed that the neonatal secretome is enriched in trophic factors and membrane-building lipids compared to adult microglia, while transcriptomic analyses of treated aged brains showed reactivation of developmental repair pathways and suppression of inflammatory signatures. Together, these results demonstrate that neonatal microglia re-engage rejuvenation-like programs in the adult CNS and highlight the importance of multifactorial strategies, integrating trophic, metabolic, and immunomodulatory cues, over single-target approaches. Our findings establish early microglial programs as a paradigm for designing new regenerative therapies for CNS disorders.