Targeting microglial MyD88 reprograms injury-activated microglia and promotes spinal cord repair via TGF-β signaling and biomimetic nanodelivery.

BACKGROUND: Following spinal cord injury (SCI), activated microglia sustain neuroinflammation and drive secondary tissue damage, and this limits functional recovery. Myeloid differentiation primary response 88 (MyD88) is a central adaptor of innate immune signaling, but whether and how microglial MyD88 regulates state transition after SCI remains unclear. This study aims to explore the impact of microglial MyD88 signaling on microglial state trajectories and tissue repair after SCI and to develop a microglia-targeted delivery strategy for therapeutic modulation. METHODS: A forceps-mediated SCI mouse model was established, and bulk and single-cell RNA sequencing were used to profile the temporal activation of microglial MyD88 signaling in the spinal cord. Pharmacological MyD88 inhibition (ST2825) and inducible microglia-specific MyD88 conditional knockout mice (Tmem119 CreERT2;Myd88 fl/fl) were used to assess neuroinflammation, tissue remodeling and downstream signaling. Functional recovery was evaluated by behavioral testing, bidirectional axonal tracing and electrophysiology. For translational validation, microglia membrane-coated, peptide-modified biomimetic nanoparticles (ST2825·DSPE@MG) were engineered to enhance microglia targeting and therapeutic efficacy. RESULTS: Transcriptomic analyses revealed rapid and sustained activation of MyD88 signaling preferentially in microglia after SCI. ST2825 suppressed pro-inflammatory outputs in vitro and in vivo while preserving trophic mediators. Microglia-specific MyD88 deletion reprogrammed injury-activated microglia from a pro-inflammatory state to a repair-associated phenotype, reducing neuronal damage, preserving axons and improving locomotor recovery. TGF-β receptor blockade with LY2109761 abolished the protective effects of MyD88 deficiency. Additionally, ST2825·DSPE@MG nanoparticles exhibited microglia-targeted uptake and conferred superior therapeutic efficacy. CONCLUSION: Our data establish MyD88 as a critical regulator of microglial reprogramming after SCI and highlight its potential as a therapeutic target for spinal cord repair.