Dendrimer Delivered shRNA Targeting the CCL20-CCR6 Axis Suppresses Complement-Mediated Microglial Synaptic Pruning and Ameliorates Chronic Neuroinflammation After Repetitive Traumatic Brain Injury

Repetitive traumatic brain injury (rTBI) induces persistent microglial activation and chronic neuroinflammation, yet the upstream signals driving long-term synaptic injury remain unclear. In this study, we identify the CCL20-CCR6 chemokine axis as a critical regulator of sustained microglial activation and complement-dependent synaptic loss after rTBI. Proteomic profiling at 30 days post-injury (dpi) showed broad normalization of complement-linked inflammatory and synaptic pathways in the cortex and hippocampus, underscoring a mechanistic link between chemokine signaling, microglial activation, and synaptic vulnerability. To therapeutically target this axis, we developed a dendrimer-based shRNA platform (shCombo-DPX) that simultaneously silences CCL20 and CCR6. Intranasal and intravenous delivery in rTBI mice effectively reduced CCL20-CCR6 expression, attenuated chronic microgliosis and astrogliosis, and suppressed complement activation. Treatment limited microglial synaptic engulfment, preserved synaptic proteins, restored BDNF levels, and improved motor, anxiety-related, and cognitive outcomes. In microglia-neuron coculture systems, CCL20 silencing reduced LPS-induced complement signaling and prevented synaptic loss, neuronal apoptosis, and BDNF depletion. Conversely, exposure to recombinant CCL20 induced dendritic degeneration, caspase-3 activation, microglial reactivity, complement dysregulation, and synaptic injury both in vitro and in vivo. Collectively, these findings establish CCL20-CCR6 as a key upstream driver of chronic complement-mediated synaptic degeneration after rTBI and support dendrimer-delivered shRNA therapy as a targeted strategy to mitigate long-term neurodegeneration.