Nanoparticles loaded with a CSF1R antagonist selectively depletes microglial cells and modulates inflammation in spinal cord injury.

Neuroinflammation is a principal event occurring after spinal cord injury (SCI). M1-like microglia are key players in the inflammatory response after injury. We hypothesize that the depletion of this microglia subtype would shift the M2/M1-like microglia balance toward a more pro-resolutive environment, favorable to SCI repair. The colony-stimulating factor 1 receptor (CSF1R) antagonist PLX5622 has been used to deplete microglia in the central nervous system. Although PLX5622 can freely cross the blood-brain barrier after systemic administration, it requires solubilization in DMSO, an organic solvent toxic for the central nervous system, while the low drug concentration that accumulates at the SCI hampers its effectiveness. Systemic administration of PLX5622 can induce side effects due to off-target accumulation. In this study, for the first time, we specifically depleted M1-like microglia by designing polymeric nanoparticles loaded with PLX5622 (PLX NPs) to locally treat spinal cord contusion. PLX NP was prepared using a microfluidic-assisted approach showing high encapsulation efficiency (approx. 84 %), nanosized dimensions (100 nm), and batch-to-batch reproducibility. PLX NP displayed selective activity in depleting M1-like microglial cells in both resting and lipopolysaccharide (LPS)-activated mixed microglial cell models while preserving non-targeted glial cells. Furthermore, locally administered PLX NP downregulated proinflammatory cytokines (e.g., TNF-α, IL-6, and IL-1β), increasing the M2/M1-like microglia ratio, thus reducing inflammation in a SCI contusion model. Our data support the hypothesis that local treatment with PLX NP, a formulation with a high translational value, reduces neuroinflammation and shifts the microglia population toward a pro-resolutive phenotype, with potential applications in SCI and central nervous system inflammatory diseases.