Nanoparticle Modulation of Immune and Vascular Microenvironment Dynamics Following Spinal Cord Injury

Trauma to the spinal cord initiates an inflammatory response causing secondary damage, which collectively can result in loss of function below the level of the injury. The unbalanced risk-benefit ratio of methylprednisolone led to development of therapeutic nanoparticles (NPs) that associate with circulating monocytes and neutrophils to reduce inflammation and secondary damage and improve functional recovery in a female mouse model of cervical hemisection spinal cord injury. Herein, we investigate the mechanisms occurring during the acute phase of injury by which NPs directly and indirectly modulate the phenotype and trafficking of monocytes and neutrophils and computationally catalog the communication network among cell types within the injury microenvironment. Using adoptive transfer to monitor trafficking, NP treatment reduced the extent of myeloid cell recruitment to the injury yet did not impact the composition of adoptively transferred monocytes or neutrophils. The proportion of inflammatory monocytes was reduced with NP treatment, and single-cell sequencing analysis indicated increased polarization toward pro-regenerative phenotypes. Sequencing analysis also demonstrated that outgoing signals from monocytes and neutrophils influenced the phenotype of numerous cell types, including endothelial cells, fibroblasts, oligodendrocyte progenitor cells, and Schwann cells. Signaling between cell compartments involves a combination of soluble and matrix signals, with NP treatment enhancing expression of genes associated with anti-inflammatory phenotypes, angiogenesis, neuroprotection, and promotion of axon outgrowth or decreasing expression of inhibitors to regeneration. Collectively, NP delivery leads to direct and indirect effects on monocytes and neutrophils, which subsequently influence gene expression and intercellular signaling networks that promote a pro-regenerative environment.