Nanoparticle-Mediated Immunomodulation as a Therapy in Acute and Chronic Spinal Cord Injury

Abstract

Spinal cord injury (SCI) causes permanent loss of sensory and motor function below the level of injury due to neuronal death. Several extrinsic factors exacerbate the injury and limit the regenerative capability of the spinal cord. First, the infiltration of myeloid cells and the cytokines that they secrete exacerbate cell death and create a pervasive inflammatory environment. Second, fibroblasts and astrocytes migrate to the injury site and form scar tissue. Third, low expression of regeneration associated genes in the CNS and the presence of myelin debris and scar tissue inhibit axonal regeneration by neurons. To combat these issues, we proposed a combinatorial approach in both acute and chronic models of a cervical hemisection injury. In an acute C5 hemisection model we investigated the window in time which poly(lactide-co-glycolide) nanoparticles (NPs) administration can successfully modulate the immune response and promote functional sparing. First, a microporous, multichannel poly(lactide-co-glycolide) PLG scaffold or “bridge” was implanted in place of the resected tissue. Second, NPs were injected intravenously following injury. These NP are phagocytosed by circulating monocytes and neutrophils, decreasing their migration to the SCI and reprogramming them towards an anti-inflammatory phenotype. At one-week post-injury, early NP intervention decreased the number of infiltrating macrophages and neutrophils but delaying treatment increased the number of neutrophils above control. All mice that received NPs had greater neuronal sparing contralateral to the injury, but mice that received NPs at early timepoints have greater neuromuscular junction innervation and motor endplate sparing. The increased sparing of neurons and neural circuits in the early NP group corresponded with increased motor function, as measured by a ladder beam test. While mice in the late NP administration group initially performed poorly on the ladder beam test, their performance improved over time. Collectively, these results suggest that early intervention with NPs can modulate the inflammatory response and preserve motor function and circuits following SCI. In a chronic C5 hemisection model, mice received PLG NPs for one week following injury to combat inflammation. Mice that received NP treatment made fewer mistakes on a ladder beam test of motor performance compared to control. At four weeks after injury, we resected scar tissue and inserted a microporous, multichannel PLG bridge. This removes inhibitory fibrotic and glial scar tissue and provides a scaffold to guide regenerating axons across the injury site. We then tested whether NP administration after the resection surgery can have a further beneficial effect. Mice that received NPs after both the initial injury and resection performed better on the ladder beam test than mice that received vehicle or NPs after only one surgery. Lastly, we administered epothilone D (epoD), a microtubule stabilizer that can penetrate the blood-brain barrier, to limit axon retraction and boost elongation. Mice that received epoD performed better on the ladder beam task than mice that did not, with mice that received both epoD and NPs performing best out of all experimental groups. Underlying this increased performance, all mice that received NPs or epoD exhibited robust axon growth into the injury and both oligodendrocyte and Schwann cell myelination of regenerating axons. Together, these results suggest that a combinatorial treatment plan that targets both inflammation and growth-inhibitory factors can improve motor performance following SCI.