Abstract
Spinal cord injury (SCI) is a devastating condition consisting of an instant primary mechanical injury followed by a secondary injury that progresses for weeks to months. The cytokine tumor necrosis factor (TNF) plays an important role in the pathophysiology of SCI. We investigated the effect of myeloid TNF ablation (peripheral myeloid cells (macrophages and neutrophils) and microglia) versus central myeloid TNF ablation (microglia) in a SCI contusion model. We show that TNF ablation in macrophages and neutrophils leads to reduced lesion volume and improved functional outcome after SCI. In contrast, TNF ablation in microglia only or TNF deficiency in all cells had no effect. TNF levels tended to be decreased 3 h post-SCI in mice with peripheral myeloid TNF ablation and was significantly decreased 3 days after SCI. Leukocyte and microglia populations and all other cytokines (IL-1β, IL-2, IL-4, IL-5, IL-6, IL-10, IL-12, and IFNγ) and chemokines (CCL2, CCL5, and CXCL1) investigated, in addition to TNFR1 and TNFR2, were comparable between genotypes. Analysis of post-SCI signaling cascades demonstrated that the MAPK kinase SAPK/JNK decreased and neuronal Bcl-XL levels increased post-SCI in mice with ablation of TNF in peripheral myeloid cells. These findings demonstrate that peripheral myeloid cell-derived TNF is pathogenic in SCI.
Highlights
Spinal cord injury (SCI) is a devastating condition resulting in paralysis below the level of injury.The primary mechanical injury is followed by a progressive secondary injury that develops for weeks to months leading to further cell death and CNS degeneration.The cytokine tumor necrosis factor (TNF) is known to be involved in the progressive secondary, neuro-inflammatory phase after SCI, affecting neuronal and axonal survival and subsequent functional outcomes [1,2]
We demonstrate that conditional ablation of TNF in macrophages and neutrophils, but not microglial TNF alone, results in decreased TNF levels, altered JNK/SAPK signaling and increased
We previously demonstrated that under physiological conditions conditional ablation of TNF in peripheral myeloid cells does not affect locomotor function, motor coordination, and neuromuscular function in male mice [20] and found the same to be true in female mice (Supplemental Table S1), except for anxiety-related behavior in the Elevated Plus Maze (EPM) (Supplemental Figure S1A–C)
Summary
The primary mechanical injury is followed by a progressive secondary injury that develops for weeks to months leading to further cell death and CNS degeneration. The cytokine tumor necrosis factor (TNF) is known to be involved in the progressive secondary, neuro-inflammatory phase after SCI, affecting neuronal and axonal survival and subsequent functional outcomes [1,2]. TNF production is rapidly increased in resident CNS cells, including microglia, followed by infiltrating leukocytes [1,2,3,4] and different studies demonstrate that TNF can display both neuroprotective and neurotoxic effects following SCI [5,6,7]. A transmembrane form (mTNF), which is enzymatically cleaved by TNF alpha converting enzyme (TACE/ADAM 17) to yield soluble TNF (solTNF) [8].
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