Investigation of microglial- versus macrophage-derived membrane-anchored tumor necrosis factor in spinal cord injury

Spinal cord injury (SCI) is a devastating neurological condition that can result in loss of sensation and motor control, having tremendous consequences for the individual affected and their care-givers. SCI initiates two main mechanisms of injury; the primary and the secondary injury. The primary injury is inevitable following SCI, but it leads to the secondary injury, which consists of a cascade of detrimental events contributing to further damage to the spinal cord. Therefore, it is also the secondary injury which are the target for therapeutic strategies following SCI. One major event of the secondary injury is the neuroinflammatory response. The neuroinflammatory response has shown to be involved in the progression of injury, however it is also essential for the repairment following SCI. The complexity of neuroinflammation following SCI is thought to be depended on the release of inflammatory mediators and activation state of immune cells and an important inflammatory mediator following SCI is tumor necrosis factor (TNF). TNF exists in a transmembrane bound form (tmTNF) and in a soluble form (solTNF), and their activity is mediated through the TNF receptors (TNFR), TNFR1 and TNFR2. Although both forms of TNF are capable to bind to both receptors, activation of TNFR1 is mostly mediated by solTNF and TNFR2 activity by tmTNF. TNF has shown to have substantial effect on the neuroinflammation and outcome following SCI. However, the heterogenic role of TNF in neuroinflammation challenge the development of an efficient anti-TNF strategy
following SCI.


Thus, the overall aim of this thesis was to increase the knowledge of TNF’s role following SCI, and to identify the temporal and cellular expression of TNF following SCI, together with the expression of TNFR1 and TNFR2. We wanted to uncover the role of microglial-derived versus leukocytederived TNF following SCI, and the dichotomy of function for tmTNF and solTNF following SCI. The current knowledge of TNF following SCI was reviewed, and TNF’s expression and function were further investigated using a moderate thoracic contusive SCI model in C57BL/6J mice or transgenic mouse strains, and with pharmacological intervention.


In Manuscripts I, we provided an overview of the current knowledge of TNF expression and the potential effect of TNF manipulation following SCI.


We found the expression of TNF to be increased in the acute and delayed phase following, in Manuscript II, together with an increase of TNFR1 and TNFR2. TNF was shown to co-localize with
CD11b+ immune cells in the delayed phase, while TNFR1 was found on degenerative MAP2+ neurons and CD68+ microglia/macrophages and TNFR2 on astrocytes and CD68+ microglia/macrophages. TNF and TNFRs was also found to co-localize with similar cell types in postmortem spinal cord tissue
from humans. Moreover, a robust neuroinflammatory response followed with activated microglia and infiltrating leukocytes.


In Manuscript III, we showed that conditional ablation of leukocyte-derived TNF in mice, decreased injury volume and improved functional recovery following SCI compared to wildtype and this seemed to be mediated by reduced apoptosis. However, conditional ablation of microglial-derived TNF did not show to alter the functional recovery following SCI compared to wildtype.
Finally, in Manuscript IV we found inhibition of solTNF using XPro1595 to improve functional recovery in mice, and to enhance a more regenerative environment following SCI, possibly by
altering the phagocytotic properties of microglia.


In conclusion, the results presented in this thesis demonstrate that TNF plays a significant role in the acute and delayed phase following SCI, and the detrimental function of TNF is mediated by peripheral leukocytes and solTNF. Thus, selectively inhibition of solTNF might represent as a promising antiTNF strategy following SCI.