Tumor necrosis factor-α antagonist suppresses local inflammatory reaction and facilitates olfactory nerve recovery following injury
Introduction
Olfactory dysfunction is a common finding in head trauma due to injury to the olfactory nerve. The incidence of olfactory dysfunction due to head trauma is reported to be 5–26% [1], [2]. Olfactory dysfunction lowers our quality of life and can be life threatening because of the inability to detect hazardous events such as fire, gas leak, and spoiled food intake [3]. Causes of olfactory dysfunction in head trauma are overextension, distortion and tearing of the olfactory nerves and contusions of the olfactory bulbs and orbitofrontal regions of the brain [4]. Although the olfactory system has a remarkable capacity for neural regeneration and recovery following injury, prognosis of head trauma patients for recovery from olfactory dysfunction is only 10–38% [2], [5] while that in patients with chronic rhinosinusitis and allergic rhinitis is reported to be better, with recovery rates of up to 68–86% [6], [7]. So, development of therapeutic management for improving olfactory dysfunction is an important clinical objective.
We previously demonstrated using an olfactory nerve injury model in mice that anti-inflammatory treatment with steroids or anti-interleukin-6 (IL-6) antibody during the acute phase of injury is effective in suppressing the inflammatory reaction and local glial scar formation and improves recovery outcomes after olfactory nerve transection (NTx) [8], [9]. In clinical practice, however, these drugs are not typically used for the treatment of head injury patients since several studies reported that steroids do not have a significant efficacy on morbidity and mortality in patients with severe head injury and there are concerns that steroids may cause serious side effects such as hypertension, hyperglycemia, infection, bone necrosis and psychosis [10], [11]. Although there are fewer concerns regarding anti-IL-6 antibody use, its administration may sometimes induce severe infection [12].
Tumor necrosis factor (TNF)-α is a key cytokine that plays a central part in inflammation, immunity and apoptosis, and is released in response to stress and injury [13]. A TNF-α antagonist, etanercept, has been reported to be an effective inhibitor of the inflammatory reaction by preventing TNF-α from combining with the TNF-α receptor expressed on surface of cells and has been used instead of steroids for the treatment of refractory inflammatory diseases such as rheumatoid arthritis [14]. In the central nervous system, an increase in the levels of TNF-α occurs shortly after injury and it is linked to many factors like cellular dysregulation, enhanced vascular permeability and impaired glutamate metabolism, and sometimes to excessive inflammatory reactions [15]. A recent study reported that administration of etanercept suppresses the severity of the response to trauma and facilitates functional recovery in experimentally induced spinal cord injury in mice [16].
The present study was designed to investigate if therapeutic intervention using TNF-α antagonist is effective in improving recovery outcomes in the olfactory system following injury in mice. In this study, we first demonstrated an increase in levels of TNF-α in the injured olfactory system using a polymerase chain reaction (PCR) assay, which would be a target of the TNF-α antagonist. Subsequently, we used histological techniques to examine the efficacy of the TNF-α antagonist on recovery outcome by measuring the degree of degeneration and regeneration of olfactory nerve fibers and the amount of injury-associated tissue (glial scar), reactive astrocytes and macrophages/microglia. We also administered an olfactory function test using avoidance conditioning behavior to odorants as well as electrophysiological recording of field potential responses to electrical stimulation of the olfactory mucosa to determine if morphological recovery parallels functional recovery in the olfactory system following therapeutic intervention.
Section snippets
Experimental animals
This study was performed using transgenic mice (OMP-tau-lacZ mice) obtained from the Jackson Laboratory (Bar Harbor, ME, USA), whose strain is derived from C57BL/6 mice. In this strain the gene sequence encoding the olfactory marker protein (OMP) has been replaced with a tau-lacZ reporter gene [17]. The OMP is expressed in all mature olfactory neurons [18] and the replacement with tau-lacZ reporter gene enables the visualization of olfactory nerve fibers and their projections to olfactory bulb
TNF-α expression
A semi quantitative PCR shows that there are significant increases in TNF-α gene expression 1 h after the NTx or later (Fig. 2A and B). Real time PCR confirmed these results as shown in Fig. 2C. While statistically significant increases in TNF-α gene expression was observed only 3 and 6 h after the NTx in real time PCR due to small number samples and multi-time point analyses of variance, these results of both PCR showed that TNF-α, the target of etanercept, increased at damaged olfactory tissues
Discussion
The present study shows that etanercept, a TNF-α antagonist, can suppress local infiltration of inflammatory cells and glial scar tissue formation and subsequently facilitate morphological and functional recovery of the olfactory system. These results are consistent with our previous reports that demonstrated olfactory nerve regeneration by systemic administration of steroid and anti-IL-6 antibody, both of which suppress local inflammation and glial scar formation in the mouse injury model [8],
Conclusions
The present study revealed that inactivation of TNF-α using its antagonist, etanercept, in an acute phase of olfactory nerve injury can contribute to ameliorate functional restoration of the olfactory system by suppressing local infiltration of inflammatory cells and glial scar tissue formation and may provide a new therapeutic strategy for the treatment of olfactory dysfunction following head injuries.
Conflict of interest
The authors declare that they have no conflict of interest.
Financial disclosures
This work was supported by JSPS KAKENHI Grant Number 26462579.
Acknowledgements
We would like to thank all members of the Department of Immunology, Mie University Graduate School of Medicine for their generous assistance in running some of the experiments included in this manuscript and Dr. Richard M. Costanzo (Virginia Commonwealth University School of Medicine, USA) for reading a draft of the manuscript and giving invaluable comments and suggestions.
References (37)
- et al.
Blockade of interleukin-6 receptor suppresses inflammatory reaction and facilitates functional recovery following olfactory system injury
Neurosci Res
(2013) - et al.
Late blocking of peripheral TNF-α is ineffective after spinal cord injury in mice
Immunobiology
(2013) - et al.
Visualizing an olfactory sensory map
Cell
(1996) - et al.
Olfactory marker protein during ontogeny: immunohistochemical localization
Dev Biol
(1980) - et al.
Identification of a neural stem cell in the adult mammalian central nervous system
Cell
(1999) - et al.
TNF alpha potentiates glutamate neurotoxicity by inhibiting glutamate uptake in organotypic brain slice cultures: neuroprotection by NF kappa B inhibition
Brain Res
(2005) - et al.
IL-6 and its soluble receptor orchestrate a temporal switch in the pattern of leukocyte recruitment seen during acute inflammation
Immunity
(2001) - et al.
Hematogenous macrophages express CD8 and distribute to regions of lesion cavitation after spinal cord injury
Exp Neurol
(2003) - et al.
Olfactory function after mild traumatic brain injury
Brain Inj
(2003) Post-traumatic anosmia
Brain
(1964)