the role of tnf-α in inflammatory olfactory loss

The LaryngoscopeVC 2011 The American Laryngological,Rhinological and Otological Society, Inc.

The Role of TNF-a in Inflammatory Olfactory Loss

Babar Sultan, MD; Lindsey A. May, BS; Andrew P. Lane, MD

Background: Despite the significant health impact of olfactory loss in chronic rhinosinusitis (CRS), the underlyingpathophysiology is incompletely understood. A transgenic mouse model of olfactory inflammation induced by tumor necrosisfactor-alpha (TNF-a) has provided new insights into the cellular and molecular basis of inflammatory olfactory loss. Here, weutilize systemic corticosteroids to suppress downstream cytokine expression, in order to study the direct role of TNF-a inCRS-associated olfactory dysfunction.

Methods: Transgenic mice were induced to express TNF-a in the olfactory epithelium for 6 weeks. In a subset of mice,1 mg/kg prednisolone was administered concurrently to inhibit downstream inflammatory responses. The olfactory epithe-lium (OE) was analyzed by histology and electro-olfactogram (EOG) recordings.

Results: Treatment with prednisolone successfully prevented inflammatory infiltration over significant regions of theOE. In areas where significant subepithelial inflammation was present, a corresponding loss of olfactory neurons wasobserved. In contrast, areas without major inflammatory changes had normal olfactory neuron layers, despite chronic localexpression of TNF-a. Prednisolone partially reversed the complete loss of olfaction in the mouse model, preserving odorantresponses that were significantly diminished compared to controls, but not absent.

Conclusions: The addition of prednisolone to the transgenic model of olfactory inflammation isolates the direct effectsof induced TNF-a expression on the OE. The finding that prednisolone treatment prevents neuronal loss in some regions ofthe OE suggests that TNF-a does not directly cause neuronal apoptosis—rather, that subepithelial inflammation or otherdownstream mediators may be responsible. At the same time, EOG results imply that TNF-a directly causes physiologic dys-function of olfactory neurons, independent of the inflammatory state. An understanding of the role of TNF-a and other inflam-matory cytokines may suggest novel therapeutic strategies for CRS-associated olfactory loss.

Key Words: TNF-a, olfaction, steroid, transgenic, rhinosinusitis.Level of Evidence: N/A.

Laryngoscope, 121:2481–2486, 2011

BACKGROUNDOlfaction has a critical impact on quality of life by

contributing to enjoyment of foods and odors while alsoserving as a warning mechanism for dangerous environ-mental hazards.1,2 The loss of the sense of smell is acommon symptom of chronic rhinosinusitis (CRS) thatcan be very debilitating to patients.3 Despite its greatclinical significance, current understanding of the patho-physiology of olfactory loss in CRS is incomplete. Twobroad mechanisms analogous to conductive and sensori-neural hearing loss have been proposed in CRS-inducedolfactory dysfunction. Conductive olfactory loss relates tophysical obstruction of odorant delivery to the olfactorycleft secondary to mucosal inflammation or abnormal-ities of the olfactory mucus.4 Sensorineural olfactory loss

is caused by damage or destruction of the neuroepithliumas a result of toxic inflammatory mediators and tissue dis-ruption from infiltrating inflammatory cells.5,6 We havepreviously proposed an additional sensorineural mecha-nism in which olfactory dysfunction can occur with anintact neuroepithelium, as a consequence of direct interac-tions between olfactory sensory neurons and inflammatorycytokines. The olfactory epithelium normally has a remark-able capacity for regeneration, with ongoing replacement ofolfactory receptor neurons occurring throughout the life ofan individual. In CRS, persistent inflammation is associ-ated with prolonged olfactory loss that may be rapidlyreversible with systemic corticosteroid treatment. This re-versal suggests that either the neuroepithelium is notseverely damaged in CRS or that it can be rapidly reconsti-tuted when inflammation is diminished with steroids.

To study the effects of inflammation on the olfactorysystem in vivo, our group has developed a transgenicmouse model in which tumor necrosis factor-alpha (TNF-a)is expressed in a temporally controlled, olfactory epitheliumspecific fashion.7 Continuous local production of TNF-awithin the olfactory mucosa results in a progressive inflam-matory infiltrate that mimics histologic features of CRS-associated olfactory loss. With this model, we have shownthat chronic TNF-a-induced inflammation causes loss ofmature receptor neurons while also suppressing the normalregenerative replacement mechanism.8 Electrical odorantresponses are lost after 5 to 7 weeks of inflammation,

From the Department of Otolaryngology—Head and Neck Surgery,Johns Hopkins University School of Medicine, Baltimore, Maryland,U.S.A.

Editor’s Note: This Manuscript was accepted for publication July11, 2011.

This article was supported by the National Institute of Deafnessand Other Communication Disorders, National Institutes of HealthGrant RO1 DC009026 (to A.P.L.).

The authors have no financial disclosures for this article.The authors have no conflicts of interest to declare.

Send correspondence to Dr. Andrew P. Lane, Department of Oto-laryngology—Head and Neck Surgery, Johns Hopkins Outpatient Center,601 N. Caroline Street, 6th floor, Baltimore, MD 21287-0910. E-mail:[email protected]

DOI: 10.1002/lary.22190

Laryngoscope 121: November 2011 Sultan et al.: TNF-a and Inflammatory Olfactory Loss

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secondary to the absence of olfactory receptor neurons.Interestingly, responses to odorants become diminishedafter 2 weeks of TNF-a production prior to the neuronsbecoming depleted, suggesting that inflammation causesfunctional impairment through another mechanism. In themouse model, expression of TNF-a initiates a downstreamcascade of multiple pro-inflammatory mediators from a vari-ety of cell types. We hypothesize that these mediators,individually or in combination, affect olfactory sensoryneurons and their progenitor cells to cause initial desensiti-zation and eventual neuronal death and suppressedregeneration. Identification of the specific cytokines and thecellular pathways they activate may lead to novel treat-ments for CRS-associated olfactory loss.

The ability of TNF-a to initiate inflammation is due inpart to NF-jB-mediated induction of multiple downstreamcytokines.9 Systemic corticosteroids have significant benefi-cial effects in CRS and a wide range of other inflammatorydiseases because they inhibit expression of pro-inflamma-tory mediators.10 A key feature of the olfactory loss mousemodel is that the expression of TNF-a is controlled by aninserted transgene regulated by exposure to a drug, ratherthan by native transcription mechanisms. As a result, corti-costeroid treatment in the mouse does not inhibit TNF-aexpression, but only the downstream expression of cytokinesinitiated by TNF-a. In this study, we take advantage of thisproperty of the mouse model to isolate the direct effects ofTNF-a on the olfactory system, separate from the secondaryeffects of inflammatory cell infiltration and consequentinflammatory mediator production.

MATERIALS AND METHODS

Inducible Olfactory Inflammation (IOI) MouseThe creation of the IOI mouse line has been described pre-

viously.7 Briefly, the reverse tetracycline transactivator genewas knocked into the olfactory-specific cyp2g1 coding regiongenerating a cyp2g1-rtTA strain. This line was crossed with aline containing the TNF-a gene under the control of a tetracy-cline-responsive element (TRE-TNF-a) to generate the IOImouse. Doxycycline (DOX) was used to induce TNF-a expressionin adult mice between the ages of 6 and 8 weeks old. IOI micewere induced to express TNF-a in the olfactory epithelium for 6weeks. In a subset of mice, 1 mg/kg prednisolone was adminis-tered concurrently. As a control, a wild-type mouse was treatedwith DOX as well as 1 mg/kg of prednisolone for 6 weeks.

Histologic AnalysesAfter sacrifice by CO2 inhalation, the mice were decapitated

and the heads were fixed and decalcified by immersion in TBD2 so-lution (Shandon, Pittsburgh, PA) for 24 hours. The heads wereembedded in paraffin, and 12-lm sections were obtained and col-lected on glass slides for hemotoxylin and eosin staining. For frozensection analysis, the mice were anesthetized by an intraperitonealinjection of 100 mg/kg of xylaket (Sigma, St. Louis, MO), before in-tracardiac perfusion with 4% paraformaldehyde. The olfactorytissue was then dissected, postfixed in 4% paraformaldehyde, andtransferred to a solution of 30% sucrose and 250 mM of EDTA for48 hours. The decalcified heads were then infiltrated with OCT tis-sue-tek compound (Miles, Elkhart, IN) and frozen on dry ice into aplastic mold. Sections of mouse olfactory tissue in OCTwere cut ona cryostat (12 lm), placed on Super-frost plus slides (Fisher Scien-tific, Pittsburgh, PA), and dried 60 minutes before use.

Olfactory Epithelium Thickness MeasurementsThe thickness of the olfactory epithelium was measured

on hematoxylin and eosin-stained tissue sections. Images wereacquired using the Zeiss Axio Imager.A2 microscope and meas-urements made using the Axiovision 4.8 software (Carl ZeissMicro-imaging, Thornwood, NJ). The measured thickness of theepithelium was from the basement membrane to the top of theolfactory knobs. All measurements were made on the same tur-binate every 200 lm from zone 4 of the olfactory epithelium ontwo sections from three mice from each data group. Values foreach individual animal were averaged and data are representedas the mean 6 SEM for each group. The range represents thethickness values for one turbinate for a representative mouse.Statistical comparisons were made using the Students t-test.

ImmunostainingCryostat sections were blocked for 1 hour in phosphate-

buffered saline (PBS) containing 5% normal secondary serumand then incubated overnight at 4�C in 5% normal serum con-taining primary antibody to keratin 5 (K5, Covance, Princeton,NJ). Primary antibodies were detected using a fluorescenttagged secondary antibody (Alexa-fluor, Invitrogen, Carlsbad,CA). Each sample was counterstained by the nuclear stain,DAPI (Vector Labs, Burlingsgame, CA). Images were viewedusing a LSM510 confocal microscope (Carl Zeiss Micro-imaging),and measurements of K5 fluorescent intensity along a zone 4turbinate were made using Axiovision 4.8 software.

Bromodeoxyuridine LabelingMice were injected i.p. with bromodeoxyuridine (BrdU)

(Sigma), 50 lg/g of body weight, 60 minutes before death. Cryo-stat sections were then incubated with 3 N HCl for 30 minutesand treated with proteinase K for 10 minutes before immuno-staining with rat anti-BrdU antibody (Abcam, Cambridge, MA).Primary antibodies were detected using a fluorescent taggedsecondary antibody (Alexa-fluor). Each sample was counter-stained by the nuclear stain, DAPI. Images were viewed usinga LSM510 confocal microscope.

Electro-olfactogram (EOG)The medial surface of the olfactory turbinates was prepared

for recording after the mouse was sacrificed using CO2. Odorant sol-utions (Aldrich, St. Louis, MO) were prepared in DMSO anddiluted with water to the working concentration just before EOG re-cording. Test odorants for air delivery were prepared at liquidconcentrations of 10�3 [final DMSO concentration of 0.2% (v/v)],and diluted to 10�4 and 10�5 M concentrations. Responses toDMSO diluent alone were measured. Odorant stimulation wasdelivered in the vapor phase as a 100-millisecond pulse by injectioninto the continuous stream of humidified air. The odorant stimuluspathway was cleaned by air between each stimulus presentationwith a minimum interval of 1 minute between two adjacent stimuli.

RESULTS

Prednisolone Treatment Markedly DecreasesTNF-a-Induced Inflammation, with RelativePreservation of Large Areas with OlfactoryEpithelium Neuron Layers

The normal mouse olfactory epithelium (OE) consistsof a superficial single layer of sustentacular cells overly-ing a densely packed layer of olfactory sensory neuronsand their progenitors. In the most basal aspect of the neu-ron layer are the multipotent stem cell populations. Below


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the basement membrane in the subepithelium lie well-demarcated axon bundles. After 6 weeks of DOX-inducedTNF-a expression, the OE is thinned with very few remain-ing mature olfactory receptor neurons, but with an intactsustentacular layer. Inflammatory cells have infiltrated thesubepithelium, distorting the subepithelial architecture, andaxon bundles are no longer evident. After 6 weeks of DOX-induced TNF-a expression concurrent with prednisolone, theOE in many areas is nearly normal with only a mild inflam-matory infiltrate in the subepithelium and irregular axonbundles (Fig. 1A). To control for interanimal heterogeneityin the extent and location of inflammation, OE thicknesswas measured completely in the same anatomic location forall three groups. The mean OE thicknesses for the no DOX,DOX, and DOX/Prednisolone mice were 43.23 lm (range:28.10–51.92 lm), 12.69 lm (range: 7.99–28.82 lm), and28.03 lm (range: 18.01–41.97 lm), respectively. These dataare graphically represented in Figure 2 and highlight thefact that prednisolone causes a significant retention of theOE neuron layer compared to DOX only, but there is stillloss of neurons when compared to wild-type mice.

Prednisolone Treatment Greatly Diminishes theLoss of Horizontal Basal Cells Associated withProlonged TNF-a Expression

Our previous studies have shown a surprising ab-sence of proliferation during active TNF-a-induced

Fig. 1. Olfactory epithelial changes during TNF-a expression. (A) With no doxycycline (No Dox), the olfactory epithelium has a normal appear-ance with intact sustentacular cells (SUS), multilayered olfactory neurons (ORN), and intact axon bundles. With 6 weeks of dox-induced TNF-aexpression (Dox), the olfactory neuronal layer is significantly thinned with significant subepithelial infiltration of infammatory cells and lack of dis-cernable axon bundles. Six weeks of dox concurrent with prednisolone (Dox/Prednisolone) results in a relatively normal olfactory epitheliumwith decreased axon bundle diameter and some infiltration of inflammatory cells in the subepithelium. Slides are stained with hemotoxylin andeosin and images were taken at 20� magnification. Scale bar represents 20 lm. (B) The basal aspect of the olfactory epithelium prior to TNF-aexpression shows a uniform layer of horizontal basal cells through the contiguous layer of K5 immunostaining (red). After 6 weeks of TNF-aexpression, there is no visible expression of K5 in the basal layer of thinned olfactory epithelium. With concurrent prednsiolone administration,the basal layer has a slight reduction in K5 expression when compared to untreated mice. The wild-type mouse had a mean intensity of 763 617 Grey levels, whereas the DOX–prednisolone mouse had a mean intensity of 575 6 43 Grey levels (P < .03). Scale bar represents 10 lm.[Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

Fig. 2. Prednisolone partially inhibits olfactory neuronal loss in miceexpressing TNF-a. The mean olfactory epithelium thicknesses forthe wild-type (No DOX), doxycycline induced TNF-a expression(DOX), and concurrent doxycycline/prednisolone administration(DOX/PRED) mice are 42.23 lm (range: 28.10–51.92 lm), 12.69 lm(range: 7.99–28.82 lm), and 28.03 lm (range: 18.01–41.97 lm).Error bars represent the SEM. *P < .02, **P < .05. [Color figure canbe viewed in the online issue, which is available atwileyonlinelibrary.com.]


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inflammation, despite progressively severe neuronaldepletion. The reason for this failure of regeneration isnot understood. Horizontal and globose basal cells lie inthe basal layer of the OE and contribute to normal turn-over and injury-induced neurogenesis.11 It has beenshown that horizontal basal cells (HBC), which are nor-mally quiescent, undergo a proliferative burst torepopulate all olfactory epithelial cell compartmentswhen there is extensive olfactory neuronal depletion.12

HBC specifically express keratin 5 (K5),13 which can beused as a marker for their identification by immunohis-tochemistry. The basal aspect of the OE prior to TNF-aexpression shows a uniform layer of HBC directly abovethe basement membrane (Fig. 1B, left side). After 6weeks of TNF-a expression, there is no visible expressionof K5 in the thinned OE, suggesting that the HBC popu-lation has either been destroyed, lost its expression ofK5, or possibly depleted itself through extensive prolifer-ation (Fig. 1B, central panel). With concurrentprednsiolone, K5 expression is maintained in areaswhere the olfactory neuron layer is preserved, althoughreduced in intensity when compared to untreated mice(Fig. 1B, right panel). Densitometric analysis revealed amean intensity of 763 6 17 Grey levels in untreatedmice, whereas the DOX–prednisolone mouse had a meanintensity of 575 6 43 Grey levels (P < .03). In areas ofinflammation and neuronal loss, K5 staining is absentin the prednisolone-treated mice. This suggests that theloss of K5-positive HBC is not caused directly by TNF-a,which is expressed throughout the entire OE, but ratheroccurs only when there is robust inflammation and wide-spread death of olfactory receptor neurons.

The Preservation of Large Areas of IntactOlfactory Neuroepithelium with PrednisoloneIs Not Consequent to Increased Proliferation

The majority of cell division during OE regenera-tion occurs in the basal compartment where theprogenitor cells are located. In order to assess the effect

of TNF-a and its downstream mediators on proliferation,we labeled dividing cells with BrdU and used an anti-BrdU antibody to visualize dividing cells in the OE. Inan adult mouse, in the absence of OE injury, neuronalturnover is relatively slow, with one to two BrdU-positivecells observed per high-power field in the OE. We havepreviously reported the absence of BrdU positive cellsduring active DOX-induced TNF-a expression. The lack ofproliferation is seen prior to the widespread loss of neu-rons and continues as long as the TNF-a is expressed, butthere is a dramatic wave of cell division and regenerationthat begins once the DOX is stopped. In concurrent DOX–prednisolone administration, areas of preserved neuroepi-thelium have one to two BrdU-positive cells per highpower field, similar to wild-type mice, but in areas withthinned neuroepithelium, there is no increase in BrdU-positive cells in response to neuronal loss (Fig. 3). Thissuggests that the preservation of OE thickness and the ol-factory neuron layer with prednisolone administrationdoes not occur through active repopulation by progenitorcells, but rather through prevention of neuronal loss. Thisis consistent with our previous hypothesis that TNF-adirectly inhibits progenitor cell proliferation.14

Prednisolone Partially Reverses the Lossof Electrical Odorant Responses

We have previously demonstrated that prolongedTNF-a-induced inflammation of the OE results in loss ofelectrical odorant responses. Shorter durations of TNF-aexpression result in a diminished amplitude of odorantresponse, even prior to the histologic loss of olfactoryneurons. In order to better understand the specific roleof TNF-a in modulating olfactory neuron function, weperformed EOG recordings in mice treated with DOXalone and with DOX–prednisolone. After 6 weeks ofDOX-induced TNF-a expression the EOG responses evenat the highest odorant concentrations are absent, corre-lating with the extremely depleted neuronal layer. Withconcurrent prednisolone, there is preservation of odorant

Fig. 3. Proliferation is not increased with concurrent prednisolone administration. Bromodeoxyuridine (BrdU) staining (red) was used as ameasure of proliferation. At baseline (No Dox), there are one to two BrdU-positive cells per high-power field in the neuroepithelium. Withdoxycycline-induced TNF-a expression for 6 weeks (Dox), there are no visible BrdU-positive cells even in the small areas of preserved neu-roepithelium. In concurrent doxycycline–prednisolone administration (Dox/Prednisolone), areas of preserved neuroepithelium have a similarstaining as the wild-type mouse. Scale bar represents 10 lm. [Color figure can be viewed in the online issue, which is available atwileyonlinelibrary.com.]


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responses, but they are significantly reduced in ampli-tude compared to littermates not treated with DOX(Fig. 4). The preservation of EOG amplitude is not pro-portional to the much greater retention of olfactoryneurons with prednisolone treatment.

DISCUSSIONIn this study, we have used the olfactory inflamma-

tion mouse model to study the effects of prolonged invivo exposure to TNF-a on the appearance and functionof the olfactory epithelium. We exploited the ability ofprednisolone to inhibit downstream expression of inflam-matory cytokines to isolate the direct effects of TNF-a.These experiments demonstrate that systemic corticoste-roids significantly prevent the profound inflammatoryinfiltrate from developing over large areas of the mouseolfactory epithelium. In regions where the olfactory epi-thelium is not inflamed, the neuronal layer is largelypreserved, the horizontal basal cell progenitors remainintact, and there is no change in neuronal proliferation.However, although the olfactory receptor neurons appearto be intact histologically, electrical odorant responsesare diminished. Taken together, these findings suggestthat the widespread loss of olfactory receptor neurons inthe inflamed state is not due directly to TNF-a, butrather results from other inflammatory mediators orphysical crowding by the inflammatory cellular infiltrateitself. Similarly, the disappearance of the horizontal ba-sal cell population is also not due to TNF-a alone,because these cells are preserved in the basal compart-ment of normal-appearing regions. An expandedanalysis of the transition areas between adjacent normaland thinned neuroepithelium in the future may yield abetter understanding of the relationship between subepi-thelial inflammation and neuroepithelial loss. The fact

that electrical odorant responses are present, but greatlyreduced, may point to a direct role of TNF-a in desensi-tizing olfactory receptor neuron function. This fits withour previously reported observation in this mouse modelthat the functional olfactory loss precedes the onset ofdramatic histologic changes.7

Systemic corticosteroids are the only consistentlyeffective treatment for CRS-associated olfactory loss inhumans, and their cellular and molecular effects havehas been studied extensively.15 In CRS, corticosteroidsgreatly reduce expression of a wide variety of immunemediators and cytokines, including TNF-a, leading to re-solution of inflammation. Although baseline levels ofTNF-a may be greater in CRSsNP than CRSwNP, a sig-nificant reduction in TNF-a levels in polyps is effectedby steroids.16,17 In addition to their anti-inflammatoryactivity, steroids have also been hypothesized to actdirectly on olfactory receptor neurons. Glucocorticoidreceptor mRNA and protein can be found in the olfactorymucosa.18 Glucocorticoids increase the expression ofcyclic nucleotide-gated (CNG) channels, which are keycomponents in olfactory receptor signal transduction,and can modulate the olfactory Na-K-ATPase.19,20 In ourmouse model, the loss of mature olfactory receptor neu-rons is greatly ameliorated by prednisolone treatment.

Although we believe that this occurs via inhibitionof inflammatory cells and their cytokine products, it isconceivable that prednisolone may act directly on olfac-tory neurons to block TNF-a-induced apoptosis.

The specific mechanisms through which TNF-aaffects olfactory neuron function have not been fullycharacterized. mRNA for TNF-a and TNF-a receptorsare found in normal olfactory epithelium.21 In vitro,TNF-a been shown to induce cell death in olfactoryepithelial explants.22 In nonolfactory neurons, Zhanget. al.23,24 have reported TNF-a-induced hypersensitivityof sensory neurons, and Soliven et al.23,24 have demon-strated TNF-a modulation of sympathetic neuroncalcium currents. These two studies highlight directeffects of TNF-a on neurons that are independent ofdownstream inflammation and mediator expression. Wepostulate that through similar intracellular mechanisms,prolonged TNF-a exposure can result in a desensitizedstate in olfactory receptor neurons. Future studies utiliz-ing TNF-a receptor knockout strains will be useful tofurther address the specific role of TNF-a versus otherdownstream inflammatory cytokines.

CRS-associated olfactory loss is a troublesomehealth condition that remains difficult to treat. Althoughthe process can be transiently responsive to systemiccorticosteroids, the side effects and long-term risks ofthis therapy are prohibitive.15 Endoscopic sinus surgeryand removal of nasal polyps can greatly improve manyCRS symptoms, but generally result in limited sustainedimprovement in olfactory function.25,26 This likelyreflects the limitation of surgical approaches to addressonly the conductive component of olfactory loss, withoutdirectly impacting the sensorineural dysfunction causedby local neuroepithelial inflammation. To that point,patients with allergic rhinitis can have hyposmia evenin the absence of significant olfactory cleft obstruction

Fig. 4. Functional loss secondary to TNF-a expression. The quan-titative assessment of electro-olfactogram (EOG) responses showsessentially no response after 6 weeks of dox-induced TNF-aexpression even at the highest odorant concentrations. With con-current prednisolone (pred), there is slight retention of responsesbut still markedly decreased when compared to controlresponses. Data reflects a minimum of four independent record-ings. Error bars represent SEM. [Color figure can be viewed in theonline issue, which is available at wileyonlinelibrary.com.]


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on computed tomography scan,27 and viral upper respi-ratory infections can result in olfactory dysfunction thatcontinues after resolution of nasal airway blockage.28,29

Further research is needed to fully elucidate the role ofTNF-a and other inflammatory cytokines in causing per-sistent olfactory dysfunction in the setting of chronicrhinosinusitis, where a mixture of conductive and sen-sorineural factors likely contribute.

CONCLUSIONSTreatment with systemic corticosteroids in a mouse

model of inflammation-induced olfactory loss sheds newlight on the underlying pathophysiology. The presentstudy suggests that although TNF-a directly alters olfac-tory neuron function and suppresses neuroepithelialregeneration, it is downstream mediators and the physi-cal infiltration of inflammatory cells that contributesmost critically to the olfactory loss phenotype in thismodel. As the impact of individual cytokines and inflam-matory mediators on olfactory neuron desensitizationand cell death are identified, the potential will exist fordevelopment of targeted therapies to restore functioneven in the face of ongoing inflammation.

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the role of tnf-α in inflammatory olfactory loss

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