The role of TNF-α in inflammatory olfactory loss.

BACKGROUND Despite the significant health impact of olfactory loss in chronic rhinosinusitis (CRS), the underlying pathophysiology is incompletely understood. A transgenic mouse model of olfactory inflammation induced by tumor necrosis factor-alpha (TNF-α) has provided new insights into the cellular and molecular basis of inflammatory olfactory loss. Here, we utilize systemic corticosteroids to suppress downstream cytokine expression, in order to study the direct role of TNF-α in CRS-associated olfactory dysfunction. METHODS Transgenic mice were induced to express TNF-α 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 epithelium (OE) was analyzed by histology and electro-olfactogram (EOG) recordings. RESULTS Treatment with prednisolone successfully prevented inflammatory infiltration over significant regions of the OE. In areas where significant subepithelial inflammation was present, a corresponding loss of olfactory neurons was observed. In contrast, areas without major inflammatory changes had normal olfactory neuron layers, despite chronic local expression of TNF-α. Prednisolone partially reversed the complete loss of olfaction in the mouse model, preserving odorant responses 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 effects of induced TNF-α expression on the OE. The finding that prednisolone treatment prevents neuronal loss in some regions of the OE suggests that TNF-α does not directly cause neuronal apoptosis--rather, that subepithelial inflammation or other downstream mediators may be responsible. At the same time, EOG results imply that TNF-α directly causes physiologic dysfunction of olfactory neurons, independent of the inflammatory state. An understanding of the role of TNF-α and other inflammatory cytokines may suggest novel therapeutic strategies for CRS-associated olfactory loss.