Otitis media: molecular impact of inflammation in the middle and inner ear--cytokines, steroids, and ion homeostasis

Cellular content plays a crucial role in Non-typeable Haemophilus influenzae infection of preinflamed Junbo mouse middle ear

Non-typeable Haemophilus influenzae (NTHi) is a major pathogen causing acute otitis media (AOM). The relationship between the cellular content of the middle ear fluid (MEF) during AOM and infection of NTHi is poorly understood. Using the Junbo mouse, a characterised NTHi infection model, we analysed the cellular content of MEF and correlated the data with NTHi titres. The MEF of the Junbo mouse was heterogeneous between ears and was graded from 1 to 5; 1 being highly serous/clear and 5 being heavily viscous/opaque. At seven-day post-intranasal inoculation, NTHi was not found in grade-1 or 2 fluids, and the proportion of MEF that supported NTHi increased with the grade. Analyses by flow cytometry indicated that the cellular content was highest in grade-4 and 5 fluids, with a greater proportion of necrotic cells and a low-live cell count. NTHi infection of the middle ear increased the cell count and led to infiltration of immune cells and changes in the cytokine and chemokine levels. Following NTHi inoculation, high-grade infected MEFs had greater neutrophil infiltration whereas monocyte infiltration was significantly higher in serous noninfected low-grade fluids. These data underline a role for immune cells, specifically monocytes and neutrophils, and cell necrosis in NTHi infection of the Junbo mouse middle ear.

Intratympanic Steroid Treatments May Improve Hearing via Ion Homeostasis Alterations and Not Immune Suppression

HYPOTHESIS

The inner ear (IE) endothelium is capable of responding to therapeutic steroids by altering the local expression of cytokine and ion homeostasis genes that impact inflammation and fluid regulation.

BACKGROUND

Glucocorticoids are often given transtympanically for hearing disorders because of their anti-inflammatory effects, but their direct impact on IE ion homeostasis and cytokine gene expression has not been studied.

METHODS

The middle ears of Balb/c mice were transtympanically injected with 5 μL of phosphate-buffered saline, prednisolone (Pred), or dexamethasone (Dex). Untreated mice were used as controls. Mice were euthanized at 6, 24, and 72 hours; the cochleas were harvested; and total RNA was isolated from the IE tissues. Expression of eight cytokine genes and 24 ion homeostasis genes was analyzed with quantitative real time reverse transcription polymerase chain reaction.

RESULTS

Phosphate-buffered saline caused upregulation of inflammatory cytokine genes that peaked at 6 hours. Surprisingly, Pred and Dex also caused upregulation of most cytokine genes. Interestingly, ion homeostasis genes were predominantly upregulated with Dex and Pred, with Pred having a larger effect.

CONCLUSION

In the murine model, intratympanic steroids caused an initial upregulation of inflammatory cytokine genes in the IE, as well as predominant upregulation of ion homeostasis genes. These findings suggest that glucocorticoids do not suppress IE inflammation but rather cause an initial inflammatory response in the IE. Thus, inflammatory gene suppression is not a likely mechanism for their hearing restorative effects. On the other hand, these steroids have a significant mineralocorticoid function, as demonstrated by increased function of ion homeostasis genes, implicating their ionic and fluid regulatory properties as a mechanism for their therapeutic effects.