Contribution of macrophages to neural survival and intracochlear tissue remodeling responses following cochlear implantation

Background: Cochlear implants (CIs) restore hearing to deafened patients. The foreign body response (FBR) following cochlear implantation (post-CI) comprises an infiltration of macrophages, other immune and non-immune cells, and fibrosis into the scala tympani, a space that is normally devoid of cells. This FBR is associated with negative effects on CI outcomes including increased electrode impedances and loss of residual acoustic hearing. This study investigates the extent to which macrophage depletion by an orally administered CSF-1R specific kinase (c-FMS) inhibitor, PLX-5622, modulates the tissue response to CI and neural health. Main text: 10- to 12-week-old CX3CR1 + /GFP Thy1 + /YFP mice on C57BL/6J/B6 background was fed chow containing 1200 mg/kg PLX5622 or control chow for the duration of the study. 7 days after starting the diet, 3-channel cochlear implants were implanted in the ear via the round window. Serial impedance and neural response telemetry (NRT) measurements were acquired throughout the study. Electric stimulation began 7 days post-CI until 28 days post-CI for 5 h/day, 5 days/week, with programming guided by NRT and behavioral responses. Cochleae harvested at 10, 28 or 56 days post-CI were cryosectioned and labeled with an antibody against α-smooth muscle actin (α-SMA) to identify myofibroblasts and quantify the fibrotic response. Using IMARIS image analysis software, the outlines of scala tympani, Rosenthal canal, modiolus, and lateral wall for each turn were traced manually to measure region volume. The density of nuclei, CX3CR1 + macrophages, Thy1 + spiral ganglion neuron (SGN) numbers, and the ratio of the α-SMA + volume/scala tympani volume were calculated. Cochlear implantation in control diet subjects caused infiltration of cells, including macrophages, into the cochlea. Fibrosis was evident in the scala tympani adjacent to the electrode array. Mice fed PLX5622 chow showed reduced macrophage infiltration throughout the implanted cochleae across all time points. However, scala tympani fibrosis was not reduced relative to control diet subjects. Further, mice treated with PLX5622 showed increased electrode impedances compared to controls. Finally, treatment with PLX5622 decreased SGN survival in implanted and contralateral cochleae. Conclusion: The data suggest that macrophages play an important role in modulating the intracochlear tissue response following CI and neural survival.