Local Macrophage-Related Immune Response Is Involved in Cochlear Epithelial Damage in Distinct Gjb2-Related Hereditary Deafness Models.

Kai Xu,Xiao-Zhou Liu,Le Xie,Yue Qiu,Wei-Jia Kong,Hui-Min Zhang,Xue Bai,Xiao-Hui Wang,Yuan Jin,Yu Sun,Sen Chen

doi:10.3389/fcell.2020.597769

Abstract

The macrophage-related immune response is an important component of the cochlear response to different exogenous stresses, including noise, ototoxic antibiotics, toxins, or viral infection. However, the role of the immune response in hereditary deafness caused by genetic mutations is rarely explored. GJB2, encoding connexin 26 (Cx26), is the most common deafness gene of hereditary deafness. In this study, two distinct Cx26-null mouse models were established to investigate the types and underlying mechanisms of immune responses. In a systemic Cx26-null model, macrophage recruitment was observed, associated with extensive cell degeneration of the cochlear epithelium. In a targeted-cell Cx26-null model, knockout of Cx26 was restricted to specific supporting cells (SCs), which led to preferential loss of local outer hair cells (OHCs). This local OHC loss can also induce a macrophage-related immune response. Common inflammatory factors, including TNF-α, IL-1β, Icam-1, Mif, Cx3cr1, Tlr4, Ccl2, and Ccr2, did not change significantly, while mRNA of Cx3cl1 was upregulated. Quantitative immunofluorescence showed that the protein expression of CX3CL1 in Deiters cells, a type of SC coupled with OHCs, increased significantly after OHC death. OHC loss caused the secondary death of spiral ganglion neurons (SGNs), while the remaining SGNs expressed high levels of CX3CL1 with infiltrated macrophages. Taken together, our results indicate that CX3CL1 signaling regulates macrophage recruitment and that enhancement of macrophage antigen-presenting function is associated with cell degeneration in Cx26-null mice.

Highlights

The inner ear has been recognized as an “immune privileged” organ, similar to the eyes or brain
Our results indicate that CX3CL1 signaling regulates macrophage recruitment and that enhancement of macrophage antigen-presenting function is associated with cell degeneration in connexin 26 (Cx26)-null mice
Quantification of the results (n = 4 mice in each group) showed that there was no significant difference in the number of CD45+ cells in all turns of the basilar membrane (BM) or the osseous spiral lamina (OSL) between the control and systemic Cx26-null groups (Figures 1H,I)

Summary

Introduction

The inner ear has been recognized as an “immune privileged” organ, similar to the eyes or brain. There is increasing evidence that immune responses are involved in the inner ear damage process induced by various exogenous stresses, including noise, ototoxic antibiotics, toxins, or viral infection (Fredelius and Rask-Andersen, 1990; Keithley and Harris, 1996; Ladrech et al, 2007; Fujioka et al, 2014; Kaur et al, 2015a). During these cochlear damage processes, macrophages/monocytes are the major immune cells contributing to cochlear immune responses (Hirose et al, 2005, 2014; Sato et al, 2010; Kaur et al, 2015b). Treatments targeting inflammation can effectively alleviate the different types of inner ear damage that have involved an immune response (Canlon et al, 2007; Wakabayashi et al, 2010; Arpornchayanon et al, 2013)

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