Activated tissue-resident macrophages contribute to hair cell insults in noise-induced hearing loss in mice.

Macrophages serve as the primary immune cell population and assume a pivotal role in the immune response within the damaged cochleae. Yet, the origin and role of macrophages in response to noise exposure remain controversial. Here, we take advantage of Ccr2 RFP/+ Cx3cr1 GFP/+ dual-reporter mice to identify the infiltrated and tissue-resident macrophages. After noise exposure, we reveal that activated resident macrophages change in morphology, increase in abundance, and migrate to the region of hair cells, leading to the loss of outer hair cells and the damage of ribbon synapses. Meanwhile, peripheral monocytes are not implicated in the noise-induced hair cell insults. These noise-induced activities of macrophages are abolished by inhibiting TLR4 signaling, resulting in alleviated insults of hair cells and partial recovery of hearing. Our findings indicate cochlear resident macrophages are pro-inflammatory and detrimental players in acoustic trauma and introduce a potential therapeutic target in noise-induced hearing loss.