Inner ear immunity

The inner ear, like all organs, interacts with the systemic immune system via lymphatic drainage and vascular circulation to protect itself from infections and stress such as acoustic trauma. The adult mammalian inner ear including the endolymphatic sac is populated with bone-marrow derived resident macrophages. Circulating macrophages continually renew the resident macrophage population. Cells within the endolymphatic sac participate in and affect inner ear immune responses, but specific mechanisms for the interactions are unknown. Resident macrophages are present within the cochlear modiolus, spiral ligament, stria vascularis, on the scala tympani surface of the basilar membrane and in the vestibular ganglia and connective tissue of the vestibular sensory epithelia. In general, the mammalian organ of Corti, on the other hand, does not contain resident macrophages. Although repair of the epithelium following hair cell death is performed by adjacent supporting cells, macrophages in the osseous spiral lamina have been seen to extend processes into the organ of Corti below the inner hair cells where they may assist in reducing synaptopathy. Systemic and middle ear bacterial infections, experimentally simulated by lipopolysaccharide (LPS) injections, cause circulating inflammatory cells to enter the inner ear from venules in the spiral ligament and modiolus. Presumably, this is a surveillance mechanism, and in the absence of cochlear infection, no action is taken, but if noise trauma or ototoxic drug exposure occurs simultaneously, a more aggressive immune response is mounted. Acoustic trauma alone induces influx of circulating immune cells. Vigorous immune responses to pathogens within the cochlea result in fibrotic tissue and osteoid formation within the fluid-filled inner ear spaces. Many of the signals for recruiting and activating immune cells have been identified, but little is known about exactly what the activated cells do, how they interact with resident macrophages and what signals terminate their activity.