Adaptive immune responses are not causal to SGN death after kanamycin-induced hair cell loss

Aminoglycoside antibiotics such as kanamycin induce sensorineural hearing loss by killing hair cells, resulting in secondary degeneration of spiral ganglion neurons (SGNs). Previous studies show that anti-inflammatory agents reduce SGN death, implicating a causal role of the immune response. This is consistent with observations of increased numbers of macrophages and lymphocytes, including T and NK cells, in the spiral ganglion after exposure to aminoglycosides. Here, we directly test the role of T cells and other lymphocytes in SGN degeneration in kanamycin-deafened rats. Homozygous RNU nude rats that lack T cells – but retain NK and B cells– show neurodegeneration similar to rats with a normal T cell complement, indicating that T cells are not necessary for neurodegeneration. Homozygous SRG rats lacking all lymphocytes (i.e., T, B, and NK cell-deficient), exhibit remarkable regional variation in the pattern of spiral ganglion degeneration post-deafening. In the basal half of the ganglion, SGN degeneration is significantly reduced in deafened SRG rats, implying a role for lymphocytes, presumably NK cells of the innate immune system, in SGN death. In the apical half of the deafened ganglion, SGN degeneration is not significantly affected by the lack of all lymphocytes, implying a role for other cellular mechanisms.