Basic Inner Ear Immunology

Due to substantial research efforts, it is now well accepted that the inner ear is fully capable of generating an immune response and that this immune response can be destructive to the

Autoimmune Inner Ear Disease TABLE 1 Epidemiology of AIED

Investigator

Year Number of patients Mean age (range) Women Men

Hughes et al. (3) Moscicki et al. (4) Rauch (5) Sismanis et al. (6) Lasak et al. (7) Harris et al. (8) Broughton et al. (9) Loveman (10) Cohen etal. (11) Matteson et al. (12)

1988 1994 1997 1997 2001

2003

2004

2004

2005 2005

42 30 20 23

52 72 61 25 62

34 28

29 13

30 53 20 25 12 12

Total

Abbreviation: AIED, autoimmune inner ear disease.

delicate inner ear tissues. In 1982, Harris injected keyhole limpet hemocyanin (KLH) systemically and into the inner ears of guinea pigs. Measurements of antibodies within the serum, perilymph, and cerebrospinal fluids were quantified using radioimmunoassay. When intradermal KLH immunization was performed, antibodies to KLH were identified in the serum and, to a lesser extent, in the perilymph. When KLH was perfused into the inner ear, serum antibodies were once again detected, but the perilymphatic levels were found in a much greater concentration compared to the intradermal injections. The contralateral inner ear had the same amount of antibodies within the perilymph as did the injected ear. This experiment proved that the inner ear is able to respond to an antigen challenge and that it can produce a local as well as a systemic response to the antigen (13,14).

Further research has elucidated the basic steps in this inner ear response. Once an antigen enters the inner ear, it must be identified and processed by immunocompetent cells to initiate the cascade of events in the immune response. In the inner ear, the endolymphatic sac has been implicated as the main antigen-processing site, with blockage or destruction of the endolymphatic sac resulting in a decreased local and systemic immune response following inner ear antigen challenges (15,16). Once the antigen has been processed by cells within and around the endolymphatic sac, these immunocompetent cells release cytokines in order to upregulate the immune response. Of these cytokines, interleukin-1 (IL-1) and interleukin-2 (IL-2) have been identified early in the cascade of events within the inner ear. IL-1 is released by macrophages when they are activated. IL-1 then causes the release of IL-2 by T helper cells. IL-2 has widespread effects and has been shown to activate T cells, B cells, polymorphonuclear cells, monocytes, and lymphocytes. IL-2 is not found within the inner ear in its resting state, but upon antigen exposure, it can be identified as early as 6 hours after exposure, peaking at 18 hours with slow diminution over the ensuing 5 days (17).

Other cytokines released early in the inner ear immune response, such as tumor necrosis factor-a (TNF-a), have been implicated in the upregulation of intercelluar adhesion molecule-1 (ICAM-1) receptors along the spiral modiolar vein, turning this vein into a high endothelial venule (18-20). The spiral modiolar vein has been identified as the primary site for influx of immunocompetent cells into the inner ear from the systemic circulation (21).

In the inner ear's immune response, the first cells to arrive are the polymorpho-nuclear cells, followed by T cells and B cells. Antibodies specific to the antigen are released as a relatively late immune response (22). Concurrent with the ingress of immunocompetent cells into the inner ear is the production of an extracellular matrix likely produced by fibrocytes within the inner ear. This extracellular matrix often results in ossification, as the inner ear has an extremely limited capacity to clear this matrix. Ossification of the cochlea invariably results in profound hearing loss (23).

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