Sulfoglucuronosyl glycolipids as putative antigens for autoimmune inner ear disease

The role of sulfoglucuronosyl glycosphingolipids in the pathogenesis of monoclonal IgM paraproteinemia and peripheral neuropathy

In IgM paraproteinemia and peripheral neuropathy, IgM M-protein secretion by B cells leads to a T helper cell response, suggesting that it is antibody-mediated autoimmune disease involving carbohydrate epitopes in myelin sheaths. An immune response against sulfoglucuronosyl glycosphingolipids (SGGLs) is presumed to participate in demyelination or axonal degeneration in the peripheral nervous system (PNS). SGGLs contain a 3-sulfoglucuronic acid residue that interacts with anti-myelin-associated glycoprotein (MAG) and the monoclonal antibody anti-HNK-1. Immunization of animals with sulfoglucuronosyl paragloboside (SGPG) induced anti-SGPG antibodies and sensory neuropathy, which closely resembles the human disease. These animal models might help to understand the disease mechanism and lead to more specific therapeutic strategies. In an in vitro study, destruction or malfunction of the blood-nerve barrier (BNB) was found, resulting in the leakage of circulating antibodies into the PNS parenchyma, which may be considered as the initial key step for development of disease.

Diagnostic tests for immunomediated hearing loss: a systematic review

OBJECTIVE

To quantitatively evaluate the diagnostic accuracy of diagnostic tests for immunomediated hearing loss.

DATA SOURCES

We searched Medline and the Cochrane Database of Systematic Reviews for potentially relevant studies.

STUDY SELECTION

Twenty-five studies met the inclusion criteria of this systematic review. The diagnosis of immunomediated hearing loss was based on the clinical presentation and the response to corticosteroid administration.

DATA EXTRACTION

The following data were extracted from the selected studies and entered into a standardised database: population demographics; exclusion and inclusion criteria; diagnostic tests; sensitivity; specificity; the number of true positive, true negative, false positive and false negative values; therapy used, including dose and duration; and delay between symptom onset and therapy commencement.

DATA SYNTHESIS

This systematic review combined data from 679 patients with immunomediated hearing loss, reported by 22 research teams. Substantial heterogeneity was found among the included studies; for this reason, summary sensitivity and specificity values were not computed.

CONCLUSIONS

The results of diagnostic tests for immunomediated hearing loss depend on many factors, and there is a risk of potential bias. This is the first time that such a systematic review has been presented; such a review is a more rigorous method of demonstrating the utility of the available diagnostic tests.

Autoimmune Mouse Antibodies Recognize Multiple Antigens Proposed in Human Immune-Mediated Hearing Loss

HYPOTHESIS

Autoimmune diseased mice with hearing loss will have autoantibodies against the various cochlear antigens proposed in clinical autoimmune inner ear disease.

BACKGROUND

Serum antibodies of patients with hearing loss recognize several proteins that are proposed as possible antigenic targets in the ear. This often leads to a clinical diagnosis of autoimmune inner ear disease, although it is not clear how these antibodies cause inner ear disease. Therefore, to better understand the relationship of autoantibodies and ear disease, an examination was made of serum autoantibodies in the MRL/MpJ-Fas(lpr) autoimmune mouse with hearing loss. Similar antibody patterns in the mouse would provide an animal model in which to investigate potential autoimmune mechanisms of this clinical ear disorder.

METHODS

Sera from MRL/MpJ-Fas(lpr) autoimmune mice and normal C3H mice were tested by the enzyme-linked immunosorbent assay technique for reactivity against various reported cochlear antigens: heat shock protein 70 (bovine, human, bacterial), laminin, heparan sulfate proteoglycan, cardiolipin, and collagen types II and IV.

RESULTS

The autoimmune mouse sera showed significantly greater antibody reactivity against all of the antigens when compared with normal mouse sera.

CONCLUSIONS

Serum antibodies from autoimmune mice recognized several putative autoantigens reported for patients with hearing loss, suggesting that comparable antigen-antibody mechanisms might be operating. However, the recognition of multiple antigens did not identify any one as being the specific target in autoimmune hearing loss. The correlation of antibodies in the MRL/MpJ-Fas(lpr) autoimmune mouse and human studies indicates this animal model should aid further investigations into potential cochlear antigens in autoimmune hearing loss.

Autoimmune sensorineural hearing loss

Autoimmune sensorineural hearing loss has been increasingly recognized as a clinical entity since its description by McCabe in 1979. Recognition and proper management of this condition is important, as it is one of the very few forms of sensorineural hearing loss that can be successfully treated by medical therapy. Recent studies have provided experimental evidence to suggest that immune processes can cause sensorineural hearing loss in animals and humans. However, antigenic targets within the inner ear are diverse and as a result conclusive evidence for specific autoimmune damage to the inner ear has been elusive. This review focuses on the recent progress in understanding of the aetio-pathogenesis of autoimmune hearing loss along with a description of the various clinical conditions in which they occur. Recent advances in the laboratory diagnosis and management of this interesting condition are also described.

Demyelination of Vestibular Nerve Axons in Unilateral Ménière's Disease

We conducted a study to determine whether vestibular nerves inpatients with unilateral Ménière's disease whose symptoms are refractory to medical management exhibit neuropathologic changes. We also endeavored to determine whether retrocochlear abnormalities are primary or secondary factors in the disease process. To these ends, we obtained vestibular nerve segments from five patients during retrosigmoid (posteriorfossa) neurectomy, immediately fixed them, and processed them for light and electron microscopy. We found that all five segments exhibited moderate to severe demyelination with axonal sparing. Moreover, we noted that reactive astrocytes produced an extensive proliferation of fibrous processes and that the microglia assumed a phagocytic role. We conclude that the possible etiologies of demyelination include viral and/or immune-mediated factors similar to those seen in other demyelinating diseases, such as multiple sclerosis and Guillain-Barré syndrome. Our findings suggest that some forms of Ménière's disease that are refractory to traditional medical management might be the result of retrocochlear pathology that affects the neuroglial portion of the vestibular nerve.

Recent studies on the roles of antiglycosphingolipids in the pathogenesis of neurological disorders

Evidence is mounting to suggest a causal role of humoral immunity arising from antiglycosphingolipid (GSL) antibodies in a variety of neurological disorders. These disorders include the demyelinating and axonal forms of Guillain-Barre syndrome, multifocal motor neuropathy, chronic inflammatory demyelinating polyradiculoneuropathy, and IgM paraproteinemia. Many claims have been made regarding other neurological disorders, which should be carefully scrutinized for their validity, based on several criteria proposed in this review. These criteria include 1) characterization of the causative antigens and immunoglobulins, 2) correlation of the pathological lesions and clinical manifestation of the antigens, 3) establishment of animal models using pure GSLs as the antigens, 4) immunopathogenic mechanisms of the neurodenerative process, 5) mechanisms for the malfunctioning of blood-nerve barrier and the ensuing leakage of circulating antibodies into peripheral nerve parenchyma, and 6) the roles of anti-GSL antibodies that may cause humorally mediated nerve dysfunction and injury as well as interference with ion channel function at the node of Ranvier, where carbohydrate epitopes are located. Finally, the origin of the anti-GSL antibodies is discussed in light of the recent circumstantial evidence pointing to a molecular mimicry mechanism with infectious agents. With a better understanding of the immunopathogenic mechanisms, it will then be possible to devise rational and effective diagnostic and therapeutic strategies for the treatment of these neurological disorders.

Autoimmune vestibulo-cochlear disorders

Autoimmune vestibulo-cochlear disorders (AVCD) represent a group of syndromes with overlapping clinical features, manifesting as sensorineural hearing loss, often associated with vertigo, tinnitus, and aural fullness, and believed to be caused by an autoimmune mechanism. Although definitive evidence of a classic "autoimmune process" is still lacking, substantial indirect evidence has accumulated to strongly indicate such a pathogenesis. Rapidly progressing AVCD is analogous to rapidly progressive glomerulonephritis in that inner ear inflammation progresses to severe, irreversible damage within 3 months of onset (and often much more quickly). Thus patients with rapidly progressive AVCD are treated with a sense of urgency. Prompt treatment with corticosteroids and other antirheumatic/immunosuppressive agents can preserve hearing and vestibular functions. We are not aware of any randomized controlled clinical trials evaluating the efficacy of antirheumatic/immunosuppressive agents in AVCD. In this article we review reports of various therapies that have been tried in this condition and our experience of etanercept therapy in AVCD.

Autoimmune inner ear disorders

There is considerable evidence to suggest that hearing and vestibular function can be influenced by autoimmune processes. A number of systemic autoimmune disorders include hearing loss and vertigo as part of their constellation of symptoms. Although classic evidence for a specific autoimmune etiology mediated by immune response directed solely at the inner ear is elusive, it appears that autoimmune damage can also exist as an entity confined to the labyrinth. Antigenic targets of autoimmunity within the labyrinth seem to be diverse. Partly because of this, the condition is difficult to diagnose. However, autoimmune disorders of the inner ear are of special interest since they are among the few forms of hearing loss that are amenable to medical treatment. Recent progress in understanding the etiology, diagnosis and treatment of autoimmune damage to the inner ear is reviewed.

Identification of beta-actin as a candidate autoantigen in autoimmune inner ear disease

It has been shown that sera from patients with autoimmune inner ear disease contain antibodies to several inner ear antigens. We report here the characterization of the 42-43 kDa protein against which a significant number of patients' sera react strongly. After separation of inner ear proteins from guinea-pig cochleas by SDS-PAGE, the band corresponding to the 42-43 kDa protein was digested with trypsin and the peptide fragments were separated by high-performance liquid chromatography. Two fractions were then subjected to amino acid sequencing by the classical automated Edman degradation. The sequence of a stretch of 15 amino acids of the first fragment was identical to that of amino acids 148-162 of beta-actin. The sequence of the 10 amino acids of the second fragment was also identical to beta-actin. On Western blots, monoclonal antibody directed against beta-actin reacted with the inner ear 42-43 kDa proteins. The serum samples from the patients and the monoclonal antibody reacted with the non-muscle actin used as antigen in Western blotting. Immunoblot analysis of inner ear proteins after two-dimensional gel electrophoresis showed a spot, corresponding to the region of the 43 kDa as compared to the protein standards. On the basis of these data it is concluded that the target 42-43 kDa protein for antibodies in sera of patients with autoimmune inner ear disease is beta-actin, a molecule, which has important and numerous functions inside cells. This is the first report to identify the cytoskeletal protein beta-actin as a candidate autoantigen in autoimmune inner ear disease.

Autoimmune inner ear disease

The known etiologies of acquired sensorineural hearing loss include acoustic trauma, physical trauma, ototoxicity, genetic predisposition, infections, Meniere's disease, aging, and autoimmune disease. Treatments are directed at eliminating or managing the underlying disease process and aiding hearing with amplification. Rarely is it possible to improve unaided hearing after sensorineural loss except when the severe to profound level of loss is reached and cochlear implantation becomes an option. Autoimmune inner ear disease, however, is a treatable cause of sensorineural hearing loss and it is important for physicians and hearing health professionals to recognize that proper early diagnosis and management strategies may result in stabilization and possibly improvement in hearing.