MRL/MP-lpr/lpr mouse as a model of immune-induced sensorineural hearing loss

The role of the stria vascularis in neglected otologic disease

The stria vascularis (SV) has been shown to play a critical role in the pathogenesis of many diseases associated with sensorineural hearing loss (SNHL), including age-related hearing loss (ARHL), noise-induced hearing loss (NIHL), hereditary hearing loss (HHL), and drug-induced hearing loss (DIHL), among others. There are a number of other disorders of hearing loss that may be relatively neglected due to being underrecognized, poorly understood, lacking robust diagnostic criteria or effective treatments. A few examples of these diseases include autoimmune inner ear disease (AIED) and/or autoinflammatory inner ear disease (AID), Meniere's disease (MD), sudden sensorineural hearing loss (SSNHL), and cytomegalovirus (CMV)-related hearing loss (CRHL). Although these diseases may often differ in etiology, there have been recent studies that support the involvement of the SV in the pathogenesis of many of these disorders. We strive to highlight a few prominent examples of these frequently neglected otologic diseases and illustrate the relevance of understanding SV composition, structure and function with regards to these disease processes. In this study, we review the physiology of the SV, lay out the importance of these neglected otologic diseases, highlight the current literature regarding the role of the SV in these disorders, and discuss the current strategies, both approved and investigational, for management of these disorders.

Role of the Stria Vascularis in the Pathogenesis of Sensorineural Hearing Loss: A Narrative Review

Sensorineural hearing loss is a common sensory impairment in humans caused by abnormalities in the inner ear. The stria vascularis is regarded as a major cochlear structure that can independently degenerate and influence the degree of hearing loss. This review summarizes the current literature on the role of the stria vascularis in the pathogenesis of sensorineural hearing loss resulting from different etiologies, focusing on both molecular events and signaling pathways, and further attempts to explore the underlying mechanisms at the cellular and molecular biological levels. In addition, the deficiencies and limitations of this field are discussed. With the rapid progress in scientific technology, new opportunities are arising to fully understand the role of the stria vascularis in the pathogenesis of sensorineural hearing loss, which, in the future, will hopefully lead to the prevention, early diagnosis, and improved treatment of sensorineural hearing loss.

Methods for Testing Immunological Factors

Hypersensitivity reactions can be elicited by various factors: either immunologically induced, i.e., allergic reactions to natural or synthetic compounds mediated by IgE, or non-immunologically induced, i.e., activation of mediator release from cells through direct contact, without the induction of, or the mediation through immune responses. Mediators responsible for hypersensitivity reactions are released from mast cells. An important preformed mediator of allergic reactions found in these cells is histamine. Specific allergens or the calcium ionophore 48/80 induce release of histamine from mast cells. The histamine concentration can be determined with the o-phthalaldehyde reaction.

Temporal bone histopathology and immunoglobulin deposition in Sjogren's syndrome

HYPOTHESIS

The histopathology of Sjogren's syndrome (SS) in the human inner ear correlates with mouse models of autoimmune inner ear disease.

BACKGROUND

SS is an autoimmune disease in which 25% of patients have sensorineural hearing loss (SNHL). The inner ear histology in a SS mouse model has shown degeneration of the stria vascularis (SV) and immunoglobulin G deposition on the basement membrane of SV blood vessels. Correlation with human temporal bone histopathology has not been addressed.

METHODS

The histopathology and immunohistochemistry of the inner ear in 4 patients with SS is described and compared with SS mouse models.

RESULTS

The histopathology of the inner ear in 3 patients with SS and SNHL showed severe loss of the intermediate cells of the SV and immunoglobulin G deposition on the basement membrane of SV blood vessels. These results parallel those of known SS mouse models. Additionally, there was shrinkage of the spiral ganglia neurons in 2 patients, whereas vestibular ganglia neurons were preserved. The fourth patient with SS and normal hearing showed only mild SV atrophy.

CONCLUSION

This is the first study describing the pathologic changes in the inner ear of 4 patients with SS. The 3 SS specimens with SNHL showed pathologic changes in the SV similar to the mouse model of autoimmune inner ear disease. Additionally, we propose that spiral ganglia neurons may be directly affected by SS pathology. These results highlight the importance of correlating the histopathology of human temporal bones with animal models to better understand inner ear disease in future research.

Immunosuppressive therapy for autoimmune inner ear disease

Autoimmune inner ear disease (AIED) is a rare disease that is diagnosed after clinical suspicion and response to corticosteroids. AIED manifests as progressive, bilateral, although often asynchronous, sensorineural hearing loss and can be associated with vestibular symptoms. Since its description as a defined disease entity in 1979, the initial mainstay of treatment remains high-dose corticosteroids. Several animal models have been developed to assist in determining efficacy of immunosuppression in AIED, and several clinical studies have also investigated the role of both steroid and steroid-sparing treatments. Here we discuss the basic science and clinical research surrounding the history of immunosuppressive therapy in AIED.

Impaired CD4+CD25+ regulatory T cell activity in the peripheral blood of patients with autoimmune sensorineural hearing loss

CD4+CD25+ regulatory T cells exert an immune regulatory function and thus play an important role in the control of self-reactivity in the pathogenesis of autoimmune inflammatory conditions. The aim of the study presented here is to perform a quantitative and functional analyses of these cells in patients with autoimmune sensorineural hearing loss (ASNHL). T cell subsets (CD4+CD25+, CD4+CD25(high), CD4+, and CD8+) from the peripheral blood of 17 patients with ASNHL, 16 patients with noise induced hearing loss (NHL), and 100 normal controls were analyzed by flow cytometry. The CD4/CD8 ratio was also analyzed. In addition, the suppressive capability of CD4+CD25+ T cells was tested in vitro by measuring their ability to suppress the proliferation and IFN-gamma secretion of CD4+CD25- T cells. No significant difference was found in the T cell subsets of ASNHL patients compared to normal controls or NHL patients, except that the proportion of CD4+ T cells was elevated in ASNHL patients. However, we did observe defective regulatory function of CD4+CD25+ T cells in patients with ASNHL. Our data supported the idea that CD4+CD25+ regulatory T cells played an immunosuppressive function in the periphery. The impaired suppressive activity of these cells may be an important factor in the pathogenesis of ASNHL.

Bone marrow transplantation as a strategy for the treatment of autoimmune hearing loss in MRL/Mp-lpr/lpr mice

Sensorineural hearing loss (SNHL) has been reported to develop as a main part of or in combination with systemic and organ-specific autoimmune diseases. The aim of the current study is to treat autoimmune SNHL in MRL/Mp-lpr/lpr (MRL/lpr) mice, a murine model of systemic autoimmune disease, using allogeneic bone marrow transplantation (BMT), which replaces recipient bone marrow cells with bone marrow cells from a non-autoimmune-prone donor. The results indicate that BMT can be used to treat SNHL; cochlear pathology, serum autoantibodies and lupus nephritis are ameliorated. Therefore, it is conceivable that the autoimmune SNHL in the MRL/lpr mice results not from defects in the cochlea, including the stria vascularis, but from defects in the bone marrow, and BMT would therefore provide a curative effect on inner ear autoimmune dysfunction associated with systemic autoimmune diseases.

Murine autoimmune hearing loss mediated by CD4+ T cells specific for inner ear peptides

Autoimmune sensorineural hearing loss (ASNHL) is characterized typically by bilateral, rapidly progressive hearing loss that responds therapeutically to corticosteroid treatment. Despite its name, data implicating autoimmunity in the etiopathogenesis of ASNHL have been limited, and targeted self-antigens have not been identified. In the current study we show that the inner ear-specific proteins cochlin and beta-tectorin are capable of targeting experimental autoimmune hearing loss (EAHL) in mice. Five weeks after immunization of SWXJ mice with either Coch 131-150 or beta-tectorin 71-90, auditory brainstem responses (ABR) showed significant hearing loss at all frequencies tested. Flow cytometry analysis showed that each peptide selectively activated CD4(+) T cells with a proinflammatory Th1-like phenotype. T cell mediation of EAHL was determined by showing significantly increased ABR thresholds 6 weeks after adoptive transfer of peptide-activated CD4(+) T cells into naive SWXJ recipients. Immunocytochemical analysis showed that leukocytic infiltration of inner ear tissues coincided with onset of hearing loss. Our study provides a contemporary mouse model for clarifying our understanding of ASNHL and facilitating the development of novel effective treatments for this clinical entity. Moreover, our data provide experimental confirmation that ASNHL may be a T cell-mediated organ-specific autoimmune disorder of the inner ear.

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.

[Immuno-pathogenesis in Meniere's disease]

Since the report of Duke in which an allergic etiology was considered to be the cause of Meniere's disease, the hypothesis that a certain type of Meniere's disease is generated through immuno-pathological mechanisms has been advocated for 70 years. During this period, another entity of immune-mediated inner ear disorders, i. e., autoimmune inner ear disease was introduced. Fundamental immunological phenomena of the inner ear have been rapidly elucidated since 1980. The endolymphatic sac is the only site which contains immuno-competent cells within the inner ear. The inner ear is capable of mounting active immune responses when appropriately stimulated and the endolymphatic sac plays an integral function for inner ear immune response. Actually, many reports have been published that link immunity and Meniere's disease with a variety of proposed immune-related etiologies from autoimmunity to non-autoimmunity. It is suggested that immune injury to the endolymphatic sac plays an important role in the pathogenesis of Meniere's disease. These functional and morphological circumstances strongly suggest that an immunological etiology of Meniere's disease is not theoretically unfounded.

Autoimmune sensorineural hearing loss: an immunologic perspective

Autoimmune sensorineural hearing loss (ASNHL) typically produces a bilateral rapidly progressive loss of hearing that may occur suddenly. The diagnosis is made by excluding ototoxicity, systemic disease, and other factors that mimic ASNHL and by showing a therapeutic response to corticosteroid treatment. Although autoantibodies and autoreactive T cells have been implicated in the etiopathogenesis of ASNHL, several central issues remain unresolved, including the relative prominence of B cell or T cell autoimmunity in the initiation and progression of ASNHL, the identity of the putative inner ear self-antigen(s) that target ASNHL, and the development and application of immunosuppressive therapies for preventing the progressive hearing loss which may be profound and require cochlear implantation. In this review, we will examine the seminal human and animal studies that have led to our current views regarding the autoimmune etiopathogenesis of ASNHL. In addition, we will address the need for developing an inner ear-specific mouse model for ASNHL that may define the stages leading to the development of ASNHL and may also provide new diagnostic markers and help develop novel and effective treatments for preventing progressive hearing loss in ASNHL.

Dystroglycan expression in the mouse cochlea

Viable dominant spotting (W(v)/W(v)) mice have a c-kit gene mutation, which impedes the migration of neural crest cells to the developing cochlea where they normally differentiate into intermediate cells (ICs). A prominent pathological feature shared by these mutants and the aging human and gerbil cochlea is thickening of the basement membrane (BM) of strial capillaries. Atrophy of strial capillaries in the aging gerbil has been associated with changes in the expression of dystroglycan (DG), a cell-surface receptor that regulates BM assembly. Here we evaluated the expression of DG in W(v)/W(v) mutant and C57BL/6J wild-type mice to investigate the possible role of ICs in regulating strial capillary BM homeostasis. The DG gene product was identified in lateral wall dissections from both W(v)/W(v) mutant and wild-type mice by reverse transcription-polymerase chain reaction. Subunit-specific antibodies were employed to localize the alpha and beta subunits of the DG heterodimer. Some sites in both wild-type and mutant mice, such as the subepithelial BM lining the scala media and regions of contact between selected epithelial cells, expressed alpha-DG alone. Other sites such as the perineural BM and the perivascular BM subtending strial capillaries and capillaries in the central portion of the auditory nerve coexpressed alpha- and beta-DG. The strong diffuse staining for alpha-DG along the basolateral membrane of strial marginal cells disappeared with advancing strial degeneration in abnormal turns of W(v)/W(v) mutants. Variations in staining intensity for both alpha- and beta-DG also occurred in the subendothelial BM of strial capillaries in turns lacking ICs and appeared to correspond with the degree of capillary atrophy. The results support the possibility that ICs play a role in the homeostasis of the strial capillary BM.

Ultrastructural and physiological defects in the cochlea of the Mpv17 mouse strain

Ultrastructural investigations were performed in young (approximately 2 months) and old (7 months) Mpv17-negative and wild-type mice. The onset, the severity and the pattern of the degeneration significantly differed between both mice strains. In the wild-type mouse strain the degenerative changes of the cochlear structures were similar to the aging pattern described for other species. In contrast, the Mpv17 mutants showed degenerative changes of the cochlear structures already at the age of 2 months. The degenerative changes were patchy arranged throughout the entire length of the cochlea and involved the organ of Corti as well as the stria vascularis epithelia with alterations of the basement membrane of the capillaries. The severe sensorineural hearing loss and degenerative changes of the cochlear structures indicate that cochlear structures, especially the outer hair cells and the intermediate cells of the stria vascularis, are vulnerable to the missing Mpv17 gene product.

Prevention of autoimmune hearing loss in MRL/lpr mice by bone marrow transplantation

We examined the effects of bone marrow transplantation (BMT) on immune-mediated inner ear diseases in MRL/Mp-lpr/lpr (MRL/lpr) mice, which manifest not only lupus nephritis but also sensorineural hearing loss (SNHL) at the age of 20 weeks. These mice were treated with cyclophosphamide (CY) and irradiation (5 Gy x 2), followed by the transplantation of bones plus bone marrow cells from allogeneic normal C57BL/6 mice at the age of 12 weeks. Hematolymphoid cells were reconstituted with donor-derived cells 3 months after BMT. Thus-treated MRL/lpr mice showed neither lupus nephritis nor SNHL even 24 weeks after BMT. No pathological findings were observed in either glomeruli or cochleae. These findings suggest that BMT can be used to prevent the development of autoimmune SNHL in MRL/lpr mice.

Steroid treatment in young MRL.MpJ-Fas(lpr) autoimmune mice prevents cochlear dysfunction

Corticosteroid therapy reverses clinical autoimmune sensorineural hearing loss, although little is known of how steroids restore normal auditory function. If suppression of systemic autoimmune processes underlies hearing restoration, then preventing autoimmune symptoms from developing should prevent cochlear dysfunction. MRL. MpJ-Fas(lpr) autoimmune mice were used to test this potential mechanism by initiating oral prednisolone treatment at 6 weeks of age, prior to autoimmune disease and hearing loss onset. The steroid treatment group was given prednisolone in their drinking water, while untreated controls were given tap water. Treatment continued for 7 months with periodic evaluations of cochlear function with auditory brainstem response (ABR) audiometry. Autoimmune mice given the steroid lived longer and did not develop levels of serum immune complexes seen in their untreated controls. Also, their ABR thresholds remained near normal throughout the 7 months of treatment, while untreated controls showed progressive threshold elevations typical for autoimmune disease. This correlation of suppressed systemic autoimmune activity and maintenance of normal cochlear function identifies one potential mechanism for autoimmune hearing loss and hearing restoration with steroid therapy. The autoimmune mouse should serve as a valuable model for future studies of the cochlear mechanisms responsive to steroid treatment in autoimmune hearing loss.

Steroid treatment improves cochlear function in the MRL.MpJ-Fas(lpr) autoimmune mouse

Corticosteroid therapy is used to reverse autoimmune sensorineural hearing loss, although little is known of the mechanism by which this occurs. This has been due to the lack of a suitable animal model with spontaneous hearing loss that is steroid responsive. The present study examined the effects of prednisolone treatment on auditory thresholds in the MRL.MpJ-Fas(lpr) autoimmune mouse to determine its suitability as such a model. Autoimmune mice at 3.5-4. 5 months of age were evaluated by pure-tone auditory brainstem response (ABR) to establish threshold elevations due to the disease. The steroid treatment group was then given prednisolone in their drinking water for 2.5 months, while untreated controls were given tap water. Significantly more steroid treated mice survived to the time of post-treatment ABR evaluation. Half of the steroid treated ears demonstrated either improvement or no change in cochlear function compared to only 25% in the untreated controls. Overall, cochlear thresholds in the untreated controls increased by 14.7 dB, whereas no significant threshold increase was seen in the steroid treated group (4.3 dB) over the treatment period. No qualitative anatomical differences were seen in the ears of those mice surviving to the end of the study. These findings establish the autoimmune mouse as a model for studies of steroid responsive mechanisms within the ear. This could apply to autoimmune sensorineural hearing loss, as well as any hearing disorder for which steroid therapy is recommended.

Strial dysfunction in the MRL-Fas mouse

The MRL-Fas(lpr) mouse, a model of multisystemic autoimmune disease, has been proposed as a potential model of autoimmune inner ear disease. Cochlear pathology, consisting of hydropic degeneration of the stria vascularis, has been documented to occur coincident with the establishment of systemic disease in this animal. Because the cochlear pathology is restricted to the stria, this study was designed to evaluate whether the endocochlear potential (EP) would be diminished in these animals because of a loss in strial Na, K-ATPase. Experimental (MRL-Fas(lpr)) mice, with established systemic disease, had auditory brain stem response thresholds and EPs recorded. MRL-+/+ mice served as controls. Animals were then euthanized, and their cochleas were processed for immunohistologic assay for the alpha1 and beta2 subunits of Na,K-ATPase. Density of staining was evaluated by use of quantitative means with densitometry image analysis of digitized images. MRL-Fas(lpr) mice revealed significant elevations in auditory brain stem response thresholds and reductions in EPs but no reductions in Na,K-ATPase levels, as evidenced by immunohistochemical assay. The reduction of EP likely occurs as a result of cellular degeneration within the stria vascularis and likely results from an abrogation of the strial perilymph/endolymph barrier and not from a reduction in strial Na, K-ATPase levels.

Ultrastructural pathology in the stria vascularis of the MRL-Fasl(lpr) mouse

The MRL-Fas(lpr) mouse, a model of multisystemic, organ nonspecific autoimmune disease, has been proposed as a model of immune-mediated inner ear disease. A preliminary study employing light microscopy indicated that it develops cochlear pathology that appeared most striking in the stria vascularis, where cells underwent edema and degeneration. However, other structures, including the inner and outer hair cells and the supporting cells, also appeared to display pathology. The current study analyzed cochlear ultrastructure using transmission electron microscopy to better delineate the cochlear lesions found in these animals. MRL-Fas(lpr) animals were allowed to develop systemic disease (20 weeks old) and then had auditory brainstem response (ABR) thresholds determined. Animals were then killed and their cochleas prepared for electron microscopy. Age-matched MRL-+/+ and BALB/c mice served as controls. Results indicated that MRL-Fas(lpr) mice demonstrated elevated ABR thresholds. In contrast to a preliminary report, the cochlear pathology was observed exclusively in the stria vascularis, where cells demonstrated hydropic degeneration. Strial capillary structure was normal as were the rest of the cellular cochlear constituents. No inflammatory infiltrate was noted. These studies confirm that the MRL-Fas(lpr) mouse develops cochlear abnormalities focused in the stria vascularis. Whether the mechanism of the cellular degeneration involves autoimmune, genetic, or uremic processes has yet to be determined.

Effects of immunosuppression on the development of cochlear disease in the MRL-Fas(lpr) mouse

OBJECTIVES

The MRL-Fas(lpr) mouse, an animal that spontaneously develops multisystemic autoimmune disease, has been proposed as model of immune-mediated inner ear disease. Previous studies revealed that this mouse manifested elevated auditory brainstem response thresholds, hydropic degeneration of strial cells, and antibody deposition within strial capillaries. As the etiology of the observed strial disease may be immune, genetic, or uremic, a study was designed to attempt to delineate between these possible etiologic factors.

STUDY DESIGN

Prospective, controlled animal study.

METHODS

Dexamethasone, which is known to suppress autoantibody production and glomerulonephritis in these animals, was administered systemically on a daily basis to experimental animals, beginning at 6 weeks of age. Control animals received no treatment. Animals were allowed to age, with control animals predictably manifesting systemic disease at 20 weeks of age, at which point all animals were sacrificed.

RESULTS

Animals receiving dexamethasone treatment manifested a significant reduction in serum immunoglobulin levels, lymphoid hyperplasia, and a significant improvement in the level of renal function. However, morphologic analysis revealed a persistence of strial disease despite the elimination of strial antibody deposition.

CONCLUSION

The results of this experiment support the hypothesis that genetic mechanisms may be responsible for the observed strial disease. Further studies are under way to confirm these findings.

Antibody deposition in the stria vascularis of the MRL-Fas(lpr) mouse

The MRL-Fas(lpr) mouse, a model of multisystemic, organ non-specific autoimmune disease, has been proposed as a model of immune-mediated inner ear disease. Preliminary studies indicate that it develops cochlear pathology focused in the stria vascularis including intracellular edema and degeneration which develops in the absence of an inflammatory infiltrate but in the presence of antibody deposition. It was thus hypothesized that the antibodies found in the stria were mediating a direct pathologic effect on this structure, without recruiting classical inflammatory mediators. It was further hypothesized that the antibodies deposited within the stria would be derived from the non-complement fixing isotypes and subclasses, which are known to be able to mediate direct pathologic effects on target tissues. This study utilized immunohistologic techniques to identify the antibody isotypes and subclasses deposited within the stria vascularis of the MRL-Fas(lpr) mouse. Results indicate that all antibody isotypes and subclasses can be identified within the stria vascularis in the absence of complement. Thus, antibody deposition was not restricted to non-complement fixing antibodies. While it is possible that antibodies are mediating direct pathologic effects within the stria, the non-specific nature of the antibody deposition may indicate that these antibodies are not responsible for the observed pathology. Rather, other mechanisms, such as metabolic and genetic etiologies, must also be considered.

Induction of immune-mediated hearing loss in SCID mice by injection of MRL/lpr mouse spleen cells

Induction of immune-mediated hearing loss in SCID mice by injection of MRL/lpr mouse spleen cells The MRL/lpr mouse, which has a mutation in the Fas gene encoding a cell-surface receptor for apoptosis, shows an accumulation of abnormal immunocompetent cells and SLE-like disease. It has recently been reported that this mouse also manifests sensorineural hearing loss (SHL) with cochlear pathology at 20 weeks of age. We examined the effects of injecting MRL/lpr spleen cells on the development of SHL in severe combined immunodeficient (SCID) mice, which originally develop neither SHL nor cochlear pathology. Immune-mediated SHL and cochlear pathology were, indeed, transferred to the SCID mice by the injection of spleen cells from the MRL/lpr mice. These findings suggest that cell-mediated immunity is involved in the development of SHL and cochlear pathology.

Human autoantibodies and monoclonal antibody KHRI-3 bind to a phylogenetically conserved inner-ear-supporting cell antigen

Autoimmunity is thought to be one cause of sensorineural hearing loss (SNHL). Sera from patients with rapidly progressive hearing loss have been shown to contain antibodies to a 68-kD protein in heterologous inner-ear tissue. Using guinea pig inner-ear tissue as the antigenic substrate and either Western blot or immunofluorescence (IF) or both, we tested sera from 74 patients suspected to have autoimmune hearing loss for inner-ear antibodies. Sera from 73 patients were tested by Western blot, and sera from 36 were tested by IF. Thirty-seven of 73 (51%) had antibody to a 68-70-kD protein by Western blot. Sera positive by IF stained supporting cells with a staining pattern like that previously observed with the KHRI-3 monoclonal antibody. There was concordance between Western blot and IF assays. Of 36 patients tested by both assays, 29/31 (94%) that were positive in Western blot were also positive by IF, three were negative by both tests, and two each were positive by one assay but negative by the other. Absorption of patient sera with human inner-ear tissue removed antibody reactivity to the guinea pig supporting cells, indicating that the antigen detected by the autoantibody is also present in the human inner ear. Absorption with an equal volume of white or red blood cells from the tissue donor did not remove the antibody reactivity to inner ear, showing that the absorption by inner-ear tissue is specific. Sera from three patients positive in both assays also stained a 68-70-kD inner-ear protein immunoprecipitated by the KHRI-3 monoclonal antibody, indicating that the monoclonal and human antibodies recognize the same antigen. The results support the hypothesis that patients with autoimmune sensorineural hearing loss produce autoantibodies to an inner-ear supporting cell antigen that is phylogenetically conserved and defined by the murine monoclonal antibody KHRI-3. Since KHRI-3 can induce hearing loss after infusion into the inner ear, it is likely that autoantibodies with the same antigenic target are also pathogenic in humans.

Cochlear Immunoglobulin in the C3H/lpr Mouse Model for Autoimmune Hearing Loss

Immunoglobulin staining was conducted in cochlear tissue from the C3H/ lpr autoimmune strain mouse to better understand the local immune processes underlying autoimmune inner ear disease. This mouse is a model for spontaneous systemic lupus erythematosus with coincident elevated cochlear thresholds. Cochleas were examined from C3H/ lpr mice at 2 months of age, before disease onset, and at 8 months of age, when systemic disease and hearing loss are manifested. Sections of these cochleas, along with cochlear sections from age-matched C3H/HeJ nonautoimmune controls, were immunocytochemically stained for IgG and IgM to identify areas of abnormal immunoglobulin activity. IgM immunoreactivity was similar in control and autoimmune cochlear tissue and did not appear to vary with disease progression. Staining was limited to the inside of capillaries in the stria vascularis and other areas within the cochlea. Similar staining patterns were seen in control animals stained for IgG. However, C3H/ lpr mice with autoimmune disease showed extensive IgG immunoreactivity spreading out from the stria vascularis capillaries into the extracapillary spaces. This increased permeability suggested that breakdown of the blood labyrinth barrier was coincident with systemic autoimmune disease.

Breakdown of Stria Vascularis Blood-Labyrinth Barrier in C3H/lpr Autoimmune Disease Mice

Sensorineural hearing loss related to autoimmune disease is a well-recognized condition, although the exact pathophysiologic mechanisms remain unclear. One current theory postulates immune complex-induced interference with blood-labyrinth barrier integrity in the stria vascularis. The C3H/ lpr autoimmune mouse was chosen to study the permeability of capillaries in the stria vascularis because this mouse model has demonstrated abnormalities of the stria vascularis and shifts in the auditory brain stem response threshold during active disease. C3H/ lpr mice with active disease were compared with younger mice without disease, as well as age-matched C3H/HeJ control mice. The mice were injected with the tracer ferritin and examined by transmission electron microscopy to evaluate the integrity of the capillary tight junctions in the stria vascularis. Four of five mice with active disease were noted to have extensive leakage of ferritin into the perivascular tissues. Neither the young, disease-free autoimmune mice nor the nonautoimmune control mice demonstrated vessel leakage. Thickening of the basement membrane was also noted in the diseased animals. The results imply that active disease leads to a breakdown in the blood-endolymph barrier, which could underlie the hearing loss accompanying autoimmune and other immune diseases.

Age-related thickening of basement membrane in stria vascularis capillaries

Ultrastructural examination was undertaken to investigate the pathogenesis of age-related atrophy of the stria vascularis (StV). Basement membrane (BM) thickness was increased in 65-85% of strial capillaries in gerbils aged 33 months or older and often exceeded by several-fold that observed in young controls. In an early stage of thickening the BM expanded slightly around the full capillary profile, after which nodular expansions of BM encircling slender cell processes were often observed at or near one or both poles of the elliptical vessel profile. As widening progressed, the BM consisted of 2-3 layers separated by cell processes in the nodules but fewer strata elsewhere. Association of slender processes of both endothelial cells and pericytes with focal thickening outside the process suggested their participation in genesis of the capillary lesion. In later stages of atrophy, pericytes degenerated and disappeared, while endothelial cells remained intact. Eventually, thick multilayered BM devoid of endothelial cells surrounded a narrow lumen occluded by debris. The age-related change in BM in the inner ear was confined to StV capillaries. Degenerative changes in StV epithelial cells occurred apparently as a secondary consequence of the capillary lesion. The pathologic alterations in marginal cells included extrusion of blebs from the luminal surface, separation and loss of basolateral interfoldings, alteration and depletion of mitochondria and nuclear pyknosis. At the end-stage of degeneration, the StV consisted of a simple or multiple layer of squamous cells lining the scala media.

Immunoglobulin deposition in thickened basement membranes of aging strial capillaries

The presence of immunoglobulins in the thickened basement membrane (BM) of aging strial capillaries was investigated as a possible indicator of autoimmunity in the genesis of atypical BM. Cochleas from young and old Mongolian gerbils raised in quiet were examined by immunostaining at the light microscopic level for IgG and IgM and for the BM components laminin (La) and type IV collagen (IV-C). Another age-graded series of cochleas was stained for IgG at the ultrastructural level. No immunoreactive IgG was detected in specimens from animals less than 6 months old. In contrast, 2 of 12 cochleas from 20- to 28-month-old gerbils and 11 of 20 cochleas from gerbils 30 months or older showed positive staining for IgG in strial capillary BM. IgM was not detected at any age. At the electron microscope level, no immunoreactive IgG was detected in the stria of cochleas younger than 30 months. However, labeling demonstrative of IgG was observed in the thickened BM of some strial capillaries in all six cochleas from gerbils older than 33 months. Lysosome-like granules in endothelial cells and the superiormost marginal cells also stained for content of IgG as did fibrillar material in edematous regions in the intrastrial space. In addition to showing accumulation of IgG, the findings confirm our prior demonstration of increased La deposition in the thickened strial capillary BM of all cochleas from old gerbils. The BM alterations appear confined to strial capillaries in old gerbils, since morphological observations and immunostaining for La and IgG failed to detect changes in BMs at any other site in a wide survey of aged gerbil organs including vessels in other regions of the affected cochleas. The results point more towards the development of an age-dependent permeability to IgG selectively in strial capillaries than to autoimmunity as an explanation of the IgG in BM.

In vivo binding and hearing loss after intracochlear infusion of KHRI-3 antibody

The IgG1 mouse monoclonal antibody (MAb) KHRI-3, binds to an antigen of 65-68 kDa expressed on inner ear supporting cells in guinea pigs. We previously showed [Nair et al. (1995) Monoclonal antibody induced hearing loss. Hear. Res. 83, 101-113] that mice carrying the KHRI-3 hybridoma develop high frequency hearing loss and loss of hair cells in the basal turn suggesting that this MAb causes immune-mediated sensorineural hearing loss. To evaluate the specificity of this effect, sterile KHRI-3 and control IgG1 preparations were infused directly into the guinea pig cochlea using Alzet mini-osmotic pumps. Assessments included: (1) hearing, measured by click auditory brain stem responses (ABRs); (2) in vivo antibody binding; and (3) the structural integrity of the organ of Corti. Nine animals were infused with KHRI-3 preparations and 5 controls were infused with control IgG1. Four guinea pigs given KHRI-3 developed 25-55 dB hearing loss. Control animals showed no difference from baseline. In vivo binding of KHRI-3 was detected in the organ of Corti in 6 of the 9 animals, including all 4 that had hearing loss. No staining was observed with control antibody. Confocal microscopy revealed that the in vivo KHRI-3 antibody binding pattern was identical to that obtained by incubating fixed tissue in vitro with KHRI-3. Histologic examination revealed an increased frequency of hair cell loss in KHRI-3 treated ears when compared to either the contralateral ears of the same guinea pigs or the IgG1 treated ears of control animals. The lesions in the infused ears of guinea pigs were scattered throughout the cochlea from base to apex. These experiments demonstrate the following points: (1) Antibodies can be chronically infused directly into the cochlea of living animals. (2) The KHRI-3 antibody binds to live supporting cells within the organ of Corti. (3) Infusion of an inner ear specific antibody affects auditory function. (4) The infusion of irrelevant antibody had no effect on the structure or function of the ear. This system provides an animal model for further studies of antibody-induced sensorineural hearing loss.

Inner ear DNA receptors in MRL/lpr autoimmune mice: potential 30 and 70 kDa link between autoimmune disease and hearing loss

Inner ear function and systemic autoimmune disease were evaluated in the MRL/lpr mouse to determine their relationship with alterations in cell surface DNA receptors of 28-30 and 68-70 kDa size. Auditory brainstem response thresholds in the autoimmune disease mice were significantly elevated as early as 2 months of age when compared to MRL/++ controls. Hearing thresholds continued to rise with progression of the disease, manifested as increasing spleen weights, antinuclear (anti-DNA) antibodies, and serum immune complexes. Cochlear membranous labyrinth cells in the autoimmune mice bound less DNA, suggesting the DNA receptors were abnormally occupied by circulating antibodies. Western blots of a murine T-cell line probed with autoimmune mouse sera demonstrated reactivity to 28-30 and 68-70 kDa proteins after disease onset. It is hypothesized that cell surface DNA binding molecules could be masked or down-regulated by circulating antibodies in autoimmune disease. This interference with DNA receptor activity may be occurring within the inner ear and underlie the cochlear dysfunction seen in autoimmune sensorineural hearing loss.

Increased laminin deposition in capillaries of the stria vascularis of quiet-aged gerbils

The distribution of laminin (LA) and type IV collagen (IV-C) in the gerbil inner ear was investigated by light and electron microscopic immunohistochemistry. Changes in protein expression were assessed from birth to old age to determine the relation of these constituents to maturation of the cochlea and development of presbyacusis. The distribution of LA paralleled that of IV-C during postnatal development, and both were visualized in the basement membrane (BM) of endothelial, epithelial and spiral ganglion cells in neonatal and young adult gerbils. Immunopositive BM underlying the stria vascularis disappeared at 8-12 days after birth coincident with the development and maturation of the strial capillaries. Immunoreactivity for LA afforded an index to the thickness of the BM and was found to increase with age only in the BM of strial capillaries. At 6 months of age, occasional strial capillaries in the apex of the cochlea showed thickening of the LA-positive BM. Abnormal deposition of LA in strial capillary BM spread to lower turns and increased in prevalence with advancing age, affecting apical and basal more than middle cochlear turns. Thickening of the capillary BM appeared to precede capillary obstruction which eventuated in complete strial atrophy. Staining for IV-C in the walls of the strial capillaries did not increase with age. The data show that LA and IV-C play important roles in postnatal development of the cochlea and that LA deposition increases with age only in the BM of strial capillaries.

Joint pathology and behavioral performance in autoimmune MRL-lpr Mice

Young autoimmune MRL-lpr mice perform more poorly than age-matched controls in tests of exploration, spatial learning, and emotional reactivity. Impaired behavioral performance coincides temporally with hyperproduction of autoantibodies, infiltration of lymphoid cells into the brain, and mild arthritic-like changes in hind paws. Although CNS mechanisms have been suggested to mediate behavioral deficits, it was not clear whether mild joint pathology significantly affected behavioral performance. Previously we observed that 11-week-old MRL-lpr mice showed a trend for disturbed performance when crossing a narrow beam. The first aim of the present study was to test the significance of this trend by increasing the sample size and, second, to examine the possibility that arthritis-like changes interfere with performance in brief locomotor tasks. For the purpose of the second goal, 18-week-old mice that differ widely in severity of joint disease were selectively taken from the population and tested in beam walking and swimming tasks. It was expected that the severity of joint inflammation would be positively correlated with the degree of locomotor impairment. The larger sample size revealed that young MRL-lpr mice perform significantly more poorly than controls on the beam-walking test, as evidenced by more foot slips and longer traversing time. However, significant correlation between joint pathology scores and measures of locomotion could not be detected. The lack of such relationship suggests that mild joint pathology does not significantly contribute to impaired performance in young, autoimmune MRL-lpr mice tested in short behavioral tasks.

Decreased auditory function in the C3H/lpr autoimmune disease mouse

To better understand autoimmune-related inner ear disease, cochlear structure and function were evaluated in the C3H/lpr autoimmune strain mouse, a model for systemic lupus erythematosus. C3H/lpr mice were examined at ages from 2 to 12 months along with age-matched C3H/HeJ controls. Autoimmune disease onset occurred at 3-4 months of age as serum immune complexes, antinuclear antibodies, and spleen weights were significantly elevated. Auditory brainstem response (ABR) audiometry showed normal auditory thresholds in C3H/lpr mice at 4 months of age, but elevated thresholds by 6 months, particularly in the high frequencies. Examination of the cochleas revealed no apparent loss of hair cells or spiral ganglion neurons, even in those mice with 50 dB SPL threshold shifts. However, changes were observed in the stria vascularis, including edematous spaces, enlarged capillaries, and thickened vessel linings. These findings imply that cochlear dysfunction in the autoimmune disease mice is the result of stria vascularis pathology.

Monoclonal antibody induced hearing loss

Monoclonal antibodies KHRI-3 and KHRI-5 identify antigens expressed on inner ear supporting cells and auditory hair cells respectively. To determine if these antibodies affect inner ear function groups of syngeneic Balb/c mice were inoculated with hybridomas KHRI-3, KHRI-5 and other Ig-secreting hybridomas. Hybridomas UM-A9, UM-7F11, the non-secreting SP2/0 myeloma and mice with no hybridoma were used as controls. Animals were tested for auditory brainstem responses (ABR) for frequencies of 4, 8, 16 and 24 kHz, before the inoculation of the hybridomas and at intervals of 6 to 10 days thereafter or daily once tumors became palpable. In normal mice there were no changes in ABR thresholds over the course of the experiment. Other control animals showed little change in ABR even when the growth of the hybridoma or myeloma tumors were far advanced. Of the KHRI-5 hybridoma bearing animals only one of seven animals exhibited threshold shifts greater than 15 dB. In contrast, most mice bearing the KHRI-3 hybridoma exhibited high frequency threshold shifts of 40-50 dB that coincided temporally with the growth of the hybridoma, the presence of circulating KHRI-3 antibody, and greatly increased immunoglobulin titers. Ears from KHRI-3-bearing mice that developed high frequency hearing loss also had a novel type of lesion in the basal turn of the cochlea that was characterized by loss of outer hair cells and absence of typical supporting cell scars. Such changes were not found in control hybridoma-bearing mice. These findings suggest that KHRI-3 antibody has an effect on hearing that is secondary to damage to the organ of Corti and loss of outer hair cells. Our results have important implications for antibody-mediated mechanisms of hearing loss and provide an animal model in which to study this phenomenon.

A substrain of NZB mouse as an animal model of autoimmune inner ear disease

A substrain of an autoimmune-prone mouse, NZB/kl, was found to show spontaneous elevation of the auditory brainstem response (ABR) threshold with age. Morphological examination of the inner ear in NZB/kl mice with high ABR thresholds revealed pathological changes confined to the stria vascularis, including marked thickening of the capillary basement membrane which contained many foamy structures, and vacuolar degeneration of the intermediate cells. Circular or granular IgM deposits and some IgG deposits were found in the stria vascularis in the mice with high ABR thresholds, suggesting that deposits of immune complexes (mainly IgM antibodies) could cause strial damage that resulted in the ABR threshold elevation. Another substrain of NZB mice, NZB/san, showed lower levels of IgM immune complexes and anti-ss DNA antibodies, and did not develop either inner ear morphological changes or a high ABR threshold. NZB/kl mice may provide a useful animal model for studying the mechanism of autoimmune inner ear disease.

Mechanisms of hearing disturbance in an autoimmune model mouse NZB/kl

A subline of the NZB mouse, NZB/kl, was found to develop severe hearing disturbances at high frequency sound at the age of 4 to 6 months. Deposition of IgG was observed on the capillary wall of the stria vascularis of the mice, but the concentration of circulating immune complex did not seem to be correlated to the deposition. Electron microscopic examination revealed that the capillaries had a thick basement membrane, and in severe cases the membrane contained foamy structures of various size. In some cases the base membrane was so thick that the capillary lumen was narrowed, and the intermediate cells seemed to be damaged. No pathological findings were found in other inner ear tissues. These results suggest that the changes in the stria vascularis were possibly caused by an autoimmune mechanism which resulted in hearing disturbance.