Autoimmunity in spontaneous myasthenia gravis in dogs

Risk Factors and Outcomes in Cats with Acquired Myasthenia Gravis (2001-2012)

Background

Acquired myasthenia gravis (MG) in cats most commonly causes generalized weakness without megaesophagus and is more often associated with a cranial mediastinal mass, compared to dogs.

Hypothesis/Objectives

To extend the clinical findings described in the report of 2000 on MG in cats (J Am Vet Med Assoc 215:55–57).

Animals

Two hundred and thirty‐five cats with MG.

Methods

Retrospective case study to evaluate the long‐term outcome and incidence of spontaneous remission in myasthenic cats. Information including signalment, clinical presentation, presence of and type of cranial mediastinal mass, treatment including surgical versus medical, survival time, and outcome including spontaneous remissions was collected and analyzed in cats diagnosed at the Comparative Neuromuscular Laboratory, University of California San Diego by detection of acetylcholine receptor antibody titers >0.3 nmol/L by immunoprecipitation radioimmunosassay.

Results

Acquired MG in cats is associated with a euthanasia rate of 58%. Abyssinian and Somali cats had an increased incidence of MG compared to mixed breed cats or cats of other breeds. A cranial mediastinal mass, most commonly thymoma, was observed in 52% of the cats, which is higher than in the previous report. Spontaneous remission is not a characteristic of MG in cats.

Conclusions and clinical importance

Myasthenia gravis in cats is a chronic disease associated with a high incidence of a cranial mediastinal mass. Spontaneous remission is not common and clinicians should warn owners of the necessity for long‐term treatment. The clinical outcome with a cranial mediastinal mass did not differ between surgical or medical treatment.

Thymoma related myasthenia gravis in humans and potential animal models

Thymoma-associated Myasthenia gravis (TAMG) is one of the anti-acetylcholine receptor MG (AChR-MG) subtypes. The clinico-pathological features of TAMG and its pathogenesis are described here in comparison with pathogenetic models suggested for the more common non-thymoma AChR-MG subtypes, early onset MG and late onset MG. Emphasis is put on the role of abnormal intratumorous T cell selection and activation, lack of intratumorous myoid cells and regulatory T cells as well as deficient expression of the autoimmune regulator (AIRE) by neoplastic thymic epithelial cells. We review spontaneous and genetically engineered thymoma models in a spectrum of animals and the extensive clinical and immunological overlap between canine, feline and human TAMG. Finally, limitations and perspectives of the transplantation of human and murine thymoma tissue into nude mice, as potential models for TAMG, are addressed.

Routine and specialized laboratory testing for the diagnosis of neuromuscular diseases in dogs and cats

The diagnosis of neuromuscular diseases can be challenging. The first step is recognition that the disease involves the neuromuscular system (muscle, neuromuscular junction, peripheral nerve, and ventral horn cells of the spinal cord). Many neuromuscular diseases share clinical signs and cannot be distinguished based on clinical examination. Routine laboratory screening, including a CBC, biochemical profile, and urinalysis, can identify some of the most common systemic abnormalities that cause muscle weakness and myalgia, such as hypo- and hyperglycemia, electrolyte disorders, or thyroid abnormalities, and may suggest a specific diagnosis, such as diabetes mellitus, hypo- or hyperadrenocorticism, renal failure, or hypothyroidism. Increased creatine kinase activity, increased cardiac troponin I concentration, and myoglobinuria are useful in detecting skeletal and cardiac muscle damage. Identification of acetylcholine receptor antibodies is diagnostic for acquired myasthenia gravis. For primary muscle or peripheral nerve diseases, tissue biopsy is the most direct way to determine specific pathology, correctly classify the disease, and determine the course of additional laboratory testing. For example, inflammatory, necrotizing, dystrophic, metabolic, or congenital myopathies require different laboratory testing procedures for further characterization. Many neuromuscular diseases are inherited or breed-associated, and DNA-based tests may already be established or may be feasible to develop after the disorder has been accurately characterized. This review focuses on both routine and specialized laboratory testing necessary to reach a definitive diagnosis and determine an accurate prognosis for neuromuscular diseases.

Possible therapeutic vaccines for canine myasthenia gravis: implications for the human disease and associated fatigue

Myasthenia gravis (MG) is caused by T cell-dependent antibodies reactive with acetylcholine receptors. These autoreactive antibodies cause muscle weakness by interfering with neuromuscular transmission via removal of acetylcholine receptors from the neuromuscular junction as well as changing the architecture of the junction itself. Consequently, muscle fatigue is a debilitating aspect of MG often leading to more general feelings of tiredness not directly due to muscle weakness. We have previously described two peptides that are mimetics of antigen receptors on certain autoreactive T and B cells that are involved in MG. When used as vaccines in the rat model of MG, these peptides prevented and ameliorated disease and muscle fatigue by blunting acetylcholine receptor antibody responses. Such disease protection resulted from vaccine-induced anergizing antibodies against acetylcholine receptor-specific T and B cell antigen receptors. The present study prospectively evaluated the efficacy of these two vaccines in spontaneous acquired MG in pet dogs. When compared to historical controls that were prospectively studied, the vaccines increased the proportion of remitted dogs from 17 to 75%. In comparison to retrospectively studied historical controls that spontaneously remitted from MG, the vaccines accelerated the rate of decline in acetylcholine receptor antibody titers which resulted in a 3-fold decrease in the mean time to remission. These results are suggestive of a new type of targeted therapy that can drive autoimmune responses into long-term remission and possibly afford a means of determining whether correction of a physical cause of muscle weakness also corrects the perception of chronic, generalized fatigue.

Haplotyping of the canine MHC without the need for DLA typing

The genomic matching technique has proven useful in MHC haplotyping in humans. We have adopted a similar approach in Australian cattle dogs and report that genotyping can be achieved with a single assay.

Thymoma and multiple thymic cysts in a dog with acquired myasthenia gravis

An anterior mediastinal cystic lesion in an 11-year-old mongrel dog was examined. The dog showed dysbasia and vomiting due to megaoesophagus, and anterior mediastinal round mass lesion, approximately 35 mm in diameter, was found by X ray. Based on clinical examinations, the dog was diagnosed as acquired myasthenia gravis and was successfully controlled by anticholinesterase treatment for approximately 4 months. The dog died of thermic stroke and was necropsied. Grossly, fatty tissues with cysts containing yellowish fluid and white nodules were found in the anterior mediastinal area. Histopathologically, multiple cysts, neoplastic tissues, and atrophic thymus were found within the examined tissues. The cysts were lined by thin wall consisting of ciliated long cuboidal and non-ciliated round cells and were filled with eosinophilic colloidal fluid. Some extended cysts contained neoplastic foci within their lumen and walls. The neoplastic tissues consisted of mixed population of large epithelial cells with abundant clear cytoplasm and large oval nuclei, and lymphocytes. Immunohistochemically, proliferating epithelial cells were intensely positive for keratin and cytokeratin, and more than half number of infiltrating lymphocytes were intensely positive for CD3 suggesting T cells. All these findings indicate the neoplastic lesion is thymoma and multiple cysts are considered as thymic or brachial cleft cysts.

Myasthenia gravis and disorders of neuromuscular transmission

Myasthenia gravis is a disorder of neuromuscular transmission that occurs in congenital and acquired autoimmune forms. Acquired myasthenia gravis is probably the most common neuromuscular disorder in dogs that can be diagnosed and treated. An early, accurate diagnosis and appropriate therapy is of utmost importance to a good clinical outcome in this disorder. This article focuses on the diagnosis and treatment of acquired myasthenia gravis in dogs and cats with brief discussions of other disorders of neuromuscular transmission, including congenital myasthenia gravis, tick paralysis, botulism, and organophosphate intoxication.

Titin and ryanodine receptor autoantibodies in dogs with thymoma and late-onset myasthenia gravis

Similar to human autoimmune myasthenia gravis (MG), canine MG occurs spontaneously and is associated with autoantibodies against the nicotinic acetylcholine receptor (AChR). In addition to AChR, human MG patients with thymoma or late-onset MG have antibodies against titin and ryanodine receptor (RyR). The objective of this study was to establish if dogs with confirmed MG (AChR antibody titer >0.6 nmol/l) also developed titin and RyR antibodies and identify possible associations with thymoma, late age of onset, or severity of clinical signs. Sera from dogs (n=430) with previously diagnosed autoimmune MG (N=415), other immune-mediated neuromuscular disorders including polymyositis (PM) and masticatory muscle myositis (N=5), and control dogs (N=10) were evaluated for the presence of titin antibodies in ELISA using MGT-30 as antigen, a peptide representing the main immunogenic region (MIR) for human titin antibodies. Titin antibody positive sera were further examined for RyR antibodies in Western blots using a RyR fusion protein (pc2-RyR) as antigen, which covers the MIR for human MG sera. Titin antibodies were found in sera of 80/430 dogs. Thymoma was present in 11/80 and age of onset was after 4 years in 66/80 titin positive dogs. Two of the titin positive dogs had PM. RyR antibodies were found in 13/80 sera (8/13 thymoma, 12/13 age of onset after 4 years, and 1/13 PM). Neither titin nor RyR antibodies were found in sera of healthy control dogs. Acute fulminating MG was described in five dogs with both titin and RyR antibodies. From these studies we conclude that titin and RyR antibodies in canine and human MG have a similar association with thymoma, late-onset MG, and possibly with more severe forms of MG.

Acquired myasthenia gravis: what we have learned from experimental and spontaneous animal models

Acquired myasthenia gravis (MG) is a disorder of neuromuscular transmission in which muscle weakness results from an autoantibody mediated depletion of acetylcholine receptors (AChR) at the neuromuscular junction. Experimental autoimmune myasthenia gravis, described in rodents and rabbits, has provided a good model of the effects of the autoimmune response against AChR and has shown that the specificities of the immune response in MG are those that would be obtained by immunization with native AChR. It has provided little information, however, about what initiates and sustains the immune response in MG. Acquired MG occurs spontaneously in dogs and may be the most common neuromuscular disorder that can be diagnosed in this species. As in human MG, an autoimmune response against AChR has been demonstrated and AChR autoantibodies have been implicated in the pathogenesis. The variability in clinical presentation, methods of diagnosis, and occurrence with other autoimmune diseases and neoplasia are identical to that of humans. Future studies of spontaneous canine autoimmune MG may provide clues to the determination of what factors initiate and sustain the autoimmune response to AChR, and in the study of specific suppression of the autoimmune response against AChR.

Review of thymic pathology in 30 cats and 36 dogs

Data are presented from 30 cats and 36 dogs in which thymic disease was recognised clinically or on postmortem examination. The diagnoses included thymic lymphoma (19 cats, 12 dogs), thymoma (five cats, 18 dogs), thymic branchial cyst formation or cystic change (one cat, four dogs), thymic hyperplasia (two cats), congenital hypoplasia (one cat, one dog), thymic haemorrhage (one cat, one dog) and thymic amyloidosis (one cat). Thymic lymphoma occurred in younger dogs and cats, and was recorded equally among domestic shorthaired and purebred (especially Siamese) cats. Eight cats with thymic lymphoma were tested for feline leukaemia virus and four were positive. Thymoma occurred more frequently in older cats and dogs, and in Labradors and German shepherd dogs. Thymic tumours were associated with paraneoplastic hypercalcaemia (six dogs), megaoesophagus (two dogs) or interface dermatitis with basement membrane immune complex deposition (one cat). Non-neoplastic thymic diseases were associated with myasthenia gravis (one cat), pemphigus foliaceus (one cat) and superficial necrolytic dermatitis (one cat).