An Animal Model of Graves’ Orbitopathy

Abstract

Graves’ orbitopathy (GO) is an inflammatory orbital disease that frequently arises as a consequence of Graves’ disease, a common autoimmune thyroid condition. The thyrotropin stimulating hormone receptor (TSHR) is the defined autoantigen in Graves’ disease and has been considered to be the immunological target in the orbital complication. Mechanistic insights into pathogenesis of GO for development of therapeutic agents remain completely undefined, due to the lack of an animal model. Earlier reported models have proved to be disappointing failures. An experimental model of GO has been developed, based upon genetic immunization of inbred mice with human TSHR ectodomain plasmid by close-field electroporation, with a high disease incidence. Inflammatory lesions characterized by CD3 + T cells and macrophages were localized in the orbital muscle tissue. Other pathological manifestations in some animals were characterized by extensive adipogenesis of orbital fat and recapitulate different aspects of pathology seen in patients with GO. Additional pathological features were reminiscent of eyelid inflammation involving chemosis, with dilated and congested orbital blood vessels. Early studies indicate the replication of the model in authors’ independent laboratories. The development of a reproducible model of GO will provide an invaluable tool to understand the pathogenic mechanisms to allow identification of novel targets for development of effective therapies for this disease.

Appendices

Commentary

Rebecca S. Bahn

Mayo Clinic College of Medicine,

Division of Endocrinology and Metabolism,

Mayo Clinic,

Rochester, MN, USA

Graves’ orbitopathy is an autoimmune ocular disease with heterogeneous clinical manifestations. While the majority of patients have a history of hyperthyroidism, some are euthyroid or even hypothyroid. Approximately, 50 % of GO patients have only mild eye irritation and redness. The other half develops more severe disease with forward protrusion of the globes (proptosis), ocular pain and swelling, or debilitating double vision. Some 3–5 % risk loss of sight. The majority experience significantly impaired quality of life. On orbital imaging, most patients show some degree of extraocular muscle enlargement and/or expansion of orbital adipose tissues. The unifying feature of the disease appears to be the presence of circulating autoantibodies directed against the thyrotropin receptor (TSHR) in essentially every patient, given the use of adequately sensitive assays. However, despite the significant morbidity that can accompany the disease, patients with GO and their physicians are left with only limited therapeutic options of having variable effectiveness.

Recent insights into pathogenesis of GO obtained through in vitro studies have undoubtedly moved the field forward. However, the study of potential novel therapy based upon those insights has been severely limited by the lack of an animal model that truly recapitulates the human disease. The robust mouse model developed by Banga, Moshkelgosha, and colleagues represents the first bona fide animal model of GO. The eye changes that develop in these animals are very similar to, and as varied as, those seen in humans with GO. Most mice show extraocular muscle enlargement and some demonstrate expansion of orbital adipose tissues. As in the human disease, all immunized mice produce measurable antibodies directed against the TSHR, whether or not they develop hyperthyroidism.

This animal model of GO, and future animal models evolving from it, will facilitate novel experimental approaches and new discoveries regarding GO pathogenesis. However, perhaps its most important contribution to the field will be to offer studies of novel therapies for established disease and approaches to GO prediction and prevention in patients with Graves’ hyperthyroidism. These in vitro studies will no doubt lead to randomized clinical trials and ultimately to more effective approaches to care patients with Graves’ disease.

Commentary

Shivani Gupta and Raymond S. Douglas

e-mail: raydougl@med.umich.edu

Kellogg Eye Center,

University of Michigan,

Ann Arbor, MI, USA

Graves’ disease is a multisystemic autoimmune disorder with a complex underlying etiology, including genetic and environmental factors. Inflammatory infiltration of the orbits is thought to be driven by autoantibodies to the thyrotropin and insulin-like growth factor-1 receptors. Animal models of GO have historically been limited by the inability to simulate the sustained milieu of autoantibodies thought to promote orbital manifestations. The authors describe a mouse model that exhibits a sustained response to the thyrotropin receptor as manifested by induced hyperthyroidism. Close-field electroporation techniques have enabled long lasting autoantibody generation to the thyrotropin receptor and robust antibody response to insulin-like growth factor-1 receptor using in vivo delivery of human purified thyroid stimulating hormone receptor (hTSHR) A-subunit plasmid protein. Histopathological evaluation of orbital tissues in this mouse model reveals the presence of extraocular muscle and perineural inflammatory infiltrates, primarily of CD3⁺ T cells. Expansion of orbital fat, chemosis, vascular congestion, and fibrosis at the later stages of disease was analogous to the human disease. Although pathological differences do exist between the animal model of GO and human disease, this model provides considerable advances to further elucidate the complex mechanisms that underlie GO.