Current concepts in the molecular pathogenesis of thyroid-associated ophthalmopathy

Abstract

Graves' disease (GD) is a common autoimmune condition. At its core, stimulatory autoantibodies are directed at the thyroid-stimulating hormone receptor (TSHR), resulting in dysregulated thyroid gland activity and growth. Closely associated with GD is the ocular condition known as thyroid-associated ophthalmopathy (TAO). The pathogenesis of TAO remains enigmatic as do the connections between the thyroid and orbit. This review highlights the putative molecular mechanisms involved in TAO and suggests how these insights provide future directions for identifying therapeutic targets. Genetic, epigenetic, and environmental factors have been suggested as contributory to the development of GD and TAO. Thyroid-stimulating hormone receptor and insulin-like growth factor receptor (IGF-1R) are expressed at higher levels in the orbital connective tissue from individuals with TAO than in healthy tissues. Together, they form a functional complex and appear to promote signaling relevant to GD and TAO. Orbital fibroblasts display an array of cell surface receptors and generate a host of inflammatory molecules that may participate in T and B cell infiltration. Recently, a population of orbital fibroblasts has been putatively traced to bone marrow-derived progenitor cells, known as fibrocytes, as they express CD45, CD34, CXCR4, collagen I, functional TSHR, and thyroglobulin (Tg). Fibrocytes become more numerous in GD and we believe traffic to the orbit in TAO. Numerous attempts at developing complete animal models of GD have been largely unsuccessful, because they lack fidelity with the ocular manifestations seen in TAO. Better understanding of the pathogenesis of TAO and development of improved animal models should greatly accelerate the identification of medical therapy for this vexing medical problem.

Trial registration: ClinicalTrials.gov NCT01868997.

Keywords: Graves' disease; autoimmune; inflammation.

Figures

Figure 1

A 61-year-old woman with TAO. (A) Frontal view demonstrating bilateral upper lid retraction (right greater than left) and bilateral lower lid retraction with inferior scleral show. Bilateral proptosis, lateral flare, chemosis, injection, caruncular edema, and significant left esotropia also are evident. (B) Worms-eye view highlighting proptosis (right greater than left).

Figure 2

Orbital CT without contrast. (A) Uninvolved orbits. (B) Patient with TAO demonstrating bilateral exophthalmos. Bilateral marked enlargement of the extraocular muscles, especially the medial, superior, and inferior rectus muscles, causing apical crowding of the optic nerve. Stranding can be observed around both optic nerves.

Figure 3

Histologic examination of orbital tissue of a patient with GD (hematoxylin and eosin, ×20). Mononuclear cell infiltrate is seen within the orbital fat compartment.

Figure 4

Increased generation of fibrocytes from PBMCs of patients with GD. There was approximately 5-fold more fibrocytes in individuals with GD compared to controls (5268 ± 1260 fibrocytes per 106 PBMCs, n = 70 versus control, 954 ± 329 fibrocytes per 106 PBMCs, n = 25, mean ± SD, P < 0.001). Reprinted with permission from Douglas RS, Afifiyan NF, Hwang CJ, et al. Increased generation of fibrocytes in thyroid-associated ophthalmopathy. J Clin Endocrinol Metab. 2010;95:430–438. Copyright 2010 The Endocrine Society.

Figure 5

(A) Similar spindle-shaped phenotypes among orbital fibroblasts, dermal fibroblasts, and fibrocytes (hematoxylin and eosin, ×20). (B) Fibrocytes from individuals with GD display cell surface receptor CD34. 1, Immunofloresence staining of CD34 in TAO-derived tissue (inset as negative control). 2, Absence of CD34 expression in healthy orbital tissue (inset as positive control). (C) Orbital fibroblasts from individuals with and without TAO display similar receptors on fibrocytes, as shown by flow cytometric analysis with anti-CD34 and anti-Col I antibodies. Reprinted with permission from Douglas RS, Afifiyan NF, Hwang CJ, et al. Increased generation of fibrocytes in thyroid-associated ophthalmopathy. J Clin Endocrinol Metab. 2010;95:430–438. Copyright 2010 The Endocrine Society.

Figure 6

Immunofloresence of the induction of hyaluronan with TGF-β in human orbital fibroblasts. Cultures were treated with nothing (controls) or TGF-β1 for 24 hours. (a, d, g) Contain images of cells stained with biotinylated HABP and demonstrate hyaluronan. (b, e, h) Contain monolayers stained with phalloidin and demonstrate actin. (c, f, i) Show cultures stained with DAPI and disclose nuclei. (a–c) Untreated controls. Hyaluronan staining appears to be perinuclear. TGF-β1 induced hyaluronan staining and formation of microvillus-like projections. Streptomyces hyaluronidase-treated fibroblasts failed to exhibit hyaluronan staining, as in (g–i). Reprinted with permission from Guo N, Woeller CF, Feldon SE, Phipps RP. Peroxisome proliferator-activated receptor γ ligands inhibit transforming growth factor-β-induced, hyaluronan-dependent, T cell adhesion to orbital fibroblasts. J Biol Chem. 2011;286:18856–18867. Copyright 2011 The American Society for Biochemistry and Molecular Biology.

Figure 7

Treatment of orbital fibroblasts with 15d-PGJ2 from different subtypes of TAO. Orbital fibroblasts were grown in the presence of 5 μM 15d-PGJ2. Type I TAO orbital fibroblasts demonstrated more adipogenesis compared to type II or orbital fibroblasts from a healthy donor, as is evidenced by Oil Red O accumulation. TED, thyroid eye disease or TAO. Reprinted with permission from Kuriyan AE, Woeller CF, O'Loughlin CW, Phipps RP, Feldon SE. Orbital fibroblasts from thyroid eye disease patients differ in proliferative and adipogenic responses depending on disease subtype. Invest Ophthalmol Vis Sci. 2013;54:7370–7377. Copyright 2013 The Association for Research in Vision and Ophthalmology.

Figure 8

Immunohistochemical analysis of TSHR immunoreactivity on orbital connective tissue from a donor with TAO. The immunostaining was conducted with a monoclonal antibody directed against TSHR (amino acids 604-764). (A) Orbital connective tissue. (B) Passage one exhibits intense staining. (C) Passage three with reduced staining. (D) Passage 5 culture fails to show staining. Reprinted with permission from Bahn RS, Dutton CM, Natt N, Joba W, Spitzweg C, Heufelder AE. Thyrotropin receptor expression in Graves' orbital adipose/connective tissues: potential autoantigen in Graves' ophthalmopathy. J Clin Endocrinol Metab. 1998;83:998–1002. Copyright 1998 The Endocrine Society.

Figure 9

Schematic illustrating the putative role of fibrocytes in the pathogenesis of TAO. CD34+ fibrocytes derive from the bone marrow and appear to be trafficked specifically to the orbit in TAO where they transition into CD34+ fibroblasts. Fibrocytes express relatively high levels of functional TSHR. Further, they can differentiate into either adipocytes or myofibroblasts in vitro. CD34+ orbital fibroblasts interact with the native residential CD34− orbital fibroblasts, resulting in dramatic reduction of expression of TSHR and other thyroid proteins. We postulate that the magnitude of this suppression may underlie susceptibility to TAO.