Glycosaminoglycans provide a binding site for thyroglobulin in orbital tissues of patients with thyroid-associated ophthalmopathy

Thyroid-associated ophthalmopathy: Emergence of teprotumumab as a promising medical therapy

Thyroid-associated ophthalmopathy (TAO) remains a vexing autoimmune component of Graves' disease that can diminish the quality of life as a consequence of its impact on visual function, physical appearance and emotional well-being. Because of its relative rarity and variable presentation, the development of highly effective and well-tolerated medical therapies for TAO has been slow relative to other autoimmune diseases. Contributing to the barriers of greater insight into TAO has been the historical absence of high-fidelity preclinical animal models. Despite these challenges, several agents, most developed for treatment of other diseases, have found their way into consideration for use in active TAO through repurposing. Among these, teprotumumab is a fully human inhibitory monoclonal antibody against the insulin-like growth factor I receptor. It has shown remarkable effectiveness in moderate to severe, active TAO in two completed multicenter, double masked, and placebo controlled clinical trials. The drug exhibits a favorable safety profile. Teprotumumab has recently been approved by the U.S. F.D.A, and may rapidly become the first line therapy for this disfiguring and potentially blinding condition.

IGF1 receptor and thyroid-associated ophthalmopathy

Thyroid-associated ophthalmopathy (TAO) is a vexing and poorly understood autoimmune process involving the upper face and tissues surrounding the eyes. In TAO, the orbit can become inflamed and undergo substantial remodeling that is disfiguring and can lead to loss of vision. There are currently no approved medical therapies for TAO, the consequence of its uncertain pathogenic nature. It usually presents as a component of the syndrome known as Graves' disease where loss of immune tolerance to the thyrotropin receptor (TSHR) results in the generation of activating antibodies against that protein and hyperthyroidism. The role for TSHR and these antibodies in the development of TAO is considerably less well established. We have reported over the past 2 decades evidence that the insulin-like growth factorI receptor (IGF1R) may also participate in the pathogenesis of TAO. Activating antibodies against IGF1R have been detected in patients with GD. The actions of these antibodies initiate signaling in orbital fibroblasts from patients with the disease. Further, we have identified a functional and physical interaction between TSHR and IGF1R. Importantly, it appears that signaling initiated from either receptor can be attenuated by inhibiting the activity of IGF1R. These findings underpin the rationale for therapeutically targeting IGF1R in active TAO. A recently completed therapeutic trial of teprotumumab, a human IGF1R inhibiting antibody, in patients with moderate to severe, active TAO, indicates the potential effectiveness and safety of the drug. It is possible that other autoimmune diseases might also benefit from this treatment strategy.

New advances in understanding thyroid-associated ophthalmopathy and the potential role for insulin-like growth factor-I receptor

Thyroid-associated ophthalmopathy (TAO), a localized periocular manifestation of the autoimmune syndrome known as Graves’ disease, remains incompletely understood. Discussions of its pathogenesis are generally focused on the thyrotropin receptor, the proposed role for which is supported by substantial evidence. Considerations of any involvement of the insulin-like growth factor-I receptor (IGF-IR) in the disease are frequently contentious. In this brief, topically focused review, I have attempted to provide a balanced perspective based entirely on experimental results that either favor or refute involvement of IGF-IR in TAO. Discussion in this matter seems particularly timely since the currently available treatments of this disfiguring and potentially sight-threatening disease remain inadequate. Importantly, no medical therapy has thus far received approval from the US Food and Drug Administration. Results from a very recently published clinical trial assessing the safety and efficacy of teprotumumab, an inhibitory human anti–IGF-IR monoclonal antibody, in active, moderate to severe TAO are extremely encouraging. That double-masked, placebo-controlled study involved 88 patients and revealed unprecedented clinical responses in the improvement of proptosis and clinical activity as well as a favorable safety profile. Should those results prove reproducible in an ongoing phase III trial, therapeutic inhibition of IGF-IR could become the basis for paradigm-shifting treatment of this vexing disease.

TSH-receptor-expressing fibrocytes and thyroid-associated ophthalmopathy

Thyroid-associated ophthalmopathy (TAO) is a vexing and undertreated ocular component of Graves disease in which orbital tissues undergo extensive remodelling. My colleagues and I have introduced the concept that fibrocytes expressing the haematopoietic cell antigen CD34 (CD34(+) fibrocytes), which are precursor cells of bone-marrow-derived monocyte lineage, express the TSH receptor (TSHR). These cells also produce several other proteins whose expression was traditionally thought to be restricted to the thyroid gland. TSHR-expressing fibrocytes in which the receptor is activated by its ligand generate extremely high levels of several inflammatory cytokines. Acting in concert with TSHR, the insulin-like growth factor 1 receptor (IGF-1R) expressed by orbital fibroblasts and fibrocytes seems to be necessary for TSHR-dependent cytokine production, as anti-IGF-1R blocking antibodies attenuate these proinflammatory actions of TSH. Furthermore, circulating fibrocytes are highly abundant in patients with TAO and seem to infiltrate orbital connective tissues, where they might transition to CD34(+) fibroblasts. My research group has postulated that the infiltration of fibrocytes into the orbit, their unique biosynthetic repertoire and their proinflammatory and profibrotic phenotype account for the characteristic properties exhibited by orbital connective tissues that underlie susceptibility to TAO. These insights, which have emerged in the past few years, might be of use in therapeutically targeting pathogenic orbit-infiltrating fibrocytes selectively by utilizing novel biologic agents that interfere with TSHR and IGF-1R signalling.

Update on the medical treatment of Graves' ophthalmopathy

PURPOSE

To review recent advances in the understanding of Graves' ophthalmopathy (GO) pathogenesis as well as discuss current and future medical management strategies.

DESIGN

Interpretive essay.

METHODS

Literature review and interpretation.

RESULTS

Medical treatment of GO has slowly evolved during the past few decades and has been hampered by a poor understanding of the disease at a cellular and molecular level. Current treatment recommendations and guidelines therefore focus on nonspecific immuno-suppression. Newer classes of treatment agents hold promise to more selectively target underlying cellular and molecular alterations in GO.

CONCLUSION

Cooperation between individual patients, physicians and between differing medical centers, together with a refined understanding of the pathogenesis of GO, will lead to newer more-effective treatments for the disease and improve patient quality of life.

Susceptibility genes in Graves' ophthalmopathy: searching for a needle in a haystack?

The variety of clinical presentations of eye changes in patients with Graves' disease suggests that complex interactions between genetic, environmental, endogenous and local factors influence the development/severity of Graves' ophthalmopathy (GO). At present, the role of genetic factors in the development of GO remains unknown. Based on small case-control association studies with candidate genes, several susceptibility loci in GO have been proposed. These are human leucocyte antigen (HLA, 6p21.3), cytotoxic T-lymphocyte antigen-4 (CTLA-4, 2q33), tumour necrosis factor (TNF, 6p21.3), interferon-gamma (IFN-gamma, 12q14), intercellular adhesion molecule-1 (ICAM-1, 19p13), and thyroid stimulating hormone receptor gene (TSH-R, 14q31). Unfortunately, these results were either not confirmed or require replication in larger studies. There are many reasons for the lack of reproducibility of association studies in GO, including poor characterization of the studied groups and small sample sizes, which may result in both false positive and negative results. Thus, the genetic background of GO remains to be elucidated in future research. However, the possibility that GO may be a genetically heterogeneous disorder, or that the development of GO may be predominantly influenced by environmental factors such as cigarette smoking, can not be disregarded.

A single chondroitin 6-sulfate oligosaccharide unit at Ser-2730 of human thyroglobulin enhances hormone formation and limits proteolytic accessibility at the carboxyl terminus. Potential insights into thyroid homeostasis and autoimmunity

We localized the site of type D (chondroitin 6-sulfate) oligosaccharide unit addition to human thyroglobulin (hTg). hTg was chromatographically separated into chondroitin 6-sulfate-containing (hTg-CS) and chondroitin 6-sulfate-devoid (hTg-CS0) molecules on the basis of their D-glucuronic acid content. In an ample number of hTg preparations, the fraction of hTg-CS in total hTg ranged from 32.0 to 71.6%. By exploiting the electrophoretic mobility shift and metachromasia conferred by chondroitin 6-sulfate upon the products of limited proteolysis of hTg, chondroitin 6-sulfate was first restricted to a carboxyl-terminal region, starting at residue 2514. A single chondroitin 6-sulfate-containing nonapeptide was isolated in pure form from the products of digestion of hTg with endoproteinase Glu-C, and its sequence was determined as LTAGXGLRE (residues 2726-2734, X being Ser2730 linked to the oligosaccharide chain). In an in vitro assay of enzymatic iodination, hTg-CS produced higher yields of 3,5,5 '-triiodothyronine (T3) (171%) and 3,5,3',5'-tetraiodothyronine (T4) (134%) than hTg-CS0. Unfractionated hTg behaved as hTg-CS. Thus, chondroitin 6-sulfate addition to a subset of hTg molecules enhanced the overall level of T4 and, in particular, T3 formation. Furthermore, the chondroitin 6-sulfate oligosaccharide unit of hTg-CS protected peptide bond Lys2714-Gly2715 from proteolysis, during the limited digestion of hTg-CS with trypsin. These findings provide insights into the molecular mechanism of regulation of the hormonogenic efficiency and of the T4/T3 ratio in hTg. The potential implications in the ability of hTg to function as an autoantigen and into the pathogenesis of thyroidal and extra-thyroidal manifestations of autoimmune thyroid disease are discussed.

Poorly specific binding of thyroglobulin to orbital fibroblasts from patients with Graves' ophthalmopathy

It has been proposed that thyroglobulin (Tg) may be involved in the pathogenesis or the progression of Graves' ophthalmopathy (GO). According to this hypothesis, following its release from the thyroid, Tg would reach orbital tissues, thereby eliciting an autoimmune aggression. In support of this, we recently found that intact Tg is present in orbital tissues of patients with GO, where it is complexed with glycosaminoglycans. In this study, we searched for additional Tg binding sites in orbital tissues, using primary cultures of orbital and skin fibroblasts from 7 GO patients who had undergone orbital decompression. Biotin-labeled Tg bound to both skin and orbital fibroblasts in a saturable manner, with constants of dissociation of approximately 75 nmol/l for skin fibroblasts and approximately 40 nmol/I for orbital fibroblasts. In an attempt to identify Tg binding sites, fibroblast extracts were blotted onto membranes that were incubated with biotin-labeled Tg, which bound especially to a protein migrating at approximately 300 kDa, present in both orbital and skin fibroblast extracts. Because no appreciable inhibition of binding of biotin-labeled Tg was produced by unlabeled Tg, we concluded that binding was poorly specific and it is unlikely to be involved in the pathogenesis of GO.

Comparison of practical methods for urinary glycosaminoglycans and serum hyaluronan with clinical activity scores in patients with Graves' ophthalmopathy

OBJECTIVE

Immunosuppressive treatment of Graves' opthalmopathy (GO) should be restricted to patients with active eye disease, but assessing disease activity is difficult. Several methods to evaluate GO activity have been introduced, but none of them is satisfactory. Glycosaminoglycans (GAGs) are complex polysaccharides that participate on the pathogenesis of GO and attempts to correlate its local increase to urinary GAGs (uGAGs) or serum hyaluronan (sHA) have been made, but the available techniques are labourious, time-consuming and difficult for routine use. The aim of the present study is to develop practical and simple methods for uGAGs and sHA and compare them to the activity and severity of thyroid-associated ophthalmopathy.

DESIGN, PATIENTS AND MEASUREMENTS

We developed a microelectrophoresis technique for uGAGs and a fluoroassay for sHA and assessed each in 152 patients with Graves' disease, 25 without GO and 127 with GO, classified according to the Clinical Activity Score (CAS). All patients had been euthyroid for > 2 months.

RESULTS

Patients with inactive disease (CAS = 2, n = 100) had uGAGs (4.2 +/- 1.3 micro g/mg/creatinine) and sHA (11.1 +/- 7.2 micro g/l) that did not differ from normal subjects (3.1 +/- 1.1 micro g/mg/creatinine, n = 138 and 13.9 +/- 9.6 micro g/l, n = 395). In contrast, patients with active eye disease (CAS = 3, n = 27) had uGAGs (8.4 +/- 2.7 micro g/mg/creatinine) and sHA (32.3 +/- 17.8 micro g/l) 2-3 times higher than those patients with inactive eye disease. Using a cut-off of 6.1 micro g/mg creatinine for uGAGs and 20.7 micro g/l for sHA we found, respectively, 85% and 81% sensitivity and 93% and 91% specificity for each test. The positive and negative predictive values were 77% and 96% for uGAGs and 71% and 95% for sHA.

CONCLUSION

Employing these two new methods we have established a significant relationship between the levels of uGAGs and/or sHA and the clinical activity of GO. Therefore, together with CAS, uGAGs determination, and, to a lesser degree, sHA, would be very useful in the discrimination from active and inactive ocular disease and aid in deciding on the best therapy for GO patients.

Role of thyroglobulin in the pathogenesis of Graves' ophthalmopathy: the hypothesis of Kriss revisited

One of the hypothesis to explain the pathogenesis of Graves' ophthalmopathy (GO) was formulated by Joseph P. Kriss in the early 1970s. He postulated that the initiating event in the pathogenesis of GO is the deposition and accumulation of thyroglobulin (Tg) in orbital tissues, followed by an autoimmune reaction against Tg. In the last 30 yrs several studies have addressed this hypothesis, through various, different experimental approaches, raising results that are both in favor and against the possibility that Tg plays a role in the pathogenesis of GO. The finding that intact Tg is present in orbital tissues of GO patients supports Kriss' hypothesis, although the role of Tg as an autoantigen seems to be unlikely, as GO is not significantly associated with serum TgAb and mice immunized with Tg do not develop GO. Whether Tg is indeed involved in the pathogenesis of GO remains to be established. Our current view is that, provided that Tg plays a role, it is unlikely the only factor involved and Tg in orbital tissues may rather reinforce or worsen a damage initiated by other mechanisms.