Progress in thyroid-associated ophthalmopathy

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.

Thyroglobulin in orbital tissues from patients with thyroid-associated ophthalmopathy: predominant localization in fibroadipose tissue

One of the hypotheses that explains the pathogenesis of thyroid-associated ophthalmopathy (TAO) is that thyroglobulin (Tg) is transported through a retrograde lymphatic route to orbital tissues (OT), where it elicits autoimmune damage. In a previous study we demonstrated the presence of intact Tg of thyroid origin in OT from three patients with TAO. The present study was undertaken to investigate this issue further, by increasing the number of patients, by analyzing the distribution of Tg in OT, and by investigating possible relations between the presence of Tg in OT and the clinical features of patients. OT was obtained from seven patients with TAO who underwent decompressive orbitotomy via a transpalpebral approach. Patients were designated P10 to P16. Inflamed palpebral skin, retrobulbar fibroadipose tissue and extraocular muscle surgical samples were collected separately. Tissue extracts were prepared by homogenization and analyzed for the presence of Tg using two different techniques. We first performed immunoprecipitation experiments, in which a rabbit polyclonal anti-Tg antibody was used to capture Tg on protein A and a mouse monoclonal anti-Tg antibody was used to re-veal captured Tg by Western blotting. Intact 330-kd Tg was detected in retrobulbar fibroadipose tissue extracts from three patients (P10, P11, and P16), whereas no Tg was detected in retrobulbar fibroadipose tissue extracts from the remaining four patients. Tg was not detected in the extraocular muscle extracts from all patients studied. In addition, intact 330-kd Tg was found in the inflamed palpebral skin extract from one patient (P10). No Tg was detected in OT extracts from two patients without thyroid or eye disease and in abdominal adipose tissue extracts from two obese patients without thyroid or eye disease. We then searched for Tg by enzyme-linked immunosorbent assay (ELISA), using the same antibodies used for immunoprecipitation. Tg was detected in retrobulbar fibroadipose tissue extracts from four patients (P10, P11, P12, and P16) and in the inflamed palpebral skin extract from patient P10, in amounts ranging from approximately 125 to approximately 400 pg/microg of tissue protein. Again, Tg was not detected in extraocular muscle extracts. A positive gradient between Tg in OT and Tg in the serum was observed in patient P12. Using an ELISA approach, we found that Tg in OT from three TAO patients (P10, P11, and P12) contained thyroxine (T4) residues (mean T(4)

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2.42 molecules per molecule of Tg), indicating that Tg had originated in the thyroid. Combining the results obtained in our previous and present study, we found a possible relation between the presence of Tg in OT and the previous treatment with radioiodine. Thus, of the seven patients (3 in the previous and 4 in the present study) in whose OT Tg was found, six had been treated with radioiodine, whereas of the three patients with no Tg in their OT none had been treated with radioiodine. In conclusion, Tg was found in OT extracts from patients with TAO by immunoprecipitation in three of seven cases and by ELISA in four of seven cases. Tg was found in retrobulbar fibroadipose tissue, but not in extraocular muscles. There was a relation between the presence of Tg in OT and the previous treatment with radioiodine. Our results support the hypothesis that Tg may play a role as a coantigen in the pathogenesis of TAO. Further studies are needed to investigate this possibility.

Identification of thyroglobulin in orbital tissues of patients with thyroid-associated ophthalmopathy

Thyroid-associated ophthalmopathy (TAO) is thought to have an autoimmune pathogenesis because of its association with autoimmune thyroid disease, in particular with Graves' disease. Nevertheless, the nature of the autoimmune reaction is unclear, and a target orbital autoantigen has not been conclusively identified. A widely discussed hypothesis is that antigens constitutively shared by the thyroid and orbital tissues are targets of an autoimmune reaction. It has been also postulated that a thyroid-soluble antigen, namely thyroglobulin (Tg), is transported to orbital tissues through the lymphatics, where it accumulates and elicits autoimmune damages in susceptible individuals. Here we have investigated whether Tg is present in orbital tissues from patients with TAO. Retrobulbar tissue specimens were obtained from three patients with Graves' disease and TAO who underwent decompressive orbitotomy, and at autopsy from two patients with no thyroid or eye disease. All patients with TAO had been previously treated with radioiodine to control Graves' hyperthyroidism. Western blot analysis with a monoclonal anti-Tg antibody showed the presence of intact Tg, both in soluble and membrane-associated fractions of orbital tissue extracts from the patients with TAO, in amounts estimated to range from approximately 320 to approximately 900 pg/microg of tissue protein. In contrast, Tg was not detected in orbital tissue extracts from patients with no thyroid or eye disease. Tg was also demonstrated in orbital tissue extracts from two of three patients with TAO by enzyme-linked immunosorbent assay (ELISA), in amounts estimated to range from approximately 450 to approximately 1000 pg/microg of protein. In addition, Tg in orbital tissue extracts from patients with TAO was immunoprecipitated by a rabbit anti-Tg antibody, suggesting that it retained its native conformation. An anti-thyroxine (T4) antibody captured in solid-phase Tg from orbital tissue extracts, showing that it contained thyroid hormone residues and had therefore originated in the thyroid. Tg-anti-Tg immune complexes were not found in orbital tissues, suggesting that if an autoimmune reaction to Tg occurs in TAO, it is likely to be cell mediated.

Somatostatin-receptor scintigraphy in Graves' disease: reproducibility and variance of orbital activity

Somatostatin-receptor (SSTR) scintigraphy using the single photon emission computed tomography (SPECT) technique allows the assessment of orbital inflammation in patients with Graves' disease. Previous studies showed differences in orbital octreotide uptake already 4 hr after injection. In this study, analysis of inter-/intra-observer variance and reproducibility in the evaluation of orbital SPECT images was performed. First, SPECT data of one representative female patient with clinically active Graves' ophthalmopathy (GO), obtained 4 hr after intravenous injection of 110 MBq 111In-pentetreotide and processed by filtered backprojection, were analyzed. Transverse SPECT images were reconstructed, an optimal orbital image was selected and predetermined regions of interests (ROIs) for both orbits were positioned by three independent observers 15 to 19 times each. In a second step, SPECT data of 8 different patients with GO were evaluated in the same manner by four independent observers 3 to 4 times each. Variance component partitioning was used to compare the order of intra- and inter-observer variation. For the right and the left orbit, the inter-observer variance proportion was 90% and 79%, whereas intra-observer variance partition was 10% and 21%, respectively. The corresponding ratios 0.11 and 0.27 summarize the comparison of sources of variance. The overall reliability was 84%, representing the patients influence on the total variance. Intra-observer reliability for both orbits was 88%, 89%, 97% and 98% (mean over orbits), respectively for observers I to IV. Using the Spearman Brown prophecy formula it follows that two replications per patient are sufficient to ensure a minimum reproducibility of 90%, which is also confirmed by the low intra-observer variation. Furthermore, intra-class correlation as a measure of (multiple) observer reproducibility was 94%. In conclusion, due to the increased inter-observer variance proportion and the high variation in intra-observer reliability, evaluations of orbital SSTR scintigraphy have to be done by the same and experienced observer leading to comparable data. But an automatic and quantitative computerized technique for evaluation of these SPECT data should be exactly reproducible and probably lead to more accurate and representative results.

[Endocrine orbitopathy 1998]

BACKGROUND

Ophthalmopathy is the most common extrathyroidal manifestation of Graves' disease, also called thyroid-associated ophthalmopathy.

CLINICS

This autoimmune disorder is characterized by a lymphocyte infiltration of the retrobulbar space. Activated T cells react with the target organ and secrete cytokines, leading to accumulation of glycosaminoglycans, interstitial edema of the peri- and retrobulbar tissue and enlargement of the extraocular muscles. During the inflammatory stage, the increased orbital pressure results in the clinical manifestation of the eye disease.

DIAGNOSIS AND TREATMENT

Thus, therapeutic immunosuppression is often used initially, and by suppressing inflammatory changes, it can result in subjective and objective improvement of the thyroid eye disease. In recent years, new pathogenetic aspects and clinical randomized trials led to modified therapy concepts.

CONCLUSION

Interdisciplinary management is recommended for rapid diagnosis and effective therapy of patients with thyroid-associated ophthalmopathy.

Beneficial effect of pentoxifylline on thyroid associated ophthalmopathy (TAO)*: a pilot study

We have previously found that pentoxifylline (Ptx) inhibited cytokine induced HLA-DR expression and glycosaminoglycan (GAG) synthesis by retroorbital fibroblasts. We have now tested the clinical efficacy of Ptx in treating TAO. Ten patients with moderately severe ophthalmopathy were selected for study. All patients were euthyroid before and during the 12 weeks of the Ptx therapy. Serum GAG, TNF-alpha, anti-TSH-receptor, anti-eye muscle, anti-thyroglobulin and anti-thyroid peroxidase antibodies were determined sequentially. At the end of 12 weeks eight of the ten patients showed improvement in soft tissue but not in proptosis or extraocular muscle involvement. At baseline the levels of GAG (5.2+/-0.92 mg/dl v.s. 0.7+/-0.14 mg/dl, p<0.001) and TNF-alpha (33.6+/-6.6 pg/ml v.s. 5.4+/-1.3 pg/ml, p<0.001) were increased in patients compared to controls. They gradually decreased in the eight patients who responded to Ptx: after 4, 8 and 12 weeks of therapy serum GAG was 3.4+/-0.42 mg/dl, 2.5+/-0.77 mg/dl (p<0.01) and 1.1+/-0.2 mg/dl (p<0.001), respectively and serum TNF-alpha was 20.9+/-4.8 pg/ml, 14.9+/-2.2 pg/ml (p<0.05) and 9.7+/-1.8 pg/ml (p<0.01), respectively. Serum GAG and TNF alpha did not fall in the two patients who did not respond. The titre of anti-eye muscle antibodies but not anti-thyroid antibodies were lower at 12 weeks. Ptx has a beneficial effect on inflammatory symptoms of TAO and associated laboratory parameters in the majority of patients.