Immunoglobulin class and subclass distribution of eye muscle and fibroblast antibodies in patients with thyroid-associated ophthalmopathy

Eye muscle antibodies in Graves' ophthalmopathy: pathogenic or secondary epiphenomenon?

The extra ocular (eye) muscles are one of the principal tissues involved in the autoimmune-mediated inflammation of Graves' ophthalmopathy (GO). Several eye muscle proteins are targeted by autoantibodies or sensitized T lymphocytes, or both, and include: G2s, which is now identified as the terminal 141 amino acids of the winged-helix transcription factor FOXP1, the flavoprotein (Fp) subunit of the mitochondrial enzyme succinate dehydrogenase, the so-called "64kDa protein", a non-tissue specific membrane protein called 1D and the calcium binding protein calsequestrin. Of these, antibodies against G2s and Fp are the most sensitive markers of eye muscle damage in patients with thyroid autoimmunity even though neither antigen is specific to eye muscle and neither antibody is specific to GO. However, the recent finding that the calsequestrin gene is 4.7 times more expressed in eye muscles than other skeletal muscles suggests that we should reconsider the possible role of anti-calsequestrin autoantibodies in ophthalmopathy. GO may comprise two main subtypes with different pathogenetic mechanisms, namely ocular myopathy in which eye muscle inflammation predominates and congestive ophthalmopathy where inflammatory changes occur in the periorbital connective tissues in the absence of eye muscle dysfunction. Anti-G2s and anti-Fp antibodies are closely associated with the ocular myopathy subtype of GO while antibodies targeting type XIII collagen, the only member of the collagen family to have a transmembrane domain, are closely linked to congestive ophthalmopathy. Since both G2s and Fp are intracellular antigens it is unlikely that either antibody causes eye muscle fiber damage in GO, although a role in the later stages of the disease when the fiber has released its cellular contents has not been excluded. Eye muscle antibodies that are cytotoxic to eye muscle cells in antibody-dependent cell-mediated cytotoxicity (ADCC) are more likely to play a role in eye muscle fiber damage since they target a putative eye muscle cell membrane antigen, the identity of which is currently being investigated. While anti-G2s and anti-Fp antibodies are probably secondary to an underlying reaction, such as cytotoxic T lymphocyte targeting of an eye muscle membrane antigen that has yet to be identified, they are reliable markers of immunologically mediated eye muscle fiber damage in patients with Graves' hyperthyroidism. In conclusion, while a pathogenic role for eye muscle antibodies has not been excluded, they are most likely secondary to cytotoxic T cell reactions in GO and, as such, good markers of this autoimmune disease.

Autoantibodies to the flavoprotein subunit of succinate dehydrogenase: analysis of specificity in autoimmune thyroid disease

OBJECTIVE

Thyroid-associated ophthalmopathy is a progressive eye disorder affecting the extraocular muscle and orbital connective tissue and is considered to have an autoimmune aetiology. A recent study reported a close relationship between serum antibodies against the flavoprotein subunit of succinate dehyhdrogenase (SDHFp) and active thyroid-associated ophthalmopathy involving eye muscle damage. The aim of the present study was to develop a sensitive and quantitative radiobinding assay for the detection of antibodies to the flavoprotein subunit of succinate dehydrogenase and to use this to determine the distribution of antibodies in different patient groups.

DESIGN AND PATIENTS

Serum samples from the following patient groups were analysed: 20 systemic lupus erythematosus; 20 Addison's disease; 26 autoimmune hypothyroidism; 28 Graves' hyperthyroidism; 12 pretibial myxoedema; 25 thyroid-associated ophthalmopathy. Sera from 20 healthy subjects were used as controls. [35S]-labelled succinate dehydrogenase flavoprotein was produced in an in vitro transcription-translation system and subsequently used in immunoprecipitation experiments with sera from patient and control groups to test for the presence of antibodies to the flavoprotein.

RESULTS

Succinate dehydrogenase flavoprotein antibodies were detected in five of the 20 (25%) patients with Addison's disease, six of the 20 (30%) with systemic lupus erythematosus, five of the 26 (19%) with autoimmune hypothyroidsm, six of the 28 (21%) with Graves' hyperthyroidism, two of the 12 (17%) with pretibial myxoedema and three of the 25 (12%) with thyroid-associated ophthalmopathy. The frequencies of flavoprotein antibodies were significantly greater than controls (P-value < 0.05) for patients with systemic lupus erythematosus (P = 0.02), but not for patients with either Addison's disease (P = 0.05), pretibial myxoedema (P = 0.13), Graves' hyperthyroidism (P = 0.07), autoimmune hypothyroidism (P = 0.06) or thyroid-associated ophthalmopathy (P = 0.24). For the patients with thyroid-associated ophthalmopathy, the frequency of SDHFp antibodies did not appear to be related to the length of time from diagnosis: the group containing samples taken less than one year from diagnosis showed no increased frequency of SDHFp antibodies when compared to controls (P = 0.10), with three of the 18 (17%) patients being positive. With respect to seven patients with thyroid-associated ophthalmopathy diagnosed for more than a year, SDHFp antibodies were not detected in any of their serum samples. In addition, the clinical severity of the disease, as recorded by the NOSPECS classification, did not correlate with the frequency of SDHFp antibodies: P = 0.13, 0.33 and 0.38, respectively, for patients with Grade II, III and IV ophthalmopathy. Similar results were also found in the case of patients with pretibial myxoedema and eye disease: P = 0.06 for patients with Grade III ophthalmopathy and, SDHFp antibodies were not detected in any of the sera taken from patients with Grade IV ophthalmopathy. In addition, no association was found between disease duration and the frequency of antibodies to the flavoprotein in this patient group.

CONCLUSIONS

Our results indicate that succinate dehydrogenase flavoprotein antibodies are not a suitable marker for thyroid-associated ophthalmopathy, at least with the assay system used, as they can be found in patients who do not have eye disease and therefore lack the disease specificity required of a diagnostic tool.

Graves' disease and thyroid associated ophthalmopathy triggered by 131I treatment of non-toxic goiter

A 39-year-old Danish woman was treated with an oral dose of 400 MBq 131I for a non-toxic goiter with compression symptoms. Serum anti-TPO and TSH receptor antibodies were negative before radioiodine therapy. The patient developed hyperthyroidism four months after treatment and ophthalmopathy after another three months. TSH receptor stimulating and blocking antibodies, eye muscle IgA and IgG were measured and an increase in stimulating TSH receptor antibodies were found concomitant with development of hyperthyroidism. IgG class antibodies against porcine eye muscle antigens and eye muscle fibroblasts were elevated and unchanged during the follow-up. A significant increase in IgA class antibodies against porcine eye muscle antigens was seen concomitant with development of ophthalmopathy. It is speculated that 131I has led to an exacerbation of eye muscle as well as TSH receptor stimulating antibodies and possibly leading to the development of ophthalmopathy, or at least is associated with it.

Thyroid-associated ophthalmopathy in black South African patients with Graves' disease: relationship to antiflavoprotein antibodies

Thyroid-associated ophthalmopathy (TAO) is a progressive eye disorder associated with Graves' hyperthyroidism, which is generally considered to have an autoimmune etiology. Eye muscle membrane proteins of 64 kd are good markers of ophthalmopathy in patients with thyroid autoimmunity. The 64-kd protein is now shown from a partial sequence to be the flavoprotein subunit (Fp) of mitochondrial succinate dehydrogenase. Hyperthyroidism due to Graves' disease is increasing in incidence among urban black female Africans, possibly because of exposure to environmental risk factors such as increased dietary iodine ingestion and stress. Ophthalmopathy is frequently observed in this clinical context, but its association with serum autoantibodies reactive with Fp has not been examined. We studied 19 black South African patients with Graves' disease during the course of prolonged antithyroid drug administration, of whom 10 had congestive ophthalmopathy, but no clinical evidence for eye muscle damage at the onset. Anti-Fp antibodies were detected in 2 of these patients, as well as in 2 of the 9 patients who did not have overt eye disease. Additionally, the antibodies became positive in 3 patients with ophthalmopathy in whom tests were negative initially, remained positive in 1 patient throughout the study period and became negative in 1 patient with positive tests initially. Ophthalmopathy did not develop in any of the 9 patients who lacked this complication on presentation. The reasons why we failed to demonstrate a close relationship between anti-Fp antibodies and the eye muscle component of ophthalmopathy are unclear although one possibility is that ocular myopathy is an uncommon manifestation in African thyrotoxic patients compared with those of Caucasian origin. The relationship between anti-Fp antibodies and eye muscle inflammation in patients with thyroid autoimmunity of different ethnic origins and environmental settings, needs to be addressed in a large prospective study.

Role of Eye Muscle Antibody Measurement in Diagnosis of Thyroid-Associated Ophthalmopathy: A Laboratory Update

OBJECTIVE

To review the current role of measurement of serum eye muscle antibodies in thyroid-associated ophthalmopathy (TAO).

METHODS

We conducted laboratory studies to determine the prevalences of serum autoantibodies reactive with eye muscle antigens in patients with active and inactive TAO, Graves' hyperthyroidism, and Hashimoto's thyroiditis as well as in normal subjects.

RESULTS

The two antigens most often recognized in immunoblotting with crude human or porcine eye muscle membranes by serum autoantibodies in patients with TAO are eye muscle membrane proteins of 55 and 64 kd. One 64-kd eye muscle protein has recently been cloned by screening a human eye muscle expression library with two different antibody probes and identified from a computer gene bank search as the calcium-binding protein calsequestrin. A fragment of a 220-kd eye muscle protein, called G2s, has also been cloned by screening the eye muscle library with affinity-purified antibodies reactive with a 55-kd eye muscle membrane protein. The prevalences of autoantibodies reactive with these two antigens in our study groups were as follows. Antibodies against calsequestrin were detected in 38% of patients with TAO for <1 year, in 17% of those with TAO for >3 years, in 17% of patients with Graves' hyperthyroidism without ophthalmopathy, in 12% of patients with Hashimoto's thyroiditis without ophthalmopathy, and in 21% of normal subjects. Antibodies reactive with the 64-kd protein were demonstrated in 62% of patients with recent-onset active TAO, in 33% with eye disease for >3 years, in 39% of patients with Graves' hyperthyroidism without ophthalmopathy, in 25% of patients with Hashimoto's thyroiditis, and in 16% of normal control subjects. Antibodies reactive with G2s fusion protein were detected in 67% of patients with recent-onset active TAO, in 46% of patients with Graves' hyperthyroidism, and in 20% of normal subjects. Antibodies reactive with the parent protein, of which G2s is a fragment, may be markers of early eye muscle swelling and inflammation, whereas those reactive with the 64-kd protein and, less often, calsequestrin are associated with established eye disease.

CONCLUSION

Measurement of serum eye muscle antibodies is recommended as an aid to the early diagnosis of ophthalmopathy in predisposed patients and first-degree relatives of patients with TAO as well as to monitor active or progressive eye disease.

The 64-kilodalton eye muscle protein is the flavoprotein subunit of mitochondrial succinate dehydrogenase: the corresponding serum antibodies are good markers of an immune-mediated damage to the eye muscle in patients with Graves' hyperthyroidism

Thyroid-associated ophthalmopathy (TAO) is a progressive eye disorder associated with thyroid autoimmunity, particularly Graves' hyperthyroidism, which is generally considered to have an autoimmune etiology. Eye muscle membrane proteins reportedly of 55 and 64 kDa are the best markers of the ophthalmopathy. The main focus of our recent studies has been to purify the pertinent proteins from porcine eye muscle membranes and characterize them. The 64-kDa protein is now shown from a partial sequence and by Western blotting using specific antibody probes to be the flavoprotein (Fp) subunit of succinate dehydrogenase and to have a correct molecular mass of 67 kDa. The protein was purified and cleaved with cyanogen bromide, and the N-terminal region of an immunoreactive partial peptide was determined. The 20-amino acid porcine sequence so obtained matched one within the Fp subunits of human and bovine succinate dehydrogenases in 20 and 18 of these positions, respectively. Succinate dehydrogenase is both a citric acid cycle enzyme and a component (complex II) of the mitochondrial respiratory chain. It is thus essential for aerobic energy production and is highly conserved. The mature human and bovine Fp subunits are 92% homologous and have a molecular mass of approximately 67 kDa, the same as our redetermined value for the 64-kDa marker protein. Sera from patients with TAO and from those with Graves' hyperthyroidism without evident ophthalmopathy highlighted the 64-kDa marker protein in crude porcine eye muscle membranes and the Fp subunit of highly purified bovine succinate dehydrogenase at the identical position on Western blots. Anti-beef Fp antibodies were detected in sera from 67% of patients with active TAO of more than 1-yr duration, in 30% with stable TAO of more than 3-yr duration, and in 30% of patients with Graves' hyperthyroidism without ophthalmopathy, but in only 7% of age- and sex-matched normal subjects. As succinate dehydrogenase is bound to the matrix (inside) surface of the mitochondrial inner membrane, it is unlikely to be accessible to circulating autoantibodies. We would postulate that eye muscle damage in ophthalmopathy is probably caused by cytotoxic antibodies or CD+ T lymphocytes targeting a cell membrane antigen, such as the thyroid and eye muscle shared protein G2s, and that presentation of succinate dehydrogenase is secondary. On the other hand, an autoantibody response to succinate dehydrogenase may be a good marker of immune-mediated damage to the eye muscle fiber and may support the idea that the extraocular muscles are targets of the autoimmune reactions of TAO.

Analysis of antibody-dependent cell-mediated cytotoxicity in autoimmune thyroid disease

There is no consensus on the role of antibody-dependent cell-mediated cytotoxicity (ADCC) in autoimmune thyroid disease; recent reports have suggested that antibodies mediating ADCC are found particularly in patients with primary myxoedema, occur less frequently in Hashimoto's thyroiditis and are absent in Graves' disease. Using an ADCC assay with a single source of effector and target cells, and expressing results as lytic units, we have found antibodies capable of mediating ADCC in 9 of 17 patients with primary myxoedema, 9 of 22 patients with Hashimoto's thyroiditis and 6 of 22 patients with Graves' disease. There was no significant difference between the groups in this distribution. Mean levels of ADCC activity were not significantly different comparing primary myxoedema and Hashimoto's thyroiditis patients, although levels were lower in Graves' disease patients compared to those with Hashimoto's thyroiditis (P < 0.05). There was no correlation between TPO antibodies (total IgG or IgG subclasses) measured by ELISA and ADCC activity. These results suggest that thyroid antigens besides TPO are involved in ADCC and that antibodies mediating ADCC are not restricted to subgroups of patients with autoimmune thyroid disease.

New assays for the measurement of serum antibodies reactive with eye muscle membrane antigens confirm their significance in thyroid-associated ophthalmopathy

Although sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (SDS-PAGE) and Western blotting are widely used to detect serum antibodies in patients with autoimmune disorders, this procedure unfolds and denatures proteins and may alter antibody binding sites. We have used nondenaturing methods for the purification of a 64-kDa eye muscle (EM) membrane antigen associated with thyroid-associated ophthalmopathy (TAO). Pig EM membrane proteins were prepared from crude homogenates by high-speed centrifugation and solubilized by hand homogenization. The 64-kDa protein was further purified by isoelectric focusing performed in the absence of SDS, detergents, reducing agents, and urea. Sera from patients with active TAO of recent onset and thyroid autoimmunity without ophthalmopathy were tested for reactivity against purified native 64-kDa protein in immunoblotting. Tests were positive in 64% of patients with TAO, in 37.5% of those with Graves' hyperthyroidism without eye disease, in 11% of patients with Hashimoto's thyroiditis without eye disease, and in 13% of normal subjects. Many of the same sera were also tested for cytotoxic activity against human EM cells in an antibody-dependent cell-mediated cytotoxicity (ADCC) assay. ADCC tests were positive in 69% of patients with TAO but in no normal subject. The specificity and sensitivity of these two tests in TAO surpass those for all other published results for orbital tissue reactive autoantibodies. Although there was a tendency for a relationship between reactivity to the 64-kDa protein and cytotoxic activity against EM cells in ADCC there were many exceptions and overall the relationship between the two tests was not significant.(ABSTRACT TRUNCATED AT 250 WORDS)