Abstract: Orbital fibroblast (OF) cells express thyrotropin receptor (TSHR), insulin-like growth factor-1 receptor (IGF-1R) and immunomodulatory molecules, and some of them can produce hyaluronic acid (GAG), inflammatory mediators and autoantibodies when given appropriate stimulation, and some of them can differentiate into adipocytes, which are the main target cells in Graves’ ophthalmopathy The main target cells of autoimmunity [1]. Therefore, inhibition of cytokine activity, reduction of B lymphocytes to inhibit antibody production, and inhibition of adipogenic differentiation of OF could be targets for new therapeutic agents in Graves’ ophthalmopathy.
Graves’ ophthalmopathy (GO), also known as thyroid-associated ophthalmopathy, is an organ-specific autoimmune disease that is both associated with and relatively independent of Graves’ disease.Garrity et al [1], by studying the characteristics of histological changes in the eyes of GO patients, concluded that OF is the main target cell of GO autoimmunity.OF is present in the expression of multiple autoantigens, and after antigen recognition, activates T cells and B cells, ultimately producing multiple cytokines and autoantibodies. Recent studies have shown that orbital 0F expresses multiple phenotypes, with some subpopulations producing GAG and inflammatory mediators in response to cytokine stimulation, and others differentiating into adipocytes. Further exploration of the role of OF in the pathogenesis of Graves’ eye disease will potentially provide new therapeutic targets for GO.
1 OF and the pathogenesis of GO
1.1 OF and cytokines
Both domestic and international studies have confirmed the presence of multiple cytokines in the retrobulbar tissue of GO patients, with Th1 cytokines predominating. Some of these cytokines can stimulate OF to proliferate, secrete large amounts of GAG, and produce various cytokines and immunomodulatory molecules, including human leukocyte antigen class II molecule (HLA-II), cell adhesion molecule-1 (ICAM-1), and heat shock protein-72 (HSP-72), thereby amplifying the local inflammatory response in the orbit [2]. Some of them can stimulate OF proliferation and differentiation into mature adipocytes, which increases the volume of retro-orbital adipose tissue and eventually leads to proptosis. In addition, studies have demonstrated that orbital OF predominantly expresses CD40, which is not found in other tissues of the organism, and that CD40 is an important B lymphocyte activator that binds to the receptor CD154, which is expressed at high levels by T lymphocytes.CD40/CD154 binding causes fibroblasts to produce various inflammatory mediators, including IL-1, IL-6, and IL-8, and to produce large amounts of GAG, which results in the accumulation of GAG in the orbit of Graves patients, causing tissue edema and enlargement of the muscle belly of the extraocular muscles [3].
1.2 OF and fat formation
CT scans show that most patients with GO have both increased intraorbital fat and enlarged extraocular muscles, while others have one or the other predominantly [1].Nishhida [4] et al. found that the increase in intraorbital adipose tissue volume in TAO patients was significantly higher than the increase in extraocular muscle volume, and the correlation coefficient between intraorbital volume and GO proptosis was significantly higher than that of extraocular muscle volume, suggesting that increased fat volume plays a very important role in the pathogenesis of GO. The correlation coefficient between orbital volume and GO proptosis was significantly higher than that of extraocular muscle volume. The increased intraorbital adipose tissue is formed by RF differentiation. It was found that transfection of mouse fibroblasts (NIH3T3) with a PPAR-γ retroviral expression vector resulted in the conversion of mouse fibroblasts into adipocytes in a PPAR-γ ligand-dependent manner [5].Sorisky [6] et al. demonstrated the presence of adipose precursor cells in human orbital adipose tissue/connective tissue, and Smith [7] also showed the presence of adipose precursor cells from the vascular stroma of newborns and from different parts of the adult body. Smith [7] also isolated OF, a subpopulation of OF, from the vascular stroma of newborns and adipose or connective tissue from different parts of the adult body that can differentiate into adipocytes, called fibroblastic adipose precursor cells. Under certain conditions of stimulation, these fibroblastic adipose precursor cells can differentiate into mature adipocytes, resulting in an increase in the volume of retrobulbar adipose tissue, which eventually leads to proptosis.
1.3 OF and TSHR
Recent studies have shown that adipocytes differentiated from OF are the main source of TSHR in the intraorbital tissues of GO patients. Studies have confirmed that TSHR mRNA and protein are expressed within the intraorbital adipose tissue of patients with Graves’ disease with or without ophthalmopathy [8], and that TSHR levels are indeed higher in GO patients than in patients without GO, suggesting that increased TSHR expression in the orbit may be involved in disease progression [9-10]. tsHR in intraorbital adipose tissue obtained during orbital decompression in GO patients
mRNA levels were positively correlated with the clinical activity of the patients further confirming this idea [11]. With increased differentiation of orbital OF to adipose, TSHR expression increases, as confirmed by in vitro culture of orbital fibrofat precursor cells in differentiation assays. mRNA levels of TSHR, PPAR-γ, leptin and lipocalin are approximately 10-fold higher in mature adipocytes than in undifferentiated cells [12]. kumar [10] et al. also found an increase in the expression of these genes in GO patients’ orbital adipose tissue had increased expression of these genes compared to normal tissue; and TSHR
mRNA levels showed a significant positive correlation with mRNA levels encoding leptin and lipocalin. In vitro experiments also confirmed that the gene expressions of leptin and TSHR were significantly increased in GO patients’ tissues.
1.4 OF and IGF-1R
Pritchard et al [13] demonstrated that OF from GO patients express IGF-1R. OF can be activated by IgG in vivo, resulting in increased IGF-1R activity and infiltration of activated T lymphocytes into inflammatory areas. Pritchard [14] suggested the presence of anti-IGF-1R antibodies in the circulating blood of GO patients, suggesting that IGF-1R may be a secondary antigen in Graves’ disease and play an important role in lymphocyte transport. In addition, IGF-1 in combination with IGF-1R could further promote massive proliferation of OF.
2 Immunotherapy of GO
Immunosuppressive agents are effective in the treatment of active GO, and GO activity can be objectively evaluated more easily using MRI, SPECT technique combined with octreotide [15] and disease activity scoring criteria drawn up by Mourits based on the clinical manifestations of the active phase. The most common and basic method is glucocorticoid therapy, and Christopher [16] et al. demonstrated that systemic high-dose intravenous hormone shock therapy alone or in combination with local radiotherapy of the eye is the most effective treatment option. In recent years, some new therapeutic targets have been proposed through research on OF at home and abroad, which will help the treatment of GO.
2.1 Inhibition of cytokine activity
Based on the interaction between cytokines and OF in the early pathogenesis of GO as a theoretical basis, monoclonal antibodies targeting pro-inflammatory response cytokines and inflammatory mediators will be particularly promising, especially TNF-a blockers or IL-1 receptor blockers [17].Durrani [18] et al. have successfully treated a GO patient with infliximab.Komorowski et al. Komorowski et al [19] also achieved good results with infliximab in an elderly female GO patient with Graves’ disease.Paridaens [20] et al treated 10 patients with moderately active GO with enalapril, resulting in a decrease in CAS scores of about 60% and a significant improvement in soft tissue changes such as conjunctival edema.Komorowski [21] also achieved good results with enalapril and infliximab Bonara et al [22] successfully treated a patient with early active GO with rituximab who did not respond to glucocorticoids. El Fassi et al [23] concluded that infliximab, enalexib, and rituximab have good potential for the treatment of GO. The cytokine modulator hexoketococine (Ptx), which inhibits HLA-DR expression and TNF-a transcription and reduces IL-1, 6 and IFN-r production, thereby inhibiting these inflammatory cytokines to stimulate OF synthesis and secretion of GAG, has also been used clinically.Balazs [24] treated 10 moderately active GO patients who were contraindicated to glucocorticoids with Ptx and found that all The patients showed significant improvement in soft tissue changes and decreased levels of TNF-a and hyaluronic acid, but there was no significant improvement in extraocular muscle viability and proptosis in the patients. finamor [25] et al. concluded that Ptx was effective in proptosis in patients with inactive GO. In addition, T-lymphocyte costimulatory signaling inhibitors such as CTLA4-Ig or alefacept can block the “second signal” needed to activate T cells and inhibit cytokine production, and by targeting these early steps of the immune response, these drugs theoretically have the dual effect of inhibiting antibody production and inflammatory cytokine By targeting these early steps of the immune response, these drugs theoretically have the dual effect of inhibiting antibody production and inflammatory cytokine secretion, which is promising [26].
2.2 Inhibition of IGF-1R binding to its receptor
Drugs that block the binding of IGF-1 to its receptor or target IGF-1R activity are another potential option for GO therapy, and they inhibit the effect of circulating blood antibodies to the IGF-1 receptor on ocular cells [13], but because of the carcinogenic-promoting effects of IGF, small-molecule inhibitors of IGF-1R antibodies, IGF-1R tyrosine kinases, and antisense RNA fragments, including anti-IGF-1R antibodies biological agents have not been actively developed. Growth inhibitor analogues can directly block the action of IGF-1 on tissues; they can also indirectly inhibit the action of IGF-1 by reducing the concentration of growth hormone in plasma and reducing the synthesis of GAG; they inhibit the release of lymphokines from T lymphocytes and suppress the action of cytokines [15], and are commonly used clinically, such as octreotide and lanreotide, and the new growth inhibitor analog SOM230 has a high growth inhibitor receptor affinity [15].
2.3 Reduction of B-lymphocytes and inhibition of antibody production
Reducing B lymphocytes, blocking their binding to CD20, and inhibiting antibody production at an early stage may be beneficial, as substantial evidence confirms the involvement of anti-TSHR and IGF-1R antibodies in the development of GO disease [15]. Currently available biologic agents are anti-B lymphocyte agents Meroval, a CD20 human-mouse chimeric monoclonal antibody (RTX). RTX acts directly against CD20 antigen, the surface antigen of normal and malignant B lymphocytes, by mediating B cell apoptosis, complement and antibody-dependent cell-mediated cytotoxicity. El Fassi [29] showed a durable remission of GO patients with low levels of TRAb but no effect on antibody levels, but RTX appeared to be ineffective in GO patients with high levels of TRAb. Recently, Komorowski [21] et al. also obtained positive results with RTX for early active GO.
2.4 Inhibition of adipose differentiation and prevention of tissue remodeling
Targeting the early stages of OF differentiation to mature adipocytes as a therapeutic target is a hot topic of current research. Valyasevi et al [12] have shown that rosiglitazone (RGZ), a PPAR-γ agonist, promotes the differentiation and maturation of orbital fibrofat precursor cells cultured in vitro, while bisphenol propane diepoxide (BADGE), a PPAR-γ antagonist, antagonizes the effect of RGZ. Starkey et al [30] demonstrated that the PPAR-γ antagonist GW9662 inhibited adipocyte differentiation and formation in an in vitro study, suggesting that a drug that specifically blocks PPAR-γ binding would be another promising treatment for GO. Starkey reported increased ocular protrusion in a patient with type 2 diabetes mellitus treated with RGZ in combination with GO [30]. Similar cases of worsening GO after administration of thiazolidinediones have been reported [31-32], and a recent study by Dorkhan et al [33] showed an increase in ocular prominence in some patients with type 2 DM after pioglitazone treatment. It is evident that PPAR-γ agonists may be contraindicated in GO patients, and PPAR-γ antagonists or drugs that inhibit the PPAR-γ signaling system will potentially be a new therapy for GO patients in the active phase. In addition, Vondrichova T et al [34] demonstrated that diclofenac has cyclooxygenase (COX-2) inhibition, anti-PPAR-γ, and fat formation inhibition, which may become a direction for the future treatment of GO.
3 Conclusion
The pathogenesis of GO and related immunotherapy have gradually received attention from several disciplines in the medical field. With further understanding of the pathogenesis of GO, it will provide new therapeutic targets for GO, but any drug related to the prevention or treatment of GO needs to be evaluated in a large number of multicenter prospective, randomized, double-blind experimental studies for its safety, dose-effect ratio, effective dose and related side effects.