Novel ways to attack inflammation in thyroid eye disease

Graves’ disease (GD) is a systemic autoimmune disease affecting 1–2% of the US adult population [1,2]. Over 90% of patients with GD present with goiter and clinical signs of hyperthyroidism while 20–50% develop thyroid eye disease (TED) [3]. Nearly half a century ago, studies showed that the sera from patients with GD contained a thyroid-stimulating antibody directed against the thyrotropin receptor (TSHR) [4–7]. These antibodies mimic the activity of thyroid-stimulating hormone (TSH), but have longer acting effects and are not subject to the physiologic hypothalamic–pituitary feedback loop. Despite recent advances in the genetic and molecular understanding of GD, the mechanisms behind inflammation patterns in seemingly unrelated anatomic regions as well as the self-limited nature of inflammation remain unclear.

Clinical course of TED

The orbital and periocular manifestations of GD are collectively termed TED. Most patients with TED experience mild self-limited ocular surface irritation, but 3–5% of patients have severe orbital congestion with pain, double vision or loss of vision [8]. Rundle was the first to divide the course of TED into active (dynamic) and inactive (static) disease phases [9]. The active phase of TED is characterized by progressive proptosis and diplopia, eyelid edema, conjunctival chemosis and injection, exposure keratitis, and even vision loss from compressive optic neuropathy. Usually within 18–24 months of the onset of active TED, inflammation resolves spontaneously and patients are left with stable proptosis, eyelid retraction and restrictive strabismus.

Current treatment of TED

Once the diagnosis of TED is made, patients who are hyperthyroid should be restored to euthyroid status and smokers are strongly encouraged to pursue smoking cessation. Both of these measures have been shown to increase response to treatment and hasten the resolution of TED [10–12]. For most patients with mild TED, supportive measures such as ocular surface lubrication are sufficient [13]. For more severe active TED, systemic glucocorticoids are considered, with the goal of decreasing orbital inflammation and minimizing inflammatory sequelae. Surgical intervention, in the form of staged orbital decompression, strabismus surgery and eyelid correction, are typically used for functional and cosmetic rehabilitation during inactive TED [14–16]. When compressive optic neuropathy occurs, however, urgent orbital decompression may prevent vision loss. As we garner more knowledge about the molecular pathways involved in TED, we seek to devise targeted therapies directed against specific steps in the TED inflammatory pathway.

Immunology of TED

The role of T cells, B cells & cytokines

Immunohistochemical studies performed on orbital specimens from patients with active TED show intense infiltration of T lymphocytes, mast cells and occasional B cells between extraocular muscle fibers and within orbital fat, suggesting that connective tissues are the primary autoimmune targets [17,18]. In the early stages of active TED, Th1 cells predominate and produce cytokines, IL-2, IFN-γ and TNF-α [19,20]. A more robust Th2-dominated immune response appears in the later stages of inflammation, with the production of IL-4, IL-5 and IL-10 [21]. B lymphocytes, which serve dual roles as immunoglobulin-secreting cells and antigen-presenting cells, produce IL-6, lymphotoxin, TNF-α and IL-10 [22]. Macrophages, fibroblasts and adipocytes produce other inflammatory mediators, including IL-1α, IL-6 and TGF-β within the orbit [23,24]. The interplay of these infiltrating immune cells with resident orbital fibroblasts is complex but is likely the lynchpin to understanding the site-specific disease process.

T-cell-targeted therapy

T-cell depletion using anti-CD3 monoclonal antibodies has been studied in vitro and in animal models of Type 1 diabetes [25,26]. The use of anti-CD3 antibodies in the absence of costimulatory signals leads to CD4⁺ T-cell anergy and tolerance [27,28]. Another T-cell-mediated pathway of interest in tumorigenesis and autoimmunity involves highly specialized T lymphocytes called regulatory T cells (Tregs). Tregs exhibit a CD4⁺CD25⁺Foxp3⁺ phenotype and when these cells are deficient, immunopathology can occur [29]. Conversely, when Tregs are replete, autoimmune diseases are tempered [30–32]. Anti-CD3 antibody depletion of pathogenic T cells increases the appearance of Tregs, thus it holds promise as a future treatment of TED by directly diminishing pathogenic T cells while indirectly enhancing beneficial regulatory T cells.

B-cell-targeted therapy

B cells are essential to the initiation of autoimmune thyroid disease [33], and autoantibody generation is also dependent on the complex interplay between B and T cells. Thus, B-cell-depleting therapies and those that interrupt interactions between cognate molecules on B-cell surfaces offer great promise in the treatment of autoimmune disease. B-cell-targeted therapies used in autoimmune disease include: rituximab (RTK; anti-CD20), eprantuzumab (anti-CD22), DT2219 (anti-CD19 and CD22) and abatacept (anti-CTLA4), among others [34]. Of these therapies, RTX, has recently garnered much attention in the TED literature. RTX is a humanized monoclonal antibody that binds the B-cell surface antigen CD20, attenuating CD20-dependent B-cell maturation. It was first used in 1997 to treat non-Hodgkin B-cell lymphomas [35], and was subsequently used in IgM antibody-related polyneuropathies and rheumatoid arthritis [36,37]. In 2006, two case reports showed significant reduction in the clinical activity in patients with TED unresponsive to steroids [38,39]. A prospective, controlled study demonstrated sustained remission of hyperthyroidism in GD patients treated with RTX even though the drug failed to influence autoantibody levels [40]. In another open, nonrandomized study of patients with TED, RTX was compared with intravenous glucocorticoid therapy [41]. Patients receiving RTX demonstrated greater improvement of the clinical activity score with fewer side effects than those treated with glucocorticoids. Our group has demonstrated rapid and sustained resolution of orbital inflammation and optic neuropathy in patients with the most severe form of TED, who are resistant to corticosteroid treatment [42]. Well-controlled, prospective and adequately powered studies remain essential to fully evaluate the role of RTX in TED.

Cytokine-targeted therapy

Disruption of the cytokine pathways in general holds substantial promise but a complete discussion is beyond the scope of this article. In particular, the TNF-α pathway has become a highly successful approach to treat rheumatoid arthritis and Crohn’s disease. In the synovium, TNF-α induces the production of proinflammatory cytokines, chemokines and cell adhesion molecules. Together, they mediate leukocyte chemoattraction, propagating inflammation. The presence of TNF-α in TED orbital tissues has previously been demonstrated [43]. Three biological anti-TNF-α agents are currently in wide clinical use, including the monoclonal antibodies infliximab and adalimumab, and etanercept, a human recombinant soluble TNF-α receptor fusion protein. Two separate reports of infliximab use in patients with TED suggest reduced inflammation and improved visual function [44,45]. Since TNF-α is present in the orbital tissue of TED patients and TNF-α inhibitors have been widely used in other autoimmune diseases, their use may merit consideration for clinical trials in TED.

Role of autoantigens

Research in recent decades suggests that multiple autoantigens may exist in TED, analogous to other autoimmune disorders. Most autoimmune diseases have several cycles of stimulation with different antigens before clinical disease becomes apparent. The role of autoantibodies to the TSHR is firmly established in the pathogenesis of hyperthyroidism in GD, but the evidence to explain the role of TSHR in TED is unclear. Several interesting correlations between TSHR antibody levels and disease activity have been reported, and TSHR mRNA has been detected in orbital tissues and orbital fibroblasts, but studies have been performed on limited sample sizes and do not explain pathogenesis [46,47].

The IGF-1 receptor (IGF-1R)/IGF-1 pathway may be another autoantigen in TED. Orbital fibroblasts in patients with TED express elevated levels of IGF-1R [48]. Treatment of these fibroblasts with either IGF-1 or pooled serum IgG from patients with GD stimulates the synthesis of powerful T-cell chemoattractants as well as the generation of hyaluronan [49]. Importantly, neither IGF-1 nor GD-IgG elicited these responses in fibroblasts from individuals without autoimmune disease. These findings suggest that increased levels of IGF-1R may play a role in the pathogenesis of GD. Evidence that TSHR and IGF-1R might be functionally linked was strengthened recently when these proteins were found to colocalize [50]. Several strategies for disrupting IGF-1R signaling have been developed recently and are currently being evaluated as therapy for cancer [51].

Role of fibroblasts & fibrocytes

Several studies have demonstrated that orbital fibroblasts, especially those from patients with GD, are uniquely susceptible to several proinflammatory cytokines. Orbital fibroblasts fail to generate adequate levels of soluble IL-1 receptor antagonist, allowing for poorly opposed IL-1β signaling, which in turn causes increased expression of the inflammatory cyclooxygenase prostaglandin-endoperoxide H synthase (PGHS)-2 [52]. Orbital fibroblasts also exhibit enhanced production of extracellular matrix components such as hyaluronan in response to these cytokines. Thus, orbital fibroblasts in GD produce proinflammatory molecules and components of connective tissue that lend themselves to the site-specific tissue remodeling occurring in TED.

Recently, our group described bone marrow-derived fibroblast-like cells called fibrocytes that are elevated in the circulation of GD patients. Fibrocytes have been shown to migrate to sites of injury and mediate site-specific inflammation and fibrosis in diseases including rheumatoid arthritis, pulmonary fibrosis and renal fibrosis [53–55]. We found that these CD34⁺ fibrocytes selectively infiltrate orbital tissues of TED patients, suggesting a migratory role for these cells in the orbital process. Most surprisingly, we found a high expression of TSHR and IGF-1R on fibrocytes from patients with GD [56]. The addition of TSH or antibodies to the TSHR to fibrocytes in culture demonstrated the production of proinflammatory cytokines including IL-6 and IL-8. Therapeutic targeting of fibrocytes or inhibiting the molecular signals of fibrocyte infiltration, such as CXCL12, may be critical for the prevention of fibrosis.

CD40 expression by T cells and fibroblasts may also play an important role in site-specific immune activation [57]. CD40 binds CD40 ligand (also known as CD154) displayed on the surface of T lymphocytes, and fibroblasts provide T-cell costimulation that results in clonal expansion of naive T lymphocytes and enhances proinflammatory cytokine production, including that of IL-1, IL-6 and IL-8. Thus, disruption of the CD40–CD40 ligand interaction may represent an important therapeutic target in TED. Administration of therapeutic blocking antibodies against CD40 ligand has already proven effective in preclinical mouse models of diabetes and inflammatory bowel disease [58,59].

Five-year view

Despite intensive study, the identity of the proximate antigenic target initiating TED, and the relationship between the orbital disease and the other components of GD, remain uncertain. Important insights concerning the pathogenesis of allied autoimmune diseases and increasing knowledge about their successful treatment should shed new light on the fundamental factors underlying TED and facilitate the development of therapies for this particularly vexing process. Unfortunately, studies of treatment modalities in TED are plagued by the inherently heterogeneous severity and natural history of TED. Meaningful clinical studies of treatment modalities will require a coordinated, multicenter effort to define disease severity and activity, similar to those proposed by the International Thyroid Eye Disease Society [101].

Financial & competing interests disclosure

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

No writing assistance was utilized in the production of this manuscript.