| Abstract: | Autoimmune response to cardiac troponin I (TnI) induces inflammation and fibrosis in the myocardium. High-mobility group box 1 (HMGB1) is a multifunctional protein that exerts proinflammatory activity by mainly binding to receptor for advanced glycation end products (RAGE). The involvement of the HMGB1–RAGE axis in the pathogenesis of inflammatory cardiomyopathy is yet not fully understood. Using the well-established model of TnI-induced experimental autoimmune myocarditis (EAM), we demonstrated that both local and systemic HMGB1 protein expression was elevated in wild-type (wt) mice after TnI immunization. Additionally, pharmacological inhibition of HMGB1 using glycyrrhizin or anti-HMGB1 antibody reduced inflammation in hearts of TnI-immunized wt mice. Furthermore, RAGE knockout (RAGE-ko) mice immunized with TnI showed no structural or physiological signs of cardiac impairment. Moreover, cardiac overexpression of HMGB1 using adeno-associated virus (AAV) vectors induced inflammation in the hearts of both wt and RAGE-ko mice. Finally, patients with myocarditis displayed increased local and systemic HMGB1 and soluble RAGE (sRAGE) expression. Together, our study highlights that HMGB1 and its main receptor, RAGE, appear to be crucial factors in the pathogenesis of TnI-induced EAM, because inhibition of HMGB1 and ablation of RAGE suppressed inflammation in the heart. Moreover, the proinflammatory effect of HMGB1 is not necessarily dependent on RAGE only. Other receptors of HMGB1 such as Toll-like receptors (TLRs) may also be involved in disease pathogenesis. These findings could be confirmed by the clinical relevance of HMGB1 and sRAGE. Therefore, blockage of one of these molecules might represent a novel therapeutic strategy in the treatment of autoimmune myocarditis and inflammatory cardiomyopathy.Inflammatory cardiomyopathy is a relatively common cause of acute heart failure in the young, for which an efficient and specific therapy is lacking. Although most patients recover completely, some present a deteriorating course. Recent work from our laboratory and others has focused on the dysregulation of the immune system as an essential modulator of disease induction and progression in heart failure (1, 2). In this context, release of cardiac troponin I (TnI) from damaged cardiomyocytes into the circulation is believed to trigger an autoimmune response to TnI (3, 4).Our group has established an animal model in which immunization with murine cardiac TnI induces severe myocardial inflammation and fibrosis, followed by severe heart failure (1). However, the exact pathomechanism and immune modulators involved in this inflammatory process are not yet fully elucidated.Extensive work has cast light on the role of high-mobility group box 1 (HMGB1) in the pathogenesis of infectious and noninfectious inflammatory diseases. HMGB1, first described as a DNA binding protein, has subsequently been associated with various pathological conditions such as cardiovascular disease (5, 6), cancer (7), and ischemia/reperfusion (I/R) injury (5). It is a key modulator of innate immune responses and regulates in part adaptive immunity (8). In response to cellular stress, HMGB1 acts as a damage-associated molecular pattern (DAMP) signal after passive release into the extracellular milieu during cell death or active secretion by mononuclear and other cell types (7). It binds to receptors such as receptor for advanced glycation end products (RAGE) and Toll-like receptors (TLRs) such as TLR-2 and -4, leading to the expression of inflammatory cytokines, chemokines, and corresponding receptors (9).Some studies describe differential effects of HMGB1- or RAGE-dependent signaling with regard to their concentration and release in a particular model or mode of application (10, 11). In a rodent model of myocardial infarction, exogenously administered HMGB1 had a beneficial effect on postinfarct myocardial remodeling (10). Kitahara and colleagues demonstrated reduced necrosis and smaller infarct size after myocardial infarction in transgenic mice overexpressing HMGB1 (12). However, in a murine model of I/R injury, our group recently showed that treatment of wild-type (wt) mice with recombinant HMGB1 increased the infarct size (6). In the same model of I/R injury, RAGE-deficient mice demonstrated significantly reduced myocardial damage compared with wt mice (6).The effect of HMGB1 and RAGE on the pathogenesis of cardiac disorders is not only described in preclinical animal models but is also investigated in human cardiac disorders. Different studies have revealed an elevated HMGB1 level in patients with heart failure correlating with disease severity (13–17). In addition to this, some studies have identified RAGE as a prognostic factor in human heart failure (16, 18, 19).In the present study, we aimed to clarify the role of HMGB1 and RAGE in an experimental model of murine autoimmune myocarditis. This experimental approach enables a reproducible sterile cardiac inflammation, as described previously (1, 20). Furthermore, the clinical relevance of both proteins should be investigated.Hence, we first studied the expression kinetics of HMGB1 in the inflamed myocardium and serum of wt mice. Then, inhibition of HMGB1 by glycyrrhizin (GL) was performed and heart tissue was analyzed in TnI-induced experimental autoimmune myocarditis (EAM) of wt mice. Additionally, RAGE knockout (RAGE-ko) mice were immunized with TnI to further study the role of RAGE signaling in this EAM model. Finally, the role of the HMGB1–RAGE axis in our model was analyzed by an adeno-associated virus (AAV)9-mediated cardiac overexpression of HMGB1. In a last step, we studied the clinical relevance of HMGB1 and RAGE in patients with myocarditis. |
