Interleukin-6-deficient mice resist development of autoimmune myocarditis associated with impaired upregulation of complement C3

The osteopontin – CD44 pathway is superfluous for the development of autoimmune myocarditis

Osteopontin (OPN) and CD44 have been implicated in the development of autoimmune diseases, including arthritis and multiple sclerosis, as well as chronic inflammatory diseases, such as atherosclerosis and colitis. To investigate their roles in autoimmune myocarditis induced by immunization with heart alpha-myosin (MyHC-alpha), a mouse model of human cardiomyopathy, we analyzed mice lacking OPN or CD44v6/v7, a CD44 isoform that binds OPN. Both, OPN(-/-) and CD44v6/v7(-/-) mice developed myocarditis with the same prevalence and severity as BALB/c wild-type controls. Furthermore, treatment of BALB/c mice with a pan-neutralizing anti-CD44 antibody did not affect the disease outcome. Consistently, expansion of MyHC-alpha-specific autoimmune CD4(+) T cells and MyHC-alpha autoantibody responses from either CD44v6/v7(-/-) mice or OPN(-/-) mice was indistinguishable from their wild-type controls. Thus, OPN and CD44v6/v7 are merely spectators rather than protagonists in autoimmune myocarditis.

MyD88 Signaling Controls Autoimmune Myocarditis Induction

BACKGROUND

Experimental autoimmune myocarditis (EAM) is a CD4+ T-cell-mediated mouse model of postviral cardiomyopathy. Activation of interleukin-1 type 1 and Toll-like receptors that share the common downstream adaptor molecule MyD88 is required for disease induction. The specific role of MyD88 in myocarditis, however, is not known.

METHODS AND RESULTS

In contrast to control littermates, MyD88(-/-) mice were protected from myocarditis after immunization with alpha-myosin heavy chain-derived peptide (MyHC-alpha) and complete Freund's adjuvant. Disease resistance of MyD88(-/-) mice resulted from impaired expansion of heart-specific CD4+ T cells after immunization. Intrinsic defects of MyD88(-/-) CD4+ T cells were excluded. In contrast, MyD88(-/-) but not MyD88(+/+) primary antigen presenting dendritic cells (DCs) were defective in their capacity to prime CD4+ T cells. This defect mainly resulted from the inability of MyD88(-/-) DCs to release tumor necrosis factor-alpha. The critical role of MyD88 signaling in DCs in the peripheral lymphatic compartments was finally proven by repetitive injection of activated, MyHC-alpha-loaded MyD88(+/+) DCs that fully restored T-cell expansion and myocarditis in MyD88(-/-) mice.

CONCLUSIONS

Autoimmune myocarditis induction depends on MyD88 signaling in self-antigen presenting cells in the peripheral compartments. We conclude that MyD88 might become a target for prevention of heart-specific autoimmunity and cardiomyopathy.

PROTECTING THE PUMP: Controlling Myocardial Inflammatory Responses

Because of the anatomy, function, and nonregenerative nature of the myocardium, inflammation in this tissue is not well tolerated. Nevertheless, various diseases of the heart are characterized by inflammatory responses involving the effector mechanisms of innate and adaptive (lymphocyte-dependent) immunity. The innate immune response to ischemia-reperfusion injury is, by far, the most common cause of myocardial inflammation. Innate responses may have beneficial influences that preserve myocardial function in the short term but may be maladaptive in chronic states. Adaptive responses in the myocardium occur with infection or loss of tolerance, and lead to myocarditis. Given the narrow margin for benefit of cardiac inflammation, special regulatory mechanisms likely raise the threshold, compared to other tissues, for the induction and persistence of adaptive immune responses. These mechanisms include strong central and peripheral T cell tolerance to heart antigens and induction of anti-inflammatory feedback mechanisms involving cytokines such as interferon-gamma.

IL-6 Increases Primed Cell Expansion and Survival

Cytochrome c-specific CD4 T cells from transgenic donors transferred to syngeneic B10.A mice expand more vigorously upon immunization if exogenous IL-6 is provided during the initial phase of immunization. The resultant increase in the frequency and number of Ag-specific cells is observed in the blood, lymph nodes, spleen, liver, and lung and persists for at least 3 mo. Treatment of immunized recipients with anti-IL-6 or use of IL-6 knockout recipients reduced the frequency of Ag-specific CD4 T cells during a comparable period, indicating that IL-6 is physiologically involved in the expansion of memory and/or effector cells and thus in the persistence of memory. IL-6 did not alter the duration of Ag-presenting activity. Both CFSE dilution studies and labeling with BrdU indicated that IL-6 does not effect proliferative rates of responding CD4 T cells. By contrast, annexin V staining was diminished in responding cells from the IL-6-treated animals, particularly among those cells that had undergone five or more divisions. These results indicate that IL-6 reduces the level of apoptosis among Ag-stimulated cells; thus, it plays a central role in determining numbers of memory and/or effector CD4 T cells in response to immunization over extended periods.

Autoimmune heart failure: new understandings of pathogenesis

Besides genetic susceptibility and infections with cardiotropic viruses, autoimmune responses against heart tissue play a major role in the pathogenesis of dilated cardiomyopathy, the most common cause of heart failure in young patients. Recent findings suggest that the combination of tissue damage resulting in release of self-antigens, together with non-specific activation of the innate immune system triggers various responses that are crucial for the development of heart-specific autoimmunity. Understanding these mechanisms is critical for the design and development of novel treatment strategies against devastating heart diseases in the future.

Dendritic cell-induced autoimmune heart failure requires cooperation between adaptive and innate immunity

Genetic susceptibility and autoimmunity triggered by microbial infections are factors implicated in the pathogenesis of dilated cardiomyopathy, the most common cause of heart failure in young patients. Here we show that dendritic cells (DCs) loaded with a heart-specific self peptide induce CD4+ T-cell-mediated myocarditis in nontransgenic mice. Toll-like receptor (TLR) stimulation, in concert with CD40 triggering of self peptide-loaded dendritic cells, was shown to be required for disease induction. After resolution of acute myocarditis, DC-immunized mice developed heart failure, and TLR stimulation of these mice resulted in relapse of inflammatory infiltrates. Injection of damaged, syngeneic cardiomyocytes also induced myocarditis in mice if TLRs were activated in vivo. DC-induced myocarditis provides a unifying theory as to how tissue damage and activation of TLRs during infection can induce autoimmunity, relapses and cardiomyopathy.