MxA expression in systemic lupus erythematosus -associated myocarditis indicates type I interferon pathway activation.

Type I interferon (IFN) pathway activation plays an important role in the pathogenesis of systemic lupus erythematosus (SLE). Myxovirus resistance protein A (MxA) is a surrogate marker of type I IFN activity; however, evidence of local IFN pathway activation in myocarditis associated with SLE remains limited. The first patient was a 51-year-old man with newly diagnosed SLE complicated by neuropsychiatric SLE, class IV lupus nephritis, erythema and myocarditis at presentation. The second patient was a 56-year-old woman with class V lupus nephritis who developed acute decompensated heart failure due to myocarditis approximately 4 months after the diagnosis of SLE. In both cases, cardiac magnetic resonance imaging and histopathological findings by endomyocardial biopsy like IgG and C1q deposition along myocardial vessel walls supported the diagnosis of myocarditis associated with SLE. Immunohistochemical staining demonstrated prominent MxA expression in cardiomyocytes in the first patient, while MxA-positive staining corresponding to CD34-positive vascular structures was observed in both patients. MxA staining was absent in control myocardial tissue from non-inflammatory dilated cardiomyopathy. These findings provide rare tissue-level evidence of MxA expression in biopsy-proven myocarditis associated with SLE resulted from local type I IFN pathway activation in this severe complication of SLE.