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Case Reports

Abstract

Herein we describe the case of a 33-year-old woman who presented with a macular rash on the bilateral inner thighs in the setting of subacute weakness, malaise, pancytopenia, transaminitis, and hyperferritinemia. Plasma EBV DNA was detected. Histologic sections of a skin biopsy demonstrated a dermal perivascular and interstitial histiocytic infiltrate with foci of karyorrhectic debris and rare neutrophils. A subset of histiocytes demonstrated engulfed erythrocytes. The overlying epidermis showed focal mild vacuolization. Immunohistochemistry revealed CD163 expression in the histiocytic infiltrate. A subset of mononuclear cells demonstrated strong myeloperoxidase (MPO) expression overlapping with CD163-positive histiocytes. S100, CD117, and CD34 were negative. PASD, GMS, Fite, and Gram stains were negative for organisms. No myeloid blasts were identified. No vasculitis was apparent. MPO-positive histiocytes represent a distinctive finding described in histiocytic necrotizing lymphadenitis and hemophagocytic lymphohistiocytosis (HLH), but this patient did not meet clinical criteria for HLH. Subsequent serology revealed anti-MDA5 positivity, and she was diagnosed with dermatomyositis with EBV infection representing a presumed trigger for immune dysregulation. Recognition of this distinct MPO/CD163-expression profile in a mononuclear cell population may provide an important morphologic clue to immune dysregulation in connective tissue disease when considered along with clinical, serologic, and other laboratory data.