A 65-year-old woman with metastatic melanoma treated with ipilimumab (3 mg/kg i.v.) and nivolumab (3 mg/kg i.v.) developed atypical chest pain, dyspnea, and fatigue 12 days later, accompanied by (A) ECG showing sinus rhythm with first-degree atrioventricular block, which progressed to complete heart block. (B) Several hours later, ECG showed ventricular tachycardia, which degenerated to ventricular fibrillation and cardiac arrest. The patient could not be resuscitated. (C) At autopsy, cardiac tissue stained with hematoxylin and eosin showed myocyte degeneration accompanied by a mononuclear cell infiltrate, immunostaining of which showed prominence of CD68, a macrophage marker (not shown). (D) Also seen was marked infiltration of T cells, as shown by immunostaining for CD3. This infiltrate included approximately equal proportions of CD4⁺ and CD8⁺ T cells (not shown). In addition, immunostaining for CD20, a B cell marker, and IgG was not detected (not shown). Scale bar: 0.1 mm. aVR, augmented vector right; aVL, augmented vector left; aVF, augmented vector foot. Reproduced with permission from the New England Journal of Medicine (13).