Histopathologic Characterization of Myocarditis Associated With Immune Checkpoint Inhibitor Therapy.

CONTEXT.—: Cardiac complications of immune checkpoint inhibitor therapy are rare, but reports of myocarditis are increasing. The findings have been described in case reports as lymphocytic myocarditis, but its histopathology is underreported. OBJECTIVE.—: To review the histology of myocardial biopsy-proven cases of immune checkpoint-associated myocarditis and provide immunohistochemical characterization of the inflammatory infiltrate. DESIGN.—: We have encountered 6 patients with biopsy-proven myocarditis in conjunction with therapy using anti-programmed death receptor-1 (PD-1)/programmed death ligand-1 (PD-L1) agents with and without cytotoxic T-lymphocyte associated protein 4 (CTLA-4) inhibitors and characterized the histopathology and immune cell profile. RESULTS.—: The myocarditis was multifocal/diffuse and characterized by a predominant CD163-positive histiocytic infiltrate, with an associated CD8+ and PD-1+ T-lymphocytic infiltrate, some of which were granzyme B positive. Cardiac myocytes showed immunoreactivity for PD-L1 in areas of injury, confirmed using 2 different anti-PD-L1 clones. Four of 6 patients recovered from their cardiac injury. One patient had residual tachycardia-bradycardia syndrome and 1 patient expired. CONCLUSIONS.—: The diffuse lymphohistiocytic myocarditis associated with this therapy is relatively distinctive, and this diagnosis is strongly suggested based on the histopathologic findings in the correct clinical setting.