A 57-year-old male with acute-type adult T cell leukemia–lymphoma (ATL) developed persistent watery, non-bloody diarrhea at a volume of 2–3 L/day following the administration of the anti-CC chemokine receptor 4 (CCR4) monoclonal antibody, mogamulizumab. An extensive examination revealed the absence of any pathogenic bacteria or parasites in his stool. Biopsied specimens from the colonic mucosa contained many small nests of apoptotic bodies in the colonic glands, which mimicked acute-colonic graft-versus-host disease. Activation of the auto-reactive immune system due to the depletion of regulatory T-cells by mogamulizumab was suspected as causative. Special attention should be paid to the risk of unique immune-related adverse events induced by mogamulizumab.
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