[Chronic graft-versus-host disease manifesting as isolated myositis].

A 26-year-old man underwent HLA-matched sibling allogeneic peripheral blood stem cell transplantation for acute myeloid leukemia with KMT2A rearrangement and maintained complete remission. On day338 post-transplant, he developed fever, myalgia, and markedly elevated creatinine kinase. We initially suspected drug-induced rhabdomyolysis and stopped the offending drugs, but his symptoms did not improve. Immunohistological findings with PD-1 staining in a muscle biopsy led to the diagnosis of graft versus host disease (GVHD)-associated myositis. The myositis resolved with prednisolone treatment based on the conventional chronic GVHD treatment protocol. Chronic GVHD with myositis alone is rare, and GVHD-associated myositis has been difficult to differentiate from other forms of myositis due to its lack of specific pathological features. Recently, PD-1 and HLA-DR expression have been reported as characteristic features of chronic GVHD-associated myositis. The pathological finding of PD-1-positive cell infiltration was useful in diagnosis and treatment in our case.