A case of myositis associated with clonal expansion of γδ T cells in peripheral blood and bone marrow

We report a 45-year-old man with myositis associated with clonal expansion of γδ T cells. He was referred to our hospital because of slowly progressive (over 10 years) muscle weakness. On neurological examination, weakness and muscle atrophy were noted in the proximal upper and lower limbs. The level of creatinine kinase (CK) was 1,436U/L. Neutropenia and monoclonal gammopathy were found in the peripheral blood. Flow cytometric analysis of peripheral blood and bone marrow revealed proliferation of CD3⁺CD4^-CD8⁺ and CD3⁺CD4^-CD8^- γδ T cells, and Southern blotting demonstrated a clonally rearranged T-cell receptor Jγ gene in peripheral blood and bone marrow. A biopsy of the right quadriceps muscle showed variations in muscle fiber size, and endomysial mononuclear cell infiltration. The expression of MHC Class I antigen was increased on the surfaces of most of muscle fibers, and TCRδ1 positive-lymphocytes invaded nonnecrotic muscle fiber. After starting treatment with cyclosporin A and steroids, his muscle weakness gradually ameliorated, the CK level decreased and neutrophils increased. Although reports of myositis associated with clonal expansion of γδ T cells are extremely rare, the present case suggests that γδ T cells might play a role in mediating myositis.