Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is a most powerful immunotherapy for hematological malignancies. However, the impact of immunological disturbances as a result of allo-HSCT is not understood well. We experienced an 11-year-old boy who presented with systemic lupus erythemathosus (SLE) 10 years after unrelated cord blood transplantation of male origin for juvenile myelomonocytic leukemia (JMML) with monosomy 7. Bone marrow examination showed complete remission without monosomy 7. Genetic analysis of peripheral blood revealed mixed chimera with recipient cells consisting of <5% of T cells, 50–60% of B cells, 60–75% of NK cells, 70–80% of macrophages, and 50–60% of granulocytes. Significance of persistent mixed chimera as a cause of SLE is discussed. 1. Introduction Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is an effective treatment to cure the hematological malignancy through the alloimmune reaction. However, excessive alloimmune reaction triggers acute graft versus host disease (aGVHD) and it attacks various organs and induces their dysfunction, which is often life-threatening. Chronic GVHD is based on more insidious alloimmune reaction and its clinical symptoms sometimes mimic autoimmune diseases. In allo-HSCT, donor cells usually replace the bone marrow cells completely, which is called complete chimera. Donor and recipient hematopoietic cells sometimes coexist stably, which is called mixed chimera. Although mixed chimera is a sign of relapse of hematological malignancy, there are cases of stable and persistent chimera without relapse. However, the impact of immunological disturbances as a result of allo-HSCT and mixed chimera is not understood well. We had an 11-year-old boy who presented with SLE 10 years after allogeneic cord blood transplantation. Interestingly, most of T cells were of donor origin, but B cells, NK cells, macrophages, and granulocytes were mixed chimera. 2. Case Report At 6-month-old, he presented with hepatosplenomegaly and skin eruption. Bone marrow examination revealed monosomy 7 and he was diagnosed as juvenile myelomonocytic leukemia through clinical evaluation. At 15-month-old, he received HLA-DRB1 one locus mismatched male-derived cord blood transplantation. Conditioning regimen was myeloablative (busulfan (140？mg/m2？？× 4) + etoposide (15？mg？/kg/day × 4) + cyclophosphamide (60？mg/kg × 2) + antithymocyte globulin (2.5？mg/kg × 4)). GVHD prophylaxis was cyclosporine and short-term methotrexate. SCT was successful with only mild GVHD. Complete chimera was determined on day 41.
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