Successful management of mixed chimerism after bone marrow transplant in beta‐thalassemia major
Authors: Gabriele Kropshofer, Sieghart Sopper, Michael Steurer, Wolfgang Schwinger, Roman Crazzolara
Journal: American Journal of Hematology (2016)

To the Editor: Allogeneic hematopoietic stem cell transplant (HSCT) is the only available curative approach for β-Thalassemia major (β-Thal) 1. Over the past three decades more than 3,000 transplantations have been performed worldwide with a disease-free survival rate of nearly 90% for otherwise highly compromised and life-limited patients 1. To overcome the non-negligible risk for transplant-related morbidity, conditioning regimens have been introduced with improved results, including intravenous busulfan, or treosulfan associated with thiotepa and fludarabine 1-4. They are sufficient to eradicate most of the defective marrow and combined with adequate immunosuppressive therapy they allow for a sustained engraftment of donor derived stem cells 2. A recent study has demonstrated that a significant proportion of transplanted patients can develop long-term and stable mixed chimerism (MC) and that complete donor engraftment is not essential for sustained hematopoiesis 3. On the other hand, graft rejection can result from early recovery of recipient hematopoiesis, suggesting that early assessment of engrafted donor marrow is crucial for the management of transplanted patients 3. Until today, little data is available how to deliver appropriate therapeutic immunomodulatory interventions in this particular situation 4. We describe the occurrence of an early graft loss at three months after allo-HSCT in a patient with β-Thal. While intensification of post-transplant immunosuppressive therapy was not able to restore graft function, interruption of immunosuppression restored permanently graft function without development of graft-versus host disease (GvHD). The female patient received matched sibling bone marrow (BM) transplantation at the age of 15 months for a transfusion-dependent β-Thal. Lucarelli risk classification was class I. Cytoreduction consisted of busulfan at 16 mg/kg and cyclophosphamide at 200 mg/kg. In addition, antithymocyte globulin was used to prevent graft failure. A dose of 3.7 × 108 unmanipulated mononuclear BM cells/kg was injected and GvHD prophylaxis consisted of 3 mg/kg per day cyclosporine and methotrexate (3 doses of 10 mg/m2 on days +1, +3, and +6). At 1 month after HSCT, the patient had 98% donor cells in unsorted peripheral blood (PB). Three months post-HSCT her PB chimerism status was 89% donor cells. Subsequently, she had further loss of donor chimerism and a gradual decline of her hemoglobin until she required resumption of regular red blood cell transfusions at a monthly basis (Fig. 1). As a consequence, ferritin incremented threefold up to 1214 µg/L. Singly picked out burst forming unit-erythroid (BFU-E) colonies revealed the presence of 45% donor cells for erythroid precursors in the BM. In contrast, 75% donor-derived cells were counted on peripheral mononuclear cells, demonstrating different chimerism between nucleated cells and RBC. Intensification of immunomodulation with the addition of mycofenolat-mofetil (MMF) at 1,500 mg/m2 daily was subsequently adopted. However, symptoms of anemia progressed and a further decline to 19% of donor-derived BFU-E in her BM at 9 months post-transplant was noted. Finally, interruption of both MMF and cyclosporine rapidly restored donor hematopoiesis, hemoglobin levels returned to 11 g/dL, no red blood cell transfusions were required and ferritin level decreased to 491 µg/L. Analysis of chimerism demonstrated establishment of persistent MC of ∼80% at 12, 15, and 18 months after HSCT. Hemoglobin over time. Hemoglobin levels (g/L; black line), % donor nucleated cells (gray line), % donor BM red cells (black triangle), red blood cell transfusions (arrows), and immunosuppression with cyclosporine (desired plasma range 150–200 ng/mL) and MMF at 1,500 mg/m2/daily over time in days post-HSCT. The occurrence of early MC after transplantation for hemoglobinopathies is associated with an increased risk of recurrence of thalassemia, mostly when the proportion of donor-derived cells in the peripheral blood or BM determined at 2 months after the transplant is less than 75% 3. Whereas the risk of disease relapse is only 13% in patients with <10% residual hematopoietic cells (RHCs), it exceeds 40% in patients with 10–25% RHCs. On the other hand, a substantial proportion of patients with MC can move into a persistent state of stable mixed chimerism without the risk of graft failure, which is defined by a minimum post-transplant follow-up period of 2 years 3. Recently a great attention has been paid to the long-term observation—3 to 5 years post-transplant—, demonstrating, that even in the presence of very low percentage of donor engrafted nucleated cells (even if no higher than 20%) patients may achieve a functioning graft status 3. In these patients the proportion of donor-derived erythroid precursors is equivalent to that observed in the mature nucleated cells, rather than that of the red blood cells. It has been suggested that in persistent MCs improved survival and selective maturation of donor RBCs might successfully contrast their host counterparts, which might be destroyed during ineffective erythropoiesis 3. In most of the cases observed so far, the evolution of persistent MC occurs within the first few months after HSCT and very rarely takes place over longer periods of time, never after 2 years from HSCT. It is likely that the mechanisms responsible for MC evolution or recurrence of thalassemia seem to be regulated by an immune mediated allo-recognition. Recent studies have shown the presence of high IL-10 expression produced by peripheral blood mononuclear cells and type-1 regulatory T-cells to play an important role in immunological tolerance in MC patients 5. Although it is not clear if the mechanisms inducing an immunological tolerance in patients with MC might also influence a different maturation of donor erythroid precursors, these observations strongly support the need of a therapeutic immunomodulation in order to prevent early graft loss. Our observation of early reduction of BFU-E colonies at 5 months after HSCT and subsequent regular red blood cell transfusion was consistent of early graft loss, and therefore an immediate intervention was required. Interestingly, intensification of immunosuppression, as is done after transplantation in malignant disorders to control regulatory T cells was not able to improve immune tolerance. In fact, after a 3-month period of combined immunosuppressive therapy the percentage of donor derived BFU-E dropped to 19%. Surprisingly, interruption of immunosuppression was soon able to restore a high and stable mixed chimerism with full graft function, characterized by a normal hemoglobin level, no red blood cell transfusion requirement, and decrease in iron stores. Most importantly, the patient has never developed any symptoms of graft-versus host disease in the 2-year follow-up after HSCT. Although limited to a single patient, this experience raises caution regarding the advisability of intensification of immunosuppression for the intentional stabilization of mixed chimerism in patients with hemoglobinopathies.