John F. Tisdale, M.D., Molecular and Clinical Hematology Branch, NHLBI and NIDDK, National Institutes of Health, Bethesda, MD
Sickle cell disease (SCD) is a recessive genetic disease that results from the production of an abnormal hemoglobin that is prone to polymerization upon deoxygenation causing severe morbidity and early mortality. Palliative therapies have been developed over the last several decades that significantly prolong lifespan, but conventional treatment remains unsatisfactory. The abnormal polymerization of hemoglobin among the erythroid progeny of hematopoietic stem cells has long supported the goal of developing therapies aimed at the hematopoietic stem cell (HSC). Allogeneic HSC transplantation is a well-established curative approach through replacement of the defective organ with that of a donor carrying the normal genotype; however, procedural toxicities limit this approach to only severely affected children(1). We and others have demonstrated the ability to achieve engraftment of allogeneic HSCs without the need for toxic myleoablative conditioning(2, 3), prompting consideration of nonmyeloablative HSC transplantion in adults with SCD. In these early studies employing high dose cyclophosphamide and fludarabine, prompt myeloid engraftment was observed in all, yet myeloid engraftment appeared to result from a donor T cell mediated alloimmune response and was associated with significant rates of graft versus host disease (GvHD) and significant attendant morbidity and mortality. We therefore went back to develop a transplantation regimen for adults with SCD for which engraftment is not dependent upon such alloreactivity and chose a low-dose radiation approach utilizing rapamycin. Rapamycin has the ability to promote T cell tolerance even when T cells are stimulated in the presence of costimulation and we confirmed this approach in vivo in a murine transplantation model with phenotypic correction observed in transgenic knockout mice that exclusively expresses human sickle Hb(4). We then initiated a clinical trial testing this approach in severely affected adults using a non-myeloablative HSCT regimen of alemtuzumab, 300 cGy of total body irradiation, and rapamycin, followed by infusion of unmanipulated G-CSF mobilized peripheral blood stem cells from HLA-matched siblings. Stable mixed chimerism and disease reversion was observed in 9 of the first 10 patients treated(5). Importantly, no patient has experienced graft versus host disease. Accrual is ongoing and has now reached 30 patients with similar results. The normalization of Hb in successfully engrafted patients was accompanied by a stabilization in brain imaging, a reduction of echocardiographic estimates of pulmonary pressure, and a reduction of hepatic iron. All engrafted patients remain without GvHD and continue to have a mixture of donor and recipient peripheral blood leukocytes. To date, 11 engrafted patients have been weaned off of immunosuppression with continued stable donor chimerism and no GvHD.
For those lacking a suitable matched sibling donor, we have now initiated a clinical protocol testing the addition of post transplant cyclophosphamide (PT-Cy) to our regimen for its ability to allow engraftment of haploidentical donors, which would open transplantation to the overwhelming majority of patients if successful.
In summary, our data support mixed chimerism as a suitable goal for HSC transplantation in SCD and have set the stage for our current work extending this approach to the haplo-identical setting.
References:
1. Walters MC, Patience M, Leisenring W, Eckman JR, Scott JP, Mentzer WC, et al. Bone marrow transplantation for sickle cell disease [see comments]. N Engl J Med. 1996;335(6):369-76.
2. Childs R, Chernoff A, Contentin N, Bahceci E, Schrump D, Leitman S, et al. Regression of metastatic renal-cell carcinoma after nonmyeloablative allogeneic peripheral-blood stem-cell transplantation. N Engl J Med. 2000;343(11):750-8.
3. Niederwieser D, Maris M, Shizuru JA, Petersdorf E, Hegenbart U, Sandmaier BM, et al. Low-dose total body irradiation (TBI) and fludarabine followed by hematopoietic cell transplantation (HCT) from HLA-matched or mismatched unrelated donors and postgrafting immunosuppression with cyclosporine and mycophenolate mofetil (MMF) can induce durable complete chimerism and sustained remissions in patients with hematological diseases. Blood. 2003;101(4):1620-9.
4. Powell JD, Fitzhugh C, Kang EM, Hsieh M, Schwartz RH, Tisdale JF. Low-dose radiation plus rapamycin promotes long-term bone marrow chimerism. Transplantation. 2005;80(11):1541-5.
5. Hsieh MM, Kang EM, Fitzhugh CD, Link MB, Bolan CD, Kurlander R, et al. Allogeneic hematopoietic stem-cell transplantation for sickle cell disease. N Engl J Med. 2009;361(24):2309-17.