Long-term correction of beta-thalassemia with minimal cellular requirement and transplantation modalities

Determination of minimal criteria, pre-transplantation regimens, and infusion modalities for effective and reproducible bone marrow (BM) therapy in beta-thalassemia is of fundamental importance for clinical application. In this study, using repopulation assays, we first established the minimal proportion of normal BM stem cells that would result in therapeutic benefit in this red blood cell (RBC) disorder. Eight groups of stable chimeric hemizygous beta-thalassemic (hemi-betathal) mice (10-89%) were systematically subjected to cellular, molecular, and patho-physiologic analyses for approximately 2 years. In the chimeric hemi-betathal groups containing 19-24% normal donor cells, all RBC parameters and consequent erythropoiesis were significantly improved. Mice in the 24% chimeric group and above had marked reduction in organ pathology including iron deposits, and survived to a normal lifespan. Altogether, these results established that a range of 19-24% normal BM cells is sufficient for long-term significant correction of the hemi-betathal phenotype. We also determined concomitantly the minimal myelosuppression radiation doses, the number of cells to be infused, and the number of infusions required in order to attain this therapeutic range in hemi-betathal mice. Importantly, with prior minimal myelosuppression with 1 or 2 Gy, and using cell doses of 40 or 60 millions, 100% of the recipients were successfully engrafted at therapeutic levels, provided the cells were administered in two doses. This study has therefore determined the therapeutic chimeric level as 19-24% of normal cells, and has also defined the minimal transplantation modalities necessary for the stable and successful correction of the hemi-betathal phenotype.