Successful correction of murine sickle cell disease with reduced stem cell requirements reinforced by fractionated marrow infusions

Minimal criteria requirements of stem cell replacement, conditioning regimen and modalities of infusion essential for cure of sickle cell disease (SCD) by bone marrow(BM)/stem cell transplantation or gene therapy must be established prior to clinical trials. The threshold of normal BM/stem cells for therapeutic correction of this red blood cell disorder was evaluated in the SAD murine SCD model from peripheral donor white blood cells. From 11 groups of stable chimeric SAD mice (5–92%) analyzed over ∼2 years, mice with ∼16% normal donor stem cells showed improvement of haematological and erythroid responses. Mice in the 26% chimeric group and above demonstrated substantial amelioration of organ pathologies with generalized decreased iron deposits, fibrosis and reached normal lifespan. Subsequently, the minimal myelosuppression concurrently with number and timing of infusions and number of BM cells was determined to reach therapeutic threshold in SAD mice. Higher myelosuppression (2 Gy vs. 1 Gy) and cell number in single infusion led to increased chimerism. Importantly, administration of three-equivalent cell subdoses within 28 h of mild myelosuppression resulted in 100% recipient engraftment at therapeutic levels. These studies established the long-term therapeutic chimeric threshold of normal white blood cells at ∼26% and determined the minimal fractionated BM/stem cell doses concomitant with mild myelosuppression for significant correction of SCD in SAD mice.