Development and Characterization of a Preclinical Total Marrow Irradiation Conditioning-based Bone Marrow Transplant Model for Sickle Cell Disease

Sickle cell disease (SCD) is a serious global health problem, and currently, the only curative option is hematopoietic stem cell transplant (HCT). However, myeloablative total body irradiation (TBI)-based HCT is associated with high mortality/morbidity in SCD patients. Therefore, reduced-intensity (2-4 Gy) total body radiation (TBI) is currently used as a conditioning regimen resulting in mixed chimerism with the rescue of the SCD disease characteristic features. However, donor chimerism gradually reduces in a few years, resulting in a relapse of the SCD features, and organ toxicities remained the primary concern for long-term survivors. Targeted marrow irradiation (TMI) is a novel technique developed to deliver radiation to the desired target while sparing vital organs and is successfully used for HCT in refractory/relapsed patients with leukemia. However, it is unknown if TMI will be an effective treatment for a hematological disorder like SCD without adverse effects seen on TBI. Therefore, we examined preclinical feasibility to determine the tolerated dose escalation, its impact on donor engraftment, and reduction in organ damage using our recently developed TMI in the humanized homozygous Berkley SCD mouse model (SS). We show that dose-escalated TMI (8:2) (8 Gy to the bone marrow and 2 Gy to the rest of the body) is tolerated with reduced organ pathology compared with TBI (4:4)-treated mice. Furthermore, with increased SCD control (AA) mice (25 million) donor BM cells, TMI (8:2)-treated mice show successful long-term engraftment while engraftment failed in TBI (2:2)-treated mice. We further evaluated the benefit of dose-escalated TMI and donor cell engraftment in alleviating SCD features. The donor engraftment in SCD mice completely rescues SCD disease features including recovery in RBCs, hematocrit, platelets, and reduced reticulocytes. Moreover, two-photon microscopy imaging of skull BM of transplanted SCD mice shows reduced vessel density and leakiness compared to untreated control SCD mice, indicating vascular recovery post-BMT.