Peripheral Blood As a Source Of Cells For Regenerative Medicine Applications In Sickle Cell Disease

Sickle cell disease (SCD) is characterized by an abnormal hemoglobin that is prone to polymerization producing abnormal red blood cells resulting in hemolysis, vasoocclusion, and severe anemia. A number of strategies targeting stem cells have been actively investigated. The stressed erythropoiesis that results may produce an increase in CD34+ stem/progenitor cells in the peripheral blood (PB), and could represent a source of stem cells for gene addition or gene correction strategies. In order to explore this possibility, we first sought to determine whether CD34+ cells are elevated in SCD PB and whether the CD34+ cell number correlated with other indicators of stressed erythropoiesis such as reticulocyte number. We thus evaluated the CD34+ cell number in the PB of SCD subjects (n=21) and compared to healthy donors (n=10). There was no difference between CD34 cell counts among SCD subjects and healthy donors: CD34+ cells counts/ µl in PB were 3.41 +/- 4.33 (n=21) in SCD subjects and 2.11 +/- 1.09 (n=10,). Additionally, there was no correlation between CD34+ cells counts and reticulocyte counts in individuals with SCD (n=17).

However, the majority of our SCD subjects are treated with hydroxyurea, which may influence CD34+ cells in the PB. Therefore, we then compared the CD34+ cell number in PB among healthy donors (n=10), SCD subjects taking hydroxyurea (HU+, n=11) and SCD subjects not taking HU (HU-, n=10). CD34+ cell counts/ µl in PB were 2.11 +/- 1.09 in healthy donors, 0.84 +/- 0.56 in HU+ and 6.25 +/- 4.92 in HU-. There was a significant difference between the CD34+ cell number between HU+ and HU- (P<0.01). Colony-forming unit (CFU) assays were also performed and enumerated after 12- 15 days culture comparing HU+ and HU- SCD patients. The number of erythroid colonies and myeloid colonies/ml in PB was 141 and 47 (n=2) in HU+, and 2000 and 636 (n=2) in HU-, respectively. The total colony number in HU+ SCD subjects was reduced to less than 10%. We also compared the CD34+ cell number within bone marrow mononuclear cells (BM) between HU+ (n=12) and HU- (n=4). CD34+ cell counts/ µl in BM was reduced at 19.6 +/- 15.9 (n=12) in HU+ subjects compared to 46.9 +/- 31.3 (n=4) in HU- subjects (P<0.05).

The concept of epigenetic memory which favors differentiation toward the donor tissue makes PB an attractive tissue source for regenerative medicine strategies in SCD, including for derivation of induced pluripotent stem (iPS) cells. However, the effects of HU may be deleterious toward this goal. To investigate this, iPS cell generation was also performed using BM stromal cells and PB derived from HU- (n=2) and HU+ (n=3) SCD subjects. Reprogramming was only successful in HU- (2/2), whereas iPS colonies were not observed in HU+ (0/3).

In conclusion, we demonstrate that circulating hematopoietic stem/progenitor cells are elevated in SCD PB at steady state, but are comparable to controls with HU treatment. Our data further suggest that while PB is a rich source of cells for regenerative medicine approaches, HU treatment reduces their frequency and efficiency. Withholding HU temporarily may thus be advisable prior to harvesting PB or BM for regenerative medicine applications. Our results are important for the design of gene targeting strategies for SCD.