Short-Term Hematopoietic Stem Cells (ST-HSC) Have Full Long-Term Capacity with Sustained but Reduced Potential Compared with LT-HSC.

Abstract 2550

Poster Board II-527

We evaluated the hypothesis that there was a homing defect between long-term (LT) hematopoietic stem cell (HSC) (KLS-Flk2-) and ST-HSC (KLS-Flk2+) that explained differences in engraftment potential and duration. Short-term HSC by definition have limited self-renewal capacity, generally described as giving rise to lymphohematopoiesis for 4–12 weeks before senescence. We performed three large animal engraftment studies into lethally ablated mice (950cGy split dose) looking at engraftment of both ST and LT-HSC cells delivered via intravenous, intraperitoneal and intra-femoral route. Two-hundred or 500 cells derived from B6/SJL mice were administered to each animal along with 300,000 recipient (C57/BLK) whole bone marrow cells for radioprotection following prior published studied [PNAS:98;14541, Stem Cells 24:1087] with optimization of flourochromes for better discrimination with our Cytopeia sorter. The animals were serially transplanted after eight months or one year to secondary recipients. In our hands, the ST-HSC engrafted animals did not lose chimerism over time. Review of the literature revealed that there were not confirmatory studies from those published from the initial one publication describing the ST-HSC. We found the ST-HSC were not short-term and persisted for one year in primary recipients and at least 3 months in secondary recipients. Engraftment kinetics favored LT-HSC over ST-HSC with engraftment examples at one year of 62% compared with 30% respectively when administered intravenously, 10% verses 4% given intra-femoral and 0.5% verse 0.3% given intraperitoneal. Chimerism was on average 50% better for the LT-HSC when compared with the ST-HSC and was irrespective of route proving that the differences seen are not due to homing deficiency but rather intrinsic differences in the two stem cell pools. Prior studies gave a maximum of 100 cells. Cell number was purposely increased for better differentiate of subtle differences in engraftment kinetics for statistical reasons. To avoid contamination of Flk2+ cells in the Flk2- cohort and vise-versa, discrimination of the gates were enhanced from that which was published prior. Double sorting of the cells confirmed that there was no appreciable cross contamination but obviously we cannot totally rule that out as a potentially confounding factor. In conclusion we found that ST-HSC as described have long-term capacity with intrinsic differences in engraftment potential that is not driven by a homing defect.