Evaluation of Serum-Free Culture Conditions Able to Support the Ex Vivo Expansion and Engraftment of Human Hematopoietic Stem Cells in the Human-to-Sheep Xenograft Model

Abstract

To develop culture conditions devoid of serum that would support the ex vivo expansion and maintenance of hematopoietic stem cells (HSC) with engraftment capability, we performed in vitro studies in which phenotypic and functional expansion of putative HSC populations were evaluated. We then used the human-sheep xenograft model to evaluate the engraftment potential of the ex vivo expanded cells. Adult human bone marrow CD34⁺-enriched cells were cultured in QBSF-60 for 14 days with or without fetal bovine serum (FBS) in the presence of interleukin-3 (IL-3), IL-6, and stem cell factor (SCF), and analyzed at days 0, 3, 7, and 14 for expansion, phenotype, clonogenic ability, and cell cycling status. Although there was a progressive expansion of numbers of cells in both groups, the group cultured with serum exhibited more than twice the expansion seen in the group without serum at all time points. The phenotypic analysis of the cultured cells showed an increase in the absolute numbers of CD34⁺ cells in both groups. However, when we evaluated the presence of CD34⁺ CD38^- cells, this population persisted in significantly higher numbers in the group cultured without serum, with maximal output of CD34⁺ CD38^- cells seen at 3 and 7 days. A higher total clonogenic potential was found in the serum-free cultures. To evaluate the in vivo engraftment potential of these cultured cells, 19 sheep fetuses were each injected i.p. with 9 × 10⁵ cells either fresh or cultured in the conditions described above. Although all the transplanted fetal sheep showed the presence of human cells in their bone marrow (BM), the highest levels of long-term engraftment in primary recipients were obtained with the fraction of cells cultured for 3 days followed by 7 days in the absence of serum. In the secondary sheep recipients, the highest level of long-term engraftment was also achieved in sheep that received cells from primary recipients that had received cultured cells in serum-free conditions for 3 days.