CD43 Defines Early Hematopoietic Progenitors Derived from Human Embryonic Stem Cells (hESC).

We have demonstrated that co-culture of hESCs with OP9 bone-marrow stromal cells induced efficient hESC differentiation into hematopoietic colony-forming cells (CFCs). During hESC/OP9 co-culture, the first CFCs were detectable 1–2 days after emergence of CD34+ cells (day 4–5 of co-culture) and 3–4 days before generation of CD45+ cells, indicating that early CFCs arose in the CD45- population. To define phenotype of early hematopoietic progenitors, we examined the expression of CD41a, CD43, CD61 and glycophorin A (CD235a) on the CD34+ cells in the course of hESC/OP9 co-culture. The first detectable CD34+ cells expressed a high level of VEGF-R2 (KDR) and were CD41a, CD43, CD61 and CD235a negative. Appearance of CFCs was accompanied with down-regulation of KDR and almost simultaneous expression of CD41a, CD43 and CD235a on the CD34+ cells. CD43+ cells gradually increased thereafter, whereas the expression of CD41a was retained on the subset of CD43+ cells that co-express CD235a and CD61. CD45+ cells emerged within CD43+ population and the most of CD45+ cells did not express CD235a, CD41a or CD61. The expression of CD34 was decreased progressively on CD43+CD41a+CD235a+CD61+ cells, but retained by CD43+CD45+ cells. To assay hematopoietic potential of defined subsets, CD34+ cells were isolated by magnetic sorting and further separated into CD34+ CD43+ and CD34+CD43− cell fractions. In addition, CD34−CD43+ cells were isolated from CD34− fraction. CD34+ selection markedly enriched all types CFCs except of E-CFCs. All GEMM-CFCs were found within CD34+ population and none within CD34−. However, CD34− cell fraction contained E-CFCs and minimal numbers of M-CFCs and GM-CFCs. CD34+ cells were also heterogenous in morphologic appearance and contained endothelial cells that were capable to endocytose Ac-LDL. Positive selection of CD43+ cells resulted in total recovery of all types of CFCs. E-CFCs, which consistently detected in the CD34- fraction, were also recovered by CD43 selection. Morphologically, CD43+ cells comprised a homogenous blast-like population and were lack of endothelial-like cells. Thus, expression of CD43 molecule (leukosialin/sialophorin) is detected on the earliest clonogenic hematopoietic progenitors before expression of CD45, persists on developing hematopoietic cells after their lineage specification and most likely defines the divergence of hematopoiesis from endothelial cells during early development in humans.