Expression of homing-associated cell adhesion molecule (H-CAM/CD44) on human CD34+ hematopoietic progenitor cells

We investigated the expression of CD44 molecule on CD34+ hematopoietic progenitor cells. Significantly lower expression of CD44 was observed on bone marrow (BM) CD34+ cells compared with circulating CD34+ cells in cord blood and peripheral blood. Using fluorescence-activated cell sorting, human CD34+ BM cells were fractionated into CD44+ and CD44− populations. Immunofluorescence analysis revealed that the majority of CD34+CD44− cells expressed B-lymphocyte–associated CD10 and CD19 antigens, whereas only a part of CD34+CD44+ cells were positive for CD19. Myeloid and erythroid progenitor cells were found predominantly in CD34+ CD44+ cell fractions. In short-term suspension cultures, cell proliferation and G1 → S transition in the cell cycle were enhanced in CD34+CD44+ cells. In contrast, a large part of CD34+CD44− cells underwent apoptotic cell death. Although co-culture with BM stromal cells could partially prevent CD34+CD44− cells from undergoing apoptosis, significant increase of apoptotic cells was consistently observed. Furthermore, CD34+CD44− cells plated on BM stromal cells could differentiate into CD34−CD44−CD10−CD19+ cells. These findings suggest that CD34+CD44− cells expressing CD19 would represent unique B-lymphocyte–committed precursors in BM, which might undergo apoptotic cell death in the early steps of B-cell differentiation.