CD34+ cells and CD34+CD38- subset from mobilized blood show different patterns of adhesion molecules compared to those from steady-state blood, bone marrow, and cord blood.

As suggested previously, a down-regulation of some cellular adhesion molecules (CAMs) on CD34(+) hematopoietic progenitor cells (HPC) may contribute to their egress from bone marrow (BM) to peripheral blood (PB) by decreasing their adhesion to BM stromal cells. Besides counting the percentage of CAM-positive cells, we decided to define clearly the antigen density (AgD) of the CAM on mobilized- and steady-state CD34(+) HPC using QIFIKIT calibration beads. Five sources of cells were compared: PB and BM from normal donors (nPB, nBM) cord blood (CB), mobilized PB obtained from leukapheresis products (LKP), and mobilized BM (mBM) samples. In our study the CAM-AgD was the lowest on CD34(+) cells in LKP which, on the contrary, contained the highest percentage of CD117(+), CD54(+), CD58(+) cell subsets. As for CB, a greater proportion of CD44(+) and CD62L(+) cells was observed in LKP than in other products. The LKP-CD34(+) cell population contained a greater percentage of CD11a(+) cells when compared to mBM, but the lowest percentage of CD49d(+) and CD49e(+) cells when compared to all products. The proportion of the CD34(+)CD38(-) immature subset expressing CD11a, CD44, CD54, or CD62L was greater in LKP than in mBM; the CD62L-AgD was higher in LKP than in mBM. This quantitative analysis clearly showed a downregulation of all CAM on LKP-CD34(+). The CD44, CD62L, CD11a, and CD54 AgD decrease appears to be specifically involved in the egress of the CD34(+) subsets into PB. The control of antigen density of these adhesion molecules is likely to be clinically important for effective mobilization of HPC as well as for rapid engraftment following HPC transplant.