Transmigration of human mPB CD34⁺ cells and monocytes through HBMEC monolayers, in response to specific chemoattractants SDF-1α (with or without SCF) and MCP-1, respectively. (A) CD99 expression on CD34⁺ cells recovered from the lower chamber (migrated cells, gray area) and from the upper chamber (nonmigrated cells, white area), 4 hours after addition of SDF-1α in the lower chamber; one representative experiment; dotted line represents isotypic control. (B) Transmigration of human monocytes through HBMEC monolayers, in response to MCP-1 (200 ng/mL). An irrelevant IgG1 was used as a negative control. Data are mean ± SEM of 6 experiments with control, 12E7, PECAM-1, and PVR, and 3 experiments with hec2 and O662. (C) Transmigration of mPB CD34⁺ cells through HBMEC monolayers, in response to SDF-1α 100 ng/mL (□) or SDF-1α plus SCF (100 ng/mL; ▪).The Immu133 antibody that binds to CD34⁺ cells and HBMECs was used as a negative control. Data are mean ± SEM of 10 experiments with control, 12E7, PECAM-1 and PVR, and 3 experiments with hec2, O662, and the combination of 12E7 and PECAM-1. In both sets of experiments, the indicated mAbs were tested at a concentration of 20 μg/mL for their ability to inhibit transmigration. The 100% value corresponds to the number of untreated cells that migrate in response to chemoattractant(s). Chemotaxis in response to chemoattractant was assessed for 4 hours. ^*P < .05.