Role of P- and E-selectins and VCAM-1 in the initial interactions of human CD34+ cells with BM microvessels. Fluorescently labeled human CD34+ cells purified from mPB were injected into P/E–/– or wild-type mice backcrossed into the NOD/SCID background. Mice were injected via carotid catheter with control rat IgG (or no Ab in two experiments) and then with labeled CD34+ cells. After 15 minutes of recording, fluorescent cells were completely cleared from the circulation. The same mouse was then injected with anti–VCAM-1 and a second bolus of fluorescent CD34+ cells. Horizontal bars represent mean values. Each dot represents a single experiment before (filled squares) or after anti–VCAM-1 injection (open circles). *P = 0.05 by paired Student t test; **P = 0.003 by unpaired Student t test compared with IgG group in wild-type animals.