Role of P-selectin and PSGL-1 in CD34+ cell interactions with BM microvessels. Fluorescently labeled CD34+ cells from mPB (a) or CB (b) were preincubated with anti–PSGL-1 (black bars) or isotype-matched control Ab and injected into wild-type or E–/– NOD/SCID mice that had been treated with anti–P-selectin (white bars) or isotype-matched control Ab (gray bars). The number of cells interacting with the BM microvasculature was determined by analysis of video recordings from fluorescence intravital microscopy experiments. *P < 0.02 compared with control Ab; **P = 0.06 compared with control Ab; #P < 0.02 compared with mPB control Ab group shown in a.