Nurselike cells obtained from B-CLL patients express high levels of functional CD31 and plexin-B1. Nurselike cells were obtained by culturing peripheral blood mononuclear cells from 4 patients with B-CLL in complete medium for 2 weeks. The appearance of the cell culture is represented in panel A at × 20 (left) and × 40 (right) magnification. A population of cells morphologically similar to nurselike cells is apparent. These cells express CD31 and plexin-B1, as shown by cytofluorography (B) and in situ immunofluorescence (C). Cells in panel C were counterstained with Syto 59 and observed using an Olympus 1 × 71 confocal microscope (oil immersion × 60 objective), and the images were acquired using the FluoView software. (D) Cells in suspension comprise CD19⁺/CD5⁺ B lymphocytes that are highly positive for CD38 and CD100. Gray profiles in panels B and D were obtained using an irrelevant isotype-matched mAb. CD31 and plexin-B1 deliver proliferation/survival signals to CD38⁺/CD100⁺ B-CLL cells, as inferred after annexin-V (E) and PI (F) stainings performed after 4-day cocultures with autologous purified B-CLL cells. The specificity of the interaction was confirmed using blocking anti-CD31 and anti–plexin-B1 mAbs. Data are the mean values obtained using B-CLL from 4 patients in 2 separate experiments; error bars represent the SD.