CD38/CD31 crosstalk up-modulates CD100 expression in a subset of B-CLL cells. B-CLL cells were purified from 3 CD38⁺ and 1 CD38^– patients (nos. 8, 11, and 17, and no. 19, respectively) and cocultured with murine irradiated L-CD31⁺ or L-mock fibroblasts previously plated on glass coverslips. After 5 days, non–tightly adhering cells were collected from culture and stained for CD100 expression (A). The remaining conjugates were fixed and stained with an anti-CD100 mAb (BB18) followed by a Texas Red–conjugated secondary antibody. Counterstaining was performed with a FITC-conjugated anti–HLA class I mAb. The coverslips were mounted on slides and the cells observed using an Olympus 1 × 71 confocal microscope (oil immersion × 60 objective), and the images acquired using the FluoView software (B).