CD100 up-regulation is restricted to the proliferating cells. (A) Purified B-CLL cells from patients nos. 8 and 13 were cultured for 5 days in the presence of agonistic anti-CD38 mAb + IL-2 and evaluated by flow cytometry using a directly labeled anti-CD100 mAb. The gray profiles represent the expression levels of the receptor in untreated cells, while the open profiles indicate the fluorescence intensity in the anti-CD38 + IL-2 samples. The percentages in parentheses refer to the CD100⁺ population. (B) The kinetics of CD100 up-regulation: the y-axis represents the MFI values of CD100 expression obtained from 7 different samples. Error bars represent the SD. Purified B-CLL cells from patient no. 6 were exposed to agonistic anti-CD38 mAb, IL-2, or a combination of the 2 for 5 days; the morphology of the resulting population was evaluated by Giemsa staining (C) and by cytofluorographic analysis (D). Gate R1 contains the proliferating cells, while R2 identifies resting and R3 the apoptotic population (as described in “Results”). The results in panel E show that the cells within the R1 gate display increased CD100 expression, while the other 2 subsets are unmodified. The open profiles reflect CD100 expression in the anti-CD38 + IL-2 condition compared with anti-CD38 alone (top row, gray profile), IL-2 alone (middle row, gray profile), and the untreated condition (bottom row, gray profile). Representative data from 7 patients.