The present study showed that engagement of CD31 delivers a survival signal in chronic lymphocytic leukaemia (CLL) cells. We describe two groups of CLL, showing different kinetics of apoptosis in vitro and distinct ratios between anti-apoptotic and pro-apoptotic proteins: CLL-I displayed low Bcl- x L/Bax and Bcl-2/Bax ratio and underwent rapid apoptosis in vitro; CLL-II had high Bcl-x L/Bax and Bcl-2/Bax ratio and were resistant to apoptosis for several days. Nurse-like cells, expressing vimentin, CD68 and CD31 were detected mainly in CLL-II cultures. Of note, CD31 cross-linking, obtained with a specific monoclonal antibody (mAb), induced phosphatidylinositol- 3-kinase-dependent Akt phosphorylation and nuclear translocation of the nuclear factor-kBp65 and p52 subunits in both CLL groups, leading to upregulation of Bcl-2 and Bcl-x L transcription and increased cell survival. Binding to CD31 + stable transfectants, could also deliver an anti-apoptotic signal in B cells of both CLL-I and CLL-II, increasing the Bcl-2 and Bcl-x L protein content, regardless the expression of CD38. On the other hand, the addition of the F(ab') 2 (that is unable to oligomerize the target molecule) of the anti-CD31 mAb prevented these effects. These data suggest that the CD31 adhesion system may play a role also in vivo in maintaining CLL survival.
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