Adhesion to fibronectin viaα4 integrin (CD49d) protects B cells from apoptosis induced by serum deprivation but not via IgM or Fas/Apo-1 receptors

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Abstract
Apoptosis is a regulated event crucial to the development and proliferation of normal and malignant B cells. We have studied the role of signals delivered via α4 integrin on apoptosis triggered by three different pathways on these cells. For apoptosis induced by serum deprivation, culturing B cells on the recombinant fibronectin fragment H89, a known ligand for α4β1 integrin, resulted in statistically significant (P < 0·005) higher viability values (68%, 65% and 67%) for Ramos, Nalm-6 and EHEB cells, respectively, than culturing cells on poly lysine (42%, 42% and 48%). An antiα4 MoAb reverted the protecting effect, thus confirming that it was due specifically to α4 engagement. Similarly, cells cultured on FN-III4-5, a recently identified fibronectin region which binds activated α4 integrin, also showed statistically significant higher viability than poly lysine cultures. α4 engagement however, did not prevent apoptosis induced on Ramos cells via surface IgM. Adhesion of IM-9 cells, a myeloma cell line carrying functional Fas receptors, to the H89 fragment neither increased cell viability upon triggering apoptosis via Fas when compared to poly lysine. These results indicate that α4 signalling may overcome B cell apoptosis induced by the lack of growth factors but does not seem to affect the IgM or Fas apoptotic pathways, thus suggesting different intracellular mechanisms for these processes.