Globotriaosyl ceramide (CD77/Gb3) in the glycolipid-enriched membrane domain participates in B-cell receptor–mediated apoptosis by regulating Lyn kinase activity in human B cells

Objective The role of CD77 expressed on a fraction of germinal center B cells, also known as glycosphyngolipid Gb3, and as a functional receptor for Shiga toxins (Stx) in B-cell receptor (BCR)–mediated apoptosis was investigated. Materials and Methods Using Stx1-sensitive Burkittʹs lymphoma Ramos cells as an in vitro model of CD77+ germinal center B cells, intracellular signaling events mediated by either Stx1 or anti-CD77 antibody were examined immunobiochemically and immunocytologically. Results We observed prompt activation of Lyn and Syk kinases leading to increased binding of these proteins to surface IgM (sIgM) in Ramos cells after Stx1 treatment. We also observed microscopic colocalization of CD77 and sIgM after stimulation with Stx1. Along with the synergism between the cross-linking of CD77 and that of sIgM in their effect on apoptosis induction, it was highly probable that CD77 cross-linking induces activation of the BCR signaling cascade. Analysis using sucrose density gradient centrifugation suggested that Stx1 binding to CD77 induced recruitment and activation of Lyn in the glycolipid-enriched membrane (GEM) fractions. Once activated, however, Lyn seemed to acquire an increased detergent solubility and moved outside of the GEM fractions. Conclusions This study describes the participation of the GEM domain in BCR-signaling cascade and suggests a possible role of CD77 as a regulator of BCR-induced apoptosis in human B cells.