Developmental differences in B cell receptor-induced signal transduction.
We have compared early signaling events at various stages of B cell differentiation using established mouse cell lines. Clustering of pre-B cell antigen receptor (BCR) or BCR induced the tyrosine phosphorylation of various proteins in all cells, although the phosphorylation pattern differed. In spite of the pre-BCR-induced tyrosine phosphorylation, we could not detect an intracellular Ca(2+) signal in pre-B cells. However, co-clustering of the pre-BCR with CD19 did induce Ca(2+) mobilization. In contrast to the immature and mature B cells, where the B cell linker protein (BLNK) went through inducible tyrosine phosphorylation upon BCR clustering, we observed a constitutive tyrosine phosphorylation of BLNK in pre-B cell lines. Both BLNK and phospholipase C (PLC)gamma were raft associated in unstimulated pre-B cells, and this could not be enhanced by pre-BCR engagement, suggesting a ligand-independent PLC gamma-mediated signaling. Further results indicate that the cell lines representing the immature stage are more sensitive to BCR-, CD19- and type II receptors binding the Fc part of IgG (Fc gamma RIIb)-mediated signals than mature B cells.