CD19 amplification of B lymphocyte Ca2+ responses: a role for Lyn sequestration in extinguishing negative regulation.

B lymphocyte antigen receptor (BCR) signals are regulated by CD19, with BCR-induced intracellular calcium ([Ca(2+)](i)) responses enhanced by CD19 co-ligation. In this study, CD19 engagement using a dimeric anti-CD19 antibody induced [Ca(2+)](i) mobilization and significantly enhanced BCR-induced [Ca(2+)](i) responses without a requirement for CD19/BCR co-ligation. Although simultaneous CD19 and BCR engagement significantly enhanced CD19/Lyn complex formation and [Ca(2+)](i) responses, downstream tyrosine phosphorylation of CD22 and multiple other cellular proteins was inhibited, as was SHP1 recruitment to phosphorylated CD22. CD19 overexpression also enhanced BCR-induced [Ca(2+)](i) responses, but down-regulated tyrosine phosphorylation of CD22 and multiple other cellular proteins following BCR ligation. Because CD19 and Lyn expression are genetically titrated in B cells, CD19 engagement may augment BCR-induced [Ca(2+)](i) responses by sequestering the available pool of functional Lyn away from downstream negative regulatory proteins such as CD22. Consistent with this, simultaneous CD19 engagement did not further enhance the BCR-induced [Ca(2+)](i) responses of Lyn- or CD22-deficient B cells. Thus, CD19 recruitment of Lyn may preferentially activate selective signaling pathways downstream of the CD19/Lyn complex to the exclusion of other downstream regulatory and effector pathways. Other receptors may also utilize a similar strategy to regulate kinase availability and downstream intermolecular signaling.