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 ([Ca2+](i)) responses enhanced by CD19 co-li gation. In this study, CD19 engagement using a dimeric anti-CD19 antibody i nduced [Ca2+](i) mobilization and significantly enhanced BCR-induced [Ca2+] (i) responses without a requirement for CD19/BCR co-ligation. Although simu ltaneous CD19 and BCR engagement significantly enhanced CD19/Lyn complex fo rmation and [Ca2+](i) responses, downstream tyrosine phosphorylation of CD2 2 and multiple other cellular proteins was inhibited, as teas SHP1 recruitm ent to phosphorylated CD22. CD19 overexpression also enhanced BCR-induced [ Ca2+](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 aug ment BCR-induced [Ca2+](i) responses by sequestering the available pool of functional Lyn away from downstream negative regulatory proteins such as CD 22. Consistent with this, simultaneous CD19 engagement did not further enha nce the BCR-induced [Ca2+](i) responses of Lyn- or CD22-deficient B cells. Thus, CD19 recruitment of Lyn may preferentially activate selective signali ng pathways downstream of the CD19/Lyn complex to the exclusion of other do wnstream regulatory and effector pathways. Other receptors may also utilize a similar strategy to regulate kinase availability and downstream intermol ecular signaling.