Rapid STAT phosphorylation via the B cell receptor. Modulatory role of CD19.

Engagement of the B cell receptor (BCR) initiates multiple signaling cascades which mediate different biological responses, depending on the stage of B cell differentiation, antigen binding affinity, and duration of stimulation. Aggregation of co-receptors such as CD19 with the antigen receptor has been suggested to modulate the signals necessary for the development and functioning of the humoral immune system. In this study, we demonstrate that engagement of the antigen receptor on peripheral blood B cells, but not naïve splenic B lymphocytes, leads to rapid phosphorylation of signal transducers and activators of transcription 1 (STAT1) on Tyr-701 and Ser-727. Interestingly, phosphorylation on tyrosine diminished with increased stimulation, whereas serine phosphorylation correlated directly with the level of BCR cross-linking. In contrast, phosphorylation of STAT3 occurs exclusively on serine and is sensitive to inhibitors of the PI3-kinase and the ERK1/2 pathways. Finally, we show that co-ligation of CD19 with the BCR results in increased tyrosine phosphorylation of STAT1 relative to BCR cross-linking alone, establishing CD19 as a positive modulator of BCR-mediated STAT activation.