B Cell Antigen Receptor Engagement Inhibits Stromal Cell–derived Factor (SDF)-1α Chemotaxis and Promotes Protein Kinase C (PKC)-induced Internalization of CXCR4

The entry of B lymphocytes into secondary lymphoid organs is a critical step in the development of an immune response, providing a site for repertoire shaping, antigen-induced activation and selection. These events are controlled by signals generated through the B cell antigen receptor (BCR) and are associated with changes in the migration properties of B cells in response to chemokine gradients. The chemokine stromal cell–derived factor (SDF)-1α is thought to be one of the driving forces during those processes, as it is produced inside secondary lymphoid organs and induces B lymphocyte migration that arrests upon BCR engagement. The signaling pathway that mediates this arrest was genetically dissected using B cells deficient in specific BCR-coupled signaling components. BCR-induced inhibition of SDF-1α chemotaxis was dependent on Syk, BLNK, Btk, and phospholipase C (Plc)γ2 but independent of Ca(2+) mobilization, suggesting that the target of BCR stimulation was a protein kinase C (PKC)-dependent substrate. This target was identified as the SDF-1α receptor, CXCR4, which undergoes PKC- dependent internalization upon BCR stimulation. Mutation of the internalization motif SSXXIL in the COOH terminus of CXCR4 resulted in B cells that constitutively expressed this receptor upon BCR engagement. These studies suggest that one pathway by which BCR stimulation results in inhibition of SDF-1α migration is through PKC-dependent downregulation of CXCR4.