B cell antigen receptor engagement inhibits stromal cell-derived factor (SDF)-1 alpha chemotaxis and promotes protein kinase C (PKC)-induced internalization of CXCR4

The entry of B lymphocytes into secondary lymphoid organs is a critical ste p in the development of an immune response, providing a site for repertoire shaping, antigen-induced activation and selection. These events are contro lled 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)-1 alpha is thought to be one of the driving forces during those processes, as it is produced inside secondary lymphoid organs and induces B lymphocyte m igration that arrests upon BCR engagement. The signaling pathway that media tes this arrest was genetically dissected using B cells deficient in specif ic BCR-coupled signaling components. BCR-induced inhibition of SDF-1 alpha chemotaxis was dependent on Syk, BLNK, Btk, and phospholipase C (Plc)gamma 2 but independent of Ca2+ mobilization, suggesting that the target of BCR s timulation was a protein kinase C (PKC)-dependent substrate. This target wa s identified as the SDF-1 alpha receptor, CXCR4, which undergoes PKC-depend ent internalization upon BCR stimulation. Mutation of the internalization m otif SSXXIL in the COOH terminus of CXCR4 resulted in B cells that constitu tively expressed this receptor upon BCR engagement. These studies suggest t hat one pathway by which BCR stimulation results in inhibition of SDF-1 alp ha migration is through PKC-dependent downregulation of CXCR4.