L-selectin stimulation enhances functional expression of surface CXCR4 in lymphocytes: implications for cellular activation during adhesion and migration

Abstract

L-selectin mediates leukocyte tethering and rolling, the first step in a sequential process of leukocyte adhesion and migration. Additionally, L-selectin has important signaling roles perhaps contributing to leukocyte activation and integrin-mediated adhesion. Because chemokines are critically involved in leukocyte activation, we questioned whether L-selectin signaling affects chemokine receptor expression and function. We observed that whereas only 5% to 15% of freshly isolated lymphocytes expressed CXCR4 on the cell surface, intracellular CXCR4 was detectable in all cells. Engagement of L-selectin by antibody cross-linking or the L-selectin ligands fucoidan or sulfatide mobilized intracellular CXCR4 to significantly increase surface CXCR4 expression but did not affect CCR5, CCR7, or β₂-integrin expression. L-selectin stimulation also inhibited stromal-derived factor 1 (SDF-1)–induced CXCR4 internalization. The combined effects of L-selectin on CXCR4 trafficking are likely important in markedly enhancing cell activation by SDF-1. Blockade of SDF-1–induced CXCR4 internalization resulted in enhanced actin polymerization on subsequent exposure to SDF-1. Physiologically more important, L-selectin stimulation increased SDF-1–induced lymphocyte adhesion and transendothelial migration, which were inhibited by anti–leukocyte function-associated antigen 1 antibodies, tyrosine kinase inhibitors, and pertussis toxin. To further corroborate the additive stimulating effects, L-selectin signaling and SDF-1 increased β₂-integrin activation. Taken together, L-selectin–mediated signals specifically enhance CXCR4 expression and function, suggesting a novel mechanism for the modulation of lymphocyte activation during cell adhesion and transmigration.