Chemoattractants Induce a Rapid and Transient Upregulation of Monocyte α4 Integrin Affinity for Vascular Cell Adhesion Molecule 1 Which Mediates Arrest: An Early Step in the Process of Emigration

Chemoattractants and chemokines induce arrest of rolling monocytes during emigration from blood into tissues. In this study, we demonstrated that α4 integrin affinity for vascular cell adhesion molecule (VCAM)-1 was upregulated rapidly and transiently by chemoattractants and stromal cell–derived factor (SDF)-1α and mediated monocyte arrest. α4 integrin affinity changes were detected and blocked using soluble VCAM-1/Fc (sVCAM-1/Fc). In a flow cytometry assay, markedly increased sVCAM-1/Fc binding to human blood monocytes or U937 cells transfected with formyl peptide (FP) receptor was detected 30 s after FP or SDF-1α treatment and declined after 2 min. In a parallel plate flow chamber assay, FP, C5a, platelet-activating factor, or SDF-1α coimmobilized with VCAM-1 induced leukocyte arrest, which was blocked by inclusion of sVCAM-1/Fc but not soluble nonimmune immunoglobulin G in the assay buffer.