Functions of domain 1 and 4 of vascular cell adhesion molecule-1 in alpha4 integrin-dependent adhesion under static and flow conditions are differentially regulated.
To assess the potential contributions of endothelial vascular cell adhesion molecule-1 (VCAM-1) to leukocyte adhesion under conditions of flow, we evaluated the effects of cytokine stimulation of HUVEC on primary capture, rolling, and stable adhesion of mono- and lymphoblastoid cells in a parallel-plate flow chamber. Stimulation of HUVEC with IL-1beta or IL-4 for 24 h induced surface expression of VCAM-1, consistent with published results. Using blocking mAbs to inhibit specific adhesion molecules, we found that a significant level of primary capture at wall shear stresses between 0.5 and 2.0 dynes/cm2 could be attributed to alpha4 integrin and VCAM-1. This primary adhesion under flow was most often characterized by abrupt stationary adhesion with little evidence of rolling. Studies with IL-1beta-stimulated HUVEC revealed that domain 1 of VCAM-1 was solely responsible for alpha4 integrin-dependent primary capture under flow, but both domains 1 and 4 were utilized in alpha4 integrin-dependent adhesion under static conditions. In contrast to results obtained with these cytokines alone, stimulation of HUVEC with a combination of IL-1beta and IL-4 markedly reduced alpha4/VCAM-1-dependent adhesion under flow, even though the surface levels of VCAM-1 were greater than with either cytokine alone, and alpha4/VCAM-1-dependent adhesion under static conditions was intact. These results demonstrate that distinct functional differences exist between the interactions of VCAM-1 and alpha4 integrin under static and flow conditions, and they reveal the potential for endothelial modulation of adhesion under flow without altering VCAM-1-dependent adhesion under static conditions.