A two-step adhesion cascade for T cell/endothelial cell interactions under flow conditions.

Neutrophil adherence to endothelial cells (ECs) under conditions of flow occurs in successive steps, including selectin-dependent primary adhesion and CD18-dependent secondary adhesion. We used a parallel-plate flow chamber to assess the steps in T cell adherence in vitro. On monolayers of L cells transfected with the EC adhesion molecules E-selectin, vascular cell adhesion molecule-1 (VCAM-1), or intercellular adhesion molecule-1 (ICAM-1), E-selectin was capable of mediating only primary adhesion, ICAM-1 was capable of mediating only secondary adhesion, and VCAM-1 was capable of mediating both primary and secondary adhesion. Studies using human umbilical vein EC monolayers stimulated for 24 h with IL-1 also revealed distinct primary and secondary steps in T cell adhesion under flow, and the secondary adhesion was inhibited > 90% by blocking both VCAM-1/alpha 4 beta 1 integrin and ICAM-1/CD18 integrin pathways. However, the primary adhesion under conditions of flow could not be attributed to any of the mechanisms known to support adhesion of leukocytes to ECs. Alone, this pathway was shown to mediate T cell rolling and was a necessary prerequisite for engagement of the two integrin pathways in this system. Thus, T cell adherence to 24-h IL-1-stimulated human umbilical vein ECs at venular wall shear stresses involves at least two successive steps, with clear molecular distinctions from the mechanisms accounting for neutrophil/EC adhesion.