TNF-α activation of arterioles and venules alters distribution and levels of ICAM-1 and affects leukocyte-endothelial cell interactions

The observation that leukocyte-endothelial cell (EC) interactions are localized to specific regions on the microvessel wall suggests that adhesion molecule distribution is not uniform. We investigated ICAM-1 distribution and leukocyte-EC interactions in blood-perfused microvessels (< 80 mu m) in cremaster muscle of anesthetized mice, using intravital confocal microscopy and immunofluorescent labeling. Variability of ICAM-1 expression directly determines leukocyte adhesion distribution within the venular microcirculation and contributes to leukocyte rolling in arterioles during inflammation. The number of rolling interactions increased with ICAM-1 intensity (r(2) = 0.69, P < 0.05), and rolling velocity was lower in regions of higher ICAM-1 intensity. In controls, venular ICAM-1 expression was approximately twofold higher than in arterioles. After TNF-alpha treatment, ICAM-1 expression was significantly increased, 2.8 +/- 0.2-fold in arterioles and 1.7 +/- 0.2-fold in venules (P < 0.05). ICAM-1 expression on activated arteriolar ECs only reached the level of control venular ICAM-1. Arteriolar but not venular ECs underwent redistribution of ICAM-1 among cells; some cells increased and some decreased ICAM-1 expression, magnifying the variability of ICAM-1. TNF-alpha treatment increased the length of bright fluorescent regions per unit vessel length (42%, control; 70%, TNF-alpha) along the arteriolar wall, whereas no significant change was observed in venules (60%, control; 63%, TNF-alpha). The spatial distribution and expression levels of adhesion molecules in the microcirculation determine the timing and placement of leukocyte interactions and hence significantly impact the inflammatory response. That arteriolar ECs respond to TNF-alpha by upregulation of ICAM-1, although in a different way compared with venules, suggests an explicit role for arterioles in inflammatory responses.