Phenotypic variations in resting and activated levels of ICAM-1 expression by cultured human aortic endothelial cells

ICAM-1 is an inducible glycoprotein important in the adhesion, activation, and transmigration of circulating leukocytes across the vascular endothelial monolayer, and it likely plays a key role in the allogeneic response. To determine the reproducibility and significance of variations in resting levels of cell surface ICAM-1, 3 individual measurements of ICAM-1 levels were performed on 26 individual isolates of human aortic endothelial cells (HAECs) both at rest and following activation by allogeneic lymphocytes, using flow cytometry. Resting HAEC ICAM-1 levels varied 10-fold (range 6-60 mean fluorescence channels) depending on the isolate studied. There were strong correlations (r = 0.71 to 0.77, P < 0.0001) between the three measurements (performed no closer than weekly intervals on separate cultures), attesting to the consistency of the phenotypic expression. Constitutive expression of ICAM-1 was not affected by cell age, based upon comparing a subset of these isolates across 3 population doublings. Levels of HAEC ICAM-1 following allogeneic lymphocyte activation varied 15-fold (range 20-300 mean fluorescent channels) and, more important, correlated with resting ICAM-1 levels (r = 0.58, P = 0.002). Finally, constitutive ICAM-1 expression was related to TNF-alpha-induced ICAM-1 levels based upon a subset of the isolates studied. These data suggest that phenotypic, and likely genetic, differences in quiescent endothelial cell adhesion molecule expression can influence inflammatory responses including alloresponsiveness to the vasculature.