Enhancement of monocyte procoagulant activity by adhesion on vascular smooth muscle cells and intercellular adhesion molecule-1-transfected Chinese hamster ovary cells

Background - Plaque erosion is a frequent finding in sudden death due to coronary thrombosis. The present study sought to investigate whether monocyte adhesion to human aortic vascular smooth muscle cells (VSMCs) induces procoagulant activity (PCA) and whether this could be mediated by intercellular adhesion molecule-1 (ICAM-1). Methods and Results - We incubated mononuclear cells (MNCs) with VSMCs and ICAM-1-transfected Chinese hamster ovary (CHO) cells, investigated monocyte tissue factor (TF) mRNA expression by Northern blot analysis and TF protein expression by ELISA, and measured PCA. Incubation of MNCs with VSMCs for 6 hours increased PCA from 0.7±0.1 to 166.0±37.9 mU/10⁵ cells (P=0.007), which could be inhibited in a dose-dependent manner by the addition of blocking anti-ICAM-1 monoclonal antibodies. Prestimulation of VSMCs with interleukin-1β enhanced surface ICAM-1 expression significantly but did not induce PCA in VSMCs. Incubation of MNCs with prestimulated VSMCs led to a further increase in PCA to 239.9±27.9 mU/10⁵ cells (P=0.02 compared with incubation with unstimulated VSMCs). Incubation of MNCs with VSMCs enhanced TF mRNA after 2 hours and significantly increased TF protein content after 6 hours. Incubation of purified monocytes with ICAM-1-transfected CHO cells increased PCA from 1.2±0.2 to 81.9±3.3 mU/10⁵ cells (P<0.001 compared with incubation with untransfected CHO cells) after 6 hours. This effect could be inhibited significantly by the addition of blocking anti-CD18, anti-CD11b, or anti- CD11c monoclonal antibodies. Similar results were obtained for MNCs. Conclusions - Monocyte adhesion to VSMCs induces TF mRNA and protein expression and monocyte PCA, which is regulated by β₂-integrin-mediated monocyte adhesion to ICAM-1 on VSMCs.