Background—The mechanisms by which lipid lowering reduces the incidence of acute thrombotic complications of coronary atheroma in clinical trials remains unknown. Tissue factor (TF) overexpressed in atheroma may accelerate thrombus formation at the sites of plaque disruption. A cell surface cytokine CD40 ligand (CD40L) enhances TF expression in vitro.

Methods and Results—To test the hypothesis that lipid lowering reduces TF expression and activity, we produced atheroma in rabbit aortas by balloon injury and cholesterol feeding for 4 months (Baseline group, n=15), followed by either a chow diet (Low group, n=10) or a continued high-cholesterol diet for 16 months (High group, n=5). Immunolocalization of TF, CD40L, and its receptor CD40 was quantified by computer-assisted color image analysis. Macrophages in atheroma of the Baseline and High groups strongly expressed TF. Intimal smooth muscle cells and endothelial cells also contained immunoreactive TF. Regions of expression of CD40L and CD40 colocalized with TF. Protein expression of TF diminished substantially in the Low group in association with reduced expression of CD40L and CD40. In situ binding of TF to factors VIIa and X, detected by digoxigenin-labeled factors VIIa and X, colocalized with TF protein in atheroma and decreased after lipid lowering. We also determined reduced TF biological activity in the Low group by use of a chromogenic assay. The level of TF mRNA detected by reverse transcription–polymerase chain reaction also decreased after lipid lowering.

Conclusions—These results suggest decreased expression and activity of TF as a novel mechanism of reduced incidence of thrombotic complications of atherosclerosis by lipid lowering.