Mononuclear Cells Adherence to Aortic Endothelium. The Effect of Transmural Pressure

Abstract

It is now generally accepted that most of the foam cells formed in the early fatty streak arise from circulating monocyte-macrophages. The effect of transmural pressure on mononuclear cells adherence to the vascular endothelium has been studied in excised rabbit thoracic aorta. Mononuclear cells were obtained from blood by centrifugation through lymphocyte separation medium (Ficoll-Paque, specific gravity 1.077). The mononuclear cell layer was collected from the interface and washed twice in RPMI containing 10 p. 100 fetal calf serum to remove contaminating platelets. The mononuclear cells were radiolabeled by incubation for 60 min in 0.1 mCi/ml of 51-chromium. After incubation, the cells were washed twice to remove free chromium, and resuspended in 5 ml RPMI containing 10 p. 100 fetal calf serum, at a concentration of 2 x 106 cells/ml. After ligature of intercostal arteries, 2 segments from the descending thoracic aorta were excised at in vivo length and under physiological pressure in order to preserve endothelial integrity. Arterial segments were incubated in oxygenated Krebs solution, at 39°C and the intraluminal solution was changed to the solution contraining the labeled mononuclear cells. The pressure was established at 70 mmHg in one segment and at 160 mmHg in the other. After 2 h incubation, the vessels were fixed under pressure. Each arterial segment was opened longitudinally and cut into 4 segments, which were laid on a microscopic slide and frozen at -20°C. The surface area of each segment was measured and sections were cut en face at 20 μm intervals from the luminal surface to the adventitia. Special care was taken to keep complete the very first section which contained the endothelium. The radioactivity of each section, normalized by its surface area, was divided by the radioactivity in one ml of intraluminal solution. This permitted the construction of the transmural profiles of mononuclear cells rate (expressed in μl/cm2). The adherence per unit surface area was obtained by multiplying the rate by the mononuclear cells concentration in the intraluminal solution (expressed in cells/mm2). Using the dimensions of the arterial segments, the adherence per unit length was then calculated (cells/mm). The transmural profiles of radioactivity across the wall showed that radioactivity was present solely in the first section. At 70 mmHg, the adherence was 277 ± 190 cells/mm2 and 1.95 ± 1.6 x 103 cells/mm. At 160 mmHg, it became 482 ± 290 cells/mm2 and 7.7 ± 4.6 x 103 cells/mm. Adherence was higher at 160 mmHg than at 70 mmHg, though statistically significant only when expressed per unit length (p < 0.01). These results suggested that the enhancement of mononuclear cell adherence with increasing transmural pressure was primarily due to distension of the wall, but might also have resulted from activation of adherence following hyperpressure.