L-selectin-mediated neutrophil recruitment in experimental rodent aneurysm formation

Background - This investigation tested the hypothesis that L-selectin is important in experimental abdominal aortic aneurysm ( AAA) formation in rodents. Methods and Results - Rat abdominal aortas were perfused with saline ( control) or porcine pancreatic elastase and studied on postperfusion days 1, 2, 4, 7, and 14 (n = 5 per treatment group per day). Neutrophil (polymorphonucleur leukocyte, PMN) and macrophage counts per high-powered field (HPF) were performed on fixed sections. L-selectin expression and protein levels in aortic tissue were determined by polymerase chain reaction and Western blot, respectively. Elastase-perfused aortic diameters were significantly increased compared with control aortas at all time points except day 1 (P < 0.05). PMN counts significantly increased in elastase-perfused aortas compared with control aortas at days 1, 2, and 4, reaching maximum levels at day 7 (40.8 versus 0.3 PMNs/HPF, P = 0.001). L- selectin mRNA expression in elastase-perfused aortas was 18 (P = 0.018), 17 (P < 0.001), and 8 times (P = 0.02) times greater than control aortas at days 1, 2, and 4, respectively. Western blot demonstrated a significant 69% increase in L- selectin protein at day 7 in elastase-as compared with saline-perfused aortas (P = 0.005). Subsequent experiments involved similar studies on postperfusion days 4, 7, and 14 of aortas from C57BL/6 wild-type (WT) mice (n = 21) and L-selectin-knockout (LKO) mice (n = 19). LKO mice had significantly smaller aortic diameters at day 14 as compared with WT mice (88% versus 123%, P = 0.02). PMN counts were significantly greater in elastase-perfused WT mouse aortas as compared with LKO mouse aortas at day 4 after perfusion (12.8 versus 4.8 PMNs/ HPF, P = 0.02). Macrophage counts were significantly greater at all time points after perfusion in elastase-perfused WT mouse aortas compared with elastase-perfused LKO mouse aortas, with a maximum difference at day 7 after perfusion (13.3 versus 0.5 macrophages/HPF, P < 0.001). Conclusion - L- selectin - mediated neutrophil recruitment may be a critical early step in AAA formation.