Platelet recruitment to lesion- prone sites is augmented in hyperlipidemic rabbits - role of p-selectin and vwf

Platelets potentially contribute to the earliest steps of atherogenesis. Experim:ntal evidence for platelet recruitment to lesion prone sites before plaque is present is lacking. We studied in vivo, ex vivo and in vitro platelet/endotheüal (EC) interactions. Platelet homing to EC overlying segmental and mesenteric artery ostia was studied in control rabbits (' I) or rabbits on 0.125% cholesterol diet for 3 months (3HC) in a novel in vivo homing assny of autologous, washed and fluorescently labeled platelets. In whole blood the label was detectable until day 3 when the aorta was harvested after pressure perfusion of heparinized rabbits, counterstained with a red fluorescent label and examined by scanning con bcal microscopy. To examine dynamic platelet recruitment to EC of C and HC rabbits i nder arterial shear (24 dyn) aortae were mounted in a flow chamber. To dissect the adhi :sion pathways, blocking studies were performed with lysophosphatidylcholine-activated hi iman EC (Ea.hy926) and TRAP-activated human platelets in a parallel plate flow chambe:. No lesions were found in C and 3HC aortae, but mild hyperlipidemia led to fibrofatty p' aque formation after 12 months ( 12HC). Out of 98 aortic ostia of 3HC rabbits, 55 had recr jited labeled platelets compared to 11 out of 63 C-ostia(p<0.0001). In average, 14.4±2.6pla'elets were found per HC aorta. 3.25+0.85 platelets homed in C aortas (n=5/4, p=0.015;. No platelets were found at the anterior aspect of the thoracic aorta. For 3HC platelets, adhesion to C aorta was 3-fold elevated compared to C platelets (97±10 vs. 32±4.6 platelets/0.9 mm2, n=5, p<0.05). Adhesion of C platelets to 3HC aorta was 3.4-fold and adhesion of |3HC platelets to 3HC aorta was 8.2-fold increased (n=5, p<0.05). No further increase was noted when 12HC platelets were used. Inhibition of vWF by mAb AJvW2 blocked 75% of plMelet translocation and 66% adhesion (p<0.05, n=3). In the human system, combined P-se!ectin (PS)- and vWF-inhibition reduced translocation by 80% and adhesion of platelets tp EC by 90%. Anti-GPIb and GPIIb/HIa-mAbs did not yield additional inhibition if vWlf was blocked. We conclude that hyperlipidemia induces EC and platelet activation early in atherogenesis, enabling specific platelet recruitment to plaque predilection sites well b ;fore lesions are developed. This interaction is, under dynamic conditions, mediated main y by PS and vWF with its ligands.