Involvement of platelet activation by P2Y₁₂ receptor in the development of transplant arteriosclerosis in mice

Background. Although activated platelets influence inflammation by intraplatelet mediators in transplantation, their mechanism of involvement in the progression of transplant arteriosclerosis has not yet been elucidated. We, therefore, investigated this question using P2Y₁₂ receptor knockout (KO) mice, in which platelets cannot be aggregated by adenosine diphosphate stimulation. Methods. Carotid arteries from 129X1 mice were orthotopically transplanted into wild-type or KO mice in a minor antigen(s)-mismatched strain combination. No immunosuppression was used. Grafts were harvested at 7, 14, 28, and 56 days after transplantation for morphometry and immunohistology. Fluorescence-activated cell sorting and quantitative real-time reverse-transcriptase polymerase chain reaction were performed at 7 and 14 days after transplantation. Results. The intima/media ratio of grafts in KO mice was significantly reduced compared with wild-type mice at 14, 28, and 56 days after transplantation. Fluorescence-activated cell sorting analysis showed a significant reduction of platelet CD154 expression and platelet-leukocyte aggregates in KO mice at 14 days after transplantation. Additionally, levels of intercellular adhesion molecule-1 and CD40 mRNA, and numbers of intercellular adhesion molecule-1- or CD40-positive cells in the grafts were lower in KO mice at 7 and 14 days after transplantation. These reductions resulted in a significant attenuation of CD45-positive leukocytes adhering to the graft vessel wall in KO mice at 14 days after transplantation. Conclusion. Diminished platelet function by P2Y₁₂ receptor deficiency attenuates initiation and strongly inhibits progression of transplant arteriosclerosis in mice by diminishing adhesion molecule expression and leukocyte accumulation in the grafts during the early phase after transplantation.