Vascular endothelial-cells injury plays a pivotal role in the pathogenesis of graft-versus-host disease (GVHD) and transplant-associated endothelial injury syndrome. Vascular endothelial cells are an exposed target tissue for immune-mediated injury during GVHD. Early endothelial injury syndromes share common features with acute GVHD. Chronic GVHD leads to a rarefaction of microvessels caused by the infiltration of alloreactive cytotoxic T lymphocytes. In this context, allogeneic reactive cytotoxic T cell may contribute to apoptosis of vascular endothelial cells. The involvement of proteinase-activated receptor (PAR-1) in regulation of apoptosis has been recently recognized in many cell types. We hypothesized that apoptosis of vascular endothelial cells induced by allogeneic cytotoxic T cell are mediated via the PAR-1. Allogeneic CD8(+) T cell, PAR-1 agonist peptide (SFLLRN) induced apoptosis of human umbilical vein endothelial cells (HUVECs) and human dermal microvascular endothelial cells (HDMECs) as assessed by AnnexinV-FITC labeling. To ascertain the mechanism of endothelial apoptosis, we determined that allogeneic CD8(+) T cell, SFLLRN enhanced cleavage of caspase-3 and led to p38MAPK activation as assessed by Western blot. The effects of allogeneic CD8(+) T cell and SFLLRN on apoptosis of vascular endothelial cells were largely prevented by a cleavage-blocking anti-human PAR-1-antibody (ATAP2) and a specific inhibitor of p38MAPK. In concert, these observations provide strong evidence that allogeneic CD8(+) T cell induces apoptosis of human vascular endothelial cells through PAR-1-dependent modulation of intrinsic apoptotic pathway via alterations of p38MAPK and caspase-3.