Increased numbers of circulating ECs are associated with systemic GVHD. 
Circulating endothelial cells (ECs) are known to reflect endothelial injury, and endothelial injury is associated with graft-versus-host disease (GVHD).  We hypothesised that circulating ECs might be associated with systemic acute graft-versus-host disease (aGVHD).  BALB/c (H-2k(d) ) mice were treated with total body irradiation and then infused with C57B/6-derived T-cell-depleted bone marrow (TCD-BM) cells or TCD-BM cells and splenocytes.  Cyclosporine was used to prevent aGVHD.  Circulating ECs and allogeneic lymphocytes were analysed by flow cytometry at multiple time points.  The morphology and ultrastructure of the endothelium were examined by light microscopy or transmission electron microscopy.  The results indicated that the number of circulating ECs peaked at day 5 after lethal irradiation in all mice; allogenic transplanted mice (TCD-BM cells and splenocytes) developed typical aGVHD beginning at day 7, exhibiting both histological and clinical symptoms of disease.  Circulating ECs peaked a second time at day 9 with aGVHD progression.  However, following the administration of CSA, an absence of or a reduction in the amount of subsequent endothelial injury was observed.  Circulating ECs might be associated with systemic aGVHD.