Background The protein C anticoagulant pathway is critical to the control of hemostasis. Thrombomodulin and a newly identified receptor for protein C/activated protein C, EPCR, are both present on endothelium. EPCR augments activation of protein C by the thrombin-thrombomodulin complex.

Methods and Results To gain a better understanding of the relationship between thrombomodulin and EPCR, we compared the cellular specificity and tissue distributions of these two receptors by using immunohistochemistry. EPCR expression was detected almost exclusively on endothelium in human and baboon tissues. In most organs, EPCR was expressed relatively intensely on the endothelium of all arteries and veins, most arterioles, and some postcapillary venules. EPCR staining was usually negative on capillary endothelial cells. In contrast, thrombomodulin was detected at high concentrations in both large vessels and capillary endothelium. Both thrombomodulin and EPCR were expressed poorly on brain capillaries. The liver sinusoids were the only capillaries in which EPCR was expressed at moderate levels and thrombomodulin was low. EPCR and thrombomodulin were both expressed on the endothelium of vasa recta in the renal medulla, the lymph node subcapsular and medullary sinuses, and some capillaries within the adrenal gland. Even in these organs the majority of capillaries were EPCR negative or stained weakly.

Conclusions These studies suggest that EPCR may be important in enhancing protein C activation on large vessels. The presence of high levels of EPCR on arterial vessels may help explain why partial protein C deficiency is a weak risk factor for arterial thrombosis.