Human brain pericytes differentially regulate expression of procoagulant enzyme complexes comprising the extrinsic pathway of blood coagulation

After vascular injury, pericytes may function in blood coagulation events that lead to thrombin formation due to their subendothelial location in the microvasculature. Pericytes from human cerebral cortex microvessels were isolated and characterized, and their ability to express and regulate procoagulant enzyme complexes was determined. Tissue factor was detected on the cell surface of cultured human brain pericytes by immunocytochemistry and was shown to form a functional complex with factor (F) VIIa to effect both FIX and FX activation. Treatment of pericytes with the calcium ionophore A23187 increased the observed tissue factor activity twofold to fivefold, which was shown to be due to an enhancement of cofactor activity and not the release of endogenous antigen stores. Pericytes also provided the appropriate membrane surface required for the assembly of a functional prothrombinase complex, so that in the presence of FVa and FXa, they effected thrombin formation 50 to 100 times faster than any other cell examined to date. In marked contrast to observations in other cell systems, pericyte expression of prothrombinase activity remained unaltered after treatment with A23187. As has been shown for platelets, the membrane receptor on pericytes for FXa assembly into the prothrombinase complex appears to at least partially consist of the FXa receptor effector cell protease receptor-1. These combined data indicate that pericytes can activate and propagate the coagulant response through the extrinsic pathway and that the activities of the required enzyme complexes can be differentially regulated in response to agonist stimulation. These observations support the concept that pericytes may play an important role in regulating coagulation events after cerebrovascular injury.