Summary
An experimental model incorporating cultured endothelial cells (EC) was used to study
the "factor VIII bypassing" activity of prothrombin complex concentrates (PCC), a
property exploited in the treatment of hemophiliacs with alloantibodies to factor
VIII. Two PCC preparations were ineffective as stimuli of tissue factor expression
by EC. However, incubation with a combination of PCC plus endotoxin (lipopolysaccharide,
LPS) or tumor necrosis factor (TNF) induced much greater tissue factor expression
than was seen in response to either substance alone. PCC expressed an additional direct
procoagulant activity at the EC surface, which could not be attributed to either thrombin
or factor Xa, and which was diminished by an anti-tissue factor antibody. Therefore
factor VIIa, which was detectable in both PCC preparations, likely provided this additional
direct procoagulant activity at the EC surface. We also excluded the possibility that
coagulation proteases contained in or generated in the presence of PCC are protected
from inactivation by AT III. Therefore, PCC can indirectly bypass factor VIII by enhancing
induced endothelial tissue factor expression, and also possess direct procoagulant
activity, probably mediated by factor VIIa.