An excess of free
heme is present in the blood during many types of
hemolytic anemia. This has been linked to organ damage caused by
heme-mediated oxidative stress and vascular
inflammation. We investigated the mechanism of
heme-induced coagulation activation in vivo.
Heme caused coagulation activation in wild-type mice that was attenuated by an anti-
tissue factor antibody and in mice expressing low levels of
tissue factor. In contrast, neither
factor XI deletion nor inhibition of
factor XIIa-mediated
factor XI activation reduced
heme-induced coagulation activation, suggesting that the intrinsic coagulation pathway is not involved. We investigated the source of
tissue factor in
heme-induced coagulation activation.
Heme increased the procoagulant activity of mouse macrophages and human PBMCs.
Tissue factor-positive staining was observed on leukocytes isolated from the blood of
heme-treated mice but not on endothelial cells in the lungs. Furthermore,
heme increased vascular permeability in the mouse lungs, kidney and heart. Deletion of
tissue factor from either myeloid cells, hematopoietic or endothelial cells, or inhibition of
tissue factor expressed by non-hematopoietic cells did not reduce
heme-induced coagulation activation. However,
heme-induced activation of coagulation was abolished when both non-hematopoietic and hematopoietic cell
tissue factor was inhibited. Finally, we demonstrated that coagulation activation was partially attenuated in sickle cell mice treated with recombinant
hemopexin to neutralize free
heme. Our results indicate that
heme promotes
tissue factor-dependent coagulation activation and induces
tissue factor expression on leukocytes in vivo. We also demonstrated that free
heme may contribute to
thrombin generation in a mouse model of
sickle cell disease.