Abstract
Myeloperoxidase (MPO) is a cationic heme enzyme stored in neutrophilic polymorphonuclear
leukocytes (PMNs) that has recently been implicated in inflammatory cell signaling
and tissue damage. Although PMNs play a critical role in both innate immunity and
vascular thrombosis, no previous study has systematically investigated the effect
of MPO on blood coagulation. Here, we show that PMN-derived MPO inhibits the procoagulant
activity (PCA) of lipidated recombinant human tissue factor (rhTF) in a time- and
concentration-dependent manner that involves, but is not entirely dependent on the
enzyme's catalytic activity. Similarly, MPO together with its substrate, H2O2, inhibited the PCA of plasma microvesicles isolated from lipopolysaccharide (LPS)-stimulated
whole blood, an effect additive to that of a function blocking TF antibody. Treatment
of whole blood with LPS or phorbol-myristate-acetate dramatically increased MPO plasma
levels, and co-incubation with 4-ABAH, a specific MPO inhibitor, significantly enhanced
the PCA in plasma supernatants. MPO and MPO/H2O2 also inhibited the PCA of activated platelets and purified phospholipids (PLs), suggesting
that modulation of negatively charged PLs, i.e., phosphatidylserine, rather than direct
interference with the TF/FVIIa initiation complex was involved. Consistently, pretreatment
of activated platelets with MPO or MPO/H2O2 attenuated the subsequent binding of lactadherin, which specifically recognizes procoagulant
PS on cell membranes. Finally, endogenously released MPO regulated the PCA of THP1
cells in an autocrine manner dependent on the binding to CD11b/CD18 integrins. Collectively,
these findings indicate that MPO is a negative regulator of PL-dependent coagulation
and suggest a more complex role of activated PMNs in haemostasis and thrombosis.
Keywords
myeloperoxidase - leukocyte activation - microparticles - integrins
