Novel considerations in the pathogenesis of the antiphospholipid syndrome: involvement of the tissue factor pathway of blood coagulation.

Abstract

The antiphospholipid syndrome (APS) is characterized by clinical manifestations such as venous and arterial thrombosis, thrombocytopenia and/or recurrent pregnancy loss, as well as the persistent presence of laboratory markers of antiphospholipid (aPL) antibodies detected in laboratory assays. Though it is generally accepted that aPL antibodies, such as anticardiolipin (aCL), anti-beta2 glycoprotein I (anti-beta2GPI), and lupus anticoagulants (LA) contribute to the pathogenesis of APS, precise mechanism(s) are yet to be fully described. It is probable that aPL antibodies bind to a range of cellular targets (e.g., platelets, endothelial cells, and monocytes), leading to thrombosis and obstetric complications. There is now increasing evidence that alterations to the tissue factor (TF) pathway of blood coagulation contribute toward hypercoagulability in patients with aPL antibodies. This article reviews current evidence that suggests changes and/or interference to the major pathway of blood coagulation may represent a novel mechanism that contributes to the development of APS.

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Novel Considerations in the Pathogenesis
of the Antiphospholipid Syndrome:
Involvement of the Tissue Factor Pathway
of Blood Coagulation
Murray Adams, Ph.D., M.A.I.M.S.1
ABSTRACT
The antiphospholipid syndrome (APS) is characterized by clinical manifestations
such as venous and arterial thrombosis, thrombocytopenia and/or recurrent pregnancy loss,
as well as the persistent presence of laboratory markers of antiphospholipid (aPL)
antibodies detected in laboratory assays. Though it is generally accepted that aPL anti-
bodies, such as anticardiolipin (aCL), anti–b2 glycoprotein I (anti-b2GPI), and lupus
anticoagulants (LA) contribute to the pathogenesis of APS, precise mechanism(s) are yet to
be fully described. It is probable that aPL antibodies bind to a range of cellular targets (e.g.,
platelets, endothelial cells, and monocytes), leading to thrombosis and obstetric compli-
cations. There is now increasing evidence that alterations to the tissue factor (TF) pathway
of blood coagulation contribute toward hypercoagulability in patients with aPL antibodies.
This article reviews current evidence that suggests changes and/or interference to the major
pathway of blood coagulation may represent a novel mechanism that contributes to the
development of APS.
KEYWORDS: Tissue factor, antiphospholipid syndrome, antiphospholipid antibodies,
tissue factor pathway inhibitor, thrombosis
The antiphospholipid syndrome (APS) is a
complex, acquired autoimmune disease characterized
by clinical manifestations of thrombosis, recurrent preg-
nancy loss and/or thrombocytopenia, coupled with the
persistent laboratory detection of antiphospholipid
(aPL) antibodies such as anticardiolipin (aCL), anti–
b2 glycoprotein I (anti-b2GPI), and lupus anticoagu-
lants (LA).1APS is associated with significant morbidity
and mortality, and despite recent advances in classifica-
tion and laboratory testing, the disease still currently
represents a considerable challenge in terms of both
diagnosis and treatment. Although it is generally ac-
cepted that aPL antibodies contribute to the clinical
manifestations of APS, how this occurs remains to be
clearly resolved. It is probable that there are a myriad of
intravascular and cellular interactions that contribute to
the development of APS rather than a single triggering
event. Such mechanisms that have been well described
include aPL antibody interference to the protein C
anticoagulant pathway,2,3interaction of aPL antibodies
1Senior Lecturer, School of Human Life Sciences, University of
Tasmania, Tasmania, Australia.
Address for correspondence and reprint requests: Murray Adams,
Ph.D., M.A.I.M.S., School of Human Life Sciences, University of
Tasmania, Tasmania, Australia (e-mail: Murray.Adams@utas.edu.au).
Antiphospholipid Antibodies and the Antiphospholipid Syndrome
I: Pathogenesis, Clinical Features, Diagnosis, and Management; Guest
Editors, Emmanuel J. Favaloro, Ph.D., M.A.I.M.S., and Richard
C.W. Wong, F.R.C.P., F.R.A.C.P., F.R.C.P.A.
Semin Thromb Hemost 2008;34:251–255. Copyright # 2008 by
Thieme Medical Publishers, Inc., 333 Seventh Avenue, New York, NY
10001, USA. Tel: +1(212) 584-4662.
DOI 10.1055/s-0028-1082268. ISSN 0094-6176.
251

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with endothelial cells resulting in increased tissue factor
(TF) expression,4,5and promotion of platelet aggrega-
tion after aPL antibody binding.6A full review of the
major proposed pathogenic mechanisms is available
elsewhere in this issue of Seminars in Thrombosis and
Hemostasis.7Whereas these mechanisms contribute
to the understanding of APS pathophysiology, new
mechanisms implicated in the development of APS
continue to be described, including involvement of
the TF pathway. The aim of this article is to focus on
and review the contribution of the TF pathway toward
the pathophysiology of APS.
TISSUE FACTOR PATHWAY
The TF pathway is the major physiologic trigger
of blood coagulation. After vessel or tissue injury, the
45-kDa integral membrane protein TF is exposed to
the circulation and triggers a sequence of reactions
culminating in thrombin generation and fibrin forma-
tion that helps to stabilize the platelet plug. Blood
coagulation has three major phases: (1) initiation, where
TF is exposed to the circulation after vessel damage to
generate trace amounts of thrombin. (2) Amplification,
where thrombin is central to overcoming the inhibitory
effect of the natural regulator of the TF pathway, tissue
factor pathway inhibitor (TFPI). This is achieved by
activation of platelets and other coagulation factors (i.e.,
factors XI, V, and VIII) that are associated with phos-
phatidylserine on damaged endothelial cells or activated
platelets to generate further thrombin. (3) Propagation,
where thrombin generation leads to the formation of
fibrin monomers that help to stabilize the platelet plug
and to form a clot8(Fig. 1).
TF was once thought to be entirely extravascular,
providing a ‘‘hemostatic envelope’’ and protection
against vascular injury. Whereas TF is easily expressed
in cultured endothelial cells in vitro using a variety of
Figure 1
vessel or tissue with the exposure of TF to the circulation resulting in the formation of the TF-FVIIa complex (initiation phase).
This complex generates trace amounts of thrombin, which is essential for feedback amplification of other coagulation factors to
(i) overcome the inhibitory effect of TFPI and (ii) generate more thrombin (amplification phase). Increased thrombin generation
results in fibrin monomer formation and eventually cross-linking of monomers to form the stable fibrin polymer (propagation
phase).
Model of blood coagulation. The tissue factor pathway (indicated in boldface) is triggered after injury to a blood
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agents, it remains controversial whether endothelial cells
constitutively express TF in vivo.9Recently, there has
been intense focus and debate on the physiologic role(s)
and measurement of plasma-borne TF microparticles
derived from various cells such as monocytes, endothelial
cells, platelets, and smooth muscle cells. TF micropar-
ticles have been reported to be elevated in a range of
diseases, including cardiovascular disease10,11and vari-
ous cancers,12,13indicating that they may contribute
significantly to thrombosis. It may therefore be reason-
able to suggest that TF microparticles have an important
role in maintaining thrombin generation during normal
hemostasis.
The major natural regulator of the TF pathway is
TFPI, a 42-kDa, three-domain, Kunitz-type protease
inhibitor constitutively produced by microvascular en-
dothelial cells.14TFPI acts in a factor Xa (FXa)-
dependent manner to inhibit TF-FVIIa complexes on
phospholipid surfaces and thus regulates thrombin
generation and fibrin formation. To date, TFPI defi-
ciency has yet to be clearly associated with either
alterations in genotype or with thrombotic phenotype.
This is probably due to heterozygous expression of
TFPI being sufficient to maintain normal hemostasis.
There is also doubt over the suitability of current assays
for TFPI in that they (i) only measure the plasma
pool accounting for ?10 to 50% of total body TFPI
and (ii) have demonstrated poor correlation between
different methods.15–17
THE TF PATHWAY IN APS
There is increasing evidence to suggest that changes to
the TF pathway of blood coagulation contribute to the
development of thrombosis in APS. A role for the TF
pathway in APS was first implicated with experiments
that demonstrated that incubation of serum from pa-
tients with systemic lupus erythematosus enhanced the
procoagulant activity of cultured human endothelial
cells,3,18–20with aPL antibodies from these patients
thought to be responsible for the increase in procoagu-
lant activity. Similarly, there is considerable evidence
suggesting that TF activity on monocytes is stimulated
by aPL antibodies from patients with APS4,5,21,22and
that monocytes from APS patients exhibit increased
expression of TF and TF mRNA.4,21Increased plasma
levels of TF antigen in APS patients have been reported,
consistent with ahypercoagulable state.4,23Furthermore,
TF microparticles have been reported to be elevated in
patients with aPL antibodies,24,25though method stand-
ardization for the measurement of TF microparticles
remains to be resolved.26It is probable that aPL anti-
body–induced TF procoagulant activity on endothelial
and other cells (e.g., monocytes), together with increased
levels of circulating TF microparticles derived from
endothelial cells as a consequence of cell damage and/or
other sources, contribute to increased TF activity and
thus hypercoagulability in APS.
Whereas data are consistent reflecting increased
TF activity in APS, there is no consensus on levels of
TFPI in APS. Thus, plasma TFPI levels are reported to
be variable in patients with aPL antibodies compared
with those of normal healthy individuals.4,23,27–30
Increased TFPI levels may be a response to increased
TF activity, though this does not effectively explain why
APS patients have a higher risk of thrombosis. Lower
levels of TFPI may be a consequence of TFPI consump-
tion during clotting or interference in the ability of TFPI
to regulate coagulation (e.g., anti-TFPI autoantibodies
and/or other cross-reacting aPL antibodies).
During the past few years, there have been reports
of anti–TFPI-like activity27,28,31as well as the identi-
fication of anti-TFPI autoantibodies in APS patients.32
Whether anti-TFPI activity and anti-TFPI autoanti-
bodies reflect the same entity, and/or involve other cross-
reacting aPL antibodies, is yet to be elucidated, though
given the heterogeneous nature of APS, it would be
reasonable to speculate that the anti-TFPI activity
reported in these studies represents one or more types
of aPL antibodies that bind to and/or inhibit TFPI.
It was reported that anti-b2GPI isolated from
APS patients has anti–TFPI-like activity.33These au-
thors demonstrated that anti-b2GPI antibodies isolated
from the IgG fraction of APS patients caused increased
FXa generation in normal plasma containing TFPI
compared with that of plasma lacking TFPI. It was
hypothesized that anti-b2GPI antibodies compete for
the same phospholipid binding sites as TFPI:FXa,
thereby interfering with phospholipid-dependent inhib-
ition of FX activation by TFPI.33A similar inhibitory
component against TFPI was later identified in the IgG
fraction of a different group of APS patients that was
associated with increased in vitro TF-induced thrombin
generation.27Further studies by this laboratory demon-
strated that purified anti-b2GPI IgG antibodies, but not
purified antiprothrombin IgG antibodies, were associ-
ated with increased TF-induced thrombin generation in
the presence of TFPI, suggesting that anti-b2GPI may
bind to and/or inhibit TFPI.31Although no cross-
reactivity was demonstrated between purified anti-
b2GPI and TFPI, these data provide evidence that
aPL antibodies impede the TF pathway, possibly
through interference to TFPI that could result in the
development of hypercoagulability and thrombosis in
APS.
Most recently, increased thrombin activity in
pooled normal plasma (PNP) supplemented with puri-
fied IgG was demonstrated in a subset of patients with
LA using the calibrated automated thrombinogram.34
Inhibition of both in vitro (rTFPI) and in vivo (heparin-
induced TFPI plasma levels) TFPI anticoagulant activity
was also reported. This study provides further evidence
NOVEL CONSIDERATIONS IN PATHOGENESIS OF APS/ADAMS
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that aPL antibodies such as LA interfere with the
normal hemostatic role of the TF pathway and that
impairment of the anticoagulant activity of TFPI in
particular may be critical in the development of hyper-
coagulability and thrombosis in APS. It was also dem-
onstrated that IgG from different patients generated
variable levels of interference toward TFPI, which could
reflect variability in the mechanisms by which aPL
antibodies cause thrombosis in different patients. This
provides further support that the pathogenesis of APS is
likely to be multifactorial, with interference by aPL
antibodies to multiple antigenic targets providing an
overall net prothrombotic environment.34This is con-
sistent with previous data,27and indeed, a review of
previous results from this laboratory (including those not
previously published), again demonstrates a wide range
of effects of purified IgG from patients with LA/aPL
antibodies on thrombin generation in PNP using a
TF-induced thrombin generation assay35(Fig. 2). These
data support the notion that both procoagulant and
anticoagulant mechanisms are affected by aPL antibod-
ies and, significantly, may also reflect the heterogeneous
clinical presentations of these patients.
CONCLUSION
Evidence from the past 10 years strongly suggests that
abnormalities and/or interference to the TF pathway of
blood coagulation may play an important role in the
pathophysiology of APS. Of particular significance has
been the demonstration that (1) aPL antibodies stim-
ulate TF expression on endothelial cells, (2) TF micro-
particles are increased in APS patients compared with
that of normal controls, (3) anti-TFPI autoantibodies
have been identified in APS patients, and (4) anti-TFPI
activity is associated with the presence of anti-b2GPI
and is correlated with increased TF-induced thrombin
generation. As well as continued investigations on the
effect of aPL antibodies on endothelial and intravascular
cells, future studies will aim to determine the individual
and combined contribution of various aPL antibodies
toward the TF pathway and the precise relationship
between aPL antibodies and TF microparticles. The
future may also see the development of target-specific
anticoagulant therapy that accounts for specific actions
of aPL antibodies on hemostatic parameters, including
the TF pathway.
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Furtherinvestigationsof
inhibitoryactivity of tissue
NOVEL CONSIDERATIONS IN PATHOGENESIS OF APS/ADAMS
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