uJournal of Reproductive Immunologyv53ŠªAP.269-277,
2002”N ŒfÚ
Autoantibodies to contact proteins in patients
with recurrent pregnancy losses
Toshitaka Sugi and Tsunehisa Makino
Department of Obstetrics and Gynecology,
Center for Growth and Reproductive Medicine,
Tokai University School of Medicine, Kanagawa,
Japan
Abstract
The kallikrein-kinin system or plasma contact
system consists of three essential plasma
proteins. These are coagulation factor XII,
prekallikrein and high molecular weight kininogen.
Deficiencies of these proteins are not associated
with clinical bleeding despite marked prolongation
of in vitro surface-activated coagulation
time. Paradoxically, studies suggest that
contact proteins have anticoagulant, profibrinolytic
functions in a physiologic millieu, on endothelial
cells. Recently, evidence has accumulated
for the presence of the kallikrein-kinin
system or plasma contact system in the fetoplacental
unit. Kinins which are released within the
placenta may play a role in regulating placental
blood flow. This suggests that the plasma
contact system may also have an important
role in pregnancy. Several studies have reported
the presence of autoantibodies to the contact
proteins in patients with recurrent early
pregnancy losses. Disruption of this system
may be a risk factor for early gestational
losses.
1. Introduction
Antiphospholipid antibodies (aPA) to anionic
phospholipids such as cardiolipin and phosphatidylserine
have been described in patients with thrombosis,
thrombocytopenia and recurrent fetal loss.
Autoantibodies reactive to zwitterionic phospholipids
such as phosphatidylethanolamine (PE) have
not been studied extensively. Most lupus
anticoagulant (LA) and aPA literature is
limited to descriptions of anionic phospholipids
such as cardiolipin, however, antiphosphatidylethanolamine
antibodies (aPE) also have been described
in patients with thrombotic diseases and
recurrent pregnancy losses (Staub et al.,
1989; Karmochkine et al., 1992, 1993; Boffa
et al., 1996; Berard et al., 1995; Sugi et
al., 1999; Gris et al., 2000; McIntyre and
Wagenknecht, 2000). We reported that certain
aPE are not specific for PE per se, but are
directed to PE-binding plasma proteins, kininogens
(Sugi and McIntyre, 1995a).
Recently, a surprising high prevalence of
factor XII deficiency among patients with
recurrent pregnancy losses and thrombosis
has been reported (Gris et al., 1997; Halbmayer
et al, 1992). Jones (1999) reported the presence
of antibodies to factor XII in patients with
aPA.
Kininogens and factor XII are known as plasma
contact proteins in the intrinsic pathway
of blood coagulation. Since contact proteins
may play an important role in pregnancy especially
in feto-placental unit, deficiencies of these
proteins and/or autoantibodies to these proteins
may be associated with pregnancy losses.
2. Plasma contact system and kallikrein-kinin
system
The plasma contact system or kallikrein-kinin
system consists of three essential plasma
proteins that interact upon binding to negatively
charged surfaces. These are coagulation factor
XII (Hageman factor), prekallikrein (Fletcher
factor), and high molecular weight kininogen
(Williams, Fitzgerald factor). Factor XII
can be activated by contact with negatively-charged
surfaces (Griffin, 1978; Silverberg et al.,
1980; Tankersley and Finlayson, 1984). Activated
factor XII (factor XIIa) converts prekallikrein
to kallikrein and kallikrein digests high
molecular weight kininogen (HK) to liberate
the vasoactive, proinflammatory mediator,
bradykinin. Factor XIIa also activates factor
XI to continue the intrinsic coagulation
cascade (Kaplan et al., 1997).
Although there are many candidate physiologic
negatively-charged surfaces that in vitro
can be associated with factor XII autoactivation,
the concept of autoactivation itself has
never been a sufficiently convincing mechanism
to explain contact system activation in vivo.
In fact, it is correct to say that the so-called
elusive physiologic, negatively charged surface
for contact system activation is actually
the assembly of contact proteins on cell
membranes (Colman et al., 1997; Motta et
al., 1998). Their specific interactions with
biologic cell membranes such as endothelial
cells and platelets indicate that assembly
and activation of this system takes place
in a physiologic milieu, independent of negatively
charged surfaces. In vivo, a negatively charged
surface is not always needed for activation.
Evidence for the presence of the kallikrein-kinin
system in fetoplacental vessels has accumulated
in several studies (Miatello et al., 1994;
Weerasinghe and Gadsby, 1992). Mutoh et al.
(1992) indicated that a kinin generating
activity of the kallikrein-kinin system is
localized within the uteroplacental unit.
Hermann et al. (1996) reported that kininogen
and plasma prekallikrein/plasma kallikrein
were present at at the endothelial cells
of placental villous capillaries. In larger
placental blood vessels and umbilical cord,
neither kininogens nor kallikreins were detected.
The co-localization of kininogen and plasma
prekallikrein/plasma kallikrein suggests
that kinins could be generated locally in
placental capillaries. The functional spectrum
of biologically active kinins, such as vasodilation,
vasoconstriction, smooth muscle contraction
and relaxation, could influence placental
blood flow regulation. Moreover, kinins could
also have anti-thrombotic/profibrinolytic
activities (Colman and Schmaier, 1997; Brown
et al., 1997). Kinins which are released
within the placenta may play a role in regulating
placental blood flow and transplacental transport
of substrates and metabolites (Hermann et
al., 1996). To influence placental circulation
and nutrient supply to the fetus effectively,
components of the kallikrein-kinin system
should be situated within or close to the
placental vasculature.
Recently, numerous studies have suggested
an associations between contact protein deficiencies
and recurrent pregnancy losses (Schved et
al., 1989; Braulke et al., 1993; Gris et
al., 1997), and between autoantibodies to
contact proteins and recurrent pregnancy
losses (McIntyre et al., 1997; Sugi et al.,
1995a; Sugi et al., 1995b; Gallimore et al.,
1998; McIntyre and Wagenknecht, 2000; Sugi
et al., 1999). Since contact proteins may
play an important role in pregnancy especially
in fetoplacental unit, deficiencies of these
proteins and/or autoantibodies to these proteins
may be associated with pregnancy losses.
3. Autoantibodies to factor XII and factor
XII deficiency
Deficiencies of contact proteins are not
associated with clinical bleeding despite
marked prolonged activated partial thromboplastin
time (aPTT), a surface-activated coagulation
protein screening test. Paradoxically, studies
suggest that contact proteins have anticoagulant,
profibrinolytic functions in a physiologic
milieu, on endothelial cells (Colman et al.,
1975; Wuepper et al., 1975; Saito et al.,
1975; Halbmayer et al., 1994; Lammle et al.,
1991; Halbmayer et al., 1992). Numerous clinical
studies suggest that contact protein deficiencies
may be associated with impaired contact factor-dependent
fibrinolysis. This result may contribute
to an increased incidence of thrombosis in
patients with congenital factor XII deficiency,
an increased incidence of factor XII deficiency
in patients with venous thrombosis, and acquired
thrombotic disorders such as myocardial infarction
and re-thrombosis of coronary arteries after
thrombolytic therapy (Mannhalter et al.,
1987; Lammle et al., 1991; Halbmayer et al.,
1992; Jespersen et al., 1992).
A surprisingly high prevalence of factor
XII deficiency among patients with recurrent
thrombosis has been reported (Mannhalter
et al, 1987; Halbmayer et al, 1992). Ten
percent of factor XII deficiencies among
patients with venous thrombosis has been
reported by Mannhalter et al. (1987). Halbmayer
et al. (1992) reported that a high prevalence
of factor XII deficiency (20%) among patients
with recurrent arterial thromboembolism and/or
myocardial infarction and an 8% incidence
of factor XII deficiency among patients with
recurrent venous thrombosis. The reduced
generation of bradykinin resulting in diminished
release of tissue plasminogen activator is
proposed as a cause of thrombosis in factor
XII-deficient patients (Levi et al., 1991).
Both vascular and placental thrombosis in
the presence of aPA or factor XII deficiency
have been reported to be associated with
recurrent fetal loss (Cowchock et al., 1986;
Schved et al., 1989). Schved et al. (1989)
reported the cases of three young women with
a factor XII deficiency (two homozygous and
one heterozygous) and a clinical history
of spontaneous abortion. Braulke et al. (1993)
reported on 8 patients with moderately reduced
level of factor XII found among 43 patients
with repeated abortions. Recently, Gris et
al. (1997) reported the prevalence of haemostasis
abnormalities in 500 unexplained primary
early recurrent aborters. They found 9.4%
of the patients with an isolated factor XII
deficiency, 7.4% of the patients with primary
aPA and 42.6% of the patients with an isolated
hypofibrinolysis (mainly high plasminogen
activator inhibitor) in the patients (reference
groups: respectively 0/150, 3/150, 2/150,
p<0.001). Willebrand disease, fibrinogen
deficiency, antithrombin, protein C or protein
S deficiencies were not more frequent in
recurrent aborters than in members of the
reference groups.
Gallimore and Winter (1998) reported a high
incidence (20.9%) of apparently true factor
XII deficiency in patients who were LA positive.
They have hypothesized that antibodies to
factor XII might be present in some patients
who are LA positive and that immune complexes
may be formed leading to reduced levels of
factor XII. They studied plasma samples from
LA positive patients for the presence of
antibodies to factor XII and reported that
many patients were positive for antibodies
to factor XII detected by ELISA and immunoblot
(Jones et al., 1999). Factor XII may be added
to the list of proteins to which antibodies
are produced in LA positive patients. These
antibodies may interfere with contact activated
fibrinolysis in such patients. Recently,
presence of autoantibodies to factor XII
was reported in factor XII deficient patients
with recurrent early pregnancy losses (Uchida
et al., 2000).
4. Autoantibodies to kininogen
The important function of the plasma kininogens
is the delivery of bradykinin, a potent biologically
active peptide. Kininogens and bradykinin
contribute to vessel patency, increased blood
flow, and anti-thrombotic/profibrinolytic
activities. Bradykinin is a stimulator of
endothelial cell prostacyclin synthesis;
an inhibitor of platelet function (Hong,
1980; Crutchley et al., 1983), nitric oxide
formation (Palmer et al., 1987) and tissue
plasminogen activator release (Brown et al,
1997). Thus, kininogens have been shown to
have antithrombin and profibrinolytic activities.
Autoantibodies reactive to zwitterionic phospholipid
such as phosphatidylethanolamine (PE) have
not been studied extensively. Most LA and
aPA literature is limited to descriptions
of anionic phospholipids such as cardiolipin,
however, aPE also have been described in
patients with thrombotic diseases (Staub
et al., 1989; Karmochkine et al., 1992, 1993;
Boffa et al., 1996; Berard et al., 1995;
McIntyre and Wagenknecht, 2000). Furthermore,
aPE have been reported as the sole aPA in
several patients (Karmochkine et al., 1992;
Karmochkine et al., 1993; Boffa et al., 1996;
Berard et al., 1995). Since PE is a major
component of both the outer and inner leaflet
of cell plasma membranes, production of aPE
may interfere with normal cell functions.
Sugi and McIntyre (1995a) reported that certain
aPE are not specific for PE per se, but are
directed to PE-binding plasma proteins, for
example, kininogens. Many aPE recognize a
kininogen-PE complex and neither PE nor kininogen
when they are presented independently (Sugi
and McIntyre, 1995a, 1996; McIntyre et al.,
1997). The kininogens bind to both negatively-charged
and zwitterionic phospholipids. Despite the
phospholipid-binding properties, kininogen-dependent
aPE are specific for the kininogen-PE complex,
as they do not recognize the kininogens when
presented on other phospholipid substrates
(Sugi and McIntyre, 1996a). This indicates
that PE induces unique antigenic conformational
changes in the kininogens which do not occur
when the kininogens bind to other phospholipids.
The female reproductive tract is the second
richest site for kininogen and its metabolic
products in the body (Hossain et al., 1995;
Hermann et al., 1996; Brann et al., 1995;
Adam et al., 1989). Adam et al. (1989) measured
12.2, 10.9, 0.4 and 1.2 mg/mg T-kininogen
in rat plasma, uterus, liver, and kidney,
respectively. The kininogen concentration
in reproductive tissues and plasma was reported
to fluctuate during ovulation, pregnancy,
and parturition (Hossain et al., 1995; Adam
et al., 1989). Why the female reproductive
system is so rich in kininogen and what governs
the fluctuation of kininogen concentrations
at the local level remains to be elucidated.
Because numerous studies (Branch et al.,
1985; Petri et al., 1997; Katano et al.,
1996) have concluded that recurrent pregnancy
loss are associated with aPA to anionic phospholipids,
and in view of the conspicuous presence of
kininogen in reproductive tissues, we decided
to test recurrent pregnancy loss patients
for aPE, especially those patients who lose
during the embryonic period (Branch and Silver,
1996). We reported that of 139 recurrent
pregnancy loss patients, 28 (20.1%), 17 (12.2%),
and 2 (1.4%) patients were positive for IgG,
IgM, and IgA aPE, respectively. A positive
test result for aPE was more frequent in
the patients with recurrent pregnancy losses
than in the members of the control group
(p=0.0002). In contrast, there was no statistically
significant difference in the incidence of
positive test results for antibodies to anionic
phospholipids between the recurrent pregnancy
loss group and the control group. In this
study, 21 patients were positive for plasma
protein-dependent IgG aPE. Nineteen (90.5%)
of these 21 patients were kininogen-dependent
(Sugi et al., 1999). Our data suggest that
aPE may represent a significant risk factor
for early recurrent pregnancy loss. Recently
Gris et al. (2000) reported that aPE were
found to be independent risk factors for
unexplained early fetal loss.
Conclusions
Plasma contact proteins have anticoagulant
and profibrinolytic activities in vivo. In
fact, an association between deficiencies
of these proteins and recurrent thrombosis
has been reported. Also deficiencies of these
proteins and antiphospholipid antibodies
are frequent haemostasis-related abnormalities
found in unexplained recurrent aborters.
Mid gestation pregnancy loss in primary antiphospholipid
syndrome patients often is associated with
anticardiolipin antibodies (Branch et al.,
1985). Gris et al. (1997) reported the association
between early recurrent miscarriages and
factor XII deficiency. Sugi et al. (1999)
reported a statistically stronger association
between recurrent miscarriages and kininogen-dependent
aPE than between recurrent miscarriages and
antibodies to anionic phospholipids for early
gestational losses. Thus, in contrast with
anticardiolipin antibody associated pregnancy
loss, early gestation pregnancy loss often
may be associated with the disruption of
plasma contact system or kallikrein-kinin
system. Because the kallikrein-kinin system
is localized within the uteroplacental unit,
it may play a role in regulating placental
blood flow and transplacental transport of
subtances and metabolites (Hermann et al.,
1996). Disruption of this system may be a
risk factor for early gestational loss.
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