Keywords
antiphospholipid antibody syndrome - infertility - recurrent abortion - antiphospholipid
antibody
Palavras-chave
síndrome do anticorpo antifosfolípide - infertilidade - aborto de repetição - anticorpo
antifosfolípide
Introduction
Antiphospholipid antibody syndrome (APS) was first described in 1983. It is defined
as a prothrombotic autoimmune disease, characterized by the presence of persistent
antiphospholipid antibodies (aPLs), thrombosis, recurrent abortion, and, occasionally,
thrombocytopenia. It can manifest itself in isolation (primary APS) or associated
with another autoimmune disease (secondary APS). The most common association is with
systemic lupus erythematosus (SLE).[1]
[2]
Antiphospholipid antibody syndrome, an acquired thrombophilia, is associated with
arterial and venous thrombosis, which can occur in unusual sites, such as hepatic
veins, visceral veins, and in the cerebral venous circulation. Any signs of thrombosis
in these sites require investigation of APS.[3]
The prevalence of the disease is unknown; however, it is estimated to affect ∼ 0.5%
of the population. The average age at the time of diagnosis is ∼ 35 years old, since
it is rare in children.[1]
The aPLs are a heterogeneous group of antibodies directed against phospholipids, situated
in the endothelial cell membrane, platelets, and other cells involved in the coagulation
cascade. Its discovery began with the discovery of cardiolipin in 1906, when Wassermann
described it as a marker for syphilis. In 1941, Pangborn isolated and identified the
antigen component in heart fragments of cattle, such as cardiolipin (diphosphatidylglycerol).
Cardiolipin is a unique phospholipid of biomembranes which have coupled phosphorylation
and electron transport, such as mitochondria. There are currently > 10,000 publications
on these antibodies, and the most researched in reproductive immunology are lupus
anticoagulant, anti-β-2-glycoprotein, and anticardiolipin.[4]
[5]
[6]
According to the revised Sapporo criteria, 2006, the diagnosis of APS occurs when
patients present at least one clinical criterion associated with a laboratorial one,
which should be performed with a minimum interval of 12 weeks of the clinical event.[7]
[8] Clinical criteria include ([Table 1]): ≥ 1previous thrombosis cases confirmed by histological or imaging tests, or cases
of obstetric morbidity, which include: unexplained death of ≥ 1 morphologically normal
fetuses with ≥ 10 weeks of gestation; ≥ 1 normal fetuses born before 34 weeks of gestation
(due to pre-eclampsia [PE], eclampsia, or placental insufficiency); or ≥ 3 spontaneous
miscarriages before 10 weeks of gestation.
Table 1
Clinical Criteria of Antiphospholipid Antibody Syndrome
|
1 Vascular thrombosis
|
|
One or more episodes of arterial or venous thrombosis in any tissue or organ, confirmed
by validated objective criteria (imaging exams or histopathology).
|
|
2 Obstetrics morbidity
|
|
(a) One or more unexplained of morphologically normal fetuses with gestational age
≥ 10 weeks (normal morphology documented by ultrasonography or direct fetus examination).
|
|
(b) One or more premature births of morphologically normal neonates with < 34 weeks
of gestational age due to severe pre-eclampsia or pre-eclampsia, or signs of placental
insufficiency.
|
|
(c) Three or more unexplained spontaneous abortions with < 10 weeks of gestational
age, excluding anatomic maternal, hormonal, and chromosomal causes.
|
Source: Adapted from Kovács et al.[9]
The laboratory criteria ([Table 2]) includes the positivity (on 2 occasions in a range of at least 12 weeks) of ≥ 1
of the following aPLs: lupus anticoagulant; anti-β-2-glycoprotein (immunoglobulin
M [IgM] and/or immunoglobulin G [IgG]); and anticardiolipin (IgM and/or IgG). Although
the Sapporo criteria are widely used in medical practice, it should be noted that
they were initially developed to define a uniform cohort of patients with APS for
clinical trials, rather than providing a practical system for clinical diagnosis.[3]
[7]
Table 2
Laboratory Criteria of Antiphospholipid Antibody Syndrome
|
1 Lupus anticoagulant present on plasma, ≥ 2 occasions, with 12 weeks of difference,
detected according to the guidelines of the International Society of Thrombosis and
Hemostasis.
|
|
2 Anticardiolipin (IgG or IgM) detected on serum or plasma, and medium or higher titles
(> 40 GPL or MPL, or > p 99), in ≥ 2 occasions, with 12 weeks of difference, through
ELISA standardized exam.
|
|
3 Anti-β-2-glycoprotein I (IgG or IgM), detected on serum or plasma (titles > 99 percentile),
in ≥ 2 occasions, with 12 weeks of difference, through ELISA standardized exam.
|
Abbreviation: ELISA, Enzyme-Linked Immunonosorbent Assay.
Source: Adapted from Kovács et al.[9]
Antiphospholipid antibodies can also be present even if the patient does not fulfill
the clinical criteria for the syndrome (nonspecific autoimmunity).[8]
It is known that the presence of these antibodies is associated with primary infertility.
They are more frequent in infertile patients and interfere with the endometrial decidualization
and, consequently, with the embryo implantation rates. Others suggest that it can
also be associated with decreased ovarian reserve. The ovarian reserve can be evaluated
by the dosages of the antimullerian hormone or by the count of antral follicles on
the ultrasonography. The antimullerian hormone is produced by growing follicles from
granulosa cells that harbor the oocytes until their maturation, and its levels remain
stable throughout the whole menstrual cycle.[9]
[10]
The pathogenesis of obstetric morbidity on APS is not yet fully understood; however,
it may happen due to the negative effect that the antiphospholipid antibodies exert
on the placental function, decreasing the trophoblastic viability and its invasiveness.
Furthermore, recent evidence suggest that the inflammatory factor can also justify
the poor obstetrics prognosis. There are experimental studies on animals that confirm
the ability of large quantities of antiphospholipid antibodies to induce fetal reabsorption
and growth retardation through the placental deposition of IgG and complement, neutrophilic
infiltration, and local secretion of tumor necrosis factor alpha (TNF-alfa).[1]
[11]
The primary treatment of the syndrome, whose main objective is to prevent thrombosis,
presents only partial success rates. Nowadays, ∼ 80% of the gestations with APS result
in live births. However, these gestations have a higher risk of developing PE, ranging
from 18 to 40%, a higher risk of restricted intrauterine growth in 5 to 15%, besides
a higher risk of premature birth.[12]
Therefore, APS is a syndrome with high prevalence rates, since 15 to 20% of the women
with recurrent miscarriage are APS carries.[13] It is necessary to expand the investigations about APS in order to understand its
pathogenic mechanisms and its association with female infertility.
Methods
The present study consists of a bibliographic review of articles found in databases,
such as PubMed, Scielo, and Bireme, from 1998 to 2018. Two articles, one from 1990
and another from 1995, were also included because of their relevance to the theme
of the study. The scientific articles related to the keywords were selected, as well
as the others related to the pathophysiology of APS. The used keywords were: síndrome do anticorpo antifosfolipídeo, infertilidade, aborto recorrente, anticorpo antifosfolipídeo, antiphospholipid syndrome, infertility, recurrent pregnancy loss, antiphospholipid antibody.
Results
Antiphospholipid antibody syndrome and primary infertility
Primary infertility is defined as the absence of gestation after 12 months of intercourse
without the use of contraceptives. The global prevalence of infertility is 9%, while
the 60-month prevalence of infertility in Europe is estimated to be 1.5%.[10]
A recent literary evaluation analyzed 31 studies, all of which showed a high and important
association between aPLs and female infertility. A total of 45% of these studies confirmed
the association between anticardiolipin antibody and infertility; however, this rate
dropped to 31% when the association with lupus anticoagulant was analyzed. Only 4
studies evaluated anti-β-2-glycoprotein, and yet, 75% of them revealed a positive
association with infertility. There was a significant difference of the positivity
of anticardiolipin antibody in infertile women, but the frequencies of anti-β-2-glycoprotein
and lupus anticoagulant antibodies were similar both in infertile and fertile women.
It must be highlighted that < 25% of the studies utilized a medium-high limit to define
the positivity of anticardiolipin and/or of anti-β-2-glicoprotein, as recommended
by international guidelines. [Table 3] reveals the estimated positivity rates of aPLs.[10]
[14]
Table 3
Positivity rates of Antiphospholipid antibodies in infertile women and in the control
group
|
aPL
|
Number of studies
|
Infertile women
Positivity
0% (0–2-5)
7% (3-7–13-3)
|
Control group
Positivity
0% (0–0)
1-6% (0–3)
2-8%
|
|
Lupus anticoagulant
|
13
|
|
Anticardiolipin
|
29
4
|
|
Anti-β-2 glicoprotein
|
7-6%
|
Source: Adapted from Chighizola et al.[10]
Antiphospholipid antibody syndrome and obstetrics morbidity
The abortion rate, when aPL levels are > 90% of the normal population values, is estimated
to be 52%. In addition, positive aPLs patients who already had an abortion have a
higher risk of obstetric morbidity in future pregnancies.[15]
Although all three antibodies (lupus anticoagulant, anticardiolipin, and anti-β-2
glycoprotein) are associated with recurrent miscarriage, the risk varies according
to the positivity of different types of antibodies. For example, the presence of anticardiolipin
antibody is associated with an odds ratio (OR) of 22.6% (95%CI [confidence interval])
for recurrent miscarriage. The presence of anti-β-2 glycoprotein antibodies increases
the chance of recurrent miscarriage from 6.8% to 22.2% when compared with women positive
for lupus anticoagulant or anticardiolipin.[16]
Fetal complications in APS include prematurity, intrauterine growth restriction (IUGR)
(due to placental insufficiency) and stillbirth. The “Euro-Phospholipid Project” cohort
study analyzed the clinical characteristics of 1,000 patients with APS during a 5-year
period, and concluded that these events complicate 28%, 11% and 7% of APS pregnancies,
respectively.[17]
A meta-analysis of 2006 revealed a positive association between the presence of anticardiolipin
IgG (between high and low titles) and recurrent abortion at < 13 weeks of gestational
age (OR 3.56; 95%CI: 1.48–8.59). Other studies suggest that circulating aPLs are the
main risk factor for 7 to 25% 1st-trimester gestational losses, whereas prevalence studies showed that 1 in 5% of these
patients are lupus anticoagulant-positive. A cohort of 500 women with a history of
recurrent abortion revealed that 9.6% were lupus anticoagulant-positive, whereas anticardiolipin
IgG and IgM were found on 3.3% and 2.2% of them, respectively.[18]
[19]
Other obstetrics morbidities, such as PE and/or placental insufficiency and IUGR are
also associated with APS. Pre-eclampsia can occur in between 2 to 8% of 1st gestations, whereas severe preeclampsia can be seen in 0.5% of the gestations in
developed countries. Most of these prospective observational studies corroborate the
association between aPL with PE and placental insufficiency. A meta-analysis of 2010
exposed that moderate to high titles of anticardiolipin are associated with preeclampsia.
Prospective and retrospective studies have shown that the persistence of high titles
of aPLs are associated with IUGR and premature birth. Data from case-control studies
indicate that aPLs were found in 50% of the patients with a history of PE or IUGR,
and in 7% of women without these morbidities.[20]
[21]
[22]
Antiphospholipid antibody syndrome and thrombosis
The association between aPLs and thrombosis is significant. Antiphospholipid antibodies
are found in ∼ 13% of the patients with history of stroke, 11% of the patients with
acute myocardial infarction, and 9.5% of the patients with a history of deep vein
thrombosis.[2] The annual rate of a first thromboembolic event on patients between 35 and 55 years
old who are not APS carriers is of ∼ 0.4%. On APS carriers, the rates are around 3.8%.
The incidence of thrombosis in patients positive for a single antiphospholipid antibody
is 1.36%, and increases to 5.3% in patients positive for all 3 antibodies. Therefore,
triple positivity presents a higher risk of occurrence and recurrence of thromboembolic
events.[23]
Discussion
Antiphospholipid antibody syndrome and primary infertility
The pathogenic mechanism that explains how aPLs induce infertility is not yet fully
understood. Some authors believe that aPLs may alter the development of the oocyte
after its secretion into the follicular fluid, since gametes or preimplantation embryos
do not come into contact with maternal blood.[10]
An alternative theory relies on the fact that the antibodies interfere on uterine
decidualization, compromising implantation. Decidualization is the transformation
of the endometrial stromal fibroblasts into specialized deciduous secretory cells,
which provide a nutritional and immunoprivileged matrix that is essential to embryonic
implantation and placental development. In vitro studies also observed the connection
between aPLs and endometrial endothelial cells, impairing angiogenesis.[10]
[24]
[25]
Evidence on the interaction between aPLs and decidua has been obtained by in vivo
studies. A 1990 study showed that mice treated with IgG aPLs showed, on histological
examination, decidual necrosis associated with intravascular deposits of IgG and fibrin.
In other studies, through the immunohistochemical analysis of the decidua of pregnant
mice, it was observed that the decidua is a preferential target for complement deposition
after treatment with IgG aPLs.[26]
[27]
[28]
Otherwise, in addition to inflammatory changes in the decidua and its interference
with embryo implantation, there is another mechanism by which APS is associated with
infertility. It also can decrease ovarian reserve.[29]
The term ovarian reserve, traditionally, has been used to describe the reproductive
potential of a woman; in other words, the quantity and quality of the oocytes she
possesses. However, ovarian reserve markers serve as a substitute for oocyte quantity,
but are poor indicators of oocyte quality. Therefore, the modern use of the term applies
to the quantity of remaining oocytes, not to oocyte quality.[30]
Thus, the ovarian reserve can be evaluated by the dosages of the antimullerian hormone
or the count of antral follicles on the ultrasonography. Both present the same sensitivity
and specificity as predictors of follicular reserve.[10]
A 2014 study assessed the ovarian reserve of patients with APS and observed that these
women had a lower count of antral follicles than those in the control group. Vega
et al, in 2016,[29] demonstrated a strong association between aPL positivity and decreasing levels of
antimullerian hormone. Thus, antiphospholipid antibodies may represent the 1st laboratory markers for early ovarian failure associated with autoimmunity. This suggests
that the presence of aPLs in women of reproductive age should be considered a risk
factor for the development of premature ovarian failure. The confirmation of this
association in future studies, therefore, could lead to the early diagnosis of this
condition in many women.[29]
[31]
Antiphospholipid antibody syndrome and obstetrics morbidity
The pathogenesis of obstetric morbidity in APS is still not completely understood.
Initially, intraplacental thrombosis was considered to be the main mechanism of poor
obstetric prognosis. However, most subsequent studies have not been able to find blood
clots in most placentas from carriers. Other hypotheses, therefore, have been suggested,
such as that aPLs induce a direct negative effect on placentation, interfering in
both trophoblastic invasion and endometrial angiogenesis.[32]
Immediately after placental implantation, the mononuclear trophoblast (cytotrophoblast)
invades the decidua of the uterus, differentiating itself into an extravilous trophoblast.
A part of this trophoblast invades the uterine spiral arteries, which supply blood
to the decidua, digesting the musculature and replacing the endothelial cells lining
these vessels. This invasion is capable of remodeling the spiral arteries and transforming
them into nonvasoactive conduits of greater caliber. Up to half of the gestational
period, the spiral arteries are remodeled throughout the decidua depth, up to one-third
of the depth of the myometrial segments. This remodeling allows a large and uninterrupted
blood supply to the fetus during the second half of the gestation, when the demand
is greater.[33]
[34]
[35]
The majority of obstetric manifestations of APS, such as preeclampsia and restricted
intrauterine growth, present failure of the extravilous trophoblast to adequately
remodel the spiral arteries. As a consequence, there is a significant decrease in
maternal blood flow to the placenta, causing ischemic injury, lack of nutrients to
the fetus, and increased blood flow velocity, which can damage the placenta.[34]
[35]
A recent meta-analysis of the histopathological findings of placentas affected by
aPLs showed five changes associated with aPLs: placental infarction, inadequate remodeling
of spiral arteries, decidual inflammation, increased number of syncytial nodes, and
decreased synovial vascular membranes. It is important to note that placentas with
aPLs rarely present evidence of intravascular or intravilous clots.[35]
[36]
The negative effect that aPLs exert on human placentation begins with the binding
of the antibodies to the trophoblast. It is known that polyclonal IgG antibodies from
APS patients and monoclonal antibodies with anti-β-2-glycoprotein activity are able
to adhere to the trophoblast and endometrial endothelial cells in vitro. The pathogenic
mechanisms by which antiphospholipid antibodies alter placentation will be described
in detail below[37]:
-
1–Antiphospholipid antibodies are able to stimulate the trophoblast to secret inflammatory
interleukins, such as IL-1 and IL-8, through the activation of the toll-like receptor
(TRL4)[35];
-
2–Antiphospholipid antibodies limit the migration of trophoblast via mediation of
apolipoprotein E receptor 2 (ApoER2), which is expressed in human trophoblast and
targets the anti-β-2-glycoprotein complexes. In addition, ApoER2 has been associated
with restricted intrauterine growth and fetal loss[35]
[38];
-
3–Antiphospholipid antibodies increase the production of extracellular vesicles of
the syncytiotrophoblast. In placentas affected by aPLs, there is an increase in syncytial
nodes, which are aggregates of syncytiotrophoblast nucleoli and aging markers of them.
They are expelled from the placenta in large vesicles as syncytial nuclear aggregates
and are responsible for activating the maternal vasculature[36]
[39];
-
4–Syncytiotrophoblast produces human chorionic gonadotropin (hCG). Antiphospholipid
antibodies reduce the growth of the syncytiotrophoblast in vitro, reducing the production
of hCG. As they prevent the formation of new syncytiotrophoblast and induce cell death,
they also cause a reduction in transplacental transport, evidenced by decreased levels
of the cholesterol transporter ABCA1 in placentas affected by APS[40]
[41];
-
5–These antibodies are also capable of blocking endometrial angiogenesis both in vitro
and in vivo. One mechanism is the inhibition of the production of factors regulated
during angiogenesis, such as vascular endothelial growth factor (VEGF). A study of
β-2-glycoprotein involvement has shown that, in contrast to its previously reported
anti-angiogenic properties, the cleaved form of β-2-glycoprotein is able to block
the activity of angiostatin, a known angiogenesis inhibitor. The inhibitory effect
of angiogenesis is demonstrated, but it is not yet known whether it is caused by the
imbalance between the cleaved and intact forms of β-2-glycoprotein.[32]
In addition to the changes described above, the inflammatory effects that the antiphospholipid
antibodies induce on the trophoblast are significant. According to Holers et al,[11] passive IgG transfer from patients with high aPL titles to pregnant mice resulted
in fetal resorption and restricted growth.
Mice models have confirmed a critical role of neutrophils in fetal developmental abnormalities.
Pregnant mice treated with aPLs had placental neutrophil infiltration, and the deleterious
effects of aPLs on fetal survival and growth were abolished by neutrophil depletion.
Neutrophil recruitment is triggered by complement activation at the maternal-fetal
interface and leads to elevated TNF-alfa levels, VEGF reduction and, finally, to abnormal
placentation and fetal death.[35]
[42]
Studies in women support the role of complement in the complications of APS pregnancies.
The C4d complement fragment, a classic pathway activation marker, is present in placentas
of women with APS and PE. Hypofunctional variants inherited from complement regulators
provide a higher risk of PE in women with APS. It is possible to suggest that complement-mediated
injury is a common pathogenic mechanism that causes abnormal placentation, fetal loss,
and restricted intrauterine growth.[35]
[43]
Antiphospholipid antibody syndrome and thrombosis
Alterations in the complement system may contribute to aPL-induced thrombosis, and
coagulation factors may activate the complement cascade.[44] The pathophysiology of thrombosis associated with APS is different from other hypercoagulability
known conditions. In APS, thrombosis can occur in virtually all vessels, in arteries
and veins, as well as in large vessels and in microcirculation. The presence of aPLs
is able to interfere with virtually all known homeostatic reactions, as shown in [Table 4]. Their presence results in diffuse thrombotic diathesis, suggesting global and general
deregulation of the hemostatic equilibrium.[45]
[46]
Table 4
Coagulation process modified by antiphospholipid antibodies
|
Inhibition of protein C activity
|
|
Inhibition of protein S activity
|
|
Inhibition of antithrombin activity
|
|
Induction of tissue factor in endothelial cells and monocytes
|
|
Inhibition of Tissue Factor Pathway Inhibitor (TPFI)
|
|
Increased prothrombin deposition, leading to increased thrombin formation
|
|
Inhibition of fibrinolysis
|
|
Activation of factor XI
|
|
Induction of platelet aggregation
|
|
Induction of endothelial cell adhesion receptors
|
Source: Adapted from Salmon et al.[46]
It is true that there are many paths by which aPLs induce thrombosis. The other main
path, besides the alteration of the homeostatic equilibrium, is the activation of
the complement system. Studies have shown that aPL-treated mice showed increased leukocyte
adhesion to endothelial cells and were able to conclude that mice deficient in C3,
C5 or C5a complement components were resistant to aPL-induced thrombophilia and endothelial
cell activation.[47]
Conclusion
Antiphospholipid antibody syndrome is an autoimmune and inflammatory disease associated
with a substantial incidence of thrombosis, obstetric morbidity, and infertility.
In recent times, aPLs have been recognized as triggers of innate immune inflammatory
pathways within the trophoblast and at the maternal-fetal interface. No longer considered
simple prothrombotic antibodies that activate the endothelial cells and platelets,
aPLs are directly responsible for the connection to the trophoblast and the alteration
of its function. The study of the pathophysiology of APS should be encouraged, since
innovative therapeutic approaches, focused on immunomodulation and inflammatory signaling
pathways, may provide important therapeutic advances for this disease, which has such
a significant impact on female fertility.
