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2. disease

Antiphospholipid Syndrome
David Keeling BSc MD MRCP MRCPath
Consultant Haematologist
Qualified from St. Bartholomew's Hospital and trained in
haematology at The Royal Free Hospital, the Royal London Hospital
and Addenbrooke's Hospital in Cambridge. Following completion of a
MD at University College, London on the nature of antiphospholipid
antibodies, he now specialises in haemostasis and thrombosis.
Correspondence: David Keeling, Oxford Haemophilia Centre, The
Churchill hospital, Oxford OX3 7LJ. Tel. 01865 225318
Fax. 01865 225608 Email. [email protected]
Introduction
In 1952 Conley and Hartmann described inhibitors of blood
coagulation in two patients with SLE who both had a biological
false positive test for syphilis1. These coagulation inhibitors are now
known to be antibodies that interfere with phospholipid dependent
coagulation tests in vitro. Antiphospholipid antibodies (aPL)
detected in this manner, that is by their ability to prolong coagulation tests, became known as lupus anticoagulants (LA) a term
originally coined by Feinstein and Rapaport in 19722. This nomenclature has become firmly established despite its misleading nature
and indeed the occurrence of thrombotic complications in patients
with the LA had already been made nine years previously3. A solid
phase radioimmunoassay that utilised cardiolipin as the antigen,
followed shortly by a microtitre solid phase ELISA were developed
in the early 1980s and became the standard method to detect anticardiolipin antibodies (aCL). Thus aPL were detected as either LA
or aCL by two distinct methodologies. Whether these antibodies
were distinct, related, or identical was a matter of debate throughout the 1980s.
aPL are not only found in patients with SLE. They are well
documented in patients with lupus-like illnesses who do not meet
the full criteria for SLE and in patients with other autoimmune diseases. In all these cases, the antibodies are associated with certain
clinical manifestations, most importantly thrombosis, thrombocytopenia and recurrent fetal loss. The concept of the antiphospholipid syndrome was proposed for the presence of aPL together
with these recognised clinical associations. Recognition of patients
with aPL and these clinical problems but who do not have any of
the major clinical or serological features of SLE, nor evidence of
any other autoimmune disease, led to the concept of the primary
antiphospholipid syndrome4. aPL have also been described secondary to various infections but in these cases they are not associated with clinical features of the antiphospholipid syndrome.
What are aPL?
Until 1990 aCL were thought to be directed against the
negatively charged headgroup of the anionic phospholipids. In
solid phase immunoassays the antibodies bound to cardiolipin,
phosphatidylserine, and phosphatidylinositol, but not to
the neutral phospholipids, phosphatidylcholine and phosphatidylethanolamine. It was then discovered that purified aCL
would only bind to anionic phospholipids in the presence of a
serum cofactor5. The cofactor was identified as β2-glycoprotein I
(β2GPI), a plasma protein known to bind to anionic phospholipids.
It is now generally accepted that so called aCL are not directed
Rila Publications Ltd. • CME Bulletin Haematology • 1999 • Volume 2 No. 1
against phospholpids at all but are antiβ2GPI antibodies (aβ2GPI).
It had also been assumed that LA were directed against anionic phospholipids. They prolong phospholipid dependent coagulation tests and are neutralised by excess phospholipid. The antibodies were suspected to interfere with the assembly of the prothrombinase and tenase complexes in which the vitamin K dependent proteins bind to anionic phospholipids in the presence of
Ca++ ions. However, in 1991 Bevers et al demonstrated that IgG
with LA activity inhibited prothrombinase activity only when
human prothrombin was the substrate6, there being no effect on
the activation of bovine prothrombin. It was proposed that LA
recognise an epitope on human prothrombin that becomes
exposed upon calcium mediated binding of prothrombin to anionic phospholipids. Some LA are clearly antiprothrombin antibodies
(aPT). To complicate the issue some aβ2GPI also have LA activity
(Type A) whilst others do not (Type B).
Clinical problems associated with
antiphospholipid antibodies
Problems that have been associated with aPL are listed in
Table 1. Clinically the significant problems are thrombosis, fetal
loss and to a lesser extent thrombocytopenia.
Thrombosis - venous and arterial (including stroke).
Fetal loss.
Thrombocytopenia.
Non-thrombotic neurological manifestations Chorea, migraine, epilepsy, Guillan-Barré syndrome.
Skin lesions - livedo reticularis, leg ulcers.
Endocardial disease.
Positive direct Coomb's test, rarely haemolytic anaemia.
Table 1. Clinical problems associated with aPL.
Thrombosis
There is strong evidence for an association between aPL and
both venous and arterial thrombosis3,7. The treatment for acute
venous thrombosis is not modified in patients with aPL. In patients
with persistent aPL the risk of recurrent thromboembolism is high.
Long term oral anticoagulation is effective in secondary prevention8. Moreover, withdrawal of anticoagulation is associated with a
high recurrence rate7. Aspirin seems inadequate for secondary prophylaxis7,8, and in their recent retrospective study Khamashta et al7
demonstrated that maintaining the INR less than 3 was unsatisfactory whilst maintaining it at 3 or more was very effective. At present it would seem that patients with aPL and a history of thrombosis should be on long term oral anticoagulant therapy to maintain an INR of 3 or more.
The presence of aPL in a patient who has never had a thrombotic event presents a dilemma. Most clinicians do not consider
this sufficient reason for primary prophylactic anticoagulation,
though prophylaxis should certainly be given to cover high risk situations such as surgery. Immunosupression with corticosteroids is
not effective in preventing venous thrombosis in patients with aPL.
After an ischaemic stroke it would seem reasonable to give warfarin for as long as aPL persist because of the otherwise high recurrence rate8. Some physicians would try aspirin in patients presenting with transient ischaemic attacks, if this is not effective warfarin
should be substituted.
11
Antiphospholipid Syndrome
Fetal Loss
Of the other clinical associations the greatest cause for concern
is spontaneous abortion and second and third trimester fetal death.
Lynch et al9 in a prospective cohort study of consecutive nulliparous pregnant women were able to show IgG aPL were associated with fetal loss (relative risk 3.5). However, a case-control study
of women presenting with a first fetal loss found aPL in 5% (17/331)
of patients and 4% (38/993) of controls10.
In vitro experiments strongly suggest a pathogenic role for aPL.
Injection of aPL into mice has resulted in decidual necrosis, fetal
abnormalities, failure of implantation and fetal death11,12. It is suggested fetal loss is a manifestation of a thrombotic tendency that
results in placental infarction.
The management of recurrent fetal loss associated with aPL
has in the past been based on anecdotal reports and mostly uncontrolled studies. Apparently good results from intervention in a few
patients needs to be balanced against the high chance of a reasonable outcome without any intervention. Although combination
of prednisolone and aspirin have been reported to be effective in
some uncontrolled series, others have found them to be ineffective.
High dose long term steroids during pregnancy are clearly associated with significant side-effects.. Aspirin and low dose heparin
have both been used alone as antithrombotic agents. A recent very
useful randomised controlled trial compared aspirin alone versus
aspirin plus heparin and found the combination to be superior13.
Pathogenesis of thrombosis in antiphospholipid
syndrome
As our understanding of these antibodies increased there was
hope that pathogenic mechanisms would be revealed and much
attention was focussed on β2GPI. This apolipoprotein is composed
of 326 amino acids, has multiple disulphide bonds, and a high carbohydrate content (19%) resulting in an apparent molecular weight
of 50 kDa. The molecule has five repeating globular (or Sushi)
domains each of about 60 amino acids with highly conserved disulphide bonds. It has several effects on the haemostatic system. It
inhibits the contact activation of the intrinsic pathway of coagulation, interferes with the assembly of the prothrombinase complex
on platelet membranes and phospholipid vesicles and also inhibits
the analogous tenase complex. It increases adenylate cyclase activity in platelet membranes and inhibits the release reaction of
platelets during ADP-induced aggregation.
If β2GPI were to act as a natural anticoagulant then perhaps
antibodies to it could predispose to thrombosis. That these antibodies might have a pathogenic role is supported by some animal
work. Naive mice immunised with β2GPI developed aCL. This
was later followed by prolongation of the PTT, thrombocytopenia,
and when the mice were mated, by a high rate of fetal resorption
in utero14. There is, however, no direct evidence that β2GPI acts as
an anticoagulant and deficiency of β2GPI is not associated with
thrombosis.
Patients with antiphospholipid syndrome have auto-antibodies
against proteins other than β2GPI and prothrombin. It is possible
that the thrombotic manifestations are due to antibodies to
proteins such as protein C, protein S, phospholipase A2, or
annexin V, or to antibodies against endothelial cells. There is,
however, no convincing evidence that any of these antibodies
could explain the antiphospholipid syndrome.
12
Galli and Bevers15 have proposed that abnormal exposure of
procoagulant phospholipid surfaces to the bloodstream may
induce binding of plasma proteins with affinity for anionic phospholipids (such as β2GPI and prothrombin). Binding of these proteins could cause a conformational change and render them
immunogenic, inducing the production of aPL. At the same time
the pathological exposure of procoagulant surfaces promotes the
phospholipid-dependent coagulation reactions and causes thrombosis. If this hypothesis is correct then aPL are just an epiphenomenon.
What tests should be done to detect aPL?
The standard aCL ELISA and LA coagulation tests are still
most often used in clinical practice. Patients may be positive in one
test and not the other, so both must be performed. In order to
exclude transient positive tests of no clinical significance, it is also
necessary to repeat the tests after an interval of six weeks or more.
An aβ2GPI ELISA could now replace the standard aCL
ELISA. This would have the advantage that non-pathogenic, transient "true" aCL associated with infection are not detected.
Although some aPT (detected by an ELISA asay) are not detected
in LA coagulation tests there is no evidence that antibodies detected in this way are associated with thrombosis. An immunological
test for aPT is therefore probably not clinically useful. Of the two
different antibodies detected as LA there is some evidence that
aβ2GPI type A are more strongly associated with thrombosis than
aPT, and the DRVVT is better at detecting these antibodies than
the KCT.
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References
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Feinstein DI, Rapaport SI. Acquired inhibitors of blood coagulation. Prog Hemost
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Bowie WEJ, Thompson JH, Pascuzzi CA, Owen GA. Thrombosis in systemic lupus erythematosus despite circulating anticoagulant. J Clin Invest. 1963;62:416-430.
Alarcon Segovia D, Sanchez Guerrero J. Primary antiphospholipid syndrome [published
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Khamashta MA, Cuadrado MJ, Mujic F, Taub NA, Hunt BJ, Hughes GR. The management
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Sthoeger ZM, Mozes E, Tartakovsky B. Anti-cardiolipin antibodies induce pregnancy failure by impairing embryonic implantation. Proc Natl Acad Sci U S A. 1993;90:6464-7.
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antibodies (or antiphospholipid antibodies) BMJ. 1997;314:253-7.
Blank M, Faden D, Tincani A, et al. Immunization with anticardiolipin cofactor (beta-2glycoprotein I) induces experimental antiphospholipid syndrome in naive mice. J
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Rila Publications Ltd. • CME Bulletin Haematology • 1999 • Volume 2 No. 1