Page 2350 - Hematology_ Basic Principles and Practice ( PDFDrive )

2092 Part XII Hemostasis and Thrombosis

myeloid cells. In addition, anti-β 2 GPI antibodies may interfere with ANTIPHOSPHOLIPID ASSAYS
the role of β 2GPI as a regulator of the complement cascade.
Complexes of β 2 GPI and human leukocyte antigen (HLA)-DR7 History
have been found in uterine decidual tissues from placentas of patients
with APS, but not on normal placentas. aPL antibodies that recognize The development of the current consensus-based clinical tests for
these β 2 GPI–HLA-DR7 complexes on cell surfaces can trigger APS stemmed from anomalous observations involving syphilis testing
complement-mediated cytotoxicity. This mechanism may play a role and coagulation assays. The immunoassays were developed in the
in the pregnancy complications of APS. early 1980s in efforts to quantify testing for syphilis by measuring
the binding of antibodies in the test serum to cardiolipin (diphos-
Direct Activation of Trophoblasts and Endometrial phatidyl glycerol), the key antigen in the serologic test for syphilis.
This assay turned out to be ineffective for detecting syphilis, but
Cells by Antiphospholipid Antibodies did quantify the biologic false-positive serologic test for syphilis
(BFP-STS), a laboratory phenomenon that was associated with
In addition to the prothrombotic effects of aPL antibodies, aPL autoimmunity. Subsequent clinical studies revealed that increased
antibodies may have direct effects on trophoblasts that induce abnor- levels of aCL antibodies detected with these assays were associated
mal trophoblast proliferation, migration and invasiveness, increased with thrombosis, spontaneous abortion, and neurologic disease. This
trophoblast apoptosis, and reduced secretion of HCG and adhesion constellation of clinical and laboratory abnormalities was recognized
molecules. Activation of MYD88 by TLR4 has been linked to as a new disorder, named the “antiphospholipid syndrome.” It was
increased levels of proinflammatory molecules (IL-8, IL-1β, mono- later discovered that, in contrast to patients who were infected
cyte chemoattractant proteins) that influence trophoblast survival. with syphilis, antibodies from patients with this syndrome did not
Downregulation of STAT3 phosphorylation decreases IL-6 expres- recognize cardiolipin directly, but rather a “cofactor” serum protein,
sion and reduced trophoblast migration. aPL antibodies have been β 2 GPI, that binds cardiolipin in the assay. It was subsequently
implicated in the disruption of maternal spiral artery transformation determined that, although β 2 GPI is the primary target antigen, aPL
and maturation and differentiation of decidual endometrial cells antibodies also may recognize several other phospholipid-binding
through activation of signaling pathways such as nuclear factor proteins.
kappa-B (NFκB). Defective placentation may result in impaired In the early 1950s the LA assays were derived from Conley’s and
blastocyte implantation. Hartmann’s report of two patients with SLE with prolonged activated
partial thromboplastin time (aPTT) values. That led to the recogni-
tion that these were a result of anticoagulants that were associated
Other Mechanisms with a BFP-STS and recurrent pregnancy loss and also with throm-
bosis, and ultimately to the recognition that the anticoagulant was a
Autoantibodies against tissue factor pathway inhibitor have also been result of antibody-mediated inhibition of phospholipid-dependent
reported in APS patients; these antibodies may prevent the regulation coagulation reactions and was part of the APS. The term lupus
of tissue factor-mediated activation of factor IX and factor X. Some anticoagulant was erroneously coined to describe these antibodies
aPL antibodies cross-react with heparin and heparinoid molecules, because initial studies were done with patients who had SLE; the
which are highly polyanionic, and inhibit their capacity to catalyze terminology has nevertheless persisted. The current aPL tests are
antithrombin. aPL antibodies show cross-reactivity with oxidized inherently limited because they do not measure a known disease
LDL and antibodies against the β 2 GPI-oxidized LDL complex may mechanism. However, as detailed later, these assays are useful as
promote atherosclerosis by attenuating oxidized LDL clearance. In surrogate reporters for thrombotic risk.
addition to promoting thrombosis, aPL antibodies may contribute to
other vascular lesions by stimulating the mammalian target of rapa-
mycin complex (mTORC) pathway. Endothelial cells and platelets Lupus Anticoagulant Tests
activated by aPL antibodies release microparticles containing proco-
agulant proteins and nucleic acids. The LA tests are performed in a variety of configurations, all of which
aim to detect the inhibition of phospholipid-dependent blood coagu-
Genetic, Genomic, and Proteomic Studies in lation reactions, These include modifications of the aPTT with
LA-sensitive and LA-insensitive reagents, the kaolin clotting time
Antiphospholipid Syndrome (KCT), the dilute Russell viper venom time (dRVVT), the tissue
thromboplastin inhibition time (TTIT), the hexagonal phase array
Although familial APS is rare, genetic factors appear to play a role in test, and the platelet neutralization procedure. The results of LA tests
the development of aPL antibodies. One study of 7 families that can vary among laboratories; although most laboratories agree on
included 30 individuals who met consensus criteria for APS con- identification of plasmas containing strongly positive LA activity, they
cluded that the inheritance pattern of aPL antibodies appeared to be frequently disagree about samples that are known to have weak LA
autosomal dominant; however, no specific linkages could be identi- activity (these are missed in approximately half the cases) and labo-
fied. A study done using peripheral blood mononuclear cells from ratories often misdiagnose factor-deficient LA-negative plasmas as
aPL antibody–positive patients found a gene-expression pattern that being LA positive.
appeared to correlate with a predisposition towards developing Despite these limitations, the presence of LA activity is more
thrombosis. Some of the genes identified encoded proteins that are predictive and more specific for the occurrence of thrombosis or
known to be involved in thrombosis, such as apolipoprotein E pregnancy loss than the aCL ELISA assays, in both patients with or
(apoE), factor X, and thromboxane. Other genes did not have a clear without lupus erythematosus. For example, a meta-analysis of the
connection with the disease process; these included hypoxia inducible risks for venous thromboembolism in individuals with aPL antibodies
factor-1alpha (HIF-1α), zinc finger proteins, matrix metalloprotein- without underlying autoimmune disease or previous thrombosis for
ase 19 (MMP19), interleukin-22 (IL-22) receptor, and hematopoietic a 15 year period reported mean ORs of 11 for LA, 3.2 for high-titer
progenitor cell antigen (CD34) precursor. aCL antibodies and 1.6 for elevated aCL antibodies. In a systematic
Proteomic studies may also provide insights on the proteins that literature review, 12 of 12 studies showed significant associations
might predict thrombotic risk in APS patients. Proteins reported between LA and thrombosis, with ORs ranging from 5.7 to 9.4. LA
to be differentially expressed in monocytes of APS patients with increased the risks of arterial and venous events to the same extent.
a history of thrombosis included annexin A1, annexin A2, ubiq- In contrast, only 15 of 28 studies showed significant associations
uitin Nedd8, Rho A protein, protein disulfide isomerase, and between aCL antibodies and thrombosis. In the Antiphospholipid
Hsp60. Antibodies Stroke Study (APASS), positivity for both LA and aCL