Antiphospholipid syndrome (APS)

Antiphospholipid Syndrome (APS) Overview

Antiphospholipid Syndrome (APS), sometimes called Hughes syndrome, is an autoimmune disorder characterized by the persistent presence of specific autoantibodies known as antiphospholipid antibodies (APLA) in the blood, associated with an increased risk of certain clinical events. The primary manifestations are venous and/or arterial thrombosis (blood clots) and/or specific pregnancy complications (obstetric morbidity).

APS can occur on its own (Primary APS) or in association with other autoimmune diseases, most commonly Systemic Lupus Erythematosus (SLE) (Secondary APS).

Antiphospholipid Antibodies (APLA)

Antiphospholipid antibodies (APLA) represent a diverse group of autoantibodies directed against negatively charged phospholipids (like cardiolipin, phosphatidylserine) or plasma proteins bound to these phospholipids (known as cofactors). The most clinically significant cofactor is β2-glycoprotein I (β2GPI).

These antibodies interfere with the normal coagulation process, paradoxically leading to a prothrombotic (clot-promoting) state in vivo, despite potentially prolonging certain phospholipid-dependent clotting tests in vitro (like the tests for lupus anticoagulant).

The interaction of APLA with phospholipids on the surface membranes of platelets and endothelial cells, along with phospholipid-binding proteins, is believed to trigger the thrombotic events seen in APS.

Clinical Manifestations of APS

The clinical spectrum of APS is broad, reflecting the potential for thrombosis to occur in vessels of any size and location.

Major Clinical Criteria:

• Vascular Thrombosis: One or more episodes of arterial, venous, or small vessel thrombosis in any tissue or organ, confirmed by imaging or pathology. Deep vein thrombosis (DVT) and pulmonary embolism (PE) are common venous events; stroke and transient ischemic attack (TIA) are common arterial events.
• Pregnancy Morbidity: Defined by one or more of the following:
  • One or more unexplained deaths of a morphologically normal fetus at or beyond the 10th week of gestation.
  • One or more premature births of a morphologically normal neonate before the 34th week of gestation due to severe preeclampsia, eclampsia, or features of placental insufficiency.
  • Three or more unexplained consecutive spontaneous abortions before the 10th week of gestation (recurrent early pregnancy loss).

Other Associated Manifestations (Not part of formal criteria but common):

• Thrombocytopenia (low platelet count)
• Livedo reticularis (a skin mottling pattern)
• Heart valve disease (vegetations, thickening)
• Neurological symptoms (migraine, seizures, chorea, cognitive dysfunction)
• Kidney disease (APS nephropathy)
• Pulmonary hypertension

"Catastrophic" APS (CAPS): A rare, life-threatening variant characterized by rapid, widespread small vessel thrombosis affecting multiple organs simultaneously.

Obstetric APS Pathogenesis: Pregnancy complications are often linked to placental issues, including thrombosis of placental vessels, placental infarcts, decidual vasculopathy (abnormal blood vessels in the uterine lining), and potentially impaired implantation, leading to fetal growth restriction, preeclampsia, or fetal loss.

Diagnosis of APS

The diagnosis of APS requires the presence of at least one clinical criterion (vascular thrombosis or pregnancy morbidity) AND at least one laboratory criterion (persistent presence of specific APLA), according to the revised Sapporo classification criteria (also known as the Sydney criteria).

Crucially, the laboratory criteria must be met on two or more occasions at least 12 weeks apart to demonstrate persistence, as transient APLA can occur, especially during infections.

The three main laboratory tests included in the diagnostic criteria are:

1. Lupus Anticoagulant (LA)
2. Anticardiolipin Antibodies (aCL) - IgG and/or IgM isotypes
3. Anti-β2-Glycoprotein I Antibodies (aβ2GPI) - IgG and/or IgM isotypes

Key Laboratory Criteria Antibodies

Lupus Anticoagulant (LA)

Lupus Anticoagulant (LA) refers to a functional laboratory finding, not a specific antibody type. It represents APLA that interfere with phospholipid-dependent coagulation tests in vitro, causing prolongation.

LA testing involves a multi-step process using plasma:

1. Screening Tests: Demonstrating prolongation of one or more phospholipid-dependent clotting assays (e.g., dilute Russell Viper Venom Time - dRVVT, silica clotting time, kaolin clotting time, certain APTT reagents sensitive to LA).
2. Mixing Studies: Showing that the prolongation is due to an inhibitor and not a coagulation factor deficiency (i.e., the prolonged clotting time is not fully corrected by mixing patient plasma with normal plasma).
3. Confirmatory Tests: Demonstrating the phospholipid-dependence of the inhibitor (i.e., showing that adding excess phospholipid shortens or corrects the prolonged clotting time).

Despite prolonging clotting tests *in vitro*, LA is strongly associated with an increased risk of thrombosis *in vivo*.

Anticardiolipin Antibodies (aCL)

Anticardiolipin Antibodies (aCL) are a major type of APLA detected using Enzyme-Linked Immunosorbent Assays (ELISA). These antibodies target cardiolipin, a negatively charged phospholipid.

However, it's now understood that the clinically significant aCL antibodies associated with APS typically require the presence of the cofactor β2-Glycoprotein I (β2GPI) to bind effectively. Standard ELISA tests for APS diagnosis are designed to detect these β2GPI-dependent aCL antibodies.

For diagnostic criteria, persistently positive medium or high titers of IgG and/or IgM aCL antibodies are considered significant. The presence of IgG aCL is often more strongly associated with thrombosis than IgM.

aCL antibodies can also be found in various autoimmune diseases (especially SLE) and sometimes transiently during infections (like syphilis - causing biological false-positive VDRL/RPR tests), but infection-related aCL are usually IgM and not β2GPI-dependent.

Anti-β2-Glycoprotein I Antibodies (aβ2GPI)

β2-Glycoprotein I (β2GPI) is a plasma protein (molecular weight ~50 kDa) that binds to anionic phospholipids. It normally circulates partly bound to lipoproteins and has some natural anticoagulant properties.

Antibodies directed against β2GPI itself (often targeting epitopes exposed when β2GPI binds to phospholipids) are highly specific for APS and strongly associated with thrombosis and pregnancy morbidity. These antibodies are detected by ELISA.

For diagnostic criteria, persistently positive IgG and/or IgM aβ2GPI antibodies (usually above the 99th percentile for the assay) are considered significant. Many consider aβ2GPI antibodies, especially the IgG isotype, to be more specific for APS than aCL antibodies.

Other Associated Antibodies

While not part of the formal diagnostic criteria, antibodies against other phospholipids or phospholipid-binding proteins may also be detected in some patients and are areas of ongoing research.

Antibodies to Other Phospholipids

Antibodies against other negatively charged phospholipids like phosphatidylserine (PS), phosphatidylinositol (PI), and phosphatidic acid (PA) can sometimes be detected. Some studies suggest associations between certain isotypes (e.g., IgG anti-PS) and specific clinical features like thrombocytopenia or preeclampsia, but their independent diagnostic value beyond the criteria antibodies is less established.

Elevated levels of various APLA (IgM anti-CL, anti-PS, anti-phosphatidylcholine) have been observed in women with severe preeclampsia, suggesting a role in this obstetric complication.

Anti-Annexin V Antibodies

Annexin V is a calcium-dependent protein found abundantly on endothelial cells and placental trophoblast cells. It binds strongly to anionic phospholipids, forming a protective anticoagulant "shield" on cell surfaces.

Antibodies against Annexin V may displace it from the cell surface, disrupting this anticoagulant barrier and potentially contributing to hypercoagulability and placental thrombosis. A reduction of Annexin V on placental villi is implicated in fetal loss syndrome in APS. While biologically plausible, anti-Annexin V antibodies are not currently part of the standard diagnostic criteria.

Anti-Prothrombin Antibodies

Prothrombin (Factor II) is a key vitamin K-dependent coagulation protein. Antibodies against prothrombin (or the prothrombin-phosphatidylserine complex) can be found in some patients with APS and SLE.

These antibodies may contribute to the LA effect by interfering with prothrombin activation or function. Some studies associate high levels of anti-prothrombin antibodies with an increased risk of venous thrombosis (DVT, PE) and potentially arterial events like myocardial infarction.

Causes and Associations

• Primary APS: Occurs in the absence of any other related disease.
• Secondary APS: Occurs in conjunction with another autoimmune disease, most commonly SLE.
• Other Associations: Transient APLA may be detected during or after certain infections (viral like HIV, hepatitis C; bacterial like syphilis), with certain medications (e.g., phenothiazines, procainamide, hydralazine, oral contraceptives), and sometimes with malignancies. However, these infection- or drug-induced antibodies are often not associated with clinical APS manifestations and may not be persistent or cofactor-dependent.

The Blood Test Procedure

Testing for APLA (aCL, aβ2GPI) and LA involves standard blood draws:

• Sample: Serum (for ELISA tests like aCL, aβ2GPI) or citrated plasma (for coagulation-based LA tests).
• Preparation: No specific fasting or preparation is usually required for the patient. However, it's important to inform the lab about any anticoagulant medications (like warfarin, heparin, direct oral anticoagulants) as these can significantly interfere with LA testing.
• Collection: Blood is drawn from a vein, typically in the arm.
• Processing: Samples are processed in a specialized laboratory. LA testing requires careful handling and timely processing of plasma.

Interpretation Considerations

• Persistence is Key: A single positive test is not sufficient for diagnosis. Positive results for LA, aCL (medium/high titer), or aβ2GPI must be confirmed on repeat testing at least 12 weeks later.
• Titers Matter (for ELISA): Low positive titers for aCL or aβ2GPI are less clinically significant than medium or high titers.
• Isotypes Matter (for ELISA): IgG antibodies are generally considered more strongly associated with clinical events than IgM. IgA antibodies are not part of the criteria but are sometimes measured.
• Triple Positivity: Patients positive for all three criteria antibodies (LA, IgG aCL, IgG aβ2GPI) are generally considered to have the highest risk for thrombotic events and pregnancy complications.
• Clinical Context: Laboratory results must always be interpreted within the patient's overall clinical picture (presence of thrombosis, pregnancy history, other symptoms, associated diseases).
• Anticoagulation Interference: Anticoagulant medications significantly interfere with LA testing. Testing should ideally be performed before starting anticoagulation or after a carefully managed temporary cessation, if clinically safe.

References

1. Miyakis, S., Lockshin, M. D., Atsumi, T., Branch, D. W., Brey, R. L., Cervera, R., ... & International consensus statement writing group. (2006). International consensus statement on an update of the classification criteria for definite antiphospholipid syndrome (APS). *Journal of Thrombosis and Haemostasis*, 4(2), 295–306. https://doi.org/10.1111/j.1538-7836.2006.01753.x
2. Giannakopoulos, B., & Krilis, S. A. (2013). The pathogenesis of the antiphospholipid syndrome. *New England Journal of Medicine*, 368(11), 1033–1044. https://doi.org/10.1056/NEJMra1112783
3. American College of Rheumatology (ACR). (n.d.). Antiphospholipid Syndrome. Retrieved from https://www.rheumatology.org/I-Am-A/Patient-Caregiver/Diseases-Conditions/Antiphospholipid-Syndrome
4. Tripodi, A., Pengo, V., & Chantarangkul, V. (2018). Laboratory diagnosis of lupus anticoagulants: where are we now?. *Journal of Thrombosis and Haemostasis*, 16(11), 2126–2136. https://doi.org/10.1111/jth.14281
5. Lab Tests Online. (n.d.). Antiphospholipid Antibodies. Retrieved from https://labtestsonline.org/tests/antiphospholipid-antibodies