Antiphospholipid antibodies and platelets

Platelet distribution width is highly associated with thrombotic events in primary antiphospholipid syndrome

OBJECTIVE

Platelet activation is a possible pathogenic process contributing to thromboembolism in antiphospholipid syndrome (APS), and platelet distribution width (PDW) is associated with platelet activation. The objective of this study was to evaluate the association between platelet indices and thrombotic events in patients with primary APS.

METHODS

This single-center cross-sectional study included 207 consecutive patients with APS treated at our institution between 2010 and 2019. Results of blood tests were recorded retrospectively from medical records.

RESULTS

Of the included patients, 135 (65.2%) were female and 72 (34.8%) were male. They were classified into thrombotic (n = 150) or non-thrombotic (n = 57) groups. PDW, mean platelet volume, and large platelet ratio were significantly higher in the thrombotic group. In univariate logistic analysis, PDW was significantly associated with an increased odds of thrombosis [odds ratio (OR) 1.554, 95% confidence interval (CI) 1.289-1.873, p<0.001]. In multivariate logistic analysis, PDW and positive lupus anticoagulant (LA) were risk factors for thrombosis. Receiver operating characteristic analysis showed that PDW, combined with a positive LA, was a reliable indicator of thrombosis, with an area under the curve of 0.796 (95% CI 0.728-0.864). The optimal cutoff value for PDW was 12.4 fl, with a sensitivity of 72.0% and specificity of 77.2%. Multivariate logistic regression of PDW tertiles showed that the odds of thrombosis increased abruptly in the highest tertile.

CONCLUSION

This study confirmed the association between PDW and thrombotic events in APS patients, supporting the theory that platelet activation is a crucial mechanism of thrombosis in APS. Key Points • This study is the first to discuss the correlation between PDW and thromboses in patients with APS. • This study provides evidence of the important role of platelet activation in the pathogenesis of APS.

The role of beta-2-glycoprotein I in health and disease associating structure with function: More than just APS

Beta-2-Glycoprotein I (β2GPI) plays a number of essential roles throughout the body. β2GPI, C-reactive protein and thrombomodulin are the only three proteins that possess the dual capability to up and down regulate the complement and coagulation systems depending upon external stimulus. Clinically, β2GPI is the primary antigen in the autoimmune condition antiphospholipid syndrome (APS), which is typically characterised by pregnancy morbidity and vascular thrombosis. This protein is also capable of adopting at least two distinct structural forms, but it has been argued that several other intermediate forms may exist. Thus, β2GPI is a unique protein with a key role in haemostasis, homeostasis and immunity. In this review, we examine the genetics, structure and function of β2GPI in the body and how these factors may influence its contribution to disease pathogenesis. We also consider the clinical implications of β2GPI in the diagnosis of APS and as a potentially novel therapeutic target.

Is there a link between mean platelet volume and thrombotic events in antiphospholipid syndrome?

The antiphospholipid syndrome (APS) is an autoimmune disease characterized by the production of antiphospholipid antibodies (aPL) that promote vascular thrombosis and pregnancy loss. APS can occur in the absence of underlying or associated disease (primary APS) or in combination with other diseases (secondary APS). Mean platelet volume (MPV) is largely regarded as a useful surrogate marker of platelet activation. We aimed to investigate if there is a relationship between MPV and thrombotic events in APS. The study consisted of 22 patients and 22 healthy controls. Group 1 is defined as all the patients in the first day of thrombotic event. Group 2 is defined as the same patient population three months after the thrombotic event. The erythrocyte sedimentation rate, C-reactive protein, white blood cell count, platelet count, and MPV levels were retrospectively recorded from patient files. Statistical analyses showed that MPV was significantly higher in group 1 than group 2 (p < 0.0001) and healthy controls (p < 0.05). However, there was no difference between group 2 and healthy controls (p = 0.888). WBC, hemoglobin and other platelet indices such as platelet distribution width and platecrit did not differ in groups. In conclusion, MPV was increased at initial thrombotic event of APS, and then it was normalized three months later by therapeutic interventions. To our knowledge, this is the first study demonstrating a correlation between MPV and thrombotic events in APS.

Complement activation on platelets: implications for vascular inflammation and thrombosis

Platelets participate in a variety of responses of the blood to injury. An emerging body of evidence suggests that these cells express an intrinsic capacity to interact with and trigger both classical and alternative pathways of complement. This activity requires cell activation with biochemical agonists and/or shear stress, and is associated with the expression of P-selectin, gC1qR, and chondroitin sulfate. Platelet mediated complement activation measurably increases soluble inflammatory mediators (C3a and C5a). Platelets may also serve as targets of classical complement activation in autoimmune conditions such as antiphospholipid syndromes (APS) and immune thrombocytopenia purpura (ITP). Retrospective correlation with clinical data suggests that enhanced platelet associated complement activation correlates with increased arterial thrombotic events in patients with lupus erythematosus and APS, and evidence of enhanced platelet clearance from the circulation in patients with ITP. Taken together, these data support a role for platelet mediated complement activation in vascular inflammation and thrombosis.

Serum complement activation on heterologous platelets is associated with arterial thrombosis in patients with systemic lupus erythematosus and antiphospholipid antibodies

Complement plays a major role in inflammation and thrombosis associated with systemic lupus erythematosus (SLE) and the antiphospholipid syndrome (APS). A cross-sectional retrospective analysis was performed to evaluate serum complement fixation on platelets and thrombotic incidence using banked sera and clinical data from patients with SLE (n = 91), SLE with antiphospholipid antibodies (aPL) or APS (n = 78) and primary aPL (n = 57) or APS (n = 96). In-situ complement fixation was measured as C1q and C4d deposition on heterologous platelets using an enzyme-linked immunosorbent assay approach. Platelet activation by patient serum in the fluid phase was assessed via serotonin release assay. Enhanced in-situ complement fixation was associated with the presence of IgG aPL and IgG anti-beta2 glycoprotein 1 antibodies (P < 0.05) and increased platelet activation (P < 0.005). Moreover, enhanced complement fixation, especially C4d deposition on heterologous platelets, was positively associated with arterial thrombotic events in patients with SLE and aPL (P = 0.039). Sera from patients with aPL possess an enhanced capacity for in-situ complement fixation on platelets. This capacity may influence arterial thrombosis risk in patients with SLE.

Platelet C4d is highly specific for systemic lupus erythematosus

OBJECTIVE

Complement-activation product C4d is deposited on normal erythrocytes, while abnormal levels have been observed on the surface of erythrocytes of patients with systemic lupus erythematosus (SLE). This study examines whether C4d also deposits on human platelet surfaces, and whether platelet-bound C4d may provide a biomarker for SLE.

METHODS

We conducted a cross-sectional study of 105 patients with SLE, 115 patients with other diseases, and 100 healthy controls. Levels of C4d on the surface of platelets were examined by flow cytometry and scanning confocal microscopy. Statistical analyses were performed to determine the clinical variables associated with platelet C4d.

RESULTS

Abnormal levels of platelet C4d were found to be highly specific for SLE. Platelet C4d was detected in 18% of patients with SLE, being 100% specific for a diagnosis of SLE compared with healthy controls and 98% specific for SLE compared with patients with other diseases (P < 0.0001). In addition, platelet C4d was significantly associated with positivity for lupus anticoagulant (P < 0.0001) and anticardiolipin antibodies of the IgG (P = 0.035) or the IgM (P = 0.016) isotype. Platelet C4d was also significantly associated with SLE disease activity according to the SLE Disease Activity Index (P = 0.039), low serum C4 (P = 0.046), an elevated erythrocyte sedimentation rate (P = 0.006), and abnormal levels of C4d on erythrocytes (P < 0.0001).

CONCLUSION

This observation suggests that platelet-bound C4d may be a useful biomarker for SLE and may be a clue to the pathogenic mechanisms responsible for the myriad thrombotic and vascular complications of lupus associated with antiphospholipid antibodies.

Clinical importance of positive test results for lupus anticoagulant and anticardiolipin antibodies

OBJECTIVES

To assess the performance of 4 clotting assays for lupus anticoagulant (LA) detection, to determine the prevalence of LA and anticardiolipin antibodies (aCL), and to correlate LA and aCL prevalence with systemic disease and thrombosis.

PATIENTS AND METHODS

We studied 664 consecutive patients at the Mayo Clinic in Rochester, Minn, who were referred for laboratory testing because of a clinical suspicion of LA or thrombophilia between June 25, 1990, and July 1, 1991.

RESULTS

Of 664 patients tested for LA, 584 also were tested for aCL. Of patients tested for both LA and aCL, 137 (235%) had positive results for one or both tests (13 [95%], LA-positive only; 76 [555%], aCL-positive only; and 48 [35.0%], positive for both). The dilute Russell viper venom time (DRVVT) was the most frequently positive LA assay (74% of the 61 patients with positive results for LA). Twenty-two patients (36.1% of the 61) had positive results for all 4 LA assays, whereas 21 (34.4% of the 61) had positive results for only 1 LA assay: activated partial thromboplastin time (3 patients [4.9%]), plasma clot time (5 patients [8.2%]), kaolin clot time (5 patients [8.2%]), or DRVVT (8 patients [13.1%]). Thromboembolism prevalence was not definitely associated with positive test results (LA only, aCL only, or LA plus aCL), nor was it strongly associated with aCL isotype or titer. Furthermore, thromboembolism prevalence was not increased when all LA assays were positive, although a history of deep venous thrombosis or pulmonary embolism was nonsignificantly associated with positive results for all 4 LA tests. The likelihood of having both LA- and aCL-positive test results was higher among patients with systemic lupus erythematosus (26 [19.0%] of 137 patients with positive results for one or both tests), but they had no more thrombotic events or fetal loss than other patients in our study group.

CONCLUSIONS

The DRVVT identified more patients with LA than the other LA tests, but more than 1 LA test was required to identify all patients with LA. Positive results were much more common for aCL than for LA. No single LA test or anticardiolipin isotype correlated with thrombosis or systemic disease in this population.

Antiphospholipid antibodies: discovery, definitions, detection and disease

Antiphospholipid antibodies (aPL) are immunoglobulins of IgG, IgM and IgA isotypes that target phospholipid (PL) and/or PL-binding plasma proteins. Detection of aPL in the laboratory is done currently by both immunoassays and functional coagulation tests. Convention defines aPL specificity in immunoassays according to the particular PL substrate present, for example aPS represents antiphosphatidylserine antibodies. This may be technically incorrect inasmuch as a particular PL may be responsible for binding and highly concentrating a specific plasma protein, the latter then becomes the target for the aPL. The binding of beta(2)GP-I (apolipoprotein H) to the negatively charged PL, cardiolipin (CL) provides a good example of this circumstance. In contrast, aPL which specifically prolong coagulation times in in vitro are called lupus anticoagulants (LA). The precise PL target(s) of the aPL responsible for LA activities are unknown and often debated. The persistent finding of aPL in patients in association with abnormal blood clotting and a myriad of neurological, obstetrical and rheumatic disorders often compounded by autoimmune diseases has led to an established clinical diagnosis termed antiphospholipid syndrome (APS). The common denominator for these APS patients is the presence of circulating aPL on two or more occasions and the observation of events attributable to abnormal or accelerated blood clotting somewhere in vivo. The purpose of this review is to collect, collate, and consolidate information concerning aPL.