Thrombin activatable fibrinolysis inhibitor (TAFI) — A possible link between coagulation and complement activation in the antiphospholipid syndrome (APS)

Complement biomarkers in the antiphospholipid syndrome - Approaches to quantification and implications for clinical management

Complement is a major driver of antiphospholipid syndrome (APS) and a promising therapeutic target in refractory and catastrophic APS. Complement testing in APS is largely limited to research settings, and reliable, rapid-turnaround biomarkers are needed to predict those at risk for adverse clinical outcomes and most likely to benefit from complement inhibition. We review complement biomarkers and their association with thrombosis and obstetric outcomes, including: (i) complement proteins and activation fragments in the fluid phase; (ii) assays that evaluate complement on cell membranes (e.g. in vivo cell-bound complement fragments, hemolytic assays, and ex vivo 'functional' cell-based assays, and (iii) sequencing of complement genes. Current studies highlight the inconsistencies in testing both between studies and various aPL/APS subgroups, suggesting that either cell-based testing or multiplex panels employing a combination of biomarkers simultaneously may be most clinically relevant. Standardization of complement assays is needed to ensure reproducibility and establish clinically relevant applications.

Plasma thrombin-activatable fibrinolysis inhibitor and the 1040C/T polymorphism are risk factors for diabetic kidney disease in Chinese patients with type 2 diabetes

Background

Inflammatory and hemostatic disorders in diabetic microangiopathy (DMA) can be linked to thrombin-activatable fibrinolysis inhibitor (TAFI) and its own gene polymorphisms. Thus, the study aimed to investigate the associations of plasma TAFI and gene polymorphisms with DMA in Chinese patients with type 2 diabetes (T2D).

Methods

Plasma TAFI of 223 patients with T2D was measured, and the genotypes and alleles of the 1040C/T, 438G/A, and 505G/A polymorphisms of the TAFI gene were analyzed. A ROC curve was constructed to evaluate the identifying power of TAFI between patients with T2D and DMA, and logistic regression analysis was used to observe the correlation of plasma TAFI and gene polymorphisms with the risk for DMA.

Results

Plasma TAFI was higher in patients with DMA than in patients with only T2D (p < 0.05). TAFI exhibited the largest area under ROC in identifying diabetic kidney disease (DKD) from only T2D (0.763, 95% CI [0.674-0.853], p < 0.01), and adjusted multivariate analysis showed a high odds ratio (OR: 15.72, 95% CI [4.573-53.987], p < 0.001) for DKD. Higher frequencies of the CT genotype and T allele of the 1040C/T polymorphism were found in DKD compared with only T2D (respectively p < 0.05), and the CT genotype exhibited a high OR (1.623, 95% CI [1.173-2.710], p < 0.05) for DKD. DKD patients with the CT genotype had higher plasma TAFI levels, while T2D and DKD patients with CC/TT genotypes had lower plasma TAFI levels.

Conclusion

Plasma TAFI and the CT genotype and T allele of the 1040C/T polymorphism are independent risk factors for DKD in Chinese T2D patients.

Pitfalls in complement analysis: A systematic literature review of assessing complement activation

Background

The complement system is an essential component of our innate defense and plays a vital role in the pathogenesis of many diseases. Assessment of complement activation is critical in monitoring both disease progression and response to therapy. Complement analysis requires accurate and standardized sampling and assay procedures, which has proven to be challenging.

Objective

We performed a systematic analysis of the current methods used to assess complement components and reviewed whether the identified studies performed their complement measurements according to the recommended practice regarding pre-analytical sample handling and assay technique. Results are supplemented with own data regarding the assessment of key complement biomarkers to illustrate the importance of accurate sampling and measuring of complement components.

Methods

A literature search using the Pubmed/MEDLINE database was performed focusing on studies measuring the key complement components C3, C5 and/or their split products and/or the soluble variant of the terminal C5b-9 complement complex (sTCC) in human blood samples that were published between February 2017 and February 2022. The identified studies were reviewed whether they had used the correct sample type and techniques for their analyses.

Results

A total of 92 out of 376 studies were selected for full-text analysis. Forty-five studies (49%) were identified as using the correct sample type and techniques for their complement analyses, while 25 studies (27%) did not use the correct sample type or technique. For 22 studies (24%), it was not specified which sample type was used.

Conclusion

A substantial part of the reviewed studies did not use the appropriate sample type for assessing complement activation or did not mention which sample type was used. This deviation from the standardized procedure can lead to misinterpretation of complement biomarker levels and hampers proper comparison of complement measurements between studies. Therefore, this study underlines the necessity of general guidelines for accurate and standardized complement analysis

Elevated levels of C3 , C4 , and CH50 of the complement system in ICU and non‐ICU patients with COVID ‐19

Purpose

SARS‐CoV‐2 infection has spread in each corner of the world. Many health systems have dealt with it intensively. The complement system is an instrumental component in the inflammatory immune response and plays a role in the activation of blood coagulation. Our understanding of the pathophysiology of SARS‐CoV‐2 is still limited but is constantly expanding. This study aimed to determine changes in the complement system in intensive care unit (ICU) and non‐ICU patients with COVID‐19.

Methods

In a cross‐sectional study, plasma levels of C3, C4, and CH50 were determined in two groups of ICU and non‐ICU patients with COVID‐19 to understand the potential effects of SARS‐CoV‐2 on the innate immune system. The assays of C3 and C4 were conducted using turbidimetry method. The CH50 test was conducted using the functional method.

Results

The present study revealed that the C3, C4, and CH50 plasma levels were 142.48 ± 30.38 mg/dL, 32.58 ± 8.78 mg/dL, and 61.74 ± 19.54%, respectively. These results indicate high levels of complement components C3 and C4 and complement function (CH50) in patients with COVID‐19 than normal ranges. Plasma levels of C3, C4, and CH50 were higher in ICU patients than in non‐ICU COVID‐19 groups.

Conclusion

These results indicate that the innate immune system was activated in both ICU and non‐ICU patients in response to SARS‐CoV‐2 infection. Further studies with a larger number of COVID‐19 patients and additional testing of complement components (C3a and C5a) may reveal the role of COVID‐19 infection in the activation of the complement system.

Pathophysiology of the Antiphospholipid Antibody Syndrome

The antiphospholipid syndrome is characterized by antibodies directed against phospholipid-binding proteins and phospholipids attached to cell membrane receptors, mitochondria, oxidized lipoproteins, and activated complement components. When antibodies bind to these complex antigens, cells are activated and the coagulation and complement cascades are triggered, culminating in thrombotic events and pregnancy morbidity that further define the syndrome. The phospholipid-binding proteins most often involved are annexins II and V, β2-glycoprotein I, prothrombin, and cardiolipin. A distinguishing feature of the antiphospholipid syndrome is the "lupus anticoagulant". This is not a single entity but rather a family of antibodies directed against complex antigens consisting of β2-glycoprotein I and/or prothrombin bound to an anionic phospholipid. Although these antibodies prolong in vitro clotting times by competing with clotting factors for phospholipid binding sites, they are not associated with clinical bleeding. Rather, they are thrombogenic because they augment thrombin production in vivo by concentrating prothrombin on phospholipid surfaces. Other antiphospholipid antibodies decrease the clot-inhibitory properties of the endothelium and enhance platelet adherence and aggregation. Some are atherogenic because they increase lipid peroxidation by reducing paraoxonase activity, and others impair fetal nutrition by diminishing placental antithrombotic and fibrinolytic activity. This plethora of destructive autoantibodies is currently managed with immunomodulatory agents, but new approaches to treatment might include vaccines against specific autoantigens, blocking the antibodies generated by exposure to cytoplasmic DNA, and selective targeting of aberrant B-cells to reduce or eliminate autoantibody production.

Association of Thrombin-Activatable Fibrinolysis Inhibitor with Acute Pulmonary Embolism

BACKGROUND

 Thrombin-activatable fibrinolysis inhibitor (TAFI) inhibits fibrinolysis and high levels may have an association with thrombosis. The aim of the current study was to investigate the association of TAFI antigen levels with pulmonary thromboembolism (PTE).

PATIENTS AND METHODS

 A case-control study was conducted with 29 patients with PTE and 17 age- and gender-matched control individuals. Plasma levels of TAFI were measured at the time of diagnosis, then at 3 and 6 months after the event.

RESULTS

 Initial TAFI levels (%) were higher in patients with PTE than in the control group Initial TAFI levels (%) were higher in patients with PTE than in the control group (190,0 [65,0-250,0] vs 133,0 [83,0-153,0]; p = 0.003). TAFI levels significantly decreased at the third and sixth months after initial diagnosis (p < 0.05). The percentage reductions in TAFI levels were 12 and 36.8% at 3 and 6 months, respectively. The Odss ratio (OR) of TAFI level for PTE was found to be 1.024 (95% CI: 1.007-1.040; p = 0.005). There was no significant correlation of initial TAFI levels with age, gender, smoking status, history of thrombosis, pulmonary artery pressure, and D-dimer levels (p > 0.05). In the sixth month of treatment, patients with residual thrombosis were seen to have similar baseline levels and reductions of TAFI as patients without residual thrombosis (p > 0.05).

CONCLUSION

 The result of this study suggests that high TAFI levels may have a role in the occurrence of PTE without impact on treatment outcome.

Impaired Fibrinolysis in the Antiphospholipid Syndrome

The pathogenesis of the antiphospholipid syndrome (APS) is complex and involves the persistent presence of antiphospholipid antibodies (aPL) in the bloodstream causing a prothrombotic condition. aPL induce excessive activation of the endothelium, monocytes, and platelets in consort with aberrations in hemostasis/clotting, fibrinolytic system, and complement activation. Impaired fibrinolysis has been found in APS patients with thrombotic as well as obstetric manifestations. Increased levels of plasminogen activator inhibitor-1 and thrombin-activatable fibrinolysis inhibitor, together with the presence of aPL against annexin-2, tissue-type plasminogen activator, and plasminogen contribute to the compromised fibrinolytic activity in these patients. Furthermore, unfavorably altered fibrin morphology, less amenable to fibrinolysis, has been proposed as a novel prothrombotic mechanism in APS. This review aims to summarize the present knowledge of the mechanisms involved in impaired fibrinolysis in APS patients. We also present a case from clinical practice as an illustration of fibrinolysis impairment in APS patients from a real-life setting.

The antiphospholipid syndrome - often overlooked cause of vascular occlusions?

The antiphospholipid syndrome (APS) was fully recognized as a clinical entity in the early 1980s. Still, more than 30 years later, the epidemiology of APS is not well described, and furthermore, APS remains a challenge in terms of both diagnostic issues and clinical praxis involving a wide range of specialties. To date, there are no diagnostic criteria for APS. The present classification criteria rely on a combination of clinical manifestations and persistently positive tests for antiphospholipid antibodies (aPL). Clinical symptoms comprise vascular thrombosis, which can affect any vascular bed, including venous, microvascular and arterial vessels, and a set of pregnancy morbidities including early and late miscarriages, foetal death and preeclampsia. APS is more frequent among patients with other autoimmune diseases, and it is especially common in systemic lupus erythematosus (SLE). Importantly, APS symptoms can present in almost any medical specialty, but general knowledge and most previous clinical studies have essentially been confined to haematology, rheumatology and obstetrics/gynaecology. However, recent data demonstrate a relatively high prevalence of aPL also in patients from the general population who suffer from vascular occlusions or pregnancy complications. It is important that these patients are recognized by the general health care since APS is a treatable condition. This review aims to summarize the present knowledge on the history, pathogenesis, clinical manifestations and treatment of APS in order to urge a wide range of clinicians to consider comprehensive assessment of all patients where the diagnosis APS may be conceivable.

Sjögren Syndrome in Systemic Lupus Erythematosus: A Subset Characterized by a Systemic Inflammatory State

OBJECTIVE

An often-neglected subset of patients with systemic lupus erythematosus (SLE) is those with secondary Sjögren syndrome (SLE-sSS). Further, primary SS overlaps and can be difficult to delineate from SLE. To shed light on the SLE-sSS subset, we investigated a large and well-characterized SLE cohort, comparing patients with SLE-sSS and SLE patients without SS (SLE-nonsSS) and controls.

METHODS

We included 504 consecutive patients with SLE, fulfilling the 1982 revised American College of Rheumatology criteria, and 319 controls from the general population, matched for age and sex to the first 319 patients. SLE-sSS was defined according to the American-European Consensus Criteria (AECC). A thorough clinical examination, including subjective and objective quantifications of sicca symptoms, was performed in all participants. Autoantibodies and 20 selected cytokines were measured by luminex and multiplex analysis, respectively.

RESULTS

SLE-sSS, as defined by AECC, occurred in 23% of the patients with SLE. In comparison to SLE-nonsSS, the SLE-sSS group was older and more frequently female. Leukopenia and peripheral neuropathy were more frequent and nephritis less frequent. Circulating levels of 6/20 investigated proinflammatory cytokines [tumor necrosis factor-α, interleukin (IL) 6, monocyte chemoattractant protein 4, macrophage inflammatory protein 1β, IL-12/IL-23p40, and interferon γ-induced protein 10], total IgG, anti-SSA/Ro52, anti-SSA/Ro60, anti-SSB/La antibodies, and rheumatoid factor (IgM and IgA) were higher in the SLE-sSS group (p < 0.05 for all comparisons).

CONCLUSION

The frequency of SLE-sSS increased with age and affected roughly one-quarter of all patients with SLE. Despite less internal organ involvement, a systemic inflammatory state with high levels of proinflammatory cytokines is present in the SLE-sSS subgroup. This is a novel observation that may affect future understanding and treatment of the SLE-sSS subset.

Complement in the Pathophysiology of the Antiphospholipid Syndrome

The antiphospholipid syndrome (APS) is characterized by thrombosis and pregnancy morbidity in the presence of antiphospholipid antibodies (aPL). Complement is a system of enzymes and regulatory proteins of the innate immune system that plays a key role in the inflammatory response to pathogenic stimuli. The complement and coagulation pathways are closely linked, and expanding data indicate that complement may be activated in patients with aPL and function as a cofactor in the pathogenesis of aPL-associated clinical events. Complement activation by aPL generates C5a, which induces neutrophil tissue factor-dependent procoagulant activity. Beta-2-glycoprotein I, the primary antigen for pathogenic aPL, has complement regulatory effects in vitro. Moreover, aPL induce fetal loss in wild-type mice but not in mice deficient in specific complement components (C3, C5). Antiphospholipid antibodies also induce thrombosis in wild type mice and this effect is attenuated in C3 or C6 deficient mice, or in the presence of a C5 inhibitor. Increased levels of complement activation products have been demonstrated in sera of patients with aPL, though the association with clinical events remains unclear. Eculizumab, a terminal complement inhibitor, has successfully been used to treat catastrophic APS and prevent APS-related thrombotic microangiopathy in the setting of renal transplant. However, the mechanisms of complement activation in APS, its role in the pathogenesis of aPL related complications in humans, and the potential of complement inhibition as a therapeutic target in APS require further study.