Autoantibodies against a complement component 1 q subcomponent contribute to complement activation and recurrent thrombosis/pregnancy morbidity in anti-phospholipid syndrome

Pediatric antiphospholipid syndrome: is it the same as an adult?

IMPORTANCE

Antiphospholipid syndrome in neonates and children is a rare, but in some cases life-threatening condition with arterial and/or venous thrombosis and/or non-thrombotic neurological, skin, ophthalmological and other manifestations.

OBSERVATIONS

This review highlights the available information about the features of pediatric APS, including the rare catastrophic form, the differences between pediatric and adult APS, and the role of genetic thrombophilia in APS manifestation.

CONCLUSIONS AND RELEVANCE

The clinical manifestations and treatment options for APS in children may differ from those in adults, and prescribing therapy can be challenging due to the unique clinical and morphological characteristics of the pediatric patient. Pediatric APS may be a predictor of the development of certain autoimmune diseases and classic manifestations of APS in adulthood, therefore, a revision of the existing criteria for the diagnosis and treatment of APS in children is necessary.

Autoantibodies against complement proteins in patients with antiphospholipid syndrome: Prevalence and clinical associations

Seventy-seven patients with antiphospholipid syndrome were tested for autoantibodies against C1q, C3, FB, FH, and C4bp. Fifty-seven patients had at least one anti-complement antibody. IgM anti-FH positivity was associated with thrombosis when anti-C3 and anti-FB were, negatively or positively, associated with various noncriteria manifestations of antiphospholipid syndrome.

Complement activation and cellular inflammation in Fabry disease patients despite enzyme replacement therapy

Defective α-galactosidase A (AGAL/GLA) due to missense or nonsense mutations in the GLA gene results in accumulation of the glycosphingolipids globotriaosylceramide (Gb3) and its deacylated derivate globotriaosylsphingosine (lyso-Gb3) in cells and body fluids. The aberrant glycosphingolipid metabolism leads to a progressive lysosomal storage disorder, i. e. Fabry disease (FD), characterized by chronic inflammation leading to multiorgan damage. Enzyme replacement therapy (ERT) with agalsidase-alfa or -beta is one of the main treatment options facilitating cellular Gb3 clearance. Proteome studies have shown changes in complement proteins during ERT. However, the direct activation of the complement system during FD has not been explored. Here, we demonstrate strong activation of the complement system in 17 classical male FD patients with either missense or nonsense mutations before and after ERT as evidenced by high C3a and C5a serum levels. In contrast to the strong reduction of lyso-Gb3 under ERT, C3a and C5a markedly increased in FD patients with nonsense mutations, most of whom developed anti-drug antibodies (ADA), whereas FD patients with missense mutations, which were ADA-negative, showed heterogenous C3a and C5a serum levels under treatment. In addition to the complement activation, we found increased IL-6, IL-10 and TGF-ß1 serum levels in FD patients. This increase was most prominent in patients with missense mutations under ERT, most of whom developed mild nephropathy with decreased estimated glomerular filtration rate. Together, our findings demonstrate strong complement activation in FD independent of ERT therapy, especially in males with nonsense mutations and the development of ADAs. In addition, our data suggest kidney cell-associated production of cytokines, which have a strong potential to drive renal damage. Thus, chronic inflammation as a driver of organ damage in FD seems to proceed despite ERT and may prove useful as a target to cope with progressive organ damage.

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.

Involvement of the complement system in immune thrombocytopenia: review of the literature

Immune thrombocytopenia (ITP) is a thrombocytopenic condition induced by autoimmune mechanisms and includes secondary ITP with underlying diseases such as connective tissue diseases (CTD). In recent years, it has been elucidated that the subsets of the ITP are associated with complement abnormalities but much remains unclear. To perform a literature review and identify the characteristics of complement abnormalities in ITP. PUBMED was used to collect the literature published up to June 2022 related to ITP and complement abnormalities. Primary and secondary ITP (CTD-related) were examined. Out of the collected articles, 17 were extracted. Eight articles were related to primary ITP (pITP) and 9 to CTD-related ITP. Analysis of the literature revealed that the ITP severity was inversely correlated with serum C3, C4 levels in both ITP subgroups. In pITP, a wide range of complement abnormalities was reported, including abnormalities of initial proteins, complement regulatory proteins, or the end products. In CTD-related ITP, reported complement abnormalities were limited to the initial proteins. Activation of the early complement system, mainly through activation of C3 and its precursor protein C4, was reported for both ITPs. On the other hand, more extensive complement activation has been reported in pITP.

Immunosuppressive Treatment in Antiphospholipid Syndrome: Is It Worth It?

The antiphospholipid syndrome (APS) is characterized by the development of venous and/or arterial thrombosis and pregnancy morbidity in patients with persistent antiphospholipid antibodies (aPL). Catastrophic antiphospholipid syndrome (CAPS) is a life-threatening form of APS occurring in about 1% of cases. Lifelong anticoagulation with vitamin K antagonists remains the cornerstone of the therapy for thrombotic APS, but frequently the use of anticoagulation may be problematic due to the increased risk of bleeding, drug interactions, or comorbidities. Immunosuppressant drugs are widely used to treat several autoimmune conditions, in which their safety and effectiveness have been largely demonstrated. Similar evidence in the treatment of primary APS is limited to case reports or case series, and studies on a large scale lack. Immunomodulatory drugs may be an emerging tool in managing such particular situations, like refractory obstetrical complications, CAPS, or so-called APS non-criteria manifestations. In addition, immunomodulatory drugs may be useful in patients experiencing recurrent thromboembolic events despite optimized anticoagulant therapy. We did a comprehensive review of literature analyzing the possible role of immunomodulation in primary APS to provide a broad overview of potentially safe and effective target treatments for managing this devastating disease.

The Immunopathology of Complement Proteins and Innate Immunity in Autoimmune Disease

The complement is a powerful cascade of the innate immunity and also acts as a bridge between innate and acquired immune defence. Complement activation can occur via three distinct pathways, the classical, alternative and lectin pathways, each resulting in the common terminal pathway. Complement activation results in the release of a range of biologically active molecules that significantly contribute to immune surveillance and tissue homeostasis. Several soluble and membrane-bound regulatory proteins restrict complement activation in order to prevent complement-mediated autologous damage, consumption and exacerbated inflammation. The crucial role of complement in the host homeostasis is illustrated by association of both complement deficiency and overactivation with severe and life-threatening diseases. Autoantibodies targeting complement components have been described to alter expression and/or function of target protein resulting in a dysregulation of the delicate equilibrium between activation and inhibition of complement. The spectrum of diseases associated with complement autoantibodies depends on which complement protein and activation pathway are targeted, ranging from autoimmune disorders to kidney and vascular diseases. Nevertheless, these autoantibodies have been identified as differential biomarkers for diagnosis or follow-up of disease only in a small number of clinical conditions. For some autoantibodies, a clear relationship with clinical manifestations has been identified, such as anti-C1q, anti-Factor H, anti-C1 Inhibitor antibodies and C3 nephritic factor. For other autoantibodies, the origin and the functional consequences still remain to be elucidated, questioning about the pathophysiological significance of these autoantibodies, such as anti-mannose binding lectin, anti-Factor I, anti-Factor B and anti-C3b antibodies. The detection of autoantibodies targeting complement components is performed in specialized laboratories; however, there is no consensus on detection methods and standardization of the assays is a real challenge. This review summarizes the current panorama of autoantibodies targeting complement recognition proteins of the classical and lectin pathways, associated proteases, convertases, regulators and terminal components, with an emphasis on autoantibodies clearly involved in clinical conditions.

Opposite Profiles of Complement in Antiphospholipid Syndrome (APS) and Systemic Lupus Erythematosus (SLE) Among Patients With Antiphospholipid Antibodies (aPL)

APS is the association of antiphospholipid antibodies (aPL) with thromboses and/or recurrent pregnancy loss (RPL). Among patients with SLE, one-third have aPL and 10–15% have a manifestation of secondary APS. Animal studies suggested that complement activation plays an important role in the pathogenesis of thrombosis and pregnancy loss in APS. We performed a cross-sectional study on complement proteins and genes in 525 patients with aPL. Among them, 237 experienced thromboses and 293 had SLE; 111 had both SLE and thromboses, and 106 had neither SLE nor thrombosis. Complement protein levels were determined by radial immunodiffusion for C4, C3 and factor H; and by functional ELISA for mannan binding lectin (MBL). Total C4, C4A and C4B gene copy numbers (GCN) were measured by TaqMan-based realtime PCR. Two to six copies of C4 genes are frequently present in a diploid genome, and each copy may code for an acidic C4A or a basic C4B protein. We observed significantly (a) higher protein levels of total C4, C4A, C4B, C3, and anticardiolipin (ACLA) IgG, (b) increased frequencies of lupus anticoagulant and males, and (c) decreased levels of complement factor H, MBL and ACLA-IgM among patients with thrombosis than those without thrombosis (N = 288). We also observed significantly lower GCNs of total C4 and C4A among aPL-positive patients with both SLE and thrombosis than others. By contrast, aPL-positive subjects with SLE had significantly reduced protein levels of C3, total C4, C4A, C4B and ACLA-IgG, and higher frequency of females than those without SLE. Patients with thrombosis but without SLE (N = 126), and patients with SLE but without thrombosis (N = 182) had the greatest differences in mean protein levels of C3 (p = 2.6 × 10⁻⁶), C4 (p = 2.2 × 10⁻⁹) and ACLA-IgG (p = 1.2 × 10⁻⁵). RPL occurred in 23.7% of female patients and thrombotic SLE patients had the highest frequency of RPL (41.0%; p = 3.8 × 10⁻¹⁰). Compared with non-RPL females, RPL had significantly higher frequency of thrombosis and elevated C4 protein levels. Female patients with homozygous C4A deficiency all experienced RPL (p = 0.0001) but the opposite was true for patients with homozygous C4B deficiency (p = 0.017). These results provide new insights and biomarkers for diagnosis and management of APS and SLE.

Pathogenic roles of anti-C1q antibodies in recurrent pregnancy loss

Recurrent pregnancy loss (RPL) is often considered idiopathic, however excessive complement activation has been observed in pregnancy related manifestations. Anti-C1q antibodies (anti-C1q) are associated with the activation of complement pathway in lupus patients, while it remains unclear in RPL. Firstly, we showed that both the prevalence and titre of anti-C1q were significantly higher in unexplained RPL than in healthy parous individuals. Secondly, we established the murine model of anti-C1q induced pregnancy loss using a monoclonal anti-mouse C1q antibody, JL-1. In mice treated with JL-1, high ratio of pregnancy loss and fetal growth restriction were frequently observed and complement activation occurred. C5a receptor (C5aR) blockade cancelled these pathogenic changes in mice treated with JL-1. In conclusion, our study reveals an association between the prevalence of anti-C1q and RPL. Additionally, our murine model has indicated that anti-C1q can induce reproductive failure, which might be ameliorated by therapy targeting the C5-C5aR axis.

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.

Clinical profiles and risk assessment in patients with antiphospholipid antibodies

INTRODUCTION

Antiphospholipid syndrome (APS) is an acquired autoimmune thrombophilia associated with the presence of persistent antiphospholipid antibodies (aPL). Owing to recent studies, not only APS patients but also incidentally-identified, asymptomatic aPL carriers are able to be stratified in terms of the risk of future thrombotic events, according to the variety and the titer of positive aPL tests and to the non-thrombotic, aPL-associated clinical manifestations. Areas covered: Here, we critically review (1) criteria manifestations of APS, (2) non-criteria manifestations of APS, (3) risk assessment in patients with APS and in aPL carriers, and (4) the potential role of primary thrombosis prophylaxis in aPL carriers. In addition, we discuss what we are currently able to do and what we need to do in the future for primary prophylaxis against a first thrombotic event. Expert commentary: We suggest a comprehensive algorithm to stratify thrombotic risk in aPL carriers, including criteria aPL, non-criteria aPL, their scoring systems, and non-criteria manifestations. However, further studies, particularly prospective randomized controlled trials, are highly warranted to establish an effective and tolerable treatment regimen for high risk aPL carriers.

Systemic lupus erythematosus: nothing stale her infinite variety

Systemic lupus erythematosus (SLE) is a representative systemic autoimmune disease that has various types of manifestations in multiple organs. Additionally, SLE is one of the most variable diseases in its epidemiology and etiology with heterogenous types of immune dysfunction. Since the word 'lupus' has first appeared in the literature in the Middle Ages, clinical/pathological knowledges have massively accumulated that contributed to the establishments and improvements of classification criteria, therapeutic agents or assessments of disease activity. Along with them, the survival rate of patients with SLE has dramatically improved. However, the mortality rate is still higher compared with the healthy population and the progress in basic, translational and clinical research are expected to lead to new insights into pathogenesis and identifying novel targets for therapy.

Antiphospholipid syndrome: an update for clinicians and scientists

PURPOSE OF REVIEW

Antiphospholipid syndrome (APS) is a leading acquired cause of thrombosis and pregnancy loss. Upon diagnosis (which is unlikely to be made until at least one morbid event has occurred), anticoagulant medications are typically prescribed in an attempt to prevent future events. This approach is not uniformly effective and does not prevent associated autoimmune and inflammatory complications. The goal of this review is to update clinicians and scientists on mechanistic and clinically relevant studies from the past 18 months, which have especially focused on inflammatory aspects of APS pathophysiology.

RECENT FINDINGS

How antiphospholipid antibodies leverage receptors and signaling pathways to activate cells is being increasingly defined. Although established mediators of disease pathogenesis (like endothelial cells and the complement system) continue to receive intensive study, emerging concepts (such as the role of neutrophils) are also receiving increasing attention. In-vivo animal studies and small clinical trials are demonstrating how repurposed medications (hydroxychloroquine, statins, and rivaroxaban) may have clinical benefit in APS, with these concepts importantly supported by mechanistic data.

SUMMARY

As anticoagulant medications are not uniformly effective and do not comprehensively target the underlying pathophysiology of APS, there is a continued need to reveal the inflammatory aspects of APS, which may be modulated by novel and repurposed therapies.

Alternative pathway activation due to low level of complement factor H in primary antiphospholipid syndrome

INTRODUCTION

Although complement activation has been proposed as a possible thrombophilic mechanism in antiphospholipid syndrome (APS), the origin of complement activation in APS remains unclear. Here, we focused on complement regulatory factors (CRF), which control the complement system to prevent damage to host tissue. We evaluated the function of two major CRF, membrane cofactor protein (MCP) and factor H (FH), in APS patients.

MATERIALS AND METHODS

In this study, we analyzed preserved serum samples from 27 patients with primary APS (PAPS), 20 with APS complicated with SLE (APS + SLE), 24 with SLE (SLE), and 25 with other connective tissue diseases (Other CTD). Serum MCP and FH levels were tested by ELISA. Autoantibodies against FH were determined by both ELISA and western-blotting.

RESULTS

Serum complement levels of PAPS were lower than those of other CTD (median C3: 82 vs 112 mg/dL, p < 0.01, C4: 15 vs 22 mg/dL, p < 0.05). Serum MCP levels did not significantly differ among the groups. Serum FH levels were significantly lower in PAPS patients compared with SLE or other CTD (median 204, 1275, and 1220 μg/mL, respectively, p < 0.01). In PAPS patients, serum FH levels were positively correlated with serum C3 levels (p < 0.01, R = 0.55), but no correlation was found with serum C4 levels (p = 0.22, R = 0.33). Autoantibodies against FH were not detected in any of our patients.

CONCLUSIONS

Activation of the alternative complement pathway due to low level of FH is one of the possible thrombophilic mechanisms in PAPS.

Antibodies Against Complement Components: Relevance for the Antiphospholipid Syndrome-Biomarkers of the Disease and Biopharmaceuticals

PURPOSE OF REVIEW

Laboratory criterion for the diagnosis of antiphospholipid syndrome (APS) is the presence of antiphospholipid antibodies (aPL Abs). Complement system has a role in mediating aPL Abs-induced thrombosis in animal models. The importance of antibodies against complement components (potential biomarkers of APS) and the importance of antibodies with beneficial anti-complement effects in APS (as biopharmaceuticals) are reviewed.

RECENT FINDINGS

Antibodies against complement components described in APS patients, so far, are anti-C1q and anti-factor H Abs, although anti-factor B Abs and anti-C5a Abs were described in animal models of APS. Clinical studies in APS patients are limited to a small number of case reports. Studies that would confirm potential role of Abs against complement components (as potential biomarkers of APS) are lacking. Lack of randomized clinical trials (that would provide complete data for confirmation of beneficial effects of biopharmaceuticals in complement inhibition) in APS is alarming.