Anti-C1q antibodies and their association with complement components in Indian systemic lupus erythematosus patients

Diagnostic test accuracy of novel biomarkers for lupus nephritis-An overview of systematic reviews

INTRODUCTION

Systemic lupus erythematosus (SLE) is a chronic autoimmune disease with multiorgan inflammatory involvement and a mortality rate that is 2.6-fold higher than individuals of the same age and sex in the general population. Approximately 50% of patients with SLE develop renal impairment (lupus nephritis). Delayed diagnosis of lupus nephritis is associated with a higher risk of progression to end-stage renal disease, the need for replacement therapy, and mortality. The initial clinical manifestations of lupus nephritis are often discrete or absent and are usually detected through complementary tests. Although widely used in clinical practice, their accuracy is limited. A great scientific effort has been exerted towards searching for new, more sensitive, and specific biomarkers in recent years. Some systematic reviews have individually evaluated new serum and urinary biomarkers tested in patients with lupus nephritis. This overview aimed to summarize systematic reviews on the accuracy of novel serum and urinary biomarkers for diagnosing lupus nephritis in patients with SLE, discussing how our results can guide the clinical management of the disease and the direction of research in this area.

METHODS

The research question is "What is the accuracy of the new serum and urinary biomarkers studied for the diagnosis of LN in patients with SLE?". We searched for systematic reviews of observational studies evaluating the diagnostic accuracy of new serum or urinary biomarkers of lupus nephritis. The following databases were included: PubMed, EMBASE, BIREME/LILACS, Scopus, Web of Science, and Cochrane, including gray literature found via Google Scholar and PROQUEST. Two authors assessed the reviews for inclusion, data extraction, and assessment of the risk of bias (ROBIS tool).

RESULTS

Ten SRs on the diagnostic accuracy of new serum and urinary BMs in LN were selected. The SRs evaluated 7 distinct BMs: (a) antibodies (anti-Sm, anti-RNP, and anti-C1q), (b) cytokines (TWEAK and MCP-1), (c) a chemokine (IP-10), and (d) an acute phase glycoprotein (NGAL), in a total of 20 review arms (9 that analyzed serum BMs, and 12 that analyzed BMs in urine). The population evaluated in the primary studies was predominantly adults. Two SRs included strictly adults, 5 reviews also included studies in the paediatric population, and 4 did not report the age groups. The results of the evaluation with the ROBIS tool showed that most of the reviews had a low overall risk of bias.

CONCLUSIONS

There are 10 SRs of evidence relating to the diagnostic accuracy of serum and urinary biomarkers for lupus nephritis. Among the BMs evaluated, anti-C1q, urinary MCP-1, TWEAK, and NGAL stood out, highlighting the need for additional research, especially on LN diagnostic panels, and attempting to address methodological issues within diagnostic accuracy research. This would allow for a better understanding of their usefulness and possibly validate their clinical use in the future.

REGISTRATION

This project is registered on the International Prospective Registry of Systematic Reviews (PROSPERO) database (CRD42020196693).

Autoantibodies against Complement Classical Pathway Components C1q, C1r, C1s and C1-Inh in Patients with Lupus Nephritis

Autoantibodies against the complement component C1q (anti-C1q) are among the main biomarkers in lupus nephritis (LN) known to contribute to renal injury. C1q, the recognition subcomponent of the complement classical pathway, forms a heterotetrameric complex with C1r and C1s, and can also associate a central complement regulator and C1 Inhibitor (C1-Inh). However, the frequency and the pathogenic relevance of anti-C1r, anti-C1s and anti-C1-Inh autoantibodies remain poorly studied in LN. In this paper, we screened for anti-C1q, anti-C1r, anti-C1s and anti-C1-Inh autoantibodies and evaluated their association with disease activity and severity in 74 LN patients followed up for 5 years with a total of 266 plasma samples collected. The presence of anti-C1q, anti-C1r, anti-C1s and anti-C1-Inh was assessed by ELISA. IgG was purified by Protein G from antigen-positive plasma and their binding to purified C1q, C1r and C1s was examined by surface plasmon resonance (SPR). The abilities of anti-C1q, anti-C1r and anti-C1s binding IgG on C1 complex formation were analyzed by ELISA. The screening of LN patients’ plasma revealed 14.9% anti-C1q positivity; only 4.2%, 6.9% and 0% were found to be positive for anti-C1r, anti-C1s and anti-C1-Inh, respectively. Significant correlations were found between anti-C1q and anti-dsDNA, and anti-nuclear antibodies, C3 and C4, respectively. High levels of anti-C1q antibodies were significantly associated with renal histologic lesions and correlated with histological activity index. Patients with the most severe disease (A class according to BILAG Renal score) had higher levels of anti-C1q antibodies. Anti-C1r and anti-C1s antibodies did not correlate with the clinical characteristics of the LN patients, did not interfere with the C1 complex formation, and were not measurable via SPR. In conclusion, the presence of anti-C1q, but not anti-C1s or anti-C1r, autoantibodies contribute to the autoimmune pathology and the severity of LN.

Diminished capacity of opsonization and immune complex solubilization, and detection of anti-C1q antibodies in sera from patients with hereditary angioedema

BACKGROUND

Hereditary angioedema (HAE) is an autosomal dominant disease caused by deficiency of C1 esterase inhibitor. Symptoms of HAE include edema, which can potentially cause suffocation. Some patients with HAE exhibit immunological abnormalities, which could prevent an accurate diagnosis. Low levels of complement components are characteristic of HAE and in other settings are thought to reduce elimination of apoptotic cells and immune complex (IC). Thus, we aimed to experimentally clarify the mechanism of immunological abnormalities using sera from HAE patients.

METHODS

Serum samples from 18 patients with HAE were collected when free from angioedema attack and compared with normal human pooled sera (NHPS) from 20 healthy volunteers. Opsonization was measured as the rate of phagocytosis of apoptotic Jurkat cells by macrophages differentiated from THP-1 cells incubated with serum. IC solubilization in serum was analyzed by quantifying peroxidase released from a synthetic IC composed of peroxidase and anti-peroxidase antibodies. Anti-C1q antibody levels were detected using an enzyme-linked immunosorbent assay.

RESULTS

Serological immunological abnormalities were detected in 12 patients. Opsonization in serum samples from each patient with HAE was lower than that in NHPS (∼20% versus 70%, respectively). The rate of IC solubilization was lower in serum from HAE patients than NHPS. Some patients had high serum anti-C1q antibody levels with increased serum IC levels.

CONCLUSIONS

Sera from patients with HAE exhibit anti-C1q antibodies, with a lower capacity for opsonization and IC solubilization. This may be associated with immunological abnormalities and should be investigated further to facilitate accurate diagnosis of HAE.

Anti-C1q Antibody is Associated with Renal and Cutaneous Manifestations in Asian Indian Patients with Systemic Lupus Erythematosus

INTRODUCTION

C1q play an important role in clearance of immune complexes and apoptotic cell debris. Impaired clearance leads to exposure of C1 native antigen and development of anti-C1q antibody formation. Anti-C1q antibody is well studied in Systemic Lupus Erythematosus (SLE). Significance of anti-C1q Ab in Indian SLE patients and their clinical manifestations is not clear.

AIM

The aim of this study was to investigate associations between anti-C1q antibody and clinical as well as serological markers of SLE.

MATERIALS AND METHODS

Retrospective study of SLE patients fulfilling either American College of Rheumatology (ACR) 1990 or Systemic Lupus International Collaborating Clinics (SLICC) 2012 classification criteria were recruited from inpatients and outpatients services of the Clinical immunology and Rheumatology Department, Christian Medical College at Vellore, India between March 2013 and January 2015. Anti-C1q antibody was assayed by ELISA (Demeditec Diagnostics GmbH, Germany). Logistic regression analysis was performed to find the association of anti-C1q antibodies with serological and clinical parameters in SLE including Lupus Nephritis (LN).

RESULTS

Sixty nine patients (54.76%) out of 126 SLE patients had LN. Anti-C1q levels were higher in patients with LN as compared to those without (p<0.05). Anti-C1q antibody was also significantly associated with positive C1q immunofluorescence staining in renal biopsy specimens (p<0.05). Overall, renal Systemic Lupus Erythematosus Disease Activity Index (SLEDAI) {OR 1.35 (1.08-1.69)}, low C4 {OR 3.11 (1.04-9.26)} and mucocutaneous manifestation {OR 4.72 (1.38-16.05)} were independently associated with anti-C1q levels in serum.

CONCLUSION

Renal SLEDAI, low C4 and mucocutaneous manifestations were independently associated with raised anti C1q antibody in SLE patients.

Autoantibodies against complement components in systemic lupus erythematosus – role in the pathogenesis and clinical manifestations

Many complement structures and a number of additional factors, i.e. autoantibodies, receptors, hormones and cytokines, are implicated in the complex pathogenesis of systemic lupus erythematosus. Genetic defects in the complement as well as functional deficiency due to antibodies against its components lead to different pathological conditions, usually clinically presented. Among them hypocomplementemic urticarial vasculitis, different types of glomerulonephritis as dense deposit disease, IgA nephropathy, atypical haemolytic uremic syndrome and lupus nephritis are very common. These antibodies cause conformational changes leading to pathological activation or inhibition of complement with organ damage and/or limited capacity of the immune system to clear immune complexes and apoptotic debris. Finally, we summarize the role of complement antibodies in the pathogenesis of systemic lupus erythematosus and discuss the mechanism of some related clinical conditions such as infections, thyroiditis, thrombosis, acquired von Willebrand disease, etc.

C1q rs292001 polymorphism and C1q antibodies in juvenile lupus and their relation to lupus nephritis

C1q deficiency is related strongly to systemic lupus erythematosus (SLE), but very few and inconsistent studies explored the single nucleotide polymorphisms of the C1q gene in relation to juvenile SLE (jSLE) and lupus nephritis (LN). The objective of this study was to analyse whether C1q rs 292001 polymorphism is associated with SLE and disease phenotype, especially nephritis, and to investigate the relation between this polymorphism and clinical data, treatment outcome, serum level of C1q protein and antibodies. Typing of C1q rs292001 polymorphism using restriction fragment length polymorphism and measuring serum levels of C1q protein and antibodies by enzyme-linked immunosorbent assay (ELISA) were performed for 130 children with SLE and 208 healthy controls. The A allele of C1q rs292001 was associated with jSLE and LN (P = 0·005 and 0·013, respectively) and the AA genotype was associated with jSLE (P = 0·036). Low serum levels of C1q protein were found in jSLE and LN (P < 0·001 and 0·009, respectively), and these levels were increased after treatment in patients with LN (P = 0·009) and active renal disease (P = 0·027). Higher titres of C1q antibodies were found in patients with LN (P = 0·015) and correlated negatively with C1q protein level (P < 0·001) and patient age (P = 0·04). The A allele and AA genotype of C1q rs292001 can be considered a susceptibility risk factor and the GG genotype could be considered protective for jSLE and LN in the studied cohort of Egyptian children. Decreased serum levels of C1q protein and increased titres of C1q antibodies may be involved in the pathogenesis of jSLE, especially LN.

Autoantibodies against C1q as a Diagnostic Measure of Lupus Nephritis: Systematic Review and Meta-analysis

OBJECTIVES

To evaluate the diagnostic accuracy of C1q autoantibodies in identifying lupus nephritis (LN) in patients with systemic lupus erythematosus (SLE).

DATA SOURCES AND METHODS

Citation indexes were searched and 370 articles published from 1977 to 2013 were evaluated. The 31 selected studies included in the meta-analysis were cross-sectional in design. Among the 31 studies, 28 compared anti-C1q antibodies in 2769 SLE patients with (n=1442) and without a history of LN (n=1327). Nine studies examined anti-C1q in 517 SLE patients with active (n=249) and inactive LN (n=268). Hierarchical summary receiver operating characteristic (HSROC) random effects models were fitted to pool estimates of accuracy across the studies.

RESULTS

Anti-C1q antibodies discriminated between patients with and without a history of LN, with a median specificity of 73.5%. The HSROC model estimated the corresponding sensitivity to be 70.4%. A hypothetical patient with a 55% prior probability of having a history of LN as opposed to no history (the median prevalence across 28 eligible studies) would have a post-test probability of 76.4% following a positive test result (positive predictive value) or 33.0% following a negative test result (negative predictive value). For discriminating active from inactive LN the median specificity of anti-C1q antibodies was 80%, with a corresponding estimated sensitivity value 75.7% based on the HSROC model. A hypothetical patient with a 56% prior probability of active as opposed to inactive LN (the median prevalence across the 9 eligible studies) would have a post-test probability of 82.8% following a positive test result or 27.9% following a negative test result.

CONCLUSIONS

Although C1q antibodies are associated with lupus nephritis the post-test probabilities are not sufficiently convincing to provide reasonable certainty of the presence or absence of history of disease/active disease.

Anti-C1q autoantibodies, novel tests, and clinical consequences

Although anti-C1q autoantibodies have been described more than four decades ago a constant stream of papers describing clinical associations or functional consequences highlights that anti-C1q antibodies are still hot and happening. By far the largest set of studies focus on anti-C1q antibodies is systemic lupus erythematosus (SLE). In SLE anti-C1q antibodies associate with involvement of lupus nephritis in such a way that in the absence of anti-C1q antibodies it is unlikely that a flare in nephritis will occur. Anti-C1q antibodies occur in several autoimmune conditions but also in healthy individuals. Although considerable progress has been made in the understanding of how anti-C1q antibodies may contribute to tissue injury there is still a lot to learn about the processes involved in the breaking of tolerance to this protein. There has been considerable improvement in the assays employed to test for the presence of anti-C1q antibodies. Hopefully with these new and standardized assays at hand larger clinical association studies will be conducted with independent replication. Such large-scale studies will reveal the true value of clinical testing for anti-C1q autoantibodies in several clinical conditions.