Overview on the role of complement-specific autoantibodies in diseases

Autoantibodies and therapeutic antibodies against complement factor H

The complement system is a crucial part of our immune defense as, upon recognition, it can kill pathogens fast and effectively. However, misguided complement activation could cause damage to host tissues. Therefore, a well-controlled regulation of the complement system is a necessity to prevent collateral damage. Regulation is achieved by several complement inhibitory proteins, acting at different levels of the complement system. One of these complement regulators is factor H, the main regulator of the alternative complement activation pathway. Factor H can regulate the complement system both in fluid-phase and on the host cell surface by, for example, acting as co-factor for factor I, inactivating C3b. The functional properties of factor H are located within different regions of the protein. Functional impairment of factor H, either because of genetic variants, competing proteins such as the factor H-related proteins and proteins from certain pathogens, but also the presence of autoantibodies will impact on complement activation. However, exact consequences are dependent on the region within factor H that is affected. Autoantibodies binding to factor H have been shown to inhibit several regulatory functions of factor H, which is observed in diseases such as membranoproliferative glomerulonephritis and atypical hemolytic uremic syndrome. As more recently the presence of anti-factor H autoantibodies has also been discovered in several other diseases, ranging from autoimmune diseases to cancer, this review provides an overview of the presence of factor H autoantibodies described in these diseases. Factor H autoantibodies are reported to have inhibitory, or enhancing, effects on factor H, depending on the epitopes that are recognized. Formal conclusions about the pathogenicity of the factor H autoantibodies in some of these diseases cannot be drawn yet. Importantly, understanding the binding and functional impact of anti-factor H (auto)antibodies will allow targeted interventions to diminish pathological consequences of anti-factor H autoantibodies but may also open up additional avenues for the use of anti-factor H antibodies as therapeutic agents.

Autoantibodies Against Factor B and Factor H Without Pathogenic Effects in a Patient with Immune Complex-Mediated Membranoproliferative Glomerulonephritis

Background: Membranoproliferative glomerulonephritis (MPGN) is an umbrella term for chronic disorders affecting the glomeruli. MPGN is often accompanied by the presence of autoantibodies against complement components. However, the actual pathogenic effects of such autoantibodies, if any, are rarely studied. In this work, we investigated the role of anti-complement autoantibodies in an IC-MPGN patient. Methods: The presence of autoantibodies, their binding site, isotype, and titer were analyzed in ELISA. Antibody-antigen complexes were detected in the patient's serum using Western blot. Autoantibodies were studied in functional assays to analyze their effects on C3 convertase, complement deposition, cofactor activity, C3b binding, and hemolysis. Results: We identified autoantibodies against factor B (FB) and factor H (FH) in the patient's serum. Both FB-, and FH-autoantibodies were of IgG2, IgG3, IgG4, and IgGκ, IgGλ isotypes. FB-autoantibodies bound to the Ba and the enzymatically active Bb part of FB. FH-autoantibodies bound to the N- and C-termini of FH and cross-reacted with FHL-1 and FHR-1 proteins. In vivo formed complexes of the autoantibodies with both FB and FH were detected in the IgG fraction isolated from the serum. The autoantibodies did not influence solid-phase C3 convertase assembly and its FH-mediated decay. The free autoantibodies had no effect on complement deposition and on FH cofactor activity but slightly reduced C3b binding to FH. The IgG fraction of the patient dose-dependently inhibited complement-mediated rabbit red blood cell lysis, and the free autoantibodies decreased the solid phase C3 convertase activity. Conclusions: This case highlights that FB- and FH-autoantibodies are not necessarily pathogenic in IC-MPGN.

Acquired drivers of C3 glomerulopathy

C3 glomerulopathy (C3G) is a group of heterogeneous ultrarare kidney diseases characterized by dysregulated activation of the complement alternative pathway (AP) leading to excessive C3 cleavage. Diagnosis relies on kidney biopsy showing predominant C3 deposition in the glomerular basement membrane, with electron microscopy differentiating between dense deposit disease (DDD) and C3 glomerulonephritis (C3GN). The main drivers of AP dysregulation in C3G are acquired rather than genetic and consist primarily of autoantibodies called nephritic factors (C3Nefs, C4Nefs and C5Nefs) that bind to and stabilize complement convertases, causing complement overactivation. Current therapies are largely supportive, and existing complement-targeting treatments, such as eculizumab, demonstrate limited efficacy. Challenges in studying C3G include variability in autoantibody detection and a lack of standardized assays, which complicates clinical interpretation. Comprehensive assessment involving autoantibody panels, complement biomarkers, functional assays and genetic testing provides a more complete understanding of disease dynamics; however, key knowledge gaps remain regarding Nef origins, mechanisms and their pathogenic role. In this review we discuss acquired drivers of C3G with an emphasis on C3Nefs and C5Nefs and suggest areas of interest that might benefit from future research.

Updates in atypical hemolytic syndrome

PURPOSE OF REVIEW

This review aims to summarize how scientific advances in complement biology have not only improved the diagnosis and management of aHUS but also continue to offer insights into the pathophysiology of complement-mediated disease that may be leveraged for future therapeutic developments.

RECENT FINDINGS

Updated information on the clinical and epidemiological features, pathophysiology, diagnosis, management, and potential for future therapeutic advancements in the treatment of aHUS are reviewed.

SUMMARY

aHUS is a rare but potentially life-threatening disease that requires prompt diagnosis and treatment as well as long-term management via a multidisciplinary team providing coordination of primary and specialty care as well as outreach and education for children and families affected by this life-long disease.

The ouroboros of autoimmunity

Human autoimmunity against elements conferring protective immunity can be symbolized by the 'ouroboros', a snake eating its own tail. Underlying infection is autoimmunity against three immunological targets: neutrophils, complement and cytokines. Autoantibodies against neutrophils can cause peripheral neutropenia underlying mild pyogenic bacterial infections. The pathogenic contribution of autoantibodies against molecules of the complement system is often unclear, but autoantibodies specific for C3 convertase can enhance its activity, lowering complement levels and underlying severe bacterial infections. Autoantibodies neutralizing granulocyte-macrophage colony-stimulating factor impair alveolar macrophages, thereby underlying pulmonary proteinosis and airborne infections, type I interferon viral diseases, type II interferon intra-macrophagic infections, interleukin-6 pyogenic bacterial diseases and interleukin-17A/F mucocutaneous candidiasis. Each of these five cytokine autoantibodies underlies a specific range of infectious diseases, phenocopying infections that occur in patients with the corresponding inborn errors. In this Review, we analyze this ouroboros of immunity against immunity and posit that it should be considered as a factor in patients with unexplained infection.

External quality assurance program for diagnostic complement laboratories: evaluation of the results of the past seven years

Introduction

The complement external quality assurance (EQA) program was first organized in 2010 by a group of researchers working in diagnostic complement laboratories. Starting in 2016, INSTAND e.V., a German, non-profit interdisciplinary scientific medical society dedicated to providing expert EQA programs for medical laboratories, started organizing the EQAs for complement diagnostic laboratories together with the same group of experienced scientists and doctors who also work as EQA experts. The aim of the current work is to provide descriptive analysis of the past seven years' complement EQA results and evaluate timeline changes in proficiency testing.

Methods

Each year, in March and October, blinded samples (normal, pathological) were sent to the participating diagnostic laboratories, where complement parameters were evaluated exactly as in daily routine samples. Since no reference method/target values exist for these parameters, and participants used different units for measurement, the reported results were compared to the stable mean (Algorithm A) of the participants using the same method/measurement units. A reported result was qualified as "passed" if it fell into the 30-50% evaluation/target range around the mean of reported results (depending on the given parameter).

Results

While the number of participating laboratories has increased in the past years (from around 120 to 347), the number of complement laboratories providing multiple determinations remained mostly unchanged (around 30 worldwide). C3, C4, C1-inhibitor antigen and activity determinations provided the best proficiency results, with >90% passing quotas in the past years, independent of the applied method. Determination of the functional activity of the three activation pathways was good in general, but results showed large variance, especially with the pathological samples. Complement factor C1q and regulators FH and FI are determined by only a few laboratories, with variable outcomes (in general in the 85-90% pass range). Activation products sC5b-9 and Bb were determined in 30 and 10 laboratories, respectively, with typical passing quotas in the 70-90% range, without a clear tendency over the past years.

Conclusion

With these accumulated data from the past seven years, it is now possible to assess sample-, method-, and evaluation related aspects to further improve proficiency testing and protocolize diagnostic complement determinations.

Autoantibodies against complement factor B in rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic inflammatory autoimmune disorder affecting the joints. Many patients carry anti-citrullinated protein autoantibodies (ACPA). Overactivation of the complement system seems to be part of the pathogenesis of RA, and autoantibodies against the pathway initiators C1q and MBL, and the regulator of the complement alternative pathway, factor H (FH), were previously reported. Our aim was to analyze the presence and role of autoantibodies against complement proteins in a Hungarian RA cohort. To this end, serum samples of 97 ACPA-positive RA patients and 117 healthy controls were analyzed for autoantibodies against FH, factor B (FB), C3b, C3-convertase (C3bBbP), C1q, MBL and factor I. In this cohort, we did not detect any patient with FH autoantibodies but detected C1q autoantibodies in four patients, MBL autoantibodies in two patients and FB autoantibodies in five patients. Since the latter autoantibodies were previously reported in patients with kidney diseases but not in RA, we set out to further characterize such FB autoantibodies. The isotypes of the analyzed autoantibodies were IgG2, IgG3, IgGκ, IgGλ and their binding site was localized in the Bb part of FB. We detected in vivo formed FB-autoanti-FB complexes by Western blot. The effect of the autoantibodies on the formation, activity and FH-mediated decay of the C3 convertase in solid phase convertase assays was determined. In order to investigate the effect of the autoantibodies on complement functions, hemolysis assays and fluid phase complement activation assays were performed. The autoantibodies partially inhibited the complement-mediated hemolysis of rabbit red blood cells, inhibited the activity of the solid phase C3-convertase and C3 and C5b-9 deposition on complement activating surfaces. In summary, in ACPA-positive RA patients we identified FB autoantibodies. The characterized FB autoantibodies did not enhance complement activation, rather, they had inhibitory effect on complement. These results support the involvement of the complement system in the pathomechanism of RA and raise the possibility that protective autoantibodies may be generated in some patients against the alternative pathway C3 convertase. However, further analyses are needed to assess the exact role of such autoantibodies.