Acute presentation and persistent glomerulonephritis following streptococcal infection in a patient with heterozygous complement factor H-related protein 5 deficiency

Diagnostic Challenges and Emerging Pathogeneses of Selected Glomerulopathies

Recent progress in glomerular immune complex and complement-mediated diseases have refined diagnostic categories and informed mechanistic understanding of disease development in pediatric patients. Herein, we discuss selected advances in 3 categories. First, membranous nephropathy antigens are increasingly utilized to characterize disease in pediatric patients and include phospholipase A2 receptor (PLA2R), Semaphorin 3B (Sema3B), neural epidermal growth factor-like 1 (NELL1), and protocadherin FAT1, as well as the lupus membranous-associated antigens exostosin 1/2 (EXT1/2), neural cell adhesion molecule 1 (NCAM1), and transforming growth factor beta receptor 3 (TGFBR3). Second, we examine advances in techniques for paraffin and light chain immunofluorescence (IF), including the former's function as a salvage technique and their necessity for diagnosis in adolescent cases of membranous-like glomerulopathy with masked IgG kappa deposits (MGMID) and proliferative glomerulonephritis with monotypic Ig deposits (PGNMID), respectively. Finally, progress in understanding the roles of complement in pediatric glomerular disease is reviewed, with specific attention to overlapping clinical, histologic, and genetic or functional alternative complement pathway (AP) abnormalities among C3 glomerulopathy (C3G), infection-related and post-infectious GN, "atypical" post-infectious GN, immune complex mediated membranoproliferative glomerulonephritis (IC-MPGN), and atypical hemolytic uremic syndrome (aHUS).

Apoptosis in post-streptococcal glomerulonephritis and mechanisms for failed of inflammation resolution

Post-streptococcal glomerulonephritis is a condition resulting from infection by group A beta-hemolytic streptococcus. The main mechanism involves the formation of immune complexes formed in the circulation or in situ on the glomerular basement membrane, which activates complement and causes various inflammatory processes. Cellular mechanisms have been reported in the induction of kidney damage represented by the infiltration of innate cells (neutrophils and monocyte/macrophages) and adaptive cells (CD4 + lymphocytes and CD8 + lymphocytes) of the immune system. These cells induce kidney damage through various mechanisms. It has been reported that nephritogenic antigens are capable of inducing inflammatory processes early, even before the formation of immune complexes. Usually, this disease progresses towards clinical and renal normalization; however, in a smaller number of patients, it evolves into chronicity and persistent kidney damage. Hypotheses have been proposed regarding the mechanisms underlying this progression to chronicity including failure to induce apoptosis and failure to phagocytose apoptotic cells, allowing these cells to undergo membrane permeabilization and release pro-inflammatory molecules into the environment, thereby perpetuating renal inflammation. Other mechanisms involved include persistent infection, genetic background of the host's complement system, tubulointerstitial changes, and pre-existing kidney damage due to old age and comorbidities.

Evaluation of the significance of complement-related genes mutations in atypical postinfectious glomerulonephritis: a pilot study

BACKGROUND

Postinfectious glomerulonephritis with C3-dominant glomerular deposition (C3-PIGN) involves C3-dominant glomerular deposition without immunoglobulin. Atypical C3-PIGN involves persistent hypocomplementemia. We investigated the clinical features and explored complement-related gene mutations in atypical PIGN patients.

METHODS

We enrolled atypical C3-PIGN patients and collected data regarding the clinical presentation and pathological characteristics and follow-up data. We measured the levels of complement associated antibodies and performed whole-exome sequencing (WES) to detect mutations in complement-related genes.

RESULTS

The analysis included six atypical C3-PIGN patients. All patients were antistreptolysin-O (ASO) positive. All patients had varying degrees of hematuria, and four patients had proteinuria. None of the patients were positive for complement-related antibodies. All patients possessed mutations of genes related to the complement pathway, including alternative complement pathway genes-CFI, CFH, CFHR3, CFHR5; the lectin pathway gene-MASP2; and the common complement pathway gene-C8A. The rare variant of CFHR3 has been reported in C3 glomerulonephritis. During 56-73 months of follow-up, the levels of urine markers in three patients recovered within 6 months, and the remaining patients had abnormal urine test results over 12 months. Patients who received glucocorticoid therapy recovered faster.

CONCLUSIONS

Our study suggested that complement-related gene mutations may be an important cause of persistent hypocomplementemia in atypical C3-PIGN patients. In addition to variations in alternate pathway-related genes, we also found variations in lectin pathway-related genes, especially MASP2 genes. Although the overall prognosis was good, atypical C3-PIGN patients exhibited a longer period for recovery. Our results suggested that atypical C3-PIGN patients should receive more medical attention and need testing for mutations in complement-related genes.

Targeted therapy in glomerular diseases

Targeted therapy has emerged as a more precise approach to treat glomerular diseases, focusing on specific molecular or cellular processes that contribute to disease development or progression. This approach complements or replaces traditional immunosuppressive therapy, optimizes supportive care, and provides a more personalized treatment strategy. In this review, we summarize the evolving understanding of pathogenic mechanisms in immune-mediated glomerular diseases and the developing targeted therapies based on these mechanisms. We begin by discussing pan-B-cell depletion, anti-CD20 rituximab, and targeting B-cell survival signaling through the BAFF/APRIL pathway. We also exam specific plasma cell depletion with anti-CD38 antibody. We then shift our focus to complement activation in glomerular diseases, which is involved in antibody-mediated glomerular diseases, such as IgA nephropathy, membranous nephropathy, ANCA-associated vasculitis, and lupus nephritis. Non-antibody-mediated complement activation occurs in glomerular diseases, including C3 glomerulopathy, complement-mediated atypical hemolytic uremic syndrome, and focal segmental glomerulosclerosis. We discuss specific inhibition of terminal, lectin, and alternative pathways in different glomerular diseases. Finally, we summarize current clinical trials targeting the final pathways of various glomerular diseases, including kidney fibrosis. We conclude that targeted therapy based on individualized pathogenesis should be the future of treating glomerular diseases.

Clinico-Pathogenic Similarities and Differences between Infection-Related Glomerulonephritis and C3 Glomerulopathy

Recently, the comprehensive concept of "infection-related glomerulonephritis (IRGN)" has replaced that of postinfectious glomerulonephritis (PIGN) because of the diverse infection patterns, epidemiology, clinical features, and pathogenesis. In addition to evidence of infection, hypocomplementemia particularly depresses serum complement 3 (C3), with endocapillary proliferative and exudative GN developing into membranoproliferative glomerulonephritis (MPGN); also, C3-dominant or co-dominant glomerular immunofluorescence staining is central for diagnosing IRGN. Moreover, nephritis-associated plasmin receptor (NAPlr), originally isolated from the cytoplasmic fraction of group A Streptococci, is vital as an essential inducer of C3-dominant glomerular injury and is a key diagnostic biomarker for IRGN. Meanwhile, "C3 glomerulopathy (C3G)", also showing a histological pattern of MPGN due to acquired or genetic dysregulation of the complement alternative pathway (AP), mimics C3-dominant IRGN. Initially, C3G was characterized by intensive "isolated C3" deposition on glomeruli. However, updated definitions allow for glomerular deposition of other complement factors or immunoglobulins if C3 positivity is dominant and at least two orders of magnitude greater than any other immunoreactant, which makes it challenging to quickly distinguish pathomorphological findings between IRGN and C3G. As for NAPlr, it was demonstrated to induce complement AP activation directly in vitro, and it aggravates glomerular injury in the development of IRGN. A recent report identified anti-factor B autoantibodies as a contributing factor for complement AP activation in pediatric patients with PIGN. Moreover, C3G with glomerular NAPlr deposition without evidence of infection was reported. Taken together, the clinico-pathogenic features of IRGN overlap considerably with those of C3G. In this review, similarities and differences between the two diseases are highlighted.

The Factor H protein family: The switchers of the complement alternative pathway

The factor H (FH) protein family is emerging as a complex network of proteins controlling the fate of the complement alternative pathway (AP) and dictating susceptibility to a wide range of diseases including infectious, inflammatory, autoimmune, and degenerative diseases and cancer. Composed, in man, of seven highly related proteins, FH, factor H-like 1, and 5 factor H-related proteins, some of the FH family proteins are devoted to down-regulating the AP, while others exert an opposite function by promoting AP activation. Recent findings have provided insights into the molecular mechanisms defining their biological roles and their pathogenicity, illustrating the relevance that the balance between the regulators and the activators within this protein family has in defining the outcome of complement activation on cell surfaces. In this review we will discuss the emerging roles of the factor H protein family, their impact in the complement cascade, and their involvement in the pathogenesis of complement-mediated diseases associated with the AP dysregulation.

Pathogenic Neisseria Bind the Complement Protein CFHR5 via Outer Membrane Porins

Neisseria meningitidis and Neisseria gonorrhoeae are important human pathogens that have evolved to bind the major negative regulator of the complement system, complement factor H (CFH). However, little is known about the interaction of pathogens with CFH-related proteins (CFHRs) which are structurally similar to CFH but lack the main complement regulatory domains found in CFH. Insights into the role of CFHRs have been hampered by a lack of specific reagents. We generated a panel of CFHR-specific monoclonal antibodies and demonstrated that CFHR5 was bound by both pathogenic Neisseria spp. We showed that CFHR5 bound to PorB expressed by both pathogens in the presence of sialylated lipopolysaccharide and enhanced complement activation on the surface of N. gonorrhoeae. Our study furthered our understanding of the interactions of CFHRs with bacterial pathogens and revealed that CFHR5 bound the meningococcus and gonococcus via similar mechanisms.

Evaluating the clinical utility of measuring levels of factor H and the related proteins

After years of disappointing clinical results, the tide has finally changed and complement targeted-therapies have become a validated and accepted treatment option for several diseases. These accomplishments have revitalized the field and brought renewed attention to the prospects that complement therapeutics can offer. Streamlining diagnostics and therapeutics is imperative in this new era of clinical use of complement therapeutics. However, the incredible success in therapeutics has not been accompanied by the development of novel standardized tools for complement testing. Complement biomarkers can assist in the risk assessment and diagnosis of diseases as well as the prediction of disease progression and treatment response. Recently, a group of complement proteins has been suggested to be highly relevant in various complement-associated disorders, namely the human factor H (FH) protein family. This family of closely related proteins consists of FH, FH-like protein 1, and five factor H-related proteins, and they have been linked to eye, kidney, infectious, vascular, and autoimmune diseases as well as cancer. The goal of this review is to provide a comprehensive overview of the available data on circulating levels of FH and its related proteins in different pathologies. In addition, we examined the current literature to determine the clinical utility of measuring levels of the FH protein family in health and disease. Finally, we discuss future steps that are needed to make their clinical translation a reality.

Genetic analysis of CFH and MCP in Egyptian patients with immune-complex proliferative glomerulonephritis

Glomerulonephritis (GN) is a complex disease with intricate underlying pathogenic mechanisms. The possible role of underlying complement dysregulation is not fully elucidated in some GN subsets, especially in the setting of autoimmunity or infection. In the current study, diagnosed cases of lupus nephritis (LN) and post-infectious GN (PIGN) were recruited for molecular genetic analysis and targeted next-generation DNA sequencing was performed for two main complement regulating genes: in the fluid phase; CFH, and on tissue surfaces; MCP. Three heterozygous pathogenic variants in CFH (Q172*, W701*, and W1096*) and one likely pathogenic heterozygous variant in MCP (C223R) have been identified in four of the studied LN cases. Additionally, among the several detected variants of uncertain significance, one novel variant (CFH:F614S) was identified in 74% of the studied LN cases and in 65% of the studied PIGN cases. This variant was detected for the first time in the Egyptian population. These findings suggest that subtle mutations may be present in complement regulating genes in patients with immune-complex mediated category of GN that may add to the disease pathogenesis. These findings also call for further studies to delineate the impact of these gene variants on the protein function, the disease course, and outcome.

Cardiac Tamponade-Associated Dense Deposit Disease: A Case Report and Review of the Literature

Pericardial effusion is an abnormal accumulation of fluid in the pericardial cavity. It can be associated with various cardiac and non-cardiac disorders. Dense deposit disease (DDD) is a rare kidney disease caused by uncontrolled activation of the alternative complement pathway. We are reporting a seven-year-old male child who was diagnosed to have DDD approved by renal biopsy and presented with shortness of breath, cough, and fever. Chest X-ray displayed cardiomegaly. Thereafter, echocardiography showed massive pericardial effusion and left ventricle compression with a risk for cardiac tamponade. He subsequently underwent pericardiocentesis with the removal of 450 ml of pericardial fluid. The patient's edema was not correlated with the described amount of drained pericardial fluid. To the best of our knowledge, this is the first reported case of significant pericardial effusion carrying the risk of cardiac tamponade associated with DDD. With this report, we would like to highlight the importance of cardiac assessment in patients with DDD, in particular those with nephrotic range proteinuria who present with cardiac symptoms and cardiomegaly.

Nephritis-Associated Plasmin Receptor (NAPlr): An Essential Inducer of C3-Dominant Glomerular Injury and a Potential Key Diagnostic Biomarker of Infection-Related Glomerulonephritis (IRGN)

Nephritis-associated plasmin receptor (NAPlr) was originally isolated from the cytoplasmic fraction of group A Streptococci, and was found to be the same molecule as streptococcal glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and plasmin receptor (Plr) on the basis of nucleotide and amino acid sequence homology. Its main functions include GAPDH activity, plasmin-binding capacity, and direct activation of the complement alternative pathway (A-P). Plasmin trapped by deposited NAPlr triggers the degradation of extracellular matrix proteins, such as glomerular basement membranes and mesangial matrix, and the accumulation of macrophages and neutrophils, leading to the induction of plasmin-related endocapillary glomerular inflammation. Deposited NAPlr at glomerular endocapillary site directly activates the complement A-P, and the endocapillary release of complement-related anaphylatoxins, C3a and C5a, amplify the in situ endocapillary glomerular inflammation. Subsequently, circulating and in situ-formed immune complexes participate in the glomerular injury resulting in NAPlr-mediated glomerulonephritis. The disease framework of infection-related glomerulonephritis (IRGN) has been further expanded. GAPDH of various bacteria other than Streptococci have been found to react with anti-NAPlr antibodies and to possess plasmin-binding activities, allowing glomerular NAPlr and plasmin activity to be utilized as key biomarkers of IRGN.

The Rational Use of Complement Inhibitors in Kidney Diseases

The development of complement inhibitors represented one of the major breakthroughs in clinical nephrology in the last decade. Complement inhibition has dramatically transformed the outcome of one of the most severe kidney diseases, the atypical hemolytic uremic syndrome (aHUS), a prototypic complement-mediated disorder. The availability of complement inhibitors has also opened new promising perspectives for the management of several other kidney diseases in which complement activation is involved to a variable extent. With the rapidly growing number of complement inhibitors tested in a rapidly increasing number of indications, a rational use of this innovative and expensive new therapeutic class has become crucial. The present review aims to summarize what we know, and what we still ignore, regarding complement activation and therapeutic inhibition in kidney diseases. It also provides some clues and elements of thoughts for a rational approach of complement modulation in kidney diseases.

Contribution of functional and quantitative genetic variants of Complement Factor H and Factor H-Related (FHR) proteins on renal pathology

The complement system is a first line of defence against infectious, tumoral or autoimmune processes, and it is constitutively regulated to avoid excessive or unspecific activation. Factor H (FH), a most relevant complement regulator, controls complement activation in plasma and on the cellular surfaces of autologous tissues. FH shares evolutionary origin and structural features with a group of plasma proteins known as FH-Related Proteins (FHRs), which could act as FH functional antagonists. Studies in patient cohorts of atypical Haemolytic-Uraemic Syndrome (aHUS), C3 Glomerulopathy (C3G), and IgA nephropathy (IgAN), have identified rare genetic variants that give rise to severe FH and FHRs dysfunctions, and are major genetic predisposing factors. These patients also have a higher frequency of a few polymorphisms whose relevance as disease risk factors is incompletely understood. In the last years, the availability of specific reagents has allowed a more precise quantitation of FH and FHRs in plasma samples from patients and controls. These studies have revealed that some aHUS, C3G or IgAN risk polymorphisms determine mild changes in FH or FHRs levels that could somehow perturb complement regulation and favour disease pathogenesis.

Thrombotic microangiopathy with transiently positive direct Coombs test in an adult with poststreptococcal acute glomerulonephritis: a case report

Background

To date, a few case reports have described the association between poststreptococcal acute glomerulonephritis (PSAGN) and hemolytic anemia/thrombocytopenia, both with or without a pathology similar to that of thrombotic microangiopathy (TMA). However, the detailed mechanism leading to the complication of TMA in PSAGN patients remains to be clarified. In contrast, infection with neuraminidase-producing Streptococcus pneumoniae is a well-known cause of TMA, and it has been reported that transient positivity of the direct Coombs test is observed in up to 90% of such patients.

Case presentation

A 44-year-old man was hospitalized for acute nephritic syndrome 3 weeks after developing pharyngitis. PSAGN was suspected owing to a low complement C3, increased antistreptolysin-O and serum creatinine (5.46 mg/dL), and hematuria/proteinuria. The throat antigen test for group A Streptococcus was positive. He developed hemolytic anemia with thrombocytopenia from hospital day 9. TMA was suspected owing to minimal coagulation abnormalities. ADAMTS-13 activity was normal, whereas the direct Coombs test was transiently positive. Renal biopsy demonstrated glomerular endocapillary proliferation without crescents, but with severe tubulitis and peritubular capillaritis on light microscopy. Immunofluorescence demonstrated C3 deposition along the glomerular capillary walls, and many subepithelial humps were observed on electron microscopy. The deposition of nephritis-associated plasmin receptor (NAPlr), a nephritogenic protein of Streptococcus pyogenes, was observed only in glomeruli. Thus, the histological diagnosis was typical PSAGN, but with atypical severe tubulointerstitial lesions. A positive direct Coombs test is often observed in pneumococcal TMA patients, which is attributed to the exposure of Thomsen–Friedenreich (T) antigen by neuraminidase. As Streptococcus pyogenes is one of the neuraminidase-producing bacteria other than Streptococcus pneumoniae, T-antigen exposure was analyzed in the renal tissue of this patient using labelled peanut lectin as a probe, which has strong and specific binding affinity for T-antigen. Exposure of T-antigen was found on tubular epithelial cells and small vessels in the tubulointerstitial area, but not in the glomeruli of this patient.

Conclusion

These findings suggest that 2 pathogenic proteins of Streptococcus pyogenes, i.e., NAPlr and neuraminidase, induced glomerular lesions of PSAGN and tubulointerstitial inflammation with TMA, respectively, resulting in severe acute kidney injury in this patient.

A case of C3 glomerulopathy with nephritis-associated plasmin receptor positivity without a history of streptococcal infection

A 17-year-old lady was diagnosed with proteinuria and microscopic hematuria within the prior 2 years to this presentation. Further tests revealed nephrotic syndrome with hematuria and hypocomplementemia. Renal histopathology showed membranoproliferative glomerulonephritis. Immunostaining and electron microscopy, suggested C3 glomerulonephritis (C3GN). Nephritis-associated plasmin receptor (NAPlr) and plasmin activity (PA), markers of infection-related glomerulonephritis, were identified in the aforementioned pathology specimens. There are several reports suggesting a causal relationship between group A streptococcal infection and C3 glomerulopathy (C3G). There are many intractable cases, whereas some have responded to immunosuppressive therapy, as did our case. However, there is currently no established gold standard treatment for this disease. We herein report a case of C3G with glomerular positive NAPlr and PA despite the absence of a streptococcal infection. Accumulation of cases such as this may help advance treatment by clarifying the etiology and pathogenic mechanism of C3G and future prognosis.

FHR-5 Serum Levels and CFHR5 Genetic Variations in Patients With Immune Complex-Mediated Membranoproliferative Glomerulonephritis and C3-Glomerulopathy

Background

Factor H-related protein 5 (FHR-5) is a member of the complement Factor H protein family. Due to the homology to Factor H, the main complement regulator of the alternative pathway, it may also be implicated in the pathomechanism of kidney diseases where Factor H and alternative pathway dysregulation play a role. Here, we report the first observational study on CFHR5 variations along with serum FHR-5 levels in immune complex-mediated membranoproliferative glomerulonephritis (IC-MPGN) and C3 glomerulopathy (C3G) patients together with the clinical, genetic, complement, and follow-up data.

Methods

A total of 120 patients with a histologically proven diagnosis of IC-MPGN/C3G were enrolled in the study. FHR-5 serum levels were measured in ELISA, the CFHR5 gene was analyzed by Sanger sequencing, and selected variants were studied as recombinant proteins in ELISA and surface plasmon resonance (SPR).

Results

Eight exonic CFHR5 variations in 14 patients (12.6%) were observed. Serum FHR-5 levels were lower in patients compared to controls. Low serum FHR-5 concentration at presentation associated with better renal survival during the follow-up period; furthermore, it showed clear association with signs of complement overactivation and clinically meaningful clusters.

Conclusions

Our observations raise the possibility that the FHR-5 protein plays a fine-tuning role in the pathogenesis of IC-MPGN/C3G.

Different Aspects of Classical Pathway Overactivation in Patients With C3 Glomerulopathy and Immune Complex-Mediated Membranoproliferative Glomerulonephritis

The rare and heterogeneous kidney disorder C3 glomerulopathy (C3G) is characterized by dysregulation of the alternative pathway (AP) of the complement system. C3G is often associated with autoantibodies stabilizing the AP C3 convertase named C3 nephritic factors (C3NeF). The role of classical pathway (CP) convertase stabilization in C3G and related diseases such as immune complex-mediated membranoproliferative glomerulonephritis (IC-MPGN) remains largely unknown. Here, we investigated the CP convertase activity in patients with C3G and IC-MPGN. Using a refined two-step hemolytic assay, we measured the stability of CP convertases directly in the serum of 52 patients and 17 healthy controls. In four patients, CP convertase activity was prolonged compared to healthy controls, i.e. the enzymatic complex was stabilized. In three patients (2 C3G, 1 IC-MPGN) the convertase stabilization was caused by immunoglobulins, indicating the presence of autoantibodies named C4 nephritic factors (C4NeFs). Importantly, the assay also enabled detection of non-immunoglobulin-mediated stabilization of the CP convertase in one patient with C3G. Prolonged CP convertase activity coincided with C3NeF activity in all patients and for up to 70 months of observation. Crucially, experiments with C3-depleted serum showed that C4NeFs stabilized the CP C3 convertase (C4bC2a), that does not contain C3NeF epitopes. All patients with prolonged CP convertase activity showed clear signs of complement activation, i.e. lowered C3 and C5 levels and elevated levels of C3d, C3bc, C3bBbP, and C5b-9. In conclusion, this work provides new insights into the diverse aspects and (non-)immunoglobulin nature of factors causing CP convertase overactivity in C3G/IC-MPGN.

Contribution of functional and quantitative genetic variants of Complement Factor H and Factor H-Related (FHR) proteins on renal pathology

The complement system is a first line of defence against infectious, tumoral or autoimmune processes, and it is constitutively regulated to avoid excessive or unspecific activation. Factor H (FH), a most relevant complement regulator, controls complement activation in plasma and on the cellular surfaces of autologous tissues. FH shares evolutionary origin and structural features with a group of plasma proteins known as FH-Related Proteins (FHRs), which could act as FH functional antagonists. Studies in patient cohorts of atypical Haemolytic-Uraemic Syndrome (aHUS), C3 Glomerulopathy (C3G), and IgA nephropathy (IgAN), have identified rare genetic variants that give rise to severe FH and FHRs dysfunctions, and are major genetic predisposing factors. These patients also have a higher frequency of a few polymorphisms whose relevance as disease risk factors is incompletely understood. In the last years, the availability of specific reagents has allowed a more precise quantitation of FH and FHRs in plasma samples from patients and controls. These studies have revealed that some aHUS, C3G or IgAN risk polymorphisms determine mild changes in FH or FHRs levels that could somehow perturb complement regulation and favour disease pathogenesis.

Latency, Anti-Bacterial Resistance Pattern, and Bacterial Infection-Related Glomerulonephritis

BACKGROUND AND OBJECTIVES

Bacterial infection-related GN occurs concurrent to or after known or unknown infections. It is important to understand the clinical implications of the bacterial isolates, antimicrobial resistance patterns, and effect of latency-based classification on kidney and patient outcomes.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

In total, 501 consecutive adults diagnosed with bacterial infection-related GN between 2005 and 2017 were included from a biopsy registry of 15,545 patients at a single center in South India, and follow-up data were collected from electronic medical records until December 2019. Latency was defined as time between resolution of infection and onset of GN, which was classified as parainfectious, peri-infectious, or postinfectious GN. Longitudinal kidney and patient outcomes were studied.

RESULTS

The mean age of the cohort was 40 (± 15) years, 6% were above 65 years, and 330 (66%) were men. Diabetes was present in 93 (19%) patients. Seventy percent (353 of 501) of patients had known infections, with the median latent period for parainfectious (115 of 353, 33%), peri-infectious (97 of 353, 27%), and postinfectious (141 of 353, 40%) GN being 0, 5 (4-7), and 15 (10-31) days, respectively. The most common predisposing organism was Streptococcus pyogenes (137 of 353, 39%). Drug-resistant nonstreptococcal bacteria were methicillin-resistant Staphylococcus aureus (25%, four of 16), extended-spectrum β-lactamases (20%, 12 of 59), and carbapenem-resistant organisms (10%, six of 59). Twenty of 22 (91%) of the drug-resistant organisms were isolated from the parainfectious group. The most common site of infection was skin in peri- (23 of 97, 24%) and postinfectious GN (61 of 141, 43%), and urinary tract in parainfectious GN (35 of 115, 30%). Of 321 patients with >3 months of follow-up, 48 (15%) developed kidney failure over a median period of 10 (2-37) months and 14 (4%) died. Parainfectious GN, eGFR<30 ml/min per 1.73 m², moderate-to-severe interstitial fibrosis and tubular atrophy, and nontreatment with renin-angiotensin system blockers were significant risk factors for progression to kidney failure by a Cox proportional-hazards model.

CONCLUSIONS

Along with clinical and histologic predictors, parainfectious GN caused predominantly by nonstreptococcal and drug-resistant bacterial infections was associated with poor kidney prognosis.

Eculizumab as a New Treatment for Severe Acute Post-infectious Glomerulonephritis: Two Case Reports

Acute post-infections glomerulonephritis (APIGN) is a frequent cause of glomerulonephritis and represents the most common cause of acute glomerulonephritis in children. It can evolve to severe acute renal failure and chronic kidney disease or even end-stage kidney disease. The precise pathophysiological mechanisms of APIGN are still incompletely understood. The implication of the alternative complement pathway and the potential benefits of C5 blockade have been recently highlighted, in particular in the presence of a C3 Nephritic Factor (C3Nef), anti-Factor B or H autoantibodies. We report two children with severe APIGN, successfully treated with eculizumab. The first patient presented a severe form of APIGN with advanced renal failure and anuria, associated with a decreased level of C3 and an increased level of soluble C5b-9, in the presence of a C3NeF autoantibody. The second case had a severe oliguric APIGN associated with low C3 level. Kidney biopsy confirmed the diagnosis of APIGN in both cases. Eculizumab allowed full renal function recovery and the avoidance of dialysis in both cases. In conclusion, the alternative and terminal complement pathways activation might be common in PIGN, and in severe cases, eculizumab might help.

Case Report: A Rare Truncating Variant of the CFHR5 Gene in IgA Nephropathy

IgA nephropathy (IgAN) is the most common primary glomerulonephritis worldwide. Despite appropriate therapy, 20–40% of affected-patients evolve toward end-stage kidney disease (ESKD). Mesangial IgA deposits are the hallmark of IgAN, and complement deposition (C3) seems to differentiate latent IgA mesangial deposits from active IgAN. Atypical hemolytic uremic syndrome (aHUS), another disease in which complement plays an important role, is caused by inherited or acquired deregulation of the alternative pathway (AP) of complement. A subgroup of IgAN shows thrombotic microangiopathy (TMA) lesions in kidney biopsies, the histological characteristic of aHUS. Genetic variants of complement Factor H (CFH), known to be present in aHUS, have been associated with rapidly progressive forms of IgAN and a clinical pattern of aHUS. Genome-wide association studies (GWAS) have confirmed that the 1q32 region, encoding for CFH and its related proteins, is an IgAN susceptibility locus. A 30 year-old man was admitted for seizures and malignant hypertension. The kidney biopsy showed IgAN associated with features of TMA. Despite five plasma exchanges, the patient remained dialysis-dependent, and ESKD was diagnosed. Functional and genetic complement analysis were performed. A monoallelic protein-truncating, likely loss-of-function variant was identified in the CFHR5 gene. Eculizumab is the treatment of aHUS. As it has been successfully used in a few cases of rapidly progressive IgAN, it was decided to administer eculizumab over a period of 12 months in addition to the usual immunosuppression for renal transplantation. After a follow-up of 3 years, there was no clinical disease recurrence. Systematic biologic and genetic screening of complement in individuals with IgAN might be useful to better delineate the role of the AP of complement in renal disease progression, and this may have therapeutic implications.

A Family Affair: Addressing the Challenges of Factor H and the Related Proteins

Inflammation is a common denominator of diseases. The complement system, an intrinsic part of the innate immune system, is a key driver of inflammation in numerous disorders. Recently, a family of proteins has been suggested to be of vital importance in conditions characterized by complement dysregulation: the human Factor H (FH) family. This group of proteins consists of FH, Factor H-like protein 1 and five Factor H-related proteins. The FH family has been linked to infectious, vascular, eye, kidney and autoimmune diseases. In contrast to FH, the functions of the other highly homologous proteins are largely unknown and, hence, their role in the different disease-specific pathogenic mechanisms remains elusive. In this perspective review, we address the major challenges ahead in this emerging area, including 1) the controversies about the functional roles of the FH protein family, 2) the discrepancies in quantification of the FH protein family, 3) the unmet needs for validated tools and 4) limitations of animal models. Next, we also discuss the opportunities that exist for the immunology community. A strong multidisciplinary approach is required to solve these obstacles and is only possible through interdisciplinary collaboration between biologists, chemists, geneticists and physicians. We position this review in light of our own perspective, as principal investigators of the SciFiMed Consortium, a consortium aiming to create a comprehensive analytical system for the quantitative and functional assessment of the entire FH protein family.

Autoimmunity in Acute Poststreptococcal GN: A Neglected Aspect of the Disease

Acute poststreptococcal GN (APSGN) is the prototype of immune complex GN and is associated with manifestations of autoimmune reactivity that have been neglected as epiphenomena. Recently, studies have demonstrated transient antifactor B autoantibodies that activate the alternative complement pathway, bringing self-immunity to a central position in the pathogenesis of APSGN. Therefore, examining other manifestations of autoimmunity that have been reported in association with poststreptococcal GN is of interest. This article reviews the renal and extrarenal manifestations of autoimmune reactivity in APSGN and considers their potential relevance in modifying the usually benign clinical course of the disease. It also discusses related aspects of the nephritogenic antigens, complement activation, and genetic elements associated with immune reactivity and their potential relevance to the familial incidence of the disease.

Low factor H-related 5 levels contribute to infection-triggered haemolytic uraemic syndrome and membranoproliferative glomerulonephritis

Dysregulation of the alternative complement pathway is a major pathogenic mechanism in two rare renal diseases: atypical haemolytic uraemic syndrome (aHUS) and membranoproliferative glomerulonephritis (MPGN). We report on a 66-year-old male with chronic hepatitis C virus (HCV) infection and a combined liver–kidney transplant that was diagnosed with MPGN at the age of 63 years and a 5-year-old boy who presented with aHUS at the age of 21 months following a Streptococcus pneumoniae infection. Both patients carried similar frameshift variants in the complement CFHR5 gene that segregate with reduced levels of factor H–related 5 (FHR-5). We conclude that low FHR-5 levels may predispose to viral and bacterial infections that then trigger different renal phenotypes.

Factors Affecting the Progression of Infection-Related Glomerulonephritis to Chronic Kidney Disease

Acute glomerulonephritis (AGN) triggered by infection is still one of the major causes of acute kidney injury. During the previous two decades, there has been a major paradigm shift in the epidemiology of AGN. The incidence of poststreptococcal acute glomerulonephritis (PSAGN), which develops after the cure of group A Streptococcus infection in children has decreased, whereas adult AGN cases have been increasing, and those associated with nonstreptococcal infections, particularly infections by Staphylococcus, are now as common as PSAGN. In adult AGN patients, particularly older patients with comorbidities, infections are usually ongoing at the time when glomerulonephritis is diagnosed; thus, the term “infection-related glomerulonephritis (IRGN)” has recently been popularly used instead of “post-infectious AGN”. The prognosis of children with PSAGN is generally considered excellent compared with that of adult IRGN cases. However, long-term epidemiological analysis demonstrated that an episode of PSAGN in childhood is a strong risk factor for chronic kidney disease (CKD), even after the complete remission of PSAGN. Although the precise mechanism of the transition from IRGN to CKD remains unknown, its clarification is important as it will lead to the prevention of CKD. In this review, we therefore focus on the possible factors that may contribute to the progression of IRGN into CKD. Four factors, namely, persistent infection, genetic background of the host’s complement system, tubulointerstitial changes, and pre-existing histological damage, are discussed.

Interaction of the Factor H Family Proteins FHR-1 and FHR-5 With DNA and Dead Cells: Implications for the Regulation of Complement Activation and Opsonization

Complement plays an essential role in the opsonophagocytic clearance of apoptotic/necrotic cells. Dysregulation of this process may lead to inflammatory and autoimmune diseases. Factor H (FH), a major soluble complement inhibitor, binds to dead cells and inhibits excessive complement activation on their surface, preventing lysis, and the release of intracellular material, including DNA. The FH-related (FHR) proteins share common ligands with FH, due to their homology with this complement regulator, but they lack the domains that mediate the complement inhibitory activity of FH. Because their roles in complement regulation is controversial and incompletely understood, we studied the interaction of FHR-1 and FHR-5 with DNA and dead cells and investigated whether they influence the regulatory role of FH and the complement activation on DNA and dead cells. FH, FHR-1, and FHR-5 bound to both plasmid DNA and human genomic DNA, where both FHR proteins inhibited FH-DNA interaction. The FH cofactor activity was inhibited by FHR-1 and FHR-5 due to the reduced binding of FH to DNA in the presence of the FHRs. Both FHRs caused increased complement activation on DNA. FHR-1 and FHR-5 bound to late apoptotic and necrotic cells and recruited monomeric C-reactive protein and pentraxin 3, and vice versa. Interactions of the FHRs with pentraxins resulted in enhanced activation of both the classical and the alternative complement pathways on dead cells when exposed to human serum. Altogether, our results demonstrate that FHR-1 and FHR-5 are competitive inhibitors of FH on DNA; moreover, FHR-pentraxin interactions promote opsonization of dead cells.

Practical management of C3 glomerulopathy and Ig-mediated MPGN: facts and uncertainties

In recent years, a substantial body of experimental and clinical work has been devoted to C3 glomerulopathy and Ig-mediated membranoproliferative glomerulonephritis. Despite the rapid accumulation of data, several uncertainties about these 2 rare forms of nephropathies persist. They concern their pathophysiology, classification, clinical course, relevance of biomarkers and of pathology findings, and assessment of the efficacy of the available therapies. The present review discusses the impact of these uncertainties on the clinical management of patients.

Clinical and Pathophysiological Insights Into Immunological Mediated Glomerular Diseases in Childhood

The kidney is often the target of immune system dysregulation in the context of primary or systemic disease. In particular, the glomerulus represents the anatomical entity most frequently involved, generally as the expression of inflammatory cell invasion or circulant or in situ immune-complex deposition. Glomerulonephritis is the most common clinical and pathological manifestation of this involvement. There are no universally accepted classifications for glomerulonephritis. However, recent advances in our understanding of the pathophysiological mechanisms suggest the assessment of immunological features, biomarkers, and genetic analysis. At the same time, more accurate and targeted therapies have been developed. Data on pediatric glomerulonephritis are scarce and often derived from adult studies. In this review, we update the current understanding of the etiologic events and genetic factors involved in the pathogenesis of pediatric immunologically mediated primitive forms of glomerulonephritis, together with the clinical spectrum and prognosis. Possible new therapeutic targets are also briefly discussed.

CFHR Gene Variations Provide Insights in the Pathogenesis of the Kidney Diseases Atypical Hemolytic Uremic Syndrome and C3 Glomerulopathy

Sequence and copy number variations in the human CFHR-Factor H gene cluster comprising the complement genes CFHR1, CFHR2, CFHR3, CFHR4, CFHR5, and Factor H are linked to the human kidney diseases atypical hemolytic uremic syndrome (aHUS) and C3 glomerulopathy. Distinct genetic and chromosomal alterations, deletions, or duplications generate hybrid or mutant CFHR genes, as well as hybrid CFHR-Factor H genes, and alter the FHR and Factor H plasma repertoire. A clear association between the genetic modifications and the pathologic outcome is emerging: CFHR1, CFHR3, and Factor H gene alterations combined with intact CFHR2, CFHR4, and CFHR5 genes are reported in atypical hemolytic uremic syndrome. But alterations in each of the five CFHR genes in the context of an intact Factor H gene are described in C3 glomerulopathy. These genetic modifications influence complement function and the interplay of the five FHR proteins with each other and with Factor H. Understanding how mutant or hybrid FHR proteins, Factor H::FHR hybrid proteins, and altered Factor H, FHR plasma profiles cause pathology is of high interest for diagnosis and therapy.

Clinicopathological features of C3 glomerulopathy in children: a single-center experience

BACKGROUND

C3 glomerulopathy (C3G) is defined by dominant glomerular deposition of C3 and minimal or no immunoglobulin, with two subtypes-dense deposit disease (DDD) and C3 glomerulonephritis (C3GN)-distinguished by features on electron microscopy (EM). Given that this rare disease has generally unfavorable yet highly variable outcomes, we sought out to review the histopathology, complement/genetic studies, and renal outcomes of pediatric patients with C3G at our institution.

METHODS

All native kidney biopsies performed in a single pediatric hospital over a 10-year period were reviewed for features of C3G. Of 589 biopsy reports, we identified 9 patients fulfilling the diagnostic criteria for C3G and retrospectively reviewed their clinical chart and renal biopsy findings.

RESULTS

We identified 4 patients with DDD, 4 with C3GN, and 1 indeterminate case, with features of both C3GN and DDD. Five patients were positive for one or more nephritic factors (C3NeF, C4NeF, C5NeF) with 1 patient additionally positive for complement factor H (CFH) autoantibody. Genetic testing done in 5 of the 9 patients failed to identify any causative mutations. Three patients showed progressive renal dysfunction over a mean follow-up period of 33 months.

CONCLUSIONS

Complement and genetic studies are now routinely recommended for patients with a histopathological diagnosis of C3G. Careful interpretation of these studies and their prognostic and therapeutic implications in conjunction with biopsy findings is needed to further understand the pathophysiology of this rare disease in children.

Panel sequencing distinguishes monogenic forms of nephritis from nephrosis in children

BACKGROUND

Alport syndrome (AS) and atypical hemolytic-uremic syndrome (aHUS) are rare forms of chronic kidney disease (CKD) that can lead to a severe decline of renal function. Steroid-resistant nephrotic syndrome (SRNS) is more common than AS and aHUS and causes 10% of childhood-onset CKD. In recent years, multiple monogenic causes of AS, aHUS and SRNS have been identified, but their relative prevalence has yet to be studied together in a typical pediatric cohort of children with proteinuria and hematuria. We hypothesized that identification of causative mutations by whole exome sequencing (WES) in known monogenic nephritis and nephrosis genes would allow distinguishing nephritis from nephrosis in a typical pediatric group of patients with both proteinuria and hematuria at any level.

METHODS

We therefore conducted an exon sequencing (WES) analysis for 11 AS, aHUS and thrombotic thrombocytopenic purpura-causing genes in an international cohort of 371 patients from 362 families presenting with both proteinuria and hematuria before age 25 years. In parallel, we conducted either WES or high-throughput exon sequencing for 23 SRNS-causing genes in all patients.

RESULTS

We detected pathogenic mutations in 18 of the 34 genes analyzed, leading to a molecular diagnosis in 14.1% of families (51 of 362). Disease-causing mutations were detected in 3 AS-causing genes (4.7%), 3 aHUS-causing genes (1.4%) and 12 NS-causing genes (8.0%). We observed a much higher mutation detection rate for monogenic forms of CKD in consanguineous families (35.7% versus 10.1%).

CONCLUSIONS

We present the first estimate of relative frequency of inherited AS, aHUS and NS in a typical pediatric cohort with proteinuria and hematuria. Important therapeutic and preventative measures may result from mutational analysis in individuals with proteinuria and hematuria.

Outcome of C3 glomerulopathy patients: largest single-centre experience from South Asia

BACKGROUND

C3 glomerulopathy (C3G) is related to dysfunction of alternative complement pathway (ACP) because of its hyperactivation. Triggering factors and genetic profile are likely to be different in developing countries as compared to the Western world. Data regarding C3G from South Asian is scanty.

STUDY DESIGN

In the present study, 115 patients of C3G from 2012 to 2017 were analyzed. Clinical details were reviewed; serological levels of C3, C4, complement factor H or B and autoantibody testing was done by nephelometry/ELISA. Limited genetics workup for CFH and CFHR5 genes was done.

RESULTS

The prevalence of C3G was 1.52%. There was no difference in demographic and histopathologic profiles of C3G patients. Majority of patients had low functional assay and C3 levels. C3 nephritic factor was present in 47.5% of DDD and 38.6% of C3GN. Autoantibodies to CFH were present more often in the patients of C3GN (29.5%) than DDD (12.5%). Autoantibodies to CFB were equally common in both groups. Past history of infections was present in one-third patients and monoclonal paraproteins were present only in two patients. No pathogenic variants were noted in CFH/CFHR5 gene. On follow-up (3.2 + 1.6 years), complete and partial remission was achieved in one-fourth patients and 26% had resistance disease. About 40% progressed to ESRD and 18 underwent renal transplantation of which nine had a post-transplant recurrence.

CONCLUSIONS

Indian cohort had some differences in the immunological and genetic profile when compared to the Western literature; most significant was the absence of monoclonal immunoglobulins as a trigger for C3G.

Regulation of regulators: Role of the complement factor H-related proteins

The complement system, while being an essential and very efficient effector component of innate immunity, may cause damage to the host and result in various inflammatory, autoimmune and infectious diseases or cancer, when it is improperly activated or regulated. Factor H is a serum glycoprotein and the main regulator of the activity of the alternative complement pathway. Factor H, together with its splice variant factor H-like protein 1 (FHL-1), inhibits complement activation at the level of the central complement component C3 and beyond. In humans, there are also five factor H-related (FHR) proteins, whose function is poorly characterized. While data indicate complement inhibiting activity for some of the FHRs, there is increasing evidence that FHRs have an opposite role compared with factor H and FHL-1, namely, they enhance complement activation directly and also by competing with the regulators FH and FHL-1. This review summarizes the current stand and recent data on the roles of factor H family proteins in health and disease, with focus on the function of FHR proteins.

Complement dysregulation in glomerulonephritis

Glomerulonephritis (GN) refers to a group of renal diseases affecting the glomeruli due to the damage mediated by immunological mechanisms. A large proportion of the disease manifestations are caused by disturbances in the complement system. They can be due to genetic errors, autoimmunity, microbes or abnormal immunoglobulins, like modified IgA or paraproteins. The common denominator in most of the problems is an overactive or misdirected alternative pathway complement activation. An assessment of kidney function, amount of proteinuria and hematuria are crucial elements to evaluate, when glomerulonephritis is suspected. However, the cornerstones of the diagnoses are renal biopsy and careful examination of the complement abnormality. Differential diagnostics between the various forms of GN is not possible based on clinical features, as they may vary greatly. This review describes the known mechanisms of complement dysfunction leading to different forms of primary GN (like IgA glomerulonephritis, dense deposit disease, C3 glomerulonephritis, post-infectious GN, membranous GN) and differences to atypical hemolytic uremic syndrome. It also covers the basic elements of etiology-directed therapy and prognosis of the most common forms of GN. Common principles in the management of GN include treatment of hypertension and reduction of proteinuria, some require immunomodulating treatment. Complement inhibition is an emerging treatment option. A thorough understanding of the basic disease mechanism and a careful follow-up are needed for optimal therapy.

Autoimmune abnormalities of the alternative complement pathway in membranoproliferative glomerulonephritis and C3 glomerulopathy

Membranoproliferative glomerulonephritis (MPGN) is a rare chronic kidney disease associated with complement activation. Recent immunofluorescence-based classification distinguishes between immune complex (IC)-mediated MPGN, with glomerular IgG and C3 deposits, and C3 glomerulopathies (C3G), with predominant C3 deposits. Genetic and autoimmune abnormalities causing hyperactivation of the complement alternative pathway have been found as frequently in patients with immune complex-associated MPGN (IC-MPGN) as in those with C3G. In the last decade, there have been great advances in research into the autoimmune causes of IC-MPGN and C3G. The complement-activating autoantibodies called C3-nephritic factors (C3NeFs), which are present in 40-80% of patients, form a heterogeneous group of autoantibodies that stabilise the C3 convertase or the C5 convertase of the alternative pathway or both. A few patients, mainly with IC-MPGN, carry autoantibodies directed against the two components of the alternative pathway C3 convertase, factors B and C3b. Finally, autoantibodies against factor H, the main regulator of the alternative pathway, have been reported in a small proportion of patients with IC-MPGN or C3G. The identification of distinct pathogenetic patterns leading to kidney injury and of targets in the complement cascade may pave the way for tailored therapies for IC-MPGN and C3G, with specific complement inhibitors in the development pipeline.

C3-Glomerulopathy Autoantibodies Mediate Distinct Effects on Complement C3- and C5-Convertases

C3 glomerulopathy (C3G) is a severe kidney disease, which is caused by defective regulation of the alternative complement pathway. Disease pathogenesis is heterogeneous and is caused by both autoimmune and genetic factors. Here we characterized IgG autoantibodies derived from 33 patients with autoimmune C3 glomerulopathy. Serum antibodies from all 33 patients as well as purified IgGs bound to the in vitro assembled C3-convertase. Noteworthy, two groups of antibodies were identified: group 1 with strong (12 patients) and group 2 with weak binding C3-convertase autoantibodies (22 patients). C3Nef, as evaluated in a standard C3Nef assay, was identified in serum from 19 patients, which included patients from group 1 as well as group 2. The C3-convertase binding profile was independent of C3Nef. Group 1 antibodies, but not the group 2 antibodies stabilized the C3-convertase, and protected the enzyme from dissociation by Factor H. Also, only group 1 antibodies induced C3a release. However, both group 1 and group 2 autoantibodies bound to the C5-convertase and induced C5a generation, which was inhibited by monoclonal anti-C5 antibody Eculizumab in vitro. In summary, group 1 antibodies are composed of C3Nef and C5Nef antibodies and likely over-activate the complement system, as seen in hemolytic assays. Group 2 antibodies show predominantly C5Nef like activities and stabilize the C5 but not the C3-convertase. Altogether, these different profiles not only reveal a heterogeneity of the autoimmune forms of C3G (MPGN), they also show that in diagnosis of C3G not all autoimmune forms are identified and thus more vigorous autoantibody testing should be performed.

Chromosome 1q31.2q32.1 deletion in an adult male with intellectual disability, dysmorphic features and obesity

Intermediate interstitial deletions of the long arm of chromosome 1 are typically associated with developmental delay and dysmorphic features. We describe the case of a 31-year-old male with intellectual disability, obesity and dysmorphic features, in whom array-comparative genomic hybridization identified a de novo 9.55 Mb deletion at 1q31.2q32.1. We discuss the genes encompassed within the deleted region; in particular, the implications of the deleted cancer-predisposing gene, CDC-73, and compare our clinical findings to other cases with similar deletions. The absence of microcephaly and growth retardation appears to differentiate more proximal interstitial 1q deletions from intermediate 1q deletions, and the presence of obesity is a newly reported phenotype within the 1q deletion spectrum. It is imperative that surveillance for CDC-73 related disorders, including parathyroid carcinoma, is considered in the management of interstitial intermediate 1q deletions.

Reference Intervals of Factor H and Factor H-Related Proteins in Healthy Children

Complement is activated as part of the innate immune defense against invading pathogens. Also, it helps to remove apoptotic debris and immune complexes from the circulation. Impaired complement function due to aberrant plasma levels of complement proteins may be indicative for complement-mediated diseases or can be involved in susceptibility for infections. To determine whether plasma levels are abnormal, reference intervals (RIs) are used from adult healthy donors. Since many complement-mediated diseases have an onset during childhood, it is important to know whether these RIs can be extrapolated to children. RIs of Factor H (FH), the crucial fluid-phase regulator, and the FH-related proteins (FHRs), its homologous counterparts, are unknown in healthy children. While FH is measured to diagnose and monitor therapy of patients with atypical hemolytic uremic syndrome, recent studies also implicated increased plasma levels of FHRs in disease. Here, we investigated the levels of FH and FHRs in healthy children using recently developed specific ELISAs. We found that levels of FH, FHR-2, and FHR-3 were equal to those found in healthy adults. Levels of FHR-4A and FHR-5 were lower in children than in adults. However, only the FHR-5 levels associated with age. The RIs of these FH family proteins now serve to support the interpretation of plasma levels in prospective and retrospective studies that can be used for routine diagnostic and monitoring purposes including pediatric patient samples.

Self-Damage Caused by Dysregulation of the Complement Alternative Pathway: Relevance of the Factor H Protein Family

The alternative pathway is a continuously active surveillance arm of the complement system, and it can also enhance complement activation initiated by the classical and the lectin pathways. Various membrane-bound and plasma regulatory proteins control the activation of the potentially deleterious complement system. Among the regulators, the plasma glycoprotein factor H (FH) is the main inhibitor of the alternative pathway and its powerful amplification loop. FH belongs to a protein family that also includes FH-like protein 1 and five factor H-related (FHR-1 to FHR-5) proteins. Genetic variants and abnormal rearrangements involving the FH protein family have been linked to numerous systemic and organ-specific diseases, including age-related macular degeneration, and the renal pathologies atypical hemolytic uremic syndrome, C3 glomerulopathies, and IgA nephropathy. This review covers the known and recently emerged ligands and interactions of the human FH family proteins associated with disease and discuss the very recent experimental data that suggest FH-antagonistic and complement-activating functions for the FHR proteins.

C3 glomerulonephritis and thrombotic microangiopathy of renal allograft after pulmonary infection in a male with concomitant two complement factor I gene variations: a case report

BACKGROUND

It has been suggested that C3 glomerulonephritis (C3GN) and atypical hemolytic-uremic syndrome (a stereotypical phenotype of thrombotic microangiopathy), two rare entities caused by complement alternative pathway dysregulation share overlapping genetic origin and can be triggered by infections.

CASE PRESENTATION

We report a case of concomitant C3GN and thrombotic microangiopathy (TMA) after pulmonary infection in a young male receiving kidney transplantation. Genetic assessment revealed two missense variations in compound heterozygous form in CFI gene (complement factor I). These two variations are segregated with disease in the core family member of this patient. Plasma CFI levels of the patient and family members were all in normal range. We considered that these two variations only impair CFI function rather than its quantity in the serum.

CONCLUSION

Our case supports that C3GN and TMA shared overlapping genetic variations and might be triggered by infection in genetically susceptible patients after kidney transplantation.

Common and rare genetic variants of complement components in human disease

Genetic variability in the complement system and its association with disease has been known for more than 50 years, but only during the last decade have we begun to understand how this complement genetic variability contributes to the development of diseases. A number of reports have described important genotype-phenotype correlations that associate particular diseases with genetic variants altering specific aspects of the activation and regulation of the complement system. The detailed functional characterization of some of these genetic variants provided key insights into the pathogenic mechanisms underlying these pathologies, which is facilitating the design of specific anti-complement therapies. Importantly, these analyses have sometimes revealed unknown features of the complement proteins. As a whole, these advances have delineated the functional implications of genetic variability in the complement system, which supports the implementation of a precision medicine approach based on the complement genetic makeup of the patients. Here we provide an overview of rare complement variants and common polymorphisms associated with disease and discuss what we have learned from them.

Maternal and Fetal Outcomes of Pregnancies in Women with Atypical Hemolytic Uremic Syndrome

Atypical HUS (aHUS) is a disorder most commonly caused by inherited defects of the alternative pathway of complement, or the proteins that regulate this pathway, and life-threatening episodes of aHUS can be provoked by pregnancy. We retrospectively and prospectively investigated 27 maternal and fetal pregnancy outcomes in 14 women with aHUS from the Vienna Thrombotic Microangiopathy Cohort. Seven pregnancies (26%) were complicated by pregnancy-associated aHUS (p-aHUS), of which three appeared to be provoked by infection, bleeding, and curettage, and three individuals were considered to have preeclampsia/HELLP syndrome before the definitive diagnosis of p-aHUS was made. Mutations in genes that encode the complement alternative pathway proteins or the molecules that regulate this pathway were detected in 71% of the women, with no relationship to pregnancy outcome. Twenty-one pregnancies (78%) resulted in a live birth, two preterm infants were stillborn, and four pregnancies resulted in early spontaneous abortions. Although short-term renal outcome was good in most women, long-term renal outcome was poor; among the 14 women, four had CKD stage 1-4, five had received a renal allograft, and three were dialysis-dependent at study end. We prospectively followed nine pregnancies of four women and treated six of these pregnancies with prophylactic plasma infusions (one pregnancy resulted in p-aHUS, one intrauterine fetal death occurred, and seven pregancies were uneventful). Our study emphasizes the frequency of successful pregnancies in women with aHUS. Close monitoring of such pregnancies for episodes of thrombotic microangiopathy is essential but, the best strategy to prevent these episodes remains unclear.

Understanding AMD by analogy: systematic review of lipid-related common pathogenic mechanisms in AMD, AD, AS and GN

RATIONALE

Age-related macular degeneration (AMD) is one of the leading causes of blindness among the elderly. Due to its complex etiology, current treatments have been insufficient. Previous studies reveal three systems closely involved in AMD pathogenesis: lipid metabolism, oxidation and inflammation. These systems are also involved in Alzheimer's disease, atherosclerosis and glomerulonephritis. Understanding commonalities of these four diseases may provide insight into AMD etiology.

OBJECTIVES

To understand AMD pathogenesis by analogy and suggest ideas for future research, this study summarizes main commonalities in disease pathogenesis of AMD, Alzheimer's disease, atherosclerosis and glomerulonephritis.

METHODS

Articles were identified through PubMed, Ovid Medline and Google Scholar. We summarized the common findings and synthesized critical differences.

RESULTS

Oxidation, lipid deposition, complement activation, and macrophage recruitment are involved in all four diseases shown by genetic, molecular, animal and human studies. Shared genetic variations further strengthen their connection. Potential areas for future research are suggested throughout the review.

CONCLUSIONS

The four diseases share many steps of an overall framework of pathogenesis. Various oxidative sources cause oxidative stress. Oxidized lipids and related molecules accumulate and lead to complement activation, macrophage recruitment and pathology. Investigations that arise under this structure may aid us to better understand AMD pathology.

Factor H-Related (FHR)-1 and FHR-2 Form Homo- and Heterodimers, while FHR-5 Circulates Only As Homodimer in Human Plasma

The complement factor H-related (FHR) proteins are hypothesized to fine-tune the regulatory role of complement factor H (FH) in the alternative pathway of the complement system. Moreover, FHR-1, FHR-2, and FHR-5 have been proposed to be dimers, which further complicates accurate analysis. As FHRs are highly similar among themselves and toward FH, obtaining specific reagents for quantification of serum levels and functional analysis is challenging. In this study, we generated antibodies and developed ELISAs to measure FHR-1, FHR-2, and FHR-5 in serum. We used both recombinant and serum-derived proteins to show that four dimers occur in human circulation: homodimers of FHR-1, FHR-2, and FHR-5, as well as FHR-1/FHR-2 heterodimers. Heterodimers containing FHR-5 were not found. In individuals with homozygous CFHR1 deletions or compound heterozygous CFHR2 missense/nonsense mutations identified in this study, the respective FHR-1 and FHR-2 homo- and heterodimers were absent. Using FRET, we found that recombinant FHR dimers exchange monomers rapidly. This was confirmed ex vivo, using FHR-1- and FHR-2-deficient sera. Of all FHR dimers, FHR-5/5 homodimers demonstrated strong binding affinity toward heparin. Specific ELISAs demonstrated that serum levels of FHR-1/1, FHR-1/2, FHR-2/2, and FHR-5/5 dimers were low compared to FH, which circulates at a 10- to 200-fold molar excess. In summary, FHR-1, FHR-2, and FHR-5 homodimerize, with FHR-1 and FHR-2 forming heterodimers as well, and equilibrate quickly in plasma.

Rare genetic variant in the CFB gene presenting as atypical hemolytic uremic syndrome and immune complex diffuse membranoproliferative glomerulonephritis, with crescents, successfully treated with eculizumab

BACKGROUND

Complement factor B gene (CFB) is an important component of the alternate pathway of complement activation that provides an active subunit that associates with C3b to form the C3 convertase, which is an essential element in complement activation. Among the complement-associated disorders, mutations and pathogenic variants in the CFB gene are relatively rare phenomena. Moreover, mutated CFB affiliation with immune-complex diffuse membranoproliferative glomerulonephritis (IC-MPGN) and atypical hemolytic uremic syndrome (aHUS) are considered a highly rare occurrence.

CASE PRESENTATION

We describe the clinical presentation, course, and pathological findings in a 7-year-old boy who has confirmed CFB heterozygous variants with pathological features compatible with IC-MPGN. Mutational analysis revealed a heterozygous variant p.Glu566Arg in exon 13 of the CFB gene. The patient did not respond to steroids and mycophenolate mofetil (MMF) therapy but responded clinically and biochemically to eculizumab treatment. This is the first case report of CFB alteration associated with IC-MPGN and aHUS that was successfully treated with eculizumab.

CONCLUSIONS

Heterozygous variants in the CFB gene can be pathogenic and associated with IC-MPGN and aHUS. Early diagnosis and prompt management can be essential in preventing end-stage renal disease. Eculizumab may provide an effective modality of treatment.

C3 glomerulopathy

C3 glomerulopathy is a recently defined entity that encompasses a group of kidney diseases caused by abnormal control of complement activation with deposition of complement component C3 in glomeruli leading to variable glomerular inflammation. Before the recognition of the unique pathogenesis of these cases, they were variably classified according to their morphological features. C3 glomerulopathy accounts for roughly 1% of all renal biopsies. Clear definition of this entity has allowed a better understanding of its pathogenesis and clinical course and is likely to lead to the design of rational therapies over the next few years.