Glomerular membrane attack complex is not a reliable marker of ongoing C5 activation in lupus nephritis

Complement Terminal Pathway Activation and Intrarenal Immune Response in C3 Glomerulopathy

Key Points

Background

C3 glomerulopathy is a rare disease resulting from an overactivation of the complement alternative pathway. Although there is also evidence of terminal pathway activation, its occurrence and consequences on the disease have been poorly studied.

Methods

We retrospectively studied a cohort of 42 patients diagnosed with C3 glomerulopathy. We performed centralized extensive characterization of histological parameters. Kidney C5b-9 staining was performed as a marker of terminal pathway activation; intrarenal immune response was characterized through transcriptomic analysis.

Results

Eighty-eight percent of biopsies showed C5b-9 deposits in glomeruli. Biopsies were grouped according to the amount of C5b-9 deposits (no or low n=15/42, 36%; intermediate n=15/42, 36%; and high n=12/42, 28%). Patients with high C5b-9 deposits significantly differed from the two other groups of patients and were characterized by a significant higher histological chronicity score (P = 0.005) and lower outcome-free survival (P = 0.001). In multivariable analysis, higher glomerular C5b-9 remained associated with poor kidney prognosis after adjustment. One third of the 847 studied immune genes were upregulated in C3 glomerulopathy biopsies compared with controls. Unsupervised clustering on differentially expressed genes identified a group of kidney biopsies enriched in high glomerular C5b-9 with high immune and fibroblastic signature and showed high chronicity scores on histological examination.

Conclusions

In a cohort of patients with C3 glomerulopathy, intrarenal terminal pathway activation was associated with specific histological phenotype and disease prognosis.

Apical tubular complement activation and the loss of kidney function in proteinuric kidney diseases

Many kidney diseases are associated with proteinuria. Since proteinuria is independently associated with kidney function loss, anti-proteinuric medication, often in combination with dietary salt restriction, comprises a major cornerstone in the prevention of progressive kidney failure. Nevertheless, complete remission of proteinuria is very difficult to achieve, and most patients with persistent proteinuria slowly progress toward kidney failure. It is well-recognized that proteinuria leads to kidney inflammation and fibrosis via various mechanisms. Among others, complement activation at the apical side of the proximal tubular epithelial cells is suggested to play a crucial role as a cause of progressive loss of kidney function. However, hitherto limited attention is given to the pathophysiological role of tubular complement activation relative to glomerular complement activation. This review aims to summarize the evidence for tubular epithelial complement activation in proteinuric kidney diseases in relation to loss of kidney function.

Overlap of Scleroderma and Class V Lupus Nephritis: A Rare Autoimmune Confluence

The autoimmune connective tissue disease scleroderma is characterized by fibrosis of the skin, blood vessels, and visceral organs. Overlap syndromes are conditions in which a patient has characteristics from two or more autoimmune disorders. The coexistence of scleroderma and lupus nephritis is rare but documented. However, it is crucial to note that both scleroderma and lupus are complex autoimmune diseases with diverse clinical presentations and can affect multiple organ systems, including the kidneys. This case report presents a unique clinical scenario of a patient with the coexistence of scleroderma and systemic lupus erythematosus with Class V lupus nephritis, highlighting the challenges in diagnosis, treatment, and the need for a multidisciplinary approach.

Neutrophil extracellular traps drive lupus flares with acute skin and kidney inflammation triggered by ultraviolet irradiation

Sunlight triggers lupus flares causing both local skin and systemic inflammation, including lupus nephritis, through poorly understood mechanisms. To address this knowledge gap, we found that UVB irradiation of asymptomatic, young female lupus-prone mice induced skin and kidney inflammation with proteinuria, accompanied by neutrophil infiltration and neutrophil extracellular trap (NET) formation. Furthermore, UVB irradiation induced co-expression of CXCR4 and cytokines/C3 by neutrophils in vitro and in vivo, in the skin and kidneys of lupus-prone mice, indicating their transmigratory and pro-inflammatory potentials. A causality study demonstrated that inhibiting CXCR4 attenuated renal neutrophil infiltration, accumulation of NETs, NET-associated cytokines/C3, and proteinuria in UVB-irradiated lupus-prone mice. Remarkably, inhibiting NETosis through a novel strategy targeting nuclear envelope integrity reduced deposition of NET-associated cytokines/C3 in skin and kidneys, attenuating proteinuria in UVB-irradiated MRL/lpr·lmnB1 Tg mice. Our investigation unveils a new mechanism by which neutrophil NETs drive the early onset of lupus flares triggered by UVB-irradiation. Targeting neutrophil transmigration and NETosis could be promising therapeutic strategies.

Microvascular C5b-9 deposition in non-lesional skin in patients with SLE and its correlation with active lupus nephritis: a prospective observational study

OBJECTIVE

Tissue damage in lupus nephritis (LN) is mediated by activation of the classical complement pathway. Complement-mediated upregulation of endothelial cell adhesion molecules is seen in dermal blood vessels of non-lesional skin of patients with active lupus. In diseases with systemic complement activation, extensive microvascular C5b-9 deposition is seen in non-lesional skin. In this study, we assess the presence of systemic complement pathway activation as determined by non-lesional skin microvascular C5b-9 deposition in patients with LN.

METHODS

Eight patients with active LN and eight patients without active LN underwent non-lesional skin biopsies. Using a diaminobenzidine technique, specimens were evaluated for microvascular C5b-9 consistent with systemic complement pathway activation.

RESULTS

Five of eight patients with active LN and one of eight patients without active LN demonstrated positive C5b-9 staining in non-lesional skin (p=0.04). Positive non-lesional C5b-9 staining has greater specificity, 87.5%, for active LN than pyuria, low complements, elevated double-stranded DNA (dsDNA) and proteinuria. Urine protein creatinine ratio was significantly higher in patients with positive non-lesional C5b-9 deposition (5.18 vs 1.20; p=0.04). C5b-9 deposition was not associated with a higher NIH Activity Index, interstitial fibrosis, dsDNA or lower complements.

CONCLUSION

This is the first study to demonstrate evidence in non-lesional skin of microvascular C5b-9 indicative of systemic complement pathway activation in LN. C5b-9 deposition is statistically more common and demonstrated greater specificity than most historical biomarkers for active LN. The findings support a potential role for microvascular C5b-9 assessment in non-lesional skin as a biomarker for LN activity.

The role of complement in glomerulonephritis-are novel therapies ready for prime time?

The complement system plays a key pathogenic role in glomerular diseases with a diverse range of aetiologies, including C3 glomerulopathy, immunoglobulin A nephropathy, membranous nephropathy, ANCA-associated vasculitis and lupus nephritis. Several novel therapies targeting complement activity have recently been developed, which have now been approved or are in the late stages of clinical development. In this review, potential benefits and challenges of targeting the complement system in glomerular disease are discussed. We summarize current understanding of the role of complement, and the novel targeted therapies that are being developed for the treatment of glomerular disease.

Complement detection in kidney biopsies - utility and challenges

PURPOSE OF REVIEW

This review discusses the important role of staining for components of the complement cascade in both native and transplant kidney biopsies. The use of complement staining as a marker of prognosis, disease activity, and as a potential future tool in identifying patients who may benefit from complement-targeted therapies is discussed.

RECENT FINDINGS

While staining for C3, C1q and C4d can yield valuable information about complement activation in kidney biopsies, to adequately assess complement activation and potential therapeutic targets, expanded staining panels looking at multiple split products and complement regulatory proteins are needed. Recent progress has been made in identifying markers of disease severity in C3 glomerulonephritis and IgA nephropathy, such as Factor H-related Protein-5, which may serve as future tissue biomarkers. In the transplant setting, the limitation of relying on C4d staining to identify antibody mediated rejection is giving way to molecular diagnostics, including The Banff Human Organ Transplant (B-HOT) panel, which includes numerous complement complement-related transcripts, with the classical, lectin, alternative, and common pathways.

SUMMARY

Staining for complement components in kidney biopsies to understand how complement is activated in individual cases may help to identify patients who may benefit from complement-targeted therapies.

Lupus, antiphospholipid syndrome, and stroke: An attempt to crossmatch

Cerebrovascular accidents (CVAs) or strokes are part of the common thrombotic manifestations of Systemic Lupus Erythematosus (SLEs) and Antiphospholipid syndrome (APS). Such neurological thrombotic events tend to occur in patients with SLE at a higher frequency when Antiphospholipid antibodies (aPLs) are present, and tend to involve the large cerebral vessels. The mechanism of stroke in SLE can be driven by complement deposition and neuroinflammation involving the blood-brain barrier although the traditional cardiovascular risk factors remain major contributing factors. Primary prevention with antiplatelet therapy and disease activity controlling agent is the basis of the management. Anticoagulation via warfarin had been a tool for secondary prevention, especially in stroke recurrence, although the debate continues regarding the target international normalized ratio (INR). The presence of either of the three criteria antiphospholipid antibodies (aPLs) and certain non-criteria aPL can be an independent risk factor for stroke. The exact mechanism for the involvement of the large cerebral arteries, especially in lupus anticoagulant (LAC) positive cases, is still to be deciphered. The data on the role of non-criteria aPL remain very limited and heterogenous, but IgA antibodies against β2GPI and the D4/5 subunit as well as aPS/PT IgG might have a contribution. Anticoagulation with warfarin has been recommended although the optimal dosing or the utility of combination with antiplatelet agents is still unknown. Minimal data is available for direct oral anticoagulants (DOACs).

Measurement of Complement Activation via Plasma-Soluble C5b-9 Comparison with Terminal Complement Complex Staining in a Series of Kidney Biopsies

INTRODUCTION

With the emergence of therapeutic complement inhibitors, there is a need to identify patients with complement-driven inflammation. C5b-9 is the terminal product of the three complement pathways and therefore a marker of total complement activation. We present a pilot study which aims to assess whether plasma soluble C5b-9 (sC5b-9) correlates with terminal complement complex (TCC) staining in kidney tissue. The secondary aim was to assess the utility of plasma sC5b-9 as part of routine workup in kidney patients undergoing kidney biopsy.

METHODS

Thirty-seven patients undergoing kidney biopsy had plasma sC5b-9 and TCC staining on kidney tissue performed. Additional blood markers including creatinine, haemoglobin, CRP, factor H, factor I, and midkine levels were also taken. These parameters were correlated with the histological diagnoses. Patients were divided into a diseased group (n = 31) and a control group (n = 6) consisting of transplanted kidneys with minor or no changes. Of the biopsies in the control group, 50% were performed as per protocol, and the other 50% were performed due to clinical need.

RESULTS

There was no correlation found between plasma sC5b-9 and TCC kidney staining. Elevated sC5b-9 levels were found in a heterogeneous group of patients but were associated with higher CRP and lower haemoglobin levels. Overall, there was more TCC kidney staining in the diseased group compared with the control group, and a trend was observed of diabetic, primary membranous nephropathy, and amyloidosis patients having more intense glomerular and peritubular/interstitial staining.

CONCLUSION

Plasma sC5b-9 as a marker of total complement activation does not correlate with TCC kidney staining. This discordance suggests that plasma sC5b-9 and TCC staining are distinct markers of disease. TCC staining reflects chronicity and tissue deposition of complement over time. Conversely, plasma sC5b-9 concentrations change rapidly and reflect systemic complement activation. Complement activation was present in a heterogeneous group of kidney disease, indicating the underlying role of complement in many disorders.

Apoptosis in lupus nephritis patients: a study of Bcl-2 to assess glomerular and tubular damage

Background

Systemic lupus erythematosus (SLE) is an immune-mediated disease, due to exposure of self-antigens, through impairment of apoptosis and failure of lymphocytic tolerance. Impaired regulation of the pro- and anti-apoptotic gene products which coordinate programmed cell death may result in autoreactive B and T cells and autoimmunity. Genetically engineered mice that over-express the anti-apoptotic molecule Bcl-2, B cell lymphoma 2 (Bcl2) in B-lymphocytes advance a lupus-like illness. Lupus nephritis (LN) is one of the most serious manifestations of this autoimmune disorder. Glomerulonephritis (GN) is caused by either impaired regulation of apoptosis and/or clearance of apoptotic cells leading to a T cell-mediated autoimmune reaction with initiation of pathological immune complex deposits.

Objective

To evaluate the correlation between Bcl2 glomerular and tubular expression and pathological findings and laboratory data in different types of SLE GN.

Results

Compared to the control group, patients with lupus nephritis have significantly higher glomerular, interstitial and tubular expression level (P value < 0.001). BCL2 expression was positively correlated with serum anti-ds-DNA, urine 24-h protein and with the chronicity index. All LN patients had significant glomerular, interstitial and tubular deposits of BCL2, P value < 0.001, P value 0.004, and P value 0.03, respectively.

Conclusion

The intrinsic pathway of apoptosis interferes not only with the pathogenesis of lupus glomerulonephritis but also interferes with the pathogenesis of tubulointerstitial lupus nephritis. tubulointerstitial lesions may not only be a result of glomerular injury but also a significant factor in lupus nephritis.

C3-dependent effector functions of complement

C3 is the central effector molecule of the complement system, mediating its multiple functions through different binding sites and their corresponding receptors. We will introduce the C3 forms (native C3, C3 [H₂ O], and intracellular C3), the C3 fragments C3a, C3b, iC3b, and C3dg/C3d, and the C3 expression sites. To highlight the important role that C3 plays in human biological processes, we will give an overview of the diseases linked to C3 deficiency and to uncontrolled C3 activation. Next, we will present a structural description of C3 activation and of the C3 fragments generated by complement regulation. We will proceed by describing the C3a interaction with the anaphylatoxin receptor, followed by the interactions of opsonins (C3b, iC3b, and C3dg/C3d) with complement receptors, divided into two groups: receptors bearing complement regulatory functions and the effector receptors without complement regulatory activity. We outline the molecular architecture of the receptors, their binding sites on the C3 activation fragments, the cells expressing them, the diversity of their functions, and recent advances. With this review, we aim to give an up-to-date analysis of the processes triggered by C3 activation fragments on different cell types in health and disease contexts.

A Histology-Guided Approach to the Management of Patients with Lupus Nephritis: Are We There Yet?

Renal involvement is a frequent complication of systemic lupus erythematosus (SLE). It occurs in up to two-thirds of patients, often early during the disease course, and is the most important predictor of the morbidity and mortality of SLE patients. Despite tremendous improvements in the approach of the lupus nephritis (LN) therapy, including the recent approval of two new disease-modifying therapies, up to 50% of patients do not obtain a renal response and up to 25% will eventually progress to end-stage renal disease (ESRD) within 10 years of diagnosis. Given the lack of correlation between clinical features and histological lesions, there is an increasing need for a histology-guided approach to the management of patients with LN. Apart from the initial diagnosis of type and severity of renal injury in SLE, the concept of a repeat kidney biopsy (either in a for-cause or a per-protocol scenario) has begun to gain increasing popularity in the nephrology community. Herein, we will provide a comprehensive overview of the most important areas of utility of the kidney biopsy in patients with LN.

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.

Urinary C5b-9 as a Prognostic Marker in IgA Nephropathy

C5b-9 plays an important role in the pathogenesis of immunoglobin A nephropathy (IgAN). We evaluated C5b-9 as a prognostic marker for IgAN. We prospectively enrolled 33 patients with biopsy-proven IgAN. We analyzed the correlation between baseline urinary C5b-9 levels, posttreatment changes in their levels, and clinical outcomes, including changes in proteinuria, estimated glomerular filtration rate (eGFR), and treatment response. Baseline urinary C5b-9 levels were positively correlated with proteinuria (r = 0.548, p = 0.001) at the time of diagnosis. Changes in urinary C5b-9 levels were positively correlated with changes in proteinuria (r = 0.644, p < 0.001) and inversely correlated with changes in eGFR (r = −0.410, p = 0.018) at 6 months after treatment. Changes in urinary C5b-9 levels were positively correlated with time-averaged proteinuria during the follow-up period (r= 0.461, p = 0.007) but were not correlated with the mean annual rate of eGFR decline (r = −0.282, p = 0.112). Baseline urinary C5b-9 levels were not a significant independent factor that could predict the treatment response in logistic regression analyses (odds ratio 0.997; 95% confidence interval, 0.993 to 1.000; p = 0.078). Currently, urinary C5b-9 is not a promising prognostic biomarker for IgAN, and further studies are needed.

Membrane attack complex (MAC) deposition in renal tubules is associated with interstitial fibrosis and tubular atrophy: a pilot study

INTRODUCTION

Treatment failures for lupus nephritis (LN) are high with 10%-30% of patients progressing to end-stage renal disease (ESRD) within 10 years. Interstitial fibrosis/tubular atrophy (IFTA) is a predictor of progression to ESRD. Prior studies suggest that tubulointerstitial injury secondary to proteinuria in LN is mediated by complement activation in the tubules, specifically through the membrane attack complex (MAC). This study aimed to investigate the associations between tubular MAC deposition with IFTA and proteinuria.

METHODS

In this cross-sectional study, LN kidney biopsies were assessed for MAC deposition by staining for Complement C9, a component of the MAC. Chromogenic immunohistochemistry was performed on paraffin-embedded human renal biopsy sections using unconjugated, murine anti-human Complement C9 (Hycult Biotech, clone X197). Tubular C9 staining intensity was analysed as present versus absent. IFTA was defined as minimal (<10%), mild (10%-24%), moderate (25%-50%) and severe (>50%).

RESULTS

Renal biopsies from 30 patients with LN were studied. There were 24 (80%) female sex, mean age (SD) was 33 (12) years old and 23 (77%) had pure/mixed proliferative LN. Tubular C9 staining was present in 7 (23%) biopsies. 27 patients had minimal-to-mild IFTA and 3 patients had moderate IFTA. Among the C9 + patients, 3 (43%) had moderate IFTA as compared with none in the C9- group, p=0.009. C9 + patients had higher median (IQR) proteinuria as compared with C9- patients: 6.2 g (3.3-13.1) vs 2.4 g (1.3-4.6), p=0.001 at the time of biopsy. There was no difference in estimated glomerular filtration rate (eGFR) between the C9 + and C9- groups.

CONCLUSION

This study demonstrated that tubular MAC deposition is associated with higher degree of IFTA and proteinuria, which are predictors of progression to ESRD. These results suggest that tubular MAC deposition may be useful in classification of LN. Understanding the role of complement in tubulointerstitial injury will also identify new avenues for LN treatment.

Natural antibody and complement activation characterize patients with idiopathic nephrotic syndrome

Focal segmental glomerulosclerosis (FSGS) and minimal change disease (MCD) are common forms of idiopathic nephrotic syndrome. The causes of these diseases are incompletely understood, but the response of patients to immunosuppressive therapies suggests that their pathogenesis is at least in part immune mediated. Preclinical and clinical research indicates that activation of the classical pathway of complement contributes to glomerular injury in FSGS. Glomerular IgM deposits are also prominent in some patients, raising the possibility that IgM is a trigger of classical pathway activation. In the present study, we examined the pattern of complement activation in the glomeruli and plasma of patients with nephrotic syndrome. We also tested whether patients with FSGS and MCD have elevated levels of natural IgM reactive with epitopes on glomerular endothelial cells and cardiolipin. We found evidence of classical pathway activation in patients with idiopathic nephrotic syndrome compared with healthy control subjects. We also detected higher levels of self-reactive IgM to both targets. Based on these results, IgM and classical pathway activation may contribute to disease pathogenesis in some patients with FSGS and MCD.NEW & NOTEWORTHY IgM is detected in biopsies from some patients with nephrotic syndrome, although this has been attributed to passive trapping of the protein. We found, however, that IgM colocalizes with complement activation fragments in some glomeruli. We also found that affected patients had higher levels of IgM reactive to glomerular endothelial cell epitopes. Thus, IgM activates the complement system in the glomeruli of some patients with nephrotic syndrome and may contribute to injury.

Improving Clinical Trials for Anticomplement Therapies in Complement-Mediated Glomerulopathies: Report of a Scientific Workshop Sponsored by the National Kidney Foundation

Blocking the complement system as a therapeutic strategy has been proposed for numerous glomerular diseases but presents myriad questions and challenges, not the least of which is demonstrating efficacy and safety. In light of these potential issues and because there are an increasing number of anticomplement therapy trials either planned or under way, the National Kidney Foundation facilitated an all-virtual scientific workshop entitled "Improving Clinical Trials for Anti-Complement Therapies in Complement-Mediated Glomerulopathies." Attended by patient representatives and experts in glomerular diseases, complement physiology, and clinical trial design, the aim of this workshop was to develop standards applicable for designing and conducting clinical trials for anticomplement therapies across a wide spectrum of complement-mediated glomerulopathies. Discussions focused on study design, participant risk assessment and mitigation, laboratory measurements and biomarkers to support these studies, and identification of optimal outcome measures to detect benefit, specifically for trials in complement-mediated diseases. This report summarizes the discussions from this workshop and outlines consensus recommendations.

Complement activation in IgA nephropathy

IgA nephropathy pathogenesis is incompletely understood, and this limits the development of disease-specific biomarkers and effective therapies. Evidence of complement activity in IgA nephropathy is well established. However, a growing body of research indicates complement activity is an important contributor to IgA nephropathy pathology. In particular, multiple associations have been identified between complement alternative, lectin and terminal pathway proteins and IgA nephropathy severity. Recently, we have also gained insight into possible mechanisms that could link glomerular IgA deposition, complement activity, glomerular inflammation and disease severity. Ongoing clinical trials of therapeutic complement inhibitors will provide insight into the importance of complement activity to IgA nephropathy pathogenesis. Further research into mechanisms of complement activity is essential to improving our understanding and management of patients with IgA nephropathy.

The Spectrum of C4d Deposition in Renal Biopsies of Lupus Nephritis Patients

Objectives

This study aimed to determine the prevalence and localization of complement factor C4d in renal biopsies from patients with lupus nephritis (LN), as well as its associations with the disease's clinico-pathological features. The correlation between arteriolar C4d deposition and renal microvascular lesions (RVLs) was further analyzed.

Methods

A total of 325 biopsy-proven LN patients were enrolled, and their clinico-pathological data were collected. C4d staining of renal biopsies was performed by immunohistochemistry. The associations between C4d deposition and the clinico-pathological features were further analyzed.

Results

C4d deposition was present in most (98.8%) renal specimens in our cohort. These deposits were localized in the glomeruli (98.2%), tubular basement membrane (TBM) (43.7%), arterioles (31.4%), and peritubular capillary (33.8%). Patients with TBM C4d staining had higher disease activity (measured with the Systemic Lupus Erythematous Disease Activity Index) and higher National Institutes of Health pathological activity and chronicity indices (all P < 0.01). Patients with arteriolar C4d deposition were more likely to develop RVLs (91.2%) compared to those with no arteriolar C4d deposition (78.0%; P = 0.004), especially with two or more types of RVLs (P < 0.001). During the mean follow-up of 55.8 months, arteriolar C4d was related to worse renal outcomes [hazard ration (HR): 2.074, 95% confidence interval (CI) 1.056-4.075, P = 0.034]. Multivariate Cox hazard analysis showed that co-deposition of arteriolar C4d and C3c was an independent risk factor (HR: 3.681, 95% CI 1.519-8.921, P = 0.004) for predicting renal outcomes.

Conclusions

C4d deposition was common in renal tissues from LN patients. TBM C4d deposition was related to the disease activity, and arteriolar C4d deposition was associated with RVLs and worse renal outcomes.

Complement and kidney disease, new insights

PURPOSE OF REVIEW

In this review, we discuss recent studies showing the importance of the complement pathway in kidney disease.

RECENT FINDINGS

Recent findings in C3 glomerulopathy (C3G) include: acute postinfectious glomerulonephritis is characterised by the presence of antifactor B antibodies; human leukocyte antigen type, but not rare complement gene variation, is associated with primary immunoglobulin-associated membranoproliferative GN and C3G. Immunohistochemistry in C3G shows that factor H related protein 5 (FHR5) is the most prevalent complement protein and correlates with kidney function. A multicentre study supported the use of mycophenolate mofetil (MMF) in C3G even after a propensity matching analysis. In immunoglobulin A nephropathy (IgAN) several studies have emphasised the importance of complement. Imbalances of circulating FH and FHR1 and FHR5, which interfere with the regulatory functions of FH, associate with IgAN. Immunohistochemistry has shown associations between glomerular FHR5 deposition and C3 activation; glomerular FHR5 associated with clinical markers of IgAN severity. Data also suggest the lectin complement pathway contributes to IgAN severity. We also discuss complement activation in thrombotic microangiopathy and other kidney diseases.

SUMMARY

Complement activity can be detected in a wide range of kidney diseases and this provides pathogenic insight and potential for therapy with the ongoing development of several drugs directed at complement activation.

Deposition of the Membrane Attack Complex in Healthy and Diseased Human Kidneys

The membrane attack complex-also known as C5b-9-is the end-product of the classical, lectin, and alternative complement pathways. It is thought to play an important role in the pathogenesis of various kidney diseases by causing cellular injury and tissue inflammation, resulting in sclerosis and fibrosis. These deleterious effects are, consequently, targeted in the development of novel therapies that inhibit the formation of C5b-9, such as eculizumab. To clarify how C5b-9 contributes to kidney disease and to predict which patients benefit from such therapy, knowledge on deposition of C5b-9 in the kidney is essential. Because immunohistochemical staining of C5b-9 has not been routinely conducted and never been compared across studies, we provide a review of studies on deposition of C5b-9 in healthy and diseased human kidneys. We describe techniques to stain deposits and compare the occurrence of deposits in healthy kidneys and in a wide spectrum of kidney diseases, including hypertensive nephropathy, diabetic nephropathy, membranous nephropathy, IgA nephropathy, lupus nephritis, C3 glomerulopathy, and thrombotic microangiopathies such as the atypical hemolytic uremic syndrome, vasculitis, interstitial nephritis, acute tubular necrosis, kidney tumors, and rejection of kidney transplants. We summarize how these deposits are related with other histological lesions and clinical characteristics. We evaluate the prognostic relevance of these deposits in the light of possible treatment with complement inhibitors.

Expanding the Role of Complement Therapies: The Case for Lupus Nephritis

The complement system is an innate immune surveillance network that provides defense against microorganisms and clearance of immune complexes and cellular debris and bridges innate and adaptive immunity. In the context of autoimmune disease, activation and dysregulation of complement can lead to uncontrolled inflammation and organ damage, especially to the kidney. Systemic lupus erythematosus (SLE) is characterized by loss of tolerance, autoantibody production, and immune complex deposition in tissues including the kidney, with inflammatory consequences. Effective clearance of immune complexes and cellular waste by early complement components protects against the development of lupus nephritis, while uncontrolled activation of complement, especially the alternative pathway, promotes kidney damage in SLE. Therefore, complement plays a dual role in the pathogenesis of lupus nephritis. Improved understanding of the contribution of the various complement pathways to the development of kidney disease in SLE has created an opportunity to target the complement system with novel therapies to improve outcomes in lupus nephritis. In this review, we explore the interactions between complement and the kidney in SLE and their implications for the treatment of lupus nephritis.

Renal deposits of complement factors as predictors of end-stage renal disease and death in patients with lupus nephritis

OBJECTIVE

Lupus nephritis (LN) increases the risks of end-stage renal disease (ESRD) and death, but these risks are difficult to estimate. Since complement factors play an essential role in the pathogenesis and are deposited in the kidneys as C1q and C3, we studied whether these deposits predict ESRD and death in patients with LN.

METHODS

We collected demographic, clinical and pathological data from 183 adult patients with LN classes II-V diagnosed with a first native kidney biopsy. Pathological data included the localization and intensity of immunofluorescence staining of C1q and C3. We obtained dates of incident ESRD and death from the United States Renal Data System and National Death Index, respectively, and evaluated survival curves and hazard ratios for ESRD and death as a composite outcome and as separate outcomes.

RESULTS

The presence and intensity of deposits of C1q and C3 in glomeruli, tubular walls and vascular walls differed between classes and were associated with known unfavourable prognostic factors, such as hypertension, hypoalbuminemia and hypocomplementemia. However, over a median follow-up of 7.5 years, their presence and intensity were associated with neither survival free of ESRD and death nor hazard ratios for ESRD and death.

CONCLUSION

Renal deposits of complement factors did not predict ESRD and death in patients with LN.

Complement activation is a crucial driver of acute kidney injury in rhabdomyolysis

Rhabdomyolysis is a life-threatening condition caused by skeletal muscle damage with acute kidney injury being the main complication dramatically worsening the prognosis. Specific treatment for rhabdomyolysis-induced acute kidney injury is lacking and the mechanisms of the injury are unclear. To clarify this, we studied intra-kidney complement activation (C3d and C5b-9 deposits) in tubules and vessels of patients and mice with rhabdomyolysis-induced acute kidney injury. The lectin complement pathway was found to be activated in the kidney, likely via an abnormal pattern of Fut2-dependent cell fucosylation, recognized by the pattern recognition molecule collectin-11 and this proceeded in a C4-independent, bypass manner. Concomitantly, myoglobin-derived heme activated the alternative pathway. Complement deposition and acute kidney injury were attenuated by pre-treatment with the heme scavenger hemopexin. This indicates that complement was activated in a unique double-trigger mechanism, via the alternative and lectin pathways. The direct pathological role of complement was demonstrated by the preservation of kidney function in C3 knockout mice after the induction of rhabdomyolysis. The transcriptomic signature for rhabdomyolysis-induced acute kidney injury included a strong inflammatory and apoptotic component, which were C3/complement-dependent, as they were normalized in C3 knockout mice. The intra-kidney macrophage population expressed a complement-sensitive phenotype, overexpressing CD11b and C5aR1. Thus, our results demonstrate a direct pathological role of heme and complement in rhabdomyolysis-induced acute kidney injury. Hence, heme scavenging and complement inhibition represent promising therapeutic strategies.

Complement activity is regulated in C3 glomerulopathy by IgG-factor H fusion proteins with and without properdin targeting domains

C3 glomerulopathy is characterized by accumulation of complement C3 within glomeruli. Causes include, but are not limited to, abnormalities in factor H, the major negative regulator of the complement alternative pathway. Factor H-deficient (Cfh^-/-) mice develop C3 glomerulopathy together with a reduction in plasma C3 levels. Using this model, we assessed the efficacy of two fusion proteins containing the factor H alternative pathway regulatory domains (FH_1-5) linked to either a non-targeting mouse immunoglobulin (IgG-FH_1-5) or to an anti-mouse properdin antibody (Anti-P-FH_1-5). Both proteins increased plasma C3 and reduced glomerular C3 deposition to an equivalent extent, suggesting that properdin-targeting was not required for FH_1-5 to alter C3 activation in either plasma or glomeruli. Following IgG-FH_1-5 administration, plasma C3 levels temporally correlated with changes in factor B levels whereas plasma C5 levels correlated with changes in plasma properdin levels. Notably, the increases in plasma C5 and properdin levels persisted for longer than the increases in C3 and factor B. In Cfh^-/- mice IgG-FH_1-5 reduced kidney injury during accelerated serum nephrotoxic nephritis. Thus, our data demonstrate that IgG-FH_1-5 restored circulating alternative pathway activity and reduced glomerular C3 deposition in Cfh^-/- mice and that plasma properdin levels are a sensitive marker of C5 convertase activity in factor H deficiency. The immunoglobulin conjugated FH_1-5 protein, through its comparatively long plasma half-life, may be a potential therapy for C3 glomerulopathy.

The Emerging Role of Renal Tubular Epithelial Cells in the Immunological Pathophysiology of Lupus Nephritis

Systemic lupus erythematosus (SLE) is a systemic, autoimmune disease that can involve virtually any organ of the body. Lupus nephritis (LN), the clinical manifestation of this disease in the kidney, is one of the most common and severe outcomes of SLE. Although a key pathological hallmark of LN is glomerular inflammation and damage, tubulointerstitial lesions have been recognized as an important component in the pathology of LN. Renal tubular epithelial cells are resident cells in the tubulointerstitium that have been shown to play crucial roles in various acute and chronic kidney diseases. In this context, recent progress has been made in examining the functional role of tubular epithelial cells in LN pathogenesis. This review summarizes recent advances in our understanding of renal tubular epithelial cells in LN, the potential role of tubular epithelial cells as biomarkers in the diagnosis, prognosis, and treatment of LN, and the future therapeutic potential of targeting the tubulointerstitium for the treatment of patients with LN.

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.

Lupus nephritis: challenges and progress

PURPOSE OF REVIEW

The management of lupus nephritis remains unsatisfactory due to insufficiently effective treatment regimens and the dearth of reliable predictors of disease onset or progression to guide individualized therapeutic decisions. This review summarizes new findings related to lupus nephritis over the last 18 months and discusses clinical needs that should be considered to advance trials of mechanism-based therapeutic strategies.

RECENT FINDINGS

Collaborative teams are addressing how to improve disease definitions and are developing predictive models for disease onset, disease response and risk of flare in individual patients. More attention is being paid to clinical trial design. Advanced technologic approaches are allowing the analysis of small amounts of human tissue and urine in unprecedented detail so as to discover new pathogenic mechanisms and identify disease biomarkers. Novel therapies continue to be tested in disease models and include new strategies to protect renal tissue from cell damage and fibrosis.

SUMMARY

The collaborative efforts of patients, clinical and translational researchers, the pharmaceutical industry and funding sources are needed to advance therapies for lupus nephritis. Specialized clinical centers can then deliver optimal and more personalized patient care that will improve patient outcomes.

Clinical promise of next-generation complement therapeutics

The complement system plays a key role in pathogen immunosurveillance and tissue homeostasis. However, subversion of its tight regulatory control can fuel a vicious cycle of inflammatory damage that exacerbates pathology. The clinical merit of targeting the complement system has been established for rare clinical disorders such as paroxysmal nocturnal haemoglobinuria and atypical haemolytic uraemic syndrome. Evidence from preclinical studies and human genome-wide analyses, supported by new molecular and structural insights, has revealed new pathomechanisms and unmet clinical needs that have thrust a new generation of complement inhibitors into clinical development for a variety of indications. This review critically discusses recent clinical milestones in complement drug discovery, providing an updated translational perspective that may guide optimal target selection and disease-tailored complement intervention.

Glomerular Complement Factor H-Related Protein 5 (FHR5) Is Highly Prevalent in C3 Glomerulopathy and Associated With Renal Impairment

Introduction

Therapeutic agents that target complement are increasingly available for glomerular diseases. However, the mechanisms linking glomerular complement deposition with inflammation and damage are incompletely understood. Complement factor H-related protein 5 (FHR5) interacts with complement C3 and is considered to promote activation. Circulating and glomerular FHR5 associates with IgA nephropathy and abnormal FHR5 associates with familial C3 glomerulopathy (C3G). We characterized glomerular FHR5 staining in C3G and assessed its relationships with histological features of glomerular injury and clinical outcome.

Methods

We developed FHR5 staining protocols for formalin-fixed paraffin-embedded (FFPE) renal tissue and applied them to surplus biopsy sections from a C3G cohort.

Results

Glomerular FHR5 was highly prevalent in native and transplant C3G and correlated with glomerular C3 and C5b-9 staining. Glomerular FHR5 staining correlated negatively with estimated glomerular filtration rate (eGFR) (P = 0.04, difference of medians 19.7 ml/min per 1.73 m²; 95% confidence interval [CI] 1.1-43.0) and positively with a membranoproliferative glomerulonephritis pattern at diagnostic biopsy (odds ratio 18; 95% CI 1.6-201; P = 0.049). Glomerular FHR5 staining intensity positively correlated with glomerular complement C3b/iC3b/C3c (Pearson's correlation coefficient [R] = 0.59; P = 0.0008), C3dg (R = 0.47; P = 0.02) and C5b9 (R = 0.44, P = 0.02).

Conclusions

Glomerular FHR5 is highly prevalent in C3G, interacts with glomerular C3, and is associated with markers of disease severity. Glomerular FHR5 likely exacerbates complement-mediated glomerular damage in C3G and its interaction with glomerular complement might be exploited to target complement therapeutic agents.