Binding of factor H to tubular epithelial cells limits interstitial complement activation in ischemic injury

Targeting the Complement Pathway in Kidney Transplantation

The complement system is paramount in the clearance of pathogens and cell debris, yet is increasingly recognized as a key component in several pathways leading to allograft injury. There is thus a growing interest in new biomarkers to assess complement activation and guide tailored therapies after kidney transplantation (KTx). C5 blockade has revolutionized post-transplant management of atypical hemolytic uremic syndrome, a paradigm of complement-driven disease. Similarly, new drugs targeting the complement amplification loop hold much promise in the treatment and prevention of recurrence of C3 glomerulopathy. Although unduly activation of the complement pathway has been described after brain death and ischemia reperfusion, any clinical attempts to mitigate the ensuing renal insults have so far provided mixed results. However, the intervention timing, strategy, and type of complement blocker need to be optimized in these settings. Furthermore, the fast-moving field of ex vivo organ perfusion technology opens new avenues to deliver complement-targeted drugs to kidney allografts with limited iatrogenic risks. Complement plays also a key role in the pathogenesis of donor-specific ABO- and HLA-targeted alloantibodies. However, C5 blockade failed overall to improve outcomes in highly sensitized patients and prevent the progression to chronic antibody-mediated rejection (ABMR). Similarly, well-conducted studies with C1 inhibitors in sensitized recipients yielded disappointing results so far, in part, because of subtherapeutic dosage used in clinical studies. The emergence of new complement blockers raises hope to significantly reduce the negative effect of ischemia reperfusion, ABMR, and nephropathy recurrence on outcomes after KTx.

A Polymeric Nanosponge as a Broad-Spectrum Reactive Oxygen Species Scavenger for Acute Kidney Injury Treatment

Acute kidney injury (AKI) is closely associated with the overproduction of reactive oxygen species (ROS), which can cause multiple organ dysfunctions without timely treatment. However, only supportive treatments are currently available for AKI in clinics. Here, we developed nanomaterials of hyperbranched polyphosphoester (PPE) containing abundant thioether (S-PPE NP) and thioketal bonds (TK-PPE NP). Our data demonstrates that S-PPE NP exhibits an excellent capability of absorbing and scavenging multiple types of ROS, including H2O2, •OH, and •O2-, via thioether oxidation to sulfone or sulfoxide; it was also determined that S-PPE NP efficiently eliminates intracellular ROS, thus preventing cellular damage. Moreover, S-PPE NP was able to efficiently accumulate in the injured kidneys of AKI-bearing mice. As a result, the administration of S-PPE NP provided a superior therapeutic effect in AKI-bearing mice by downregulating ROS- and inflammation-related signaling pathways, thus reducing cell apoptosis. This thioether-containing polymer represents a promising broad-spectrum ROS scavenger that can be used for effective AKI treatments.

Complement activation and kidney transplantation; a complex relationship

Although kidney transplantation is the best treatment for end stage kidney disease, the benefits are limited by factors such as the short fall in donor numbers, the burden of immunosuppression and graft failure. Although there have been improvements in one-year outcomes, the annual rate of graft loss beyond the first year has not significantly improved, despite better therapies to control the alloimmune response. There is therefore a need to develop alternative strategies to limit kidney injury at all stages along the transplant pathway and so improve graft survival. Complement is primarily part of the innate immune system, but is also known to enhance the adaptive immune response. There is increasing evidence that complement activation occurs at many stages during transplantation and can have deleterious effects on graft outcome. Complement activation begins in the donor and occurs again on reperfusion following a period of ischemia. Complement can contribute to the development of the alloimmune response and may directly contribute to graft injury during acute and chronic allograft rejection. The complexity of the relationship between complement activation and allograft outcome is further increased by the capacity of the allograft to synthesise complement proteins, the contribution complement makes to interstitial fibrosis and complement's role in the development of recurrent disease. The better we understand the role played by complement in kidney transplant pathology the better placed we will be to intervene. This is particularly relevant with the rapid development of complement therapeutics which can now target different the different pathways of the complement system. Combining our basic understanding of complement biology with preclinical and observational data will allow the development and delivery of clinical trials which have best chance to identify any benefit of complement inhibition.

The susceptibility of the kidney to alternative pathway activation-A hypothesis

The glomerulus is often the prime target of dysregulated alternative pathway (AP) activation. In particular, AP activation is the key driver of two severe kidney diseases: atypical hemolytic uremic syndrome and C3 glomerulopathy. Both conditions are associated with a variety of predisposing molecular defects in AP regulation, such as genetic variants in complement regulators, autoantibodies targeting AP proteins, or autoantibodies that stabilize the AP convertases (C3- and C5-activating enzymes). It is noteworthy that these are systemic AP defects, yet in both diseases pathologic complement activation primarily affects the kidneys. In particular, AP activation is often limited to the glomerular capillaries. This tropism of AP-mediated inflammation for the glomerulus points to a unique interaction between AP proteins in plasma and this particular anatomic structure. In this review, we discuss the pre-clinical and clinical data linking the molecular causes of aberrant control of the AP with activation in the glomerulus, and the possible causes of this tropism. Based on these data, we propose a model for why the kidney is so uniquely and frequently targeted in patients with AP defects. Finally, we discuss possible strategies for preventing pathologic AP activation in the kidney.

Factor H related proteins modulate complement activation on kidney cells

Complement activation at a particular location is determined by the balance of activating and inhibitory proteins. Factor H is a key regulator of the alternative pathway of complement, and genetic or acquired impairments in Factor H are associated with glomerular injury. The human Factor H-related proteins (FHRs) comprise a family of five proteins that are structurally related to Factor H. Variations in the genes or expression levels of the FHRs are also associated with glomerular disease, although the mechanisms of glomerular protection/injury are incompletely understood. To explore the role of the FHRs on complement regulation/dysregulation in the kidney, we expressed and purified recombinant murine FHRs (FHRs A, B, C and E). These four distinct FHRs contain binding regions with high amino acid sequence homology to binding regions within Factor H, but we observed different interactions of the FHRs with Factor H binding ligands, including heparin and C3d. There was differential binding of the FHRs to the resident kidney cell types (mesangial, glomerular endothelial, podocytes, and tubular epithelial). All four FHRs caused complement dysregulation on kidney cell surfaces in vitro, although the magnitude of the effect differed among the FHRs and also varied among the different kidney cells. However, only FHR E caused glomerular complement dysregulation when injected in vivo but did not exacerbate injury when injected into mice with ischemic acute kidney injury, an alternative pathway-mediated model. Thus, our experiments demonstrate that the FHRs have unique, and likely context-dependent, effects on the different cell types within the kidney.

Extracellular vesicles derived from patients with antibody-mediated rejection induce tubular senescence and endothelial to mesenchymal transition in renal cells

Extracellular vesicles (EV) are emerging mediators in several diseases. However, their role in the pathophysiology of antibody-mediated allograft rejection (AMR) has been poorly investigated. Here, we investigated the role of EV isolated from AMR patients in inducing tubular senescence and endothelial to mesenchymal transition (EndMT) and analyzed their miRNA expression profile. By multiplex bead flow cytometry, we characterized the immunophenotype of plasma AMR-derived EV and found a prevalent platelet and endothelial cell origin. In vitro, AMR-derived EV induced tubular senescence by upregulating SA-β Gal and CDKN1A mRNA. Furthermore, AMR-derived EV induced EndMT. The occurrence of tubular senescence and EndMT was confirmed by analysis of renal biopsies from the same AMR patients. Moreover, AMR-derived EV induced C3 gene upregulation and CFH downregulation in tubular epithelial cells, with C4d deposition on endothelial cells. Interestingly, RNase-mediated digestion of EV cargo completely abrogated tubular senescence and EndMT. By microarray analysis, miR-604, miR-515-3p, miR-let-7d-5p, and miR-590-3p were significantly upregulated in EV from AMR group compared with transplant controls, whereas miR-24-3p and miR-29a-3p were downregulated. Therefore, EV-associated miRNA could act as active player in AMR pathogenesis, unraveling potential mechanisms of accelerated graft senescence, complement activation and early fibrosis that might lead to new therapeutic intervention.

Role of Complement System in Kidney Transplantation: Stepping From Animal Models to Clinical Application

Kidney transplantation is a life-saving strategy for patients with end-stage renal diseases. Despite the advances in surgical techniques and immunosuppressive agents, the long-term graft survival remains a challenge. Growing evidence has shown that the complement system, part of the innate immune response, is involved in kidney transplantation. Novel insights highlighted the role of the locally produced and intracellular complement components in the development of inflammation and the alloreactive response in the kidney allograft. In the current review, we provide the updated understanding of the complement system in kidney transplantation. We will discuss the involvement of the different complement components in kidney ischemia-reperfusion injury, delayed graft function, allograft rejection, and chronic allograft injury. We will also introduce the existing and upcoming attempts to improve allograft outcomes in animal models and in the clinical setting by targeting the complement system.

The role of properdin and Factor H in disease

The complement system consists of three pathways (alternative, classical, and lectin) that play a fundamental role in immunity and homeostasis. The multifunctional role of the complement system includes direct lysis of pathogens, tagging pathogens for phagocytosis, promotion of inflammatory responses to control infection, regulation of adaptive cellular immune responses, and removal of apoptotic/dead cells and immune complexes from circulation. A tight regulation of the complement system is essential to avoid unwanted complement-mediated damage to the host. This regulation is ensured by a set of proteins called complement regulatory proteins. Deficiencies or malfunction of these regulatory proteins may lead to pro-thrombotic hematological diseases, renal and ocular diseases, and autoimmune diseases, among others. This review focuses on the importance of two complement regulatory proteins of the alternative pathway, Factor H and properdin, and their role in human diseases with an emphasis on: (a) characterizing the main mechanism of action of Factor H and properdin in regulating the complement system and protecting the host from complement-mediated attack, (b) describing the dysregulation of the alternative pathway as a result of deficiencies, or mutations, in Factor H and properdin, (c) outlining the clinical findings, management and treatment of diseases associated with mutations and deficiencies in Factor H, and (d) defining the unwanted and inadequate functioning of properdin in disease, through a discussion of various experimental research findings utilizing in vitro, mouse and human models.

Renal diseases and the role of complement: Linking complement to immune effector pathways and therapeutics

The complement system is an ancient and phylogenetically conserved key danger sensing system that is critical for host defense against pathogens. Activation of the complement system is a vital component of innate immunity required for the detection and removal of pathogens. It is also a central orchestrator of adaptive immune responses and a constituent of normal tissue homeostasis. Once complement activation occurs, this system deposits indiscriminately on any cell surface in the vicinity and has the potential to cause unwanted and excessive tissue injury. Deposition of complement components is recognized as a hallmark of a variety of kidney diseases, where it is indeed associated with damage to the self. The provenance and the pathophysiological role(s) played by complement in each kidney disease is not fully understood. However, in recent years there has been a renaissance in the study of complement, with greater appreciation of its intracellular roles as a cell-intrinsic system and its interplay with immune effector pathways. This has been paired with a profusion of novel therapeutic agents antagonizing complement components, including approved inhibitors against complement components (C)1, C3, C5 and C5aR1. A number of clinical trials have investigated the use of these more targeted approaches for the management of kidney diseases. In this review we present and summarize the evidence for the roles of complement in kidney diseases and discuss the available clinical evidence for complement inhibition.

The importance of sialic acid, pH and ion concentration on the interaction of uromodulin and complement factor H

Uromodulin (UMOD) can bind complement factor H (cFH) and inhibit the activation of complement alternative pathway (AP) by enhancing the cofactor activity of cFH on degeneration of C3b. UMOD, an N-glycans-rich glycoprotein, is expressed in thick ascending limb of Henle's loop where the epithelia need to adapt to gradient change of pH and ion concentration. ELISA-based cofactor activity of cFH and erythrocytes haemolytic assay was used to measure the impact of native and de-glycosylated UMOD on the functions of cFH. The binding assay was performed under different pH and ion concentrations, using ELISA. The levels of sialic acid on UMOD, from healthy controls and patients with chronic kidney disease (CKD), were also detected by lectin-ELISA. It was shown that removal of glycans decreased the binding between UMOD and cFH and abolished the ability of enhancing C3b degradation. In acidic condition, the binding became stronger, but it reduced as sodium concentration increased. A significant decrease of α-2,3 sialic acids on UMOD was observed in CKD patients compared with that of healthy individuals. The sialic acids on UMOD, local pH and sodium concentration could impact the binding capacity between UMOD and cFH and thus regulate the activation of complement AP.

Role of C5aR1 and C5L2 Receptors in Ischemia-Reperfusion Injury

The role of C5a receptors (C5aR1 and C5L2) in renal ischemia-reperfusion injury (IRI) is uncertain. We generated an in vitro model of hypoxia/reoxygenation with human proximal tubule epithelial cells to mimic some IRI events. C5aR1, membrane attack complex (MAC) and factor H (FH) deposits were evaluated with immunofluorescence. Quantitative polymerase chain reaction evaluated the expression of C5aR1, C5L2 genes as well as genes related to tubular injury, inflammation, and profibrotic pathways. Additionally, C5aR1 and C5L2 deposits were evaluated in kidney graft biopsies (KB) from transplant patients with delayed graft function (DGF, n = 12) and compared with a control group (n = 8). We observed higher immunofluorescence expression of C5aR1, MAC and FH as higher expression of genes related to tubular injury, inflammatory and profibrotic pathways and of C5aR1 in the hypoxic cells; whereas, C5L2 gene expression was unaffected by the hypoxic stimulus. Regarding KB, C5aR1 was detected in the apical and basal membrane of tubular epithelial cells, whereas C5L2 deposits were observed in endothelial cells of peritubular capillaries (PTC). DGF-KB showed more frequently diffuse C5aR1 staining and C5L2 compared to controls. In conclusion, C5aR1 expression is increased by hypoxia and IRI, both in vitro and in human biopsies with an acute injury. C5L2 expression in PTC could be related to endothelial cell damage during IRI.

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.

Activation of final complement components after kidney transplantation as a marker of delayed graft function severity

Background

Ischaemia-reperfusion (I/R) damage is a relevant cause of delayed graft function (DGF). Complement activation is involved in experimental I/R injury, but few data are available from kidney transplant (KT) patients. We studied the dynamics of membrane attack complex (C5b-9) as a soluble fraction (SC5b-9) and the histological deposit pattern of C3b, complement Factor H (FH) and C5b-9 in DGF patients.

Methods

We evaluated SC5b-9 levels in 59 recipients: 38 with immediate graft function and 21 with DGF. The SC5b-9 was measured at admission for KT and 7 days after KT. DGF-kidney biopsies (n = 12) and a control group of 1-year protocol biopsies without tissue damage (n = 4) were stained for C5b-9, C3b and FH.

Results

SC5b-9 increased significantly in DGF patients (Day 0: 6621 ± 2202 mAU/L versus Day 7: 9626 ± 4142  mAU/L; P = 0.006), while it remained stable in non-DGF patients. Days 0-7 increase >5% was the better cut-off associated with DGF versus non-DGF patient discrimination (sensitivity = 81%). In addition, SC5b-9 increase was related to DGF duration and worse graft function, and independently associated with DGF occurrence. SC5b-9, C3b and FH stains were observed in tubular epithelial cells basal membrane. DGF-kidney biopsies showed a more frequently high-intensity stain, a higher number of tubules with positive stain and larger perimeter of tubules with positive stains for SC5b-9, C3b and FH than control patients.

Conclusions

Both SC5b-9 levels and SC5b-9, C3b and FH deposits in tubular epithelial cells basal membrane are highly expressed in patients experiencing DGF. SC5b-9 levels increase could be useful as a marker of DGF severity.

Circulating FH Protects Kidneys From Tubular Injury During Systemic Hemolysis

Intravascular hemolysis of any cause can induce acute kidney injury (AKI). Hemolysis-derived product heme activates the innate immune complement system and contributes to renal damage. Therefore, we explored the role of the master complement regulator Factor H (FH) in the kidney's resistance to hemolysis-mediated AKI. Acute systemic hemolysis was induced in mice lacking liver expression of FH (hepatoFH-/-, ~20% residual FH) and in WT controls, by phenylhydrazine injection. The impaired complement regulation in hepatoFH-/- mice resulted in a delayed but aggravated phenotype of hemolysis-related kidney injuries. Plasma urea as well as markers for tubular (NGAL, Kim-1) and vascular aggression peaked at day 1 in WT mice and normalized at day 2, while they increased more in hepatoFH-/- compared to the WT and still persisted at day 4. These were accompanied by exacerbated tubular dilatation and the appearance of tubular casts in the kidneys of hemolytic hepatoFH-/- mice. Complement activation in hemolytic mice occurred in the circulation and C3b/iC3b was deposited in glomeruli in both strains. Both genotypes presented with positive staining of FH in the glomeruli, but hepatoFH-/- mice had reduced staining in the tubular compartment. Despite the clear phenotype of tubular injury, no complement activation was detected in the tubulointerstitium of the phenylhydrazin-injected mice irrespective of the genotype. Nevertheless, phenylhydrazin triggered overexpression of C5aR1 in tubules, predominantly in hepatoFH-/- mice. Moreover, C5b-9 was deposited only in the glomeruli of the hemolytic hepatoFH-/- mice. Therefore, we hypothesize that C5a, generated in the glomeruli, could be filtered into the tubulointerstitium to activate C5aR1 expressed by tubular cells injured by hemolysis-derived products and will aggravate the tissue injury. Plasma-derived FH is critical for the tubular protection, since pre-treatment of the hemolytic hepatoFH-/- mice with purified FH attenuated the tubular injury. Worsening of acute tubular necrosis in the hepatoFH-/- mice was trigger-dependent, as it was also observed in LPS-induced septic AKI model but not in chemotherapy-induced AKI upon cisplatin injection. In conclusion, plasma FH plays a key role in protecting the kidneys, especially the tubules, against hemolysis-mediated injury. Thus, FH-based molecules might be explored as promising therapeutic agents in a context of AKI.

Donor Urinary C5a Levels Independently Correlate With Posttransplant Delayed Graft Function

BACKGROUND

Accumulating evidence implicates the complement cascade as pathogenically contributing to ischemia-reperfusion injury and delayed graft function (DGF) in human kidney transplant recipients. Building on observations that kidney injury can initiate in the donor before nephrectomy, we tested the hypothesis that anaphylatoxins C3a and C5a in donor urine before transplantation associate with risk of posttransplant injury.

METHODS

We evaluated the effects of C3a and C5a in donor urine on outcomes of 469 deceased donors and their corresponding 902 kidney recipients in a subset of a prospective cohort study.

RESULTS

We found a threefold increase of urinary C5a concentrations in donors with stage 2 and 3 acute kidney injury (AKI) compared donors without AKI (P < 0.001). Donor C5a was higher for the recipients with DGF (defined as dialysis in the first week posttransplant) compared with non-DGF (P = 0.002). In adjusted analyses, C5a remained independently associated with recipient DGF for donors without AKI (relative risk, 1.31; 95% confidence interval, 1.13-1.54). For donors with AKI, however, urinary C5a was not associated with DGF. We observed a trend toward better 12-month allograft function for kidneys from donors with C5a concentrations in the lowest tertile (P = 0.09). Urinary C3a was not associated with donor AKI, recipient DGF, or 12-month allograft function.

CONCLUSIONS

Urinary C5a correlates with the degree of donor AKI. In the absence of clinical donor AKI, donor urinary C5a concentrations associate with recipient DGF, providing a foundation for testing interventions aimed at preventing DGF within this high-risk patient subgroup.

Complement Inhibitor CRIg/FH Ameliorates Renal Ischemia Reperfusion Injury via Activation of PI3K/AKT Signaling

Complement activation is involved in the pathogenesis of ischemia reperfusion injury (IRI), which is an inevitable process during kidney transplantation. Therefore, complement-targeted therapeutics hold great potential in protecting the allografts from IRI. We observed universal deposition of C3d and membrane attack complex in human renal allografts with delayed graft function or biopsy-proved rejection, which confirmed the involvement of complement in IRI. Using FB-, C3-, C4-, C5-, C5aR1-, C5aR2-, and C6-deficient mice, we found that all components, except C5aR2 deficiency, significantly alleviated renal IRI to varying degrees. These gene deficiencies reduced local (deposition of C3d and membrane attack complex) and systemic (serum levels of C3a and C5a) complement activation, attenuated pathological damage, suppressed apoptosis, and restored the levels of multiple local cytokines (e.g., reduced IL-1β, IL-9, and IL-12p40 and increased IL-4, IL-5, IL-10, and IL-13) in various gene-deficient mice, which resulted in the eventual recovery of renal function. In addition, we demonstrated that CRIg/FH, which is a targeted complement inhibitor for the classical and primarily alternative pathways, exerted a robust renoprotective effect that was comparable to gene deficiency using similar mechanisms. Further, we revealed that PI3K/AKT activation, predominantly in glomeruli that was remarkably inhibited by IRI, played an essential role in the CRIg/FH renoprotective effect. The specific PI3K antagonist duvelisib almost completely abrogated AKT phosphorylation, thus abolishing the renoprotective role of CRIg/FH. Our findings suggested that complement activation at multiple stages induced renal IRI, and CRIg/FH and/or PI3K/AKT agonists may hold the potential in ameliorating renal IRI.

The mTORC1 signaling modulated by intracellular C3 activation in Paneth cells promotes intestinal epithelial regeneration during acute injury

Complement activation is associated with regional inflammation during acute gastrointestinal injury (AGI). This study is designed to explore how intracellular C3 activation in Paneth cells (PCs) affects regeneration of intestinal epithelium during AGI. AGI was induced in wildtype C57BL/6 mice, with sham operation employed as control. Exogenous C3 (1 mg, I.P.) was applied at 6 h post-surgery. Intestinal crypts harvested from ileum were cultured with presence or absence of C3 (20 μg/ml), with small interfering RNA against BST1 and complement activation inhibitor selectively applied in vitro. The intestinal integrity, percentage of PCs and intestinal stem cells (ISCs) were evaluated. Importantly, cADPR, C3 fragments, and S6-related proteins were detected in PCs to inspect the mammalian target of rapamycin complex 1 (mTORC1) signaling. AGI caused breakdown of intestinal mucosa integrity and regional inflammation. Exogenous C3 by itself failed to promote the growth of intestinal epithelium, but distinctly enhanced the activity of PCs via intracellular activation, which subsequently supported the expansion of ISCs inside of intestinal crypts. Inhibition of C3 activation was associated with decreased expressions of S6, S6K1 and cADPR, with blocking BST1 found to depress cADPR only. Collectively, these data confirmed intracellular activation of C3 in PCs enhanced expansion of ISCs in response to acute injury. The mTORC1 signaling pathway in PCs contributed to this crosstalk during exogenous C3 treatment.

Distribution of exogenous complement factor H in mice in vivo

Factor H is an important regulator of complement activation in plasma and on cell surfaces in both humans and mice. If FH function is compromised, inappropriate complement activation on self-surfaces can have disastrous effects as seen in the kidney diseases atypical haemolytic uremic syndrome (aHUS) and C3 glomerulopathy. As FH constructs have been proposed to be used in treatment for these diseases, we studied the distribution of exogenous FH fragments in mice. Full-length mFH, mFH1-5 and mFH18-20 fragments were radiolabelled, and their distribution was examined in WT, FH^-/- and FH^-/- C3^-/- mice in vivo. Whole body scintigraphy revealed accumulation of radioactivity in the abdominal part of the mice, but also to the thyroid gland and urinary bladder. At organ level in WT mice, some full-length FH accumulated in internal organs, but most of it remained in the circulation. Both of the mFH fragments accumulated in the kidneys and were excreted in urine. For mFH1-5, urinary secretion is the likely cause for the accumulation. Concentration of mFH18-20 to kidneys was slower, and at tissue level, mFH18-20 was localized at the proximal tubuli in WT and FH^-/- C3^-/- mice. No C3-independent binding to glomeruli was detected. In conclusion, these results show that glomerular glycosaminoglycans and sialic acids alone do not collect FH in kidneys. Deposition of C3 fragments is also needed, which implies that in aHUS, the problem is in simultaneous recognition of C3 fragments and glycosaminoglycans or sialic acids by FH, not just the inability of FH to recognize glomerular endothelium as such.

Complement factor H protects mice from ischemic acute kidney injury but is not critical for controlling complement activation by glomerular IgM

Natural IgM binds to glomerular epitopes in several progressive kidney diseases. Previous work has shown that IgM also binds within the glomerulus after ischemia/reperfusion (I/R) but does not fully activate the complement system. Factor H is a circulating complement regulatory protein, and congenital or acquired deficiency of factor H is a strong risk factor for several types of kidney disease. We hypothesized that factor H controls complement activation by IgM in the kidney after I/R, and that heterozygous factor H deficiency would permit IgM-mediated complement activation and injury at this location. We found that mice with targeted heterozygous deletion of the gene for factor H developed more severe kidney injury after I/R than wild-type controls, as expected, but that complement activation within the glomeruli remained well controlled. Furthermore, mice that are unable to generate soluble IgM were not protected from renal I/R, even in the setting of heterozygous factor H deficiency. These results demonstrate that factor H is important for limiting injury in the kidney after I/R, but it is not critical for controlling complement activation by immunoglobulin within the glomerulus in this setting. IgM binds to glomerular epitopes after I/R, but it is not a significant source of injury.

Modulation of the Alternative Pathway of Complement by Murine Factor H-Related Proteins

Factor H (FH) is a key alternative pathway regulator that controls complement activation both in the fluid phase and on specific cell surfaces, thus allowing the innate immune response to discriminate between self and foreign pathogens. However, the interrelationships between FH and a group of closely related molecules, designated the FH-related (FHR) proteins, are currently not well understood. Whereas some studies have suggested that human FHR proteins possess complement regulatory abilities, recent studies have shown that FHR proteins are potent deregulators. Furthermore, the roles of the FHR proteins have not been explored in any in vivo models of inflammatory disease. In this study, we report the cloning and expression of recombinant mouse FH and three FHR proteins (FHR proteins A-C). Results from functional assays show that FHR-A and FHR-B proteins antagonize the protective function of FH in sheep erythrocyte hemolytic assays and increase cell-surface C3b deposition on a mouse kidney proximal tubular cell line (TEC) and a human retinal pigment epithelial cell line (ARPE-19). We also report apparent KD values for the binding interaction of mouse C3d with mouse FH (3.85 μM), FHR-A (136 nM), FHR-B (546 nM), and FHR-C (1.04 μM), which directly correlate with results from functional assays. Collectively, our work suggests that similar to their human counterparts, a subset of mouse FHR proteins have an important modulatory role in complement activation. Further work is warranted to define the in vivo context-dependent roles of these proteins and determine whether FHR proteins are suitable therapeutic targets for the treatment of complement-driven diseases.

The Murine Factor H-Related Protein FHR-B Promotes Complement Activation

Factor H-related (FHR) proteins consist of varying number of complement control protein domains that display various degrees of sequence identity to respective domains of the alternative pathway complement inhibitor factor H (FH). While such FHR proteins are described in several species, only human FHRs were functionally investigated. Their biological role is still poorly understood and in part controversial. Recent studies on some of the human FHRs strongly suggest a role for FHRs in enhancing complement activation via competing with FH for binding to certain ligands and surfaces. The aim of the current study was the functional characterization of a murine FHR, FHR-B. To this end, FHR-B was expressed in recombinant form. Recombinant FHR-B bound to human C3b and was able to compete with human FH for C3b binding. FHR-B supported the assembly of functionally active C3bBb alternative pathway C3 convertase via its interaction with C3b. This activity was confirmed by demonstrating C3 activation in murine serum. In addition, FHR-B bound to murine pentraxin 3 (PTX3), and this interaction resulted in murine C3 fragment deposition due to enhanced complement activation in mouse serum. FHR-B also induced C3 deposition on C-reactive protein, the extracellular matrix (ECM) extract Matrigel, and endothelial cell-derived ECM when exposed to mouse serum. Moreover, mouse C3 deposition was strongly enhanced on necrotic Jurkat T cells and the mouse B cell line A20 by FHR-B. FHR-B also induced lysis of sheep erythrocytes when incubated in mouse serum with FHR-B added in excess. Altogether, these data demonstrate that, similar to human FHR-1 and FHR-5, mouse FHR-B modulates complement activity by promoting complement activation via interaction with C3b and via competition with murine FH.

Properdin: a tightly regulated critical inflammatory modulator

The complement alternative pathway is a powerful arm of the innate immune system that enhances diverse inflammatory responses in the human host. Key to the effects of the alternative pathway is properdin, a serum glycoprotein that can both initiate and positively regulate alternative pathway activity. Properdin is produced by many different leukocyte subsets and circulates as cyclic oligomers of monomeric subunits. While the formation of non‐physiological aggregates in purified properdin preparations and the presence of potential properdin inhibitors in serum have complicated studies of its function, properdin has, regardless, emerged as a key player in various inflammatory disease models. Here, we review basic properdin biology, emphasizing the major hurdles that have complicated the interpretation of results from properdin‐centered studies. In addition, we elaborate on an emerging role for properdin in thromboinflammation and discuss the potential utility of properdin inhibitors as long‐term therapeutic options to treat diseases marked by increased formation of platelet/granulocyte aggregates. Finally, we describe the interplay between properdin and the alternative pathway negative regulator, Factor H, and how aiming to understand these interactions can provide scientists with the most effective ways to manipulate alternative pathway activation in complex systems.

The bittersweet taste of tubulo-interstitial glycans

Recently, interesting work was published by Farrar et al. [1] showing the interaction of fucosylated glycoproteins on stressed tubular epithelial cells with collectin-11 leading to complement activation via the lectin route of complement. This elegant work stimulated us to evaluate the dark side (bittersweet taste) of tubulo-interstitial glycans in kidney tissue damage. As will be discussed, glycans not only initiate tubular complement activation but also orchestrate tubulo-interstitial leucocyte recruitment and growth factor responses. In this review we restrict ourselves to tubulo-interstitial damage mainly by proteinuria, ischaemia-reperfusion injury and transplantation, and we discuss the involvement of endothelial and tubular glycans in atypical and Escherichia coli-mediated haemolytic uraemic syndrome. As will be seen, fucosylated, mannosylated, galactosylated and sialylated oligosaccharide structures along with glycosaminoglycans comprise the most important glycans related to kidney injury pathways. Up to now, therapeutic interventions in these glycan-mediated injury pathways are underexplored and warrant further research.

Intracellular Activation of Complement 3 Is Responsible for Intestinal Tissue Damage during Mesenteric Ischemia

Intestinal ischemia followed by reperfusion leads to local and remote organ injury attributed to inflammatory response during the reperfusion phase. The extent to which ischemia contributes to ischemia/reperfusion injury has not been thoroughly studied. After careful evaluation of intestinal tissue following 30 min of ischemia, we noticed significant local mucosal injury in wild-type mice. This injury was drastically reduced in C3-deficient mice, suggesting C3 involvement. Depletion of circulating complement with cobra venom factor eliminated, as expected, injury recorded at the end of the reperfusion phase but failed to eliminate injury that occurred during the ischemic phase. Immunohistochemical studies showed that tissue damage during ischemia was associated with increased expression of C3/C3 fragments primarily in the intestinal epithelial cells, suggesting local involvement of complement. In vitro studies using Caco2 intestinal epithelial cells showed that in the presence of LPS or exposure to hypoxic conditions the cells produce higher C3 mRNA as well as C3a fragment. Caco2 cells were also noted to produce cathepsins B and L, and inhibition of cathepsins suppressed the release of C3a. Finally, we found that mice treated with a cathepsin inhibitor and cathepsin B-deficient mice suffer limited intestinal injury during the ischemic phase. To our knowledge, our findings demonstrate for the first time that significant intestinal injury occurs during ischemia prior to reperfusion and that this is due to activation of C3 within the intestinal epithelial cells in a cathepsin-dependent manner. Modulation of cathepsin activity may prevent injury of organs exposed to ischemia.

Targeting Complement Pathways During Cold Ischemia and Reperfusion Prevents Delayed Graft Function

The complement system plays a critical role in ischemia-reperfusion injury (IRI)-mediated delayed graft function (DGF). To better understand the roles of complement activation pathways in IRI in kidney transplantation, donor kidneys were treated ex vivo with terminal complement pathway (TP) inhibitor, anti-rat C5 mAb 18A10, or complement alternative pathway (AP) inhibitor TT30 for 28 h at 4°C pretransplantation in a syngeneic kidney transplantation rat model. All 18A10- and 67% of TT30-pretreated grafts, but only 16.7% of isotype control-pretreated grafts, survived beyond day 21 (p < 0.01). Inhibitor treatment in the final 45 min of 28-h cold ischemia (CI) similarly improved graft survival. Systemic posttransplant treatment with 18A10 resulted in 60% increased graft survival beyond day 21 (p < 0.01), while no TT30-treated rat survived > 6 days. Our results demonstrate that AP plays a prominent role during CI and that blocking either the AP or, more effectively the TP prevents ischemic injury and subsequent DGF. Multiple complement pathways may be activated and contribute to reperfusion injury; blocking the TP, but not the AP, posttransplant is effective in preventing reperfusion injury and increasing graft survival. These results demonstrate the feasibility of using complement inhibitors for prevention of DGF in humans.

Interaction of uromodulin and complement factor H enhances C3b inactivation

Recent studies suggest that uromodulin plays an important role in chronic kidney diseases. It can interact with several complement components, various cytokines and immune system cells. Complement factor H (CFH), as a regulator of the complement alternative pathway, is also associated with various renal diseases. Thus, we have been suggested that uromodulin regulates complement activation by interacting with CFH during tubulointerstitial injury. We detected co‐localization of uromodulin and CFH in the renal tubules by using immunofluorescence. Next, we confirmed the binding of uromodulin with CFH in vitro and found that the affinity constant (K_D) of uromodulin binding to CFH was 4.07 × 10⁻⁶M based on surface plasmon resonance results. The binding sites on CFH were defined as the short consensus repeat (SCR) units SCR1–4, SCR7 and SCR19–20. The uromodulin‐CFH interaction enhanced the cofactor activity of CFH for factor I‐mediated cleavage of C3b to iC3b. These results indicate that uromodulin plays a role via binding and enhancing the function of CFH.

Properdin-Mediated C5a Production Enhances Stable Binding of Platelets to Granulocytes in Human Whole Blood

Enhanced levels of platelet/granulocyte aggregates (PGAs) are found in patients suffering from many different inflammatory vascular diseases, and their formation in animal models of vascular disease is associated with increased thromboinflammation and worsened outcomes. The complement system, a part of the innate immune system, influences PGA formation, but the mechanisms for its effects are unknown. In this study, we have defined complement-mediated mechanisms that enhance PGA formation in human whole blood stimulated with thrombin receptor-activating peptide (TRAP) using ex vivo flow cytometry assays. We demonstrate that physiological properdin, a positive regulator of complement alternative pathway activity, increases PGA formation when added to TRAP-stimulated blood. All physiological properdin forms increase PGA formation, but properdin tetramers are the most efficient at increasing complement activity and PGA formation. Inhibition of endogenous properdin, either circulating in the blood or produced locally by leukocytes, impairs TRAP-mediated PGA formation to the same level as specific inhibition of either the alternative or classical pathway. Additionally, blocking the interaction of C5a with its cellular receptor prevents properdin-mediated increases in PGA formation. Adding either properdin tetramers or C5a to whole blood increases CD11b expression on granulocytes, and this increase is prevented by blockade of the C5a-C5a receptor axis. Finally, we demonstrate that the effects of properdin on PGA formation are tightly regulated by Factor H. Cumulatively, our data indicate that properdin enhances PGA formation via increased production of C5a, and that inhibition of properdin function has therapeutic potential to limit thromboinflammation in diseases characterized by increased PGA formation.

Renoprotective Mechanism of Remote Ischemic Preconditioning Based on Transcriptomic Analysis in a Porcine Renal Ischemia Reperfusion Injury Model

Ischemic preconditioning (IPC) is a well-known phenomenon in which tissues are exposed to a brief period of ischemia prior to a longer ischemic event. This technique produces tissue tolerance to ischemia reperfusion injury (IRI). Currently, IPC’s mechanism of action is poorly understood. Using a porcine single kidney model, we performed remote IPC with renal IRI and evaluated the IPC mechanism of action. Following left nephrectomy, 15 female Yorkshire pigs were divided into three groups: no IPC and 90 minutes of warm ischemia (control), remote IPC immediately followed by 90 minutes of warm ischemia (rIPCe), and remote IPC with 90 minutes of warm ischemia performed 24 hours later (rIPCl). Differential gene expression analysis was performed using a porcine-specific microarray. The microarray analysis of porcine renal tissues identified 1,053 differentially expressed probes in preconditioned pigs. Among these, 179 genes had altered expression in both the rIPCe and rIPCl groups. The genes were largely related to oxidation reduction, apoptosis, and inflammatory response. In the rIPCl group, an additional 848 genes had altered expression levels. These genes were primarily related to immune response and inflammation, including those coding for cytokines and cytokine receptors and those that play roles in the complement system and coagulation cascade. In the complement system, the membrane attack complex was determined to be sublytic, because it colocalized with phosphorylated extracellular signal-regulated kinase. Furthermore, alpha 2 macroglobulin, tissue plasminogen activator, uterine plasmin trypsin inhibitor, and arginase-1 mRNA levels were elevated in the rIPCl group. These findings indicate that remote IPC produces renoprotective effects through multiple mechanisms, and these effects develop over a long timeframe rather than immediately following IPC.

Efficacy of Targeted Complement Inhibition in Experimental C3 Glomerulopathy

C3 glomerulopathy refers to renal disorders characterized by abnormal accumulation of C3 within the kidney, commonly along the glomerular basement membrane (GBM). C3 glomerulopathy is associated with complement alternative pathway dysregulation, which includes functional defects in complement regulator factor H (FH). There is no effective treatment for C3 glomerulopathy. We investigated the efficacy of a recombinant mouse protein composed of domains from complement receptor 2 (CR2) and FH (CR2-FH) in two models of C3 glomerulopathy with either preexisting or triggered C3 deposition along the GBM. FH-deficient mice spontaneously develop renal pathology associated with abnormal C3 accumulation along the GBM and secondary plasma C3 deficiency. CR2-FH partially restored plasma C3 levels in FH-deficient mice 2 hours after intravenous injection. CR2-FH specifically targeted glomerular C3 deposits, reduced the linear C3 reactivity assessed with anti-C3 and anti-C3b/iC3b/C3c antibodies, and prevented further spontaneous accumulation of C3 fragments along the GBM. Reduction in glomerular C3d and C9/C5b-9 reactivity was observed after daily administration of CR2-FH for 1 week. In a second mouse model with combined deficiency of FH and complement factor I, CR2-FH prevented de novo C3 deposition along the GBM. These data show that CR2-FH protects the GBM from both spontaneous and triggered C3 deposition in vivo and indicate that this approach should be tested in C3 glomerulopathy.

Antepartum or immediate postpartum renal biopsies in preeclampsia/eclampsia of pregnancy: new morphologic and clinical findings

BACKGROUND

Preeclampsia (PE) and eclampsia remain leading causes of maternal and fetal mortality worldwide. The kidney is considered the first and most severely affected organ in women with PE/eclampsia. In this study, we analyzed new morphologic features of kidney biopsies and clinical findings in patients with PE or eclampsia at our hospital.

METHODS

Eight patients with PE/eclampsia underwent renal biopsies during the antepartum (3/8) or postpartum (5/8) period. Maternal clinical findings, major serological indices, neonatal outcomes, and renal histopathologic and immunofluorescent characteristics were reviewed for each case.

RESULTS

Most patients had abnormal serum cholesterol (8/8), triglyceride (6/8), albumin (7/8), and uric acid (5/8). The ratio of blood urea nitrogen (BUN) to serum creatinine (SCr) was elevated in all patients. Five of eight newborns survived. Various degrees of morphologic change were present in the renal glomeruli, and were associated with proteinuria. All patients had deposition of complement factor 4 (C4) in the renal glomeruli and seven had deposition of immunoglobulin M (IgM).

CONCLUSION

Endotheliosis, vacuolation of podocytes, proliferation of mesangial cells, and protein casts in the tubule lumens were found in the kidneys of women with PE/eclampsia. Immune depositions of C4 and IgM are major contributors to renal lesions in preeclamptic patients, whose neonates can generally survive. Eclampsia can occur without increased blood pressure.

Complement as a multifaceted modulator of kidney transplant injury

Improvements in clinical care and immunosuppressive medications have positively affected outcomes following kidney transplantation, but graft survival remains suboptimal, with half-lives of approximately 11 years. Late graft loss results from a confluence of processes initiated by ischemia-reperfusion injury and compounded by effector mechanisms of uncontrolled alloreactive T cells and anti-HLA antibodies. When combined with immunosuppressant toxicity, post-transplant diabetes and hypertension, and recurrent disease, among other factors, the result is interstitial fibrosis, tubular atrophy, and graft failure. Emerging evidence over the last decade unexpectedly identified the complement cascade as a common thread in this process. Complement activation and function affects allograft injury at essentially every step. These fundamental new insights, summarized herein, provide the foundation for testing the efficacy of various complement antagonists to improve kidney transplant function and long-term graft survival.

Properdin has an ascendancy over factor H regulation in complement-mediated renal tubular damage

Background

Urinary (U)-complement components have been detected in patients with proteinuric renal diseases, and complement activation via the alternative pathway (AP) is believed to play a role in renal tubular damage. The present study aimed to examine the regulation of complement AP activation in patients with renal tubular damage by focusing on the balance between properdin (P) and factor H (fH).

Methods

In the in vivo studies, U concentrations of P, fH and membrane attack complex (MAC) were measured in patients with renal diseases using an enzyme-linked immunosorbent assay (ELISA), and their relationships with the clinical data were evaluated. In the in vitro studies, human proximal tubular epithelial cells (PTECs) were incubated with normal human serum (NHS), P-depleted serum (PDS), purified P and/or fH. Changes in cell morphology and phenotype were assessed by microscopy, real-time polymerase chain reaction (PCR), immunostaining and a cell viability assay.

Results

The U-P, fH and MAC concentrations were significantly higher in patients with renal disease than in normal controls and correlated with the U-protein and tubular damage markers. Furthermore, multivariate analysis revealed a relationship between P levels and tubular damage markers. There were no significant changes in morphology and mRNA expression in the AP components (P, fH, fB, C3, C5 and C9) after the addition of up to 25% NHS. Dose-dependent depositions of P or fH were observed after the addition of P or fH on PTECs. Depositions of P were not inhibited by fH in a mixture of a fixed concentration of P and a variable concentration of fH, and vice versa. Preincubation with the fixed concentration of P before the addition of NHS or PDS increased the depositions of P, C3 and MAC compared with incubation with intact NHS or intact PDS only; the depositions of C3 and MAC showed a serum-dependent trend. Preincubation with P before NHS addition significantly suppressed cell viability without causing morphological changes.

Conclusions

In the pathogenesis of renal tubular damage, P can directly bind to PTECs and may accelerate AP activation by surpassing fH regulation.

Tissue-specific deletion of Crry from mouse proximal tubular epithelial cells increases susceptibility to renal ischemia-reperfusion injury

The murine cell surface protein Crry (complement receptor 1-related protein/gene y) is a key complement regulator with similar activities to human membrane cofactor protein (MCP) and decay-accelerating factor. MCP has a critical role in preventing complement-mediated tissue injury and its mutation has been implicated in several human kidney diseases. The study of Crry in mice has relevance to understanding MCP activity in human diseases; however, such efforts have been hampered by the embryonic lethality phenotype of Crry gene knockout. Here we used a conditional gene-targeting approach and deleted Crry from the mouse proximal tubular epithelial cells where Crry is prominently expressed. Absence of Crry from proximal tubular epithelial cells resulted in spontaneous C3 deposition on the basolateral surface but no apparent renal disease in unchallenged mice. However, mice deficient in Crry on proximal tubular epithelial cells developed exacerbated renal injury when subjected to renal ischemia-reperfusion, showing increased blood urea nitrogen levels, higher tubular injury scores, more tubular epithelial cell apoptosis, and inflammatory infiltrates. Renal ischemia-reperfusion injury in the Crry conditional knockout mice was prevented by blocking C3 and C5 activation using an anti-properdin or anti-C5 monoclonal antibody (mAb), respectively. Thus, Crry has a critical role in protecting proximal tubular epithelial cells during ischemia-reperfusion challenge. Our results highlight the latent risk for inflammatory kidney injury associated with defects in membrane complement regulators.

The role of the complement system in acute kidney injury

Acute kidney injury is a common and severe clinical problem. Patients who develop acute kidney injury are at increased risk of death despite supportive measures such as hemodialysis. Research in recent years has shown that tissue inflammation is central to the pathogenesis of renal injury, even after nonimmune insults such as ischemia/reperfusion and toxins. Examination of clinical samples and preclinical models has shown that activation of the complement system is a critical cause of acute kidney injury. Furthermore, complement activation within the injured kidney is a proximal trigger of many downstream inflammatory events within the renal parenchyma that exacerbate injury to the kidney. Complement activation also may account for the systemic inflammatory events that contribute to remote organ injury and patient mortality. Complement inhibitory drugs have now entered clinical use and may provide an important new therapeutic approach for patients suffering from, or at high risk of developing, acute kidney injury.

Identification of a novel mode of complement activation on stimulated platelets mediated by properdin and C3(H2O)

Elevated numbers of activated platelets circulate in patients with chronic inflammatory diseases, including atherosclerosis and coronary disease. Activated platelets can activate the complement system. Although complement activation is essential for immune responses and removal of spent cells from circulation, it also contributes to inflammation and thrombosis, especially in patients with defective complement regulation. Proinflammatory activated leukocytes, which interact directly with platelets in response to vascular injury, are among the main sources of properdin, a positive regulator of the alternative pathway. The role of properdin in complement activation on stimulated platelets is unknown. Our data show that physiological forms of human properdin bind directly to human platelets after activation by strong agonists in the absence of C3, and bind nonproportionally to surface CD62P expression. Activation of the alternative pathway on activated platelets occurs when properdin is on the surface and recruits C3b or C3(H2O) to form C3b,Bb or a novel cell-bound C3 convertase [C3(H2O),Bb], which normally is present only in the fluid phase. Alternatively, properdin can be recruited by C3(H2O) on the platelet surface, promoting complement activation. Inhibition of factor H-mediated cell surface complement regulation significantly increases complement deposition on activated platelets with surface properdin. Finally, properdin released by activated neutrophils binds to activated platelets. Altogether, these data suggest novel molecular mechanisms for alternative pathway activation on stimulated platelets that may contribute to localization of inflammation at sites of vascular injury and thrombosis.

Detection of complement activation using monoclonal antibodies against C3d

During complement activation the C3 protein is cleaved, and C3 activation fragments are covalently fixed to tissues. Tissue-bound C3 fragments are a durable biomarker of tissue inflammation, and these fragments have been exploited as addressable binding ligands for targeted therapeutics and diagnostic agents. We have generated cross-reactive murine monoclonal antibodies against human and mouse C3d, the final C3 degradation fragment generated during complement activation. We developed 3 monoclonal antibodies (3d8b, 3d9a, and 3d29) that preferentially bind to the iC3b, C3dg, and C3d fragments in solution, but do not bind to intact C3 or C3b. The same 3 clones also bind to tissue-bound C3 activation fragments when injected systemically. Using mouse models of renal and ocular disease, we confirmed that, following systemic injection, the antibodies accumulated at sites of C3 fragment deposition within the glomerulus, the renal tubulointerstitium, and the posterior pole of the eye. To detect antibodies bound within the eye, we used optical imaging and observed accumulation of the antibodies within retinal lesions in a model of choroidal neovascularization (CNV). Our results demonstrate that imaging methods that use these antibodies may provide a sensitive means of detecting and monitoring complement activation-associated tissue inflammation.

Novel roles of complement in renal diseases and their therapeutic consequences

The complement system functions as a part of the innate immune system. Inappropriate activation of the complement pathways has a deleterious effect on kidneys. Recent advances in complement research have provided new insights into the pathogenesis of glomerular and tubulointerstitial injury associated with complement activation. A new disease entity termed 'C3 glomerulopathy' has recently been proposed and is characterized by isolated C3 deposition in glomeruli without positive staining for immunoglobulins. Genetic and functional studies have demonstrated that several different mutations and disease variants, as well as the generation of autoantibodies, are potentially associated with its pathogenesis. The data from comprehensive analyses suggest that complement dysregulation can also be associated with hemolytic uremic syndrome and more common glomerular diseases, such as IgA nephropathy and diabetic kidney disease. In addition, animal studies utilizing genetically modified mice have begun to elucidate the molecular pathomechanisms associated with the complement system. From a diagnostic point of view, a noninvasive, MRI-based method for detecting C3 has recently been developed to serve as a novel tool for diagnosing complement-mediated kidney diseases. While novel therapeutic tools related to complement regulation are emerging, studies evaluating the precise roles of the complement system in kidney diseases will still be useful for developing new therapeutic approaches.

Essential role of surface-bound complement factor H in controlling immune complex-induced arthritis

Factor H (fH) is an endogenous negative regulator of the alternative pathway (AP) that binds polyanions as well as complement activation fragments C3b and C3d. The AP is both necessary and sufficient to develop collagen Ab-induced arthritis (CAIA) in mice; the mechanisms whereby normal control of the AP is overcome and injury develops are unknown. Although primarily a soluble circulating protein, fH can also bind to tissues in a manner dependent on the carboxyl-terminal domain containing short consensus repeats 19 and 20. We examined the role of fH in CAIA by blocking its binding to tissues through administration of a recombinant negative inhibitor containing short consensus repeats 19 and 20 (rfH19-20), which impairs fH function and amplifies surface AP activation in vitro. Administration of rfH19-20, but not control rfH3-5, significantly worsened clinical disease activity, histopathologic injury, and C3 deposition in the synovium and cartilage in wild-type and fH(+/-) mice. In vitro studies demonstrated that rfH19-20 increased complement activation on cartilage extracts and injured fibroblast-like synoviocytes, two major targets of complement deposition in the joint. We conclude that endogenous fH makes a significant contribution to inhibition of the AP in CAIA through binding to sites of immune complex formation and complement activation.

CR2-mediated targeting of complement inhibitors: bench-to-bedside using a novel strategy for site-specific complement modulation

Recent approval of the first human complement pathway-directed therapeutics, along with high-profile genetic association studies, has catalyzed renewed biopharmaceutical interest in developing drugs that modulate the complement system. Substantial challenges remain, however, that must be overcome before widespread application of complement inhibitors in inflammatory and autoimmune diseases becomes possible. Among these challenges are the following: (1) defining the complement pathways and effector mechanisms that cause tissue injury in humans and determining whether the relative importance of each varies by disease, (2) blocking or modulating, using traditional small molecule or biologic approaches, the function of complement proteins whose circulating levels are very high and whose turnover rates are relatively rapid, especially in the setting of acute and chronic autoimmune diseases, and (3) avoiding infectious complications or impairment of other important physiological functions of complement when using systemically active complement-blocking agents. This chapter will review data that address these challenges to therapeutic development, with a focus on the development of a novel strategy of blocking specific complement pathways by targeting inhibitors using a recombinant portion of the human complement receptor type 2 (CR2/CD21) which specifically targets to sites of local complement C3 activation where C3 fragments are covalently fixed. Recently, the first of these CR2-targeted proteins has entered human phase I studies in the human disease paroxysmal nocturnal hemoglobinuria. The results of murine translational studies using CR2-targeted inhibitors strongly suggest that a guiding principle going forward in complement therapeutic development may well be to focus on developing strategies to modulate the pathway as precisely as possible by physically localizing therapeutic inhibitory effects.

Which pathways trigger the role of complement in ischaemia/reperfusion injury?

Investigations into the role of complement in ischemia/reperfusion (I/R) injury have identified common effector mechanisms that depend on the production of C5a and C5b-9 through the cleavage of C3. These studies have also defined an important role for C3 synthesized within ischemic kidney. Less clear however is the mechanism of complement activation that leads to the cleavage of C3 in ischemic tissues and to what extent the potential trigger mechanisms are organ dependent - an important question which informs the development of therapies that are more selective in their ability to limit the injury, yet preserve the other functions of complement where possible. Here we consider recent evidence for each of the three major pathways of complement activation (classical, lectin, and alternative) as mediators of I/R injury, and in particular highlight the role of lectin molecules that increasingly seem to underpin the injury in different organ models and in addition reveal unusual routes of complement activation that contribute to organ damage.

Defective complement inhibitory function predisposes to renal disease

The role of the complement system in mediating human renal disease has long been recognized in immune-complex excess syndromes such as systemic lupus erythematosus and in dense deposit disease in which no immunoglobulin (Ig) is present. Over the past 15 years, mutations in complement regulatory genes have been demonstrated to predispose to thrombotic microangiopathies including atypical hemolytic uremic syndrome, C3 and C1q glomerulopathies, and preeclampsia. Excessive complement activation on an endothelial cell, due to either an autoantibody or a regulatory protein deficiency, sets up a procoagulant state in these diseases as well as in the antiphospholipid syndrome. Knowledge of the genes involved and the functional consequences of alterations in their structure has led to therapy that blocks complement activation.

Mouse model of ischemic acute kidney injury: technical notes and tricks

Renal ischemia-reperfusion leads to acute kidney injury (AKI), a major kidney disease associated with an increasing prevalence and high mortality rates. A variety of experimental models, both in vitro and in vivo, have been used to study the pathogenic mechanisms of ischemic AKI and to test renoprotective strategies. Among them, the mouse model of renal clamping is popular, mainly due to the availability of transgenic models and the relatively small animal size for drug testing. However, the mouse model is generally less stable, resulting in notable variations in results. Here, we describe a detailed protocol of the mouse model of bilateral renal ischemia-reperfusion. We share the lessons and experiences gained from our laboratory in the past decade. We further discuss the technical issues that account for the variability of this model and offer relevant solutions, which may help other investigators to establish a well-controlled, reliable animal model of ischemic AKI.

Factor h and properdin recognize different epitopes on renal tubular epithelial heparan sulfate

During proteinuria, renal tubular epithelial cells become exposed to ultrafiltrate-derived serum proteins, including complement factors. Recently, we showed that properdin binds to tubular heparan sulfates (HS). We now document that factor H also binds to tubular HS, although to a different epitope than properdin. Factor H was present on the urinary side of renal tubular cells in proteinuric, but not in normal renal tissues and colocalized with properdin in proteinuric kidneys. Factor H dose-dependently bound to proximal tubular epithelial cells (PTEC) in vitro. Preincubation of factor H with exogenous heparin and pretreatment of PTECs with heparitinase abolished the binding to PTECs. Surface plasmon resonance experiments showed high affinity of factor H for heparin and HS (K(D) values of 32 and 93 nm, respectively). Using a library of HS-like polysaccharides, we showed that chain length and high sulfation density are the most important determinants for glycosaminoglycan-factor H interaction and clearly differ from properdin-heparinoid interaction. Coincubation of properdin and factor H did not hamper HS/heparin binding of one another, indicating recognition of different nonoverlapping epitopes on HS/heparin by factor H and properdin. Finally we showed that certain low anticoagulant heparinoids can inhibit properdin binding to tubular HS, with a minor effect on factor H binding to tubular HS. As a result, these heparinoids can control the alternative complement pathway. In conclusion, factor H and properdin interact with different HS epitopes of PTECs. These interactions can be manipulated with some low anticoagulant heparinoids, which can be important for preventing complement-derived tubular injury in proteinuric renal diseases.

Taming lupus-a new understanding of pathogenesis is leading to clinical advances

Systemic lupus erythematosus (SLE) is an autoimmune disease that is characterized by the loss of tolerance to nuclear self antigens, the production of pathogenic autoantibodies and damage to multiple organ systems. Over the years, patients with SLE have been managed largely with empiric immunosuppressive therapies, which are associated with substantial toxicities and do not always provide adequate control of the disease. The development of targeted therapies that specifically address disease pathogenesis or progression has lagged, largely because of the complex and heterogeneous nature of the disease, as well as difficulties in designing uniform outcome measures for clinical trials. Recent advances that could improve the treatment of SLE include the identification of genetic variations that influence the risk of developing the disease, an enhanced understanding of innate and adaptive immune activation and regulation of tolerance, dissection of immune cell activation and inflammatory pathways and elucidation of mechanisms and markers of tissue damage. These discoveries, together with improvements in clinical trial design, form a platform from which to launch the development of a new generation of lupus therapies.