The C5a receptor is expressed by human renal proximal tubular epithelial cells

Identification of a genetic region linked to tolerance to MRSA infection using Collaborative Cross mice

Staphylococcus aureus (S. aureus) colonizes humans asymptomatically but can also cause opportunistic infections, ranging from mild skin infections to severe life-threatening conditions. Resistance and tolerance are two ways a host can survive an infection. Resistance is limiting the pathogen burden, while tolerance is limiting the health impact of a given pathogen burden. In previous work, we established that collaborative cross (CC) mouse line CC061 is highly susceptible to Methicillin-resistant S. aureus infection (MRSA, USA300), while CC024 is tolerant. To identify host genes involved in tolerance after S. aureus infection, we crossed CC061 mice and CC024 mice to generate F1 and F2 populations. Survival after MRSA infection in the F1 and F2 generations was 65% and 55% and followed a complex dominant inheritance pattern for the CC024 increased survival phenotype. Colonization in F2 animals was more extreme than in their parents, suggesting successful segregation of genetic factors. We identified a Quantitative Trait Locus (QTL) peak on chromosome 7 for survival and weight change after infection. In this QTL, the WSB/EiJ (WSB) allele was present in CC024 mice and contributed to their MRSA tolerant phenotype. Two genes, C5ar1 and C5ar2, have high-impact variants in this region. C5ar1 and C5ar2 are receptors for the complement factor C5a, an anaphylatoxin that can trigger a massive immune response by binding to these receptors. We hypothesize that C5a may have altered binding to variant receptors in CC024 mice, reducing damage caused by the cytokine storm and resulting in the ability to tolerate a higher pathogen burden and longer survival.

Amiloride Reduces Urokinase/Plasminogen-Driven Intratubular Complement Activation in Glomerular Proteinuria

SIGNIFICANCE STATEMENT

Proteinuria predicts accelerated decline in kidney function in CKD. The pathologic mechanisms are not well known, but aberrantly filtered proteins with enzymatic activity might be involved. The urokinase-type plasminogen activator (uPA)-plasminogen cascade activates complement and generates C3a and C5a in vitro / ex vivo in urine from healthy persons when exogenous, inactive, plasminogen, and complement factors are added. Amiloride inhibits uPA and attenuates complement activation in vitro and in vivo . In conditional podocin knockout (KO) mice with severe proteinuria, blocking of uPA with monoclonal antibodies significantly reduces the urine excretion of C3a and C5a and lowers tissue NLRP3-inflammasome protein without major changes in early fibrosis markers. This mechanism provides a link to proinflammatory signaling in proteinuria with possible long-term consequences for kidney function.

BACKGROUND

Persistent proteinuria is associated with tubular interstitial inflammation and predicts progressive kidney injury. In proteinuria, plasminogen is aberrantly filtered and activated by urokinase-type plasminogen activator (uPA), which promotes kidney fibrosis. We hypothesized that plasmin activates filtered complement factors C3 and C5 directly in tubular fluid, generating anaphylatoxins, and that this is attenuated by amiloride, an off-target uPA inhibitor.

METHODS

Purified C3, C5, plasminogen, urokinase, and urine from healthy humans were used for in vitro / ex vivo studies. Complement activation was assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, immunoblotting, and ELISA. Urine and plasma from patients with diabetic nephropathy treated with high-dose amiloride and from mice with proteinuria (podocin knockout [KO]) treated with amiloride or inhibitory anti-uPA antibodies were analyzed.

RESULTS

The combination of uPA and plasminogen generated anaphylatoxins C3a and C5a from intact C3 and C5 and was inhibited by amiloride. Addition of exogenous plasminogen was sufficient for urine from healthy humans to activate complement. Conditional podocin KO in mice led to severe proteinuria and C3a and C5a urine excretion, which was attenuated reversibly by amiloride treatment for 4 days and reduced by >50% by inhibitory anti-uPA antibodies without altering proteinuria. NOD-, LRR- and pyrin domain-containing protein 3-inflammasome protein was reduced with no concomitant effect on fibrosis. In patients with diabetic nephropathy, amiloride reduced urinary excretion of C3dg and sC5b-9 significantly.

CONCLUSIONS

In conditions with proteinuria, uPA-plasmin generates anaphylatoxins in tubular fluid and promotes downstream complement activation sensitive to amiloride. This mechanism links proteinuria to intratubular proinflammatory signaling. In perspective, amiloride could exert reno-protective effects beyond natriuresis and BP reduction.

CLINICAL TRIAL REGISTRY NAME AND REGISTRATION NUMBER

Increased Activity of a Renal Salt Transporter (ENaC) in Diabetic Kidney Disease, NCT01918488 and Increased Activity of ENaC in Proteinuric Kidney Transplant Recipients, NCT03036748 .

The Complement C3a and C5a Signaling in Renal Diseases: A Bridge between Acute and Chronic Inflammation

The complement system, a cornerstone of the innate immune defense, typically confers protection against pathogens. However, in various clinical scenarios the complement's defensive actions can harm host cells, exacerbating immune and inflammatory responses. The central components C3 and C5 undergo proteolytic cleavage during complement activation, yielding small active fragments C3a and C5a anaphylatoxins. Traditionally, these fragments were associated with inflammation via the specific receptors C3a receptor (R), C5aR1 and C5aR2. Recent insights, however, spotlight the excessive C3a/C3aR and C5a/C5aR1 signaling as culprits in diverse disorders of inflammatory and autoimmune etiology. This is particularly true for several kidney diseases, where the potential involvement of anaphylatoxins in renal damage is supported by the enhanced renal expression of their receptors and the high levels of C3a and C5a in both plasma and urine. Furthermore, the production of complement proteins in the kidney, with different renal cells synthesizing C3 and C5, significantly contributes to local tissue injury. In the present review, we discuss the different aspects of C3a/C3aR and C5a/C5aR signaling in acute and chronic kidney diseases and explore the therapeutic potential of emerging targeted drugs for future clinical applications.

The C5aR1 complement receptor: A novel immunomodulator of insulin action in skeletal muscle

The complement system constitutes an integral component of the innate immune system and plays a critical role in adaptive immunity. Activation of this system engenders the production of complement peptide fragments, including C5a, which engage G-protein coupled receptors predominantly expressed in immune-associated cells, such as neutrophils, initiating pro-inflammatory responses. Intriguingly, our investigation has unveiled the presence of C5a receptor 1 (C5aR1) expression within skeletal muscle, a key metabolic tissue and primary target of insulin. Herein, we demonstrate that C5aR1 activation by C5a in differentiated human skeletal muscle cells elicits acute suppression of insulin signalling. This suppression manifests as impaired insulin-dependent association between IRS1 and the p85 subunit of PI3-kinase, a 50% reduction in Akt phosphorylation, and a 60% decline in insulin-stimulated glucose uptake. This impairment in insulin signalling is associated with a three-fold elevation in intramyocellular diacylglycerol (DAG) levels and a two-fold increase in cytosolic calcium content, which promote PKC-mediated IRS1 inhibition via enhanced phosphorylation at IRS1 Ser1101. Significantly, our findings demonstrate that structurally diverse C5aR1 antagonists, along with genetic deletion or stable silencing of C5aR1 by 80% using short-hairpin RNA, effectively attenuate repression of insulin signalling by C5a in LHCN-M2 human skeletal myotubes. These results underscore the potential of heightened C5aR1 activation, characteristic of obesity and chronic inflammatory conditions, to detrimentally impact insulin function within skeletal muscle cells. Additionally, the study suggests that agents targeting the C5a-C5aR axis, originally devised for mitigating complement-dependent inflammatory conditions, may offer therapeutic avenues to ameliorate immune-driven insulin resistance in key peripheral metabolic tissues, including skeletal muscle.

A Transcriptome Array-Based Approach to Link SGLT-2 and Intrarenal Complement C5 Synthesis in Diabetic Nephropathy

Diabetic nephropathy is a common microvascular complication of diabetes mellitus. It is characterized by progressive chronic kidney disease (CKD) with decline of kidney function by hyperfiltration. On a mechanistic level, activation of the complement system has been implicated in the pathogenesis of diabetic nephropathy. Therefore, here we pursued a transcriptome array-based approach to link intrarenal SGLT-2 and the synthesis of distinct complement components in diabetic nephropathy. Publicly available datasets for SLC5A2 (encoding SGLT-2) and complement system components were extracted specifically from microdissected tubulointerstitial (healthy controls: n = 31, diabetic nephropathy: n = 17) and glomerular compartments (healthy controls: n = 21, diabetic nephropathy: n = 12). First, we compared tubulointerstitial and glomerular log2SLC5A2 mRNA expression levels and confirmed a predominant synthesis within the tubulointerstitial compartment. Among various complement components and receptors, the only significant finding was a positive association between SLC5A2 and the tubulointerstitial synthesis of the complement component C5 in diabetic nephropathy (p = 0.0109). Finally, intrarenal expression of SLC5A2 was associated predominantly with pathways involved in metabolic processes. Interestingly, intrarenal complement C5 synthesis was also associated with enrichment of metabolic signaling pathways, overlapping with SLC5A2 for "metabolism" and "biological oxidations". These observations could be of relevance in the pathogenesis of diabetic nephropathy and implicate a mechanistic link between SGLT-2 and intrarenal complement synthesis.

C5a-licensed phagocytes drive sterilizing immunity during systemic fungal infection

Systemic candidiasis is a common, high-mortality, nosocomial fungal infection. Unexpectedly, it has emerged as a complication of anti-complement C5-targeted monoclonal antibody treatment, indicating a critical niche for C5 in antifungal immunity. We identified transcription of complement system genes as the top biological pathway induced in candidemic patients and as predictive of candidemia. Mechanistically, C5a-C5aR1 promoted fungal clearance and host survival in a mouse model of systemic candidiasis by stimulating phagocyte effector function and ERK- and AKT-dependent survival in infected tissues. C5ar1 ablation rewired macrophage metabolism downstream of mTOR, promoting their apoptosis and enhancing mortality through kidney injury. Besides hepatocyte-derived C5, local C5 produced intrinsically by phagocytes provided a key substrate for antifungal protection. Lower serum C5a concentrations or a C5 polymorphism that decreases leukocyte C5 expression correlated independently with poor patient outcomes. Thus, local, phagocyte-derived C5 production licenses phagocyte antimicrobial function and confers innate protection during systemic fungal infection.

The C5a/C5aR1 Axis Contributes to the Pathogenesis of Acute Cystitis Through Enhancement of Adhesion and Colonization of Uropathogenic E. coli

Our previous work using a murine model of pyelonephritis demonstrated that the C5a/C5aR1 axis plays a pathogenic role in acute kidney infection. In this study, we report that the C5a/C5aR1 axis also plays a pathogenic role in acute bladder infection. C5aR1-deficient mice had reduced bladder bacterial load and attenuated bladder tissue injury, which is associated with reduced expression of terminal α-mannosyl residues (Man) (a potential ligand for type 1 fimbriae of E. coli) at the luminal surface of the bladder epithelium and reduced early bacterial colonization of the bladder. In vitro, C5a stimulation enhanced mannose expression in and facilitated bacterial adhesion/colonization to human bladder epithelial cells. C5a stimulation also upregulated the activation of ERK1/2 and NF-κB signaling and gene expression of proinflammatory cytokines (i.e., Il6, Il1b, Cxcl1, Ccl2) in the epithelial cells, which could drive pro-inflammatory responses leading to tissue injury. Administration of the C5aR1 antagonist effectively reduced bladder bacterial load and tissue injury. Thus, our findings demonstrate a previously unknown pathogenic role for the C5a/C5aR1 axis in bladder infection and suggest that the C5a/C5aR1 axis-mediated upregulation of Man expression, enhancement of bacterial adhesion/colonization, and excessive inflammatory responses contribute to acute bladder infection. These findings improve our understanding of the pathogenesis of bladder infection with therapeutic implications for UTI.

Urinary C3 levels associated with sepsis and acute kidney injury-A pilot study

Acute kidney injury (AKI) is an abrupt deterioration of renal function often caused by severe clinical disease such as sepsis, and patients require intensive care. Acute-phase parameters for systemic inflammation are well established and used in routine clinical diagnosis, but no such parameters are known for AKI and inflammation at the local site of tissue damage, namely the nephron. Therefore, we sought to investigate complement factors C3a/C3 in urine and urinary sediment cells. After the development of a C3a/C3-specific mouse monoclonal antibody (3F7E2), urine excretion from ICU sepsis patients was examined by dot blot and immunoblotting. This C3a/C3 ELISA and a C3a ELISA were used to obtain quantitative data over 24 hours for 6 consecutive days. Urine sediment cells were analyzed for topology of expression. Patients with severe infections (n = 85) showed peak levels of C3a/C3 on the second day of ICU treatment. The majority (n = 59) showed C3a/C3 levels above 20 μg/ml at least once in the first 6 days after admission. C3a was detectable on all 6 days. Peak C3a/C3 levels correlated negatively with peak C-reactive protein (CRP) levels. No relationship was found between peak C3a/C3 with peak leukocyte count, age, or AKI stage. Analysis of urine sediment cells identified C3a/C3-producing epithelial cells with reticular staining patterns and cells with large-granular staining. Opsonized bacteria were detected in patients with urinary tract infections. In critically ill sepsis patients with AKI, urinary C3a/C3 inversely correlated with serum CRP. Whether urinary C3a/C3 has a protective function through autophagy, as previously shown for cisplatin exposure, or is a by-product of sepsis caused by pathogenic stimuli to the kidney must remain open in this study. However, our data suggest that C3a/C3 may function as an inverse acute-phase parameter that originates in the kidney and is detectable in urine.

Immunohistochemical identification of complement peptide C5a receptor 1 (C5aR1) in non-neoplastic and neoplastic human tissues

The complement component C5a and its receptor C5aR1 are involved in the development of numerous inflammatory diseases. In addition to immune cells, C5aR1 is expressed in neoplastic cells of multiple tumour entities, where C5aR1 is associated with a higher proliferation rate, advanced tumour stage, and poor patient outcomes. The aim of the present study was to obtain a broad expression profile of C5aR1 in human non-neoplastic and neoplastic tissues, especially in tumour entities not investigated in this respect so far. For this purpose, we generated a novel polyclonal rabbit antibody, {5227}, against the carboxy-terminal tail of C5aR1. The antibody was initially characterised in Western blot analyses and immunocytochemistry using transfected human embryonic kidney (HEK) 293 cells. It was then applied to a large series of formalin-fixed, paraffin-embedded non-neoplastic and neoplastic human tissue samples. C5aR1 was strongly expressed by different types of immune cells in the majority of tissue samples investigated. C5aR1 was also present in alveolar macrophages, bronchial, gut, and bile duct epithelia, Kupffer cells, occasionally in hepatocytes, proximal renal tubule cells, placental syncytiotrophoblasts, and distinct stem cell populations of bone marrow. C5aR1 was also highly expressed in the vast majority of the 32 tumour entities investigated, where a hitherto unappreciated high prevalence of the receptor was detected in thyroid carcinomas, small-cell lung cancer, gastrointestinal stromal tumours, and endometrial carcinomas. In addition to confirming published findings, we found noticeable C5aR1 expression in many tumour entities for the first time. Here, it may serve as an interesting target for future therapies.

Complement C5a Induces Renal Injury in Diabetic Kidney Disease by Disrupting Mitochondrial Metabolic Agility

The sequelae of diabetes include microvascular complications such as diabetic kidney disease (DKD), which involves glucose-mediated renal injury associated with a disruption in mitochondrial metabolic agility, inflammation, and fibrosis. We explored the role of the innate immune complement component C5a, a potent mediator of inflammation, in the pathogenesis of DKD in clinical and experimental diabetes. Marked systemic elevation in C5a activity was demonstrated in patients with diabetes; conventional renoprotective agents did not therapeutically target this elevation. C5a and its receptor (C5aR1) were upregulated early in the disease process and prior to manifest kidney injury in several diverse rodent models of diabetes. Genetic deletion of C5aR1 in mice conferred protection against diabetes-induced renal injury. Transcriptomic profiling of kidney revealed diabetes-induced downregulation of pathways involved in mitochondrial fatty acid metabolism. Interrogation of the lipidomics signature revealed abnormal cardiolipin remodeling in diabetic kidneys, a cardinal sign of disrupted mitochondrial architecture and bioenergetics. In vivo delivery of an orally active inhibitor of C5aR1 (PMX53) reversed the phenotypic changes and normalized the renal mitochondrial fatty acid profile, cardiolipin remodeling, and citric acid cycle intermediates. In vitro exposure of human renal proximal tubular epithelial cells to C5a led to altered mitochondrial respiratory function and reactive oxygen species generation. These experiments provide evidence for a pivotal role of the C5a/C5aR1 axis in propagating renal injury in the development of DKD by disrupting mitochondrial agility, thereby establishing a new immunometabolic signaling pathway in DKD.

Complement C5a inhibition moderates lipid metabolism and reduces tubulointerstitial fibrosis in diabetic nephropathy

Background

Complement C5 mediates pro-inflammatory responses in many immune-related renal diseases. Given that the C5a level is elevated in diabetes, we investigated whether activation of C5a/C5aR signalling plays a pathogenic role in diabetic nephropathy (DN) and the therapeutic potential of C5a inhibition for renal fibrosis.

Methods

Human renal biopsies from patients with DN and control subjects were used for immunohistochemical staining of complement C5 components. Renal function and tubulointerstitial injury were compared between db/m mice, vehicle-treated mice and C5a inhibitor-treated db/db mice. A cell culture model of tubule epithelial cells (HK-2) was used to demonstrate the effect of C5a on the renal fibrotic pathway.

Results

Increased levels of C5a, but not of its receptor C5aR, were detected in renal tubules from patients with DN. The intensity of C5a staining was positively correlated with the progression of the disease. In db/db mice, administration of a novel C5a inhibitor, NOX-D21, reduced the serum triglyceride level and attenuated the upregulation of diacylglycerolacyltransferase-1 and sterol-regulatory element binding protein-1 expression and lipid accumulation in diabetic kidney. NOX-D21-treated diabetic mice also had reduced serum blood urea nitrogen and creatinine levels with less glomerular and tubulointerstitial damage. Renal transforming growth factor beta 1 (TGF-β1), fibronectin and collagen type I expressions were reduced by NOX-D21. In HK-2 cells, C5a stimulated TGF-β production through the activation of the PI3K/Akt signalling pathway.

Conclusions

Blockade of C5a signalling by NOX-D21 moderates altered lipid metabolism in diabetes and improved tubulointerstitial fibrosis by reduction of lipid accumulation and TGF-β-driven fibrosis in diabetic kidney.

The multifaceted role of complement in kidney transplantation

Increasing evidence indicates an integral role for the complement system in the deleterious inflammatory reactions that occur during critical phases of the transplantation process, such as brain or cardiac death of the donor, surgical trauma, organ preservation and ischaemia-reperfusion injury, as well as in humoral and cellular immune responses to the allograft. Ischaemia is the most common cause of complement activation in kidney transplantation and in combination with reperfusion is a major cause of inflammation and graft damage. Complement also has a prominent role in antibody-mediated rejection (ABMR) owing to ABO and HLA incompatibility, which leads to devastating damage to the transplanted kidney. Emerging drugs and treatment modalities that inhibit complement activation at various stages in the complement cascade are being developed to ameliorate the damage caused by complement activation in transplantation. These promising new therapies have various potential applications at different stages in the process of transplantation, including inhibiting the destructive effects of ischaemia and/or reperfusion injury, treating ABMR, inducing accommodation and modulating the adaptive immune response.

Epithelial C5aR1 Signaling Enhances Uropathogenic Escherichia coli Adhesion to Human Renal Tubular Epithelial Cells

Recent work in a murine model of ascending urinary tract infection has suggested that C5a/C5aR1 interactions play a pathogenic role in the development of renal infection through enhancement of bacterial adhesion/colonization to renal tubular epithelial cells (RTECs). In the present study, we extended these observations to human. We show that renal tubular epithelial C5aR1 signaling is involved in promoting uropathogenic Escherichia coli (UPEC) adhesion/invasion of host cells. Stimulation of primary cultures of RTEC with C5a resulted in significant increases in UPEC adhesion/invasion of the RTEC. This was associated with enhanced expression of terminal α-mannosyl residues (Man) (a ligand for type 1 fimbriae of E. coli) in the RTEC following C5a stimulation. Mechanism studies revealed that C5aR1-mediated activation of ERK1/2/NF-κB and upregulation of proinflammatory cytokine production (i.e., TNF-α) is at least partly responsible for the upregulation of Man expression and bacterial adhesion. Clinical sample studies showed that C5aR1 and Man were clearly detected in the renal tubular epithelium of normal human kidney biopsies, and UPEC bound to the epithelium in a d-mannose-dependent manner. Additionally, C5a levels were significantly increased in urine of urinary tract infection patients compared with healthy controls. Our data therefore demonstrate that, in agreement with observations in mice, human renal tubular epithelial C5aR1 signaling can upregulate Man expression in RTEC, which enhances UPEC adhesion to and invasion of RTEC. It also suggests the in vivo relevance of upregulation of Man expression in renal tubular epithelium by C5a/C5aR1 interactions and its potential impact on renal infection.

Novel insights into the expression pattern of anaphylatoxin receptors in mice and men

The anaphylatoxins (AT) C3a and C5a play important roles as mediators of inflammation. Further, they regulate and control multiple innate and adaptive immune responses through binding and activation of their cognate G protein-coupled receptors, i.e. C3a receptor (C3aR), C5a receptor 1 (C5aR1) and C5a receptor 2 (C5aR2), although the latter lacks important sequence motifs for G protein-coupling. Based on their pleiotropic functions, they contribute not only to tissue homeostasis but drive, perpetuate and resolve immune responses in many inflammatory diseases including infections, malignancies, autoimmune as well as allergic diseases. During the past few years, transcriptome expression data provided detailed insights into AT receptor tissue mRNA expression. In contrast, our understanding of cellular AT receptor expression in human and mouse tissues under steady and inflammatory conditions is still sketchy. Ligand binding studies, flow cytometric and immunohistochemical analyses convincingly demonstrated tissue-specific C5aR1 expression in various cells of myeloid origin. However, a detailed map for C3aR or C5aR2 expression in human or mouse tissue cells is still lacking. Also, reports about AT expression in lymphoid cells is still controversial. To understand the multiple roles of the ATs in the innate and adaptive immune networks, a detailed understanding of their receptor expression in health and disease is required. Recent findings obtained with novel GFP or tdTomato AT-receptor knock-in mice provide detailed insights into their expression pattern in tissue immune and stroma cells. Here, we will provide an update about our current knowledge of AT receptor expression pattern in humans and mice.

Targeted complement inhibition and microvasculature in transplants: a therapeutic perspective

Active complement mediators play a key role in graft-versus-host diseases, but little attention has been given to the angiogenic balance and complement modulation during allograft acceptance. The complement cascade releases the powerful proinflammatory mediators C3a and C5a anaphylatoxins, C3b, C5b opsonins and terminal membrane attack complex into tissues, which are deleterious if unchecked. Blocking complement mediators has been considered to be a promising approach in the modern drug discovery plan, and a significant number of therapeutic alternatives have been developed to dampen complement activation and protect host cells. Numerous immune cells, especially macrophages, develop both anaphylatoxin and opsonin receptors on their cell surface and their binding affects the macrophage phenotype and their angiogenic properties. This review discusses the mechanism that complement contributes to angiogenic injury, and the development of future therapeutic targets by antagonizing activated complement mediators to preserve microvasculature in rejecting the transplanted organ.

C3a and C5a receptor antagonists ameliorate endothelial-myofibroblast transition via the Wnt/β-catenin signaling pathway in diabetic kidney disease

BACKGROUND

Endothelial-myofibroblast transition (EndMT) has been implicated in the pathogenesis of diabetic renal fibrosis. In this study, the effect of the complement fragments C3a/C5a and their receptor antagonists C3aRA and C5aRA on EndMT in diabetic kidney disease (DKD) and the possible mechanisms were investigated.

METHODS

The coexpression of CD31 with α-smooth muscle (α-SMA), C3a receptor (C3aR) and C5a receptor (C5aR) was detected in human renal biopsy tissue obtained from patients with early and advanced DKD and in normal renal tissues from patients with renal-cell carcinoma. The effects of C3aRA and C5aRA on EndMT and the expression of C3a/C3aR, C5a/C5aR, α-SMA, CD31, TGFβ, FN and β-catenin were examined in a streptozotocin (STZ)-induced rat model of DKD and in human renal glomerular endothelial cells (HRGECs) cultured in high glucose and with C3a/C5a, and DKK1 (a Wnt/β-catenin inhibitor).

RESULTS

Double-labeling of α-SMA, C3aR, C5aR and CD31 was detected in the glomerulus of renal tissues obtained from biopsies of patients with DKD. Upregulated expression of α-SMA, TGF-β, FN and β-catenin and downregulated expression of CD31 were detected in the GECs of diabetic rats. The expression of these proteins was inhibited by treatment with C3aRA/C5aRA. In vitro, C3aRA/C5aRA and DKK1 ameliorated the high glucose-induced EndMT and the subsequent expression of α-SMA, TGFβ, FN and β-catenin in HRGECs.

CONCLUSIONS

The blockade of C3aR/C5aR and the downstream Wnt/β-catenin pathway may prevent EndMT and alleviate fibrosis in the glomeruli of individuals with DKD.

Does this case hold the answer to one of the worse types of pain in medicine--that of loin pain haematuria syndrome (LPHS)

A patient with loin pain haematuria syndrome suffering chronic throbbing pulsing pain overlaid with prolonged periods of incapacitating colic and overnight vomiting was presented 10 months following diagnosis. Ultrasound was normal. No renal or ureteral stones, or filling defects were seen on CT. At cytoscopy, bladder and urethra were normal, and bloody urine effluxed from the left ureteric orifice. The ureters were normal at diagnosis, and developed new abutting non-penetrating calcifications by 8 months. Pain episodes of complete incapacitating intensity of 2-4 h duration were reduced to 10 min with 5 mg crushed tadalafil administered at onset. If tadalafil was delayed to after onset, the original course of agony resulted. Daily tadalafil reduced loin pain intensity, but not the exacerbations. Tadalafil efficacy may indicate that the pain exacerbations are due to spasm of ureter smooth muscle. 5 mg tadalafil taken at onset alleviated severe loin pain exacerbations in this case of loin pain haematuria syndrome.

Complement activation in progressive renal disease

Chronic kidney disease (CKD) is common and the cause of significant morbidity and mortality. The replacement of functioning nephrons by fibrosis is characteristic of progressive disease. The pathways that lead to fibrosis are not fully understood, although chronic non-resolving inflammation in the kidney is likely to drive the fibrotic response that occurs. In patients with progressive CKD there is histological evidence of inflammation in the interstitium and strategies that reduce inflammation reduce renal injury in pre-clinical models of CKD. The complement system is an integral part of the innate immune system but also augments adaptive immune responses. Complement activation is known to occur in many diverse renal diseases, including glomerulonephritis, thrombotic microangiopathies and transplant rejection. In this review we discuss current evidence that complement activation contributes to progression of CKD, how complement could cause renal inflammation and whether complement inhibition would slow progression of renal disease.

Monitoring and cell-specific deletion of C5aR1 using a novel floxed GFP-C5aR1 reporter knock-in mouse

Many of the biological properties of C5a are mediated through activation of its receptor (C5aR1), the expression of which has been demonstrated convincingly on myeloid cells, such as neutrophils, monocytes, and macrophages. In contrast, conflicting results exist regarding C5aR1 expression in dendritic cells (DCs) and lymphoid lineage cells. In this article, we report the generation of a floxed GFP-C5aR1 reporter knock-in mouse. Using this mouse strain, we confirmed strong C5aR1 expression in neutrophils from bone marrow, blood, lung, and spleen, as well as in peritoneal macrophages. Further, we show C5aR1 expression in lung eosinophils, lung- and lamina propria-resident and alveolar macrophages, bone marrow-derived DCs, and lung-resident CD11b(+) and monocyte-derived DCs, whereas intestinal and pulmonary CD103(+) DCs stained negative. Also, some splenic NKT cells expressed GFP, whereas naive NK cells and B2 cells lacked GFP expression. Finally, we did not observe any C5aR1 expression in naive or activated CD4(+) Th cells in vitro or in vivo. Mating the floxed GFP-C5aR1 mouse strain with LysMCre mice, we were able to specifically delete C5aR1 in neutrophils and macrophages, whereas C5aR1 expression was retained in DCs. In summary, our findings suggest that C5aR1 expression in mice is largely restricted to cells of the myeloid lineage. The novel floxed C5aR1 reporter knock-in mouse will prove useful to track C5aR1 expression in experimental models of acute and chronic inflammation and to conditionally delete C5aR1 in immune cells.

Complement-mediated cellular injury

Complement activation and recruitment of inflammatory leukocytes is an important defense mechanism against bacterial infection. However, complement also can mediate cellular injury and contribute to the pathogenesis of various diseases. With the appreciation that the C5b-9 membrane attack complex can injure cells in the absence of leukocytes, a role for the terminal complement pathway in inducing cell injury and kidney disease was shown in several experimental models, including the rat passive Heymann nephritis model of human membranous nephropathy. In podocytes, sublytic C5b-9 activates a variety of downstream pathways including protein kinases, lipid metabolism, reactive oxygen species, growth factors/gene transcription, endoplasmic reticulum stress, and the ubiquitin-proteasome system, and it impacts the integrity of the cytoskeleton and slit diaphragm proteins. C5b-9 also injures other kidney cells, including mesangial, glomerular endothelial, and tubular epithelial cells, and it contributes to the pathogenesis of mesangial-proliferative glomerulonephritis, thrombotic microangiopathy, and acute kidney injury. Conversely, certain C5b-9 signals limit complement-induced injury, or promote recovery of cells. In addition to C5b-9, complement cleavage products, such as C5a and C1q, can injure kidney cells. Thus, the complement system contributes to various kidney pathologies by causing cellular damage in both an inflammation-dependent and inflammation-independent manner.

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.

New insights for C5a and C5a receptors in sepsis

The complement system plays a central role in inflammation and immunity. Among the complement activation products, C5a is one of the most potent inflammatory peptides with a broad spectrum of functions. There is strong evidence for complement activation including elevated plasma level of C5a in humans and animals with sepsis. C5a exerts its effects through the C5a receptors. Of the two receptors that bind C5a, the C5aR (CD88) is known to mediate signaling activity, whereas the function of another C5a binding receptor, C5L2, remains largely unknown. Here, we review the critical role of C5a in sepsis and summarize evidence indicating that both C5aR and C5L2 act as regulating receptors for C5a during sepsis.

C3a and C5a promote renal ischemia-reperfusion injury

Renal ischemia reperfusion injury triggers complement activation, but whether and how the small proinflammatory fragments C3a and C5a contribute to the pathogenesis of this injury remains to be elucidated. Using C3aR-, C5aR-, or C3aR/C5aR-deficient mice and models of renal ischemia-reperfusion injury, we found that deficiency of either or both of these receptors protected mice from injury, but the C3aR/C5aR- and C5aR-deficient mice were most protected. Protection from injury was associated with less cellular infiltration and lower mRNA levels of kidney injury molecule-1, proinflammatory mediators, and adhesion molecules in postischemic kidneys. Furthermore, chimera studies showed that the absence of C3aR and C5aR on renal tubular epithelial cells or circulating leukocytes attenuated renal ischemia-reperfusion injury. In vitro, C3a and C5a stimulation induced inflammatory mediators from both renal tubular epithelial cells and macrophages after hypoxia/reoxygenation. In conclusion, although both C3a and C5a contribute to renal ischemia-reperfusion injury, the pathogenic role of C5a in this injury predominates. These data also suggest that expression of C3aR and C5aR on both renal and circulating leukocytes contributes to the pathogenesis of renal ischemia-reperfusion injury.

C5a and its receptors in human anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis

Introduction

The complement system is crucial for the development of antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV). In particular, C5a plays a central role. In this study, plasma and urinary levels of C5a as well as renal C5a receptors (CD88 and C5L2) expression were investigated in patients with AAV.

Methods

Twenty-four patients with AAV in the active phase, 19 patients with AAV in the remission phase, and 20 patients with lupus nephritis (LN) were included. Plasma and urinary levels of C5a were measured with enzyme-linked immunosorbent assay (ELISA). The staining of CD88 and C5L2 in renal specimens was detected with immunohistochemistry.

Results

The level of plasma C5a was significantly higher in patients with AAV in the active phase than that in patients in remission, that in patients with LN, and that in normal controls. The urinary C5a level was significantly higher in patients with AAV in the active phase than that in patients in remission and that in normal controls, but not significantly different between patients with active AAV and patients with LN. The mean optical density of CD88 staining in the tubulointerstitium was significantly lower in AAV patients than that in normal controls (0.0052 ± 0.0011 versus 0.029 ± 0.0042; P = 0.005). The mean optical density of C5L2 in glomeruli was significantly higher in AAV patients than that in normal controls (0.013 ± 0.0027 versus 0.0032 ± 0.0006; P < 0.001). The mean optical density of CD88 staining closely correlated with the initial eGFR (r = 0.835; P < 0.001) in AAV patients. Double-labeling immunofluorescence assay suggested that CD88 did not express on neutrophils, monocytes, or macrophages, but C5L2 expressed on neutrophils (or monocytes) and macrophages.

Conclusion

The elevated plasma and urinary C5a levels indicated complement activation in human AAV. The level of renal CD88 expression could reflect the disease severity of ANCA-associated glomerulonephritis. CD88 expression was downregulated, and C5L2 was upregulated in ANCA-associated glomerulonephritis.

Crosstalk between complement and Toll-like receptor activation in relation to donor brain death and renal ischemia-reperfusion injury

Two central pathways of innate immunity, complement and Toll-like receptors (TLRs), play an important role in the pathogenesis of renal injury inherent to kidney transplantation. Recent findings indicate close crosstalk between complement and TLR signaling pathways. It is suggested that mitogen activated protein kinases (MAPKs) might be the key molecules linking both the complement and TLR pathways together. Complement and TLRs are important mediators of renal ischemia-reperfusion injury (IRI). Besides IRI, complement C3 can also be upregulated and activated in the kidney before transplantation as a direct result of brain death (BD) in the donor. This local upregulation and activation of complement in the donor kidney has been proven to be detrimental for renal allograft outcome. Also TLR4 and several of its major ligands are upregulated by donor BD compared to living donors. Important and in line with the observations above, kidney transplant recipients have a benefit when receiving a kidney from a TLR4 Asp299Gly/Thr399Ile genotypic donor. The role of complement and TLRs and crosstalk between these two innate immune systems in relation to renal injury during donor BD and ischemia-reperfusion are focus of this review. Future strategies to target complement and TLR activation in kidney transplantation are considered.

Attenuation of IgG immune complex-induced acute lung injury by silencing C5aR in lung epithelial cells

Acute lung injury (ALI) in mouse lung occurs after distal airway deposition of IgG immune complexes (IgGICs), resulting in a breakdown of the vascular-airway barrier, causing intrapulmonary edema, hemorrhage, and accumulation of neutrophils [polymorphonuclear leukocytes (PMNs)] in the alveolar compartment, these changes being complement (C5a) and C5a receptor (C5aR) dependent. In this ALI model, C5aR expression (protein) was found to occur on upper (bronchial) and lower (alveolar) airway epithelial cells. An adenovirus construct (siRNA) was used to silence mRNA for C5aR in the lung. Under such conditions, C5aR protein was markedly reduced on lung epithelial cells, resulting in much reduced leakage of albumin into the lung, diminished buildup of PMNs, and lower levels of proinflammatory mediators in bronchoalveolar lavage fluids. These studies indicate that bronchial and alveolar epithelial cell C5aR is up-regulated and greatly contributes to inflammation and injury in the lung. The use of siRNA administered into the airways avoids systemic suppression of C5aR, which might compromise innate immunity. It is possible that such an intervention might be employed in humans with ALI or acute respiratory distress syndrome as well as in upper-airway inflammatory diseases, such as chronic obstructive pulmonary disease and asthma, where there is evidence for complement activation and buildup of PMNs.

Adiponectin is present in the urine in its native conformation, and specifically reduces the secretion of MCP-1 by proximal tubular cells

AIM

To determine whether adiponectin detected in urine is present in its native form and if adiponectin receptors (AdipoR) present and functional in proximal tubular (HK-2) cells.

BACKGROUND

Adiponectin is a protein with anti-inflammatory, anti-atherogenic and insulin-sensitizing properties. It has previously been detected antigenically in the urine in several forms of renal disease.

METHODS

We compared the isoform distribution of urinary adiponectin in patients with proteinuric and non-proteinuric renal disease with that of matched controls using chromatography and enzyme-linked immunosorbent assay. We examined whether AdipoR were present in HK-2 cells by real-time reverse transcription polymerase chain reaction. Their functionality was investigated by determining the effect of recombinant adiponectin on adenosine monophosphate-activated protein kinase phosphorylation using western blotting, and on the secretion of monocyte chemotactic protein-1 and C3 using enzyme-linked immunosorbent assays.

RESULTS

Adiponectin in the urine is physiologically intact and largely present as the low molecular weight isoform. Subjects with urinary protein >150 mg/L excreted significantly more adiponectin and its high and low molecular weight isoforms than those with <150 mg/L. mRNA for AdipoR were present in HK-2 cells, with levels of mRNA for AdipoR1 being 20 times greater than those for AdipoR2. Ligation of AdipoR on proximal tubular cells increased phosphorylation of adenosine monophosphate-activated protein kinase, and downregulated the secretion of the inflammatory cytokine monocyte chemotactic protein-1, but not of C3.

CONCLUSION

Physiologically relevant isoforms of adiponectin are present in the urine of normal subjects and those with proteinuria. In addition, functional receptors for adiponectin are present in HK-2 cells. Abnormal levels of adiponectin in the urine may therefore activate these receptors, potentially resulting in anti-inflammatory activity.

Pharmacological targeting of C5a receptors during organ preservation improves kidney graft survival

Cadaveric renal transplants suffer frequently from delayed graft function, which is associated with increased risk for long-term graft survival loss. One-third of kidney grafts that are stored in current organ preservation solutions experience delayed graft function, demonstrating the urgent need for improvement. Although ischaemic graft injury is complex in nature, complement activation is considered important to the process. Here we show that pharmacological targeting of the complement 5a receptor (C5aR) during cold ischaemia has a protective effect on early kidney graft survival, inflammation and apoptosis in a mouse model of syngeneic kidney transplantation. Graft survival of kidneys that were stored in University of Wisconsin solution in the presence of a C5aR antagonist increased from 29% to 57%. Increased graft survival was associated with less tubular damage and apoptosis, protection from sustained C5aR expression and decreased production of tumour necrosis factor-alpha and macrophage inflammatory protein-2. In a translational approach, we determined C5aR expression in paediatric living-related and cadaveric allografts. C5aR expression was significantly higher in all compartments of kidneys from cadaveric compared with kidneys from living-related donors. C5aR expression in cadaveric kidneys correlated positively with cold ischaemia time, renal dysfunction and the frequency of apoptotic tubular cells, suggesting a novel role for C5a in delayed graft function pathogenesis. Supplementing organ preservation solutions with C5aR inhibitors may improve early graft function following cadaveric kidney transplantation.

Overexpression of innate immune response genes in a model of recessive polycystic kidney disease

Defects in the primary cilium/basal body complex of renal tubular cells cause polycystic kidney disease (PKD). To uncover pathways associated with disease progression, we determined the kidney transcriptome of 10-day-old severely and mildly affected cpk mice, a model of recessive PKD. In the severe phenotype, the most highly expressed genes were those associated with the innate immune response including many macrophage markers, particularly those associated with a profibrotic alternative activation pathway. Additionally, gene expression of macrophage activators was dominated by the complement system factors including the central complement component 3. Additional studies confirmed increased complement component 3 protein levels in both cystic and non-cystic epithelia in the kidneys of cpk compared to wild-type mice. We also found elevated complement component 3 activation in two other mouse-recessive models and human-recessive PKD. Our results suggest that abnormal complement component 3 activation is a key element of progression in PKD.

The Many Effects of Complement C3- and C5-Binding Proteins in Renal Injury

The complement system is an important component of the innate immune system and a modulator of adaptive immunity. The entire complement system is focused on C3 and C5. Thus, there are proteins that activate C3 and C5, those that regulate this activation, and those that transduce the effects of C3 and C5 activation products; each can affect the kidney in renal injury. The normal kidney has the inherent capacity to protect itself from complement activation through cellular expression of decay-accelerating factor, membrane cofactor protein (in human beings), and Crry (in rodents). In addition, plasma factor H protects vascular spaces in the kidney. Although the main function of these proteins is to limit complement activation, there is now considerable evidence that they can transduce signals on engagement in immune cells. The G-protein-coupled 7-span transmembrane receptors for C3a and C5a, and the integral membrane complement receptors (CR) for C3b, iC3b, and C3dg, are expressed outside the kidney, particularly in cells of hematopoietic and immune lineage. These are important in renal injury through their infiltration of the kidney and/or by affecting kidney-directed immune responses. There is mounting evidence that intrinsic glomerular and tubular cell C3aR and C5aR expression and activation also can affect renal injury. CR1 on podocytes and the beta2 integrins CR3 and CR4 in kidney dendritic cells have functions that remain poorly defined. Cells of the kidney also have the capacity to produce and activate their own complement proteins. Thus, intrinsic renal cells express decay-accelerating factor, membrane cofactor protein, Crry, C3aR, C5aR, CR1, CR3, and CR4. These can be engaged by C3 and C5 activation products derived from systemic and local pools in renal injury. Given their capacity to provide signals that influence kidney cellular behavior, their activation can have substantial effects in renal injury. Defining these in a cell- and disease-specific fashion is an exciting challenge for future research.

Decay-accelerating factor but not CD59 limits experimental immune-complex glomerulonephritis

The complex balance between the pro-activating and regulatory influences of the complement system can affect the pathogenesis of immune complex-mediated glomerulonephritis (ICGN). Key complement regulatory proteins include decay accelerating factor (DAF) and CD59, which inhibit C3 activation and C5b-9 generation, respectively. Both are glycosylphosphatidylinositol-linked cell membrane proteins, which are widely distributed in humans and mice. Chronic serum sickness induced by daily immunization with horse spleen apoferritin over 6 weeks was used to induce ICGN in DAF-, CD59- and DAF/CD59-deficient mice, with wild-type littermate mice serving as controls. Both DAF and DAF/CD59-deficient mice had an increased incidence of GN relative to wild-type controls associated with significantly increased glomerular C3 deposition. Disease expression in CD59-deficient mice was no different than wild-type controls. DAF- and DAF/CD59-deficient mice also had increased monocyte chemoattractant protein-1 mRNA expression and glomerular infiltration with CD45(+) leukocytes. Our findings suggest that activation of C3 is strongly associated with experimental ICGN while downstream formation of C5b-9 is of lesser pathogenic importance in this model.

Complement C5 mediates experimental tubulointerstitial fibrosis

Renal fibrosis is the final common pathway of most progressive renal diseases. C5 was recently identified as a risk factor for liver fibrosis. This study investigated the role of C5 in the development of renal tubulointerstitial fibrosis by (1) induction of renal fibrosis in wild-type and C5(-/-) mice by unilateral ureteral ligation (UUO) and (2) investigation of the effects of a C5a receptor antagonist (C5aRA) in UUO. In C5(-/-) mice, when compared with wild-type controls, markers of renal fibrosis (Sirius Red, type I collagen, fibronectin, alpha-smooth muscle actin, vimentin, and infiltrating macrophages) were significantly reduced on day 5 of UUO. On day 10, fibronectin mRNA and protein expression were still reduced in the C5(-/-) mice. Cortical mRNA of all PDGF isoforms and of TGF-beta(1) (i.e., central mediators of renal disease) were significantly reduced in C5(-/-) mice when compared with controls. Renal tubular cell expression of the C5aR was sparse in normal cortex but markedly upregulated after UUO. Treatment of wild-type UUO mice with C5aRA also led to a significant reduction of cortical Sirius Red staining, fibronectin protein expression, and PDGF-B mRNA expression on day 5. Neither genetic C5 deficiency nor C5aRA treatment caused any histologic changes in the nonobstructed kidneys. In cultured murine cortical tubular cells, C5a stimulated production of TGF-beta(1), and this was inhibited by C5aRA. Using a combined genetic and pharmacologic approach, C5, in particular C5a, is identified as a novel profibrotic factor in renal disease and as a potential new therapeutic target.

Self, non-self, and danger: a complementary view

Complement is a sophisticated system of molecules that is critical to the functional integrity of the body. Initially considered as a defense system to ward off infections, it becomes increasingly clear that the complement system is one of the most important humoral systems to sense danger, i.e., to recognize conserved patterns on pathogens and on altered/damaged self. In addition to this important role in danger recognition, the complement system has the ability to translate the danger information into an adequate cellular innate or adaptive immune response. This is accomplished by two distinct mechanisms: (a) danger sensors that have recognized altered cells or pathogens can directly activate cell-bound receptors (e.g., C1q/C1q receptor interaction), and/or (b) danger sensors initiate cleavage of complement factors C3 and C5, the fragments of which acquire the ability to bind to complement receptors and/or regulators. It is the specific interaction of the danger sensors and of the cleavage fragments with distinct cell-bound receptors/regulators that directs the immune response toward an innate or an adaptive phenotype. Further, the expression pattern of the complement receptors critically impacts the shape of the immune response. Complement has the ability to discriminate between physiological and pathological danger, i.e., physiological cell death and death in response to injury. In the former case, cells are merely flagged for enhanced phagocytosis (by C3 fragments) without accompanying inflammation (through CR3), whereas in the latter case inflammatory signals are accessorily triggered (e.g., by the release of ATs, which recruit and activate neutrophils, eosinophils, etc.). This function is of major importance for apoptotic cell clearance and tissue repair but plays also important roles in fibrotic tissue remodeling in response to chronic tissue injury. Further, complement cleavage fragments may prevent the development of maldaptive immune responses at the mucosal surface. Here, complement fragment C5a does not act as a danger transmitter but as a "homeostasis transmitter," as its interaction with the C5a receptor on DCs provides a signal that prevents DCs from activating CD4+ T cells. The generation of regulatory T cells in response to CD46 ligation may have a similar function, as injured cells lose CD46 expresssion, which may lead to decreased proliferation of Tregs and, consecutively, increased production of T effector cells. Although we are still at the beginning of understanding the complex interaction patterns within the complement system, recent data suggest substantial crosstalk between the signaling pathways downstream of complement receptors and other receptors of the innate immune system that function as immune sensors and/or transmitters (i.e., TLRs, FcgammaRs130,131). Given the importance of complement as a sensor and effector system of innate and adaptive immune responses, a complement-related view of the immune system might help to unravel some enigmas of autoimmunity, allergy, and transplantation.

The role of complement in danger sensing and transmission

Self-non-self discrimination has long been considered the main function of the immune system. Increasing evidence supports the view of the immune system as a network of complex danger sensors and transmitters in which self-non-self discrimination is only one facet. To meet the challenge of danger sensing, the immune system carries a large stock of germline-encoded, highly conserved molecules that can recognize microbial as well as modified host structures. Among those are the Toll-like receptors (TLR), which comprise a dozen membrane-bound pattern-recognition receptors that directly link danger recognition to danger transmission through activation of several distinct cellular signaling pathways. Here, I discuss the function and biology of a complex, evolutionary ancient system, the complement system, which has long been considered critical to host defense. In contrast to TLRs, the complement system senses danger by a panel of soluble molecules that can directly bind to specific complement receptors and/or initiate a complex cascade of proteolytic events that lead to the generation of soluble complement fragments able to bind to another, distinct set of specific complement receptors. As I will outline in this review, complement- mediated danger sensing and the complex transition of this information into distinct cellular activation profiles is critical for tissue homeostasis under steady-state conditions and in response to infection and cell injury. Furthermore, I will discuss recent findings that support a concept of intense cross-talk between the complement system and TLRs, which defines the quality and the magnitude of immune responses in vivo.

Proteomic profiling of urinary protein excretion in the factor H-deficient mouse

BACKGROUND

Since the 1970s a variety of experimental techniques have been employed in an attempt to identify urinary biomarkers of renal injury. While these approaches have met with some success, modern proteomic tools now permit broad based high-throughput analysis of the urinary proteome.

METHODS

Using the ICAT isotopic labeling based LC/MS/MS approach, comparative urinary protein profiling was performed in a murine model of membranoproliferative glomerulonephritis. Paired samples were analyzed mice with a targeted deletion of the complement regulatory protein factor H (FH-/-) and control mice.

RESULTS

25 distinct urinary proteins were identified of which 7 were differentially expressed in the FH-/- mice. Two proteins were markedly altered in the urine of FH-/- mice compared to controls: uromodulin (5.5-fold lower) and the MHC class II molecule H2e (8.6-fold higher). Differential expression was confirmed by Western blot and RT-PCR. Immunofluorescent staining demonstrated a marked increased expression of H2e and a reduction of uromodulin expression in the tubular epithelium of FH-/- mice.

CONCLUSIONS

These findings provide insight into early complement-dependent alterations in tubular protein expression which may play critical roles in the development of tubulointerstitial disease, and provide experimental support for the use of urinary proteomic profiling in murine models of renal injury.

Increased concentration of the complement split product C5a in acute pyelonephritis during pregnancy

OBJECTIVE

Pregnant women with acute pyelonephritis develop acute respiratory distress syndrome (ARDS) more frequently than non-pregnant women. The reasons for this remain unknown. The complement system is a complex set of self-assembling proteins that have been implicated in the pathophysiology of ARDS and sepsis. The purpose of this study was to determine if activation of the complement system occurs in pregnant women with acute pyelonephritis.

METHODS

A cross-sectional study was conducted to determine the plasma concentrations of C3a, C4a and C5a (i.e., complement split products) in pregnant patients with acute pyelonephritis (n=38) and normal pregnant women (n=38). The complement split products C3a, C4a and C5a were measured using ELISA. Data were analyzed using non-parametric statistics.

RESULTS

1) The median plasma concentration of C5a in pregnant patients with acute pyelonephritis was significantly higher than that in normal pregnant women (p<0.001); 2) there was no statistical difference in the median plasma concentration of C3a and C4a between the two groups (p>0.05); and 3) concentrations of C3a, C4a and C5a were not different among patients with acute pyelonephritis with and without bacteremia.

CONCLUSIONS

1) Pyelonephritis in pregnant women is associated with an increased plasma concentration of C5a, but not C3a and C4a; and 2) an excess of C5a can predispose pregnant women to develop ARDS and multi-organ failure in pyelonephritis. This finding may have clinical implications since blocking C5a improves ARDS in experimental sepsis.

C5a receptor deficiency attenuates T cell function and renal disease in MRLlpr mice

The development and progression of systemic lupus erythematosus (SLE) is strongly associated with complement activation and deposition. To characterize the role of C5a and its receptor (C5aR) in SLE, C5aR-deficient mice were backcrossed nine generations onto the lupus-like MRL(lpr) genetic background. Evidence is presented that C5aR modulates both renal injury and T cell responses in MRL(lpr) mouse. C5aR-deficient MRL(lpr) mice had prolonged viability, with a mean survival time of 33.0 wk compared with 22.6 wk in control mice. Renal injury was also attenuated in the C5aR-/- MRL(lpr) mice. At 20 wk of age C5aR-/- MRL(lpr) mice had a complete absence of glomerular crescents and marked reductions in glomerular hypercellularity. There was no difference in the degree of glomerular C3 deposition; however, IgG deposits were reduced in the C5aR-/- MRL(lpr) mice. The reduction in glomerular injury was also associated with a four-fold decrease in renal CD4+ T cell infiltrates. Whereas there were modest differences in total IgG anti-dsDNA antibody titers, C5aR-deficient mice had 3.5-fold higher levels of IgG1 and 15-fold lower levels of IgG2a anti-dsDNA antibody titers compared to controls. The differences in anti-dsDNA IgG subclasses were associated with reduced CD4+ Th-1 responses in the C5aR-/- MRL(lpr) mice, including diminished production of IL-12p70, IFN-gamma, and increased expression of the Th-2 transcription factor GATA-3. These findings indicate that the C5aR plays a major role in modulating complement-dependent renal injury and T helper cell Th-1 responses in the MRL(lpr) mouse.

C5a promotes development of experimental lupus nephritis which can be blocked with a specific receptor antagonist

The MRL/lpr murine SLE model has widespread complement activation and deposition of complement fragments in affected tissues. The potent anaphylatoxin C5a has the potential to play a key role in the pathogenesis of lupus nephritis. We found that renal expression of C5aR mRNA and protein was significantly increased in MRL/lpr mice compared to control MRL/+ mice. To examine the role of C5a signaling through C5aR, a specific small molecule antagonist (a) of C5aR was administered continuously to MRL/lpr mice from 13 to 19 wks of age. Littermate controls were given vehicle alone. The progressive impairment in renal function exhibited in the control group was prevented by C5aRa treatment. Infiltration of neutrophils and macrophages into kidneys was significantly reduced in animals treated with C5aRa compared to controls. Furthermore, renal expression of IL-1beta and MIP-2 mRNA as well as the extent of apoptosis were significantly decreased with blockade of C5aR, indicating their dependence upon signals delivered through C5aR. Thus, pharmacological blockade of C5aR reduces disease manifestations in experimental lupus nephritis. These data support an important role for the C5a anaphylatoxin in lupus nephritis, and that blockade of C5aR represents a potentially viable treatment for human lupus nephritis.

Complement Factor H Limits Immune Complex Deposition and Prevents Inflammation and Scarring in Glomeruli of Mice with Chronic Serum Sickness

Factor H is the major complement regulator in plasma. Abnormalities in factor H have been implicated in membranoproliferative glomerulonephritis in both humans and experimental animals. It has been shown that factor H on rodent platelets functions analogously to human erythrocyte complement receptor 1 in its role to traffic immune complexes to the mononuclear phagocyte system. C57BL/6 factor H-deficient mice (Cfh(-/-)) and wild-type (wt) controls were immunized daily for 5 wk with heterologous apoferritin to study the chronic serum sickness GN model. Immunizations were started in 6- to 8-wk-old mice, which was before the development of spontaneous membranoproliferative glomerulonephritis in some Cfh(-/-) animals. Glomerular deposition of IgG immune complexes in glomeruli was qualitatively and quantitatively increased in Cfh(-/-) mice compared with wt mice. Consistent with the increase in glomerular immune complexes and possibly because of alternative pathway complement activation, Cfh(-/-) mice had increased glomerular C3 deposition. Wt mice developed no glomerular pathology. In contrast, Cfh(-/-) mice developed diffuse proliferative GN with focal crescents and glomerulosclerosis. In addition, there was significantly increased expression of collagen IV, fibronectin, and laminin mRNA in Cfh(-/-) glomeruli. These data show a role for platelet-associated factor H to process immune complexes and limit their accumulation in glomeruli. Once deposited in glomeruli, excessive complement activation can lead to glomerular inflammation and the rapid development of a scarring phenotype.

Renal expression of the C3a receptor and functional responses of primary human proximal tubular epithelial cells

Although complement activation and deposition have been associated with a variety of glomerulopathies, the pathogenic mechanisms by which complement directly mediates renal injury remain to be fully elucidated. Renal parenchymal tissues express a limited repertoire of receptors that directly bind activated complement proteins. We report the renal expression of the receptor for the C3 cleavage product C3a, a member of the anaphylatoxin family. C3aR is highly expressed in normal human and murine kidney, as demonstrated by immunohistochemistry and in situ hybridization. Its distribution is limited to epithelial cells only, as glomerular endothelial and mesangial cells showed no evidence of C3aR expression. The C3aR is also expressed by primary renal proximal tubular epithelial cells in vitro as demonstrated by FACS, Western blot, and RT-PCR. In vitro C3aR is functional in terms of its capacity to bind 125I-labeled C3a and generate inositol triphosphate. Finally, using microarray analysis, four novel genes were identified and confirmed as transcriptionally regulated by C3aR activation in proximal tubular cells. These studies define a new pathway by which complement activation may directly modulate the renal response to immunologic injury.

Excessive matrix accumulation in the kidneys of MRL/lpr lupus mice is dependent on complement activation

Complement receptor 1-related gene/protein y (Crry) in rodents is a potent membrane complement regulator that inhibits complement C3 activation by both classical and alternative pathways. Complement inhibition with Crry as the recombinant protein Crry-Ig has been demonstrated to prevent MRL/MpJ-Tnfrsf6(lpr) (MRL/lpr) mice from developing proteinuria and renal failure. Crry-Ig-treated mice also showed less glomerulosclerosis compared with control MRL/lpr mice. To clarify how complement inhibition with Crry might affect renal scarring in lupus nephritis, gene transcript profiling was performed comparing Crry-Ig-treated MRL/lpr mice to control-treated MRL/lpr mice as well as to the MRL/+ strain control. Altered gene expression was confirmed by quantitative PCR, and protein quantity with either immunoblotting or immunofluorescence microscopy. Collagens I, III, IV, and VI were overexpressed in control MRL/lpr mice, whereas complement inhibition with Crry reduced the overexpression of these extracellular matrix components toward normal. Plasminogen activator inhibitor 1, connective tissue growth factor, and TGF-beta1 were upregulated in MRL/lpr mice compared with MRL/+ mice and were normalized by Crry-Ig treatment, suggesting that the product of these genes may contribute to the progressive glomerulosclerosis in MRL/lpr mice in a complement-dependent fashion. Thus, complement inhibition with Crry has a prominent effect on matrix-related genes and proteins, which translates into improvement in functional renal disease.

Carboxypeptidase N: a pleiotropic regulator of inflammation

Carboxypeptidase N (CPN) is a plasma zinc metalloprotease, which consists of two enzymatically active small subunits (CPN1) and two large subunits (CPN2) that protect the protein from degradation. CPN cleaves carboxy-terminal arginines and lysines from peptides found in the bloodstream such as complement anaphylatoxins, kinins, and creatine kinase MM (CK-MM). By removing only one amino acid, CPN has the ability to change peptide activity and receptor binding. CPN is a member of a larger family of carboxypeptidases, many of which also cleave arginine and lysine. Because of the highly conserved active sites and the possible redundant functions of carboxypeptidases, it has been difficult to elucidate the role of CPN in disease processes. The future use of gene ablation technology may be the most appropriate way to understand the function of CPN in vivo.

C5a mutants are potent antagonists of the C5a receptor (CD88) and of C5L2: position 69 is the locus that determines agonism or antagonism

The anaphylatoxin C5a exerts a plethora of biologic activities critical in the pathogenesis of systemic inflammatory diseases. Recently, we reported on a C5a mutant, jun/fos-A8, as a potent antagonist for the human and mouse C5a receptor (CD88). Addressing the molecular mechanism accounting for CD88 receptor antagonism by site-directed mutagenesis, we found that a positively charged amino acid at position 69 is crucial. Replacements by either hydrophobic or negatively charged amino acids switched the CD88 antagonist jun/fos-A8 to a CD88 agonist. In addition to CD88, the seven-transmembrane receptor C5L2 has recently been found to provide high affinity binding sites for C5a and its desarginated form, C5adesArg74. A jun/fos-A8 mutant in which the jun/ fos moieties and amino acids at positions 71-73 were deleted, A8Delta71-73, blocked C5a and C5adesArg74 binding to CD88 and C5L2. In contrast, the cyclic C5a C-terminal analog peptide AcF-[OP-d-ChaWR] inhibited binding of the two anaphylatoxins to CD88 but not to C5L2, suggesting that the C5a core segment is important for high affinity binding to C5L2. Both receptors are coexpressed on human monocytes and the human mast cell line HMC-1; however, C5L2 expression on monocytes is weaker as compared with HMC-1 cells and highly variable. In contrast, no C5L2 expression was found on human neutrophils. A8Delta71-73 is the first antagonist that blocks C5a and C5adesArg74 binding to both C5a receptors, CD88 and C5L2, making it a valuable tool for studying C5L2 functions and for blocking the biological activities of C5a and C5adesArg74 in mice and humans.

Distribution of rat C5a anaphylatoxin receptor

The anaphylatoxin, complement 5a (C5a), plays a key role in mediating various inflammatory reactions following complement activation. Several investigators have reported that C5a receptor (C5aR) is expressed in non-myeloid cells under certain conditions or in different cell lines. In our study, the abundance of C5aR-positive myeloid cells in rats depended on the organs examined. C5aR was usually expressed at the site of exposure to pathogens, such as in salivary gland or lung, and was up-regulated in liver in the inflammatory state induced by lipopolysaccharide (LPS) administration. Furthermore, the increased expression of C5aR antigen was not accompanied by an increase in C5aR mRNA in Kupffer cells following LPS challenge.

Complement factor C5a mediates renal ischemia-reperfusion injury independent from neutrophils

The complement system has been shown to mediate renal ischemia-reperfusion (I/R) injury. However, the contribution of complement factor C5a to I/R injury, in particular in the kidney, remains to be established. In this study, we investigated the impact of blocking the C5aR pathway on the inflammatory response and on the renal function in a murine model of I/R injury. First, we analyzed C5aR expression in kidneys of healthy mice. Intriguingly, we found expression on mesangial, as well as on tubular epithelial, cells. After I/R injury, C5aR expression was up-regulated in tubular epithelial cells. In addition, mRNA levels of CXC chemokines and TNF-alpha increased significantly and kidneys were heavily infiltrated by neutrophils. Blocking the C5aR pathway by a specific C5a receptor antagonist (C5aRA) abrogated up-regulation of CXC chemokines but not of TNF-alpha and reduced neutrophil infiltration by >50%. Moreover, application of the C5aRA significantly reduced loss of renal function. This improvement of function was independent of the presence of neutrophils because neutrophil depletion by mAb NIMP-R14 did not affect the protective effect of C5aRA treatment. Furthermore, blocking of the C5aR pathway had no influence on renal apoptosis. These data provide evidence that C5a is crucially involved in the pathogenesis of renal I/R injury by modulation of neutrophil-dependent as well as neutrophil-independent pathways, which include the regulation of CXC chemokines but not TNF-alpha or apoptotic pathways.

Administration of a soluble recombinant complement C3 inhibitor protects against renal disease in MRL/lpr mice

Complement receptor 1-related gene/protein y (Crry) in rodents is a potent membrane complement regulator that inhibits complement C3 activation by both classical and alternative pathways. To clarify the role of complement in lupus nephritis, MRL/lpr mice were given Crry as a recombinant protein (Crry-Ig) from 12 to 24 wk of age. Control groups were given saline or normal mouse IgG. Sera and urine were collected biweekly. Only 1 of 20 (5%) Crry-Ig-treated mice developed renal failure (BUN > 50 mg/dl) compared with 18 of 38 (47.4%) mice in control groups (P = 0.001). BUN levels at 24 wk were reduced from 68.8 +/- 9.7 mg/dl in control groups to 38.5 +/- 3.9 mg/dl in the Crry-Ig-treated group (P < 0.01). Urinary albumin excretion at 24 wk was also significantly reduced from 5.3 +/- 1.4 mg/mg creatinine in the control groups to 0.5 +/- 0.2 mg/mg creatinine in the Crry-Ig-treated group (P < 0.05). Of the histologic data at 24 wk, there was a significant reduction in scores for glomerulosclerosis and C3d, IgG, IgG3, and IgA staining intensity in glomeruli in complement-inhibited animals. Crry-Ig-treated animals were also protected from vasculitic lesions. Although there was no effect on relevant autoimmune manifestations such as anti-double stranded DNA titers or cryoglobulin IgG3 levels, circulating immune complex levels were markedly higher in complement-inhibited animals. Thus, inhibition of complement activation with Crry-Ig significantly reduces renal disease in MRL/lpr lupus mice. The data support the strategy of using recombinant complement C3 inhibitors to treat human lupus nephritis.

A small molecule C5a receptor antagonist protects kidneys from ischemia/reperfusion injury in rats

BACKGROUND

C5a has been implicated in numerous pathophysiological conditions, including ischemia/reperfusion (I/R) injury of the kidney. We examined whether a novel and specific C5a receptor antagonist, the cyclic compound AcF-[OPdChaWR] could moderate I/R-induced renal injury in rats.

METHODS

Female Wistar rats were subjected to renal ischemia (60 min) and reperfusion (5 h). Rats were treated with either 1 mg/kg IV in 5% ethanol/saline or 10 mg/kg PO in 25% ethanol/saline prior to ischemia. I/R injury was characterized by significant tissue hemorrhage with increased microvascular permeability, elevated renal tissue levels of tumor necrosis factor-alpha (TNF-alpha) and myeloperoxidase (MPO), increased serum levels of creatinine and aspartate aminotransferase (AST) and hematuria.

RESULTS

Pre-ischemic treatment with the C5a receptor (C5aR) antagonist (1 mg/kg IV or 10 mg/kg PO) substantially inhibited or prevented I/R-induced hematuria, vascular leakage, tissue levels of TNF-alpha and MPO, and serum levels of AST and creatinine. Histological examination of kidneys from antagonist pretreated I/R animals showed a marked reduction in tissue damage compared to drug-free I/R rats. This antagonist, however, did not inhibit complement-mediated lysis of red blood cells, suggesting unimpaired formation of the membrane attack complex (MAC).

CONCLUSIONS

The results demonstrate for the first time that a selective antagonist of both human and rat C5a receptors, given either intravenously or orally, significantly protects the kidney from I/R injury in the rat. We conclude that C5a is an important pathogenic agent in renal I/R injury, and that C5a receptor antagonists may be useful therapeutic agents for the pretreatment of anticipated renal reperfusion injury in humans.

C5a is important in the tubulointerstitial component of experimental immune complex glomerulonephritis

Interstitial injury is the hallmark of glomerulonephritis which is progressing to end-stage renal disease (ESRD). In humans and experimental animals, we have shown that interstitial disease is accompanied by up-regulation of complement components in tubular epithelial cells. Glomerulonephritis was induced in mice by the intraperitoneal injection of horse spleen apoferritin (HSA) and lipopolysaccharide (LPS). In addition to wild-type C57/B6 mice, animals in which the C5a receptor had been deleted (C5aR KO) were used. Animals were killed after 3 or 6 weeks, and kidneys harvested. At three weeks, both groups had evidence of mild mesangial matrix expansion and increased cellularity; there were no crescents, sclerotic lesions, or interstitial disease. At six weeks, glomerular lesions were advanced, but identical in the two groups. Both groups had evidence of an identical pattern of C3 gene expression in the tubular epithelium by in situ hybridization. There was a marked difference, however, in the extent of interstitial injury. Wild-type animals had significantly greater numbers of infiltrating interstitial cells, greater expansion of the peritubular space, more tubular atrophy, and more apoptotic tubular cells than did C5aR KOs. The anaphylotoxic fragment of C5, C5a, is not likely to be important in the glomerular component of this model of progressive glomerulonephritis. On the other hand, the interstitial component is markedly attenuated in knockout animals. These data support a role for complement in the interstitial component of this glomerulonephritis model. They are consistent with our hypotheses of a role for complement in the progression of some forms of glomerulonephritis to ESRD.