Mannan-binding lectin recognizes structures on ischaemic reperfused mouse kidneys and is implicated in tissue injury

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.

Identification and Verification of Potential Biomarkers in Renal Ischemia-Reperfusion Injury by Integrated Bioinformatic Analysis

Background

Renal ischemia-reperfusion injury (RIRI) plays an important role in the poor prognosis of patients with renal transplants. However, the potential targets and mechanism of IRI are still unclear.

Method

Differential gene expression (DEG) analysis and weighted correlation network analysis (WGCNA) were performed on the GSE27274 dataset. Pathway enrichment analysis on the DEGs was performed. To identify the hub DEGs, we constructed a protein-protein interaction (PPI) network. Finally, the hub genes were verified, and candidate drugs were screened from the DsigDB database.

Results

A hundred DEGs and four hub genes (Atf3, Psmb6, Psmb8, and Psmb10) were screened out. Pathway enrichment analysis revealed that 100 DEGs were mainly enriched in apoptosis and the TNF signaling pathway. The four hub genes were verified in animal models and another dataset (GSE148420). Thereafter, a PPI network was used to identify the four hub genes (Atf3, Psmb6, Psmb8, and Psmb10). Finally, eight candidate drugs were identified as potential drugs.

Conclusion

Three hub genes (Psmb6, Psmb8, and Psmb10) were associated with RIRI and could be potential novel biomarkers for RIRI.

Complement-targeting therapeutics for ischemia-reperfusion injury in transplantation and the potential for ex vivo delivery

Organ shortages and an expanding waitlist have led to increased utilization of marginal organs. All donor organs are subject to varying degrees of IRI during the transplant process. Extended criteria organs, including those from older donors and organs donated after circulatory death are especially vulnerable to ischemia-reperfusion injury (IRI). Involvement of the complement cascade in mediating IRI has been studied extensively. Complement plays a vital role in the propagation of IRI and subsequent recruitment of the adaptive immune elements. Complement inhibition at various points of the pathway has been shown to mitigate IRI and minimize future immune-mediated injury in preclinical models. The recent introduction of ex vivo machine perfusion platforms provides an ideal window for therapeutic interventions. Here we review the role of complement in IRI by organ system and highlight potential therapeutic targets for intervention during ex vivo machine preservation of donor organs.

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.

A potent truncated form of human soluble CR1 is protective in a mouse model of renal ischemia-reperfusion injury

The complement system is a potent mediator of ischemia–reperfusion injury (IRI), which detrimentally affects the function and survival of transplanted kidneys. Human complement receptor 1 (HuCR1) is an integral membrane protein that inhibits complement activation by blocking the convertases that activate C3 and C5. We have previously reported that CSL040, a truncated form of recombinant soluble HuCR1 (sHuCR1), has enhanced complement inhibitory activity and improved pharmacokinetic properties compared to the parent molecule. Here, we compared the capacity of CSL040 and full-length sHuCR1 to suppress complement-mediated organ damage in a mouse model of warm renal IRI. Mice were treated with two doses of CSL040 or sHuCR1, given 1 h prior to 22 min unilateral renal ischemia and again 3 h later. 24 h after reperfusion, mice treated with CSL040 were protected against warm renal IRI in a dose-dependent manner, with the highest dose of 60 mg/kg significantly reducing renal dysfunction, tubular injury, complement activation, endothelial damage, and leukocyte infiltration. In contrast, treatment with sHuCR1 at a molar equivalent dose to 60 mg/kg CSL040 did not confer significant protection. Our results identify CSL040 as a promising therapeutic candidate to attenuate renal IRI and demonstrate its superior efficacy over full-length sHuCR1 in vivo.

Pattern Recognition Molecules of Lectin Complement Pathway in Ischemic Stroke

Purpose

The current study aimed to investigate in an Armenian population the levels of pattern recognition molecules (PRMs) of lectin complement pathway (LCP), MBL (mannan-binding lectin) and M-ficolin in plasma in ischemic stroke (IS), and the possible association of 11 single nucleotide polymorphisms (SNPs) in MBL2, FCN1 and FCN2 genes.

Patients and Methods

A total of 122 patients with IS and 150 control subjects were included in this study. Immunofluorometric assays (TRIFMAs) and real-time polymerase chain reactions with TaqMan probes were conducted.

Results

According to the results, the levels of M-ficolin in IS patients are significantly higher than in control subjects, and the MBL2 rs11003125 and rs12780112 SNPs, as well as MBL2 rs12780112*T and FCN1 rs10120023*T minor alleles (MAs) are negatively associated with the risk of IS. Further, MBL2 rs11003125 and rs1800450 SNPs and the carriage of their MAs, as well as FCN1 rs2989727 SNP and the carriage of FCN1 rs10120023*T MA significantly alter plasma MBL and M-ficolin levels in IS patients, respectively. Five common haplotypes in MBL2 gene and three common haplotypes in FCN1 and FCN2 genes were revealed, among which CGTC was negatively associated with IS and decreasing MBL plasma levels in IS.

Conclusion

In conclusion, we suggest that LCP PRMs are associated with the risk of developing IS, and may also participate in pathological events leading to post-ischemic brain damage. This study emphasizes the important contribution of alterations of LCP PRMs on genomic and proteomic levels to the pathomechanisms of ischemic stroke, at least in an Armenian population.

Complement Initiation Varies by Sex in Intestinal Ischemia Reperfusion Injury

Intestinal ischemia reperfusion (IR)-induced tissue injury represents an acute inflammatory response with significant morbidity and mortality. The mechanism of IR-induced injury is not fully elucidated, but recent studies suggest a critical role for complement activation and for differences between sexes. To test the hypothesis that complement initiation differs by sex in intestinal IR, we performed intestinal IR on male and female WT C57B6L/, C1q-/-, MBL-/-, or properdin (P)-/- mice. Intestinal injury, C3b and C5a production and ex vivo secretions were analyzed. Initial studies demonstrated a difference in complement mRNA and protein in male and female WT mice. In response to IR, male C1q-, MBL- and P-deficient mice sustained less injury than male WT mice. In contrast, only female MBL-/- mice sustained significantly less injury than female wildtype mice. Importantly, wildtype, C1q-/- and P-/- female mice sustained significant less injury than the corresponding male mice. In addition, both C1q and MBL expression and deposition increased in WT male mice, while only elevated MBL expression and deposition occurred in WT female mice. These data suggested that males use both C1q and MBL pathways, while females tend to depend on lectin pathway during intestinal IR. Females produced significantly less serum C5a in MBL-/- and P-/- mice. Our findings suggested that complement activation plays a critical role in intestinal IR in a sex-dependent manner.

Mannose-binding lectin (MBL) and the lectin complement pathway play a role in cutaneous ischemia and reperfusion injury

Objectives

Cutaneous ischemia/reperfusion (CI/R) injury has shown to play a significant role in chronic wounds such as decubitus ulcers, diabetic foot ulcers, atherosclerotic lesions, and venous stasis wounds. CI/R also plays a role in free tissue transfer in reconstructive microsurgery and has been linked to clinical burn-depth progression after thermal injury. While the role of the complement system has been elucidated in multiple organ systems, evidence is lacking with respect to its role in the skin. Therefore, we evaluated the role of the complement system in CI/R injury.

Methods

Using a single pedicle skin flap mouse model of acute CI/R, we performed CI/R in wild-type (WT) mice and complement knock out (KO) mice, deficient in either C1q (C1q KO; classical pathway inhibition), mannose-binding lectin (MBL null; lectin pathway inhibition) or factor B (H2Bf KO; alternative pathway inhibition). Following 10 h ischemia and 7 days reperfusion, mice were sacrificed, flaps harvested and flap viability assessed via Image J software. The flap necrotic area was expressed as % total flap area. In another group, mice were sacrificed following CI/R with 10 h ischemia and 48 h reperfusion. Two cranial skin flap samples were taken for gene expression analysis of IL1b, IL6, TNFα, ICAM1, VCAM1, IL10, IL13 using real-time polymerase chain reaction (RT-PCR).

Results

Following CI/R, MBL null mice had a statistically significant smaller %necrotic flap area compared to WT mice (10.6 vs. 43.1%; p<0.05) suggesting protection from CI/R. A significantly reduced mean %necrotic flap area was not seen in either C1q KO or H2Bf KO mice relative to WT (22.9 and 31.3 vs. 43.1%; p=0.08 and p=0.244, respectively). There were no statistically significant differences between groups for markers of inflammation (TNFα, ICAM1, VCAM1, IL1b, IL6). In contrast, mRNA levels of IL10, a regulator of inflammation, were significantly increased in the MBL null group (p=0.047).

Conclusions

We demonstrated for the first time a significant role of MBL and the lectin complement pathway in ischemia/reperfusion injury of the skin and a potential role for IL10 in attenuating CI/R injury, as IL10 levels were significantly increased in the tissue from the CI/R-protected MBL null group.

MASP-2 Is a Heparin-Binding Protease; Identification of Blocking Oligosaccharides

It is well-known that heparin and other glycosaminoglycans (GAGs) inhibit complement activation. It is however not known whether fractionation and/or modification of GAGs might deliver pathway-specific inhibition of the complement system. Therefore, we evaluated a library of GAGs and their derivatives for their functional pathway specific complement inhibition, including the MASP-specific C4 deposition assay. Interaction of human MASP-2 with heparan sulfate/heparin was evaluated by surface plasmon resonance, ELISA and in renal tissue. In vitro pathway-specific complement assays showed that highly sulfated GAGs inhibited all three pathways of complement. Small heparin- and heparan sulfate-derived oligosaccharides were selective inhibitors of the lectin pathway (LP). These small oligosaccharides showed identical inhibition of the ficolin-3 mediated LP activation, failed to inhibit the binding of MBL to mannan, but inhibited C4 cleavage by MASPs. Hexa- and pentasulfated tetrasaccharides represent the smallest MASP inhibitors both in the functional LP assay as well in the MASP-mediated C4 assay. Surface plasmon resonance showed MASP-2 binding with heparin and heparan sulfate, revealing high Kon and Koff rates resulted in a Kd of ~2 μM and confirmed inhibition by heparin-derived tetrasaccharide. In renal tissue, MASP-2 partially colocalized with agrin and heparan sulfate, but not with activated C3, suggesting docking, storage, and potential inactivation of MASP-2 by heparan sulfate in basement membranes. Our data show that highly sulfated GAGs mediated inhibition of all three complement pathways, whereas short heparin- and heparan sulfate-derived oligosaccharides selectively blocked the lectin pathway via MASP-2 inhibition. Binding of MASP-2 to immobilized heparan sulfate/heparin and partial co-localization of agrin/heparan sulfate with MASP, but not C3b, might suggest that in vivo heparan sulfate proteoglycans act as a docking platform for MASP-2 and possibly prevent the lectin pathway from activation.

Inflammaging and Complement System: A Link Between Acute Kidney Injury and Chronic Graft Damage

The aberrant activation of complement system in several kidney diseases suggests that this pillar of innate immunity has a critical role in the pathophysiology of renal damage of different etiologies. A growing body of experimental evidence indicates that complement activation contributes to the pathogenesis of acute kidney injury (AKI) such as delayed graft function (DGF) in transplant patients. AKI is characterized by the rapid loss of the kidney's excretory function and is a complex syndrome currently lacking a specific medical treatment to arrest or attenuate progression in chronic kidney disease (CKD). Recent evidence suggests that independently from the initial trigger (i.e., sepsis or ischemia/reperfusions injury), an episode of AKI is strongly associated with an increased risk of subsequent CKD. The AKI-to-CKD transition may involve a wide range of mechanisms including scar-forming myofibroblasts generated from different sources, microvascular rarefaction, mitochondrial dysfunction, or cell cycle arrest by the involvement of epigenetic, gene, and protein alterations leading to common final signaling pathways [i.e., transforming growth factor beta (TGF-β), p16 ink4a , Wnt/β-catenin pathway] involved in renal aging. Research in recent years has revealed that several stressors or complications such as rejection after renal transplantation can lead to accelerated renal aging with detrimental effects with the establishment of chronic proinflammatory cellular phenotypes within the kidney. Despite a greater understanding of these mechanisms, the role of complement system in the context of the AKI-to-CKD transition and renal inflammaging is still poorly explored. The purpose of this review is to summarize recent findings describing the role of complement in AKI-to-CKD transition. We will also address how and when complement inhibitors might be used to prevent AKI and CKD progression, therefore improving graft function.

Recent advances into the role of pattern recognition receptors in transplantation

Pattern recognition receptors (PRRs) are germline-encoded sensors best characterized for their critical role in host defense. However, there is accumulating evidence that organ transplantation induces the release or display of molecular patterns of cellular injury and death that trigger PRR-mediated inflammatory responses. There are also new insights that indicate PRRs are able to distinguish between self and non-self, suggesting the existence of non-clonal mechanisms of allorecognition. Collectively, these reports have spurred considerable interest into whether PRRs or their ligands can be targeted to promote transplant survival. This review examines the mounting evidence that PRRs play in transplant-mediated inflammation. Given the large number of PRRs, we will focus on members from four families: the complement system, toll-like receptors, the formylated peptide receptor, and scavenger receptors through examining reports of their activity in experimental models of cellular and solid organ transplantation as well as in the clinical setting.

Post-transplant Alternative Complement Pathway Activation Influences Kidney Allograft Function

The complement system is one of the crucial pathophysiological mechanisms that directly influence the function of a transplanted kidney. Since the complement pathways’ activation potential can be easily determined via their functional activity measurement, we focused on fluctuation in the cascade activity in the early post-transplant period. The aim of the study was to relate the kidney transplantation-induced complement system response to allograft outcome. Forty-two kidney recipients (aged: 53.5 [37–52], 17 females/25 males) and 24 healthy controls (aged: 40.5 [34–51], 13 females/11 males) were enrolled in the study. The functional activities of alternative, classical, and lectin pathways were determined before and in the first week after transplantation using Wielisa^®-kit. We observed that the baseline functional activity of the alternative pathway (AP) was higher in chronic kidney disease patients awaiting transplantation compared to healthy controls and that its level depended on the type of dialysis. AP-functional activity was decreased following transplantation procedure and its post-transplant level was related to allograft function. The baseline and transplantation-induced functional activities of the classical and lectin pathways were not influenced by dialysis type and were not associated with transplant outcome. Moreover, our study showed that intraoperative graft surface cooling had a protective effect on AP activation. Our study confirms the influence of dialysis modality on persistent AP complement activation and supports the role of AP in an early phase after kidney transplantation and allograft outcome.

Chimeric Proteins Containing MAP-1 and Functional Domains of C4b-Binding Protein Reveal Strong Complement Inhibitory Capacities

The complement system is a tightly regulated network of proteins involved in defense against pathogens, inflammatory processes, and coordination of the innate and adaptive immune responses. Dysregulation of the complement cascade is associated with many inflammatory disorders. Thus, inhibition of the complement system has emerged as an option for treatment of a range of different inflammatory diseases. MAP-1 is a pattern recognition molecule (PRM)-associated inhibitor of the lectin pathway of the complement system, whereas C4b-binding protein (C4BP) regulates both the classical and lectin pathways. In this study we generated chimeric proteins consisting of MAP-1 and the first five domains of human C4BP (C4BP¹⁻⁵) in order to develop a targeted inhibitor acting at different levels of the complement cascade. Two different constructs were designed and expressed in CHO cells where MAP-1 was fused with C4BP¹⁻⁵ in either the C- or N-terminus. The functionality of the chimeric proteins was assessed using different in vitro complement activation assays. Both chimeric proteins displayed the characteristic Ca²⁺-dependent dimerization and binding to PRMs of native MAP-1, as well as the co-factor activity of native C4BP. In ELISA-based complement activation assays they could effectively inhibit the lectin and classical pathways. Notably, MAP-1:C4BP¹⁻⁵ was five times more effective than rMAP-1 and rC4BP¹⁻⁵ applied at the same time, emphasizing the advantage of a single inhibitor containing both functional domains. The MAP-1/C4BP chimeras exert unique complement inhibitory properties and represent a novel therapeutic approach targeting both upstream and central complement activation.

Functional Activity of the Complement System in Deceased Donors in Relation to Kidney Allograft Outcome

Complement activation is considered one of the mediators of renal ischemia-reperfusion injury. Elevated levels of C5b-9, C3a, and C5a are detected in sera of deceased kidney donors. The goal of the study was to characterize the functional activity of complement pathways in donor sera and to assess their influence on transplant outcome.

MATERIALS AND METHODS

Sixty-four deceased kidney donors (age 45 ± 16 years; 28 female, 36 male) and 27 healthy controls (age 42 ± 12 years; 14 female, 13 male) were enrolled in the study. The results of transplantation for the respective 122 kidney recipients were included in the analysis. The functional activities of classical (CP), lectin (LP), and alternative (AP) pathways were measured using Wielisa-kit (reference normal level = 100%). In most cases, decreased functional activity reflects the activation status of the pathway.

RESULTS

The median (interquartile range) functional activities of the pathways in donor sera were CP 118 (89-150)%, LP 80 (20-127)%, and AP 74 (50-89)%, and did not differ from the control values CP 110 (102-115)%, LP 81 (26-106)%, AP 76 (61-88)%. The frequency of pathway activation observed in controls was CP 0%, LP 11%, and AP 0%. Deceased donors did not differ in activation of classical (11%) and lectin (13%) pathways, but presented a higher rate of alternative pathway activation (19%, P = .03). No significant influence of any pathway functional activity or its activation was proved to influence the transplant outcome.

CONCLUSION

Complement activation via alternative pathway was observed in diseased donor sera. No predictive potential of donor complement functional activity on the transplant outcome could be proved.

Initiation of the inflammatory response after renal ischemia/reperfusion injury during renal transplantation

Ischemia/reperfusion injury (IRI) occurs commonly during renal transplantation. It has been well demonstrated that the inflammatory response has an important role in the pathogenesis and pathological processes of IRI. However, the signaling events that trigger the activation of the inflammatory response are less clear. Accumulated evidence has identified the role of various injury factors released from or exposed in ischemic, damaged, or dying cells, which serve as initiators of the inflammatory response and exacerbate kidney injury after renal IRI. Signaling pathways triggered by these endogenous molecules that activate different pathogen recognition receptors have also been widely investigated. Here, we review the molecular signaling molecules that initiate the inflammatory response during renal IRI and that provide potential therapeutic options for the disease.

Targeted Delivery of Neutralizing Anti-C5 Antibody to Renal Endothelium Prevents Complement-Dependent Tissue Damage

Complement activation is largely implicated in the pathogenesis of several clinical conditions and its therapeutic neutralization has proven effective in preventing tissue and organ damage. A problem that still needs to be solved in the therapeutic control of complement-mediated diseases is how to avoid side effects associated with chronic neutralization of the complement system, in particular, the increased risk of infections. We addressed this issue developing a strategy based on the preferential delivery of a C5 complement inhibitor to the organ involved in the pathologic process. To this end, we generated Ergidina, a neutralizing recombinant anti-C5 human antibody coupled with a cyclic-RGD peptide, with a distinctive homing property for ischemic endothelial cells and effective in controlling tissue damage in a rat model of renal ischemia/reperfusion injury (IRI). As a result of its preferential localization on renal endothelium, the molecule induced complete inhibition of complement activation at tissue level, and local protection from complement-mediated tissue damage without affecting circulating C5. The ex vivo binding of Ergidina to surgically removed kidney exposed to cold ischemia supports its therapeutic use to prevent posttransplant IRI leading to delay of graft function. Moreover, the finding that the ex vivo binding of Ergidina was not restricted to the kidney, but was also seen on ischemic heart, suggests that this RGD-targeted anti-C5 antibody may represent a useful tool to treat organs prior to transplantation. Based on this evidence, we propose preliminary data showing that Ergidina is a novel targeted drug to prevent complement activation on the endothelium of ischemic kidney.

Activation of complement factor B contributes to murine and human myocardial ischemia/reperfusion injury

The pathophysiology of myocardial injury that results from cardiac ischemia and reperfusion (I/R) is incompletely understood. Experimental evidence from murine models indicates that innate immune mechanisms including complement activation via the classical and lectin pathways are crucial. Whether factor B (fB), a component of the alternative complement pathway required for amplification of complement cascade activation, participates in the pathophysiology of myocardial I/R injury has not been addressed. We induced regional myocardial I/R injury by transient coronary ligation in WT C57BL/6 mice, a manipulation that resulted in marked myocardial necrosis associated with activation of fB protein and myocardial deposition of C3 activation products. In contrast, in fB-/- mice, the same procedure resulted in significantly reduced myocardial necrosis (% ventricular tissue necrotic; fB-/- mice, 20 ± 4%; WT mice, 45 ± 3%; P < 0.05) and diminished deposition of C3 activation products in the myocardial tissue (fB-/- mice, 0 ± 0%; WT mice, 31 ± 6%; P<0.05). Reconstitution of fB-/- mice with WT serum followed by cardiac I/R restored the myocardial necrosis and activated C3 deposition in the myocardium. In translational human studies we measured levels of activated fB (Bb) in intracoronary blood samples obtained during cardio-pulmonary bypass surgery before and after aortic cross clamping (AXCL), during which global heart ischemia was induced. Intracoronary Bb increased immediately after AXCL, and the levels were directly correlated with peripheral blood levels of cardiac troponin I, an established biomarker of myocardial necrosis (Spearman coefficient = 0.465, P < 0.01). Taken together, our results support the conclusion that circulating fB is a crucial pathophysiological amplifier of I/R-induced, complement-dependent myocardial necrosis and identify fB as a potential therapeutic target for prevention of human myocardial I/R injury.

Complement in Kidney Transplantation

The complement system is considered to be an important part of innate immune system with a significant role in inflammation processes. The activation can occur through classical, alternative, or lectin pathway, resulting in the creation of anaphylatoxins C3a and C5a, possessing a vast spectrum of immune functions, and the assembly of terminal complement cascade, capable of direct cell lysis. The activation processes are tightly regulated; inappropriate activation of the complement cascade plays a significant role in many renal diseases including organ transplantation. Moreover, complement cascade is activated during ischemia/reperfusion injury processes and influences delayed graft function of kidney allografts. Interestingly, complement system has been found to play a role in both acute cellular and antibody-mediated rejections and thrombotic microangiopathy. Therefore, complement system may represent an interesting therapeutical target in kidney transplant pathologies.

Complement in renal transplantation: The road to translation

Renal transplantation is the treatment of choice for patients with end-stage renal disease. The vital role of the complement system in renal transplantation is widely recognized. This review discusses the role of complement in the different phases of renal transplantation: in the donor, during preservation, in reperfusion and at the time of rejection. Here we examine the current literature to determine the importance of both local and systemic complement production and how complement activation contributes to the pathogenesis of renal transplant injury. In addition, we dissect the complement pathways involved in the different phases of renal transplantation. We also review the therapeutic strategies that have been tested to inhibit complement during the kidney transplantation. Several clinical trials are currently underway to evaluate the therapeutic potential of complement inhibition for the treatment of brain death-induced renal injury, renal ischemia-reperfusion injury and acute rejection. We conclude that it is expected that in the near future, complement-targeted therapeutics will be used clinically in renal transplantation. This will hopefully result in improved renal graft function and increased graft survival.

Effect of Optimization of Glycaemic Control on Mannan-Binding Lectin in Type 1 Diabetes

OBJECTIVE

Mannan-binding lectin (MBL) concentration in plasma is increased in subjects with type 1 diabetes and associated with increased mortality and risk of diabetic nephropathy. Recent findings show that pancreas transplantation reduces MBL concentration. Whether the increased MBL concentration is reversed by improved glycaemic control remains unknown. We investigated the effects of improved glycaemic control on MBL concentration in patients with type 1 diabetes.

METHODS

We measured MBL, fructosamine, and HbA_1cat baseline and after 6 weeks in 52 type 1 diabetic patients following the change from conventional insulin therapy to insulin pump therapy.

RESULTS

After initiation of insulin pump therapy, the total daily insulin dose was significantly reduced (from 51 ± 18 IE/day to 39 ± 13 IE/day, P < 0.0001). There was a significant decrease in HbA_1c from 8.6% to 7.7% (from 70 mmol/mol to 61 mmol/mol, P < 0.0001) and in fructosamine levels (from 356 μmol/L to 311 μmol/L, P < 0.0001). MBL levels decreased by 10% from 2165 μg/L (IQR 919-3389 μg/L) at baseline to 1928 μ/L (IQR 811-2758 μg/L) at follow-up (P = 0.005), but MBL change was not significantly correlated with changes in insulin dose, HbA_1c, or fructosamine.

CONCLUSIONS

MBL concentration decreased following the initiation of insulin pump therapy in patients with type 1 diabetes and did not correlate with changes in glycaemic control.

Diabetes Is Associated with Increased Autoreactivity of Mannan-Binding Lectin

Mannan-binding lectin (MBL) has been reported to be involved in the pathophysiology of diabetic nephropathy. MBL is a pattern-recognition molecule of the innate immune system that initiates the lectin pathway of the complement system upon recognition of evolutionary conserved pathogen-associated molecular patterns or to altered self-tissue. Our group have previously shown direct effects of MBL on diabetes-induced kidney damage, and we hypothesized that MBL may cause autoactivation of the complement system via binding to neoepitopes induced by hyperglycemia. In the present study, we induced diabetes in MBL knockout mice and in wild type C57BL/6J mice by low-dose streptozotocin injection and measured blood glucose and urine albumin-to-creatinine ratio to monitor development of diabetes. After 24 weeks, fluorescently labelled recombinant MBL was injected intravenously in diabetic MBL knockout mice after which the distribution was investigated using in vivo fluorescence imaging. Mice were subjected to in vivo and ex vivo imaging 24 hours after injection. MBL was found to accumulate in the kidneys of diabetic mice as compared to healthy control mice (p < 0.0001). These findings support the hypothesis of a significant role of MBL and the complement system in the pathophysiology of diabetic nephropathy.

Role of the lectin complement pathway in kidney transplantation

In the last 15 years two major advances in the role of complement in the kidney transplant have come about. The first is that ischaemia reperfusion injury and its profound effect on transplant outcome is dependent on the terminal product of complement activation, C5b-9. The second key observation relates to the function of the small biologically active fragments C3a and C5a released by complement activation in increasing antigen presentation and priming the T cell response that results in transplant rejection. In both cases local synthesis of C3 principally by the renal tubule cells plays an essential role that overshadows the role of the circulating pool of C3 generated largely by hepatocyte synthesis. More recent efforts have investigated the molecules expressed by renal tissue that can trigger complement activation. These have revealed a prominent effect of collectin-11 (CL-11), a soluble C-type lectin that is expressed in renal tissue and aligns with its major ligand L-fucose at sites of complement activation following ischaemic stress. Biochemical studies have shown that interaction between CL-11 and L-fucose results in complement activation by the lectin complement pathway, precisely targeting the innate immune response to the ischaemic tubule surface. Therapeutic approaches to reduce inflammatory and immune stimulation in ischaemic kidney have so far targeted C3 or its activation products and several are in clinical trials. The finding that lectin-fucose interaction is an important trigger of lectin pathway complement activation within the donor organ opens up further therapeutic targets where intervention could protect the donor kidney against complement.

Complement-here, there and everywhere, but what about the transplanted organ?

The part of the innate immune system that communicates and effectively primes the adaptive immune system was termed "complement" by Ehrlich to reflect its complementarity to antibodies having previously been described as "alexine" (i.e protective component of serum) by Buchner and Bordet. It has been established that complement is not solely produced systemically but may have origin in different tissues where it can influence organ specific functions that may affect the outcome of transplanted organs. This review looks at the role of complement in particular to kidney transplantation. We look at current literature to determine whether blockade of the peripheral or central compartments of complement production may prevent ischaemic reperfusion injury or rejection in the transplanted organ. We also review new therapeutics that have been developed to inhibit components of the complement cascade with varying degrees of success leading to an increase in our understanding of the multiple triggers of this complex system. In addition, we consider whether biomarkers in this field are effective markers of disease or treatment.

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.

de Bruin R, N. M. Ijzermans J, van Kooten C, Dor FJMF. Mannan-Binding Lectin Is Involved in the Protection against Renal Ischemia/Reperfusion Injury by Dietary Restriction

Preoperative fasting and dietary restriction offer robust protection against renal ischemia/reperfusion injury (I/RI) in mice. We recently showed that Mannan-binding lectin (MBL), the initiator of the lectin pathway of complement activation, plays a pivotal role in renal I/RI. Based on these findings, we investigated the effect of short-term DR (30% reduction of total food intake) or three days of water only fasting on MBL in 10-12 weeks old male C57/Bl6 mice. Both dietary regimens significantly reduce the circulating levels of MBL as well as its mRNA expression in liver, the sole production site of MBL. Reconstitution of MBL abolished the protection afforded by dietary restriction, whereas in the fasting group the protection persisted. These data show that modulation of MBL is involved in the protection against renal I/RI induced by dietary restriction, and suggest that the mechanisms of protection induced by dietary restriction and fasting may be different.

Complement involvement in kidney diseases: From physiopathology to therapeutical targeting

Complement cascade is involved in several renal diseases and in renal transplantation. The different components of the complement cascade might represent an optimal target for innovative therapies. In the first section of the paper the authors review the physiopathology of complement involvement in renal diseases and transplantation. In some cases this led to a reclassification of renal diseases moving from a histopathological to a physiopathological classification. The principal issues afforded are: renal diseases with complement over activation, renal diseases with complement dysregulation, progression of renal diseases and renal transplantation. In the second section the authors discuss the several complement components that could represent a therapeutic target. Even if only the anti C5 monoclonal antibody is on the market, many targets as C1, C3, C5a and C5aR are the object of national or international trials. In addition, many molecules proved to be effective in vitro or in preclinical trials and are waiting to move to human trials in the future.

Innate immunity in solid organ transplantation: an update and therapeutic opportunities

Innate immunity is increasingly recognized as a major player in transplantation. In addition to its role in inflammation in the early post-transplant period, innate immunity shapes the differentiation of cells of adaptive immunity, with a capacity to promote either rejection or tolerance. Emerging data indicate that innate allorecognition, a characteristic previously limited to lymphocytes, is involved in allograft rejection. This review briefly summarizes the physiology of each component of the innate immune system in the context of transplantation and presents the current or promising therapeutic applications, such as cellular, anticomplement and anticytokine therapies.

The role of complement in the pathogenesis of renal ischemia-reperfusion injury and fibrosis

The complement system is a major component of innate immunity and has been commonly identified as a central element in host defense, clearance of immune complexes, and tissue homeostasis. After ischemia-reperfusion injury (IRI), the complement system is activated by endogenous ligands that trigger proteolytic cleavage of complement components via the classical, lectin and/or alternative pathway. The result is the formation of terminal complement components C3a, C5a, and the membrane attack complex (C5b-9 or MAC), all of which play pivotal roles in the amplification of the inflammatory response, chemotaxis, neutrophil/monocyte recruitment and activation, and direct tubular cell injury. However, recent evidence suggests that complement activity transcends innate host defense and there is increasing data suggesting complement as a regulator in processes such as allo-immunity, stem cell differentiation, tissue repair, and progression to fibrosis. In this review, we discuss recent advances addressing the role of complement as a regulator of IRI and renal fibrosis after organ donation for transplantation. We will also briefly discuss currently approved therapies that target complement activity in kidney ischemia-reperfusion and transplantation.

Versatility of the complement system in neuroinflammation, neurodegeneration and brain homeostasis

The immune response after brain injury is highly complex and involves both local and systemic events at the cellular and molecular level. It is associated to a dramatic over-activation of enzyme systems, the expression of proinflammatory genes and the activation/recruitment of immune cells. The complement system represents a powerful component of the innate immunity and is highly involved in the inflammatory response. Complement components are synthesized predominantly by the liver and circulate in the bloodstream primed for activation. Moreover, brain cells can produce complement proteins and receptors. After acute brain injury, the rapid and uncontrolled activation of the complement leads to massive release of inflammatory anaphylatoxins, recruitment of cells to the injury site, phagocytosis and induction of blood brain barrier (BBB) damage. Brain endothelial cells are particularly susceptible to complement-mediated effects, since they are exposed to both circulating and locally synthesized complement proteins. Conversely, during neurodegenerative disorders, complement factors play distinct roles depending on the stage and degree of neuropathology. In addition to the deleterious role of the complement, increasing evidence suggest that it may also play a role in normal nervous system development (wiring the brain) and adulthood (either maintaining brain homeostasis or supporting regeneration after brain injury). This article represents a compendium of the current knowledge on the complement role in the brain, prompting a novel view that complement activation can result in either protective or detrimental effects in brain conditions that depend exquisitely on the nature, the timing and the degree of the stimuli that induce its activation. A deeper understanding of the acute, subacute and chronic consequences of complement activation is needed and may lead to new therapeutic strategies, including the ability of targeting selective step in the complement cascade.

The role of complement in the pathogenesis of renal ischemia-reperfusion injury and fibrosis

The complement system is a major component of innate immunity and has been commonly identified as a central element in host defense, clearance of immune complexes, and tissue homeostasis. After ischemia-reperfusion injury (IRI), the complement system is activated by endogenous ligands that trigger proteolytic cleavage of complement components via the classical, lectin and/or alternative pathway. The result is the formation of terminal complement components C3a, C5a, and the membrane attack complex (C5b-9 or MAC), all of which play pivotal roles in the amplification of the inflammatory response, chemotaxis, neutrophil/monocyte recruitment and activation, and direct tubular cell injury. However, recent evidence suggests that complement activity transcends innate host defense and there is increasing data suggesting complement as a regulator in processes such as allo-immunity, stem cell differentiation, tissue repair, and progression to fibrosis. In this review, we discuss recent advances addressing the role of complement as a regulator of IRI and renal fibrosis after organ donation for transplantation. We will also briefly discuss currently approved therapies that target complement activity in kidney ischemia-reperfusion and transplantation.

Lessons learned from mice deficient in lectin complement pathway molecules

The lectin pathway of the complement system is initiated when the pattern-recognition molecules, mannose-binding lectin (MBL), ficolins or collectin-11, bind to invading pathogens or damaged host cells. This leads to activation of MBL/ficolin/collectin-11 associated serine proteases (MASPs), which in turn activate downstream complement components, ultimately leading to elimination of the pathogen. Mice deficient in the key molecules of lectin pathway of complement have been generated in order to build knowledge of the molecular mechanisms of the lectin pathway in health and disease. Despite differences in the genetic arrangements of murine and human orthologues of lectin pathway molecules, the knockout mice have proven to be valuable models to explore the effect of deficiency states in humans. In addition, new insight and unexpected findings on the diverse roles of lectin pathway molecules in complement activation, pathogen infection, coagulation, host tissue injury and developmental biology have been revealed by in vivo investigations. This review provides an overview of the mice deficient in lectin pathway molecules and highlights some of the most important findings that have resulted from studies of these.

Ischemia/reperfusion Injury and its Consequences on Immunity and Inflammation

Ischemia/reperfusion injury (IRI), an inherent component of transplantation, affects organ quality and transplant outcomes. Although the complexity of the pathophysiology is recognized, detailed mechanisms remain unclear, and strategies preventing the consequences of IRI have been challenging. Of critical significance appears the link between IRI, the initiation of innate immune responses, and the (potential) augmentation of adaptive immunity. An improved understanding of those complex mechanisms and interactions may pave the way for more effective treatment strategies.

Mannan-binding lectin-associated serine protease 2 is critical for the development of renal ischemia reperfusion injury and mediates tissue injury in the absence of complement C4

Mannan-binding lectin-associated serine protease 2 (MASP-2) has been described as the essential enzyme for the lectin pathway (LP) of complement activation. Since there is strong published evidence indicating that complement activation via the LP critically contributes to ischemia reperfusion (IR) injury, we assessed the effect of MASP-2 deficiency in an isogenic mouse model of renal transplantation. The experimental transplantation model used included nephrectomy of the remaining native kidney at d 5 post-transplantation. While wild-type (WT) kidneys grafted into WT recipients (n=7) developed acute renal failure (control group), WT grafts transplanted into MASP-2-deficient recipients (n=7) showed significantly better kidney function, less C3 deposition, and less IR injury. In the absence of donor or recipient complement C4 (n=7), the WT to WT phenotype was preserved, indicating that the MASP-2-mediated damage was independent of C4 activation. This C4-bypass MASP-2 activity was confirmed in mice deficient for both MASP-2 and C4 (n=7), where the protection from postoperative acute renal failure was no greater than in mice with MASP-2 deficiency alone. Our study highlights the role of LP activation in renal IR injury and indicates that injury occurs through MASP-2-dependent activation events independent of C4.

Heterocomplex formation between MBL/ficolin/CL-11-associated serine protease-1 and -3 and MBL/ficolin/CL-11-associated protein-1

The activity of the complement system is tightly controlled by many fluid-phase and tissue-bound regulators. Mannose-binding lectin (MBL)/ficolin/collectin-11-associated protein-1 (MAP-1) is a recently discovered plasma protein that acts as an upstream inhibitor of the lectin complement pathway (LCP). It has previously been shown that MAP-1 can compete with the MBL/ficolin/collectin-11-associated serine proteases (MASPs) in binding to MBL and the ficolins. However, this mechanism may only partly explain the inhibitory complement effect of MAP-1. We hypothesized that MAP-1 is also involved in heterocomplex formation with the MASPs thereby breaking the stoichiometry of the activation complexes of the LCP, which could represent an alternative mechanism of MAP-1-mediated complement inhibition. We assessed the heterocomplex formation with ELISA, size-exclusion chromatography, and immunoblotting using both recombinant proteins and serum/plasma. We found that rMAP-1 can engage in heterocomplexes with rMASP-1 and rMASP-3 in a calcium-dependent manner. Moreover, we discovered that rMASP-1 and rMASP-3 also form heterocomplexes under these conditions. Complexes containing both MAP-1 and MASP-1 or -3 were detected in normal human serum and plasma, and depletion of the LCP recognition molecules from ficolin-3-deficient human serum showed that free circulating heterocomplexes also exist in the blood, although the major part appears to be associated with the LCP recognition molecules. Altogether, these findings suggest that MASPs can associate in various combinations and bring new perspectives to the complexity of lectin pathway-driven complement activation.

Impact of mannose-binding lectin deficiency on radiocontrast-induced renal dysfunction

Contrast-induced nephropathy (CIN) is the third leading cause of acute renal failure in hospitalized patients. Endothelial dysfunction, renal medullary ischemia, and tubular toxicity are regarded as the most important factors in the pathogenesis of CIN. Mannose-binding lectin (MBL), a pattern recognition protein of the lectin pathway of complement, has been found to aggravate and mediate tissue damage during experimental renal ischemia/reperfusion (I/R) injury which was alleviated by inhibition with C1 inhibitor, a potent MBL, and lectin pathway inhibitor. In this paper, we highlight the potential role of MBL in the pathogenesis of human CIN. In experimental I/R models, MBL was previously found to induce tubular cell death independent of the complement system. In addition, after binding to vascular endothelial cells, MBL and its associated serine proteases were able to trigger a proinflammatory reaction and contribute to endothelial dysfunction. In humans, urinary MBL was increased after administration of contrast media and in individuals with CIN. Moreover, individuals with normal/high MBL levels were at increased risk to develop radiocontrast-induced renal dysfunction. Hence, MBL and the lectin pathway seem to be a promising target given that a licensed, powerful, human recombinant inhibitor exits to be added to the scarce armamentarium currently available for prophylaxis of CIN.

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.

The innate immune system and transplantation

The sensitive and broadly reactive character of the innate immune system makes it liable to activation by stress factors other than infection. Thermal and metabolic stresses experienced during the transplantation procedure are sufficient to trigger the innate immune response and also augment adaptive immunity in the presence of foreign antigen on the donor organ. The resulting inflammatory and immune reactions combine to form a potent effector response that can lead to graft rejection. Here we examine the evidence that the complement and toll-like receptor systems are central to these pathways of injury and present a formidable barrier to transplantation. We review extensive information about the effector mechanisms that are mediated by these pathways, and bring together what is known about the damage-associated molecular patterns that initiate this sequence of events. Finally, we refer to two ongoing therapeutic trials that are evaluating the validity of these concepts in man.

The complement cascade and renal disease

Serum complement cascade, a part of innate immunity required for host protection against invading pathogens, is also a mediator of various forms of disease and injury. It is activated by classical, lectin, and alternative pathways that lead to activation of C3 component by C3 convertases, release of C3b opsonin, C5 conversion and eventually membrane attack complex formation. The tightly regulated activation process yields also C3a and C5a anaphylatoxins, which target a broad spectrum of immune and non-immune cells. The review discusses the involvement of the complement cascade in kidney disease pathogenesis and injury. The role of the complement pathways in autoantibody-mediated forms of glomerulonephritis (lupus nephritis, anti-glomerular basement membrane disease, anti-neutrophil cytoplasmic autoantibody-induced or membranoproliferative glomerulonephritis, membranous nephropathy), C3 glomerulopathy, atypical forms of hemolytic uremic syndrome, ischemic-reperfusion injury of transplanted kidney, and antibody-mediated renal allograft rejection are discussed. The disturbances in complement activation and regulation with underlying genetics are presented and related to observed pathology. Also promising strategies targeting the complement system in complement-related disorders are mentioned.

Mannan binding lectin: a two-faced regulator of renal allograft injury?

Complement activation plays an important role in the pathogenesis of renal allograft injury after kidney transplantation. There are three known pathways of complement activation, namely, classical, alternative, and lectin pathways. In renal allograft injury, contradictory results were reported about the role of the lectin pathway activated via mannan binding lectin (MBL). This Commentary discusses the findings by Bay et al. (this issue), who demonstrated a protective role for circulating MBL in the recipient, particularly in non-HLA immunized recipients.

Emerging role of the mannose-binding lectin-dependent pathway of complement activation in clinical organ transplantation

PURPOSE OF REVIEW

Over the past decade, the role of the complement system in solid organ transplantation has received increased attention. A number of experimental and epidemiological studies have suggested that the lectin pathway plays a role in infectious complications, rejection and long-term outcome after transplantation. This review discusses recent data on the role of the lectin pathway in solid organ transplantation.

RECENT FINDINGS

Studies on the role of mannose-binding lectin (MBL) in organ transplantation have shown an association of MBL-deficient states with an increased risk of infection after liver and simultaneous pancreas-kidney transplantation. On the contrary, a high MBL status in the recipient has been associated with poorer organ survival and increased rejection associated damage in various transplant settings. Experimental data points towards a role for MBL in ischemia-reperfusion damage in various organs. Several lines of evidence suggest that MBL may contribute to immunoglobulin-mediated complement activation in both ischemia-reperfusion and rejection. The interaction of MBL with IgM may be of particular importance in this setting.

SUMMARY

We review recent epidemiological data on the role of MBL in solid organ transplantation. We relate these findings to the emerging experimental data and attempt to explain some of the conflicting results on beneficial and harmful effects of the lectin pathway.

Molecular mechanisms of liver ischemia reperfusion injury: Insights from transgenic knockout models

Ischemia reperfusion injury is a major obstacle in liver resection and liver transplantation surgery. Understanding the mechanisms of liver ischemia reperfusion injury (IRI) and developing strategies to counteract this injury will therefore reduce acute complications in hepatic resection and transplantation, as well as expanding the potential pool of usable donor grafts. The initial liver injury is initiated by reactive oxygen species which cause direct cellular injury and also activate a cascade of molecular mediators leading to microvascular changes, increased apoptosis and acute inflammatory changes with increased hepatocyte necrosis. Some adaptive pathways are activated during reperfusion that reduce the reperfusion injury. IRI involves a complex interplay between neutrophils, natural killer T-cells cells, CD4+ T cell subtypes, cytokines, nitric oxide synthases, haem oxygenase-1, survival kinases such as the signal transducer and activator of transcription, Phosphatidylinositol 3-kinases/Akt and nuclear factor κβ pathways. Transgenic animals, particularly genetic knockout models, have become a powerful tool at elucidating mechanisms of liver ischaemia reperfusion injury and are complementary to pharmacological studies. Targeted disruption of the protein at the genetic level is more specific and maintained than pharmacological inhibitors or stimulants of the same protein. This article reviews the evidence from knockout models of liver IRI about the cellular and molecular mechanisms underlying liver IRI.

Blocking properdin, the alternative pathway, and anaphylatoxin receptors ameliorates renal ischemia-reperfusion injury in decay-accelerating factor and CD59 double-knockout mice

Complement is implicated in the pathogenesis of ischemia-reperfusion injury (IRI). The activation pathway(s) and effector(s) of complement in IRI may be organ specific and remain to be fully characterized. We previously developed a renal IRI model in decay-accelerating factor (DAF) and CD59 double-knockout (DAF(-/-)CD59(-/-)) mice. In this study, we used this model to dissect the pathway(s) by which complement is activated in renal IRI and to evaluate whether C3aR- or C5aR-mediated inflammation or the membrane attack complex was pathogenic. We crossed DAF(-/-)CD59(-/-) mice with mice deficient in various complement components or receptors including C3, C4, factor B (fB), factor properdin (fP), mannose-binding lectin, C3aR, C5aR, or Ig and assessed renal IRI in the resulting mutant strains. We found that deletion of C3, fB, fP, C3aR, or C5aR significantly ameliorated renal IRI in DAF(-/-)CD59(-/-) mice, whereas deficiency of C4, Ig, or mannose-binding lectin had no effect. Treatment of DAF(-/-)CD59(-/-) mice with an anti-C5 mAb reduced renal IRI to a greater degree than did C5aR deficiency. We also generated and tested a function-blocking anti-mouse fP mAb and showed it to ameliorate renal IRI when given to DAF(-/-)CD59(-/-) mice 24 h before, but not 4 or 8 h after, ischemia/reperfusion. These results suggest that complement is activated via the alternative pathway during the early phase of reperfusion, and both anaphylatoxin-mediated inflammation and the membrane attack complex contribute to tissue injury. Further, they demonstrate a critical role for properdin and support its therapeutic targeting in renal IRI.

Complement depletion protects lupus-prone mice from ischemia-reperfusion-initiated organ injury

Ischemia-reperfusion (IR) injury causes a vigorous immune response that is amplified by complement activation, leading to local and remote tissue damage. Using MRL/lpr mice, which are known to experience accelerated tissue damage after mesenteric IR injury, we sought to evaluate whether complement inhibition mitigates organ damage. We found that complement depletion with cobra venom factor protected mice from local and remote lung tissue damage. Protection from injury was associated with less complement (C3) and membrane attack complex deposition, less neutrophil infiltration, and lower levels of local proinflammatory cytokine production. In addition, complement depletion was able to decrease the level of oxidative stress as measured by glutathione peroxidase 1 mRNA levels and superoxide dismutase activity. Furthermore, blockage of C5a receptor protected MRL/lpr mice from local tissue damage, but not from remote lung tissue damage. In conclusion, although treatments with cobra venom factor and C5a receptor antagonist were able to protect mice from local tissue damage, treatment with C5a receptor antagonist was not able to protect mice from remote lung tissue damage, implying that more factors contribute to the development of remote tissue damage after IR injury. These data also suggest that complement inhibition at earlier, rather than late, stages can have clinical benefit in conditions that are complicated with IR injury.

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.