Lack of a functional alternative complement pathway ameliorates ischemic acute renal failure in mice

Aseptic injury to epithelial cells alters cell surface complement regulation in a tissue specific fashion

We have recently shown that oxidative stress of ARPE-19 cells alters the expression of the cell surface complement regulatory proteins DAF and CD59, and permits increased activation of complement when the cells are subsequently exposed to serum. Based upon these results, we hypothesized that RPE cells respond to cellular stress as if it is infection, and reduce their surface expression of complement regulatory proteins to foster the local immune response. To test this hypothesis, we examined whether cellular hypoxia would produce a similar change in ARPE-19 cells. In addition, we asked whether this response to oxidative stress is universal in all epithelial cells, by examining the expression of complement regulatory proteins on the surface of the renal and pulmonary epithelial cells. We found that the expression of complement regulatory proteins is altered by aseptic cellular stressors such as hypoxia and oxidative stress, but the response to these conditions differs from tissue to tissue. In RPE cells oxidative stress reduces the expression of the cell surface complement regulators and sensitizes the cells to complement mediated injury. This specific response is not seen in epithelial cells from the lung or kidney, and is not induced by hypoxia. These studies help explain the unique mechanisms by which uncontrolled complement activation may contribute to the development of AMD.

Early complement factors in the local tissue immunocomplex generated during intestinal ischemia/reperfusion injury

Recent work reveals that the innate immune system is able to recognize self-targets and initiate an inflammatory response similar to that of pathogens. One novel example of this innate autoimmunity is ischemia/reperfusion (I/R) injury, in which reperfusion of the ischemic tissues elicits an acute inflammatory response activated by natural IgM (nIgM) binding to ischemia-specific self-antigens, which are non-muscle myosin heavy chains type II (NMHC-II) subtype A and C. Subsequently, the complement lectin pathway is activated and eventually tissue injury occurs. Although earlier studies in the intestinal model showed that the classical complement pathway did not initiate I/R injury, C1q deposition was still observed in the local injured tissues by imaging analysis. Moreover, the involvement of the alternative complement pathway became unclear due to conflicting reports using different knockout mice. To explore the immediate downstream pathway following nIgM-ischemic antigen interaction, we isolated the nIgM-ischemic antigen immunocomplexes from the local tissue of animals treated in the intestinal I/R injury model, and examined the presence of initial molecules of three complement pathways. Our results showed that mannan-binding lectin (MBL), the early molecule of the lectin pathway, was present in the nIgM-ischemic Ag immunocomplex. In addition, C1q, the initial molecule of the classical pathway was also detected on the immunocomplex. However, Factor B, the early molecule in the alternative pathway, was not detected in the immunocomplex. To further examine the role of the alternative pathway in I/R injury, we utilized Factor B knockout mice in the intestinal model. Our results showed that Factor B knockout mice were not protected from local tissue injury, and their complement system was activated in the local tissues by nIgM during I/R. These results indicated that the lectin complement pathway operates immediately downstream of the nIgM-ischemic antigen interaction during intestinal I/R. Furthermore, the classical complement pathway also appears to interact with the of nIgM-ischemic antigen immunocomplex. Finally, the alternative complement pathway is not involved in I/R injury induction in the current intestinal model.

Structure-activity relationships for substrate-based inhibitors of human complement factor B

Human complement is a cascading network of plasma proteins important in immune defense, cooperatively effecting recognition, opsonization, destruction, and removal of pathogens and infected/damaged cells. Overstimulated or unregulated complement activation can result in immunoinflammatory diseases. Key serine proteases in this cascade are difficult to study due to their multiprotein composition, short lifetimes, formation on membranes, or serum circulation as inactive zymogens. Factor B is inactive at pH 7, but a catalytically active serine protease under alkaline conditions, enabling structure-activity relationship studies for 63 substrate-based peptide inhibitors with 4-7 residues and a C-terminal aldehyde. A potent factor B inhibitor was hexpeptide Ac-RLTbaLAR-H (IC(50) 250 nM, pH 9.5), which at pH 7 also blocked formation of membrane attack complex via the "alternative pathway" of complement activation and inhibited human complement mediated lysis of rabbit erythrocytes. Inhibitors of factor B may be valuable probes and drug leads for complement mediated immunity and disease.

Targeted inhibition of the complement alternative pathway with complement receptor 2 and factor H attenuates collagen antibody-induced arthritis in mice

The alternative pathway (AP) of complement is required for the induction of collagen Ab-induced arthritis (CAIA) in mice. The objective of this study was to examine the effect of a recombinant AP inhibitor containing complement receptor 2 and factor H (CR2-fH) on CAIA in mice. CR2 binds to tissue-fixed activation fragments of C3, and the linked fH is a potent local inhibitor of the AP. CAIA was induced in C57BL/6 mice by i.p. injections of 4 mAb to type II collagen (CII) on day 0 and LPS on day 3. PBS or CR2-fH (250 or 500 microg) were injected i.p. 15 min after the mAb to CII on day 0 and 15 min after LPS on day 3; the mice were sacrificed on day 10. The disease activity score (DAS) was decreased significantly (p < 0.001) in both groups receiving CR2-fH compared with the PBS. Histology scores for inflammation, pannus, bone damage, and cartilage damage decreased in parallel with the DAS. C3 deposition in the synovium and cartilage was significantly reduced (p < 0.0001) in the mice treated with CR2-fH. In vitro studies with immune complexes containing type II collagen and mAb to CII showed that CR2-fH specifically inhibited the AP with minimal effect on the classical pathway (CP) and no effect on the lectin pathway (LP). The relative potency of CR2-fH in vitro was superior to mAbs to factor B and C5. Thus, CR2-fH specifically targets and inhibits the AP of complement in vitro and is effective in CAIA in vivo.

Static and dynamic contact angles of water droplet on a solid surface using molecular dynamics simulation

The present study investigates the variation of static contact angle of a water droplet in equilibrium with a solid surface in the absence of a body force and the dynamic contact angles of water droplet moving on a solid surface for different characteristic energies using the molecular dynamics simulation. With increasing characteristic energy, the static contact angle in equilibrium with a solid surface in the absence of a body force decreases because the hydrophobic surface changes its characteristics to the hydrophilic surface. In order to consider the effect of moving water droplet on the dynamic contact angles, we apply the constant acceleration to an individual oxygen and hydrogen atom. In the presence of a body force, the water droplet changes its shape with larger advancing contact angle than the receding angle. The dynamic contact angles are compared with the static contact angle in order to see the effect of the presence of a body force.

Complement-mediated ischemia-reperfusion injury: lessons learned from animal and clinical studies

Ischemia-reperfusion (I/R) injury provides a substantial limitation to further improvements in the development of therapeutic strategies for ischemia-related diseases. Studies in animal I/R models, including intestinal, hindlimb, kidney, and myocardial I/R models, have established a key role of the complement system in mediation of I/R injury using complement inhibitors and knock-out animal models. As complement activation has been shown to be an early event in I/R injury, inhibiting its activation or its components may offer tissue protection after reperfusion. However, clinical study results using complement inhibitors have largely been disappointing. Therefore, identification of a more specific pathogenic target for therapeutic intervention seems to be warranted. For this purpose more detailed knowledge of the responsible pathway of complement activation in I/R injury is required. Recent evidence from in vitro and in vivo models suggests involvement of both the classic and the lectin pathways in I/R injury via exposition of neo-epitopes in ischemic membranes. However, most of these findings have been obtained in knock-out murine models and have for a large part remained unconfirmed in the human setting. The observation that the relative role of each pathway seems to differ among organs complicates matters further. Whether a defective complement system protects from I/R injury in humans remains largely unknown. Most importantly, involvement of mannose-binding lectin as the main initiator of the lectin pathway has not been demonstrated at tissue level in human I/R injury to date. Thus, conclusions drawn from animal I/R studies should be extrapolated to the human setting with caution.

IRF-1 promotes inflammation early after ischemic acute kidney injury

Acute renal ischemia elicits an inflammatory response that may exacerbate acute kidney injury, but the regulation of the initial signals that recruit leukocytes is not well understood. Here, we found that IFN regulatory factor 1 (IRF-1) was a critical, early proinflammatory signal released during ischemic injury in vitro and in vivo. Within 15 min of reperfusion, proximal tubular cells of the S3 segment produced IRF-1, which is a transcription factor that activates proinflammatory genes. Transgenic knockout of IRF-1 ameliorated the impairment of renal function, morphologic injury, and inflammation after acute ischemia. Bone marrow chimera experiments determined that maximal ischemic injury required IRF-1 expression by both leukocytes and radioresistant renal cells, the latter identified as S3 proximal tubule cells in the outer medulla by in situ hybridization and immunohistochemistry. In vitro, reactive oxygen species, generated during ischemia/reperfusion injury, stimulated expression of IRF-1 in an S3 proximal tubular cell line. Taken together, these data suggest that IRF-1 gene activation by reactive oxygen species is an early signal that promotes inflammation after ischemic renal injury.

Mechanisms of human complement factor B induction in sepsis and inhibition by activated protein C

To investigate the potential role of the local expression of alternative complement factor B (hBf) in human sepsis, we examined the induction of Bf gene expression in human peripheral blood monocytes (PBMCs) from patients with septic shock and the mechanisms of hBf gene regulation by tumor necrosis factor (TNF)-α, interferon (IFN)-γ, and lipopolysaccharide (LPS) in human monocytes. PBMCs from septic shock patients showed increased hBf mRNA expression when compared with control patients. Costimulation with TNF-α and IFN-γ or stimulation with LPS demonstrated a time- and dose-dependent induction of hBf mRNA expression in human PBMCs. A region of the hBf promoter between −735 and +128 bp was found to mediate IFN-γ, TNF-α, and LPS responsiveness as well as the synergistic effect of IFN-γ/TNF-α on hBf promoter activity. Site-directed mutagenesis of a IFN-γ-activation site (GAS) cis element (−90 to −82 bp) abrogated IFN-γ responsiveness. Mutagenesis of a nuclear factor (NF)-κB cis element at −466 to −456 bp abrogated TNF-α and LPS responsiveness of the Bf promoter. Thus hBf gene expression is induced in PBMCs from septic shock patients, and the induction of hBf by IFN-γ, TNF-α, and LPS is through GAS and NF-κB cis-binding sites on the hBf promoter. Furthermore, activated protein C (APC) inhibited LPS-stimulated hBf promoter activity and protein expression in human monocytes suggesting that the beneficial effect of APC therapy in sepsis may in part be due to inhibition of complement induction and/or activation via the alternative pathway.

Amelioration of renal ischemia-reperfusion injury by inhibition of IL-6 production in the poloxamer 407-induced mouse model of hyperlipidemia

It is largely unknown whether hyperlipidemia is involved in the pathobiology of renal ischemia-reperfusion (I/R) injury that is an important cause of acute kidney injury. Here we studied the effect of experimental dyslipidemia on renal I/R injury. Renal I/R injury was less severe in hyperlipidemic mice treated with poloxamer 407 than in the control mice. Cytokine analyses revealed decreased levels of renal and serum IL-6 in the hyperlipidemic mice after renal I/R. Protection from renal I/R injury in the hyperlipidemic mice was diminished by administration of recombinant IL-6. Concanavalin A-induced IL-6 release from cultured splenocytes derived from the hyperlipidemic mice was lower than that from splenocytes of normal mice. In hypercholesterolemic apolipoprotein E-knockout mice, in which renal I/R injury is less severe than in control mice, renal I/R-induced IL-6 production was also less than that in controls. In angiopoietin-like 3-deficient mice, which were hypolipidemic, renal dysfunction and renal IL-6 level after I/R were similar to those of control mice. Our data indicate that the presence of experimental hyperlipidemia may be associated with a decreased risk of renal I/R injury, possibly mediated by reduced renal IL-6 production after the insult and extend the notion that an anti-IL6 agent would be useful for the treatment of acute kidney injury.

Ischemia-mediated aggregation of the actin cytoskeleton is one of the major initial events resulting in ischemia-reperfusion injury

Ischemia-reperfusion (IR) injury represents a major clinical challenge, which contributes to morbidity and mortality during surgery. The critical role of natural immunoglobulin M (IgM) and complement in tissue injury has been demonstrated. However, cellular mechanisms that result in the deposition of natural IgM and the activation of complement are still unclear. In this report, using a murine intestinal IR injury model, we demonstrated that the beta-actin protein in the small intestine was cleaved and actin filaments in the columnar epithelial cells were aggregated after a transient disruption during 30 min of ischemia. Ischemia also led to deposition of natural IgM and complement 3 (C3). A low dose of cytochalasin D, a depolymerization reagent of the actin cytoskeleton, attenuated this deposition and also attenuated intestinal tissue injury in a dose-dependent manner. In contrast, high doses of cytochalasin D failed to worsen the injury. These data indicate that ischemia-mediated aggregation of the actin cytoskeleton, rather than its disruption, results directly in the deposition of natural IgM and C3. We conclude that ischemia-mediated aggregation of the actin cytoskeleton leads to the deposition of natural IgM and the activation of complement, as well as tissue injury.

The innate immune response in ischemic acute kidney injury

Kidney ischemia reperfusion injury is a major cause of morbidity in both allograft and native kidneys. Ischemia reperfusion-induced acute kidney injury is characterized by early, alloantigen-independent inflammation. Major components of the innate immune system are activated and participate in the pathogenesis of acute kidney injury, plus prime the allograft kidney for rejection. Soluble members of innate immunity implicated in acute kidney injury include the complement system, cytokines, and chemokines. Toll-like receptors (TLRs) are also important contributors. Effector cells that participate in acute kidney injury include the classic innate immune cells, neutrophils and macrophages. Recent data has unexpectedly identified lymphocytes as participants of early acute kidney injury responses. In this review, we will focus on immune mediators that participate in the pathogenesis of ischemic acute kidney injury.

Discrimination between host and pathogens by the complement system

Pathogen-specific complement activation requires direct recognition of pathogens and/or the absence of complement control mechanisms on their surfaces. Antibodies direct complement activation to potential pathogens recognized by the cellular innate and adaptive immune systems. Similarly, the plasma proteins MBL and ficolins direct activation to microorganisms expressing common carbohydrate structures. The absence of complement control proteins permits amplification of complement by the alternative pathway on any unprotected surface. The importance of complement recognition molecules (MBL, ficolins, factor H, C3, C1q, properdin, and others) to human disease are becoming clear as analysis of genetic data and knock out animals reveals links between complement proteins and specific diseases.

Gene expression analysis reveals new possible mechanisms of vancomycin-induced nephrotoxicity and identifies gene markers candidates

Vancomycin, one of few effective treatments against methicillin-resistant Staphylococcus aureus, is nephrotoxic. The goals of this study were to (1) gain insights into molecular mechanisms of nephrotoxicity at the genomic level, (2) evaluate gene markers of vancomycin-induced kidney injury, and (3) compare gene expression responses after iv and ip administration. Groups of six female BALB/c mice were treated with seven daily iv or ip doses of vancomycin (50, 200, and 400 mg/kg) or saline, and sacrificed on day 8. Clinical chemistry and histopathology demonstrated kidney injury at 400 mg/kg only. Hierarchical clustering analysis revealed that kidney gene expression profiles of all mice treated at 400 mg/kg clustered with those of mice administered 200 mg/kg iv. Transcriptional profiling might thus be more sensitive than current clinical markers for detecting kidney damage, though the profiles can differ with the route of administration. Analysis of transcripts whose expression was changed by at least twofold compared with vehicle saline after high iv and ip doses of vancomycin suggested the possibility of oxidative stress and mitochondrial damage in vancomycin-induced toxicity. In addition, our data showed changes in expression of several transcripts from the complement and inflammatory pathways. Such expression changes were confirmed by relative real-time reverse transcription–polymerase chain reaction. Finally, our results further substantiate the use of gene markers of kidney toxicity such as KIM-1/Havcr1, as indicators of renal injury.

Gene silencing of complement C5a receptor using siRNA for preventing ischemia/reperfusion injury

Ischemia/reperfusion (I/R) injury in organ transplantation significantly contributes to graft failure and is untreatable using current approaches. I/R injury is associated with activation of the complement system, leading to the release of anaphylatoxins, such as C5a, and the formation of the membrane attack complex. Here, we report a novel therapy for kidney I/R injury through silencing of the C5a receptor (C5aR) gene using siRNA. Mice were injected with 50 microg of C5aR siRNA 2 days before induction of ischemia. Renal ischemia was then induced through clamping of the renal vein and artery of the left kidney for 25 minutes. The therapeutic effects of siRNA on I/R were evaluated by assessment of renal function, histopathology, and inflammatory cytokines. siRNA targeting C5aR efficiently inhibited C5aR gene expression both in vitro and in vivo. Administering C5aR siRNA to mice preserved renal function from I/R injury, as evidenced by reduced levels of serum creatinine and blood urea nitrogen in the treated groups. Inhibition of C5aR also diminished in vivo production of the pro-inflammatory cytokine tumor necrosis factor-alpha and chemokines MIP-2 and KC, resulting in the reduction of neutrophils influx and cell necrosis in renal tissues. This study demonstrates that siRNA administration represents a novel approach to preventing renal I/R injury and may be used in a variety of clinical settings, including transplantation and acute tubular necrosis.

Complement and renal transplantation: from donor to recipient

Long-term kidney graft survival is affected by different variables including donor condition, ischemia-reperfusion injury, and graft rejection during the transplantation process. The complement system is an important mediator of renal ischemia-reperfusion injury and in rejecting allografts. However, donor complement C3 seems to be crucial in renal transplantation-related injury as renal injury is attenuated in C3 deficient kidney grafts. Interestingly, before ischemia-reperfusion induced C3 expression, C3 is already induced in donors suffering from brain death. Therefore, strategies targeting complement activation in the brain-dead donor may increase graft viability and transplant outcome.

Dextran sulfate facilitates anti-CD4 mAb-induced long-term rat cardiac allograft survival after prolonged cold ischemia

Ischemia/reperfusion injury leads to activation of graft endothelial cells (EC), boosting antigraft immunity and impeding tolerance induction. We hypothesized that the complement inhibitor and EC-protectant dextran sulfate (DXS, MW 5000) facilitates long-term graft survival induced by non-depleting anti-CD4 mAb (RIB 5/2). Hearts from DA donor rats were heterotopically transplanted into Lewis recipients treated with RIB 5/2 (20 mg/kg, days-1,0,1,2,3; i.p.) with or without DXS (grafts perfused with 25 mg, recipients treated i.v. with 25 mg/kg on days 1,3 and 12.5 mg/kg on days 5,7,9,11,13,15). Cold graft ischemia time was 20 min or 12 h. Median survival time (MST) was comparable between RIB 5/2 and RIB 5/2+DXS-treated recipients in the 20-min group with >175-day graft survival. In the 12-h group RIB 5/2 only led to chronic rejection (MST = 49.5 days) with elevated alloantibody response, whereas RIB 5/2+DXS induced long-term survival (MST >100 days, p < 0.05) with upregulation of genes related to transplantation tolerance. Analysis of the 12-h group treated with RIB 5/2+DXS at 1-day posttransplantation revealed reduced EC activation, complement deposition and inflammatory cell infiltration. In summary, DXS attenuates I/R-induced acute graft injury and facilitates long-term survival in this clinically relevant transplant model.

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.

Antiphospholipid antibodies and pregnancy loss: a disorder of inflammation

The antiphospholipid syndrome (APS) is a leading cause of miscarriage and maternal and fetal morbidity. APS is characterized by thrombosis and pregnancy loss that occur in the presence of antiphospholipid (aPL) antibodies. Using a mouse model of APS induced by passive transfer of human aPL antibodies, we have shown that complement activation plays an essential and causative role in pregnancy loss and fetal growth restriction, and that blocking activation of the complement cascade rescues pregnancies. Conventional treatment for APS patients is sub-anticoagulant doses of heparin throughout pregnancy. Could heparin prevent pregnancy loss by inhibiting complement? In our experimental model of APS, heparin inhibits activation of complement on trophoblasts in vivo and in vitro, and anticoagulation in and of itself is not sufficient to prevent pregnancy complications. These studies underscore the importance of inflammation in fetal injury associated with aPL antibodies and raise the importance of developing and testing targeted complement inhibitory therapy for patients with APS.

[The relevance of the inflammatory response in the injured brain]

Research efforts in recent years have defined traumatic brain injury (TBI) as a predominantly immunological and inflammatory disorder. This perception is based on the fact that the overwhelming neuroinflammatory response in the injured brain contributes to the development of posttraumatic edema and to neuropathological sequelae which are, in large part, responsible for the adverse outcome. While the "key" mediators of neuroinflammation, such as the cytokine cascade and the complement system, have been clearly defined by studies in experimental TBI models, their exact pathways of interaction and pathophysiological implications remain to be further elucidated. This lack of knowledge is partially due to the concept of a "dual role" of the neuroinflammatory response after TBI. This notion implies that specific inflammatory molecules may mediate diverse functions depending on their local concentration and kinetics of expression in the injured brain. The inflammation-induced effects range from beneficial aspects of neuroprotection to detrimental neurotoxicity. The lack of success in pushing anti-inflammatory therapeutic concepts from"bench to bedside" for patients with severe TBI strengthens the further need for advances in basic research on the molecular aspects of the neuroinflammatory network in the injured brain. The present review summarizes the current knowledge from experimental studies in this field of research and discusses potential future targets of investigation.

Profiling the enzymatic properties and inhibition of human complement factor B

Human complement factor B is the crucial catalytic component of the C3 convertase enzyme that activates the alternative pathway of complement-mediated immunity. Although a serine protease in its own right, factor B circulates in human serum as an inactive zymogen and there is a crystal structure only for the inactive state of factor B and various fragments. To provide greater insight to the catalytic function and properties of factor B, we have used short para-nitroanilide derivatives of 4- to 15-residue peptides as substrates to profile the catalytic properties of factor B. Among factors found to influence catalytic activity of factor B was an unusual dependence on pH. Non-physiological alkaline conditions strongly promoted substrate cleavage by factor B, consistent with a pH-accessible conformation of the enzyme that may be critical for catalytic function. Small N-terminal extensions to conventional hexapeptide para-nitroanilide substrates significantly increased catalytic activity of factor B, which was more selective for its cleavage site than trypsin. The new chromogenic assay enabled optimization of catalysis conditions, the profiling of different substrate sequences, and the development of the first reversible and competitive substrate-based inhibitor of factor B. The inhibitor was also shown to prevent in vitro formation of C3a from C3 by factor B, by synthetic and by natural C3 convertase of the alternative complement activation pathway, and to block formation of membrane attack complex. The availability of a reversible substrate-based inhibitor that could stabilize the active conformation of factor B, in conjunction with a pH-promoted higher processing activity, may offer a new avenue to obtain crystal structures of factor B and C3 convertase in an active conformation.

Absence of functional alternative complement pathway alleviates lupus cerebritis

The complement inhibitor, Crry, which blocks both the classical and alternative pathways, alleviates CNS disease in the lupus model, MRL/MpJ-Tnfrsf6lpr (MRL/lpr) mice. To understand the role of the alternative pathway, we studied mice deficient in a key alternative pathway protein, complement factor B (fB). Immune deposits (IgG and C3) were reduced in the brains of MRL/lpr fB-deficient (fB-/-MRL/lpr) compared to fB-sufficient (MRL/lpr) mice, indicating reduced complement activation. Reduced neutrophil infiltration (22% of MRL/lpr mice) and apoptosis (caspase-3 activity was reduced to 33% of MRL/lpr mice) in these mice indicates that the absence of the alternative pathway was neuroprotective. Furthermore, expression of phospho (p)-Akt (0.16+/-0.02 vs. 0.35+/-0.13, p<0.03) was increased, while expression of p-PTEN (0.40+/-0.06 vs. 0.11+/-0.07, p<0.05) was decreased in fB-/-MRL/lpr mice compared to their MRL/lpr counterparts. The expression of fibronectin, laminin and collagen IV was significantly decreased in fB-/-MRL/lpr mice compared to MRL/lpr mice, indicating that in the lupus setting, tissue integrity was maintained in the absence of the alternative pathway. Absence of fB reduced behavioral alterations in MRL/lpr mice. Our results suggest that in lupus, the alternative pathway may be the key mechanism through which complement activation occurs in brain, and therefore it might serve as a therapeutic target for lupus cerebritis.

The role of complement in inflammatory diseases from behind the scenes into the spotlight

Our understanding of the biology of the complement system has undergone a drastic metamorphosis since its original discovery. This system, which was traditionally primarily described as a "complement" to humoral immunity, is now perceived as a central constituent of innate immunity, defending the host against pathogens, coordinating various events during inflammation, and bridging innate and adaptive immune responses. Complement is an assembly of proteins found in the blood and body fluids and on cell surfaces. Soluble complement components form the proteolytic cascade, whose activation leads to the generation of complement effectors that target various cells involved in the immune response. Membrane-bound receptors and regulators transmit signals from complement effectors to target cells and limit complement activation to the surfaces of pathogens and damaged or activated host cells. The multiple interconnections among complement proteins, immune cells, and mediators provide an excellent mechanism to protect the organism against infections and support the repair of damaged tissues. However, disturbances in this "defense machinery" contribute to the pathogenesis of various diseases. The role of complement in various inflammatory disorders is multifaceted; for example, the activation of complement can significantly contribute to inflammation-mediated tissue damage, whereas inherited or acquired complement deficiencies highly favor the development of autoimmunity.

ROLE OF THE ALTERNATIVE PATHWAY IN THE EARLY COMPLEMENT ACTIVATION FOLLOWING MAJOR TRAUMA

Complement activation has been reported after major trauma. However, little is known about the clinical relevance and the mechanisms of complement activation early after trauma. Therefore, the aim of this study was to measure complement activation, to identify the roles of injury severity and hypoperfusion, to determine the predominant activated pathway, and to identify the clinical significance of early complement activation in trauma patients. A total of 208 adult trauma patients were enrolled in this prospective single-center cohort study of major trauma patients. Blood samples were obtained within 30 min after injury before any significant fluid resuscitation. Complement (C5b-9) was activated early after trauma, correlated with injury severity and tissue hypoperfusion, and was associated with increased mortality rate and with the development of organ failure such as acute lung injury and acute renal failure. The alternative pathway seems to be the predominant activated complement pathway early after trauma. However, the classical and/or the lectin pathway initiated complement activation because of the correlation between plasma levels of C4d and C3a/C5b-9. Finally, in patients with low C3a levels, C5b-9 levels correlated with plasma levels of prothrombin fragments 1 + 2, a marker of thrombin generation, suggesting additional C3-independent complement activation by thrombin after severe trauma. In summary, complement activation via its amplification by the alternative pathway is observed early after trauma and correlates with injury severity, tissue hypoperfusion, and worse clinical outcomes. Besides complement activation by the classical and/or lectin pathways, there is an independent association between thrombin generation and complement activation.

The role of complement and Toll-like receptors in organ transplantation

The innate immune system not only participates in host defence but also contributes to the control of adaptive immune responses. Complement and Toll-like receptors (TLR) are key components of innate immunity. Emerging evidence suggests their activation is involved in all major aspects of transplantation. This paper reviews the current understanding of how the complement and TLR on impact transplant injury.

Mesangial immune complex glomerulonephritis due to complement factor D deficiency

Complement factor D is a serine protease essential for the activation of the alternative pathway and is expressed in the kidney, adipocytes, and macrophages. Factor D is found at relatively high levels in glomeruli suggesting that this component of the complement cascade could influence renal pathophysiology. In this study, we utilize mice with a targeted deletion of the activating complement factor D gene and compare these results to mice with targeted deletion of the inhibitory complement factor H gene. Eight-month-old mice with a deleted factor D gene spontaneously develop albuminuria and have reduced creatinine clearance due to mesangial immune complex glomerulonephritis. These mesangial deposits contain C3 and IgM. In contrast to the mesangial location of the immune deposits in the factor D-deficient mice, age-matched factor H-deficient mice develop immune deposits along the glomerular capillary wall. Our observations suggest that complement factor D or alternative pathway activation is needed to prevent spontaneous accumulation of C3 and IgM deposits within the mesangium. Our studies show that the complement factor D gene knockout mice are a novel model of spontaneous mesangial immune complex glomerulonephritis.

The alternative pathway of complement is activated in the glomeruli and tubulointerstitium of mice with adriamycin nephropathy

The complement system effectively identifies and clears invasive pathogens as well as injured host cells. Uncontrolled complement activation can also contribute to tissue injury, however, and inhibition of this system may ameliorate many types of inflammatory injury. Several studies have demonstrated that the filtration of complement proteins into the renal tubules, as occurs during proteinuric renal disease, causes tubular inflammation and injury. In the present study, we tested the hypothesis that activation of the complement system in the urinary space requires an intact alternative pathway. Using a model of adriamycin-induced renal injury, which induces injury resembling focal segmental glomerulosclerosis, we examined whether mice deficient in factor B would be protected from the development of progressive tubulointerstitial injury. Complement activation was attenuated in the glomeruli and tubulointerstitium of mice with congenital deficiency of factor B (fB-/-) compared with wild-type controls, demonstrating that complement activation does occur through the alternative pathway. Deficiency in factor B did not significantly protect the mice from tubulointerstitial injury. However, treatment of wild-type mice with an inhibitory monoclonal antibody to factor B did delay the development of renal failure. These results demonstrate that complement activation in this nonimmune complex-mediated model of progressive renal disease requires an intact alternative pathway.

TLR2 Is Constitutively Expressed within the Kidney and Participates in Ischemic Renal Injury through Both MyD88-Dependent and -Independent Pathways

TLRs are an evolutionarily conserved family of cell membrane proteins believed to play a significant role in innate immunity and the response to tissue injury, including that induced by ischemia. TLR signaling pathways activate transcription factors that regulate expression of prosurvival proteins, as well as proinflammatory cytokines and chemokines through one of two proximal adapter proteins, MyD88 or Toll/IL-1R domain-containing adaptor-inducing IFN-beta (Trif). Our study defines the constitutive protein expression of TLR2 in kidneys of humans and mice, and provides insight into the signaling mechanisms by which a deficiency of TLR2 protects from ischemic organ injury. Our study compared and contrasted the effects of renal ischemia in wild-type mice and mice deficient in TLR2, MyD88, Trif, and MyD88xTrif. TLR2 protein was evident in many cell types in the kidney, including renal tubules of the outer stripe of the medulla, glomeruli, and in the renal vasculature. The pattern of protein expression was similar in humans and mice. The absence of TLR2, MyD88, and MyD88xTrif conferred both physiologic and histologic protection against sublethal ischemia at 24 h. Interestingly, TLR2-deficient mice were better protected from ischemic renal injury than those deficient for the adapter protein MyD88, raising the intriguing possibility that TLR-2-dependent/MyD88-independent pathways also contribute to kidney injury. We conclude that TLR2 protein is constitutively expressed in the kidney and plays an important role in the pathogenesis of acute ischemic injury by signaling both MyD88-dependent and MyD88-independent pathways.

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.

C3a is required for the production of CXC chemokines by tubular epithelial cells after renal ishemia/reperfusion

The complement system is one of the major ways by which the body detects injury to self cells, and the alternative pathway of complement is rapidly activated within the tubulointerstitium after renal ischemia/reperfusion (I/R). In the current study, we investigate the hypothesis that recognition of tubular injury by the complement system is a major mechanism by which the systemic inflammatory response is initiated. Gene array analysis of mouse kidney following I/R initially identified MIP-2 (CXCL2) and keratinocyte-derived chemokine (KC or CXCL1) as factors that are produced in a complement-dependent fashion. Using in situ hybridization, we next demonstrated that these factors are expressed in tubular epithelial cells of postischemic kidneys. Mouse proximal tubular epithelial cells (PTECs) in culture were then exposed to an intact alternative pathway and were found to rapidly produce both chemokines. Selective antagonism of the C3a receptor significantly attenuated production of MIP-2 and KC by PTECs, whereas C5a receptor antagonism and prevention of membrane attack complex (MAC) formation did not have a significant effect. Treatment of PTECs with an NF-kappaB inhibitor also prevented full expression of these factors in response to an intact alternative pathway. In summary, alternative pathway activation after renal I/R induces production of MIP-2 and KC by PTECs. This innate immune system thereby recognizes hypoxic injury and triggers a systemic inflammatory response through the generation of C3a and subsequent activation of the NF-kappaB system.

Mouse podocyte complement factor H: the functional analog to human complement receptor 1

Complement factor H (Cfh) is a key plasma protein in humans and animals that serves to limit alternative pathway complement activation in plasma, as well as in local sites such as capillaries of the glomerulus and eye. It was shown that rodent Cfh on platelets is the functional analogue to human erythrocyte complement receptor 1 with a role that is distinct from plasma Cfh and that Cfh is also on cultured rodent podocytes. For investigation of the role of Cfh in the kidney, renal transplants were performed between wild-type (WT) and Cfh(-/-) C57BL/6 mice. For these studies, bilateral native nephrectomies were done so that renal function was dependent solely on the transplanted kidney. Chronic serum sickness was induced by active immunization with apoferritin. Diffuse proliferative glomerulonephritis (GN) occurred in WT kidneys that were transplanted into Cfh(-/-) recipients (n = 8) but not into WT recipients (n = 14), consistent with the importance of plasma Cfh to dictate outcome in this disease model. Relative to the WT recipients of WT kidneys, WT mice with Cfh(-/-) kidneys (n = 12) developed glomerular disease features, including increased albuminuria (82.8 +/- 7.0 versus 45.1 +/- 3.6 microg/mg creatinine; P < 0.001) and blood urea nitrogen levels (54.4 +/- 6.1 versus 44.2 +/- 3.7 mg/dl; P < 0.01). In addition, they had substantial glomerular capillary wall deposits of IgG and C3, which by electron microscopy were present in subendothelial and subepithelial immune deposits, whereas WT kidneys in WT hosts had almost exclusive mesangial deposits. The IgG deposits in Cfh(-/-) kidneys were adjacent to Cfh-deficient podocytes, whereas WT kidneys in a Cfh(-/-) host had podocyte-associated Cfh with absent IgG deposits. These data suggest that locally produced podocyte Cfh is important to process immune complexes in the subepithelial space, where it also limits complement activation. Just as in platelets, rodent podocytes seem to use Cfh as the functional surrogate for human complement receptor 1.

The simple design of complement factor H: Looks can be deceiving

The complement system is a powerful component of innate immunity which recognizes and facilitates the elimination of pathogens and unwanted host material. Since complement can also lead to host tissue injury and inflammation, strict regulation of its activation is important. One of the key regulators is complement factor H (CFH), a protein with an ever-expanding list of relevant functions. Inherited mutations in CFH can account for membranoproliferative glomerulonephritis (MPGN) type II, atypical hemolytic uremic syndrome, and age-related macular degeneration. The former can be associated with excessive systemic complement activation from dysfunctional CFH, while the latter two are associated with mutations affecting the ability of CFH to bind to anionic surfaces such as on endothelial cells and glomerular and retinal capillary walls. Mice with targeted deletion of CFH can spontaneously develop MPGN and have increased susceptibility to models of GN. In the rodent, CFH on platelets functions as the immune adherence receptor, analogous to CR1 on primate erythrocytes. In mice, platelets lacking CFH are unable to effectively clear immune complexes which results in their accumulation in glomeruli. The same switch also appears to be true in the rodent podocyte where CFH is present in place of CR1 in human podocytes. Thus, CFH has a variety of functions which can affect the diverse roles the complement system plays in health and disease.

Inhibition of Complement Component C3 Reduces Vein Graft Atherosclerosis in Apolipoprotein E3–Leiden Transgenic Mice

BACKGROUND

Venous bypass grafts may fail because of development of intimal hyperplasia and accelerated atherosclerosis. Inflammation plays a major role in these processes. Complement is an important part of the immune system and participates in the regulation of inflammation. The exact role of complement in the process of accelerated atherosclerosis of vein grafts has not yet been explored, however.

METHODS AND RESULTS

To assess the role of complement in the development of vein graft atherosclerosis, a mouse model, in which a venous interposition was placed in the common carotid artery, was used. In this model, vein graft thickening appeared within 4 weeks. The expression of complement components was studied with the use of immunohistochemistry on sections of the thickened vein graft. C1q, C3, C9, and the regulatory proteins CD59 and complement receptor-related gene y could be detected in the lesions 4 weeks after surgery. Quantitative mRNA analysis for C1q, C3, CD59, and complement receptor-related gene y revealed expression of these molecules in the thickened vein graft, whereas C9 did not show local mRNA expression. Furthermore, interference with C3 activation with complement receptor-related gene y-Ig was associated with reduced vein graft thickening, reduced C3 and C9 deposition, and reduced inflammation as assessed by analysis of influx of inflammatory cells, such as leukocytes, T cells, and monocytes. In addition, changes in apoptosis and proliferation were observed. When C3 was inhibited by cobra venom factor, a similar reduction in vein graft thickening was observed.

CONCLUSIONS

The complement cascade is involved in vein graft thickening and may be a target for therapy in vein graft failure disease.

A novel interpretation of immune redundancy and duality in reperfusion injury with important implications for intervention in ischaemic disease

The majority of ischaemia related injury occurs upon tissue reperfusion. Knock-out mouse models have recently shed light on the underlying molecular mechanisms, and suggest that this may be the result of an innate autoimmune response. Based on these new findings we present a novel model of immune redundancy and duality in reperfusion injury. Natural antibody, mannan-binding lectin and toll-like receptor 4 are three pre-formed innate immune receptors that recognise pathogenic molecular patterns. Removing either significantly ameliorates reperfusion injury. We propose that these three receptors serve as key parallel recognition elements that respond to the same or similar ischaemic neo-antigens, of which at least one may have a lipopolysaccharide-like motif. This would fit both with the ligand preference of the three receptors, and the observation that giving monoclonal antibody to lipopolysaccharide reduces reperfusion injury. The consequent injury caused by receptor activation appears to be mainly related to the complement anaphylatoxins, and less to phagocytes, oxidative radicals, and the membrane attack complex. C5a levels in particular are predictive of overall injury, and we suggest this anaphylatoxin causes most of reperfusion injury via both direct toxic effects and a generalised immune activation. The former is illustrated by the recent observation that excess C5a alone can cause cardiac dysfunction. As for the latter, there is evidence that adaptive immunity (especially CD4+ cells) and other serum cascades (coagulation and kallikrein) are involved, and may have been recruited by complement. Furthermore, excess C5a can cause innate immune overactivation that paralyses neutrophils, reduces complement lytic function, and leads to systemic inflammation. This is analogous to what happens in sepsis, and would explain the passive role in IRI of normal immune effectors. Finally, there is a duality complement's function in reperfusion, as some elements are conductive of damage, whilst others may help inflammatory resolution. Most important among the latter are the opsonins, like C3b and apparently C1q, which help macrophages clear apoptosing cells before they undergo secondary necrosis. This model has important implications for clinical interventions. Firstly, redundancy means that inhibiting multiple receptors may achieve a larger mortality reduction than the small and inconsistent one seen in the published monotherapy trials. Secondly, duality means that a non-specific inhibition of complement would reduce both injury and resolution. Therefore, a specific inhibition of the lectin pathway and/or an inhibition of the downstream effectors upon which the receptors converge (e.g. C5a) seem to be a better interceptive strategy.

Complement mediators in ischemia–reperfusion injury

BACKGROUND

Ischemia-reperfusion (I/R) injury occurs when a tissue is temporarily deprived of blood supply and the return of the blood supply triggers an intense inflammatory response. Pathologically, increased complement activity can cause substantial damage to blood vessels, tissues and also facilitate leukocyte activation and recruitment following I/R injury. Herein, previously published studies are reported and critically reviewed.

METHODS

Medline and the World Wide Web were searched and the relevant literature was classified under the following categories: (1) Complement pathways; (2) The complement system and the inflammatory response; (3) Complement in ischemia-reperfusion injuries; and (4) Therapeutic approaches against complement in I/R injuries.

RESULTS AND CONCLUSIONS

I/R injury is a common clinical event with the potential to seriously affect, and sometimes kill, the patient and is a potent inducer of complement activation that results in the production of a number of inflammatory mediators. Complement activation leads to the release of biologically active potent inflammatory complement substances including the anaphylatoxins (C3a and C5a) and the cytolytic terminal membrane attack complement complex C5b-9 (MAC). The use of specific complement inhibitors to block complement activation at various levels of the cascade has been shown to prevent or reduce local tissue injury after I/R. Several agents that inhibit all or part of the complement system, such as soluble complement receptor type 1 (sCR1), C1 inhibitor (C1-INH), C5a monoclonal antibodies, a C5a receptor antagonist and soluble CD59 (sCD59) have been shown to reduce I/R injury of various organs. The novel inhibitors of complement products may eventually find wide clinical application because there are no effective drug therapies currently available to treat I/R injuries.

Triggers of inflammation after renal ischemia/reperfusion

Renal ischemia/reperfusion (I/R) is a common cause of acute renal failure (ARF). Ischemic ARF is associated with tubulointerstitial inflammation, and studies using animal models have demonstrated that the inflammatory response to I/R exacerbates the resultant renal injury. Ischemic ARF involves complement activation, the generation of cytokines and chemokines within the kidney, and infiltration of the kidney by leukocytes. Recent work has revealed some of the events and signals that trigger the inflammatory response to aseptic, hypoxic injury of the kidney. In many ways, the inflammatory reaction to this injury resembles that seen during ascending urinary infection, and it may represent a general response of the tubular epithelial cells (TECs) to stress or injury. A greater understanding of the signals that trigger the inflammatory response may permit the development of effective therapies to ameliorate ischemic ARF.

Natural antibodies, autoantibodies and complement activation in tissue injury

Ischemia/reperfusion-induced tissue damage is a significant problem occurring in multiple clinical conditions. Antibodies and complement activation contribute significantly to this pathology. Mice deficient in complement receptors 1 and 2 fail to produce a component of the natural antibody repertoire that binds to ischemia-conditioned tissues and activate complement. In contrast, mice prone to autoimmunity display accelerated tissue injury that results from the binding of autoantibodies to injured tissues. The specificity and production of natural antibodies, their role in autoimmunity and the mode of complement activation are reviewed from the perspective of the processes involved in ischemia/reperfusion-induced tissue damage.

Preventing renal ischemia-reperfusion injury using small interfering RNA by targeting complement 3 gene

The complement system is one of the important mediators of renal ischemia-reperfusion injury (IRI). We hypothesized that efficient silencing of C3, which is the central component on which all complement activation pathways converge, could be achieved using small interfering RNA (siRNA), and that this would result in overall inhibition of complement activation, thereby preventing IRI in kidneys. A series of experiments was conducted, using a mouse model of IRI and vector-delivered C3-specific siRNA. We demonstrated the following: (1) renal expression of C3 increases as a result of IRI; (2) by incorporation into a pRNAT U6.1 vector, siRNA can be delivered to renal cells in vivo; (3) systemically delivered siRNA is effective in reducing the expression of C3 in an experimentally induced mouse kidney model of IRI; (4) similarly, siRNA reduces complement-mediated IRI-related effects, both in terms of renal injury (as evidenced by renal function and histopathology examination) and mouse mortality and (5) silencing the production of C3 diminishes in vivo production of TNF-alpha. This study implies that siRNA represents a novel approach to preventing IRI in kidneys and might be used in a variety of clinical settings, including transplantation and acute tubular necrosis.

Investigations on the Involvement of the Lectin Pathway of Complement Activation in Anaphylaxis

BACKGROUND

Systemic anaphylaxis is the most severe form of immediate hypersensitivity reaction. The activation of the complement system occurs during anaphylactic shock. The purpose of this study was to determine in a mouse model whether the lectin pathway of complement activation is involved in anaphylaxis.

METHODS

To see whether the lectin pathway is involved in anaphylactic shock, serum mannan-binding lectin (MBL) levels were measured after passive anaphylaxis. Also MBL expression and binding to potential ligands were investigated. To determine whether complement or mast cell activation is essential for hypothermia in anaphylactic shock, mouse strains deficient in MBL-A and MBL-C, C1q, factors B and C2, C5, C5aR, or mast cells were tested.

RESULTS

After antigenic challenge a marked drop in body temperature as well as a rapid decrease in serum MBL levels were observed. The decrease of serum MBL levels in shock could not be attributed to MBL binding to immune complexes or tissues, but an interaction of MBL with mast cell-derived proteoglycans was seen. In contrast to mast cell-deficient mice, none of the complement-deficient mouse strains were protected from shock-associated hypothermia.

CONCLUSIONS

These results indicate that neither MBL nor activation of the complement cascade is crucial for the induction of anaphylaxis. In contrast mast cell activation is associated with the development of hypothermia and possibly the observed decrease in serum MBL levels.

Alternative Complement Pathway Activation Is Essential for Inflammation and Joint Destruction in the Passive Transfer Model of Collagen-Induced Arthritis

Activation of each complement initiation pathway (classical, alternative, and lectin) can lead to the generation of bioactive fragments with resulting inflammation in target organs. The objective of the current study was to determine the role of specific complement activation pathways in the pathogenesis of experimental anti-type II collagen mAb-passive transfer arthritis. C57BL/6 mice were used that were genetically deficient in either the alternative pathway protein factor B (Bf(-/-)) or in the classical pathway component C4 (C4(-/-)). Clinical disease activity was markedly decreased in Bf(-/-) compared with wild-type (WT) mice (0.5 +/- 0.22 (n = 6) in Bf(-/-) vs 8.83 +/- 0.41 (n = 6) in WT mice (p < 0.0001)). Disease activity scores were not different between C4(-/-) and WT mice. Analyses of joints showed that C3 deposition, inflammation, pannus, cartilage, and bone damage scores were all significantly less in Bf(-/-) as compared with WT mice. There were significant decreases in mRNA levels of C3, C4, CR2, CR3, C3aR, and C5aR in the knees of Bf(-/-) as compared with C4(-/-) and WT mice with arthritis; mRNA levels for complement regulatory proteins did not differ between the three strains. These results indicate that the alternative pathway is absolutely required for the induction of arthritis following injection of anti-collagen Abs. The mechanisms by which these target organ-specific mAbs bypass the requirements for engagement of the classical pathway remain to be defined but do not appear to involve a lack of alternative pathway regulatory proteins.

Reduced neuronal cell death after experimental brain injury in mice lacking a functional alternative pathway of complement activation

Background

Neuroprotective strategies for prevention of the neuropathological sequelae of traumatic brain injury (TBI) have largely failed in translation to clinical treatment. Thus, there is a substantial need for further understanding the molecular mechanisms and pathways which lead to secondary neuronal cell death in the injured brain. The intracerebral activation of the complement cascade was shown to mediate inflammation and tissue destruction after TBI. However, the exact pathways of complement activation involved in the induction of posttraumatic neurodegeneration have not yet been assessed. In the present study, we investigated the role of the alternative complement activation pathway in contributing to neuronal cell death, based on a standardized TBI model in mice with targeted deletion of the factor B gene (fB-/-), a "key" component required for activation of the alternative complement pathway.

Results

After experimental TBI in wild-type (fB+/+) mice, there was a massive time-dependent systemic complement activation, as determined by enhanced C5a serum levels for up to 7 days. In contrast, the extent of systemic complement activation was significantly attenuated in fB-/- mice (P < 0.05,fB-/- vs. fB+/+; t = 4 h, 24 h, and 7 days after TBI). TUNEL histochemistry experiments revealed that posttraumatic neuronal cell death was clearly reduced for up to 7 days in the injured brain hemispheres of fB-/- mice, compared to fB+/+ littermates. Furthermore, a strong upregulation of the anti-apoptotic mediator Bcl-2 and downregulation of the pro-apoptotic Fas receptor was detected in brain homogenates of head-injured fB-/- vs. fB+/+ mice by Western blot analysis.

Conclusion

The alternative pathway of complement activation appears to play a more crucial role in the pathophysiology of TBI than previously appreciated. This notion is based on the findings of (a) the significant attenuation of overall complement activation in head-injured fB-/- mice, as determined by a reduction of serum C5a concentrations to constitutive levels in normal mice, and (b) by a dramatic reduction of TUNEL-positive neurons in conjunction with an upregulation of Bcl-2 and downregulation of the Fas receptor in head-injured fB-/- mice, compared to fB+/+ littermates. Pharmacological targeting of the alternative complement pathway during the "time-window of opportunity" after TBI may represent a promising new strategy to be pursued in future studies.

Therapeutic strategy with a membrane-localizing complement regulator to increase the number of usable donor organs after prolonged cold storage

A shortage of donor organs and increasing dependence on marginal grafts with prolonged ischemic times have meant that new methods are needed to prevent postischemic damage. Herein is reported a new strategy aimed to protect donor kidney from complement-mediated postischemic damage and therefore increase the number of successful transplants. Rat donor kidneys were perfused with a membrane-localizing complement regulator derived from human complement receptor type 1 (APT070) and then subjected to prolonged periods of cold storage (at 4 degrees C). A relationship was found between the duration of cold ischemia and the extent of complement-mediated tubule damage and loss of graft function. After 16 h of cold storage, APT070-treated kidneys that were transplanted into syngeneic recipients showed a significant increase in the number of surviving grafts, compared with control-treated grafts (63.6 versus 26.3%). Surviving grafts also displayed less acute tubular injury and better preservation of renal function. These results not only enhance the understanding of the mechanism by which prolonged cold ischemia reduces immediate graft survival but also provide essential information about the effectiveness of membrane-localizing complement regulator with prolonged cold storage. This could lead to more effective strategies for improving the use of severely ischemic donor organs.

The central role of the alternative complement pathway in human disease

The complement system is increasingly recognized as important in the pathogenesis of tissue injury in vivo following immune, ischemic, or infectious insults. Within the complement system, three pathways are capable of initiating the processes that result in C3 activation: classical, alternative, and lectin. Although the roles that proinflammatory peptides and complexes generated during complement activation play in mediating disease processes have been studied extensively, the relative contributions of the three activating pathways is less well understood. Herein we examine recent evidence that the alternative complement pathway plays a key and, in most instances, obligate role in generating proinflammatory complement activation products in vivo. In addition, we discuss new concepts regarding the mechanisms by which the alternative pathway is activated in vivo, as recent clinical findings and experimental results have provided evidence that continuous active control of this pathway is necessary to prevent unintended targeting and injury to self tissues.

Treatment with an inhibitory monoclonal antibody to mouse factor B protects mice from induction of apoptosis and renal ischemia/reperfusion injury

Complement activation in the kidney after ischemia/reperfusion (I/R) seems to occur primarily via the alternative complement pathway. The ability of an inhibitory mAb to mouse factor B, a necessary component of the alternative pathway, to protect mice from ischemic acute renal failure was tested. Treatment with the mAb prevented the deposition of C3b on the tubular epithelium and the generation of systemic C3a after renal I/R. Treated mice had significantly lower increases in serum urea nitrogen and developed significantly less morphologic injury of the kidney after I/R. For gaining insight into potential mechanisms of protection, the activity of caspases within the kidney also was measured, and it was found that caspases-2, -3, and -9 increased in a complement-dependent manner after renal I/R. Apoptotic cells were detected by terminal deoxynucleotidyl transferase catalyzed labeling of DNA fragments, and mice in which the alternative pathway was inhibited demonstrated significantly less apoptosis than control mice. Thus, use of an inhibitory mAb to mouse factor B effectively prevented activation of complement in the kidney after I/R and protected the mice from necrotic and apoptotic injury of the tubules.

Local extravascular pool of C3 is a determinant of postischemic acute renal failure

The third complement component (C3) is an acute phase protein that plays a central role in reperfusion injury in several organ models. To investigate the contribution of local synthesis of C3 and distinguish it from that of circulating complement mainly produced by hepatic synthesis, we employed a mouse renal isograft model. Our model demonstrated a close relationship between the extent of intrarenal expression of C3 and cold-ischemia induced injury. Ischemic C3-positive donor kidneys transplanted into C3-positive or C3-negative recipients developed widespread tissue damage and severe acute renal failure. In contrast, ischemic C3-negative isografts exhibited only mild degrees of functional and structural disturbance, even when transplanted into normal C3-positive recipients. Thus local synthesis of C3, mostly identified in the tubular epithelium, was essential for complement-mediated reperfusion damage, whereas circulating C3 had a negligible effect. Our results suggest a two-compartment model for the pathogenic function of C3, in which the extravascular compartment is the domain of local synthesis of C3, and where the role of circulating C3 is redundant. Our data cast new light on the mechanism of complement-mediated tissue injury in nonimmunological disorders, and challenges the longstanding dogma that circulating components are the main complement effectors of extravascular tissue damage.