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

Anaphylatoxins: their role in bacterial infection and inflammation

Activation of the complement system plays a crucial role in the pathogenesis of infection and inflammation. Especially the complement activation products C3a and C5a, known as the anaphylatoxins, are potent proinflammatory mediators. In addition to their evident role in innate immunity, it is clear that the anaphylatoxins also play a role in regulation of adaptive immune responses. The anaphylatoxins play a role in a variety of infectious and inflammatory diseases like sepsis, ischemia-reperfusion injury, immune complex diseases, and hypersensitivity diseases like asthma. In this review we discuss the role of anaphylatoxins in infection and inflammation. Furthermore, we focus on bacterial complement evasion strategies that can provide tools for further research on pathogenesis of infectious diseases and a better understanding of the role of complement and anaphylatoxins in infection and inflammation.

New concepts of complement in allorecognition and graft rejection

In transplantation, activation of complement has largely been equated to antibody-mediated rejection, but complement is also important in recognition of apoptotic and necrotic cells as well as in modifying antigen presentation to T cells and B cells. As a part of the innate immune system, complement is one of the first responses to injury, and it can determine the direction and magnitude of the subsequent responses. Consequently, the effects of complement in allorecognition and graft rejection are increased when organs are procured from cadaver donors because these organs sustain a series of stresses from brain death, prolonged life support, ischemia and finally reperfusion that initiate proinflammatory processes and tissue injury. In addition, these organs are transplanted to patients, who frequently have been sensitized to histocompatibility antigens as the result of transfusions, pregnancies or transplants. Complement activation generates a series of biologically active effector molecules that can modulate graft rejection by directly binding to the graft or by modifying the response of macrophages, T and B cells of the recipient. However, complement is regulated and the process of regulation produces split products that can decrease as well as increase immune responses. Small animal models have been developed to test these variables. The guide for evaluating results from these models remains clinical findings because there are significant differences between the rodent and human complement systems.

Decay-accelerating factor attenuates remote ischemia-reperfusion-initiated organ damage

Complement activation contributes to the expression of local and remote organ injury in animal models of ischemia-reperfusion (IR). We demonstrate here that a soluble form of decay-accelerating factor (DAF) protects normal C57Bl/6 and autoimmunity-prone B6.MRL/lpr mice subjected to hindlimb IR from remote intestinal and lung injury without affecting the degree of local skeletal muscle injury. In addition, DAF treatment attenuates remote organ injury in mice subjected to mesenteric IR. Soluble DAF allowed the deposition of complement 3 in local and remote injury sites while it limited the presence of terminal membrane attack complex and did not increase animal susceptibility to sepsis. These data provide evidence that soluble DAF might offer clinical benefit to patients suffering remote intestinal or lung damage in response to muscle or other organ injury.

Function, structure and therapeutic potential of complement C5a receptors

Complement fragment (C)5a is a 74 residue pro-inflammatory polypeptide produced during activation of the complement cascade of serum proteins in response to foreign surfaces such as microorganisms and tissue damaged by physical or chemical injury. C5a binds to at least two seven-transmembrane domain receptors, C5aR (C5R1, CD88) and C5L2 (gpr77), expressed ubiquitously on a wide variety of cells but particularly on the surface of immune cells like macrophages, neutrophils and T cells. C5aR is a classical G protein-coupled receptor that signals through G alpha i and G alpha 16, whereas C5L2 does not appear to couple to G proteins and has no known signalling activity. Although C5a was first described as an anaphylatoxin and later as a leukocyte chemoattractant, the widespread expression of C5aR suggested more general functionality. Our understanding of the physiology of C5a has improved significantly in recent years through exploitation of receptor knockout and knocking mice, C5 and C5a antibodies, soluble recombinant C5a and C5a analogues and newly developed receptor antagonists. C5a is now also implicated in non-immunological functions associated with developmental biology, CNS development and neurodegeneration, tissue regeneration, and haematopoiesis. Combined receptor mutagenesis, molecular modelling, structure-activity relationship studies and species dependence for ligand potency on C5aR have been helpful for identifying ligand binding sites on the receptor and for defining mechanisms of receptor activation and inactivation. This review will highlight major developments in C5a receptor research that support C5aR as an important therapeutic target. The intriguing possibilities raised by the existence of a non-signalling C5a receptor are also discussed.

C5a Receptor-Deficient Mice Are Protected from Thrombophilia and Endothelial Cell Activation Induced by Some Antiphospholipid Antibodies

Recent findings indicate that complement activation--involving specifically C3 and C5--contributes to antiphospholipid (aPL)-mediated thrombosis. Two complement effector pathways are initiated by the cleavage of C5, C5a and C5b, which leads to the formation of the C5b-9 membrane attack complex. To delineate and distinguish the role of C5a from the C5b-9 membrane attack complex seeded by C5b, we examined the in vivo effects (thrombosis) of aPL on C5a receptor-deficient (C5aR-/-) mice. C5aR-/- and C5aR+/+ mice were injected with IgM or with IgG from two different patients with APS (IgM-APS or IgG-APS) or with control IgM or IgG (IgM-NHS or IgG-NHS) twice. Complement fixing activity of the Ig fractions and anticardiolipin activity in the sera of the mice were determined by enzyme-linked immunosorbent assay. Surgical procedures to study thrombus dynamics were performed. IgM-APS but not IgG-APS fixed C1q to cardiolipin-coated plates. IgM-APS significantly enhanced thrombus size in C5aR+/+ mice compared to C5aR+/+ mice treated with IgM-NHS (3198 +/- 2361 microm2 versus 585 +/- 460 microm2). C5aR-/- mice treated with IgM-APS showed a significant reduction in thrombi size as compared with C5aR+/+ mice (676 +/- 690 microm2 versus 3198 +/- 2361 microm2; P = 0.001). IgG-APS enhanced thrombus formation significantly in C5aR+/+ when compared to IgG-NHS-treated mice (3507 +/- 965 microm2 versus 1321 +/- 798 microm2), and these effects were not altered in C5aR-/- mice (3400 +/- 1681 microm2). The data indicate that C5aR-/- mice are protected from the thrombogenic effects of some aPL antibodies.

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.

Transdermal pharmacology of small molecule cyclic C5a antagonists

Overproduction or underregulation of the proinflammatory complement component C5a has been implicated in numerous immune and inflammatory conditions. Therefore, targeting the C5a receptor (C5aR) has become an innovative strategy for antiinflammatory drug development. The novel cyclic peptide C5aR antagonist, AcF-[OP(D-Cha)WR] (PMX53), attenuates injury in numerous animal models of inflammation following intravenous, subcutaneous, intraperitoneal, and oral administration. In the present study the transdermal pharmacology of PMX53 and three analogs designed with increased lipophilicity, hydrocinnamate-[OP(D-Cha)WCit] (PMX200), AcF-[OP(D-Cha)WCit] (PMX201) and hydrocinnamate-[OP(D-Cha)WR] (PMX205), have been examined in order to assess their transdermal permeability and inhibitory effect on C5a-mediated lipopolysaccharide (LPS)-induced systemic responses. In the rat, PMX53, PMX201, and PMX205, were bioavailable following topical dermal administration (10 mg/50 cm2 site/rat). All analogs functionally antagonized neutropenia and hypotension induced by systemic challenge with LPS (1 mg/kg i.v.). Interestingly, PMX200 attenuated LPS-induced neutropenia more effectively than other analogs, despite undetectable (<5 ng/ml) circulating levels following topical administration. In conclusion, we have demonstrated that cyclic peptide C5aR antagonists can penetrate transdermally sufficiently to have systemic effects. However, increasing lipophilicity in these compounds did not result in increased blood levels. Nonetheless, topical application of C5aR antagonists produced circulating levels of the drugs that antagonized the LPS-induced systemic responses of neutropenia and hypotension. This suggests that these small-molecule C5aR antagonists may be developed for topical administration for the treatment of local and systemic inflammatory conditions in the human and veterinary pharmaceutical markets.

Protection of Renal Ischemia Injury using Combination Gene Silencing of Complement 3 and Caspase 3 Genes

BACKGROUND

Ischemia/reperfusion (I/R) injury occurs in clinical kidney transplantation, which results in graft dysfunction and rejection. It has been documented that I/R injury is associated with complement activation and renal cell apoptosis. The purpose of this study was to develop a strategy to prevent I/R injury using small interfering RNA (siRNA) that target complement 3 (C3) and caspase 3 genes.

METHODS

siRNA-expression vectors were constructed to target C3 and caspase 3 genes. Gene silencing efficacy was assessed using real-time polymerase chain reaction. In vivo gene silencing was performed by hydrodynamic injection with C3 and caspase 3 siRNA. Renal I/R injury was induced through clamping the renal vein and artery for 25 min. I/R injury was evaluated using kidney histopathology, blood urea nitrogen (BUN), serum levels of creatinine, and survival.

RESULTS

Effective gene silencing was first confirmed in vitro. Notably upregulated expression of C3 and caspase 3 genes was observed from 2 to 48 hr after I/R injury, which were effectively and specifically inhibited by C3 and caspase 3 siRNA. In comparison with control mice, serum levels of creatinine and BUN were also significantly decreased in C3 and caspase 3 siRNA-treated mice. Furthermore, the therapeutic effect of siRNA was assessed in a severe, lethal I/R injury experiment, in which siRNA treatment significantly reduced mortality. Tissue histopathology showed an overall reduction in injury area in siRNA-treated mice.

CONCLUSIONS

This is the first demonstration that renal I/R injury can be prevented through silencing the complement gene and apoptosis gene, highlighting the potential for siRNA-based clinical therapy.

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.

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.

Complement inhibitors selectively attenuate injury following administration of cobra venom factor to rats

Systemic activation of complement is a pathophysiological response common to severe disturbances such as hemorrhagic shock, major burn injury and sepsis. Intravenous infusion of cobra venom factor (CVF) has been used as an animal model of acute respiratory distress syndrome (ARDS), and reliably and selectively induces rapid intravascular activation of the complement system, leading to acute organ damage. In the present study, we have used different complement inhibitors to investigate the roles of complement products in CVF-induced responses in the rat. Rats were treated with either a C5a receptor antagonist (C5aRA, AcF-[OP(d-Cha)WR], 1 mg/kg i.v. or 10 mg/kg p.o.), a C3a receptor antagonist (C3aRA, N(2)-[(2,2-diphenylethoxy)acetyl]-l-arginine, 0.1 mg/kg i.v.) or a convertase inhibitor, rosmarinic acid (RMA, 10 mg/kg i.v.), prior to CVF-induced complement challenge. Intravenous CVF resulted in hallmark events evident in the development of ARDS, including systemic neutropenia followed by neutrophil migration to the lung and bronchoalveolar vascular leakage, blood pressure alterations, and an increase in TNFalpha levels in both serum and bronchoalveolar lavage fluid. These hemodynamic changes were differentially inhibited by antagonism of C5a receptors, C3a receptors or by inhibition of the entire complement cascade using RMA. This evidence strongly implicates complement factors in the development of lung injury associated with systemic complement activation and identifies complement inhibition as a potential therapeutic target for acute syndromes such as ARDS and other severe systemic shock states mediated by activation of complement.

Folic acid 5 or 15 mg/d similarly reduces plasma homocysteine in patients with moderate-advanced chronic renal failure

BACKGROUND

Hyperhomocysteinaemia is an independent risk factor for cardiovascular disease with a remarkable prevalence in patients with chronic renal failure (CRF). Low doses of folic acid (FA) with or without vitamin B6 and B12 has been shown to effectively reduce plasma homocysteine (Hcy). The aim of this study was to compare the short-term effects of two different oral doses of FA (5 vs 15 mg/d) on plasma Hcy levels in subjects suffering from moderate-severe CRF.

METHODS

A double-blind, double-dummy, comparative, two-stage randomised study was performed. Seventeen patients aged 45-71 years, with glomerular filtration rates between 15.4-50 mL/min 1.73/m2 were randomly assigned to receive FA 5 mg/d (FA-5, n: 8) or FA 15 mg/d (FA-15, n: 9) for 30 days. At the end of this 30-day double-blind period, all the participants were placed on FA 5 mg/d (open period), for 5 additional months. Both groups were also supplemented with vitamins B1, B6 and B12 throughout the trial. Blood samples were drawn at 0, 15, 30, 90 and 180 days to assess Hcy, complete blood count (CBC) and sequential multichannel analysis (SMA). Chest X-ray and a 12-lead electrocardiogram (ECG) were also performed.

RESULTS

Plasma Hcy (mean +/- SEM) decreased from 27.9 +/- 1.4 (baseline) to 15.1 +/- 0.6, 13.3 +/- 0.9, 14.1 +/- 0.5 and 13.8 +/- 0.5 micromol/L (FA-5) and from 28.8 +/- 2.7 to 15.6 +/- 1.2, 14.4 +/- 1.3, 13.0 +/- 0.7 and 13.1 +/- 0.6 micromol/L (FA-15) at days 15, 30, 90 and 180, respectively. (P < 0.01 from day 15 to 180 vs baseline for both groups with no differences between them). Renal function remained stable throughout the entire period of the study in all but one patient in whom it deteriorated to pre-end stage disease. No adverse cardiovascular events developed during the trial.

CONCLUSION

Both folic acid doses induced a significant and similar decrease in plasma Hcy in subjects with moderate-severe chronic renal failure. The possible dose-related effect of this approach in reducing the risk of accelerated sclerotic vascular disease and cardiovascular events in this especially vulnerable population should be a matter of further investigation.

The complement system in the pathophysiology of pregnancy

A fully active complement system deriving from the maternal circulation as well as from local production by various cell source is present in the placenta. The role of this system at the placental level, as in any other tissue in the body, is to protect both the fetus and the mother against infectious and other toxic agents. As fetal tissues are semi-allogeneic and alloantibodies commonly develop in the mother, the placenta is potentially subject to complement-mediated immune attack at the feto-maternal interface with the potential risk of fetal loss. Uncontrolled complement activation is prevented in successful pregnancy by the three regulatory proteins DAF, MCP and CD59 positioned on the surface of trophoblasts. The critical role played by these complement regulators is supported by the embryonic lethality observed in mice deficient in the complement regulator Crry. Excessive complement activation in the placenta places the fetus at risk for growth restriction or death. The role played by the complement system in the fetal damage induced by anti-phospholipid antibodies in a mouse model will be examined.

Strategies of therapeutic complement inhibition

The involvement of complement in the pathogenesis of a great number of partly life threatening diseases defines the importance to develop inhibitors which specifically interfere with its deleterious action. Endogenous soluble complement-inhibitors, antibodies or low molecular weight antagonists, either blocking key proteins of the cascade reaction or neutralizing the action of the complement-derived anaphylatoxins have successfully been tested in various animal models over the past years. Promising results consequently led to first clinical trials. This review is focused on different approaches for the development of inhibitors, on their site of action in the cascade, on possible indications for complement inhibition based on experimental animal data, and on potential side effects of such treatment.

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.

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.

Allograft rejection: effect of local synthesis of complement

The complement system is known for its ability to participate in non-specific inflammation and membrane injury as well as contributing to antigen-specific immune stimulation. In renal transplantation, the complement cascade behaves true to form in that both non-immune- and immune-mediated destruction of the renal tubules are complement dependent. What is remarkable, however, is the extent of involvement of local synthesis of complement in both of these injuries, suggesting that the extravascular tissue compartment is the domain of local synthesis, whereas the effect of circulating complement is much less. This creates a new paradigm for studying the influence of local synthesis of complement in other organ-based diseases and underlines the need for tissue-targeting strategies in successful therapeutic development.

Urokinase-induced activation of the gp130/Tyk2/Stat3 pathway mediates a pro-inflammatory effect in human mesangial cells via expression of the anaphylatoxin C5a receptor

Glomerular mesangial cells (MCs) are central to the pathogenesis of progressive glomeruli-associated renal diseases. However, molecular mechanisms underlying changes in MC functions still remain poorly understood. Here, we show that in MCs, the urokinase-type plasminogen activator (uPA) induces, via its specific receptor (uPAR, CD87), upregulated expression of the complement anaphylatoxin C5a receptor (C5aR, CD88), and modulates C5a-dependent functional responses. This effect is mediated via the interaction of the uPA-specific receptor (uPAR, CD87) and gp130, a signal transducing subunit of the receptor complexes for the IL-6 cytokine family. The Janus kinase Tyk2 and the transcription factor Stat3 serve as downstream components in the signaling cascade resulting in upregulation of C5aR expression. In vivo, expression of C5aR and uPAR was increased in the mesangium of wild-type mice in a lipopolysaccharide (LPS)-induced model of inflammation, whereas in uPAR(-/-) animals C5aR expression remained unchanged. This is the first demonstration in vitro and in vivo that uPA acts in MCs as a modulator of immune responses via control of immune-competent receptors. The data suggest a novel role for uPA/uPAR in glomeruli-associated renal failure via a signaling cross-talk between the fibrinolytic and immune systems.

Targeted complement inhibition by C3d recognition ameliorates tissue injury without apparent increase in susceptibility to infection

Previous studies indicate a pivotal role for complement in mediating both local and remote injury following ischemia and reperfusion of the intestine. Here, we report on the use of a mouse model of intestinal ischemia/reperfusion injury to investigate the strategy of targeting complement inhibition to sites of complement activation by linking an iC3b/C3dg-binding fragment of mouse complement receptor 2 (CR2) to a mouse complement-inhibitory protein, Crry. We show that the novel CR2-Crry fusion protein targets sites of local and remote (lung) complement activation following intestinal ischemia and reperfusion injury and that CR2-Crry requires a 10-fold lower dose than its systemic counterpart, Crry-Ig, to provide equivalent protection from both local and remote injury. CR2-Crry has a significantly shorter serum half-life than Crry-Ig and, unlike Crry-Ig, had no significant effect on serum complement activity at minimum effective therapeutic doses. Furthermore, the minimum effective dose of Crry-Ig significantly enhanced susceptibility to infection in a mouse model of acute septic peritonitis, whereas the effect of CR2-Crry on susceptibility to infection was indistinguishable from that of PBS control. Thus, compared with systemic inhibition, CR2-mediated targeting of a complement inhibitor of activation improved bioavailability, significantly enhanced efficacy, and maintained host resistance to infection.

Increased potency of a novel complement factor 5a receptor antagonist in a rat model of inflammatory bowel disease

We have previously shown that complement factor 5a (C5a) plays a role in the pathogenesis of 2,4,6-trinitrobenzene sulfonic acid (TNBS)-induced colitis in rats by using the selective, orally active C5a antagonist AcF-[OP(d-Cha)WR]. This study tested the efficacy and potency of a new C5a antagonist, hydrocinnamate (HC)-[OP(d-Cha)WR], which has limited intestinal lumenal metabolism, in this model of colitis. Analogs of AcF-[OP(d-Cha)WR] were examined for their susceptibility to alimentary metabolism in the rat using intestinal mucosal washings. One metabolically stable analog, HC-[OP(d-Cha)WR], was then evaluated pharmacokinetically and investigated at a range of doses (0.03-10 mg/kg/day p.o.) in the 8-day rat TNBS-colitis model, against the comparator drug AcF-[OP(d-Cha)WR]. Using various amino acid substitutions, it was determined that the AcF moiety of AcF-[OP(d-Cha)WR] was responsible for the metabolic instability of the compound in intestinal mucosal washings. The analog HC-[OP(d-Cha)WR], equiactive in vitro to AcF-[OP(d-Cha)WR], was resistant to intestinal metabolism, but it displayed similar oral bioavailability to AcF-[OP(d-Cha)WR]. However, in the rat TNBS-colitis model, HC-[OP(d-Cha)WR] was effective at reducing mortality, colon edema, colon macroscopic scores, and increasing food consumption and body weights, at 10- to 30-fold lower oral doses than AcF-[OP(d-Cha)WR]. These studies suggest that resistance to intestinal metabolism by HC-[OP(d-Cha)WR] may result in increased local concentrations of the drug in the colon, thus affording efficacy with markedly lower oral doses than AcF-[OP(d-Cha)WR] against TNBS-colitis. This large increase in potency and high efficacy of this compound makes it a potential candidate for clinical development against intestinal diseases such as inflammatory bowel disease.

Fc receptors and their interaction with complement in autoimmunity

Genetic studies in mice indicate a crucial role for Fc receptors (FcR) in antibody-mediated autoimmune diseases. Like other immune regulatory receptor pairs, the FcR system is constituted by activating and inhibitory receptors that bind the same ligand, the Fc portion of Ig. Analyses of animal models have shown that the inhibitory Fc receptor, FcgammaRIIB can suppress antibody-mediated autoimmunity, whereas activating-type FcR, such as FcgammaRIII promote disease development. This review summarizes recent advances of FcR, as obtained from gene deletion studies in mice, and highlights the importance of factors that interact with FcR in autoimmunity. There is emerging evidence for an indispensable role of the complement component C5a in the regulation of FcR and the sensing of FcR-dependent effector cell responses. On the other hand, FcR might be alternatives to serum complement in the generation of C5a at sites of inflammation. Thus, FcR and complement interact with each other at the level of C5a by linking regulatory events with effector cell activities in autoimmunity. This connecting pathway is now proposed to be a promising new therapeutic target for the treatment of inflammation and autoimmune disease in both mice and humans.

ROLE OF C5A IN INFLAMMATORY RESPONSES

The complement system not only represents an effective innate immune mechanism of host defense to eradicate microbial pathogens, but it is also widely involved in many forms of acute and chronic inflammatory diseases including sepsis, acute lung injury, ischemia-reperfusion injury, and asthma, to give just a few examples. The complement-activated product, C5a, displays powerful biological activities that lead to inflammatory sequelae. C5a is a strong chemoattractant and is involved in the recruitment of inflammatory cells such as neutrophils, eosinophils, monocytes, and T lymphocytes, in activation of phagocytic cells and release of granule-based enzymes and generation of oxidants, all of which may contribute to innate immune functions or tissue damage. Accumulating data suggest that C5a provides a vital bridge between innate and adaptive immune functions, extending the roles of C5a in inflammation. Herein, we review human and animal data describing the cellular and molecular mechanisms of C5a in the development of inflammatory disorders, sepsis, acute lung injury, ischemia-reperfusion injury, and asthma.

Molecular mechanisms of acute renal failure following ischemia/reperfusion

Acute renal failure (ARF) necessitating renal replacement therapy is a common problem associated with high morbidity and mortality in the critically ill. Hypotension, followed by resuscitation, is the most common etiologic factor, mimicked by ischemia/reperfusion (I/R) in animal models. Although knowledge of the pathophysiology of ARF in the course of this condition is increasingly detailed, the intracellular and molecular mechanisms leading to ARF are still incompletely understood. This review aims at describing the role of cellular events and signals, including collapse of the cytoskeleton, mitochondrial and nuclear changes, in mediating cell dysfunction, programmed cell death (apoptosis), necrosis and others. Insight into the molecular pathways in the various elements of the kidney, such as vascular endothelium and smooth muscle and tubular epithelium leading to cell damage upon I/R will, hopefully, open new therapeutic modalities, to mitigate the development of ARF after hypotensive episodes and to promote repair and resumption of renal function once ARF has developed.

Effect of synthetic protease inhibitor gabexate mesilate on the attenuation of ischemia/reperfusion injury in canine kidney autotransplantation

BACKGROUND

Kidneys from non-heart-beating donors are associated with delayed graft function and a high rejection rate due to the long period of warm ischemia. Gabexate mesilate (GM), a synthetic serine protease inhibitor, has been shown to improve organ function by suppressing cytokine activity and neutrophil function after ischemia/reperfusion. In this study, we evaluated the effect of GM on renal function after warm ischemia in a canine kidney autotransplantation model.

METHODS

After 60 minutes of warm ischemia, the left kidney was transplanted into the iliac fossa, and the right kidney was removed. The control group (n = 7) and GM group (n = 7) were evaluated for serum creatinine and blood urea nitrogen (BUN) concentrations, renal tissue blood flow, resistive index, pulsatility index, interleukin (IL)-1beta and tumor necrosis factor (TNF)-alpha mRNA expression levels in peripheral blood mononuclear cells, apoptotic index, CD10 immunolabeling as an indicator of brush border injury, and standard histopathology.

RESULTS

Compared with controls, administration of GM resulted in lower serum creatinine concentrations (11.3 +/- 2.4 vs 5.2 +/- 3.3 mg/dL at 72 hours; P = .04) and BUN concentrations (188 +/- 26 mg/dL vs 98 +/- 41 mg/dL at 72 hours; P = .04), as well as better tissue blood flow, improvement of brush border injury and apoptotic index (each P < .05). The expression of IL-1beta and TNF-alpha mRNA did not change after administration of GM.

CONCLUSIONS

The present study shows that GM protected renal function after warm ischemia/reperfusion by inhibition of serine proteases, maintenance of tissue blood flow, and amelioration of tubular apoptosis.

Innate autoimmunity

The adaptive immune system has evolved highly specific pattern recognition proteins and receptors that, when triggered, provide a first line of host defense against pathogens. Studies reveal that these innate recognition proteins are also self-reactive and can initiate inflammation against self-tissues in a similar manner as with pathogens. This specific event is referred to as "innate autoimmunity." In this review, we describe two classes of autoimmune responses, that is, reperfusion injury and fetal loss syndrome, in which the recognition and injury are mediated by innate immunity. Both disorders are common and are clinically important. Reperfusion injury (RI) represents an acute inflammatory response after a reversible ischemic event and subsequent restoration of blood flow. Findings that injury is IgM and complement dependent and that a single natural antibody prepared from a panel of B-1 cell hybridomas can restore injury in antibody-deficient mice suggest that RI is an autoimmune-type disorder. Fetal loss syndrome is also an antibody- and complement-dependent disorder. Although both immune and natural antibodies are likely involved in recognition of phospholipid self-antigens, inhibition of the complement pathway in rodent models can block fetal loss. As new innate recognition proteins and receptors are identified, it is likely that innate responses to self represent frequent events and possibly underlie many of the known chronic autoimmune disorders normally attributable to dysregulation of adaptive immunity.

The mannose-binding lectin-pathway is involved in complement activation in the course of renal ischemia-reperfusion injury

Ischemia-reperfusion (I/R) is an important cause of acute renal failure (ARF). The complement system appears to be essentially involved in I/R injury. However, via which pathway the complement system is activated and in particular whether the mannose-binding lectin (MBL)-pathway is activated is unclear. This tempted us to study the activation and regulation of the MBL-pathway in the course of experimental renal I/R injury and in clinical post-transplant ARF. Mice subjected to renal I/R displayed evident renal MBL-depositions, depending on the duration of warm ischemia, in the early reperfusion phase. Renal deposition of C3, C6 and C9 was observed in the later reperfusion phase. The deposition of MBL-A and -C completely co-localized with the late complement factor C6, showing that MBL is involved in complement activation in the course of renal I/R injury. Moreover, the degree of early MBL-deposition correlated with complement activation, neutrophil-influx, and organ-failure observed in the later reperfusion phase. In serum of mice subjected to renal I/R MBL-A, levels increased in contrast to MBL-C levels, which dropped evidently. In line, liver mRNA levels for MBL-A increased, whereas MBL-C levels decreased. Renal MBL mRNA levels rapidly dropped in the course of renal I/R. Finally, in human biopsies, MBL-depositions were observed early after transplantation of ischemically injured kidneys. In line with our experimental data, in ischemically injured grafts displaying post-transplant organ-failure extensive MBL depositions were observed in peritubular capillaries and tubular epithelial cells. In conclusion, in experimental renal I/R injury and clinical post-transplant ARF the MBL-pathway is activated, followed by activation of the complement system. These data indicate that the MBL-pathway is involved in ischemia-induced complement activation.

Complement in Renal Transplantation

Previous research and therapy in renal transplantation largely focused on the cellular arm of the adaptive immune response. Evidence is emerging that innate immune mechanisms, particularly complement, play a greater role in inflammatory and immune responses against the graft than has been previously recognized. Alternative complement pathway activation appears to mediate renal ischaemia/reperfusion injury, and proximal tubular cells may be both the source and the site of attack of complement components in this setting. Locally produced complement also plays a role in the development of both cellular and antibody-mediated immune responses against the graft. C4d staining has emerged as a useful marker of humoral rejection both in the acute and in the chronic setting and led to renewed interest in the significance of anti-donor antibody formation. A number of therapies are in development which inhibit complement or reduce local synthesis, and may lead to an improved clinical outcome following renal transplantation.

The role of the complement system in ischemia-reperfusion injury

Ischemia-reperfusion (I/R) injury is a common clinical event with the potential to seriously affect, and sometimes kill, the patient. Interruption of blood supply causes ischemia, which rapidly damages metabolically active tissues. Paradoxically, restoration of blood flow to the ischemic tissues initiates a cascade of pathology that leads to additional cell or tissue injury. I/R is a potent inducer of complement activation that results in the production of a number of inflammatory mediators. The use of specific inhibitors to block complement activation has been shown to prevent local tissue injury after I/R. Clinical and experimental studies in gut, kidney, limb, and liver have shown that I/R results in local activation of the complement system and leads to the production of the complement factors C3a, C5a, and the membrane attack complex. The novel inhibitors of complement products may find wide clinical application because there are no effective drug therapies currently available to treat I/R injuries.

CD59a deficiency exacerbates ischemia-reperfusion injury in mice

The terminal complement components C5a and the membrane attack complex are involved in the pathogenesis of ischemia-reperfusion injury in many organs. CD59 is the major regulator of membrane attack complex formation. Mice deficient in the Cd59a gene (mCd59a-/-) were used to investigate the role of CD59 in renal ischemia-reperfusion injury. Unilateral ischemia-reperfusion injury was induced by clamping the left renal pedicle for 30 minutes under general anesthetic. Mice were studied at 72 hours and 2 weeks after ischemia-reperfusion injury. mCd59a-/- mice developed significantly greater tubular injury (P = 0.01), tubulointerstitial apoptosis (P = 0.02), and neutrophil influx (P = 0.04) than controls at 72 hours after ischemia-reperfusion. Two weeks after ischemia-reperfusion, mCd59a-/- mice exhibited more severe tubular damage predominantly in a corticomedullary distribution than controls (P = 0.02). Quantification of interstitial leukocytes revealed significantly greater numbers of infiltrating lymphocytes (but not macrophages) in mCd59a-/- mice than controls (P = 0.04) at 2 weeks. At both time points, significantly more C9 (as a marker of membrane attack complex) deposition occurred in a peritubular distribution in mCd59a-/- mice than controls. In conclusion, these results demonstrate that the lack of CD59a, by allowing unregulated membrane attack complex deposition, exacerbates both the tubular injury and the interstitial leukocyte infiltrate after ischemia-reperfusion injury in mice.

Complement C5a receptor is essential for the optimal generation of antiviral CD8+ T cell responses

The complement system has been long regarded as an important effector of the innate immune response. Furthermore, complement contributes to various aspects of B and T cell immunity. Nevertheless, the role of complement in CD8(+) T cell antiviral responses has yet to be fully delineated. We examined the CD8(+) T cell response in influenza type A virus-infected mice treated with a peptide antagonist to C5aR to test the potential role of complement components in CD8(+) T cell responses. We show that both the frequency and absolute numbers of flu-specific CD8(+) T cells are greatly reduced in C5aR antagonist-treated mice compared with untreated mice. This reduction in flu-specific CD8(+) T cells is accompanied by attenuated antiviral cytolytic activity in the lungs. These results demonstrate that the binding of the C5a component of complement to the C5a receptor plays an important role in CD8(+) T cell responses.

Selective adsorption of homocysteine using an HFR-ON LINE technique

HFR-ON LINE (double chamber HDF with reinfusion of ultrafiltrate regenerated through a charcoal-resin cartridge) is a novel method which combines the processes of diffusion, convection, and adsorbance. We have investigated the effect of such a treatment on the homocysteine (Hcy) levels in ten patients with a mean Hcy level of 57.6 micromol/L (range 24.1-119.7 micromol/L). We have measured the Hcy, folate, and vitamin B12 predialysis and postdialysis, and in the ultrafiltrate precartridge and postcartridge at 10, 120, and 240 min. The mean Hcy levels were 57.6 and 35.3 micromol/L (range 9.9-80.3 micromol/L) (P = 0.005) predialysis and postdialysis, respectively, while folate and vitamin B12 were unchanged. Precartridge and postcartridge Hcy levels were 11.6 vs. 2.5 micromol/L (P = 0.005), 9.3 vs. 3.9 micromol/L (P = 0.005), and 7.7 vs. 4.6 micro mol/L (P = 0.012) at the three time points considered, while folate and vitamin B12 were essentially undetectable. These preliminary data, which need confirmation in a long-term study, seem to indicate that HFR-ON LINE is able to reduce Hcy levels not only through a likely reduction of uremic toxins, but also through an actual removal of Hcy by adsorbance onto the charcoal-resin cartridge.

Comparative anti-inflammatory activities of antagonists to C3a and C5a receptors in a rat model of intestinal ischaemia/reperfusion injury

1. Complement activation is implicated in the pathogenesis of intestinal ischaemia-reperfusion injury (I/R), although the relative importance of individual complement components is unclear. A C3a receptor antagonist N(2)-[(2,2-diphenylethoxy)acetyl]-l-arginine (C3aRA) has been compared with a C5a receptor antagonist (C5aRA), AcF-[OPdChaWR], in a rat model of intestinal I/R. 2. C3aRA (IC(50)=0.15 microm) and C5aRA (IC(50)=0.32 microm) bound selectively to human polymorphonuclear leukocyte (PMN) C3a and C5a receptors, respectively. Effects on circulating neutrophils and blood pressure in the rat were also assessed. 3. Anaesthetised rats, subjected to intestinal ischaemia (30 min) and reperfusion (120 min), were administered intravenously with either (A) the C3aRA (0.1-1.0 mg x kg(-1)); the C5aRA (1.0 mg x kg(-1)); the C3aRA+C5aRA (each 1.0 mg x kg(-1)); or vehicle, 45 min prior, or (B) the C3aRA (1.0 mg x kg(-1)) or vehicle, 120 min prior to reperfusion. 4. The C3aRA and C5aRA, administered 45 min prior to reperfusion, displayed similar efficacies at ameliorating several disease markers (increased oedema, elevated ALT levels and mucosal damage) of rat intestinal I/R. The combination drug treatment did not result in greater injury reduction than either antagonist alone. However, doses of the C3aRA (0.01-10 mg x kg(-1)) caused transient neutropaenia, and the highest dose (10 mg x kg(-1)) also caused a rapid and transient hypertension. 5. The C3aRA (1.0 mg x kg(-1)), delivered 120 min prior to reperfusion to remove the global effect of C3aRA-induced neutrophil sequestration, did not attenuate the markers of intestinal I/R, despite persistent C3aR antagonism at this time. 6. C3aR antagonism does not appear to be responsible for the anti-inflammatory actions of this C3aRA in intestinal I/R in the rat. Instead, C3aRA-mediated global neutrophil tissue sequestration during ischaemia and early reperfusion may account for the protective effects observed.

Potent cyclic antagonists of the complement C5a receptor on human polymorphonuclear leukocytes. Relationships between structures and activity

Human C5a is a plasma protein with potent chemoattractant and pro-inflammatory properties, and its overexpression correlates with severity of inflammatory diseases. C5a binds to its G protein-coupled receptor (C5aR) on polymorphonuclear leukocytes (PMNLs) through a high-affinity helical bundle and a low-affinity C terminus, the latter being solely responsible for receptor activation. Potent and selective C5a antagonists are predicted to be effective anti-inflammatory drugs, but no pharmacophore for small molecule antagonists has yet been developed, and it would significantly aid drug design. We have hypothesized that a turn conformation is important for activity of the C terminus of C5a and herein report small cyclic peptides that are stable turn mimics with potent antagonism at C5aR on human PMNLs. A comparison of solution structures for the C terminus of C5a, small acyclic peptide ligands, and cyclic antagonists supports the importance of a turn for receptor binding. Competition between a cyclic antagonist and either C5a or an acyclic agonist for C5aR on PMNLs supports a common or overlapping binding site on the C5aR. Structure-activity relationships for 60 cyclic analogs were evaluated by competitive radioligand binding with C5a (affinity) and myeloperoxidase release (antagonist potency) from human PMNLs, with 20 compounds having high antagonist potencies (IC(50), 20 nM-1 microM). Computer modeling comparisons reveal that potent antagonists share a common cyclic backbone shape, with affinity-determining side chains of defined volume projecting from the cyclic scaffold. These results define a new pharmacophore for C5a antagonist development and advance our understanding of ligand recognition and receptor activation of this G protein-coupled receptor.

Acute renal failure in the newborn

Acute renal failure in the newborn is a common problem and is typically classified as prerenal, intrinsic renal disease including vascular insults, and obstructive uropathy. In the newborn, renal failure may have a prenatal onset in congenital diseases such as renal dysplasia with or without obstructive uropathy and in genetic diseases such as autosomal recessive polycystic kidney disease. Acute renal failure in the newborn is also commonly acquired in the postnatal period because of hypoxic ischemic injury and toxic insults. Nephrotoxic acute renal failure in newborns is usually associated with aminoglycoside antibiotics and nonsteroidal anti-inflammatory medications used to close a patent ductus arteriosis. Alterations in renal function occur in approximately 40% of premature newborns who have received indomethacin and such alterations are usually reversible. Renal artery thrombosis and renal vein thrombosis will result in renal failure if bilateral or if either occurs in a solitary kidney. Cortical necrosis is associated with hypoxic/ischemic insults due to perinatal anoxia, placenta abruption and twin-twin or twin-maternal transfusions with resultant activation of the coagulation cascade. As in older children, hospital acquired acute renal failure is newborns is frequently multifactorial in origin. Although the precise incidence and prevalence of acute renal failure in the newborn is unknown, several studies have shown that acute renal failure is common in the neonatal intensive care unit. Recent interesting studies have demonstrated that some newborns may have genetic risks factors for acute renal failure. Once intrinsic renal failure has become established, management of the metabolic complications of acute renal failure continues to involve appropriate management of fluid balance, electrolyte status, acid-base balance, nutrition and the initiation of renal replacement therapy when appropriate. Renal replacement therapy may be provided by peritoneal dialysis, intermittent hemodialysis, or hemofiltration with or without a dialysis circuit. The preferential use of hemofiltration by pediatric nephrologists is increasing while the use of peritoneal dialysis is decreasing except for neonates and small infants. Peritoneal dialysis has been a major modality of therapy for acute renal failure in the neonate when vascular access may be difficult to maintain. In the newborn, the prognosis and recovery from acute renal failure is highly dependent upon the underlying etiology of the acute renal failure. Factors that are associated with mortality include multiorgan failure, hypotension, need for pressors, hemodynamic instability, and need for mechanical ventilation and dialysis. The mortality and morbidity of newborns with acute renal failure is much worse in neonates with multiorgan failure. Newborns who have suffered substantial loss of nephrons as may occur in cortical necrosis are at risk for late development of renal failure after apparent recovery from the initial insult. Similarly, hypoxic/ischemic and nephrotoxic injury to the developing kidney can result is decreased nephron number. Newborns with acute renal failure need life-long monitoring of their renal function, blood pressure, and urinalysis. Typically, the late development of chronic renal failure will first becomes apparent with the development of hypertension, proteinuria, and eventually an elevated blood urea nitrogen and creatinine.

A potent human C5a receptor antagonist protects against disease pathology in a rat model of inflammatory bowel disease

The complement system is implicated in the pathogenesis of human inflammatory bowel disease, but the specific role of C5a has never been examined. We have compared the efficacy of an orally active human C5a receptor antagonist (AcPhe[Orn-Pro-D-cyclohexylalanine-Trp-Arg]), prednisolone, and infliximab against trinitrobenzene sulfonic acid (TNBS)-induced colitis in rats. The drugs were administered either 2 days before or 24 h after TNBS instillation, and rats were then examined after 8 days. Drug-free colitis control rats showed severe disease pathology with significant mortality (39%). Rats pre or posttreated with the C5a antagonist (10 mg/kg/day peroral, 0.3 mg/kg/day s.c.) had reduced mortality and significantly improved macroscopic scores, colon edema, colon myeloperoxidase levels, reduced concentrations of TNF-alpha levels in the colon and serum, and had greater food intake resulting in greater weight gains than colitis-only rats. Rats pretreated with prednisolone (1 mg/kg/day s.c.) displayed significant improvement in parameters measured, but posttreatment was ineffective. Single dose pretreatment with the TNF-alpha inhibitor infliximab (3 mg/kg i.v.) also had significant improvements in the parameters measured. Rats pretreated with a combination of the C5a antagonist and prednisolone showed no greater improvements than either drug alone. These findings suggest a central role for complement, particularly C5a, in the pathology of TNBS-induced colitis in rats, indicating a possible therapeutic role for C5a antagonists in inflammatory bowel disease.

Complement C5a receptor antagonist attenuates multiple organ injury in a model of ruptured abdominal aortic aneurysm

OBJECTIVE

Abdominal aortic aneurysm (AAA) rupture is associated with a systemic inflammatory response syndrome, characterized by increased microvascular permeability and neutrophil sequestration, leading to multiorgan dysfunction. We examined the role of a novel complement factor 5a (C5aR) receptor antagonist, the cyclic peptide AcF-(OpdChaWR), in attenuation of pathologic complement activation and tissue injury in a model of AAA rupture.

METHODS

Anesthetized rats were randomized to sham (control) or shock and clamp (s+c) groups. Animals in the s+c group underwent 1 hour of hemorrhagic shock (mean arterial blood pressure < or =50 mm Hg), followed by 45 minutes of supramesenteric aortic clamping, then 2 hours of resuscitated reperfusion. Animals in the s+c group were randomized to receive an intravenous bolus of C5aR antagonist at 1 mg/kg or saline solution control at the end of hemorrhagic shock. Intestinal and pulmonary permeability to iodine 125-labeled albumin was measured as an indicator of microvascular permeability. Tissue myeloperoxidase activity, proinflammatory cytokine tissue necrosis factor-alpha (TNF-alpha) protein and mRNA, and C5aR mRNA levels were measured as indicators of neutrophil sequestration and inflammatory signaling, respectively.

RESULTS

Lung permeability index was significantly increased in the s+c group compared with the sham group (4.43 +/- 0.96 vs 1.30 +/- 0.17; P <.01), and prevented with treatment with C5aR antagonist (1.74 +/- 0.50; P <.03). Lung myeloperoxidase activity was significantly increased in the the s+c group compared with the sham group (2.41 +/- 0.34 U/mg vs 1.03 +/- 0.29 U/mg; P <.009), and significantly attenuated with treatment with C5aR antagonist (1.11 +/- 0.09 U/mg; P <.006). Lung TNF-alpha protein levels were significantly elevated in both s+c groups, whereas lung TNF-alpha mRNA expression was significantly downregulated in both s+c groups compared with the sham group. Intestinal permeability index was significantly increased in animals in the s+c groups during reperfusion, compared with sham (P <.001), which was attenuated in early reperfusion with treatment with C5a receptor antagonist. Data represent mean +/- SEM, group comparisons with analysis of variance and post hoc Scheffé test.

CONCLUSIONS

These results indicate that a potent antagonist of C5a receptor protects the rat intestine and lung from neutrophil-associated injury in a model of AAA rupture. These data suggest that complement-mediated inflammation can be modulated at the C5a receptor level, independent of proinflammatory TNF-alpha production, and prevent acute local and remote organ injury.

Protective effects of a potent c5a receptor antagonist on experimental acute limb ischemia-reperfusion in rats

OBJECTIVES

The capacity of a potent C5a receptor antagonist to inhibit various parameters of local and remote organ injury following lower limb ischemia-reperfusion (I/R) in rats was investigated.

METHODS

Rats were subjected to 2 h bilateral hindlimb ischemia and 4 h reperfusion. Drug-treated rats received AcF-[OPdChaWR] (1 mg/kg) iv either 10 min before ischemia or 10 min prior to reperfusion, or orally (10 mg/kg) 30 min prior to ischemia. Levels of circulating creatine kinase (CK), lactate dehydrogenase (LDH), alanine and aspartate aminotransferase (ALT/AST), creatinine, blood urea nitrogen (BUN), polymorphonuclear leukocytes (PMNs), and calcium (Ca(++)) and potassium (K(+)) ions were determined. Other parameters measured included urinary protein levels, muscle edema, and myeloperoxidase (MPO) concentrations in the lung, liver, and muscle along with liver homogenate tumor necrosis factor-alpha (TNF-alpha) concentrations.L RESULTS: imb I/R injury was characterized by significant elevations of CK, LDH, ALT, AST, creatinine, BUN, proteinuria, PMNs, serum K(+), muscle edema, organ MPO, and liver homogenate TNF-alpha concentrations, but a significant reduction in serum Ca(2+) concentrations. When rats were treated with AcF-[OPdChaWR], there were significant improvements in all these parameters.

CONCLUSIONS

These results indicate a pivotal role for C5a in inducing local and remote organ injury and suggest a possible new drug therapeutic category for preventing anticipated tissue injury associated with I/R.

A key role of C5a/C5aR activation for the development of sepsis

In recent studies, evidence has been provided for complement activation early during the onset of experimental sepsis. Excessive production of the anaphylatoxin C5a thereby appears to elicit various harmful effects. Blockade of C5a or C5a receptor (C5aR) at the start of experimental sepsis has been demonstrated to greatly improve survival in rodents. There is evidence that C5a, during the onset of sepsis, enhances the production of various proinflammatory mediators in different cell types. Besides its known, other proinflammatory effects, recent work suggested an inhibitory role of C5a for innate-immune functions of phagocytic cells (phagocytosis, reactive oxygen species production, chemotaxis) during experimental sepsis. This review article provides an overview of the important role of C5a/C5aR activation for the onset and development of sepsis.

Complement C5a receptors and neutrophils mediate fetal injury in the antiphospholipid syndrome

Antiphospholipid syndrome (APS) is defined by recurrent pregnancy loss and thrombosis in the presence of antiphospholipid (aPL) Ab's. Currently, therapy for pregnant women with APS is focused on preventing thrombosis, but anticoagulation is only partially successful in averting miscarriage. We hypothesized that complement activation is a central mechanism of pregnancy loss in APS and tested this in a model in which pregnant mice receive human IgG containing aPL Ab's. Here we identify complement component C5 (and particularly its cleavage product C5a) and neutrophils as key mediators of fetal injury, and we show that Ab's or peptides that block C5a-C5a receptor interactions prevent pregnancy complications. The fact that F(ab)'2 fragments of aPL Ab's do not mediate fetal injury and that C4-deficient mice are protected from fetal injury suggests that activation of the complement cascade is initiated via the classical pathway. Studies in factor B-deficient mice, however, indicate that alternative pathway activation is required and amplifies complement activation. In contrast, activating Fc gamma Rs do not play an important role in mediating aPL Ab-induced fetal injury. Our findings identify the key innate immune effectors engaged by pathogenic autoantibodies that mediate poor pregnancy outcomes in APS and provide novel and important targets for prevention of pregnancy loss in APS.

The tyrosine kinase inhibitor tyrphostin AG126 reduces renal ischemia/reperfusion injury in the rat

BACKGROUND

We investigate the effects of tyrphostin AG126, an inhibitor of tyrosine kinase activity, on the renal dysfunction and injury caused by ischemia/reperfusion (I/R) of the kidney.

METHODS

Tyrphostin AG126 (5 mg/kg intraperitoneally) was administered to male Wistar rats 30 minutes prior to bilateral renal ischemia for 45 minutes followed by reperfusion for up to 48 hours. Biochemical markers of renal dysfunction and injury were measured and renal sections assessed for renal injury. Expression of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) and formation of nitrotyrosine and poly (ADP) ribose (PAR) were assessed using immunohistochemistry. Rat proximal tubular cells (PTCs) were incubated with interferon-gamma (100 IU/mL), bacterial lipopolysaccharide (10 microg/mL), and with increasing concentrations of tyrphostin AG126 (0.0001-1 mmol/L) for 24 hours. Nitric oxide production was measured in both plasma from rats subjected to I/R and in incubation medium from PTCs.

RESULTS

After 6 hours of reperfusion, tyrphostin AG126 significantly reduced the increase in serum and urinary indicators of renal dysfunction and injury caused by I/R and reduced histologic evidence of renal injury. Tyrphostin AG126 also improved renal function (after 24 and 48 hours of reperfusion) and reduced the histologic signs of renal injury (after 48 hours of reperfusion). Tyrphostin AG126 reduced the expression of iNOS and nitric oxide levels in both rat plasma and in PTC cultures, as well as expression of COX-2. Tyrphostin AG126 also reduced nitrotyrosine and PAR formation, suggesting reduction of nitrosative stress and poly (ADP-ribose) polymerase (PARP) activation, respectively.

CONCLUSION

Taken together, these results show that tyrphostin AG126 significantly reduces the renal dysfunction and injury caused by I/R of the kidney. We propose that inhibition of tyrosine kinase activity may be useful against renal I/R injury.

Characterisation of C5a receptor agonists from phage display libraries

C5a des-Arg(74) has a 10- to 100-fold lower receptor binding affinity than intact C5a and is only a partial agonist. We have used phage display selection from randomly mutated C5a des-Arg(74) libraries to isolate variant proteins that can activate C5a receptors with similar potency to C5a. Here we explore the interactions of three variants (V1-3) with C5aR mutated at residues involved in the differential response. The mutant Asp(282)Arg-C5aR is preferentially activated by C5a des-Arg(74), probably due to repulsion between Arg(74) of C5a and the substituent Arg(282). In accordance with this hypothesis, V2 (with a polar C-terminus which has no Arg residue) but not V1 (with a C-terminal Arg residue at position 73) could activate Asp(282)Arg-C5aR. V3, with a very hydrophobic C-terminus, was the most potent agonist at Asp(282)Arg-C5aR. Arg(175) is a potential counterion for the C-terminal carboxylate of C5a. C5aR mutated to either Ala or Asp at this position lost nearly all responsiveness to both C5a and C5a des-Arg(74), suggesting that mutation of Arg(175) caused a non-specific loss of receptor conformation and a loss of signalling capacity. However, V3 could still activate Arg(175)Asp/Ala-C5aR with the same potency as wild-type C5aR, demonstrating that the mutant receptors retained high signalling capability and showed a specific loss of responsiveness. Thus C5a des-Arg(74) variants produced by phage display are potentially useful tools for the dissection of ligand-receptor interactions.

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

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

Role of the complement system in rejection

The complement system plays a complex role in transplantation, beginning with effects on reperfusion injury and continuing with stimulation of the adaptive immune response. Recent evidence has emphasised the importance of the late components of the complement cascade in the mediation of post-ischaemic damage, which are apparently triggered by the classical, alternative or lectin pathways of complement activation, depending on the organ affected. In studies of renal allograft rejection, the local synthesis of complement component C3 seems to influence the T-cell response more strongly than circulating complement protein, raising the possibility that there is co-operation between locally derived C3 and antigen presentation in the graft. Class switching of alloantibody to a high-affinity IgG response is also highly dependent on C3. In addition, the finding that capillary-bound C4d is a robust marker for humoral rejection has started a new investigation into the significance of alloantibodies in acute and chronic allograft rejection. There are several selective and nonselective inhibitors suitable for clinical development; clearly it is time for more concerted effort to evaluate their role in clinical transplantation.