Hydrophobic effect and hydrogen bonds account for the improved activity of a complement inhibitor, compstatin

Activation of polymorphonuclear leukocytes by candidate biomaterials for an implantable glucose sensor

BACKGROUND

Continuous monitoring of glucose by implantable microfabricated devices offers key advantages over current transcutaneous glucose sensors that limit usability due to their obtrusive nature and risk of infection. A successful sensory implant should be biocompatible and retain long-lasting function. Polymorphonuclear leukocytes (PMN) play a key role in the inflammatory system by releasing enzymes, cytokines, and reactive oxygen species, typically as a response to complement activation. The aim of this study was to perform an in vitro analysis of PMN activation as a marker for biocompatibility of materials and to evaluate the role of complement in the activation of PMN.

METHODS

Fifteen candidate materials of an implantable glucose sensor were incubated in lepirudin-anticoagulated whole blood. The cluster of differentiation molecule 11b (CD11b) expression on PMN was analyzed with flow cytometry and the myeloperoxidase (MPO) concentration in plasma was analyzed with enzyme-linked immunosorbent assay. Complement activation was prevented by the C3 inhibitor compstatin or the C5 inhibitor eculizumab.

RESULTS

Three of the biomaterials (cellulose ester, polyamide reverse osmosis membrane, and polyamide thin film membrane), all belonging to the membrane group, induced a substantial and significant increase in CD11b expression and MPO release. The changes were virtually identical for these two markers. Inhibition of complement with compstatin or eculizumab reduced the CD11b expression and MPO release dose dependently and in most cases back to baseline. The other 12 materials did not induce significant PMN activation.

CONCLUSION

Three of the 15 candidate materials triggered PMN activation in a complement-dependent manner and should therefore be avoided for implementation in implantable microsensors.

Ornithodoros moubata complement inhibitor is an equally effective C5 inhibitor in pigs and humans

Experimental evidence suggests that C inhibition and more particularly combined inhibition of C and the TLR coreceptor CD14 may be of therapeutic benefit in sepsis and other inflammatory conditions. A barrier to the testing and further development of many inhibitors is that their activity is species specific. Pig is a relevant species for experimental models of human disease, and this study undertakes a comprehensive comparison of the inhibitory efficacy of the C5 inhibitor Ornithodoros moubata C inhibitor (OmCI) in human and porcine whole blood ex vivo models of Escherichia coli-induced sepsis. The effect of OmCI on complement activity in pigs undergoing E. coli sepsis was also examined. Porcine and human serum, and whole blood anticoagulated with lepirudin, was incubated with E. coli and the effect of OmCI investigated. The ex vivo results were virtually identical in pig and human. OmCI completely ablated the activity of all three C pathways at 0.64 μM. E. coli-induced C activation and expression of CD11b (wCD11R3 in the pig), was abolished ex vivo at 0.32 μM OmCI. Combining anti-CD14 and OmCI reduced the formation of IL-8 and TNF-α more potently than the single inhibitors. OmCI also efficiently bound E. coli-induced leukotriene B(4) in pig and human plasma. In support of our ex vivo findings, in vivo the activity of all C pathways was inhibited at 0.6 mg OmCI/kg pig. In conclusion, OmCI efficiently inhibited pig and human C activation, has accompanying anti-inflammatory effects and is a promising candidate inhibitor for further in vivo studies of sepsis.

Neisseria meningitidis and Escherichia coli are protected from leukocyte phagocytosis by binding to erythrocyte complement receptor 1 in human blood

The initial interaction of Gram-negative bacteria with erythrocytes and its implications on leukocyte phagocytosis and oxidative burst in human whole blood were examined. Alexa-labeled Escherichia coli, wild-type H44/76 N. meningitidis and the H44/76lpxA lipopolysaccharide (LPS)-deficient mutant were incubated with whole blood using lepirudin as anticoagulant which has no adverse effects on complement. Bacteria free in plasma, bound to erythrocytes or phagocytized by granulocytes and monocytes were quantified using flow cytometry. The effects of the C3 inhibitor compstatin, a C5a receptor antagonist (C5aRa) and a complement receptor 1 (CR1)-blocking antibody (3D9) were examined. Most bacteria (80%) immediately bound to erythrocytes. The binding gradually declined over time, with a parallel increase in phagocytosis. Complement inhibition with compstatin reduced erythrocyte binding and bacterial C3 opsonization. In contrast, the C5aRa efficiently reduced phagocytosis, but did not affect the binding of bacteria to erythrocytes. The anti-CR1 blocking mAb dose-dependently reduced bacterial binding to erythrocytes to nil, with subsequent increased phagocytosis and oxidative burst. LPS had no effect on these processes since similar results were obtained using an LPS-deficient N. meningitidis mutant. In vivo experiments in a pig model of sepsis showed limited binding of bacteria to erythrocytes, consistent with the facts that erythrocyte CR1 receptors are absent in non-primates and that the bacteria were mainly found in the lungs. In conclusion, complement-dependent binding of Gram-negative bacteria to erythrocyte CR1 decreases phagocytosis and oxidative burst by leukocytes in human whole blood.

Synthesis and activity of thioether-containing analogues of the complement inhibitor compstatin

Disulfide bonds are essential for the structural stability and biological activity of many bioactive peptides. However, these bonds are labile to reducing agents, which can limit the therapeutic utility of such peptides. Substitution of a disulfide bond with a reduction-resistant cystathionine bridge is an attractive means of improving stability while imposing minimal structural perturbation to the peptide. We have applied this approach to the therapeutic complement inhibitor compstatin, a disulfide-containing peptide currently in clinical trials for age-related macular degeneration, in an effort to maintain its potent activity while improving its biological stability. Thioether-containing compstatin analogues were produced via solid-phase peptide synthesis utilizing orthogonally protected cystathionine amino acid building blocks and solid-supported peptide cyclization. Overall, the affinity of these analogues for their biological target and potent inhibition of complement activation were largely maintained when compared to those of the parent disulfide-containing peptides. Thus, the improved stability to reduction conferred by the thioether bond makes this new class of compstatin peptides a promising alternative for therapeutic applications. Additionally, the versatility of this synthesis allows for exploration of disulfide-to-thioether substitution in a variety of other therapeutic peptides.

Mesenchymal stromal cells engage complement and complement receptor bearing innate effector cells to modulate immune responses

Infusion of human third-party mesenchymal stromal cells (MSCs) appears to be a promising therapy for acute graft-versus-host disease (aGvHD). To date, little is known about how MSCs interact with the body's innate immune system after clinical infusion. This study shows, that exposure of MSCs to blood type ABO-matched human blood activates the complement system, which triggers complement-mediated lymphoid and myeloid effector cell activation in blood. We found deposition of complement component C3-derived fragments iC3b and C3dg on MSCs and fluid-phase generation of the chemotactic anaphylatoxins C3a and C5a. MSCs bound low amounts of immunoglobulins and lacked expression of complement regulatory proteins MCP (CD46) and DAF (CD55), but were protected from complement lysis via expression of protectin (CD59). Cell-surface-opsonization and anaphylatoxin-formation triggered complement receptor 3 (CD11b/CD18)-mediated effector cell activation in blood. The complement-activating properties of individual MSCs were furthermore correlated with their potency to inhibit PBMC-proliferation in vitro, and both effector cell activation and the immunosuppressive effect could be blocked either by using complement inhibitor Compstatin or by depletion of CD14/CD11b-high myeloid effector cells from mixed lymphocyte reactions. Our study demonstrates for the first time a major role of the complement system in governing the immunomodulatory activity of MSCs and elucidates how complement activation mediates the interaction with other immune cells.

A new generation of potent complement inhibitors of the Compstatin family

Compstatin family peptides are potent inhibitors of the complement system and promising drug candidates against diseases involving under-regulated complement activation. Compstatin is a 13-residue cyclized peptide that inhibits cleavage of complement protein C3, preventing downstream complement activation. We present three new compstatin variants, characterized by tryptophan replacement at positions 1 and/or 13. Peptide design was based on physicochemical reasoning and was inspired by earlier work, which identified tryptophan substitutions at positions 1 and 13 in peptides with predicted C3c binding abilities [Bellows M.L., Fung H.K., Taylor M.S., Floudas C.A., López de Victoria A., Morikis D. (2010) Biophys J; 98: 2337-2346]. The new variants preserve distinct polar and nonpolar surfaces of compstatin, but have altered local interaction capabilities with C3. All three peptides exhibited potent C3 binding by surface plasmon resonance and potent complement inhibition by enzyme-linked immunosorbent assay. We also present enzyme-linked immunosorbent assay data and detailed surface plasmon resonance kinetic data of three peptides from previous computational design.

Artificial surface-induced inflammation relies on complement factor 5: proof from a deficient person

BACKGROUND

Exposing blood to artificial surfaces results in an inflammatory response, including complement activation and cytokine release. The aim of this investigation was to study complement-dependency and independency in artificial surface-induced inflammation in human whole blood from a patient with a genetic deficiency of complement factor 5 (C5).

METHODS

Whole blood from a C5-deficient patient, C5 protein reconstituted blood, and blood from a control subject was used. The complement inhibitor compstatin (C3 inhibitor) and a C5a receptor antagonist were used to block complement. Blood was circulated in closed loops of polyvinyl chloride tubing. Leukocyte CD11b expression and release of granule enzymes (myeloperoxidase, elastase, lactoferrin), cytokines (interleukins, chemokines, and growth factors; n = 27) as well as complement activation were measured after incubation.

RESULTS

In C5-deficient blood, there was no formation of the terminal complement complex, as opposed to reconstituted or control blood. Release of granule enzymes was partly dependent on C3, revealed by a compstatin-dependent effect in C5-deficient blood, and partly C5a-dependent as evident from the reconstitution and control blood. The chemokines interleukin-8 and monocyte chemoattractant protein-1 were also highly complement dependent, the effect being C5a-mediated, whereas platelet-derived and vascular endothelial growth factors were partly complement dependent. Interferon-γ increased in a complement-independent manner, whereas the rest of the cytokines did not respond to the surface. Leukocyte expression of CD11b was only marginally increased in deficient blood exposed to the surface, whereas reconstitution induced a considerable, C5a-dependent increase, comparable with that of the control.

CONCLUSIONS

The polyvinyl chloride surface induced a defined inflammatory response, which largely depended on C5.

Novel analogues of the therapeutic complement inhibitor compstatin with significantly improved affinity and potency

Compstatin is a 13-residue disulfide-bridged peptide that inhibits a key step in the activation of the human complement system. Compstatin and its derivatives have shown great promise for the treatment of many clinical disorders associated with unbalanced complement activity. To obtain more potent compstatin analogues, we have now performed an N-methylation scan of the peptide backbone and amino acid substitutions at position 13. One analogue (Ac-I[CVW(Me)QDW-Sar-AHRC](NMe)I-NH(2)) displayed a 1000-fold increase in both potency (IC(50) = 62 nM) and binding affinity for C3b (K(D) = 2.3 nM) over that of the original compstatin. Biophysical analysis using surface plasmon resonance and isothermal titration calorimetry suggests that the improved binding originates from more favorable free conformation and stronger hydrophobic interactions. This study provides a series of significantly improved drug leads for therapeutic applications in complement-related diseases, and offers new insights into the structure-activity relationships of compstatin analogues.

Selective inhibition of the lectin pathway of complement with phage display selected peptides against mannose-binding lectin-associated serine protease (MASP)-1 and -2: significant contribution of MASP-1 to lectin pathway activation

The complement system, an essential part of the innate immune system, can be activated through three distinct routes: the classical, the alternative, and the lectin pathways. The contribution of individual activation pathways to different biological processes can be assessed by using pathway-selective inhibitors. In this paper, we report lectin pathway-specific short peptide inhibitors developed by phage display against mannose-binding lectin-associated serine proteases (MASPs), MASP-1 and MASP-2. On the basis of the selected peptide sequences, two 14-mer peptides, designated as sunflower MASP inhibitor (SFMI)-1 and SFMI-2, were produced and characterized. SFMI-1 inhibits both MASP-1 and MASP-2 with a K(I) of 65 and 1030 nM, respectively, whereas SFMI-2 inhibits only MASP-2 with a K(I) of 180 nM. Both peptides block the lectin pathway activation completely while leaving the classical and the alternative routes intact and fully functional, demonstrating that of all complement proteases only MASP-1 and/or MASP-2 are inhibited by these peptides. In a C4 deposition inhibitor assay using preactivated MASP-2, SFMI-2 is 10-fold more effective than SFMI-1 in accordance with the fact that SFMI-2 is a more potent inhibitor of MASP-2. Surprisingly, however, out of the two peptides, SFMI-1 is much more effective in preventing C3 and C4 deposition when normal human serum containing zymogen MASPs is used. This suggests that MASP-1 has a crucial role in the initiation steps of lectin pathway activation most probably by activating MASP-2. Because the lectin pathway has been implicated in several life-threatening pathological states, these inhibitors should be considered as lead compounds toward developing lectin pathway blocking therapeutics.

Anti-inflammatory effects of C1-Inhibitor in porcine and human whole blood are independent of its protease inhibition activity

C1-Inhibitor (C1-INH) is an important biological inhibitor, regulating several protein cascade systems. Recent research has shown that the molecule exhibits properties not dependent on its protease inhibition activity. Serum and whole blood from pigs and humans were pre-incubated with C1-INH, iC1-INH or the complement inhibitors SPICE or compstatin. Whole, live Escherichia coli were then added for further incubation. Complement activation, a range of cytokines, chemokines and growth factors, as well as the leukocyte activation markers wCD11R3 (pig) and CD11b (human) were measured. Both C1-INH and iC1-INH dose-dependently and significantly (P<0.05) reduced a range of E. coli-induced pro-inflammatory cytokines and chemokines in porcine and human whole blood, as well as growth factors in human whole blood. Differences between the two forms of C1-INH and between the two species were modest. Most of these anti-inflammatory effects could not be explained by complement inhibition, as specific complement inhibitors had minor effect on several of the mediators. C1-Inhibitor had no inhibitory effect on E. coli-induced complement activation, while iC1-INH enhanced complement activation. The presented data indicate that C1-INH has broad anti-inflammatory effects in E. coli-induced inflammation in pig and human whole blood. These effects are largely independent of the protease inhibition activity.

Cell membrane modification for rapid display of bi-functional peptides: a novel approach to reduce complement activation

Ischemia and reperfusion of organs is an unavoidable consequence of transplantation. Inflammatory events associated with reperfusion injury are in part attributed to excessive complement activation. Systemic administration of complement inhibitors reduces reperfusion injury but leaves patients vulnerable to infection. Here, we report a novel therapeutic strategy that decorates cells with an anti-complement peptide. An analog of the C3 convertase inhibitor Compstatin (C) was synthesized with a hexahistidine (His(6)) tag to create C-His(6). To decorate cell membranes with C-His(6), fusogenic lipid vesicles (FLVs) were used to incorporate lipids with nickel (Ni(2+)) tethers into cell membranes, and these could then couple with C-His(6). Ni(2+) tether levels to display C-His(6) were modulated by changing FLV formulation, FLV incubation time and FLV levels. SKOV-3 cells decorated with C-His(6) effectively reduced complement deposition in a classical complement activation assay. We conclude that our therapeutic approach appears promising for local ex vivo treatment of transplanted organs to reduce complement-mediated reperfusion injury.

Complement inhibition decreases the procoagulant response and confers organ protection in a baboon model of Escherichia coli sepsis

Severe sepsis leads to massive activation of coagulation and complement cascades that could contribute to multiple organ failure and death. To investigate the role of the complement and its crosstalk with the hemostatic system in the pathophysiology and therapeutics of sepsis, we have used a potent inhibitor (compstatin) administered early or late after Escherichia coli challenge in a baboon model of sepsis-induced multiple organ failure. Compstatin infusion inhibited sepsis-induced blood and tissue biomarkers of complement activation, reduced leucopenia and thrombocytopenia, and lowered the accumulation of macrophages and platelets in organs. Compstatin decreased the coagulopathic response by down-regulating tissue factor and PAI-1, diminished global blood coagulation markers (fibrinogen, fibrin-degradation products, APTT), and preserved the endothelial anticoagulant properties. Compstatin treatment also improved cardiac function and the biochemical markers of kidney and liver damage. Histologic analysis of vital organs collected from animals euthanized after 24 hours showed decreased microvascular thrombosis, improved vascular barrier function, and less leukocyte infiltration and cell death, all consistent with attenuated organ injury. We conclude that complement-coagulation interplay contributes to the progression of severe sepsis and blocking the harmful effects of complement activation products, especially during the organ failure stage of severe sepsis is a potentially important therapeutic strategy.

Complement anaphylatoxin C5a contributes to hemodialysis-associated thrombosis

Thrombosis is a common complication of end-stage renal disease, particularly in patients on hemodialysis. Although substantial progress has been made in preventing thrombotic complications in various other groups of patients, the mechanisms of thrombosis during hemodialysis require clarification. In this report, we demonstrate that complement activation triggered by hemodialysis biomaterials, and the subsequent generation of the complement anaphylatoxin C5a, results in the expression of functionally active tissue factor (TF) in peripheral blood neutrophils. Because TF is a key initiator of coagulation in vivo, we postulate that the recurring complement activation that occurs during long-term hemodialysis contributes to thrombosis in dialyzed end-stage renal disease patients. Furthermore, we found that complement contributed to the induction of granulocyte colony-stimulating factor, which has been implicated in the pathogenesis of thrombosis in patients treated with the recombinant form of this molecule. Importantly, the inhibition of complement activation attenuated the TF expression and granulocyte colony-stimulating factor induction in blood passing through a hemodialysis circuit, suggesting that the complement system could become a new therapeutic target for preventing thrombosis in patients with chronic renal failure who are maintained on hemodialysis.

An ex vivo loop system models the toxicity and efficacy of PEGylated and unmodified adenovirus serotype 5 in whole human blood

Polyethylene glycol coating (PEGylation) of adenovirus serotype 5 (Ad5) has been shown to effectively reduce immunogenicity and increase circulation time of intravenously administered virus in mouse models. Herein, we monitored clot formation, complement activation, cytokine release and blood cell association upon addition of uncoated or PEGylated Ad5 to human whole blood. We used a novel blood loop model where human blood from healthy donors was mixed with virus and incubated in heparin-coated PVC tubing while rotating at 37 degrees C for up to 8 h. Production of the complement components C3a and C5a and the cytokines IL-8, RANTES and MCP-1 was significantly lower with 20K-PEGylated Ad5 than with uncoated Ad5. PEGylation prevented clotting and reduced Ad5 binding to blood cells in blood with low ability to neutralize Ad5. The effect was particularly pronounced in monocytes, granulocytes, B-cells and T-cells, but could also be observed in erythrocytes and platelets. In conclusion, PEGylation of Ad5 can reduce the immune response mounted in human blood, although the protective effects are rather modest in contrast to published mouse data. Our findings underline the importance of developing reliable models and we propose the use of human whole blood models in pre-clinical screening of gene therapy vectors.

Complement component C3 binds to activated normal platelets without preceding proteolytic activation and promotes binding to complement receptor 1

It has been reported that complement is activated on the surface of activated platelets, despite the presence of multiple regulators of complement activation. To reinvestigate the mechanisms by which activated platelets bind to complement components, the presence of complement proteins on the surfaces of nonactivated and thrombin receptor-activating peptide-activated platelets was analyzed by flow cytometry and Western blot analyses. C1q, C4, C3, and C9 were found to bind to thrombin receptor-activating peptide-activated platelets in lepirudin-anticoagulated platelet-rich plasma (PRP) and whole blood. However, inhibiting complement activation at the C1q or C3 level did not block the binding of C3 to activated platelets. Diluting PRP and chelating divalent cations also had no effect, further indicating that the deposition of complement components was independent of complement activation. Furthermore, washed, activated platelets bound added C1q and C3 to the same extent as platelets in PRP. The use of mAbs against different forms of C3 demonstrated that the bound C3 consisted of C3(H(2)O). Furthermore, exogenously added soluble complement receptor 1 was shown to bind to this form of platelet-bound C3. These observations indicate that there is no complement activation on the surface of platelets under physiological conditions. This situation is in direct contrast to a number of pathological conditions in which regulators of complement activation are lacking and thrombocytopenia and thrombotic disease are the ultimate result. However, the generation of C3(H(2)O) represents nonproteolytic activation of C3 and after factor I cleavage may act as a ligand for receptor binding.

Structure-kinetic relationship analysis of the therapeutic complement inhibitor compstatin

Compstatin is a 13-residue peptide that inhibits activation of the complement system by binding to the central component C3 and its fragments C3b and C3c. A combination of theoretical and experimental approaches has previously allowed us to develop analogs of the original compstatin peptide with up to 264-fold higher activity; one of these analogs is now in clinical trials for the treatment of age-related macular degeneration (AMD). Here we used functional assays, surface plasmon resonance (SPR), and isothermal titration calorimetry (ITC) to assess the effect of modifications at three key residues (Trp-4, Asp-6, Ala-9) on the affinity and activity of compstatin and its analogs, and we correlated our findings to the recently reported co-crystal structure of compstatin and C3c. The K(D) values for the panel of tested analogs ranged from 10(-6) to 10(-8) M. These differences in binding affinity could be attributed mainly to differences in dissociation rather than association rates, with a >4-fold range in k(on) values (2-10 x 10(5) M(-1) s(-1)) and a k(off) variation of >35-fold (1-37 x 10(-2) s(-1)) being observed. The stability of the C3b-compstatin complex seemed to be highly dependent on hydrophobic effects at position 4, and even small changes at position 6 resulted in a loss of complex formation. Induction of a beta-turn shift by an A9P modification resulted in a more favorable entropy but a loss of binding specificity and stability. The results obtained by the three methods utilized here were highly correlated with regard to the activity/affinity of the analogs. Thus, our analyses have identified essential structural features of compstatin and provided important information to support the development of analogs with improved efficacy.

Suppression of drusen formation by compstatin, a peptide inhibitor of complement C3 activation, on cynomolgus monkey with early-onset macular degeneration

For the past 10 years, number of evidence has shown that activation of complement cascade has been associated with age-related macular degeneration (AMD). The genome wide association study in American population with dominantly dry-type AMD has revealed strong association with single nucleotide polymorphism (SNP) of complement genes. Protein composition of drusen, a deposit observed in sub-retinal space between Bruch's membrane and retinal pigment epithelial (RPE), contains active complement molecules in human and monkey. These evidences have leaded us to consider the possibility of suppressing complement cascade in the retina to delay or reverse the onset of AMD. To test is hypothesis we used the C3 inhibitor Compstatin on primate model with early-onset macular degeneration which develop drusen in less than 2 years after birth. Our preliminary result showed drusen disappearance after 6 months of intravitreal injection.

Complement activation by CpG in a human whole blood loop system: mechanisms and immunomodulatory effects

Phosphorothioate oligodeoxynucleotides can activate complement, and experimental murine studies have revealed differential effects upon simultaneous TLR stimulation and complement activation compared with either event alone. We set out to investigate the immune stimulatory effects of CpG 2006 in fresh non-anticoagulated human blood with or without presence of active complement. We also sought to elucidate the mechanism behind complement activation upon stimulation with phosphorothioate CpG 2006. In a human blood loop system, both backbone and sequence-specific effects by CpG were counteracted by selective inhibition of C3. Furthermore, DNA backbone-mediated CD40 and CD83 expression on monocytes and sequence-specific IL-6 and TNF production were reduced by complement inhibition. CpG-induced complement activation occurred via either the classical or the alternative pathway and deposits of both IgM and properdin, two activators of complement, were detected on CpG after incubation with EDTA plasma. Quartz crystal microbalance with dissipation monitoring demonstrated alternative pathway convertase build-up onto CpG as a likely pathway to initiate and sustain complement activation. Specific inhibition of C3 suppressed CpG 2006 uptake into monocytes indicating that C3 fragments are involved in CpG internalization. The interplay between complement and TLR9 signaling demonstrated herein warrants further investigation.

Recent developments in low molecular weight complement inhibitors

As a key part of the innate immune system, complement plays an important role not only in defending against invading pathogens but also in many other biological processes. Inappropriate or excessive activation of complement has been linked to many autoimmune, inflammatory, and neurodegenerative diseases, as well as ischemia-reperfusion injury and cancer. A wide array of low molecular weight complement inhibitors has been developed to target various components of the complement cascade. Their efficacy has been demonstrated in numerous in vitro and in vivo experiments. Though none of these inhibitors has reached the market so far, some of them have entered clinical trials and displayed promising results. This review provides a brief overview of the currently developed low molecular weight complement inhibitors, including short peptides and synthetic small molecules, with an emphasis on those targeting components C1 and C3, and the anaphylatoxin receptors.

Dissecting the instant blood-mediated inflammatory reaction in islet xenotransplantation

BACKGROUND

A massive destruction of transplanted tissue occurs immediately following transplantation of pancreatic islets from pig to non-human primates. The detrimental instant blood-mediated inflammatory reaction (IBMIR), triggered by the porcine islets, is a likely explanation for this tissue loss. This reaction may also be responsible for mediating an adaptive immune response in the recipient that requires a heavy immunosuppressive regimen.

MATERIALS AND METHODS

Low molecular weight dextran sulfate (LMW-DS) and the complement inhibitor Compstatin were used in a combination of in vitro and in vivo studies designed to dissect the xenogeneic IBMIR in a non-human primate model of pancreatic islet transplantation. Adult porcine islets (10,000 IEQs/kg) were transplanted intraportally into three pairs of cynomolgus monkeys that had been treated with LMW-DS or heparin (control), and the effects on the IBMIR were characterized. Porcine islets were also incubated in human blood plasma in vitro to assess complement inhibition by LMW-DS and Compstatin.

RESULTS

Morphological scoring and immunohistochemical staining revealed that the severe islet destruction and macrophage, neutrophilic granulocyte, and T-cell infiltration observed in the control (heparin-treated) animals were abrogated in the LMW-DS-treated monkeys. Both coagulation and complement activation were significantly reduced in monkeys treated with LMW-DS, but IgM and complement fragments were still found on the islet surface. This residual complement activation could be inhibited by Compstatin in vitro.

CONCLUSIONS

The xenogeneic IBMIR in this non-human primate model is characterized by an immediate binding of antibodies that triggers deleterious complement activation and a subsequent clotting reaction that leads to further complement activation. The effectiveness of LMW-DS (in vivo and in vitro) and Compstatin (in vitro) in inhibiting this IBMIR provides the basis for a protocol that can be used to abrogate the IBMIR in pig-human clinical islet transplantation.

Development of a new pharmacophore model that discriminates active compstatin analogs

Compstatin and its active peptide analogs can potentially be used for therapeutic purposes because their binding to the third component of complement prohibits its conversion into the proteolytically activated form of the third component of complement, thus inhibiting complement cascades in all three complement pathways. Mallik and Morikis built three quasi-dynamic pharmacophore models for compstatin peptide analogs before, but only nine compstatin peptide analogs were incorporated in their study and the most active compstatin analog had only medium inhibitory activity. Since then, many more compstatin analogs have been synthesized and their inhibitory activities tested. Furthermore, the X-ray structure of AcCompNH2-V4W-H9A bound to the third component of complement has become available (PDB ID: 2QKI). In this paper, we utilized all the new information and built a new pharmacophore model using a distinct approach. Our model demonstrated good performance in a separate test set of 82 compstatin analogs: it accurately identified 70% of the analogs of medium or high inhibitory activities and misclassified only 8.5% of the analogs of low or no inhibitory activities. The results proved our pharmacophore model to be a filter of great sensitivity and specificity.

Compstatin: a complement inhibitor on its way to clinical application

Therapeutic modulation of the human complement system is considered a promising approach for treating a number of pathological conditions. Owing to its central position in the cascade, component C3 is a particularly attractive target for complement-specific drugs. Compstatin, a cyclic tridecapeptide, which was originally discovered from phage-display libraries, is a highly potent and selective C3 inhibitor that demonstrated clinical potential in a series of experimental models. A combination of chemical, biophysical, and computational approaches allowed a remarkable optimization of its binding affinity towards C3 and its inhibitory potency. With the recent announcement of clinical trials with a compstatin analog for the treatment of age-related macular degeneration, another important milestone has been reached on its way to a drug. Furthermore, the release of a co-crystal structure of compstatin with C3c allows a detailed insight into the binding mode and paves the way to the rational design of peptides and mimetics with improved activity. Considering the new incentives and the promising pre-clinical results, compstatin seems to be well equipped for the challenges on its way to a clinical therapeutic.

The artificial surface-induced whole blood inflammatory reaction revealed by increases in a series of chemokines and growth factors is largely complement dependent

Exposing blood to an artificial surface results in a systemic inflammatory response, including cytokine release and complement activation. We studied the artificial surface-induced inflammation in human whole blood using an extensive panel of inflammatory mediators including proinflammatory cytokines, chemokines and growth-factors and investigated the role of the complement system in the induction of this response. Using multiplex technology, 27 different inflammatory mediators were measured after circulating blood for 4 hours in polyvinyl chloride tubing. The C3 inhibitor compstatin was used to block complement activation. A significant (p < 0.05) increase in 14 of the 27 mediators was induced by the surface, of which 7 were chemokines (IL-8, MCP-1, MIP-1alpha, MIP-1beta, RANTES, eotaxin and IP-10) and 5 were growth-factors (G-CSF, GM-CSF, VEGF, PDGF and FGF). The traditional proinflammatory cytokines like IL-1beta, TNFalpha and IL-6 were not induced, although IL-6, as well as IL-15 and IL-17 increased if the surface was coated with highly bioincompatible laminaran. Inhibition of complement activation with compstatin significantly (p < 0.05) reduced the formation of 12 of the 14 mediators. For 10 of the 12 mediators, the inhibition was by 2/3 or more, for the remaining two the inhibition was more moderate. A highly biocompatible heparin-coated PVC surface was used as negative control and completely abolished the whole inflammatory response. The artificial surface PVC markedly induced a broad spectrum of chemokines and growth-factors, which was largely dependent on activation of complement.

Complement activation triggered by chondroitin sulfate released by thrombin receptor-activated platelets

BACKGROUND

Chondroitin sulfate (CS) is a glycosaminoglycan released by activated platelets.

OBJECTIVE

Here we test the hypothesis that CS released by activated platelets can trigger complement activation in the fluid phase.

METHODS AND RESULTS

Thrombin receptor-activating peptide (TRAP)-6 was used to activate platelets in platelet-rich plasma and blood, anticoagulated with the thrombin inhibitor lepirudin. TRAP activation induced fluid-phase complement activation, as reflected by the generation of C3a and sC5b-9, which could be attenuated by the C3 inhibitor compstatin. Chondroitinase ABC treatment of supernatants from activated platelets totally inhibited the activation, indicating that platelet-derived CS had initiated the complement activation. Furthermore, addition of purified CS to plasma strongly triggered complement activation. C1q was identified as the recognition molecule, as it bound directly to CS, and CS-triggered complement activation could be restored in C1q-depleted serum by adding purified C1q. TRAP activation of whole blood increased the expression of CD11b on leukocytes and generation of leukocyte-platelet complexes. It was demonstrated that these leukocyte functions were dependent on C3 activation and signaling via C5a, as this expression could be inhibited by compstatin and by a C5aR antagonist.

CONCLUSIONS

We conclude that platelets trigger complement activation in the fluid phase by releasing CS, which leads to inflammatory signals mediated by C5a.

Stages of meningococcal sepsis simulated in vitro, with emphasis on complement and Toll-like receptor activation

The clinical presentation of meningococcal disease is closely related to the number of meningococci in the circulation. This study aimed to examine the activation of the innate immune system after being exposed to increasing and clinically relevant concentrations of meningococci. We incubated representative Neisseria meningitidis serogroup B (ST-32) and serogroup C (ST-11) strains and a lipopolysaccharide (LPS)-deficient mutant (the 44/76 lpxA mutant) in human serum and whole blood and measured complement activation and cytokine secretion and the effect of blocking these systems. HEK293 cells transfected with Toll-like receptors (TLRs) were examined for activation of NF-kappaB. The threshold for cytokine secretion and activation of NF-kappaB was 10(3) to 10(4) meningococci/ml. LPS was the sole inflammation-inducing molecule at concentrations up to 10(5) to 10(6) meningococci/ml. The activation was dependent on TLR4-MD2-CD14. Complement contributed to the inflammatory response at >or=10(5) to 10(6) meningococci/ml, and complement activation increased exponentially at >or=10(7) bacteria/ml. Non-LPS components initiated TLR2-mediated activation at >or=10(7) bacteria/ml. As the bacterial concentration exceeded 10(7)/ml, TLR4 and TLR2 were increasingly activated, independent of CD14. In this model mimicking human disease, the inflammatory response to N. meningitidis was closely associated with the bacterial concentration. Therapeutically, CD14 inhibition alone was most efficient at a low bacterial concentration, whereas addition of a complement inhibitor may be beneficial when the bacterial load increases.

Mannose-binding lectin (MBL) facilitates opsonophagocytosis of yeasts but not of bacteria despite MBL binding

Mannose-binding lectin (MBL) is a serum protein of the innate immune system. After binding to a microorganism, MBL in complex with MBL-associated serine proteases activates the complement system, resulting in cleavage of complement factor C3. Cleaved C3 on the surface of the microorganism mediates opsonization for clearance, but the impact of MBL on subsequent phagocytosis has not been widely studied. We investigated the role of MBL in complement activation and phagocytosis of various bacteria and yeast species by flow cytometry. We measured both the C3 deposition during serum opsonization of fluorescent-labeled microorganisms as well as subsequent uptake of these microorganisms by human neutrophils. In MBL-deficient sera, a consistently decreased C3 deposition on both zymosan and Candida albicans was found and a reduced phagocytosis by neutrophils that was restored by exogenous MBL. This indicates that the lectin pathway of complement activation is important for the opsonophagocytosis of yeasts. In contrast, the C1q-dependent classical pathway dominated in the opsonization and phagocytosis of Staphylococcus aureus, Streptococcus pneumoniae, and Escherichia coli, whereas no effect of MBL was found. Both the lectin and the classical pathway of complement activation were highly amplified by the alternative route for opsonophagocytosis by neutrophils of yeast as well as microbial species. In summary, our data demonstrate that yeast species are preferentially opsonized and subsequently phagocytosed via activation of the lectin pathway of complement, whereas the uptake of bacterial strains was found to be largely MBL independent.

Cytokine secretion depends on Galalpha(1,3)Gal expression in a pig-to-human whole blood model

Transplants from alpha1,3-galactosyltransferase (Gal) gene-knockout pigs to nonhuman primates are largely protected from hyperacute but not acute humoral xenograft rejection. The present study investigates the role of Gal in cytokine responses using a novel pig-to-human whole blood in vitro model, developed for species-specific analysis of porcine and human cytokines. Porcine (n = 7) and human (n = 27) cytokines were measured using ELISA or multiplex technology, respectively. Porcine aortic endothelial cells from control (Gal(+/+)) and Gal-deficient (Gal(-/-)) pigs were incubated with human lepirudin anticoagulated whole blood from healthy donors. E-selectin expression was measured by flow cytometry. The C3 inhibitor compstatin and a C5aR antagonist were used to study the role of complement. Cytokine species specificity was documented, enabling detection of 2 of 7 porcine cytokines and 13 of 27 human cytokines in one single sample. Gal(+/+) porcine aortic endothelial cells incubated with human whole blood showed a marked complement C5b-9 dependent up-regulation of E-selectin and secretion of porcine IL-6 and IL-8. In contrast, Gal(-/-) cells responded with E-selectin and cytokine expression which was so weak that the role of complement could not be determined. Human IL-6, IL-8, IFN-gamma, MIP-1alpha, MIP-1beta, eotaxin, and RANTES were detected in the Gal(+/+) system, but virtually no responses were seen in the Gal(-/-) system (p = 0.03). The increase in human cytokine release was largely complement dependent and, in contrast to the porcine response, mediated through C5a. Species-specific analysis of cytokine release revealed a marked, complement-dependent response when Gal(+/+) pig cells were incubated with human whole blood, compared with Gal(-/-) cells which induced virtually no cytokine release.

Acute antibody-mediated complement activation mediates lysis of pancreatic islets cells and may cause tissue loss in clinical islet transplantation

BACKGROUND

Clinical islet transplantation is associated with loss of transplanted islets necessitating tissue from more than one donor to obtain insulin independence. The instant blood-mediated inflammatory reaction (IBMIR) is one explanation to the tissue loss. Complement activation is an important cytotoxic component of the IBMIR, and in the present study, we have investigated this component in detail.

METHODS

Isolated human islets were analyzed by large particle flow cytometry and confocal microscopy after incubation in human ABO-compatible hirudin-plasma.

RESULTS

After incubation in plasma, the islets bound IgG and IgM, CIq, C4, C3 and C9. The binding of C3b/iC3b was evident already after 5 min. The binding of C3b/iC3b and the generation of C3a and sC5b-9 were inhibited by the complement inhibitor Compstatin. Lysis as reflected by propidium iodide (PI) staining and release of C-peptide was also inhibited by Compstatin. There were significant correlations between IgM/IgG versus C3b/iC3b and between sC5b-9 and C-peptide.

CONCLUSION

The conclusion is that complement is activated by natural IgG and IgM antibodies already after 5 min. The complement activation leads to lysis of cells of the pancreatic islets. This very rapid reaction may be an essential entity of the damage induced by the IBMIR in clinical islet transplantation.

Dynamic structural changes during complement C3 activation analyzed by hydrogen/deuterium exchange mass spectrometry

Proteolytic cleavage of component C3 to C3b is a central step in the activation of complement. Whereas C3 is largely biologically inactive, C3b is directly involved in various complement activities. While the recently described crystal structures of C3 and C3b provide a molecular basis of complement activation, they do not reflect the dynamic changes that occur in solution. In addition, the available C3b structures diverge in some important aspects. Here we have utilized hydrogen/deuterium exchange coupled with mass spectrometry (HDX-MS) to investigate relative changes in the solution-phase structures of C3 and C3b. By combining two forms of mass spectrometry we could maximize the primary sequence coverage of C3b and demonstrate the feasibility of this method for large plasma proteins. While the majority of the 82 peptides that could be followed over time showed only minor alterations in HDX, we observed clear changes in solvent accessibility for 16 peptides, primarily in the alpha-chain (alpha'NT, MG6-8, CUB, TED, C345C domains). Most of these peptides could be directly linked to the structural transitions visible in the crystal structures and revealed additional information about the probability of the structural variants of C3b. In addition, a discontinuous cluster of seven peptides in the MG3, MG6, LNK and alpha'NT domains showed a decreased accessibility after activation to C3b. Although no gross conformational changes are detected in the crystal structure, this area may reflect a structurally flexible region in solution that contributes to C3 activation and function.

Complement-targeted therapeutics

The complement system is a central component of innate immunity and bridges the innate to the adaptive immune response. However, it can also turn its destructive capabilities against host cells and is involved in numerous diseases and pathological conditions. Modulation of the complement system has been recognized as a promising strategy in drug discovery, and a large number of therapeutic modalities have been developed. However, successful marketing of complement-targeted drugs has proved to be more difficult than initially expected, and many strategies have been discontinued. The US Food and Drug Administration's approval of the first complement-specific drug, an antibody against complement component C5 (eculizumab; Soliris), in March 2007, was a long-awaited breakthrough in the field. Approval of eculizumab validates the complement system as therapeutic target and might facilitate clinical development of other promising drug candidates.

CD59 efficiently protects human NT2-N neurons against complement-mediated damage

The complement regulatory protein CD59 controls cell survival by the inhibition of C5b-9 formation on the cell membrane. Loss of CD59 increases the susceptibility of cells to complement-mediated damage and lysis. Deposition of IgM can induce complement activation with subsequent cell death. We have previously demonstrated the presence of CD59 on human NT2-N neurons. In this study, we investigated the functional role of CD59 for NT2-N cell survival after IgM-mediated complement activation. Complement activation was induced on NT2-N neurons with human serum following incubation with the IgM monoclonal antibody A2B5 reacting with a neuronal cell membrane epitope. Deposition of C1q and C5b-9 was detected on the cell membrane and sC5b-9 in the culture supernatant. Specific inhibition of complement was obtained by the C3 inhibitor compstatin, and by anti-C5/C5a MoAb. CD59 was blocked by the MoAb BRIC 229. Membrane damage of propidium iodide-stained NT2-N cells was confirmed by immunofluorescence microscopy and degeneration of neuronal processes was shown with crystal violet staining. A2B5, but not the irrelevant control IgM antibody, induced complement activation on NT2-N neurons after incubation with a human serum, as detected by the deposition of C1q. A marked membrane deposition of C5b-9 on NT2-N neurons with accompanying cell death and axonal degeneration was found after the blocking of CD59 with MoAb BRIC 229 but not with an isotype-matched control antibody. Compstatin and anti-C5 monoclonal antibodies which blocked C5 activation efficiently inhibited complement activation. In conclusion, CD59 is essential for protecting human NT2-N neurons against complement-mediated damage, which is known to occur in a number of clinical conditions including stroke.

Characterization of Ehp, a secreted complement inhibitory protein from Staphylococcus aureus

We report here the discovery and characterization of Ehp, a new secreted Staphylococcus aureus protein that potently inhibits the alternative complement activation pathway. Ehp was identified through a genomic scan as an uncharacterized secreted protein from S. aureus, and immunoblotting of conditioned S. aureus culture medium revealed that the Ehp protein was secreted at the highest levels during log-phase bacterial growth. The mature Ehp polypeptide is composed of 80 residues and is 44% identical to the complement inhibitory domain of S. aureus Efb (extracellular fibrinogen-binding protein). We observed preferential binding by Ehp to native and hydrolyzed C3 relative to fully active C3b and found that Ehp formed a subnanomolar affinity complex with these various forms of C3 by binding to its thioester-containing C3d domain. Site-directed mutagenesis demonstrated that Arg(75) and Asn(82) are important in forming the Ehp.C3d complex, but loss of these side chains did not completely disrupt Ehp/C3d binding. This suggested the presence of a second C3d-binding site in Ehp, which was mapped to the proximity of Ehp Asn(63). Further molecular level details of the Ehp/C3d interaction were revealed by solving the 2.7-A crystal structure of an Ehp.C3d complex in which the low affinity site had been mutationally inactivated. Ehp potently inhibited C3b deposition onto sensitized surfaces by the alternative complement activation pathway. This inhibition was directly related to Ehp/C3d binding and was more potent than that seen for Efb-C. An altered conformation in Ehp-bound C3 was detected by monoclonal antibody C3-9, which is specific for a neoantigen exposed in activated forms of C3. Our results suggest that increased inhibitory potency of Ehp relative to Efb-C is derived from the second C3-binding site in this new protein.

Structure of compstatin in complex with complement component C3c reveals a new mechanism of complement inhibition

Undesired complement activation is a major cause of tissue injury in various pathological conditions and contributes to several immune complex diseases. Compstatin, a 13-residue peptide, is an effective inhibitor of the activation of complement component C3 and thus blocks a central and crucial step in the complement cascade. The precise binding site on C3, the structure in the bound form, and the exact mode of action of compstatin are unknown. Here we present the crystal structure of compstatin in complex with C3c, a major proteolytic fragment of C3. The structure reveals that the compstatin-binding site is formed by the macroglobulin (MG) domains 4 and 5. This binding site is part of the structurally stable MG-ring formed by domains MG 1-6 and is far away from any other known binding site on C3. Compstatin does not alter the conformation of C3c, whereas compstatin itself undergoes a large conformational change upon binding. We propose a model in which compstatin sterically hinders the access of the substrate C3 to the convertase complexes, thus blocking complement activation and amplification. These insights are instrumental for further development of compstatin as a potential therapeutic.

Conformational analysis of compstatin analogues with molecular dynamics simulations in explicit water

The cyclic 13-residue peptide compstatin is a potential therapeutic agent against the unregulated activation of the complement system. A thorough knowledge of its structural and dynamical properties in solution may assist the design of improved complement inhibitors. NMR studies have suggested that the 5-8 segment of free compstatin folds into a critical for activity 5-8 beta turn and the rest of the peptide is mainly disordered. Earlier computational studies of compstatin analogues with a polar-hydrogen/generalized-Born approximation reproduced the 5-8 turn, but also indicated the formation of beta-hairpin or alpha-helical elements and the existence of interactions between certain charged or aromatic sidechains. However, these features are absent or partly present in the NMR spectra, due to extensive conformational averaging. In order to check the compstatin properties with a more rigorous model of the intra- and intermolecular interactions, we conduct here 98-ns all-atom/explicit-water simulations of three compstatin analogues with variable activity; a native analogue, the more active mutant V4W/H9A and the inactive mutant Q5G. The 5-8 beta-turn population is in good accord with NMR. For the systems studied here, the simulations suggest that the 5-8 turn population does not correlate strictly with activity, in agreement with earlier mutational studies. Furthermore, they show structural differences among the analogues outside the 5-8 region. The possible role of these differences in activity is discussed. The probability of beta-hairpin or alpha-helix elements is much smaller with respect to the polar-hydrogen/GB simulations, and the persistent Trp4-Trp7 or Asp6-Arg11 sidechain interactions of the earlier GB studies are not reproduced. The present simulations extend the NMR data and improve our understanding of the properties of compstatin and related analogues.

A method for the covalent capture and screening of diverse small molecules in a microarray format

This protocol describes a robust method for the covalent capture of small molecules with diverse reactive functional groups in microarray format, and outlines a procedure for probing small-molecule microarrays (SMMs) with proteins of interest. A vapor-catalyzed, isocyanate-mediated surface immobilization scheme is used to attach bioactive small molecules, natural products and small molecules derived from diversity-oriented synthesis pathways. Additionally, an optimized methodology for screening SMMs with purified proteins and cellular lysates is described. Finally, a suggested model for data analysis that is compatible with commercially available software is provided. These procedures enable a platform capability for discovering novel interactions with potential applications to immunoglobulin profiling, comparative analysis of cellular states and ligand discovery. With the appropriate materials and experimental setup, the printing of SMMs can be completed in 14 hours over 3 days. Screening and data analysis requires 2 days. A detailed timeline is provided.

A simple, yet highly accurate, QSAR model captures the complement inhibitory activity of compstatin

Compstatin is a 13-residue cyclic peptide inhibitor of complement activation that was originally identified through phage-mediated presentation of a peptide library to C3b. Recent efforts to improve its activity have led to a rich dataset of complement analogs, with the most active analog being approximately 260 times more active than the parent compstatin. In the present work, a highly transparent quantitative structure-activity relationship model (Radj2=0.89) with four parameters is presented that captures important physico-chemical and geometrical properties of the analog molecules with regard to activity. The number of aromatic bonds and hydrophobicity of the fourth residue of compstatin correlated strongly with activity. Also important were the hydrophobic patch size near the disulfide bond and the solvent-accessible surface area occupied by nitrogen atoms of basic amino acid residues.

The role of complement in biomaterial-induced inflammation

Biomaterials are regularly used in various types of artificial tissues and organs, such as oxygenators, plasmapheresis equipment, hemodialysers, catheters, prostheses, stents, vascular grafts, miniature pumps, sensors and heart aids. Although progress has been made regarding bioincompatibility, many materials and procedures are associated with side effects, in particular bioincompatibility-induced inflammation, infections and subsequent loss of function. After cardiopulmonary bypass, coagulopathies can occur and lead to cognitive disturbances, stroke and extended hospitalization. Hemodialysis is associated with anaphylatoid reactions that cause whole-body inflammation and may contribute to accelerated arteriosclerosis. Stents cause restenosis and, in severe cases, thrombotic reactions. This situation indicates that there is still a need to try to understand the mechanisms involved in these incompatibility reactions in order to be able to improve the biomaterials and to develop treatments that attenuate the reactions and thereby reduce patients' discomfort, treatment time and cost. This overview deals with the role of complement in the incompatibility reactions that occur when biomaterials come in contact with blood and other body fluids.