Peptide inhibitor of complement C1 modulates acute intravascular hemolysis of mismatched red blood cells in rats

Humanization of a mouse anti-human complement C6 monoclonal antibody as a potential therapeutic for certain complement-mediated diseases

The assembly of tissue-damaging membrane attack complexes (MACs; C5b-9) is a major mechanism by which excessive complement activation causes diseases. We previously developed a mouse anti-human C6 monoclonal antibody (mAb) 1C9 that selectively inhibits the assembly of MACs in human and non-human primates. In this project, we found that 1C9 also cross-reacted with rat and guinea pig C6, and determined its binding domains on C6 using different truncated C6 proteins. We then humanized the anti-C6 mAb by molecular modeling and complementarity-determining region grafting. After screening a library of 276 humanized variants with different combinations of humanized light and heavy chains in biophysical assays, we identified clone 3713 with the best developability profile, and an increased affinity against C6 when compared with the parental 1C9 mAb. This humanized 3713 mAb inhibited human, monkey, and rat complement-mediated hemolysis in vitro, and more importantly, it significantly reduced complement-mediated hemolysis in vivo in rats. These results demonstrated the successful humanization of the anti-C6 mAb and suggested that the humanized 3713 mAb could be further developed as a new therapeutic that selectively targets MAC for certain complement-mediated pathological conditions.

Complement and platelets: prothrombotic cell activation requires membrane attack complex-induced release of danger signals

Complement activation in the diseases paroxysmal nocturnal hemoglobinuria (PNH) and atypical hemolytic uremic syndrome (aHUS) results in cytolysis and fatal thrombotic events, which are largely refractory to anticoagulation and/or antiplatelet therapy. Anticomplement therapy, however, efficiently prevents thrombotic events in PNH and aHUS, but the underlying mechanisms remain unresolved. We show that complement-mediated hemolysis in whole blood induces platelet activation similarly to activation by adenosine 5'-diphosphate (ADP). Blockage of C3 or C5 abolished platelet activation. We found that human platelets failed to respond functionally to the anaphylatoxins C3a and C5a. Instead, complement activation did lead to prothrombotic cell activation in the whole blood when membrane attack complex (MAC)-mediated cytolysis occurred. Consequently, we demonstrate that ADP receptor antagonists efficiently inhibited platelet activation, although full complement activation, which causes hemolysis, occurred. By using an established model of mismatched erythrocyte transfusions in rats, we crossvalidated these findings in vivo using the complement inhibitor OmCI and cobra venom factor. Consumptive complement activation in this animal model only led to a thrombotic phenotype when MAC-mediated cytolysis occurred. In conclusion, complement activation only induces substantial prothrombotic cell activation if terminal pathway activation culminates in MAC-mediated release of intracellular ADP. These results explain why anticomplement therapy efficiently prevents thromboembolisms without interfering negatively with hemostasis.

The EPICC Family of Anti-Inflammatory Peptides: Next Generation Peptides, Additional Mechanisms of Action, and In Vivo and Ex Vivo Efficacy

The EPICC peptides are a family of peptides that have been developed from the sequence of the capsid protein of human astrovirus type 1 and previously shown to inhibit the classical and lectin pathways of complement. The EPICC peptides have been further optimized to increase aqueous solubility and identify additional mechanisms of action. Our laboratory has developed the lead EPICC molecule, PA-dPEG24 (also known as RLS-0071), which is composed of a 15 amino acid peptide with a C-terminal monodisperse 24-mer PEGylated moiety. RLS-0071 has been demonstrated to possess other mechanisms of action in addition to complement blockade that include the inhibition of neutrophil-driven myeloperoxidase (MPO) activity, inhibition of neutrophil extracellular trap (NET) formation as well as intrinsic antioxidant activity mediated by vicinal cysteine residues contained within the peptide sequence. RLS-0071 has been tested in various ex vivo and in vivo systems and has shown promise for the treatment of both immune-mediated hematological diseases where alterations in the classical complement pathway plays an important pathogenic role as well as in models of tissue-based diseases such as acute lung injury and hypoxic ischemic encephalopathy driven by both complement and neutrophil-mediated pathways (i.e., MPO activity and NET formation). Next generation EPICC peptides containing a sarcosine residue substitution in various positions within the peptide sequence possess aqueous solubility in the absence of PEGylation and demonstrate enhanced complement and neutrophil inhibitory activity compared to RLS-0071. This review details the development of the EPICC peptides, elucidation of their dual-acting complement and neutrophil inhibitory activities and efficacy in ex vivo systems using human clinical specimens and in vivo efficacy in animal disease models.

Peptide inhibition of acute lung injury in a novel two-hit rat model

Acute lung injury (ALI) often causes severe trauma that may progress to significant morbidity and mortality. ALI results from a combination of the underlying clinical condition of the patient (e.g., inflammation) with a secondary insult such as viral pneumonia or a blood transfusion. While the secondary insult may be variable, the rapidly progressive disease process leading to pulmonary failure is typically mediated by an overwhelming innate immunological or inflammatory reaction driven by excessive complement and neutrophil-mediated inflammatory responses. We recently developed a ‘two-hit’ ALI rat model mediated by lipopolysaccharide followed by transfusion of incompatible human erythrocytes resulting in complement activation, neutrophil-mediated ALI and free DNA in the blood indicative of neutrophil extracellular trap formation. The objective of this study was to evaluate the role of peptide inhibitor of complement C1 (RLS-0071), a classical complement pathway inhibitor and neutrophil modulator in this animal model. Adolescent male Wistar rats were infused with lipopolysaccharide followed by transfusion of incompatible erythrocytes in the presence or absence of RLS-0071. Blood was collected at various time points to assess complement C5a levels, free DNA and cytokines in isolated plasma. Four hours following erythrocyte transfusion, lung tissue was recovered and assayed for ALI by histology. Compared to animals not receiving RLS-0071, lungs of animals treated with a single dose of RLS-0071 showed significant reduction in ALI as well as reduced levels of C5a, free DNA and inflammatory cytokines in the blood. These results demonstrate that RLS-0071 can modulate neutrophil-mediated ALI in this novel rat model.

Delayed Hemolytic Transfusion Reaction in a Patient With Sickle Cell Disease and the Role of the Classical Complement Pathway: A Case Report

A 14-year-old female patient with sickle cell disease developed a severe delayed hemolytic transfusion reaction (DHTR) leading to multiple transfusions and intensive care management. To better understand the extent to which the classical complement pathway was contributing to her DHTR, we utilized the complement hemolysis using human erythrocytes (CHUHE) assay and the classical complement pathway inhibitor, PIC1. Residual discarded de-identified plasma and erythrocytes from the patient obtained from routine phlebotomy was acquired. These reagents were used in the CHUHE assay in the presence of increasing concentrations of PIC1. Complement-mediated hemolysis of the patient's erythrocytes occurred in her plasma and complement permissive buffer. Increasing concentrations of PIC1 dose-dependently inhibited hemolysis to levels found for the negative control - complement inhibitor buffer. Complement-mediated hemolysis was demonstrated by the CHUHE assay for this patient with sickle cell disease and severe DHTR. PIC1 inhibition of hemolysis suggested that the classical complement pathway was contributing to her DHTR.

Classical complement activation on human erythrocytes in subjects with systemic lupus erythematosus and a history of autoimmune hemolytic anemia

INTRODUCTION

Autoimmune hemolytic anemia (AIHA) is a serious manifestation of systemic lupus erythematosus (SLE) associated with significant morbidity and mortality. In order to more fully understand the causative pathways, we utilized sera from subjects with SLE and active AIHA, or a history of AIHA, to evaluate the classical complement pathway, anti-erythrocyte antibodies, and immune complexes.

METHODS

To evaluate antibody-mediated complement activation on the surface of erythrocytes, as occurs in AIHA, blood type O erythrocytes were incubated with sera from 19 subjects with SLE and a history of AIHA. Circulating anti-erythrocyte antibodies and immune complexes were measured with ELISA-based assays.

RESULTS

In total, 90% of subjects with SLE and a history of AIHA, but not active clinical hemolysis, had measurable anti-erythrocyte antibodies. Of those with anti-erythrocyte antibody, 53% demonstrated complement opsonization on the erythrocyte surface >twofold above negative control and 29% generated the anaphylatoxin C5a.

CONCLUSIONS

For subjects with SLE and a history of AIHA, the persistence of circulating anti-erythrocyte antibodies and resultant erythrocyte complement opsonization and anaphylatoxin generation suggests the possibility that these complement effectors contribute to chronic morbidity and risk of AIHA relapse.

A complement-mediated rat xenotransfusion model of platelet refractoriness

BACKGROUND

Platelet refractoriness remains a challenging clinical dilemma although significant advancements have been made in identifying human leukocyte antigen (HLA) matched or HLA compatible units. Antiplatelet antibodies are the major risk factor for immune-mediated platelet refractoriness, yet the role of antibody-initiated complement-mediated platelet destruction remains poorly understood.

STUDY DESIGN AND METHODS

Human complement-mediated opsonization and killing of platelets was assayed ex vivo using antibody-sensitized human platelets incubated with complement-sufficient human sera. A new animal model of platelet refractoriness utilizing Wistar rats transfused with human platelets is described.

RESULTS

Human platelets sensitized with anti-platelet antibodies were rapidly opsonized with iC3b upon incubation in human sera. This opsonization could be completely blocked with a classical pathway complement inhibitor, PA-dPEG24. Complement activation decreased platelet viability, which was also reversible with complement inhibitor PA-dPEG24. A new rat model of platelet refractoriness was developed that demonstrated some platelet removal from the blood stream was complement mediated.

CONCLUSIONS

Complement activation initiated by anti-platelet antibodies leads to complement opsonization and decreased platelet viability. A new rat model of platelet refractoriness was developed that adds a new tool for elucidating the mechanisms of platelet refractoriness.

Incompatible erythrocyte transfusion with lipopolysaccharide induces acute lung injury in a novel rat model

Acute transfusion reactions can manifest in many forms including acute hemolytic transfusion reaction, allergic reaction and transfusion-related acute lung injury. We previously developed an acute hemolytic transfusion reaction rat model mediated by transfusion of incompatible human erythrocytes against which rats have preexisting antibodies resulting in classical complement pathway mediated intravascular hemolysis. In this study, the acute hemolytic transfusion reaction model was adapted to yield an acute lung injury phenotype. Adolescent male Wistar rats were primed in the presence or absence of lipopolysaccharide followed by transfusion of incompatible erythrocytes. Blood was collected at various time points during the course of the experiment to determine complement C5a levels and free DNA in isolated plasma. At 4 hours, blood and lung tissue were recovered and assayed for complete blood count and histological acute lung injury, respectively. Compared to sham animals or animals receiving increasing amounts of incompatible erythrocytes (equivalent to a 15–45% transfusion) in the absence of lipopolysaccharide, lungs of animals receiving lipopolysaccharide and a 30% erythrocyte transfusion showed dramatic alveolar wall thickening due to neutrophil infiltration. C5a levels were significantly elevated in these animals indicating that complement activation contributes to lung damage. Additionally, these animals demonstrated a significant increase of free DNA in the blood over time suggestive of neutrophil extracellular trap formation previously associated with transfusion-related acute lung injury in humans and mice. This novel ‘two-hit’ model utilizing incompatible erythrocyte transfusion in the presence of lipopolysaccharide yields a robust acute lung injury phenotype.

Extracellular DNA traps in inflammation, injury and healing

Following strong activation signals, several types of immune cells reportedly release chromatin and granular proteins into the extracellular space, forming DNA traps. This process is especially prominent in neutrophils but also occurs in other innate immune cells such as macrophages, eosinophils, basophils and mast cells. Initial reports demonstrated that extracellular traps belong to the bactericidal and anti-fungal armamentarium of leukocytes, but subsequent studies also linked trap formation to a variety of human diseases. These pathological roles of extracellular DNA traps are now the focus of intensive biomedical research. The type of pathology associated with the release of extracellular DNA traps is mainly determined by the site of trap formation and the way in which these traps are further processed. Targeting the formation of aberrant extracellular DNA traps or promoting their efficient clearance are attractive goals for future therapeutic interventions, but the manifold actions of extracellular DNA traps complicate these approaches.

Ceftriaxone-Induced Immune Hemolytic Anemia: In Vitro Reversal with Peptide Inhibitor of Complement C1 (PIC1)

We report a case of ceftriaxone-induced immune hemolytic anemia in a 10-year-old with chronic active Epstein-Barr virus disease and hemophagocytic lymphohistiocytosis. After chemotherapy, she became febrile and received ceftriaxone. She rapidly developed respiratory failure and anemia. Her direct antiglobulin test was positive for IgG and C3. To confirm this was ceftriaxone-induced complement-mediated hemolysis, we adapted the complement hemolysis using human erythrocytes (CHUHE) assay by adding exogenous ceftriaxone to the patient's serum which enhanced lysis of her erythrocytes. We confirmed that ceftriaxone initiated a classical complement pathway-mediated hemolysis by in vitro reversal with peptide inhibitor of complement C1 (PIC1).

Inhibition of Immune Complex Complement Activation and Neutrophil Extracellular Trap Formation by Peptide Inhibitor of Complement C1

Two major aspects of systemic lupus erythematosus (SLE) pathogenesis that have yet to be targeted therapeutically are immune complex-initiated complement activation and neutrophil extracellular trap (NET) formation by neutrophils. Here, we report in vitro testing of peptide inhibitor of complement C1 (PIC1) in assays of immune complex-mediated complement activation in human sera and assays for NET formation by human neutrophils. The lead PIC1 derivative, PA-dPEG24, was able to dose-dependently inhibit complement activation initiated by multiple types of immune complexes (IC), including C1-anti-C1q IC, limiting the generation of pro-inflammatory complement effectors, including C5a and membrane attack complex (sC5b-9). In several instances, PA-dPEG24 achieved complete inhibition with complement effector levels equivalent to background. PA-dPEG24 was also able to dose-dependently inhibit NET formation by human neutrophils stimulated by PMA, MPO, or immune complex activated human sera. In several instances PA-dPEG24 achieved complete inhibition with NETosis with quantitation equivalent to background levels. These results suggest that PA-dPEG24 inhibition of NETs occurs by blocking the MPO pathway of NET formation. Together these results demonstrate that PA-dPEG24 can inhibit immune complex activation of the complement system and NET formation. This provides proof of concept that peptides can potentially be developed to inhibit these two important contributors to rheumatologic pathology that are currently untargeted by available therapies.

Peptide Inhibitor of Complement C1 (PIC1) demonstrates antioxidant activity via single electron transport (SET) and hydrogen atom transfer (HAT)

Reactive oxygen species (ROS) are natural byproducts of oxidative respiration that are toxic to organs and tissues. To mitigate ROS damage, organisms have evolved a variety of antioxidant systems to counteract these harmful molecules, however in certain pathological conditions these protective mechanisms can be overwhelmed. We have recently demonstrated that Peptide Inhibitor of Complement C1 (PIC1) mitigates peroxidase activity of the heme bearing proteins myeloperoxidase, hemoglobin, and myoglobin through a reversible process. To determine if this property of PIC1 was antioxidant in nature, we tested PIC1 in a number of well-established antioxidant assays. PIC1 showed dose-dependent antioxidant activity in a total antioxidant (TAC) assay, hydroxyl radical antioxidant capacity (HORAC) assay, oxygen radical antioxidant capacity (ORAC) assay as well as the thiobarbituric acid reactive substances (TBARS) assay to screen for PIC1 antioxidant activity in human plasma. The antioxidant activity of PIC1 in the TAC assay, as well as the HORAC/ORAC assay demonstrated that this peptide acts via the single electron transport (SET) and hydrogen atom transfer (HAT) mechanisms, respectively. Consistent with this mechanism of action, PIC1 did not show activity in a metal chelating activity (MCA) assay. PIC1 contains two vicinal cysteine residues and displayed similar antioxidant activity to the well characterized cysteine-containing tripeptide antioxidant molecule glutathione (GSH). Consistent with the role of the cysteine residues in the antioxidant activity of PIC1, oxidation of these residues significantly abrogated antioxidant activity. These results demonstrate that in addition to its described complement inhibiting activity, PIC1 displays in vitro antioxidant activity.

Peptide Inhibitor of Complement C1 Inhibits the Peroxidase Activity of Hemoglobin and Myoglobin

Hemoglobin is the natural carrier of oxygen in red blood cells (RBCs). While intracellular hemoglobin provides life-sustaining oxygen transport, extracellular free hemoglobin displays toxicity due to inherent peroxidase activity generating reactive oxygen species that subsequently react with the hemoglobin molecule to produce toxic heme degradation products resulting in free radicals, oxidative stress damage, and lipid peroxidation. We have recently demonstrated that Peptide Inhibitor of Complement C1 (PIC1) inhibits peroxidase activity of the heme-based enzyme myeloperoxidase. To elucidate whether PIC1 could inhibit peroxidase activity of hemoglobin, we evaluated the consequence of PIC1 on RBC lysates, methemoglobin, and myoglobin using tetramethylbenzidine (TMB) as an oxidation target. PIC1 reversibly and dose-dependently prevented TMB oxidation to tetramethylbenzidine diimine by RBC lysates, methemoglobin, and myoglobin, having comparable activity to the inhibitor 4-aminobenzoic acid hydrazide. PIC1 inhibited TMB oxidation of RBC lysates similar to L-cysteine suggesting that the two cysteine residues contained in PIC1 may mediate peroxidase activity. PIC1 also inhibited heme destruction by NaOCl for RBC lysates, hemoglobin, and myoglobin as assayed by preservation of the Soret absorbance peak in the presence of NaOCl and reduction in free iron release. In conclusion, PIC1 inhibits peroxidase activity of hemoglobin and myoglobin likely via an antioxidant mechanism.

Inhibition of Myeloperoxidase Activity in Cystic Fibrosis Sputum by Peptide Inhibitor of Complement C1 (PIC1)

Myeloperoxidase is the major peroxidase enzyme in neutrophil granules and implicated in contributing to inflammatory lung damage in cystic fibrosis. Free myeloperoxidase is present in cystic fibrosis lung fluid and generates hypochlorous acid. Here we report a new inhibitor of myeloperoxidase activity, Peptide Inhibitor of Complement C1 (PIC1). Using TMB as the oxidizing substrate, PIC1 inhibited myeloperoxidase activity in cystic fibrosis sputum soluble fractions by an average of a 3.4-fold decrease (P = 0.02). PIC1 also dose-dependently inhibited myeloperoxidase activity in a neutrophil lysate or purified myeloperoxidase by up to 28-fold (P < 0.001). PIC1 inhibited myeloperoxidase activity similarly, on a molar basis, as the specific myeloperoxidase inhibitor 4-Aminobenzoic acid hydrazide (ABAH) for various oxidizing substrates. PIC1 was able to protect the heme ring of myeloperoxidase from destruction by NaOCl, assayed by spectral analysis. PIC1 incubated with oxidized TMB reversed the oxidation state of TMB, as measured by absorbance at 450 nm, with a 20-fold reduction in oxidized TMB (P = 0.02). This result was consistent with an antioxidant mechanism for PIC1. In summary, PIC1 inhibits the peroxidase activity of myeloperoxidase in CF sputum likely via an antioxidant mechanism.

Viral-derived complement inhibitors: current status and potential role in immunomodulation

The complement system is one of the body's major innate immune defense mechanisms in vertebrates. Its function is to detect foreign bodies and promote their elimination through opsonisation or lysis. Complement proteins play an important role in the immunopathogenesis of several disorders. However, excessive complement activation does not confer more protection but instead leads to several autoimmune and inflammatory diseases. With inappropriate activation of the complement system, activated complement proteins and glycoproteins may damage both healthy and diseased tissues. Development of complement inhibitors represents an effective approach in controlling dysregulated complement activity and reducing disease severity, yet few studies have investigated the nature and role of novel complement inhibitory proteins of viral origin. Viral complement inhibitors have important implications in understanding the importance of complement inhibition and their role as a promising novel therapeutic approach in diseases caused by dysregulated complement function. In this review, we discuss the role and importance of complement inhibitors derived from several viruses in the scope of human inflammatory and autoimmune diseases.