Factor H mediated cell surface protection from complement is critical for the survival of PNH erythrocytes

Evoked Weibel-Palade Body Exocytosis Modifies the Endothelial Cell Surface by Releasing a Substrate-Selective Phosphodiesterase

Weibel Palade bodies (WPB) are lysosome-related secretory organelles of endothelial cells. Commonly known for their main cargo, the platelet and leukocyte receptors von-Willebrand factor (VWF) and P-selectin, WPB play a crucial role in hemostasis and inflammation. Here, the authors identify the glycerophosphodiester phosphodiesterase domain-containing protein 5 (GDPD5) as a WPB cargo protein and show that GDPD5 is transported to WPB following uptake from the plasma membrane via an unique endocytic transport route. GDPD5 cleaves GPI-anchored, plasma membrane-resident proteins within their GPI-motif, thereby regulating their local activity. The authors identify a novel target of GDPD5 , the complement regulator CD59, and show that it is released from the endothelial surface by GDPD5 following WPB exocytosis. This results in increased deposition of complement components and can enhance local inflammatory and thrombogenic responses. Thus, stimulus-induced WPB exocytosis can modify the endothelial cell surface by GDPD5-mediated selective release of a subset of GPI-anchored proteins.

Mechanisms by which Factor H protects Trypanosoma cruzi from the alternative pathway of complement

Chagas disease, a chronic disabling disease caused by the protozoan Trypanosoma cruzi, has no standardized treatment or preventative vaccine. The infective trypomastigote form of T. cruzi is highly resistant to killing by the complement immune system. Factor H (FH), a negative regulator of the alternative pathway (AP) of complement on cell surfaces and in blood, contains 20 short consensus repeat domains. The four N-terminal domains of FH inactivate the AP, while the other domains interact with C3b/d and glycan markers on cell surfaces. Various pathogens bind FH to inactivate the AP. T. cruzi uses its trans-sialidase enzyme to transfer host sialic acids to its own surface, which could be one of the approaches it uses to bind FH. Previous studies have shown that FH binds to complement-opsonized T. cruzi and parasite desialylation increases complement-mediated lysis of trypomastigotes. However, the molecular basis of FH binding to T. cruzi remain unknown. Only trypomastigotes, but not epimastigotes (non-infective, complement susceptible) bound FH directly, independent of C3 deposition, in a dose-dependent manner. Domain mapping experiments using 3-5 FH domain fragments showed that domains 5-8 competitively inhibited FH binding to the trypomastigotes by ~35% but did not decrease survival in complement. FH-Fc or mutant FH-Fc fusion proteins (3-11 contiguous FH domains fused to the IgG Fc) also did not kill trypomastigotes. FH-related protein-5, whose domains bear significant sequence identity to all known polyanion-binding FH domains (6-7, 10-14, 19-20), fully inhibited FH binding to trypomastigotes and reduced trypomastigote survival to < 24% in the presence of serum. In conclusion, we have elucidated the role of FH in complement resistance of trypomastigotes.

SAR443809: a selective inhibitor of the complement alternative pathway, targeting complement factor Bb

Dysregulated activation of the complement system is implicated in the onset or progression of several diseases. Most clinical-stage complement inhibitors target the inactive complement proteins present at high concentrations in plasma, which increases target-mediated drug disposition and necessitates high drug levels to sustain therapeutic inhibition. Furthermore, many efforts are aimed at inhibiting only terminal pathway activity, which leaves opsonin-mediated effector functions intact. We describe the discovery of SAR443809, a specific inhibitor of the alternative pathway C3/C5 convertase (C3bBb). SAR443809 selectively binds to the activated form of factor B (factor Bb) and inhibits alternative pathway activity by blocking the cleavage of C3, leaving the initiation of classical and lectin complement pathways unaffected. Ex vivo experiments with patient-derived paroxysmal nocturnal hemoglobinuria erythrocytes show that, although terminal pathway inhibition via C5 blockade can effectively inhibit hemolysis, proximal complement inhibition with SAR443809 inhibits both hemolysis and C3b deposition, abrogating the propensity for extravascular hemolysis. Finally, intravenous and subcutaneous administration of the antibody in nonhuman primates demonstrated sustained inhibition of complement activity for several weeks after injection. Overall, SAR443809 shows strong potential for treatment of alternative pathway-mediated disorders.

With complements: C3 inhibition in the clinic

C3 is a key complement protein, located at the nexus of all complement activation pathways. Extracellular, tissue, cell-derived, and intracellular C3 plays critical roles in the immune response that is dysregulated in many diseases, making it an attractive therapeutic target. However, challenges such as very high concentration in blood, increased acute expression, and the elevated risk of infections have historically posed significant challenges in the development of C3-targeted therapeutics. This is further complicated because C3 activation fragments and their receptors trigger a complex network of downstream effects; therefore, a clear understanding of these is needed to provide context for a better understanding of the mechanism of action (MoA) of C3 inhibitors, such as pegcetacoplan. Because of C3's differential upstream position to C5 in the complement cascade, there are mechanistic differences between pegcetacoplan and eculizumab that determine their efficacy in patients with paroxysmal nocturnal hemoglobinuria. In this review, we compare the MoA of pegcetacoplan and eculizumab in paroxysmal nocturnal hemoglobinuria and discuss the complement-mediated disease that might be amenable to C3 inhibition. We further discuss the current state and outlook for C3-targeted therapeutics and provide our perspective on which diseases might be the next success stories in the C3 therapeutics journey.

Protein therapeutics and their lessons: Expect the unexpected when inhibiting the multi-protein cascade of the complement system

Over a century after the discovery of the complement system, the first complement therapeutic was approved for the treatment of paroxysmal nocturnal hemoglobinuria (PNH). It was a long-acting monoclonal antibody (aka 5G1-1, 5G1.1, h5G1.1, and now known as eculizumab) that targets C5, specifically preventing the generation of C5a, a potent anaphylatoxin, and C5b, the first step in the eventual formation of membrane attack complex. The enormous clinical and financial success of eculizumab across four diseases (PNH, atypical hemolytic uremic syndrome (aHUS), myasthenia gravis (MG), and anti-aquaporin-4 (AQP4) antibody-positive neuromyelitis optica spectrum disorder (NMOSD)) has fueled a surge in complement therapeutics, especially targeting diseases with an underlying complement pathophysiology for which anti-C5 therapy is ineffective. Intensive research has also uncovered challenges that arise from C5 blockade. For example, PNH patients can still face extravascular hemolysis or pharmacodynamic breakthrough of complement suppression during complement-amplifying conditions. These "side" effects of a stoichiometric inhibitor like eculizumab were unexpected and are incompatible with some of our accepted knowledge of the complement cascade. And they are not unique to C5 inhibition. Indeed, "exceptions" to the rules of complement biology abound and have led to unprecedented and surprising insights. In this review, we will describe initial, present and future aspects of protein inhibitors of the complement cascade, highlighting unexpected findings that are redefining some of the mechanistic foundations upon which the complement cascade is organized.

Initiation of the alternative pathway of complement and the history of "tickover"

The evolutionary history of complement suggests that the alternative pathway arose prior to the arrival of the classical and lectin pathways. In these pathways, target specificity is provided by antibodies and sugar specific lectins. While these efficient initiation systems dominate activation on most targets, the alternative pathway produces most of the C3b and 80%-90% of the C5b-9. While the tickover process, originally proposed by Peter Lachmann, provided ancient hosts with a crude self/non-self-discriminatory system that initiated complement attack on everything foreign, tickover clearly plays a more minor role in complement activation in modern organisms possessing classical and lectin pathways. Spontaneous activation of the alternative pathway via tickover may play a major role in human pathologies where tissue damage is complement-mediated. The molecular mechanism of tickover is still not convincingly proven. Prevailing hypotheses include (a) spontaneous hydrolysis of the thioester in C3 forming the C3b-like C3(H₂ O) in solution and (b) "enhanced tickover" in which surfaces cause specific or non-specific contact activated conformational changes in C3. Theoretical considerations, including computer simulations, suggest that the latter mechanism is more likely and that more research needs to be devoted to understanding interactions between biological surfaces and C3.

A Rare Case of Atypical Hemolytic Uremia Syndrome Triggered by Influenza Vaccination

Atypical hemolytic uremic syndrome (aHUS) occurs in patients with defective alternative complement pathways, making them susceptible to thrombotic microangiopathy (thrombocytopenia, intravascular hemolysis, and renal failure), and is usually triggered by infectious agents. Influenza and Streptococcus pneumonia are known triggers for aHUS. However, influenza vaccination triggering aHUS is rarely reported. We present a 30-year-old male who presented with chills, abdominal discomfort, and night sweats after receiving the influenza vaccine. The patient had thrombocytopenia, elevated creatinine, blood urea nitrogen, liver enzymes, and bilirubin with schistocytes with peripheral smear. ADAMTS13 activity was normal so the patient was diagnosed with aHUS. The patient improved with eculizumab and was ultimately found to have a mutation in CD46, which made him susceptible to aHUS. This case shows patients with dysregulated alternative complement pathways may be predisposed to develop aHUS after receiving influenza vaccination.

The role of properdin and Factor H in disease

The complement system consists of three pathways (alternative, classical, and lectin) that play a fundamental role in immunity and homeostasis. The multifunctional role of the complement system includes direct lysis of pathogens, tagging pathogens for phagocytosis, promotion of inflammatory responses to control infection, regulation of adaptive cellular immune responses, and removal of apoptotic/dead cells and immune complexes from circulation. A tight regulation of the complement system is essential to avoid unwanted complement-mediated damage to the host. This regulation is ensured by a set of proteins called complement regulatory proteins. Deficiencies or malfunction of these regulatory proteins may lead to pro-thrombotic hematological diseases, renal and ocular diseases, and autoimmune diseases, among others. This review focuses on the importance of two complement regulatory proteins of the alternative pathway, Factor H and properdin, and their role in human diseases with an emphasis on: (a) characterizing the main mechanism of action of Factor H and properdin in regulating the complement system and protecting the host from complement-mediated attack, (b) describing the dysregulation of the alternative pathway as a result of deficiencies, or mutations, in Factor H and properdin, (c) outlining the clinical findings, management and treatment of diseases associated with mutations and deficiencies in Factor H, and (d) defining the unwanted and inadequate functioning of properdin in disease, through a discussion of various experimental research findings utilizing in vitro, mouse and human models.

Halting targeted and collateral damage to red blood cells by the complement system

The complement system is an important defense mechanism against pathogens; however, in certain pathologies, the system also attacks human cells, such as red blood cells (RBCs). In paroxysmal nocturnal hemoglobinuria (PNH), RBCs lack certain complement regulators which sensitize them to complement-mediated lysis, while in autoimmune hemolytic anemia (AIHA), antibodies against RBCs may initiate complement-mediated hemolysis. In recent years, complement inhibition has improved treatment prospects for these patients, with eculizumab now the standard of care for PNH patients. Current complement inhibitors are however not sufficient for all patients, and they come with high costs, patient burden, and increased infection risk. This review gives an overview of the underlying pathophysiology of complement-mediated hemolysis in PNH and AIHA, the role of therapeutic complement inhibition nowadays, and the high number of complement inhibitors currently under investigation, as for almost every complement protein, an inhibitor is being developed. The focus lies with novel therapeutics that inhibit complement activity specifically in the pathway that causes pathology or those that reduce costs or patient burden through novel administration routes.

Development of an anti-human complement C6 monoclonal antibody that inhibits the assembly of membrane attack complexes

Membrane attack complexes (MACs; C5b-9) assembled after complement activation can directly injure self-tissues, leading to various diseases. Eculizumab, a monoclonal antibody (mAb) against complement component C5, is being used in the clinic to treat diseases in which MAC-mediated tissue damage is a primary cause. However, C5 is not a selective target for MAC assembly inhibition, and some patients respond incompletely or not at all to the eculizumab treatment. Therefore, C6, the next essential component in the terminal pathway of complement activation, may be an alternative target for the selective inhibition of MAC formation. Surprisingly, few reports describe a functional blockade of C6 using a specific mAb. Here, we report the development of an anti-human C6 mAb (clone 1C9) that recognizes C6 both in free circulation and within C5b6 complexes. This mAb blocked C7 binding to C5b6 complexes and consequently inhibited MAC formation and protected affected paroxysmal nocturnal hemoglobinuria patient red blood cells from MAC-mediated damage in vitro. In addition, this mAb cross-reacts with rhesus monkey but not mouse complement C6. Finally, 1C9 significantly reduced human complement-mediated intravascular hemolysis in vivo in a mouse model. These results suggest that the anti-C6 mAb holds promise as a new therapeutic agent that selectively targets MAC for many complement-mediated pathological conditions.

The dual role of complement in cancers, from destroying tumors to promoting tumor development

Complement is an important branch of innate immunity; however, its biological significance goes far beyond the scope of simple nonspecific defense and involves a variety of physiological functions, including the adaptive immune response. In this review, to unravel the complex relationship between complement and tumors, we reviewed the high diversity of complement components in cancer and the heterogeneity of their production and activation pathways. In the tumor microenvironment, complement plays a dual regulatory role in the occurrence and development of tumors, affecting the outcomes of the immune response. We explored the differential expression levels of various complement components in human cancers via the Oncomine database. The gene expression profiling interactive analysis (GEPIA) tool and Kaplan-Meier plotter (K-M plotter) confirmed the correlation between differentially expressed complement genes and tumor prognosis. The tumor immune estimation resource (TIMER) database was used to statistically analyze the effect of complement on tumor immune infiltration. Finally, with a view to the role of complement in regulating T cell metabolism, complement could be a potential target for immunotherapies. Targeting complement to regulate the antitumor immune response seems to have potential for future treatment strategies. However, there are still many complex problems, such as who will benefit from this therapy and how to select the right therapeutic target and determine the appropriate drug concentration. The solutions to these problems depend on a deeper understanding of complement generation, activation, and regulatory and control mechanisms.

Tuning the Functionality by Splicing: Factor H and Its Alternative Splice Variant FHL-1 Share a Gene but Not All Functions

The alternative pathway regulator Factor H-like protein 1 (FHL-1) is composed of the first 7 N-terminal complement control protein domains of Factor H (FH) and protects host surfaces from uncontrolled complement attack. Although FHL-1 shares the N-terminal regulatory domains with FH, it was thought to be a weaker regulator. Recently, the regulatory activity of FHL-1 was shown to be comparable to FH. Nonetheless, the question remained whether FHL-1 is an indispensable, unique regulator. The discovery that FHL-1 is the predominant regulator on Bruch’s membrane, a critical site for the onset and progression of age-related-macular degeneration (AMD), showed that FHL-1 is essential for complement regulation. A common single nucleotide polymorphism in FH/FHL-1 that predisposes for AMD underlines the important role of FHL-1 in this context. Reports that some cancer tissues specifically upregulate FHL-1 expression, thereby evading immune surveillance, suggests a pronounced regulatory activity of the splice variant. Several microorganisms specifically recruit FHL-1 to evade complement attack. From a phylogenetic point of view, FHL-1 appears much later than other complement regulators, which could imply a specific role that is possibly not systemic but rather tissue specific. This review focuses on the current knowledge of FHL-1 and its physiological and pathophysiological roles.

Modeling the activation of the alternative complement pathway and its effects on hemolysis in health and disease

The complement system is a powerful mechanism of innate immunity poised to eliminate foreign cells and pathogens. It is an intricate network of >35 proteins, which, once activated, leads to the tagging of the surface to be eliminated, produces potent chemoattractants to recruit immune cells, and inserts cytotoxic pores into nearby lipid surfaces. Although it can be triggered via different pathways, its net output is largely based on the direct or indirect activation of the alternative pathway. Complement dysregulation or deficiencies may cause severe pathologies, such as paroxysmal nocturnal hemoglobinuria (PNH), where a lack of complement control proteins leads to hemolysis and life-threatening anemia. The complexity of the system poses a challenge for the interpretation of experimental data and the design of effective pharmacological therapies. To address this issue, we developed a mathematical model of the alternative complement pathway building on previous modelling efforts. The model links complement activation to the hemolytic activity of the terminal alternative pathway, providing an accurate description of pathway activity as observed in vitro and in vivo, in health and disease. Through adjustment of the parameters describing experimental conditions, the model was capable of reproducing the results of an array of standard assays used in complement research. To demonstrate its clinical applicability, we compared model predictions with clinical observations of the recovery of hematological biomarkers in PNH patients treated with the complement inhibiting anti-C5 antibody eculizumab. In conclusion, the model can enhance the understanding of complement biology and its role in disease pathogenesis, help identifying promising targets for pharmacological intervention, and predict the outcome of complement-targeting pharmacological interventions.

The complement system as a part of the innate immunity builds a first line of defense of the body against foreign and altered host structures, such as pathogens and damaged cells. In contrast to the adaptive immune system, which takes days to weeks to mount a response, the complement system can react within seconds to minutes and therefore enables the body to quickly react to infections. Dysregulation of the system, however, has been identified as a major driver or significant contributor to several pathologies, ranging from the pathologic lysis of body’s own erythrocytes to the progressive loss of eyesight. The pathway’s highly diverse and interconnected architecture makes it difficult to identify promising points of therapeutic intervention and to predict clinical study outcomes. Building on previous modelling efforts, in this work we developed a mathematical model of the alternative pathway of the complement system that can explain experimental observations acquired over more than 50 years of complement research. These include results acquired in the laboratory as well as in patients. We envision that this work can be used to support the development of novel treatments for complement-mediated diseases at several key points such as target selection or design of clinical studies.

Reduced red blood cell surface level of Factor H as a mechanism underlying paroxysmal nocturnal hemoglobinuria

The absence of the cell-surface complement inhibitors CD55 and CD59 is considered the mechanism underlying the complement-mediated destruction of affected red blood cells (RBCs) in paroxysmal nocturnal hemoglobinuria (PNH) patients, but Factor H (FH), a fluid-phase complement inhibitor, has also been proposed to be involved. However, the status of FH on the PNH patient RBC surface is unclear and its precise role in PNH pathogenesis remains to be further defined. In this study, we identified significantly lower levels of surface-bound FH on the affected CD59^- RBCs than on the unaffected CD59⁺ RBCs. Although this reduction in surface-bound FH on PNH RBCs was accompanied by decreased surface sialic acid levels, the enzymatic removal of sialic acids from these RBCs did not significantly affect the levels of surface-bound FH. We further observed higher surface levels of FH on the C3b/iC3b/C3d^high RBCs than on C3b/iC3b/C3d^low RBCs within the affected PNH RBCs of patients treated with eculizumab. Finally, we determined that enhanced surface levels of FH on CD55/CD59-deficient RBCs from mice and PNH patients protected against complement-mediated hemolysis. Taken together, our results suggest that a reduced surface level of FH is another important mechanism underlying the pathogenesis of PNH.

Properdin Is a Key Player in Lysis of Red Blood Cells and Complement Activation on Endothelial Cells in Hemolytic Anemias Caused by Complement Dysregulation

The complement system alternative pathway (AP) can be activated excessively in inflammatory diseases, particularly when there is defective complement regulation. For instance, deficiency in complement regulators CD55 and CD59, leads to paroxysmal nocturnal hemoglobinuria (PNH), whereas Factor H mutations predispose to atypical hemolytic uremic syndrome (aHUS), both causing severe thrombohemolysis. Despite eculizumab being the treatment for these diseases, benefits vary considerably among patients. Understanding the molecular mechanisms involved in complement regulation is essential for developing new treatments. Properdin, the positive AP regulator, is essential for complement amplification by stabilizing enzymatic convertases. In this study, the role of properdin in red blood cell (RBC) lysis and endothelial cell opsonization in these AP-mediated diseases was addressed by developing in vitro assays using PNH patient RBCs and human primary endothelial cells, where the effects of inhibiting properdin, using novel monoclonal antibodies (MoAbs) that we generated and characterized, were compared to other complement inhibitors. In in vitro models of PNH, properdin inhibition prevented hemolysis of patient PNH type II and III RBCs more than inhibition of Factor B, C3, and C5 (>17-fold, or >81-fold, or >12-fold lower molar IC₉₀ values, respectively). When tested in an in vitro aHUS hemolysis model, the anti-properdin MoAbs had 11-fold, and 86-fold lower molar IC₉₀ values than inhibition of Factor B, or C3, respectively (P < 0.0001). When comparing target/inhibitor ratios in all hemolysis assays, inhibiting properdin was at least as efficient as the other complement inhibitors in most cases. In addition, using in vitro endothelial cell assays, the data indicate a critical novel role for properdin in promoting complement activation on human endothelial cells exposed to heme (a hemolysis by-product) and rH19-20 (to inhibit Factor H cell-surface protection), as occurs in aHUS. Inhibition of properdin or C3 in this system significantly reduced C3 fragment deposition by 75%. Altogether, the data indicate properdin is key in promoting RBC lysis and complement activation on human endothelial cells, contributing to the understanding of PNH and aHUS pathogenesis. Further studies to determine therapeutic values of inhibiting properdin in complement-mediated diseases, in particular those that are characterized by AP dysregulation, are warranted.

Coordinated roles for glycans in regulating the inhibitory function of CD22 on B cells

CD22 is an inhibitory B cell co-receptor that recognizes sialic acid-containing glycoconjugates as ligands. Interactions with its glycan ligands are key to regulating the ability of CD22 to modulate B cell function, the most widely explored of which is antagonizing B cell receptor (BCR) signaling. Most importantly, interactions of CD22 with ligands on the same cell (cis) control the organization of CD22 on the cell surface, which minimizes co-localization with the BCR. In contrast with the modest ability of CD22 to intrinsically dampen BCR signaling, glycan ligands presented on another cell (trans) along with an antigen drawn CD22 and the BCR together within an immunological synapse, strongly inhibiting BCR signaling. New concepts are emerging for how CD22 controls B cell function, such as changes in glycosylation at different stages of B cell differentiation, specifically on GC B cells. Related to these changes, new players, such galectin-9, have been discovered that regulate cell surface nanoclusters of CD22. Roles of glycan ligands in controlling CD22 are the primary focus of this review as we highlight the ability of CD22 to modulate B cell function.

Different Levels of Incomplete Terminal Pathway Inhibition by Eculizumab and the Clinical Response of PNH Patients

Background: Eculizumab blocks the lytic complement pathway by inhibiting C5 and has become the standard of care for certain complement-mediated diseases. Previously, we have shown that strong complement activation in vitro overrides the C5 inhibition by Eculizumab, which accounts for residual terminal pathway activity.

Results: Here we show that the levels of residual hemolysis in ex vivo assays differ markedly (up to 3.4-fold) across sera collected from different paroxysmal nocturnal hemoglobinuria (PNH) patients on Eculizumab treatment. This large variability of residual activity was also found in sera of healthy donors, thus cross-validating the findings in patients. While PNH patients with residual lytic activities of 11–30% exhibited hemolysis levels around the upper limit of normal (i.e., plasma LDH of ~250 u/L), as expected for PNH patients on Eculizumab therapy, we found sustained and markedly increased LDH levels of around 400 u/L for the patient with the highest residual activity of 37%. Furthermore, the clinical history of nine out of 14 PNH patients showed intravascular breakthrough hemolysis at the time of documented infections despite ample amounts of administered Eculizumab and/or experimentally determined excess over C5.

Conclusion: The occurrence of extraordinary high levels of residual terminal pathway activity in PNH patients receiving Eculizumab is rare, but can impair the suppression of hemolysis. The commonly observed low levels of residual terminal pathway activity seen for most PNH patients can exacerbate during severe infections and, thus, can cause pharmacodynamic breakthrough hemolysis in PNH patients treated with Eculizumab.

Highly Variable Sialylation Status of Donor-Specific Antibodies Does Not Impact Humoral Rejection Outcomes

Clinical outcome in antibody-mediated rejection (AMR) shows high inter-individual heterogeneity. Sialylation status of the Fc fragment of IgGs is variable, which could modulate their ability to bind to C1q and/or Fc receptors. In this translational study, we evaluated whether DSA sialylation influence AMR outcomes. Among 938 kidney transplant recipients for whom a graft biopsy was performed between 2004 and 2012 at Lyon University Hospitals, 69 fulfilled the diagnosis criteria for AMR and were enrolled. Sera banked at the time of the biopsy were screened for the presence of DSA by Luminex. The sialylation status of total IgG and DSA was quantified using Sambucus nigra agglutinin-based chromatography. All patients had similar levels of sialylation of serum IgGs (~2%). In contrast, the proportion of sialylated DSA were highly variable (median = 9%; range = 0–100%), allowing to distribute the patients in two groups: high DSA sialylation (n = 44; 64%) and low DSA sialylation (n = 25; 36%). The two groups differed neither on the intensity of rejection lesions (C4d, ptc, and g; p > 0.05) nor on graft survival rates (Log rank test, p = 0.99). in vitro models confirmed the lack of impact of Fc sialylation on the ability of a monoclonal antibody to trigger classical complement cascade and activate NK cells. We conclude that DSA sialylation status is highly variable but has not impact on DSA pathogenicity and AMR outcome.

Complement Component 3 Negatively Regulates Antibody Response by Modulation of Red Blood Cell Antigen

Red blood cell (RBC) alloimmunization can make it difficult to procure compatible RBCs for future transfusion, directly leading to increased morbidity and mortality in transfusion-dependent patients. However, the factors that regulate RBC alloimmunization remain incompletely understood. As complement has been shown to serve as a key adjuvant in the development of antibody (Ab) responses against microbes, we examined the impact of complement on RBC alloimmunization. In contrast to the impact of complement component 3 (C3) in the development of an immune response following microbial exposure, transfusion of C3 knockout (C3 KO) recipients with RBCs expressing KEL (KEL RBCs) actually resulted in an enhanced anti-KEL Ab response. The impact of C3 appeared to be specific to KEL, as transfusion of RBCs bearing another model antigen, the chimeric HOD antigen (hen egg lysozyme, ovalbumin and Duffy), into C3 KO recipients failed to result in a similar increase in Ab formation. KEL RBCs experienced enhanced C3 deposition and loss of detectable target antigen over time when compared to HOD RBCs, suggesting that C3 may inhibit Ab formation by impacting the accessibility of the target KEL antigen. Loss of detectable KEL on the RBC surface did not reflect antigen masking by C3, but instead appeared to result from actual removal of the KEL antigen, as western blot analysis demonstrated complete loss of detectable KEL protein. Consistent with this, exposure of wild-type B6 or C3 KO recipients to KEL RBCs with reduced levels of detectable KEL antigen resulted in a significantly reduced anti-KEL Ab response. These results suggest that C3 possesses a unique ability to actually suppress Ab formation following transfusion by reducing the availability of the target antigen on the RBC surface.

Mechanisms of Complement-Mediated Damage in Hematological Disorders

The complement cascade is an ancient defense system that destroys and eliminates threats to normal homeostasis in the bloodstream and tissues. Although multiple controls keep complement in check to minimize innocent bystander injury to normal cells and tissues, defects in complement regulation due to mutations in, or autoantibodies to, complement control proteins underlie the pathogenesis of several hemolytic diseases including paroxysmal nocturnal hemoglobinuria, and atypical hemolytic uremic syndrome. In autoimmune hemolytic anemias complement plays an important role in erythrocyte destruction mediated by antierythrocyte antibodies. The pathogenic mechanisms of these hemolytic diseases are discussed, with an emphasis on pivotal steps in complement activation.

Consequences of dysregulated complement regulators on red blood cells

The complement system represents the first line of defense that is involved in the clearance of pathogens, dying cells and immune complexes via opsonization, induction of an inflammatory response and the formation of a lytic pore. Red blood cells (RBCs) are very important for the delivery of oxygen to tissues and are continuously in contact with complement proteins in the blood plasma. To prevent complement activation on RBCs, various complement regulatory proteins can be found in plasma and on the cell membrane. RBCs are special cells without a nucleus and having a slightly different make-up of complement regulators than nucleated cells, as membrane cofactor protein (MCP) is not expressed and complement receptor 1 (CR1) is highly expressed. Decreased expression and/or function of complement regulatory proteins may result in unwanted complement activation and accelerated removal of RBCs. This review describes complement regulation on RBCs and the consequences when this regulation is out of balance.

Protection of host cells by complement regulators

The complement cascade is an ancient immune-surveillance system that not only provides protection from pathogen invasion but has also evolved to participate in physiological processes to maintain tissue homeostasis. The alternative pathway (AP) of complement activation is the evolutionarily oldest part of this innate immune cascade. It is unique in that it is continuously activated at a low level and arbitrarily probes foreign, modified-self, and also unaltered self-structures. This indiscriminate activation necessitates the presence of preformed regulators on autologous surfaces to spare self-cells from the undirected nature of AP activation. Although the other two canonical complement activation routes, the classical and lectin pathways, initiate the cascade more specifically through pattern recognition, their activity still needs to be tightly controlled to avoid excessive reactivity. It is the perpetual duty of complement regulators to protect the self from damage inflicted by inadequate complement activation. Here, we review the role of complement regulators as preformed mediators of defense, explain their common and specialized functions, and discuss selected cases in which alterations in complement regulators lead to disease. Finally, rational engineering approaches using natural complement inhibitors as potential therapeutics are highlighted.

Properdin: a tightly regulated critical inflammatory modulator

The complement alternative pathway is a powerful arm of the innate immune system that enhances diverse inflammatory responses in the human host. Key to the effects of the alternative pathway is properdin, a serum glycoprotein that can both initiate and positively regulate alternative pathway activity. Properdin is produced by many different leukocyte subsets and circulates as cyclic oligomers of monomeric subunits. While the formation of non‐physiological aggregates in purified properdin preparations and the presence of potential properdin inhibitors in serum have complicated studies of its function, properdin has, regardless, emerged as a key player in various inflammatory disease models. Here, we review basic properdin biology, emphasizing the major hurdles that have complicated the interpretation of results from properdin‐centered studies. In addition, we elaborate on an emerging role for properdin in thromboinflammation and discuss the potential utility of properdin inhibitors as long‐term therapeutic options to treat diseases marked by increased formation of platelet/granulocyte aggregates. Finally, we describe the interplay between properdin and the alternative pathway negative regulator, Factor H, and how aiming to understand these interactions can provide scientists with the most effective ways to manipulate alternative pathway activation in complex systems.

Eculizumab treatment: stochastic occurrence of C3 binding to individual PNH erythrocytes

BACKGROUND

C5 blockade by eculizumab prevents complement-mediated intravascular hemolysis in paroxysmal nocturnal hemoglobinuria (PNH). However, C3-bound PNH red blood cells (RBCs), arising in almost all treated patients, may undergo extravascular hemolysis reducing clinical benefits. Despite the uniform deficiency of CD55 and of CD59, there are always two distinct populations of PNH RBCs, with (C3+) and without (C3-) C3 binding.

METHODS

To investigate this paradox, the phenomenon has been modeled in vitro by incubating RBCs from eculizumab untreated PNH patients with compatible sera containing eculizumab, and by assessing the C3 binding after activation of complement alternative pathway.

RESULTS

When RBCs from untreated patients were exposed in vitro to activated complement in the context of C5-blockade, there was the prompt appearance of a distinct C3+ PNH RBC population whose size increased with time and also with the rate of complement activation. Eventually, all PNH RBCs become C3+ to the same extent, without differences between old and young (reticulocytes) PNH RBCs.

CONCLUSIONS

This study indicates that the distinct (C3+ and C3-) PNH RBC populations are not intrinsically different; rather, they result from a stochastic all-or-nothing phenomenon linked to the time-dependent cumulative probability of each individual PNH red cell to be exposed to levels of complement activation able to trigger C3 binding. These findings may envision novel approaches to reduce C3 opsonization and the subsequent extravascular hemolysis in PNH patients on eculizumab.

Paroxysmal nocturnal haemoglobinuria

Paroxysmal nocturnal haemoglobinuria (PNH) is a clonal haematopoietic stem cell (HSC) disease that presents with haemolytic anaemia, thrombosis and smooth muscle dystonias, as well as bone marrow failure in some cases. PNH is caused by somatic mutations in PIGA (which encodes phosphatidylinositol N-acetylglucosaminyltransferase subunit A) in one or more HSC clones. The gene product of PIGA is required for the biosynthesis of glycosylphosphatidylinositol (GPI) anchors; thus, PIGA mutations lead to a deficiency of GPI-anchored proteins, such as complement decay-accelerating factor (also known as CD55) and CD59 glycoprotein (CD59), which are both complement inhibitors. Clinical manifestations of PNH occur when a HSC clone carrying somatic PIGA mutations acquires a growth advantage and differentiates, generating mature blood cells that are deficient of GPI-anchored proteins. The loss of CD55 and CD59 renders PNH erythrocytes susceptible to intravascular haemolysis, which can lead to thrombosis and to much of the morbidity and mortality of PNH. The accumulation of anaphylatoxins (such as C5a) from complement activation might also have a role. The natural history of PNH is highly variable, ranging from quiescent to life-threatening. Therapeutic strategies include terminal complement blockade and bone marrow transplantation. Eculizumab, a monoclonal antibody complement inhibitor, is highly effective and the only licensed therapy for PNH.

HUS and atypical HUS

Hemolytic uremic syndrome (HUS) is a thrombotic microangiopathy characterized by intravascular hemolysis, thrombocytopenia, and acute kidney failure. HUS is usually categorized as typical, caused by Shiga toxin-producing Escherichia coli (STEC) infection, as atypical HUS (aHUS), usually caused by uncontrolled complement activation, or as secondary HUS with a coexisting disease. In recent years, a general understanding of the pathogenetic mechanisms driving HUS has increased. Typical HUS (ie, STEC-HUS) follows a gastrointestinal infection with STEC, whereas aHUS is associated primarily with mutations or autoantibodies leading to dysregulated complement activation. Among the 30% to 50% of patients with HUS who have no detectable complement defect, some have either impaired diacylglycerol kinase ε (DGKε) activity, cobalamin C deficiency, or plasminogen deficiency. Some have secondary HUS with a coexisting disease or trigger such as autoimmunity, transplantation, cancer, infection, certain cytotoxic drugs, or pregnancy. The common pathogenetic features in STEC-HUS, aHUS, and secondary HUS are simultaneous damage to endothelial cells, intravascular hemolysis, and activation of platelets leading to a procoagulative state, formation of microthrombi, and tissue damage. In this review, the differences and similarities in the pathogenesis of STEC-HUS, aHUS, and secondary HUS are discussed. Common for the pathogenesis seems to be the vicious cycle of complement activation, endothelial cell damage, platelet activation, and thrombosis. This process can be stopped by therapeutic complement inhibition in most patients with aHUS, but usually not those with a DGKε mutation, and some patients with STEC-HUS or secondary HUS. Therefore, understanding the pathogenesis of the different forms of HUS may prove helpful in clinical practice.

Incomplete inhibition by eculizumab: mechanistic evidence for residual C5 activity during strong complement activation

Eculizumab inhibits the terminal, lytic pathway of complement by blocking the activation of the complement protein C5 and shows remarkable clinical benefits in certain complement-mediated diseases. However, several reports suggest that activation of C5 is not always completely suppressed in patients even under excess of eculizumab over C5, indicating that residual C5 activity may derogate the drug's therapeutic benefit under certain conditions. By using eculizumab and the tick-derived C5 inhibitor coversin, we determined conditions ex vivo in which C5 inhibition is incomplete. The degree of such residual lytic activity depended on the strength of the complement activator and the resulting surface density of the complement activation product C3b, which autoamplifies via the alternative pathway (AP) amplification loop. We show that at high C3b densities required for binding and activation of C5, both inhibitors reduce but do not abolish this interaction. The decrease of C5 binding to C3b clusters in the presence of C5 inhibitors correlated with the levels of residual hemolysis. However, by employing different C5 inhibitors simultaneously, residual hemolytic activity could be abolished. The importance of AP-produced C3b clusters for C5 activation in the presence of eculizumab was corroborated by the finding that residual hemolysis after forceful activation of the classical pathway could be reduced by blocking the AP. By providing insights into C5 activation and inhibition, our study delivers the rationale for the clinically observed phenomenon of residual terminal pathway activity under eculizumab treatment with important implications for anti-C5 therapy in general.

Small-molecule factor D inhibitors targeting the alternative complement pathway

Complement is a key component of the innate immune system, recognizing pathogens and promoting their elimination. Complement component 3 (C3) is the central component of the system. Activation of C3 can be initiated by three distinct routes-the classical, the lectin and the alternative pathways-with the alternative pathway also acting as an amplification loop for the other two pathways. The protease factor D (FD) is essential for this amplification process, which, when dysregulated, predisposes individuals to diverse disorders including age-related macular degeneration and paroxysmal nocturnal hemoglobinuria (PNH). Here we describe the identification of potent and selective small-molecule inhibitors of FD. These inhibitors efficiently block alternative pathway (AP) activation and prevent both C3 deposition onto, and lysis of, PNH erythrocytes. Their oral administration inhibited lipopolysaccharide-induced AP activation in FD-humanized mice. These data demonstrate the feasibility of inhibiting the AP with small-molecule antagonists and support the development of FD inhibitors for the treatment of complement-mediated diseases.

Selectivity of C3-opsonin targeted complement inhibitors: A distinct advantage in the protection of erythrocytes from paroxysmal nocturnal hemoglobinuria patients

Paroxysmal nocturnal hemoglobinuria (PNH) is characterized by complement-mediated cell lysis due to deficiency of GPI-anchored complement regulators. Blockage of the lytic pathway by eculizumab is the only available therapy for PNH patients and shows remarkable benefits, but regularly yields PNH erythrocytes opsonized with fragments of complement protein C3, rendering such erythrocytes prone to extravascular hemolysis. This effect is associated with insufficient responsiveness seen in a subgroup of PNH patients. Novel C3-opsonin targeted complement inhibitors act earlier in the cascade, at the level of activated C3 and are engineered from parts of the natural complement regulator Factor H (FH) or complement receptor 2 (CR2). This inhibitor class comprises three variants of "miniFH" and the clinically developed "FH-CR2" fusion-protein (TT30). We show that the approach of FH-CR2 to target C3-opsonins was more efficient in preventing complement activation induced by foreign surfaces, whereas the miniFH variants were substantially more active in controlling complement on PNH erythrocytes. Subtle differences were noted in the ability of each version of miniFH to protect human PNH cells. Importantly, miniFH and FH-CR2 interfered only minimally with complement-mediated serum killing of bacteria when compared to untargeted inhibition of all complement pathways by eculizumab. Thus, the molecular design of each C3-opsonin targeted complement inhibitor determines its potency in respect to the nature of the activator/surface providing potential functionality in PNH.

Comparative Analysis of Novel Complement-Targeted Inhibitors, MiniFH, and the Natural Regulators Factor H and Factor H-like Protein 1 Reveal Functional Determinants of Complement Regulation

The serum proteins factor H (FH), consisting of 20 complement control protein modules (CCPs), and its splice product FH-like protein 1 (FHL-1; consisting of CCPs 1-7) are major regulators of the alternative pathway (AP) of complement activation. The engineered version of FH, miniFH, contains only the N- and C-terminal portions of FH linked by an optimized peptide and shows ∼ 10-fold higher ex vivo potency. We explored the hypothesis that regulatory potency is enhanced by unmasking of a ligand-binding site in the C-terminal CCPs 19-20 that is cryptic in full-length native FH. Therefore, we produced an FH variant lacking the central domains 10-15 (FHΔ10-15). To explore how avidity affects regulatory strength, we generated a duplicated version of miniFH, termed midiFH. We compared activities of FHΔ10-15 and midiFH to miniFH, FH, and FHL-1. Relative to FH, FHΔ10-15 exhibited an altered binding profile toward C3 activation products and a 5-fold-enhanced complement regulation on a paroxysmal nocturnal hemoglobinuria patient's erythrocytes. Contrary to dogma, FHL-1 and FH exhibited equal regulatory activity, suggesting that the role of FHL-1 in AP regulation has been underestimated. Unexpectedly, a substantially increased avidity for complement opsonins, as seen in midiFH, did not potentiate the inhibitory potential on host cells. In conclusion, comparisons of engineered and native FH-based regulators have identified features that determine high AP regulatory activity on host cells. Unrestricted availability of FH CCPs 19-20 and an optimal spatial orientation between the N- and C-terminal FH regions are key.

Long Standing Eculizumab Treatment without Anticoagulant Therapy in High-Risk Thrombogenic Paroxysmal Nocturnal Hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) is an ultra-orphan disease affecting all hematopoietic cell types. The abnormality of red blood cells in this disease predisposes to intravascular complement-mediated hemolysis. Eculizumab is an orphan drug used to treat this rare disease. Thrombosis is the key cause of death in PNH patients in about 40% to 67% of cases. We report the case of a woman presenting with PNH complicated with serious Budd-Chiari syndrome thrombosis and with a stent inserted in the portal vein. She refused to take any anticoagulant treatment since she commenced eculizumab 4 years before. No thrombotic events happened since that time. This case could add an extra benefit for eculizumab, which could be used as an anti-thromboembolic prophylactic agent in PNH, especially in patients with thrombocytopenia, where the use of anticoagulant agents is extremely hazardous. More randomized studies might establish the use of eculizumab without anticoagulants to avoid serious bleeding that could happen in thrombocytopenic PNH patients.

The role of complement in C3 glomerulopathy

C3 glomerulopathy describes a spectrum of disorders with glomerular pathology associated with C3 cleavage product deposition and with defective complement action and regulation (Fakhouri et al., 2010; Sethi et al., 2012b). Kidney biopsies from these patients show glomerular accumulation or deposition of C3 cleavage fragments, but no or minor deposition of immunoglobulins (Appel et al., 2005; D'Agati and Bomback, 2012; Servais et al., 2007; Sethi and Fervenza, 2011). At present the current situation asks for a better definition of the underlining disease mechanisms, for precise biomarkers, and for a treatment for this disease. The complement system is a self activating and propelling enzymatic cascade type system in which inactive, soluble plasma components are activated spontaneously and lead into an amplification loop (Zipfel and Skerka, 2009). Activation of the alternative pathway is spontaneous, occurs by default, and cascade progression leads to amplification by complement activators. The system however is self-controlled by multiple regulators and inhibitors, like Factor H that control cascade progression in fluid phase and on surfaces. The activated complement system generates a series of potent effector components and activation products, which damage foreign-, as well as modified self cells, recruit innate immune cells to the site of action, coordinate inflammation and the response of the adaptive immune system in form of B cells and T lymphocytes (Kohl, 2006; Medzhitov and Janeway, 2002; Ogden and Elkon, 2006; Carroll, 2004; Kemper and Atkinson, 2007; Morgan, 1999; Muller-Eberhard, 1986; Ricklin et al., 2010). Complement controls homeostasis and multiple reactions in the vertebrate organism including defense against microbial infections (Diaz-Guillen et al., 1999; Mastellos and Lambris, 2002; Nordahl et al., 2004; Ricklin et al., 2010). In consequence defective control of the spontaneous self amplifying cascade or regulation is associated with numerous human disorders (Ricklin and Lambris, 2007; Skerka and Zipfel, 2008; Zipfel et al., 2006). Understanding the exact action and regulation of this sophisticated homeotic cascade system is relevant to understand disease pathology of various complement associated human disorders. Furthermore this knowledge is relevant for a better diagnosis and appropriate therapy. At present diagnosis of C3 glomerulopathy is primarily based on the kidney biopsy, and histological, immmunohistological and electron microscopical evaluation (D'Agati and Bomback, 2012; Fakhouri et al., 2010; Medjeral-Thomas et al., 2014a,b; Sethi et al., 2012b). The challenge is to define the actual cause of the diverse glomerular changes or damages, to define how C3 deposition results in the reported glomerular changes, the location of the cell damage and the formation of deposits.

Paroxysmal nocturnal hemoglobinuria: a complement-mediated hemolytic anemia

Paroxysmal nocturnal hemoglobinuria is manifests with a chronic hemolytic anemia from uncontrolled complement activation, a propensity for thrombosis and marrow failure. The hemolysis is largely mediated by the alternative pathway of complement. Clinical manifestations result from the lack of specific cell surface proteins, CD55 and CD59, on PNH cells. Complement inhibition by eculizumab leads to dramatic clinical improvement. While this therapeutic approach is effective, there is residual complement activity resulting from specific clinical scenarios as well as from upstream complement components that can account for suboptimal responses in some patients. Complement inhibition strategies are an area of active research.

Assessing complement blockade in patients with paroxysmal nocturnal hemoglobinuria receiving eculizumab

CH50 activity reflects C5 blockade in PNH patients treated with eculizumab and is directly related to circulating free eculizumab levels. Both CH50 and free eculizumab level markers look promising for the monitoring of complement blockade in patients with PNH receiving eculizumab.

A novel method for direct measurement of complement convertases activity in human serum

Complement convertases are enzymatic complexes that play a central role in sustaining and amplification of the complement cascade. Impairment of complement function leads directly or indirectly to pathological conditions, including higher infection rate, kidney diseases, autoimmune- or neurodegenerative diseases and ischaemia-reperfusion injury. An assay for direct measurement of activity of the convertases in patient sera is not available. Existing assays testing convertase function are based on purified complement components and, thus, convertase formation occurs under non-physiological conditions. We designed a new assay, in which C5 blocking compounds enabled separation of the complement cascade into two phases: the first ending at the stage of C5 convertases and the second ending with membrane attack complex formation. The use of rabbit erythrocytes or antibody-sensitized sheep erythrocytes as the platforms for convertase formation enabled easy readout based on measurement of haemolysis. Thus, properties of patient sera could be studied directly regarding convertase activity and membrane attack complex formation. Another advantage of this assay was the possibility to screen for host factors such as C3 nephritic factor and other anti-complement autoantibodies, or gain-of-function mutations, which prolong the half-life of complement convertases. Herein, we present proof of concept, detailed description and validation of this novel assay.

Structural basis for sialic acid-mediated self-recognition by complement factor H

The serum protein complement factor H (FH) ensures downregulation of the complement alternative pathway, a branch of innate immunity, upon interaction with specific glycans on host cell surfaces. Using ligand-based NMR, we screened a comprehensive set of sialylated glycans for binding to FH and solved the crystal structure of a ternary complex formed by the two C-terminal domains of FH, a sialylated trisaccharide and the complement C3b thioester-containing domain. Key residues in the sialic acid binding site are conserved from mice to men, and residues linked to atypical hemolytic uremic syndrome cluster within this binding site, suggesting a possible role for sialic acid as a host marker also in other mammals and a critical role in human renal complement homeostasis. Unexpectedly, the FH sialic acid binding site is structurally homologous to the binding sites of two evolutionarily unrelated proteins. The crystal structure also advances our understanding of bacterial immune evasion strategies.

Complement in paroxysmal nocturnal hemoglobinuria: exploiting our current knowledge to improve the treatment landscape

Paroxysmal nocturnal hemoglobinuria (PNH) is a rare hematological disorder associated with an acquired deficiency in glycophosphatidylinositol-anchor biosynthesis that renders erythrocytes susceptible to complement attack. Intravascular hemolysis via the membrane attack complex is a clinical hallmark of the disease, and C5 blockade is currently the only approved treatment for PNH. However, residual anemia is an emerging observation for many PNH patients receiving anti-C5 treatment. A range of complement-targeted therapeutic approaches, encompassing surface-directed inhibition of C3 convertases, blockade of membrane attack complex assembly or C3 interception using peptidic inhibitors, has yielded promising results and offers leverage for even more effective treatment of PNH. This article discusses recent advances in this rapidly evolving field, integrating critical perspectives from preclinical PNH models and diverse complement modulation strategies with genetic insights and therapy response profiles. It also evaluates the relative efficacy, limitations and benefits afforded by C3 or C5 inhibition in the context of PNH therapeutics.

Peptide inhibitors of C3 activation as a novel strategy of complement inhibition for the treatment of paroxysmal nocturnal hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) is characterized by complement-mediated intravascular hemolysis due to the lack of CD55 and CD59 on affected erythrocytes. The anti-C5 antibody eculizumab has proven clinically effective, but uncontrolled C3 activation due to CD55 absence may result in opsonization of erythrocytes, possibly leading to clinically meaningful extravascular hemolysis. We investigated the effect of the peptidic C3 inhibitor, compstatin Cp40, and its long-acting form (polyethylene glycol [PEG]-Cp40) on hemolysis and opsonization of PNH erythrocytes in an established in vitro system. Both compounds demonstrated dose-dependent inhibition of hemolysis with IC50 ∼4 µM and full inhibition at 6 µM. Protective levels of either Cp40 or PEG-Cp40 also efficiently prevented deposition of C3 fragments on PNH erythrocytes. We further explored the potential of both inhibitors for systemic administration and performed pharmacokinetic evaluation in nonhuman primates. A single intravenous injection of PEG-Cp40 resulted in a prolonged elimination half-life of >5 days but may potentially affect the plasma levels of C3. Despite faster elimination kinetics, saturating inhibitor concentration could be reached with unmodified Cp40 through repetitive subcutaneous administration. In conclusion, peptide inhibitors of C3 activation effectively prevent hemolysis and C3 opsonization of PNH erythrocytes, and are excellent, and potentially cost-effective, candidates for further clinical investigation.

Anti-Complement Treatment in Paroxysmal Nocturnal Hemoglobinuria: Where we Stand and Where we are Going

Paroxysmal nocturnal hemoglobinuria (PNH) is a clonal, non-malignant, hematological disorder characterized by the expansion of hematopoietic stem cells and progeny mature blood cells which are deficient in some surface proteins, including the two complement regulators CD55 and CD59. PNH is the paradigm of diseases implying complement dysregulation as main pathogenic mechanism; in fact, PNH erythrocytes are uncapable to modulate on their surface physiologic complement activation, which eventually leads to the typical clinical hallmark of PNH - the chronic complement-mediated intravascular anemia. Indeed, due to the lack of CD55 complement is continuously activated on erythrocyte surface, which subsequently enables the terminal lytic complement because of the lack of CD59, finally resulting in erythrocyte lysis. The availability of eculizumab as the first complement inhibitor for clinical use renewed the interest for this rare hematological disease. Indeed, in the last decad the anti-C5 monoclonal antibody has proven effective for the treatment of PNH, resulting in a sustained control of complement-mediated intravascular hemolysis, with a remarkable clinical benefit. Anti-complement treatment allowed transfusion independence in at least half of PNH patients receiving eculizumab, with adequate control of all hemolysis-associated symptoms even in almost all remaining patients. In addition, the risk of thromboembolic events - an other clinical hallmark of PNH, which significantly affects prognosis and survival - seems substantially reduced on eculizumab treatment, apparently resulting in improved survival. Even with all these remarkable effects, eculizumab treatment does not result in hemoglobin normalization, and most patients remain anemic. It has been demonstrated that this is due to persistent activation of the early phases of complement activation (upstream the C5), leading to complement-mediated extravascular hemolysis. Ongoing researches are focusing on possible strategies to improve current anti-complement therapies, aiming to develop second-generation complement therapeutics. Here we review PNH and its complement-mediated pathophysiology, summarizing available data on anti-complement treatment; we'll also discuss recent pathogenic insights which drive the development of novel strategies of complement inhibition.

Antigen modulation confers protection to red blood cells from antibody through Fcγ receptor ligation

Autoantibodies and alloantibodies can damage self-tissue or transplanted tissues through either fixation of complement or ligation of FcγRs. Several pathways have been described that imbue self-tissues with resistance to damage from complement fixation, as a protective measure against damage from these Abs. However, it has been unclear whether parallel pathways exist to provide protection from FcγR ligation by bound Abs. In this article, we describe a novel pathway by which cell surface Ag is specifically decreased as a result of Ab binding (Ag modulation) to the extent of conferring protection to recognized cells from Fcγ-dependent clearance. Moreover, the Ag modulation in this system requires FcγR ligation. Together, these findings provide unique evidence of self-protective pathways for FcγR-mediated Ab damage.

Complement factor H-derived short consensus repeat 18-20 enhanced complement-dependent cytotoxicity of ofatumumab on chronic lymphocytic leukemia cells

The antitumor activity of monoclonal antibodies in the treatment of chronic lymphocytic leukemia is mediated mainly by antibody-dependent cellular cytotoxicity and complement-dependent cytotoxicity. Unfortunately, the efficacy of complement-dependent cytotoxicity is strongly restricted due to the expression and acquisition of regulators of complement activation by lymphocytic leukemia cells. Whereas the role of membrane regulators of complement activation, such as CD55 and CD59, has been investigated in detail in chronic lymphocytic leukemia, the involvement of soluble regulators of complement activation, such as complement factor H, has not yet been reported. Propidium iodide staining was performed to investigate the efficacy of ofatumumab and factor H-derived short-consensus-repeat 18-20 in the induction of complement-dependent cytotoxicity on primary chronic lymphocytic leukemia cells from 20 patients. Deposition of complement C3 fragments was monitored by western blot analysis. Expression of CD20, CD55 or CD59 was determined by FACS analysis. Replacement of factor H with short consensus repeat 18-20 significantly increased the susceptibility of primary chronic lymphocytic leukemia cells to ofatumumab-induced complement-dependent cytotoxicity. More importantly, addition of short-consensus-repeat 18-20 was able to overcome complement- resistance occurring during treatment with ofatumumab alone. Use of short consensus repeat 18-20 is likely to prolong the turnover time of active C3b fragments generated on the target cells following ofatumumab-induced complement activation, thereby improving specific killing of chronic lymphocytic leukemia cells by complement-dependent cytotoxicity. The relative contribution of factor H to the protection of chronic lymphocytic leukemia cells against complement-dependent cytotoxicity was comparable to that of CD55. Our data suggest that, by abrogating factor H function, short consensus repeat 18-20 may provide a novel approach that improves the complement-dependent efficacy of therapeutic monoclonal antibodies.

Identification of a novel mode of complement activation on stimulated platelets mediated by properdin and C3(H2O)

Elevated numbers of activated platelets circulate in patients with chronic inflammatory diseases, including atherosclerosis and coronary disease. Activated platelets can activate the complement system. Although complement activation is essential for immune responses and removal of spent cells from circulation, it also contributes to inflammation and thrombosis, especially in patients with defective complement regulation. Proinflammatory activated leukocytes, which interact directly with platelets in response to vascular injury, are among the main sources of properdin, a positive regulator of the alternative pathway. The role of properdin in complement activation on stimulated platelets is unknown. Our data show that physiological forms of human properdin bind directly to human platelets after activation by strong agonists in the absence of C3, and bind nonproportionally to surface CD62P expression. Activation of the alternative pathway on activated platelets occurs when properdin is on the surface and recruits C3b or C3(H2O) to form C3b,Bb or a novel cell-bound C3 convertase [C3(H2O),Bb], which normally is present only in the fluid phase. Alternatively, properdin can be recruited by C3(H2O) on the platelet surface, promoting complement activation. Inhibition of factor H-mediated cell surface complement regulation significantly increases complement deposition on activated platelets with surface properdin. Finally, properdin released by activated neutrophils binds to activated platelets. Altogether, these data suggest novel molecular mechanisms for alternative pathway activation on stimulated platelets that may contribute to localization of inflammation at sites of vascular injury and thrombosis.

An engineered construct combining complement regulatory and surface-recognition domains represents a minimal-size functional factor H

Complement is an essential humoral component of innate immunity; however, its inappropriate activation leads to pathology. Polymorphisms, mutations, and autoantibodies affecting factor H (FH), a major regulator of the alternative complement pathway, are associated with various diseases, including age-related macular degeneration, atypical hemolytic uremic syndrome, and C3 glomerulopathies. Restoring FH function could be a treatment option for such pathologies. In this article, we report on an engineered FH construct that directly combines the two major functional regions of FH: the N-terminal complement regulatory domains and the C-terminal surface-recognition domains. This minimal-size FH (mini-FH) binds C3b and has complement regulatory functions similar to those of the full-length protein. In addition, we demonstrate that mini-FH binds to the FH ligands C-reactive protein, pentraxin 3, and malondialdehyde epitopes. Mini-FH was functionally active when bound to the extracellular matrix and endothelial cells in vitro, and it inhibited C3 deposition on the cells. Furthermore, mini-FH efficiently inhibited complement-mediated lysis of host-like cells caused by a disease-associated FH mutation or by anti-FH autoantibodies. Therefore, mini-FH could potentially be used as a complement inhibitor targeting host surfaces, as well as to replace compromised FH in diseases associated with FH dysfunction.

Rational engineering of a minimized immune inhibitor with unique triple-targeting properties

Inadequate control of the complement system is the underlying or aggravating factor in many human diseases. Whereas treatment options that specifically target the alternative pathway (AP) of complement activation are considered highly desirable, no such option is available in the clinic. In this study, we present a successful example of protein engineering, guided by structural insight on the complement regulator factor H (FH), yielding a novel complement-targeted therapeutic (mini-FH) with clinical potential. Despite a 70% reduction in size, mini-FH retained and in some respects exceeded the regulatory activity and cell surface-recognition properties of its parent protein FH, including the recently described recognition of sites of oxidative stress. Importantly, the chosen design extended the functional spectrum of the inhibitor, as mini-FH showed increased binding to the surface-bound opsonins iC3b and C3dg when compared with FH. Thus, mini-FH is equipped with a unique and clinically valuable triple-targeting profile toward diseased host cells, through its binding to sites of ongoing complement activation, markers of oxidative damage, and host surface-specific polyanions. When assessed in a clinically relevant AP-mediated disease model of paroxysmal nocturnal hemoglobinuria, mini-FH largely outperformed FH and indicated advantages over clinically evaluated AP inhibitors. Thus, the rational engineering of a streamlined FH construct not only provided insight into the function of a key complement regulator, but also yielded a novel inhibitor that combines a triple-targeting approach with high AP-specific inhibitory activity (IC50 ~ 40 nM), which may pave the way toward new options for the treatment of complement-mediated diseases.

Combination of factor H mutation and properdin deficiency causes severe C3 glomerulonephritis

Factor H (fH) and properdin both modulate complement; however, fH inhibits activation, and properdin promotes activation of the alternative pathway of complement. Mutations in fH associate with several human kidney diseases, but whether inhibiting properdin would be beneficial in these diseases is unknown. Here, we found that either genetic or pharmacological blockade of properdin, which we expected to be therapeutic, converted the mild C3 GN of an fH-mutant mouse to a lethal C3 GN with features of human dense deposit disease. We attributed this phenotypic change to a differential effect of properdin on the dynamics of alternative pathway complement activation in the fluid phase and the cell surface in the fH-mutant mice. Thus, in fH mutation-related C3 glomerulopathy, additional factors that impact the activation of the alternative pathway of complement critically determine the nature and severity of kidney pathology. These results show that therapeutic manipulation of the complement system requires rigorous disease-specific target validation.

Initiation and regulation of complement during hemolytic transfusion reactions

Hemolytic transfusion reactions represent one of the most common causes of transfusion-related mortality. Although many factors influence hemolytic transfusion reactions, complement activation represents one of the most common features associated with fatality. In this paper we will focus on the role of complement in initiating and regulating hemolytic transfusion reactions and will discuss potential strategies aimed at mitigating or favorably modulating complement during incompatible red blood cell transfusions.