The renaissance of complement therapeutics

Bacterial killing by complement requires membrane attack complex formation via surface-bound C5 convertases

The immune system kills bacteria by the formation of lytic membrane attack complexes (MACs), triggered when complement enzymes cleave C5. At present, it is not understood how the MAC perturbs the composite cell envelope of Gram-negative bacteria. Here, we show that the role of C5 convertase enzymes in MAC assembly extends beyond the cleavage of C5 into the MAC precursor C5b. Although purified MAC complexes generated from preassembled C5b6 perforate artificial lipid membranes and mammalian cells, these components lack bactericidal activity. In order to permeabilize both the bacterial outer and inner membrane and thus kill a bacterium, MACs need to be assembled locally by the C5 convertase enzymes. Our data indicate that C5b6 rapidly loses the capacity to form bactericidal pores; therefore, bacterial killing requires both in situ conversion of C5 and immediate insertion of C5b67 into the membrane. Using flow cytometry and atomic force microscopy, we show that local assembly of C5b6 at the bacterial surface is required for the efficient insertion of MAC pores into bacterial membranes. These studies provide basic molecular insights into MAC assembly and bacterial killing by the immune system.

Emerging Roles of Complement in Psychiatric Disorders

The complement system consists of more than 30 proteins that have long been known to participate to the immune defence against pathogens and to the removal of damaged cells. Their role, however, extends beyond immunity and clearance of altered "self" components in the periphery. In particular, complement proteins can be induced by all cell types in the brain. Recent discoveries highlight the role of complement in normal and pathological brain development. Specifically, the complement system mediates synaptic pruning, a developmental process whereby supernumerary synapses are eliminated in the immature brain. The complement system has been implicated in pathological synapse elimination in schizophrenia, West Nile virus infection, and lupus, all of which are associated with psychiatric manifestations. Complement also contributes to synapse loss in neurodegenerative conditions. This review provides a brief overview of the well-studied role of complement molecules in immunity. The contribution of complement to embryonic and adult neurogenesis, neuronal migration, and developmental synaptic elimination in the normal brain is reviewed. We discuss the role of complement in synapse loss in psychiatric and neurological diseases and evaluate the therapeutic potential of complement-targeting drugs for brain disorders.

The role of properdin in complement-mediated renal diseases: a new player in complement-inhibiting therapy?

Properdin is known as the only positive regulator of the complement system. Properdin promotes the activity of this defense system by stabilizing its key enzymatic complexes: the complement alternative pathway (AP) convertases. Besides, some studies have indicated a role for properdin as an initiator of complement activity. Though the AP is a powerful activation route of the complement system, it is also involved in a wide variety of autoimmune and inflammatory diseases, many of which affect the kidneys. The role of properdin in regulating complement in health and disease has not received as much appraisal as the many negative AP regulators, such as factor H. Historically, properdin deficiency has been strongly associated with an increased risk for meningococcal disease. Yet only recently had studies begun to link properdin to other complement-related diseases, including renal diseases. In the light of the upcoming complement-inhibiting therapies, it is interesting whether properdin can be a therapeutic target to attenuate AP-mediated injury. A full understanding of the basic concepts of properdin biology is therefore needed. Here, we first provide an overview of the function of properdin in health and disease. Then, we explore its potential as a therapeutic target for the AP-associated renal diseases C3 glomerulopathy, atypical hemolytic uremic syndrome, and proteinuria-induced tubulointerstitial injury. Considering current knowledge, properdin-inhibiting therapy seems promising in certain cases. However, knowing the complexity of properdin's role in renal pathologies in vivo, further research is required to clarify the exact potential of properdin-targeted therapy in complement-mediated renal diseases.

C5aR1 regulates T follicular helper differentiation and chronic graft-versus-host disease bronchiolitis obliterans

CD4+ follicular helper T (Tfh) cells are specialized providers of T cell help to B cells and can function as pathogenic mediators of murine antibody-dependent chronic graft-versus-host disease (GvHD). Using a parent→F1 model of lupus-like chronic GvHD, in which Tfh cell and germinal center (GC) B cell differentiation occurs over 14 days, we demonstrate that absence of CD4+ T cell-expressed C5a receptor 1 (C5ar1) or pharmacological C5aR1 blockade abrogated generation/expansion of Tfh cells, GC B cells, and autoantibodies. In a Tfh cell-dependent model of chronic GvHD manifested by bronchiolitis obliterans syndrome (BOS), C5aR1 antagonism initiated in mice with established disease ameliorated BOS and abolished the associated differentiation of Tfh and GC B cells. Guided by RNA-sequencing data, mechanistic studies performed using murine and human T cells showed that C5aR1 signaling amplifies IL-6-dependent expression of the transcription factor c-MAF and the cytokine IL-21 via phosphorylating phosphokinase B (AKT) and activating the mammalian target of rapamycin (mTOR). In addition to linking C5aR1-initiated signaling to Tfh cell differentiation, our findings suggest that C5aR1 may be a useful therapeutic target for prevention and/or treatment of individuals with Tfh cell-dependent diseases, including those chronic GvHD patients who have anti-host reactive antibodies.

Complement Inhibitor CRIg/FH Ameliorates Renal Ischemia Reperfusion Injury via Activation of PI3K/AKT Signaling

Complement activation is involved in the pathogenesis of ischemia reperfusion injury (IRI), which is an inevitable process during kidney transplantation. Therefore, complement-targeted therapeutics hold great potential in protecting the allografts from IRI. We observed universal deposition of C3d and membrane attack complex in human renal allografts with delayed graft function or biopsy-proved rejection, which confirmed the involvement of complement in IRI. Using FB-, C3-, C4-, C5-, C5aR1-, C5aR2-, and C6-deficient mice, we found that all components, except C5aR2 deficiency, significantly alleviated renal IRI to varying degrees. These gene deficiencies reduced local (deposition of C3d and membrane attack complex) and systemic (serum levels of C3a and C5a) complement activation, attenuated pathological damage, suppressed apoptosis, and restored the levels of multiple local cytokines (e.g., reduced IL-1β, IL-9, and IL-12p40 and increased IL-4, IL-5, IL-10, and IL-13) in various gene-deficient mice, which resulted in the eventual recovery of renal function. In addition, we demonstrated that CRIg/FH, which is a targeted complement inhibitor for the classical and primarily alternative pathways, exerted a robust renoprotective effect that was comparable to gene deficiency using similar mechanisms. Further, we revealed that PI3K/AKT activation, predominantly in glomeruli that was remarkably inhibited by IRI, played an essential role in the CRIg/FH renoprotective effect. The specific PI3K antagonist duvelisib almost completely abrogated AKT phosphorylation, thus abolishing the renoprotective role of CRIg/FH. Our findings suggested that complement activation at multiple stages induced renal IRI, and CRIg/FH and/or PI3K/AKT agonists may hold the potential in ameliorating renal IRI.

Emerging immunotherapies for autoimmune kidney disease

Autoimmunity is a leading cause of chronic kidney disease and loss of native and transplanted kidneys. Conventional immunosuppressive therapies can be effective but are non-specific, noncurative, and risk serious side effects such as life-threatening infection and cancer. Novel therapies and targeted interventions are urgently needed. In this brief review we explore diverse strategies currently in development and under consideration to interrupt underlying disease mechanisms in immune-mediated renal injury. Because autoantibodies are prominent in diagnosis and pathogenesis in multiple human glomerulopathies, we highlight several promising therapies that interfere with functions of early mediators (IgG and complement) of the effector arm and with an epicenter (the germinal center) for induction of humoral immunity.

CIMT 2018: Pushing frontiers in cancer immunotherapy — Report on the 16th Annual Meeting of the Association for Cancer Immunotherapy

The 16th Annual Meeting of the Association for Cancer Immunotherapy (CIMT), Europe’s largest meeting series of its kind, took place in Mainz, Germany from 15–17 May, 2018. Cutting-edge advancements in cancer immunotherapy were discussed among more than 700 scientists under the motto “Pushing Frontiers in Cancer Immunotherapy”. This meeting report is a summary of some of the CIMT 2018 highlights.

A Single-Domain Antibody Targeting Complement Component C5 Acts as a Selective Inhibitor of the Terminal Pathway of the Complement System and Thus Functionally Mimicks the C-Terminal Domain of the Staphylococcus aureus SSL7 Protein

The complement system is an efficient anti-microbial effector mechanism. On the other hand abnormal complement activation is involved in the pathogenesis of multiple inflammatory and hemolytic diseases. As general inhibition of the complement system may jeopardize patient health due to increased susceptibility to infections, the development of pathway-specific complement therapeutics has been a long-lasting goal over the last decades. In particular, pathogen mimicry has been considered as a promising approach for the design of selective anti-complement drugs. The C-terminal domain of staphylococcal superantigen-like protein 7 (SSL7), a protein secreted by Staphylococcus aureus, was recently found to be a specific inhibitor of the terminal pathway of the complement system, providing selective inhibition of cell lysis mediated by the membrane attack complex (MAC). We describe here the selection by phage display of a humanized single-domain antibody (sdAb) mimicking the C-terminal domain of SSL7. The antibody, called sdAb_E4, binds complement C5 with an affinity in the low micromolar range. Furthermore, sdAb_E4 induces selective inhibition of MAC-mediated lysis, allowing inhibition of red blood cell hemolysis and inhibition of complement deposition on apopto-necrotic cells, while maintaining efficient bactericidal activity of the complement terminal pathway. Finally, we present preliminary results indicating that sdAb_E4 may also be efficient in inhibiting hemolysis of erythrocytes from patients with paroxysmal nocturnal hemoglobinuria. Our data provide a proof of concept for the design of a selective MAC inhibitor capable of retaining complement bacteriolytic activity and this study opens up promising perspectives for the development of an sdAb_E4-derived therapeutics with application in the treatment of complement-mediated hemolytic disorders.

The utility of complement assays in clinical immunology: A comprehensive review

The multi-tasking organ liver, which is the major synthesis site of most serum proteins, supplies humoral components of the innate, - including proteins of the complement system; and, less intensely, also of the acquired immune system. In addition to hepatocyte origins, C1q, factor D, C3, C7 and other protein components of the complement system are produced at various body locations by monocytes/macrophages, lymphocytes, adipocytes, endometrium, enterocytes, keratinocytes and epithelial cells; but the contribution of these alternate sites to the total serum concentrations is slight. The two major exceptions are factor D, which cleaves factor B of the alternative pathway derived largely from adipocytes, and C7, derived largely from polymorphonuclear leukocytes and monocytes/macrophages. Whereas the functional meaning of the extrahepatic synthesis of factor D remains to be elucidated, the local contribution of C7 may up- or downregulate the complement attack. The liver, however, is not classified as part of the immune system but is rather seen as victim of autoimmune diseases, a point that needs apology. Recent histological and cell marker technologies now turn the hands to also conceive the liver as proactive autoimmune disease catalyst. Hosting non-hepatocytic cells, e.g. NK cells, macrophages, dendritic cells as well as T and B lymphocytes, the liver outreaches multiple sites of the immune system. Immunopharmacological follow up of liver transplant recipients teaches us on liver-based presence of ABH-glycan HLA phenotypes and complement mediated ischemia/regeneration processes. In clinical context, the adverse reactions of the complement system can now be curbed by specific drug therapy. This review extends on the involvement of the complement system in liver autoimmune diseases and should allow to direct therapeutic opportunities.

Interpretation of Serological Complement Biomarkers in Disease

Complement system aberrations have been identified as pathophysiological mechanisms in a number of diseases and pathological conditions either directly or indirectly. Examples of such conditions include infections, inflammation, autoimmune disease, as well as allogeneic and xenogenic transplantation. Both prospective and retrospective studies have demonstrated significant complement-related differences between patient groups and controls. However, due to the low degree of specificity and sensitivity of some of the assays used, it is not always possible to make predictions regarding the complement status of individual patients. Today, there are three main indications for determination of a patient's complement status: (1) complement deficiencies (acquired or inherited); (2) disorders with aberrant complement activation; and (3) C1 inhibitor deficiencies (acquired or inherited). An additional indication is to monitor patients on complement-regulating drugs, an indication which may be expected to increase in the near future since there is now a number of such drugs either under development, already in clinical trials or in clinical use. Available techniques to study complement include quantification of: (1) individual components; (2) activation products, (3) function, and (4) autoantibodies to complement proteins. In this review, we summarize the appropriate indications, techniques, and interpretations of basic serological complement analyses, exemplified by a number of clinical disorders.

Unraveling the Molecular Mechanisms Underlying Complement Dysregulation by Nephritic Factors in C3G and IC-MPGN

Membranoproliferative glomerulonephritis (MPGN) was recently classified as C3 glomerulopathies (C3G), and immune-complex (IC) mediated MPGN. Dysregulation of the complement alternative pathway, driven by acquired and/or genetic defects, plays a pathogenetic role in C3G. However, alternative pathway abnormalities were also found in IC-MPGN. The most common acquired drivers are the C3 nephritic factors (C3NeFs), heterogeneous autoantibodies that stabilize the C3 convertase, C3bBb. C3NeFs are traditionally detected by hemolytic assays based on sheep erythrocyte lysis, which however do not provide a direct molecular estimation of C3bBb formation and decay. We set up a microplate/western blot assay that specifically detects and quantifies C3bBb, and its precursor, the C3 proconvertase C3bB, to investigate the complex mechanistic effects of C3NeFs from patients with primary IC-MPGN (n = 13) and C3G (n = 13). In the absence of properdin, 9/26 patients had C3NeF IgGs stabilizing C3bBb against spontaneous and FH-accelerated decay. In the presence of properdin the IgGs of all but one patient had C3bBb-stabilizing activity. Properdin-independent C3NeFs were identified mostly in DDD patients, while properdin-dependent C3NeFs associated with either C3GN or IC-MPGN and with higher incidence of nephrotic syndrome. When we grouped patients based on our recent cluster analysis, patients in cluster 3, with highly electron-dense intramembranous deposits, low C3, and mostly normal sC5b-9 levels, had a higher prevalence of properdin-independent C3NeFs than patients in clusters 1 and 2. Conversely, about 70% of cluster 1 and 2 patients, with subendothelial, subepithelial, and mesangial deposits, low C3 levels and high sC5b-9 levels, had properdin-dependent C3NeFs. The flexibility of the assay allowed us to get deep insights into C3NeF mechanisms of action, showing that: (1) most C3NeFs bind strongly and irreversibly to C3 convertase; (2) C3NeFs and FH recognize different epitopes in C3 convertase; (3) C3NeFs bind rapidly to C3 convertase and antagonize the decay accelerating activity of FH on newly formed complexes; (4) C3NeFs do not affect formation and stability of the C3 proconvertase. Thus, our study provides a molecular approach to detecting and characterizing C3NeFs. The results highlight different mechanisms of complement dysregulation resulting in different complement profiles and patterns of glomerular injury, and this may have therapeutic implications.

Safety profile after prolonged C3 inhibition

The central component of the complement cascade, C3, is involved in various biological functions, including opsonization of foreign bodies, clearance of waste material, activation of immune cells, and triggering of pathways controlling development. Given its broad role in immune responses, particularly in phagocytosis and the clearance of microbes, a deficiency in complement C3 in humans is often associated with multiple bacterial infections. Interestingly, an increased susceptibility to infections appears to occur mainly in the first two years of life and then wanes throughout adulthood. In view of the well-established connection between C3 deficiency and infections, therapeutic inhibition of complement at the level of C3 is often considered with caution or disregarded. We therefore set out to investigate the immune and biochemical profile of non-human primates under prolonged treatment with the C3 inhibitor compstatin (Cp40 analog). Cynomolgus monkeys were dosed subcutaneously with Cp40, resulting in systemic inhibition of C3, for 1 week, 2 weeks, or 3 months. Plasma concentrations of both C3 and Cp40 were measured periodically and complete saturation of plasma C3 was confirmed. No differences in hematological, biochemical, or immunological parameters were identified in the blood or tissues of animals treated with Cp40 when compared to those injected with vehicle alone. Further, skin wounds showed no signs of infection in those treated with Cp40. In fact, Cp40 treatment was associated with a trend toward accelerated wound healing when compared with the control group. In addition, a biodistribution study in a rhesus monkey indicated that the distribution of Cp40 in the body is associated with the presence of C3, concentrating in organs that accumulate blood and produce C3. Overall, our data suggest that systemic C3 inhibition in healthy adult non-human primates is not associated with a weakened immune system or susceptibility to infections.

C3a receptor antagonism as a novel therapeutic target for chronic rhinosinusitis

Chronic rhinosinusitis with nasal polyps (CRSwNP) is an inflammatory disease with an unknown etiology. Recent studies have implicated the complement system as a potential modulator of disease immunopathology. We performed proteomic pathway enrichment analysis of differentially increased proteins, and found an enrichment of complement cascade pathways in the nasal mucus of individuals with CRSwNP as compared to control subjects. Sinonasal mucus levels of complement 3 (C3) correlated with worse subjective disease severity, whereas no significant difference in systemic C3 levels could be determined in plasma samples. Given that human sinonasal epithelial cells were the predominate sinonasal source of C3 and complement anaphylatoxin 3a (C3a) staining, we focused on their role in in vitro studies. Baseline intracellular C3 levels were higher in CRSwNP cells, and following exposure to Aspergillus fumigatus (Af) extract, they released significantly more C3 and C3a. Inhibition of complement 3a receptor (C3aR) signaling led to a decrease in Af-induced C3 and C3a release, both in vitro and in vivo. Finally, we found in vivo that C3aR deficiency or inhibition significantly reduced inflammation and CRS development in a mouse model of Af-induced CRS. These findings demonstrate that local sinonasal complement activation correlates with subjective disease severity, and that local C3aR antagonism significantly ameliorates Af-induced CRS in a rodent model.

Complement and Bacterial Infections: From Molecular Mechanisms to Therapeutic Applications

Complement is a complex protein network of plasma, and an integral part of the innate immune system. Complement activation results in the rapid clearance of bacteria by immune cells, and direct bacterial killing via large pore-forming complexes. Here we review important recent discoveries in the complement field, focusing on interactions relevant for the defense against bacteria. Understanding the molecular interplay between complement and bacteria is of great importance for future therapies for infectious and inflammatory diseases. Antibodies that support complement-dependent bacterial killing are of interest for the development of alternative therapies to treat infections with antibiotic-resistant bacteria. Furthermore, a variety of novel therapeutic complement inhibitors have been developed to prevent unwanted complement activation in autoimmune inflammatory diseases. A better understanding of how such inhibitors may increase the risk of bacterial infections is essential if such therapies are to be successful.

Materials Nanoarchitectonics for Mechanical Tools in Chemical and Biological Sensing

In this Focus Review, nanoarchitectonic approaches for mechanical-action-based chemical and biological sensors are briefly discussed. In particular, recent examples of piezoelectric devices, such as quartz crystal microbalances (QCM and QCM-D) and a membrane-type surface stress sensor (MSS), are introduced. Sensors need well-designed nanostructured sensing materials for the sensitive and selective detection of specific targets. Nanoarchitectonic approaches for sensing materials, such as mesoporous materials, 2D materials, fullerene assemblies, supported lipid bilayers, and layer-by-layer assemblies, are highlighted. Based on these sensing approaches, examples of bioanalytical applications are presented for toxic gas detection, cell membrane interactions, label-free biomolecular assays, anticancer drug evaluation, complement activation-related multiprotein membrane attack complexes, and daily biodiagnosis, which are partially supported by data analysis, such as machine learning and principal component analysis.

Developments in anti-complement therapy; from disease to clinical trial

The complement system is well known for its role in innate immunity and in maintenance of tissue homeostasis, providing a first line of defence against infection and playing a key role in flagging apoptotic cells and debris for disposal. Unfortunately complement also contributes to pathogenesis of a number of diseases; in some cases driving pathology, and in others amplifying or exacerbating the inflammatory and damaging impact of non-complement disease triggers. The role of complement in pathogenesis of an expanding number of diseases has driven industry and academia alike to develop an impressive arsenal of anti-complement drugs which target different proteins and functions of the complement cascade. Evidence from genetic and biochemical analyses, combined with improved identification of complement biomarkers and supportive data from sophisticated animal models of disease, has driven a drug development landscape in which the indications selected for clinical trial cluster in three 'target' tissues: the kidney, eye and vasculature. While the disease triggers may differ, complement activation and amplification is a common feature in many diseases which affect these three tissues. An abundance of drugs are in clinical development, some show favourable progression whereas others experience significant challenges. However, these hurdles in themselves drive an ever-evolving portfolio of 'next-generation' drugs with improved pharmacokinetic and pharmacodynamics properties. In this review we discuss the indications which are in the drug development 'spotlight' and review the relevant indication validation criteria. We present current progress in clinical trials, highlighting successes and difficulties, and look forward to approval of a wide selection of drugs for use in man which give clinicians choice in mechanistic target, modality and route of delivery.

Be on Target: Strategies of Targeting Alternative and Lectin Pathway Components in Complement-Mediated Diseases

The complement system has moved into the focus of drug development efforts in the last decade, since its inappropriate or uncontrolled activation has been recognized in many diseases. Some of them are primarily complement-mediated rare diseases, such as paroxysmal nocturnal hemoglobinuria, C3 glomerulonephritis, and atypical hemolytic uremic syndrome. Complement also plays a role in various multifactorial diseases that affect millions of people worldwide, such as ischemia reperfusion injury (myocardial infarction, stroke), age-related macular degeneration, and several neurodegenerative disorders. In this review, we summarize the potential advantages of targeting various complement proteins with special emphasis on the components of the lectin (LP) and the alternative pathways (AP). The serine proteases (MASP-1/2/3, factor D, factor B), which are responsible for the activation of the cascade, are straightforward targets of inhibition, but the pattern recognition molecules (mannose-binding lectin, other collectins, and ficolins), the regulatory components (factor H, factor I, properdin), and C3 are also subjects of drug development. Recent discoveries about cross-talks between the LP and AP offer new approaches for clinical intervention. Mannan-binding lectin-associated serine proteases (MASPs) are not just responsible for LP activation, but they are also indispensable for efficient AP activation. Activated MASP-3 has recently been shown to be the enzyme that continuously supplies factor D (FD) for the AP by cleaving pro-factor D (pro-FD). In this aspect, MASP-3 emerges as a novel feasible target for the regulation of AP activity. MASP-1 was shown to be required for AP activity on various surfaces, first of all on LPS of Gram-negative bacteria.

The effect on the immunology laboratory of the expansion in complement therapeutics

The approval in the US and Europe of Eculizumab in 2007 marked a change in complement therapeutics, and with it the landscape for complement testing in the clinical immunology laboratory changed. The change had begun even before that when C1-Inhibitor preparations were approved in the 1980s in Europe. There are now two classes of approved drugs that may impact the immunology laboratory, with two dozen more with novel modalities and potential indications that are in various stages of development. Every pathway and about every component of complement has been targeted by these drug development programs, and the modalities of the drugs in development are diverse. These developments will likely result in more laboratories offering more complement testing, so this review looks forward to some of those possible changes in testing.

Functional Characterization of Alternative and Classical Pathway C3/C5 Convertase Activity and Inhibition Using Purified Models

Complement is essential for the protection against infections; however, dysregulation of complement activation can cause onset and progression of numerous inflammatory diseases. Convertase enzymes play a central role in complement activation and produce the key mediators of complement: C3 convertases cleave C3 to generate chemoattractant C3a and label target cells with C3b, which promotes phagocytosis; C5 convertases cleave C5 into chemoattractant C5a, and C5b, which drives formation of the membrane attack complex. Since convertases mediate nearly all complement effector functions, they are ideal targets for therapeutic complement inhibition. A unique feature of convertases is their covalent attachment to target cells, which effectively confines complement activation to the cell surface. However, surface localization precludes detailed analysis of convertase activation and inhibition. In our previous work, we developed a model system to form purified alternative pathway (AP) C5 convertases on C3b-coated beads and quantify C5 conversion via functional analysis of released C5a. Here, we developed a C3aR cell reporter system that enables functional discrimination between C3 and C5 convertases. By regulating the C3b density on the bead surface, we observe that high C3b densities are important for conversion of C5, but not C3, by AP convertases. Screening of well-characterized complement-binding molecules revealed that differential inhibition of AP C3 convertases (C3bBb) and C5 convertases [C3bBb(C3b)_n] is possible. Although both convertases contain C3b, the C3b-binding molecules Efb-C/Ecb and FHR5 specifically inhibit C5 conversion. Furthermore, using a new classical pathway convertase model, we show that these C3b-binding proteins not only block AP C3/C5 convertases but also inhibit formation of a functional classical pathway C5 convertase under well-defined conditions. Our models enable functional characterization of purified convertase enzymes and provide a platform for the identification and development of specific convertase inhibitors for treatment of complement-mediated disorders.

Self-Damage Caused by Dysregulation of the Complement Alternative Pathway: Relevance of the Factor H Protein Family

The alternative pathway is a continuously active surveillance arm of the complement system, and it can also enhance complement activation initiated by the classical and the lectin pathways. Various membrane-bound and plasma regulatory proteins control the activation of the potentially deleterious complement system. Among the regulators, the plasma glycoprotein factor H (FH) is the main inhibitor of the alternative pathway and its powerful amplification loop. FH belongs to a protein family that also includes FH-like protein 1 and five factor H-related (FHR-1 to FHR-5) proteins. Genetic variants and abnormal rearrangements involving the FH protein family have been linked to numerous systemic and organ-specific diseases, including age-related macular degeneration, and the renal pathologies atypical hemolytic uremic syndrome, C3 glomerulopathies, and IgA nephropathy. This review covers the known and recently emerged ligands and interactions of the human FH family proteins associated with disease and discuss the very recent experimental data that suggest FH-antagonistic and complement-activating functions for the FHR proteins.

New Analogs of the Complement C3 Inhibitor Compstatin with Increased Solubility and Improved Pharmacokinetic Profile

Improper regulation of complement is associated with various pathologies, and the clinical demand for compounds that can regulate complement activation is therefore imperative. Cp40, an analog of the peptide compstatin, inhibits all complement pathways at the level of the central component C3. We have further developed Cp40, using either PEGylation at the N-terminus or insertion of charged amino acids at the C-terminus. The PEGylated analogs are highly soluble and retained their inhibitory activity, with C3b binding affinity dependent on the length of the PEG chain. The addition of two or three residues of lysine, in turn, not only improved the peptide's solubility but also increased the binding affinity for C3b while retaining its inhibitory potency. Three of the new derivatives showed improved pharmacokinetic profiles in vivo in non-human primates. Given their compelling solubility and pharmacokinetic profiles, these new Cp40 analogs should broaden the spectrum of administration routes, likely reducing dosing frequency during chronic treatment and potentially expanding their range of clinical application.

Expanding Complement Therapeutics for the Treatment of Paroxysmal Nocturnal Hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) is widely regarded as an archetypal complement-mediated disorder that has propelled complement drug discovery in recent decades. Its pathology is driven by chronic complement dysregulation resulting from the lack of the glycosyl phosphatidyl inositol-linked regulators DAF and CD59 on susceptible erythrocytes. This complement imbalance fuels persistent C3 activation on affected erythrocytes, which culminates in chronic complement-mediated intravascular hemolysis. The clinical application of eculizumab, a humanized anti-C5 antibody that blocks terminal pathway activation, has led to drastic improvement of therapeutic outcomes but has also unveiled hitherto elusive pathogenic mechanisms that are now known to contribute to the clinical burden of a significant proportion of patients with PNH. These emerging clinical needs have sparked a true resurgence of complement therapeutics that offer the promise of even more effective, disease-tailored therapies for PNH. Here, we review the current state of complement therapeutics with a focus on the clinical development of C3-targeted and alternative pathway-directed drug candidates for the treatment of PNH. We also discuss the relative advantages and benefits offered by each complement-targeting approach, including translational considerations that might leverage a more comprehensive clinical intervention for PNH.

Properdin: A multifaceted molecule involved in inflammation and diseases

Properdin, the widely known positive regulator of the alternative pathway (AP), has undergone significant investigation over the last decade to define its function in inflammation and disease, including its role in arthritis, asthma, and kidney and cardiovascular diseases. Properdin is a glycoprotein found in plasma that is mainly produced by leukocytes and can positively regulate AP activity by stabilizing C3 and C5 convertases and initiating the AP. Promotion of complement activity by properdin results in changes in the cellular microenvironment that contribute to innate and adaptive immune responses, including pro-inflammatory cytokine production, immune cell infiltration, antigen presenting cell maturation, and tissue damage. The use of properdin-deficient mouse models and neutralizing antibodies has contributed to the understanding of the mechanisms by which properdin contributes to promoting or preventing disease pathology. This review mainly focusses on the multifaceted roles of properdin in inflammation and diseases, and how understanding these roles is contributing to the development of new disease therapies.

Complement analysis in the era of targeted therapeutics

Complement immunobiology, and with it complement analysis, has undergone a renaissance in the past decade. Classically, complement analysis was limited number of testing C3, C4 in a routine laboratory with the possible addition of CH50 with all other analysis being performed at only few highly esoteric laboratories. This diagnostics expanding beyond specialized laboratories is the result of the growing recognition of the role played by complement dysfunction in many more diseases and disorders and the concomitant increase in interest in complement targeting therapeutics. In response, laboratories specializing in complement analysis have joined with the International Complement Society and the IUIS to coordinate efforts to standardize and improve complement testing, ongoing efforts that have already borne fruit. A recognition of the power of complement analysis has brought forward new testing but also realization of the importance of post-draw specimen handling to limit ex vivo activation, as well as the sometimes large difference between testing laboratory results. The increased usefulness of complement analysis and efforts to standardize and expand it means the future is strong for complement analysis.

Identification of complement inhibitory activities of two chemotherapeutic agents using a high-throughput cell imaging-based screening assay

Excessive complement activation contributes significantly to the pathogeneses of various diseases. Currently, significant developmental research efforts aim to identify complement inhibitors with therapeutic uses have led to the approval of one inhibitor for clinical use. However, most existing complement inhibitors are based on monoclonal antibodies, which have many drawbacks such as high costs and limited administration options. With this report, we establish an inexpensive, cell imaging-based high-throughput assay for the large-scale screening of potential small molecule complement inhibitors. Using this assay, we screened a library containing 3115 bioactive chemical compounds and identified cisplatin and pyridostatin as two new complement inhibitors in addition to nafamostat mesylate, a compound with known complement inhibitory activity. We further demonstrated that cisplatin and pyridostatin inhibit C5 convertases in the classical pathway of complement activation but have no effects on the alternative pathway of complement activation. In summary, this work has established a simple, large-scale, high-throughput assay for screening novel complement inhibitors and discovered previously unknown complement activation inhibitory activities for cisplatin and pyridostatin.

Complement systems C4, C3 and CH50 not subject to a circadian rhythm

Background:

The circadian fluctuations in the blood levels of selected components of the complement system are ill-defined. Some authors found nadir serum levels of C4 and C3 components, together with C3a at nighttime, while others reported insomnia when pro-inflammatory components exhibited increased serum levels. In this study, we quantitatively estimate the morning and evening daytime serum levels of CH50, C4, C3, put into context with C-reactive protein (CRP), cortisol, parathyroid hormone (PTH) and 25(OH)vitamin D at 07:00 A.M. and at 07:00 P.M.

Methods:

Seven healthy adult women and 11 men who were voluntary participants agreed to a fasting venipuncture in the morning after having normally eaten through the day and in the evening. The C4 and C3 serum levels were measured on a Cobas (Roche Diagnostics, Switzerland) modular analyzer, CH50 was estimated using the COMPL300 enzyme-linked immunosorbent assay (ELISA) of Wieslab (Malmö, Sweden). CRP, 25(OH)vitamin D, PTH and cortisol concentrations were assessed with electro-chemiluminescence immunoassay (ECLIA) on the Roche Cobas 6000 platform; IgG was measured using nephelometry (Siemens, Germany).

Results:

With the exception of higher PTH levels in the evening [3.12–5.46, 95% confidence interval (CI)] compared to the morning (2.93–4.65, 95% CI), the mean and median values of C4, C3, CH50 as well as CRP, PTH and 25(OH)vitamin D fell within the established reference intervals. Cortisol levels were measured as an internal positive control for diurnal fluctuations (morning: 294–522 nmol/L, 95% CI; evening: 106–136 nmol/L, 95% CI).

Conclusions:

The concentrations of the assessed complement components C4 and C3 as well as CH50 surrogate assay did not yield significantly different values between early morning and evening. This does not exclude their participation in the circadian metabolome; this pilot study with healthy participants suggests that patients with an autoimmune disease in remission can give their blood samples independently during daytime with or without fasting.

Complement Activation and Inhibition in Autoimmune Hemolytic Anemia: Focus on Cold Agglutinin Disease

The classical complement pathway and, to some extent, the terminal pathway, are involved in the immune pathogenesis of autoimmune hemolytic anemia (AIHA). In primary cold agglutinin disease (CAD), secondary cold agglutinin syndrome and paroxysmal cold hemoglobinuria, the hemolytic process is entirely complement dependent. Complement activation also plays an important pathogenetic role in some warm-antibody AIHAs, especially when immunoglobulin M is involved. This review describes the complement-mediated hemolysis in AIHA with a major focus on CAD, in which activation of the classical pathway is essential and particularly relevant for complement-directed therapy. Several complement inhibitors are candidate therapeutic agents in CAD and other AIHAs, and some of these drugs seem very promising. The relevant in vitro findings, early clinical data and future perspectives are reviewed.

Treatment of antiphospholipid syndrome beyond anticoagulation

Antiphospholipid syndrome (APS) is a systemic autoimmune disorder marked by thrombosis and/or pregnancy morbidity in the presence of antiphospholipid antibodies (aPL). At the present time, treatment is primarily focused on anticoagulation. However, there is increasing awareness of the mechanisms involved in APS pathogenesis, which has led to the trial of novel therapies targeting those mechanisms. Following a brief review of the etiopathogenesis of and current management strategies in APS, this paper focuses on the evidence for these potential, targeted APS treatments, e.g., hydroxychloroquine, statins, rituximab, belimumab, eculizumab, defibrotide, sirolimus, and peptide therapy.

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.