Complement-mediated Activation Research Articles

Complement-Mediated Two-Step NETosis: Serum-Induced Complement Activation and Calcium Influx Generate NADPH Oxidase-Dependent NETs in Serum-Free Conditions.

The complement system and neutrophils play crucial roles in innate immunity. Neutrophils release neutrophil extracellular traps (NETs), which are composed of decondensed DNA entangled with granular contents, as part of their innate immune function. Mechanisms governing complement-mediated NET formation remain unclear. In this study, we tested a two-step NETosis mechanism, as follows: classical complement-mediated neutrophil activation in serum and subsequent NET formation in serum-free conditions, using neutrophils from healthy donors, endothelial cells, and various assays (Fluo-4AM, DHR123, and SYTOX), along with flow cytometry and confocal microscopy. Our findings reveal that classical complement activation on neutrophils upregulated the membrane-anchored complement regulators CD46, CD55, and CD59. Additionally, complement activation increased CD11b on neutrophils, signifying activation and promoting their attachment to endothelial cells. Complement activation induced calcium influx and citrullination of histone 3 (CitH3) in neutrophils. However, CitH3 formation alone was insufficient for NET generation. Importantly, NET formation occurred only when neutrophils were in serum-free conditions. In such environments, neutrophils induced NADPH oxidase-dependent reactive oxygen species (ROS) production, leading to NET formation. Hence, we propose that complement-mediated NET formation involves a two-step process, as follows: complement deposition, neutrophil priming, calcium influx, CitH3 formation, and attachment to endothelial cells in serum. This is followed by NADPH-dependent ROS production and NET completion in serum-free conditions. Understanding this process may unveil treatment targets for pathologies involving complement activation and NET formation.

Can complement activation be the missing link in antiphospholipid syndrome?

APS is an autoimmune disorder with life-threatening complications that, despite therapeutic advantages, remains associated with thrombotic recurrences and treatment failure. The role of complement activation in APS pathogenesis is increasingly recognised, specifically in obstetric APS. However, its exact role in thrombotic APS and on the severity of the disease is not yet fully elucidated. Further mechanistic studies are needed to delineate the role of complement activation in the various APS clinical manifestations with aim to identify novel markers of disease severity, together with clinical trials to evaluate the efficacy of complement inhibition in APS. This could ultimately improve risk stratification in APS, patient tailored targeted therapy with complement inhibition identified as an adjunctive treatment. This article reviews current findings and challenges about complement activation in APS, discusses the potential role of platelet mediated complement activation in this setting and provides an overview on clinical implications and current therapeutics.

Constitutive immunity is influenced by avian influenza virus-induced modification of gut microbiota in Eurasian teal (Anas crecca)

Understanding the dynamics of migrant birds' gut microbial communities is essential for evaluating their ecological interactions, since these birds act as vectors for zoonotic viruses and their gut microbiome may have exceptional relationship with zoonotic viral infection. The Eurasian teal duck Anas crecca traverses continents during migration, combining and providing intercontinental links for avian influenza viruses (AIV) of different origins. The present study aimed to investigate how the AIV infection affects gut microbial composition and evaluate the consequent physiological stress and constitutive immunity of teal birds. Samples were collected from 2 flocks during their migratory stopover in northern Egypt. An important shift in gut microbiota of AIV-infected individuals has been detected by RT-PCR. In healthy teal, firmicutes dominated followed by proteobacteria, while the structure was reversed in infected birds. Infection with AIV significantly increased the stress hormone corticosterone, accompanied by a significant increase in both oxidative stress markers and antioxidants. Constitutive immunity, measured by plasma bactericidal effect against E. coli, the nonspecific natural antibodies, and the mediated complement activation, was reduced in AIV-infected teal birds. Constitutive immunity parameters were proportionally correlated to the firmicutes and inversely to the proteobacteria abundances, but not to the viral positivity. In conclusion, the present study provides initial evidence of the alteration of the gut microbiome in the Eurasian teal Anas crecca by AIV infection and demonstrates that the AIV-induced reduction in constitutive immunity is a consequence of the shift in microbiome composition rather than the virus infection itself or its induced stress.

Peripheral Ulcerative Keratitis: A Serious Corneal Inflammation

Abstract
 Introduction : Peripheral ulcerative keratitis (PUK) is a rare, but severe sight-threatening inflammatory condition that affects the cornea. PUK is characterized by peripheral corneal ulceration, stromal thinning, and neovascularization that can lead to corneal perforation and vision loss. Although PUK can occur in individuals without underlying systemic disease, it is most commonly associated with autoimmune disorders.
 Case Illustration : A 53-year-old woman presented to the ophthalmology clinic with complaints of redness, foreign body sensation, and blurred vision in her both eye for the past three days. On examination, slit-lamp biomicroscopy revealed a peripheral corneal ulcer. She had a history of rheumatoid arthritis for the past 2 months and was being treated with methotrexate and metylprednisolone. Laboratory investigations revealed elevated Anti-CCP IgG, which confirmed the presence of rheumatoid arthritis.
 Discussion : PUK is most commonly associated with autoimmune disorders, particularly rheumatoid arthritis, but may also occur in individuals with infectious, degenerative or traumatic corneal injuries. Immune- mediated inflammation, complement activation, and tissue destruction are thought to be involved in the pathogenesis of PUK. The diagnosis of PUK requires a thorough ophthalmic examination, including slit-lamp biomicroscopy, and laboratory investigations for underlying autoimmune disorders.
 Conclusion : PUK is a rare but serious ocular disease that requires prompt diagnosis and management to prevent severe complications such as corneal perforation and vision loss. Although PUK can occur in individuals without underlying systemic disease, it is most commonly associated with autoimmune disorders, particularly rheumatoid arthritis.

Targeting the tissue factor coagulation initiation complex prevents antiphospholipid antibody development

AbstractAntiphospholipid antibodies (aPL) in primary or secondary antiphospholipid syndrome (APS) are a major cause for acquired thrombophilia, but specific interventions preventing autoimmune aPL development are an unmet clinical need. Although autoimmune aPL cross react with various coagulation regulatory proteins, lipid-reactive aPL, including those derived from patients with COVID-19, recognize the endolysosomal phospholipid lysobisphosphatidic acid presented by the cell surface–expressed endothelial protein C receptor. This specific recognition leads to complement-mediated activation of tissue factor (TF)–dependent proinflammatory signaling and thrombosis. Here, we show that specific inhibition of the TF coagulation initiation complex with nematode anticoagulant protein c2 (NAPc2) prevents the prothrombotic effects of aPL derived from patients with COVID-19 in mice and the aPL–induced proinflammatory and prothrombotic activation of monocytes. The induction of experimental APS is dependent on the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase complex, and NAPc2 suppresses monocyte endosomal reactive oxygen species production requiring the TF cytoplasmic domain and interferon-α secretion from dendritic cells. Latent infection with murine cytomegalovirus causes TF cytoplasmic domain–dependent development of persistent aPL and circulating phospholipid-reactive B1 cells, which is prevented by short-term intervention with NAPc2 during acute viral infection. In addition, treatment of lupus prone MRL-lpr mice with NAPc2, but not with heparin, suppresses dendritic-cell activation in the spleen, aPL production and circulating phospholipid-reactive B1 cells, and attenuates lupus pathology. These data demonstrate a convergent TF-dependent mechanism of aPL development in latent viral infection and autoimmune disease and provide initial evidence that specific targeting of the TF initiation complex has therapeutic benefits beyond currently used clinical anticoagulant strategies.

Complement receptor type 1 and 2 (CR1 and CR2) gene polymorphisms and plasma protein levels are associated with the Dengue disease severity

The pathological outcome of dengue disease results from complex interactions between dengue virus (DENV) and host genetics and immune response. Complement receptor types 1 and 2 (CR1 and CR2) mediate complement activation through the alternative pathway. This study investigated the possible association of genetic polymorphisms and plasma levels of CR1 and CR2 with dengue disease. A total of 267 dengue patients and 133 healthy controls were recruited for this study. CR1 and CR2 gene polymorphisms were analyzed by Sanger sequencing, while plasma CR1 and CR2 levels were measured by ELISA. The frequency of the CR1 minor allele rs6691117G was lower in dengue patients and those with severe dengue compared to healthy controls. Plasma CR1 and CR2 levels were decreased in dengue patients compared to healthy controls (P < 0.0001) and were associated with platelet counts. CR1 levels were lower in dengue patients with warning signs (DWS) compared to those without DWS, while CR2 levels were decreased according to the severity of the disease and after 5 days (T1) and 8 days (T2) of follow-up. CR2 levels were decreased in dengue patients positive for anti-DENV IgG and IgM and patients with bleeding and could discriminate DWS and SD from dengue fever patients (AUC = 0.66). In conclusion, this study revealed a reduction in CR2 levels in dengue patients and that the CR1 SNP rs6691117A/G is associated with the dengue severity. The correlation of CR2 levels with platelet counts suggests that CR2 could be an additional biomarker for the prognosis of severe dengue disease.

Systemic Lupus Erythematosus in Children.

Systemic lupus erythematosus (SLE) is a systemic autoimmune disease, that mainly affects skin, joints and kidneys but can affect any organ in the body. It is characterized by presence of multiple autoantibodies like ANA, antibodies to dsDNA and RNA associated proteins. The major mechanism leading to tissue damage includes immune complex mediated complement activation, interferon alpha release by plasmacytoid dendritic cells, NETosis by neutrophils as well as defects in monocytes leading to poor clearance of cellular debris and direct cellular dysfunction mediated by antibodies. A child can present with pyrexia of unknown origin, immune mediated cytopenias, malar rash, oral ulcers, serositis, glomerulonephritis or nervous system dysfunction. As renal disease has a bearing on the long term impact, all children should have urine exam and blood pressure measurement done to rule out renal disease.The treatment varies depending on the severity and organs involved. In life or organ threatening situations, pulse methylprednisolone is used. Hydroxychloroquine, Mycophenolate mofetil, Azathioprine and Cyclophosphamide are the commonly used drugs in SLE. Over the years the prognosis of SLE has improved probably due to early diagnosis and better use of immunosuppressive treatment, regular follow up and treatment of co-morbidities. The 10-year survival now approaches 90% and with advent of new and targeted therapy it is hoped that the morbidity and organ damage can also be minimized.

The pathogenic anti-PLA2R autoantibodies cooperatively elicit the activation of complement in primary membranous nephropathy

Abstract Primary membranous nephropathy (pMN) is an autoimmune disease associated with autoantibody-mediated immune complexes and the deposition of activated complement proteins on the glomerulus. The phospholipase A2 receptor 1 (PLA2R) has been identified as a major antigen for autoantibodies in 80% of pMN patients. Antibodies of IgG1 and IgG3, but not IgG4, can activate complement. Unexpectedly, in pMN patients, the PLA2R autoantibodies are IgG4 predominance, and those of IgG1 and IgG3 are less abundant in the affected tissues, arguing the role of PLA2R autoantibodies in the pathologies of pMN. To study the molecular basis of individual PLA2R-reactive IgGs mediating complement activation in pMN, we employed a single-cell capture method and generated 16 anti-PLA2R monoclonal antibodies (mAbs). We characterize mAbs properties including epitope, affinity and gene feature. Then we adopted the CDC assay with PLA2R-podocytes to investigate the complement activation of the mAbs. Our data show that the combination of CTLD1 and any other epitope showed a synergistic effect that enhanced CDC activation, which single epitope anti-PLA2R mAb cannot achieve. Surprisingly, IgG1 mAb can still promote the CDC when pair with IgG4 mAb in synergistic effects. Moreover, polyclonal anti-PLA2R IgGs from pMN patients but not healthy volunteers can induce CDC. Next, we found that a group of patients only have CysR-IgG3, neither CTLD1-IgG1 nor IgG3; when we add CTLD1-IgG1 mAb can enhance CDC activation. Our data identify an essential role of complement-reactive IgGs, presumably IgG1 and IgG3 synergies between epitopes synergistic effects, and highlight the importance of immune complexes in complement activation by PLA2R autoantibodies, which underlies pMN.

A novel C1qDC (PoC1qDC) with a collagen domain in Paralichthys olivaceus mediates complement activation and against bacterial infection

Complement C1q domain containing protein (C1qDC) is a vital recognition molecule and has an important effect on immunity. The C1qDCs exhibit opsonic activity in fish, while the mechanisms of C1qDCs in activation complement still remain unclear. This study explored immunological characteristics of a C1qDC from Japanese flounder (Paralichthys olivaceus) (PoC1qDC). PoC1qDC consists of 296 amino acid residues, possessing a collagen domain and a C1q domain. According to our results, PoC1qDC was expressed in 9 diverse tissue samples and showed up-regulation after bacterial challenge. Recombinant PoC1qDC (rPoC1qDC) activated normal serum bactericidal and hemolytic activities by interaction with Japanese flounder IgM, but not enhanced the complement activity of C3-depeleted serum. rPoC1qDC was significantly bound to various bacterial species and agglutination activity against Edwardsiella piscicida and Streptococcus iniae. Furthermore, rPoC1qDC showed direct interaction with peripheral blood leucocytes while enhancing phagocytic and chemotactic activity. When PoC1qDC was overexpressed in Japanese flounder before E. piscicida infection, bacterial replication was significantly inhibited in fish tissues. Consistently, when PoC1qDC expression in Japanese flounder was knocked down, bacterial replication was significantly enhanced. The above findings first suggested the role of PoC1qDC in teleost in mediating complement activation by interaction with IgM, which can positively influence bacterial infection.

Endothelial-platelet interaction in sepsis

Introduction. Endothelial cells and platelets are actively involved in pathogenetic processes in sepsis — an unregulated reaction of the host organism in response to infection, leading, on the one hand, to the development of prothrombotic, and on the other hand, to hemorrhagic readiness.Aim — to summarize the current information on the mechanisms of intercellular interaction between endotheliocytes and platelets in sepsis.Main findings. The development of multiple organ failure, which increases the likelihood of an unfavorable outcome of sepsis, is partly due to endothelial dysfunction, as well as the involvement of platelets in the pathogenetic process. Under physiological conditions, an anatomically and functionally intact endothelium is important to prevent microvascular thrombosis. Although platelets are most associated with hemostasis, they perform many other functions, including participation in inflammatory processes through complement activation, interaction with leukocytes and monocytes, participation in host defense against infection, and regulation of vascular tone. There is also abundant evidence that suggests that the processes regulating hemostasis evolved as a component of the inflammatory response to infection. Many of these interaction points occur on the surface of endothelial cells, linking these two cell types, endotheliocytes and platelets, in initiating and regulating blood clotting and inflammation. Various mechanisms may contribute to direct and indirect platelet activation in sepsis, including pathogen-induced platelet activation, pathogen- and inflammation-induced endothelial and leukocyte activation, and complement-mediated platelet activation.

IMMUNE CHECKPOINT INHIBITOR-INDUCED DIABETIC KETOACIDOSIS: A RARE SIDE EFFECT

IMMUNE CHECKPOINT INHIBITOR-INDUCED DIABETIC KETOACIDOSIS: A RARE SIDE EFFECT

The role of the complement system in kidney glomerular capillary thrombosis.

The complement system is part of the innate immune system. The crucial step in activating the complement system is the generation and regulation of C3 convertase complexes, which are needed to generate opsonins that promote phagocytosis, to generate C3a that regulates inflammation, and to initiate the lytic terminal pathway through the generation and activity of C5 convertases. A growing body of evidence has highlighted the interplay between the complement system, coagulation system, platelets, neutrophils, and endothelial cells. The kidneys are highly susceptible to complement-mediated injury in several genetic, infectious, and autoimmune diseases. Atypical hemolytic uremic syndrome (aHUS) and lupus nephritis (LN) are both characterized by thrombosis in the glomerular capillaries of the kidneys. In aHUS, congenital or acquired defects in complement regulators may trigger platelet aggregation and activation, resulting in the formation of platelet-rich thrombi in the kidneys. Because glomerular vasculopathy is usually noted with immunoglobulin and complement accumulation in LN, complement-mediated activation of tissue factors could partly explain the autoimmune mechanism of thrombosis. Thus, kidney glomerular capillary thrombosis is mediated by complement dysregulation and may also be associated with complement overactivation. Further investigation is required to clarify the interaction between these vascular components and develop specific therapeutic approaches.

NR4A1-3 nuclear receptor activity and immune cell dysregulation in rheumatic diseases.

The development and progression of immune-mediated rheumatic disease (IMRD) involves dysfunction of innate and adaptive immune cell populations leading to altered responses including inflammasome activation, dysregulated cytokine networks, increased immune cell numbers and multifaceted cell-cell communication. Several rheumatic diseases are further characterized by the presence of autoantibodies, immune complex mediated complement activation and the deficit of peripheral immune tolerance due to reduced regulatory T-lymphocyte cell function. Ultimately, in rheumatic disease the loss in cellular and tissue homeostasis culminates in the advancement of chronic inflammation. The three members of the NR4A subfamily of nuclear receptors are immediate early genes, and act as potent transcriptional responders to changes in the cellular and tissue microenvironment. Subfamily members are rapidly expressed in diseases characterized by inflammation and function to control the differentiation and activity of innate and adaptive immune cells in a cell-type and cell-context specific manner. Rheumatic disease including rheumatoid-, psoriatic-, osteo-arthritis and systemic sclerosis display altered NR4A1-3 activity in controlling immune cell migration and function, production of paracrine signaling molecules, synovial tissue hyperplasia, and regulating cartilage turn-over in vivo. Additionally, NR4A1-3 activities mediate cytokine, prostanoid and growth factor signaling to control angiogenesis, modulate the regulatory functions of mesenchymal stromal cells, alter the activation status of dendritic cells, influence the generation of peripheral myeloid and T-lymphocyte lineages and promote the maintenance of functional regulatory T-cells. Further reports uncover the potential of moderating NR4A 1-3 receptors as therapeutic targets in altering immune tolerance, pathological angiogenesis and controlling inflammation in several models of disease.

Using Bio-Layer Interferometry to Evaluate Anti-PEG Antibody-Mediated Complement Activation.

The purpose of this study was to develop a Bio-layer interferometry (BLI) system that could be an alternative approach for the direct evaluation of anti-polyethylene glycol (PEG) immunoglobulin M (IgM)-mediated complement activation of the accelerated blood clearance (ABC) phenomenon. Complement activation is well known to play an important role in the clearance of PEGylated and non-PEGylated nanomedicines following intravenous injection. This complement system is also thought to be responsible for the ABC phenomenon wherein repeated injections of PEGylated products are bound by anti-PEG antibodies. This study used three different sources of anti-PEG antibodies: HIK-M09 monoclonal antibodies (mAbs); HIK-M11 mAbs; and antiserum containing polyclonal anti-PEG IgMs. 1,2-Distearoyl-sn-glycero-3-phosphoethanolamine-n-[methoxy (polyethylene glycol)-2000] (mPEG2000-DSPE) was immobilized as an antigen on aminopropyl silane biosensor chips of BLI. All anti-PEG IgMs in the sources increased the signals (thickness of the layer around the sensor tip) regarding binding of anti-PEG antibodies to PEG on the chips. In all anti-PEG IgM sources, further increases in the signals were observed when incubated in naïve mouse serum, which is a complement source, but not in heat inactivated (56 °C, 30 min) mouse serum, which abolishes complement activity. These findings show that the complement activation mediated via anti-PEG IgMs, which occurred on the sensor chips, was detected via BLI analysis. The complement activation induced by all anti-PEG IgM sources was confirmed via conventional enzyme-linked immunosorbent assay (ELISA), which is the conventional mode for detection of complement activation. Our study results show that BLI is a simple alternative method for the detection of complement activation.

The pleotropic effects of fluvastatin on complement-mediated T-cell activation in hypercholesterolemia

T-cells orchestrate the inflammatory responses in atherosclerosis, and their function is modified by the lipoprotein milieu and complement activity. We investigated the effects of fluvastatin on the expression of complement decay-accelerating factor (DAF/CD55) antigen, and the levels of transcription factors in circulating T-cells in hypercholesterolemia. The hypercholesterolemic state was associated with the upregulation of DAF expression on circulating T-cells and increased levels nuclear factor kappa B (NF-kB) and interferon regulatory factor 4 (IRF4). Notably, the elevated levels of DAF and NF-kB expression persisted following treatment with fluvastatin. Therefore, the pleiotropic effects of fluvastatin are partially ascribed to its ability to mediate T-cell activation and regulate complement activity. Consequently, enhanced therapeutic interventions that targets complement-induced T-cell activation may be important in mitigating the development of atherosclerosis and major cardiovascular events in individuals with hypercholesterolemia.

Complement Factor D as a Strategic Target for Regulating the Alternative Complement Pathway.

The complement system is central to first-line defense against invading pathogens. However, excessive complement activation and/or the loss of complement regulation contributes to the development of autoimmune diseases, systemic inflammation, and thrombosis. One of the three pathways of the complement system, the alternative complement pathway, plays a vital role in amplifying complement activation and pathway signaling. Complement factor D, a serine protease of this pathway that is required for the formation of C3 convertase, is the rate-limiting enzyme. In this review, we discuss the function of factor D within the alternative pathway and its implication in both healthy physiology and disease. Because the alternative pathway has a role in many diseases that are characterized by excessive or poorly mediated complement activation, this pathway is an enticing target for effective therapeutic intervention. Nonetheless, although the underlying disease mechanisms of many of these complement-driven diseases are quite well understood, some of the diseases have limited treatment options or no approved treatments at all. Therefore, in this review we explore factor D as a strategic target for advancing therapeutic control of pathological complement activation.

Lectin Pathway Mediates Complement Activation by SARS-CoV-2 Proteins.

Early and persistent activation of complement is considered to play a key role in the pathogenesis of COVID-19. Complement activation products orchestrate a proinflammatory environment that might be critical for the induction and maintenance of a severe inflammatory response to SARS-CoV-2 by recruiting cells of the cellular immune system to the sites of infection and shifting their state of activation towards an inflammatory phenotype. It precedes pathophysiological milestone events like the cytokine storm, progressive endothelial injury triggering microangiopathy, and further complement activation, and causes an acute respiratory distress syndrome (ARDS). To date, the application of antiviral drugs and corticosteroids have shown efficacy in the early stages of SARS-CoV-2 infection, but failed to ameliorate disease severity in patients who progressed to severe COVID-19 pathology. This report demonstrates that lectin pathway (LP) recognition molecules of the complement system, such as MBL, FCN-2 and CL-11, bind to SARS-CoV-2 S- and N-proteins, with subsequent activation of LP-mediated C3b and C4b deposition. In addition, our results confirm and underline that the N-protein of SARS-CoV-2 binds directly to the LP- effector enzyme MASP-2 and activates complement. Inhibition of the LP using an inhibitory monoclonal antibody against MASP-2 effectively blocks LP-mediated complement activation. FACS analyses using transfected HEK-293 cells expressing SARS-CoV-2 S protein confirm a robust LP-dependent C3b deposition on the cell surface which is inhibited by the MASP-2 inhibitory antibody. In light of our present results, and the encouraging performance of our clinical candidate MASP-2 inhibitor Narsoplimab in recently published clinical trials, we suggest that the targeting of MASP-2 provides an unsurpassed window of therapeutic efficacy for the treatment of severe COVID-19.

Development of a Complement-Mediated Hemolysis Assay for Immune Globulin Intravenous Products and Characterization of Lots Associated with Clinical Hemolysis

Introduction: Immune Globulin Intravenous (Human) (IGIV), as known as Intravenous Gamma-Globulin (IVIg), are therapeutic polyclonal immunoglobulin preparations manufactured from pooled human plasma. Initially licensed for replacement therapy of primary immune deficiencies, IGIV indications have expanded to include various autoimmune and inflammatory conditions. Hemolytic reactions associated with IGIV infusion were historically considered as uncommon, but seem to be more frequent over the past 5-6 years, coinciding with widespread manufacturing changes to increase IgG yield, and use of higher dosages. Anti-A and anti-B IgG isoagglutinins in the IGIV preparations are believed to mediate these events. In 2014, FDA co-organized a workshop to discuss strategies to address hemolytic complications of IGIV infusions (Scott 2015). Although US products all comply with an anti-A and anti-B specification of ≤1:64, using the European Pharmacopoeia direct hemagglutination test (DHAT) (Thorp 2010), hemolytic events have persisted. The DHAT is semi-quantitative, imprecise, and does not measure hemolytic function. However, hemolysin tests have been difficult to standardize, especially for highly concentrated IGIV products (Scott 2015).This study reports development of a complement-dependent hemolysis assay (CDHA) suitable for testing IGIV products. We also tested a positive control preparation that is available from NIBSC (code 14/160), consisting of an IGIV lot that caused clinical hemolysis. Finally, we demonstrated differences among IGIV brands with respect to hemolytic activity.Methods: To provide consistent reagents, serum from whole blood donations were frozen as a large volume source of complement and a droplet-freezing method was used to store Red Blood Cells (Schmid 2011; Figure 1). Critical assay conditions were studied, including enzymatic treatment of RBC, removal of unbound IgG, complement source and concentration, divalent cation concentrations, and pH. Hemolytic activities of IGIV products and a positive control IGIV preparation were compared to the hemagglutination results tested using DHAT method. The CDHA was also used to evaluate hemolytic activity of IgG subclasses.Results: The most critical parameters for reproducible CDHA were complement source and preparation method, and assay pH. A pH of 6.5 (0.1M Tri-BIS buffer) was the most suitable condition for all of the tested IGIV analytes. Intra-laboratory variation was minimized by use of frozen RBCs and a consistent highly active complement preparation. The DHAT and CDHA results correlated with each other for most IGIV product lots (Figure 2). Hemolytic activity in IGIV lots varied among, in some cases within, brands. Enrichment of IgG subclass fractions from NIBSC 14/160 showed the highest hemolytic activity was present in IgG2 (Figure 3). In contrast, studies of recombinant anti-A monoclonal antibodies expressing the same anti-A F(ab'), but subclass-specific Fc receptors showed greatest activity of IgG3, consistent with evidence that IgG3 is the most potent complement-activating subclass. The role of IgG2 in hemolysis is not entirely surprising, as saccharide antigens often elicit IgG2 responses, and IgG2 can mediate complement activation (Diebolder 2014). Further subclass studies are planned.Conclusion: Hemolytic activity of IGIV products mediated by anti-A isoagglutinins can be reproducibly measured in IGIV products (Figure 4). Compared to the DHAT method, CDHA provides a continuous readout, is less variable, amenable to use of a standard curve, and more precise. Historical challenges to reproducibility are addressed through biological reagent standardization and attention to critical conditions. Studies comparing the CDHA and DHAT methods across a wider range of products and hemolytic lots will discern whether the CDHA provides a better measure of product safety than the current DHAT lot release assay. The CDHA can be helpful for characterization of new manufacturing methods and products, and investigation of hemolytic events. Studies of hemolysis-implicated lots may provide a basis for improvements in product safety. [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

Fibrinogen and Clot Structure Is Affected By Eculizumab in Patients with Paroxysmal Nocturnal Haemoglobinuria

Background Paroxysmal nocturnal haemoglobinuria (PNH) is a haematopoietic stem cell disorder in which an acquired somatic mutation of the PIGA gene in a haematopoietic stem cell results in the deficiency of key complement regulatory molecules, namely CD59 and CD55, from the surface of blood cells leading to an increased susceptibility of blood cells to complement-mediated activation +/- intravascular haemolysis and thrombosis. Thrombosis has been the leading cause of death in PNH. Multiple mechanisms of thrombosis in PNH have been proposed, including cross talk between the coagulation and complement system. This may lead to denser clots which are more resistant to fibrinolysis. Treatment with eculizumab, which inhibits complement factor C5 and thereby prevents terminal complement activation, reduces thromboembolic events in PNH, indicating that complement activation plays a significant role in the pathophysiology of thrombosis in this disease. Patients with other thrombotic disorders are known to form denser fibrin clots that incur greater resistance to fibrinolysis. It has not previously been explored whether changes to clot architecture contribute to thrombosis in PNH. Furthermore, the role of eculizumab in clot architecture is undocumented.Aims Characterise fibrin clot structure in patients with PNH and determine the effect of eculizumab on clot structure.Methods Informed consent was obtained from all participants. 53 patients from the PNH National Service (Leeds) were recruited. They were grouped according to the proportion of PNH granulocytes, <40% (n=18), 40> - < 90 %(n=18) and >90% (n=17) and reanalysed according to treatment with (n=19) or without (n=32) eculizumab. Plasma samples were obtained and ex vivo fibrin clot structure and function was analysed by permeation analysis of clot pore structure, turbidity analysis of clot formation and fibrinolysis, and fibrin ultrastructure by laser scanning confocal microscopy and scanning electron microscopy, according to methodology and protocols well established in our laboratory. The proportion of PNH granulocytes was measured by flow cytometry and fibrinogen levels by Clauss. P values of <0.05 were taken to indicate significance.Results The proportion of PNH granulocytes correlated with fibrinogen (r= -0.35, p=0.048), maximum absorbency (r= -0.34, p=0.046) and clot density (r= -0.36, p=0.04). Seventy per cent of patients with large proportions of PNH granulocytes were on eculizumab. Analysis of patients not on eculizumab showed a trend towards proportion of PNH granulocytes correlating positively with fibrinogen levels (r=0.42, p=ns). Patients on eculizumab had lower fibrinogen levels (3.1±0.2 v 3.6±0.1g/L, P=0.02), maximum absorbance (0.31±0.1 v 0.40±0.1, p=0.028) and fibre density/100µm (18.5±4.4 v 23.9±4.2, p=0.006), indicative of a less dense clot.Summary/Conclusions This is the first study to demonstrate that patients on eculizumab showed reduced fibrinogen, fibre thickness and fibre density. Our data indicate that the beneficial effects of eculizumab in PNH is partly due to reduced complement activation and attenuated inflammatory response, leading to lower fibrinogen levels and a less thrombotic fibrin clot structure. These novel data help to explain the impact of complement inhibition in this disease. [Display omitted] DisclosuresArnold:Alexion Pharmaceuticals, Inc.: Honoraria. Griffin:Alexion Pharmaceuticals, Inc.: Honoraria. Munir:Janssen: Honoraria; Alexion Pharmaceuticals, Inc.: Honoraria; Roche: Honoraria; AbbVie: Honoraria; Gilled: Honoraria. Hillmen:Gilead: Consultancy, Honoraria, Research Funding; Janssen: Consultancy, Honoraria, Research Funding; Alexion Pharmaceuticals, Inc.: Consultancy, Honoraria; Pharmacyclics LLC, an AbbVie Company: Honoraria, Research Funding; Celgene: Research Funding; AbbVie: Consultancy, Honoraria, Research Funding; Roche: Consultancy, Honoraria, Research Funding; Novartis: Honoraria, Research Funding; GSK: Consultancy, Honoraria, Research Funding. Richards:Alexion Pharmaceuticals, Inc.: Honoraria. Hill:Alexion Pharmaceuticals, Inc.: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees.

PAMPs, DAMPs and SAMPs: Host Glycans are Self‐Associated Molecular Patterns, but subject to Microbial Molecular Mimicry

It has long been known that Pathogen-Associated Molecular Patterns (PAMPs, e.g., lipo-oligosaccharides, LOS) and Danger-Associated Molecular Patterns (DAMPs, e.g., fragments of endogenous hyaluronan) can be glycans or modified glycans. We have more recently emphasized that host glycans can also serve as Self-Associated Molecular Patterns (SAMPs), e.g., sialic acid-bearing glycans that are recognized by CD33-related Siglecs on innate immune cells to dampen responses, or by complement Factor H to dampen complement-mediated activation. However, several commensal or pathogenic organisms have evolved successful molecular mimicry of sialylated SAMPs, so as to hijack these mechanisms and suppress innate immune responses against them. Paired activating CD33rSiglecs have emerged in some lineages, presumably to counter hijacking by microbial molecular mimicry of sialylated SAMPs. Meanwhile, sialylated host SAMPs also have to evolve rapidly to escape from other pathogens that utilize them as targets to initiate infection. The ongoing pressures of these often-competing selection forces can explain our finding that the sialic-acid binding domains of CD33rSiglecs and Factor H show much higher than expected Ka/Ks ratios (an excess of non-synonymous substitutions changing encoded amino acids, over synonymous substitutions). Examples can also be found of lineage-specific complete loss of sialic binding e.g., via mutation of an essential arginine residue in some CD33rSiglec binding pocket––or even whole-sale elimination of the binding domain e.g., alternate splicing out of the amino-terminal V-set domain of CD33/Siglec-3. Such evolutionary changes can be either fixed or polymorphic within a given species. Further complexity arises because CD33rSiglec and Factor H can be directly engaged via protein:protein interactions, both with endogenous proteins such as Hsp70 and Vimentin, and with surface proteins of commensals e.g., the PorB protein of N. gonorrhea. The overall outcome is inter-species or intra-species variations in the intensity of innate immune responses, which can be reflected in relative susceptibility or resistance to infectious agents and/or harmful aggravation of endogenous inflammatory diseases. Perhaps because of the major changes in the sialome resulting from inactivation of the CMAH gene in the Homo lineage leading to humans, these and many other types of evolutionary changes appear to be very prominent in our species and in our pathogen regimes. This presentation will provide an overview of these topics and examples of our recent work in these areas. Varki, A.: Since there are PAMPs and DAMPs, there must be SAMPs? Glycan “self-associated molecular patterns” dampen innate immunity, but pathogens can mimic them. Glycobiology, 21:1121-1124, 2011. Padler-Karavani, V., Hurtado-Ziola, N., Chang, Y.-C., Sonnenburg, J., Ronaghy, A., Yu, H., Verhagen, A., Nizet, V., Chen, X., Varki, N., Varki, A., Angata, T.: Rapid Evolution of Binding Specificities and Expression Patterns of Inhibitory CD33-related Siglecs in Primates. FASEB J. 28:1280-93, 2014. Laubli, H. and Varki, A. Sialic acid–binding immunoglobulin-like lectins (Siglecs) detect self-associated molecular patterns to regulate immune responses. Cell. Mol. Life Sci. Springer, International. 2019.