Complement Attack Research Articles

Alternative pathway activation in pregnancy, a measured amount "complements" a successful pregnancy, too much results in adverse events.

During pregnancy, the maternal host must adapt in order to enable growth of the fetus. These changes affect all organ systems and are designed both to protect the fetus and to minimize risk to the mother. One of the most prominent adaptations involves the immune system. The semi-allogenic fetoplacental unit has non-self components and must be protected against attack from the host. This requires both attenuation of adaptive immunity and protection from innate immune defense mechanisms. One of the key innate immune players is complement, and it is important that the fetoplacental unit is not identified as non-self and subjected to complement attack. Adaptation of the complement response must, however, be managed in such a way that maternal protection against infection is not compromised. As the complement system also plays a significant facilitating role in many of the stages of a normal pregnancy, it is also important that any necessary adaptation to accommodate the semi-allogenic aspects of the fetoplacental unit does not compromise this. In this review, both the physiological role of the alternative pathway of complement in facilitating a normal pregnancy, and its detrimental participation in pregnancy-specific disorders, are discussed.

A Case of PNH with Abdominal Pain, AKI and Hemolytic Anemia

Paroxysmal nocturnal hemoglobinuria (PNH) is a rare hematopoietic stem cell disorder affecting 1 to 10 people per million. With an associated 10-year mortality, PNH is a life-threatening disorder where the complement system of the body's innate immune system prematurely lyses red blood cells. It can be characterized by a number of clinical features, the most common being hemolytic anemia, pancytopenia, and thrombosis. 45 year old African American male presented with a chief complaint of shortness of breath x 2-3 days. Reports associated dry, nonproductive cough, congestion, myalgias onset one week prior. He was taking two tablets of Advil daily since symptom onset. He has a past medical history of right sided BKA after motor vehicle accident and currently uses a prosthetic leg. No other significant past medical history noted. On ED admission, the patient was febrile to 103.2F, tachycardic to 110 beats per minute, tachypneic to 26 breaths per minute, and normotensive to 128/64. On physical exam, there was conjunctival pallor, no scleral icterus, and right BKA. The remainder of the physical exam was unremarkable. Labs on admission were notable for anemia (Hgb 3.9, baseline of 9.2, 2 years prior), AKI (Cr 7.3, baseline of 0.9), elevated LDH 2336, and low Haptoglobin <10. Platelet count WNL throughout admission however there was a drop from 242 to 180 between Day 1 and 2 of admission. Peripheral smear did not show schistocytes however ADAMTS13 sent per heme/onc consult. Anemia improved to Hgb 7.6 on Day 3 of admission status post 4 units packed red blood cell transfusion. Workup for hemolytic anemia with AKI was pursued: Direct Coombs test negative, PNH screen sent. SPEP and UPEP unremarkable. On Day 6 of admission, flow cytometry showed evidence of PNH clone upon analysis of RBCs, granulocytes, and monocytes. AKI was attributed to ATN s/t heme-pigment deposition vs NSAID use and Cr trended down to 2.9 prior to discharge. On day 7 of admission, patient developed severe abdominal pain, not associated with fevers, chills, nausea, vomiting, or diarrhea. Pain moderately improved with Norco, ultrasound imaging was negative, and unable to CTA given renal function. Pt was started empirically on therapeutic anticoagulation with Lovenox, as the literature reports a high incidence of venous and/or arterial thrombosis in PNH w/granulocyte count >50% (89.8% in this patient). Patient was discharged the next day with close outpatient hematology follow-up with plan to start eculizumab. Given patient's presentation of fever, symptomatic anemia, and AKI the differential diagnosis included: hemoglobinopathies, red cell membrane disorders, PNH, DIC, TTP, HUS, AIHA, and infections such as malaria or HIV. It was difficult to identify PNH as the cause in this patient on ED presentation as there was no reported urine discoloration. Though classified as an acquired disorder through mutation of a gene on the X chromosome, it occurs in equal distribution among men and women. PNH specifically occurs through the mutation of a gene involved in the synthesis of glycosylphosphatidylinositol (GPI), an anchor protein for complement regulatory proteins that prevent complement-induced hemolysis of red blood cells. The resulting GPI deficiency leads to the unregulated formation of complement attack complexes that damage the cell membrane causing them to lyse. This intravascular hemolysis is seen as decreased levels of Hgb and hemoglobinuria that leads to elevated levels of LDH as seen with this patient. The degree of hemolysis and PNH clone size can increase risk of thrombosis, the leading cause of death in patients with PNH. Strong indicators for the start of PNH therapy are severe anemia, thrombosis, renal dysfunction, and dyspnea. The typical management of PNH in patients with thrombosis is shown in this case as the patient is put on therapeutic anticoagulation and plans for eculizumab, a monoclonal antibody that is a complement inhibitor used to prevent hemolysis. This PNH therapy will likely be administered lifelong and decrease the need for RBC transfusions as well as decrease thromboembolic rate. If the patient is unresponsive to eculizumab they become a strong candidate for allogeneic stem transplantation. This patient meets the popular standard for starting primary prophylaxis for management of thrombosis with a PNH clone size >50%.

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.

Mapping of the Complement C9 Binding Region on Clonorchis sinensis Paramyosin.

Heliminthic paramyosin is a multifunctional protein that not only acts as a structural protein in muscle layers but as an immune-modulatory molecule interacting with the host immune system. Previously, we found that paramyosin from Clonorchis sinensis (CsPmy) is bound to human complement C9 protein (C9). To analyze the C9 binding region on CsPmy, overlapping recombinant fragments of CsPmy were produced and their binding activity to human C9 was investigated. The fragmental expression of CsPmy and C9 binding assays revealed that the C9 binding region was located at the C-terminus of CsPmy. Further analysis of the C-terminus of CsPmy to narrow the C9 binding region on CsPmy indicated that the region flanking 731Leu–780Leu was a potent C9 binding region. The CsPmy fragments corresponding to the region effectively inhibited human C9 polymerization. These results provide a precise molecular basis for CsPmy as a potent immunomodulator to evade host immune defenses by inhibiting complement attack.

Effects of the complement system on antibody formation and function: implications for transplantation.

In antibody-mediated allograft rejection, donor-reactive antibodies cause transplant injury in part via complement activation. New mechanistic insights indicate complement also modulates development of humoral immune responses. Herein we review recent data that describes how complement affects antibody formation and we discuss therapeutic implications. Extravasating T cells interacting with integrins express and activate intracellular complement that drives immune-metabolic adaptations vital for CD4 + helper cells. Marginal zone B cells can acquire intact major histocompatibility complexes from dendritic cells via complement-dependent trogocytosis for presentation to T cells. Activated B cells in germinal centers receive co-stimulatory signals from T-helper cells. These germinal center B cells undergo coordinate shifts in surface complement regulator expression that permit complement receptor signaling on the germinal center B cells required for affinity maturation. The positively selected, high-affinity B cells can differentiate into plasma cells that produce donor-HLA-reactive antibodies capable of ligating endothelial, among other, graft cells. Subsequent sublytic complement attack can stimulate endothelial cells to activate CD4 + and CD8 + T cells, promoting cellular and humoral rejection. Newly developed complement inhibitors are being tested to prevent/treat transplant rejection. The complement system influences T-cell, B-cell and endothelial-cell activation, and thereby contributes allograft injury. Emerging therapeutic strategies targeting complement activation have the potential to prevent or abrogate transplant injury and improve transplant outcomes.

Ex Vivo Test for Measuring Complement Attack on Endothelial Cells: From Research to Bedside

As part of the innate immune system, the complement system plays a key role in defense against pathogens and in host cell homeostasis. This enzymatic cascade is rapidly triggered in the presence of activating surfaces. Physiologically, it is tightly regulated on host cells to avoid uncontrolled activation and self-damage. In cases of abnormal complement dysregulation/overactivation, the endothelium is one of the primary targets. Complement has gained momentum as a research interest in the last decade because its dysregulation has been implicated in the pathophysiology of many human diseases. Thus, it appears to be a promising candidate for therapeutic intervention. However, detecting abnormal complement activation is challenging. In many pathological conditions, complement activation occurs locally in tissues. Standard routine exploration of the plasma concentration of the complement components shows values in the normal range. The available tests to demonstrate such dysregulation with diagnostic, prognostic, and therapeutic implications are limited. There is a real need to develop tools to demonstrate the implications of complement in diseases and to explore the complex interplay between complement activation and regulation on human cells. The analysis of complement deposits on cultured endothelial cells incubated with pathologic human serum holds promise as a reference assay. This ex vivo assay most closely resembles the physiological context. It has been used to explore complement activation from sera of patients with atypical hemolytic uremic syndrome, malignant hypertension, elevated liver enzymes low platelet syndrome, sickle cell disease, pre-eclampsia, and others. In some cases, it is used to adjust the therapeutic regimen with a complement-blocking drug. Nevertheless, an international standard is lacking, and the mechanism by which complement is activated in this assay is not fully understood. Moreover, primary cell culture remains difficult to perform, which probably explains why no standardized or commercialized assay has been proposed. Here, we review the diseases for which endothelial assays have been applied. We also compare this test with others currently available to explore complement overactivation. Finally, we discuss the unanswered questions and challenges to overcome for validating the assays as a tool in routine clinical practice.

Screening of Surface-Exposed Lipoproteins of Leptospira Involved in Modulation of Host Innate Immune Response.

Leptospira, a zoonotic pathogen, is capable of causing both chronic and acute infection in a susceptible host. Surface-exposed lipoproteins play a major role in modulating the host immune response by activating the innate cells like macrophages and dendritic cells or evading complement attack and killing by phagocytes like neutrophils to favor pathogenesis and establish infection. In this study, we screened some surface-exposed lipoproteins known to be involved in pathogenesis to assess their possible role in immune modulation (innate immune activation or evasion). Surface proteins of the Len family (LenB, LenD, and LenE), Lsa30, Loa22, and Lipl21 were purified in recombinant form and then tested for their ability to activate macrophages of the different host (mouse, human, and bovine). These proteins were tested for binding with complement regulators like Factor H (FH), C4 Binding Protein (C4BP), and host protease Plasminogen (PLG) and also as nucleases to access their possible role in innate immune evasion. Our results show that, of various proteins tested, Loa22 induced strong innate activation and Lsa30 was least stimulatory, as evident from the production of pro-inflammatory cytokines (interleukin-6 and tumor necrosis factor–α) and expression of surface markers [CD80, CD86, and major histocompatibility complex class II (MHCII)]. All the tested proteins were able to bind to FH, C4BP, and PLG; however, Loa22 showed strong binding to PLG correlating to plasmin activity. All the proteins except Loa22 showed nuclease activity, albeit with a requirement of different metal ions. The nuclease activity of these proteins correlated to in vitro degradation of neutrophil extracellular trap (NET). In conclusion, our results indicate that these surface proteins are involved in innate immune modulation and may play a critical role in assisting the bacteria in invading and colonizing the host tissue for persistent infection.

A synthetic protein as efficient multitarget regulator against complement over-activation

The complement system constitutes the innate defense against pathogens. Its dysregulation leads to diseases and is a critical determinant in many viral infections, e.g., COVID-19. Factor H (FH) is the main regulator of the alternative pathway of complement activation and could be a therapy to restore homeostasis. However, recombinant FH is not available. Engineered FH versions may be alternative therapeutics. Here, we designed a synthetic protein, MFHR13, as a multitarget complement regulator. It combines the dimerization and C5-regulatory domains of human FH-related protein 1 (FHR1) with the C3-regulatory and cell surface recognition domains of human FH, including SCR 13. In summary, the fusion protein MFHR13 comprises SCRs FHR11-2:FH1-4:FH13:FH19-20. It protects sheep erythrocytes from complement attack exhibiting 26 and 4-fold the regulatory activity of eculizumab and human FH, respectively. Furthermore, we demonstrate that MFHR13 and FHR1 bind to all proteins forming the membrane attack complex, which contributes to the mechanistic understanding of FHR1. We consider MFHR13 a promising candidate as therapeutic for complement-associated diseases.

Combating Complement's Deleterious Effects on Nanomedicine by Conjugating Complement Regulatory Proteins to Nanoparticles (Adv. Mater. 8/2022)

Nanomedicine Immediately upon encountering blood, nanoparticles are attacked by complement proteins. Complement binding produces an anaphylaxis-like reaction and causes nanoparticles to be cleared from circulation by phagocytes. To defend nanoparticles, in article number 2107070, Jacob W. Myerson, Jacob S. Brenner, and co-workers demonstrate how to conjugate nanoparticles to humans' natural complement inhibitor, Factor I, which defends the nanoparticle from complement attack, thus ameliorating all the above problems.

Coronavirus disease 2019-induced hypercoagulability and its clinical implications.

Coronavirus disease 2019 is the disease produced by severe acute respiratory syndrome-coronavirus-2, which is introduced into the host's cell thanks to the angiotensin-converting enzyme 2 receptor. Once there, it uses the cell's machinery to multiply itself. In this process, it generates an immune response that stimulates the lymphocytes to produce cytokines and reactive oxygen species that begin to deteriorate the endothelial cell. Complement activation, through the complement attack complex and C5a, contributes to this endothelial damage. The different mediators further promote the expression of adhesion molecules on the endothelial surface, which encourages all blood cells to adhere to the endothelial surface to form small conglomerates, called clots, which obstruct the lumen of the small blood vessels. Furthermore, the mediators of clot lysis are inhibited. All this promotes a prothrombotic environment within the pulmonary capillaries that is reflected in the elevation of D-dimer. The only solution for this cascade of events seems to be the implementation of an effective anticoagulation protocol that early counteracts the changes induced by thrombi in the pulmonary circulation and reflected in the functioning of the right ventricle.

Total coumarin derivates from Hydrangea paniculata attenuate renal injuries in cationized-BSA induced membranous nephropathy by inhibiting complement activation and interleukin 10-mediated interstitial fibrosis

BackgroundTotal coumarins extracted from Hydrangea. Paniculata, Sieb (HP) have showed renal protective effect in several experimental acute and chronic kidney diseases. PurposeThe aim of current study is to evaluate renal protective effect of HP against cationized-BSA (c-BSA) induced experimental membranous nephritis (MN), and further investigate its underlying mechanisms. MethodsRat MN model was established by intravenous injection of 5 mg c-BSA for consecutive 14 days, and after albuminuria confirmed, HP was orally administrated with 7.5, 15, 30 mg/kg for nine weeks. The renal function was measured and histopathological injuries were observed. RNA sequencing was used to analyze the altered signaling pathways in kidneys. Pharmacokinetics was performed to investigate the pharmacodynamics of major ingredients in HP and possible metabolites. Discover X platform helped to clarify the possible molecular mechanisms of major compound in HP. ResultsHP administration could significantly improve the renal function, and ameliorate the dyslipidemia and histopathological injuries. mRNA sequencing demonstrated that HP had anti-inflammation and anti-fibrosis effects possible through down-regulating the complement activation and PI3K-AKT pathways. Pharmacokinetics demonstrated that skimmin and 7-hydoxycoumarin (7-HC) were major compound or metabolite in plasma after oral administration. Based on Discover X platform, we confirmed that skimmin and 7-HC inhibited the IL10 production by inflammatory macrophages through blocking PI3K-AKT and NFκB signaling pathways. Finally, we demonstrated that HP protected tubulointerstitium from complement attack by reducing the C3 self-production and auto-cleavage in tubular cells. ConclusionsHP has a renal protective effect, and its drug development may provide one alternative strategy to treat immune-mediated nephropathy.

The Protease SplB of Staphylococcus aureus Targets Host Complement Components and Inhibits Complement-Mediated Bacterial Opsonophagocytosis.

ABSTRACTStaphylococcus aureus is an opportunistic pathogen that can cause life-threatening infections, particularly in immunocompromised individuals. The high-level virulence of S. aureus largely relies on its diverse and variable collection of virulence factors and immune evasion proteins, including the six serine protease-like proteins SplA to SplF. Spl proteins are expressed by most clinical isolates of S. aureus, but little is known about the molecular mechanisms by which these proteins modify the host’s immune response for the benefit of the bacteria. Here, we identify SplB as a protease that inactivates central human complement proteins, i.e., C3, C4, and the activation fragments C3b and C4b, by preferentially cleaving their α-chains. SplB maintained its proteolytic activity in human serum, degrading C3 and C4. SplB further cleaved the components of the terminal complement pathway, C5, C6, C7, C8, and C9. In contrast, the important soluble human complement regulators factor H and C4b-binding protein (C4BP), as well as C1q, were left intact. Thereby, SplB reduced C3b-mediated opsonophagocytosis by human neutrophils as well as C5b-9 deposition on the bacterial surface. In conclusion, we identified the first physiological substrates of the S. aureus extracellular protease SplB. This enzyme inhibits all three complement pathways and blocks opsonophagocytosis. Thus, SplB can be considered a novel staphylococcal complement evasion protein.IMPORTANCE The success of bacterial pathogens in immunocompetent humans depends on the control and inactivation of host immunity. S. aureus, like many other pathogens, efficiently blocks host complement attack early in infection. Aiming to understand the role of the S. aureus-encoded orphan proteases of the Spl operon, we asked whether these proteins play a role in immune escape. We found that SplB inhibits all three complement activation pathways as well as the lytic terminal complement pathway. This blocks the opsonophagocytosis of the bacteria by neutrophils. We also clarified the molecular mechanisms: SplB cleaves the human complement proteins C3, C4, C5, C6, C7, C8, and C9 as well as factor B but not the complement inhibitors factor H and C4BP. Thus, we identify the first physiological substrates of the extracellular protease SplB of S. aureus and characterize SplB as a novel staphylococcal complement evasion protein.

Immune Landscape of CMV Infection in Cancer Patients: From "Canonical" Diseases Toward Virus-Elicited Oncomodulation.

Human Cytomegalovirus (HCMV) is an immensely pervasive herpesvirus, persistently infecting high percentages of the world population. Despite the apparent robust host immune responses, HCMV is capable of replicating, evading host defenses, and establishing latency throughout life by developing multiple immune-modulatory strategies. HCMV has coexisted with humans mounting various mechanisms to evade immune cells and effectively win the HCMV-immune system battle mainly through maintaining its viral genome, impairing HLA Class I and II molecule expression, evading from natural killer (NK) cell-mediated cytotoxicity, interfering with cellular signaling, inhibiting apoptosis, escaping complement attack, and stimulating immunosuppressive cytokines (immune tolerance). HCMV expresses several gene products that modulate the host immune response and promote modifications in non-coding RNA and regulatory proteins. These changes are linked to several complications, such as immunosenescence and malignant phenotypes leading to immunosuppressive tumor microenvironment (TME) and oncomodulation. Hence, tumor survival is promoted by affecting cellular proliferation and survival, invasion, immune evasion, immunosuppression, and giving rise to angiogenic factors. Viewing HCMV-induced evasion mechanisms will play a principal role in developing novel adapted therapeutic approaches against HCMV, especially since immunotherapy has revolutionized cancer therapeutic strategies. Since tumors acquire immune evasion strategies, anti-tumor immunity could be prominently triggered by multimodal strategies to induce, on one side, immunogenic tumor apoptosis and to actively oppose the immune suppressive microenvironment, on the other side.

Uridine diphosphate-N-acetylglucosamine-2-epimerase/N-acetylmannosamine kinase deletion in mice leads to lethal intracerebral hemorrhage during embryonic development

Among the enzymes of the biosynthesis of sialoglycoconjugates, uridine diphosphate-N-acetylglucosamine-2-epimerase/N-acetylmannosamine kinase (GNE), catalyzing the first essential step of the sialic acid (Sia) de novo biosynthesis, and cytidine monophosphate (CMP)-Sia synthase (CMAS), activating Sia to CMP-Sia, are particularly important. The knockout of either of these enzymes in mice is embryonically lethal. While the lethality of Cmas-/- mice has been attributed to a maternal complement attack against asialo fetal placental cells, the cause of lethality in Gne-deficient embryos has remained elusive. Here, we advanced the significance of sialylation for embryonic development through detailed histological analyses of Gne-/- embryos and placentae. We found that Gne-/- embryonic and extraembryonic tissues are hyposialylated rather than being completely deficient of sialoglycans, which holds true for Cmas-/- embryos. Residual sialylation of Gne-/- cells can be explained by scavenging free Sia from sialylated maternal serum glycoconjugates via the lysosomal salvage pathway. The placental architecture of Gne-/- mice was unaffected, but severe hemorrhages in the neuroepithelium with extensive bleeding into the cephalic ventricles were present at E12.5 in the mutants. At E13.5, the vast majority of Gne-/- embryos were asystolic. This phenotype persisted when Gne-/- mice were backcrossed to a complement component 3-deficient background, confirming distinct pathomechanisms of Cmas-/- and Gne-/- mice. We conclude that the low level of sialylation observed in Gne-/- mice is sufficient both for immune homeostasis at the fetal-maternal interface and for embryonic development until E12.5. However, formation of the neural microvasculature is the first critical process, depending on a higher degree of sialylation during development of the embryo proper.

Abstract LB067: Development of a novel bispecific antibody targeting PD-L1 and CD55 for cancer therapy

Abstract Background: Immune checkpoint interaction helps tumors resist immunity-induced apoptosis. Blocking PD-1/PD-L1 leads to remarkable clinical responses; however, a large portion of patients develop acquired resistance after initial therapy. Many tumor-specific antigens (TSAs) and tumor-associated antigens (TAAs) have been identified. TSAs are restricted to cancer cells only but TAAs are elevated on tumor cells and minimally expressed on healthy cells. Therefore, co-targeting TAAs/TSAs would show benefits over monotherapy. CD55 is a TAA that regulates complement activation pathway and expressed on cells exposed to the complement system to protect against complement attack. Cancer cells escape this attack by raising CD55 expression. Additionally, CD55 acts as a virus receptor for internalization. Based on the roles of CD55, we designed PD-L1/CD55 bispecific antibody (GB262) with weaker CD55 binding arm to rule out the possibility of lethality to healthy cells but maintain its CD55 binding and internalization ability specifically to cancer cells. Moreover, co-binding to PD-L1 and CD55 leads to co-internalization of PD-L1 by CD55, enhancing the blocking of PD-1/PD-L1 interaction. Methods: Co-localization and levels of PD-L1 and CD55 on cancer cells were analyzed by microscopy and FACS. T-cell activation was studied using PD-L1+/CD55+ cells and PBMCs. CD97 and CD55 blocking was performed using FACS. Cancer killing, ADCC and CDC were performed by culturing PDL-1+/CD55+ cells with PBMCs, NK cells and human complement serum (HCS), respectively. GB262 internalization and its effects on intracellular PD-L1 were studied by pHrodo and western blot. Saporin-conjugated GB262 efficacy was evaluated by cell binding, internalization and cancer killing. In vivo study was performed by engrafting PBMCs and PANC-1 in B-NDG mice. Results: PD-L1 significantly co-localized with CD55 on cancer cells from different indications. While high PD-L1 binding was maintained, a significant reduction in CD55 binding was observed by GB262 to their receptors. GB262 also blocked the interaction between CD97 and CD55 more efficiently than monoclonals. GB262 induced stronger T-cell activation and cancer killing in presence of PBMCs. Similarly, higher level of ADCC and CDC were also evident by GB262. The degree of GB262 internalization was CD55 level dependent on cells surface and leads to intracellular PD-L1 degradation. When conjugated to saporin, GB262-saporin significantly induced cancer killing in absence of effector cells or HCS, whereas GB262 or saporin had no effect. Both GB262 and GB262-saporin suppressed tumor growth in B-NDG mice models. Conclusion: GB262 is the first bispecific antibody that not only releases cancer repression on T-cell activation, but also releases cancer repression on CDC. By conjugating to a toxin, GB262 could serve as an ADC to target multiple cancer cells. GB262 has potential to serve as a novel therapy for many cancers. Citation Format: Amit K. Chaudhary, Jiadong Shi, Wenyan Cai, Bo Wang, Mohd S. Zaman, Cai Huang, Jun Lin, Steven Z. Kan, Joe Zhou, Jianbo Dong, Yue Liu. Development of a novel bispecific antibody targeting PD-L1 and CD55 for cancer therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr LB067.

Complement Decay-Accelerating Factor is a modulator of influenza A virus lung immunopathology.

Clearance of viral infections, such as SARS-CoV-2 and influenza A virus (IAV), must be fine-tuned to eliminate the pathogen without causing immunopathology. As such, an aggressive initial innate immune response favors the host in contrast to a detrimental prolonged inflammation. The complement pathway bridges innate and adaptive immune system and contributes to the response by directly clearing pathogens or infected cells, as well as recruiting proinflammatory immune cells and regulating inflammation. However, the impact of modulating complement activation in viral infections is still unclear. In this work, we targeted the complement decay-accelerating factor (DAF/CD55), a surface protein that protects cells from non-specific complement attack, and analyzed its role in IAV infections. We found that DAF modulates IAV infection in vivo, via an interplay with the antigenic viral proteins hemagglutinin (HA) and neuraminidase (NA), in a strain specific manner. Our results reveal that, contrary to what could be expected, DAF potentiates complement activation, increasing the recruitment of neutrophils, monocytes and T cells. We also show that viral NA acts on the heavily sialylated DAF and propose that the NA-dependent DAF removal of sialic acids exacerbates complement activation, leading to lung immunopathology. Remarkably, this mechanism has no impact on viral loads, but rather on the host resilience to infection, and may have direct implications in zoonotic influenza transmissions.

Hijacking Factor H for Complement Immune Evasion.

The complement system is an essential player in innate and adaptive immunity. It consists of three pathways (alternative, classical, and lectin) that initiate either spontaneously (alternative) or in response to danger (all pathways). Complement leads to numerous outcomes detrimental to invaders, including direct killing by formation of the pore-forming membrane attack complex, recruitment of immune cells to sites of invasion, facilitation of phagocytosis, and enhancement of cellular immune responses. Pathogens must overcome the complement system to survive in the host. A common strategy used by pathogens to evade complement is hijacking host complement regulators. Complement regulators prevent attack of host cells and include a collection of membrane-bound and fluid phase proteins. Factor H (FH), a fluid phase complement regulatory protein, controls the alternative pathway (AP) both in the fluid phase of the human body and on cell surfaces. In order to prevent complement activation and amplification on host cells and tissues, FH recognizes host cell-specific polyanionic markers in combination with complement C3 fragments. FH suppresses AP complement-mediated attack by accelerating decay of convertases and by helping to inactivate C3 fragments on host cells. Pathogens, most of which do not have polyanionic markers, are not recognized by FH. Numerous pathogens, including certain bacteria, viruses, protozoa, helminths, and fungi, can recruit FH to protect themselves against host-mediated complement attack, using either specific receptors and/or molecular mimicry to appear more like a host cell. This review will explore pathogen complement evasion mechanisms involving FH recruitment with an emphasis on: (a) characterizing the structural properties and expression patterns of pathogen FH binding proteins, as well as other strategies used by pathogens to capture FH; (b) classifying domains of FH important in pathogen interaction; and (c) discussing existing and potential treatment strategies that target FH interactions with pathogens. Overall, many pathogens use FH to avoid complement attack and appreciating the commonalities across these diverse microorganisms deepens the understanding of complement in microbiology.

Evaluation of Combinatory Effects of Plasmodium Circumsporozoite Protein and Complement Regulatory Protein Expression of Recombinant Baculovirus Vectors.

Baculovirus vectors (BVs) are safely able to transduce foreign genes and express them in mammalian cells. However, the transduction activity of BVs is strongly reduced by the attack of serum complement, which is one of the major obstacles in the use of BVs for in vivo gene transfer. One strategy to overcome this problem is the display of complement regulatory proteins (CRPs) on BV virions. We previously developed CD46-decay accelerating factor (DAF)-CD59 triple fusion type BV showing potent complement resistance. We also developed BVs expressing Plasmodium circumsporozoite protein (CSP) to enhance transduction efficacy in hepatic cells. In this study, we investigated the combination of CSP and CRPs in a BV system to evaluate transduction efficacy along with complement resistance. To accomplish the combination of CSP and CRPs, we generated insect Sf9 cells stably expressing CRPs, to which CSP type BV was infected. The BVs collected from these infected cells were confirmed to possess both CSP and CRPs in virions. We demonstrated that CSP-CD46-DAF-CD59 type BV, containing both CSP and CD46-DAF-CD59, showed a significant increase in transduction efficacy in human hepatoma HepG2 cells under intact serum exposure compared with control type BV or CSP type BV, retaining both advantages of CSP and CD46-DAF-CD59. Collectively, these results demonstrated that the utilization of stably expressing Sf9 cells to introduce the protein products of interest, e.g., CRPs into BVs, would be useful strategy to generate BVs with novel functions such as resistance against serum complement attack.

C4b Binding Protein Acts as an Innate Immune Effector Against Influenza A Virus.

C4b Binding Protein (C4BP) is a major fluid phase inhibitor of the classical and lectin pathways of the complement system. Complement inhibition is achieved by binding to and restricting the role of activated complement component C4b. C4BP functions as a co-factor for factor I in proteolytic inactivation of both soluble and cell surface-bound C4b, thus restricting the formation of the C3-convertase, C4b2a. C4BP also accelerates the natural decay/dissociation of the C3 convertase. This makes C4BP a prime target for exploitation by pathogens to escape complement attack, as seen in Streptococcus pyogenes or Flavivirus. Here, we examined whether C4BP can act on its own in a complement independent manner, against pathogens. C4BP bound H1N1 and H3N2 subtypes of Influenza A Virus (IAV) most likely via multiple sites in Complement Control Protein (CCP) 1-2, 4-5, and 7-8 domains of its α-chain. In addition, C4BP CCP1-2 bound H3N2 better than H1N1. C4BP bound three IAV envelope proteins: Haemagglutinin (~70 kDa), Neuraminidase (~55 kDa), and Matrix protein 1 (~25kDa). C4BP suppressed H1N1 subtype infection into the lung epithelial cell line, A549, while it promoted infection by H3N2 subtype. C4BP restricted viral entry for H1N1 but had the opposite effect on H3N2, as evident from experiments using pseudo-typed viral particles. C4BP downregulated mRNA levels of pro-inflammatory IFN-α, IL-12, and NFκB in the case of H1N1, while it promoted a pro-inflammatory immune response by upregulating IFN- α, TNF-α, RANTES, and IL-6 in the case of H3N2. We conclude that C4BP differentially modulates the efficacy of IAV entry, and hence, replication in a target cell in a strain-dependent manner, and acts as an entry inhibitor for H1N1. Thus, CCP containing complement proteins such as factor H and C4BP may have additional defense roles against IAV that do not rely on the regulation of complement activation.

Hemolytic Tests Exploring Factor H Functional Activities.

Impairment of the complement regulatory protein Factor H (FH) is implicated in the physiopathological mechanisms of different diseases like atypical hemolytic and uremic syndrome and C3 glomerulopathies. It may be due to genetic abnormalities or acquired with the development of autoantibodies. FH has several ligands; therefore, the exploration of its functions requires to perform different tests. Among them, two hemolytic tests are very useful because they give specific and complementary information about FH functions. The first one is dedicated to explore the FH capacity to dissociate the alternative pathway C3 convertase, whereas the second one is designed to explore the capacity of FH to bind cell surfaces and to protect them from complement attack. This chapter describes the procedures to perform these two hemolytic tests, exploring in a complementary way the FH functionality.