C3b Inactivator Research Articles

Staphylococcal protein Ecb impairs complement receptor-1 mediated recognition of opsonized bacteria

Staphyloccus aureus is a major human pathogen leading frequently to sepsis and soft tissue infections with abscesses. Multiple virulence factors including several immune modulating molecules contribute to its survival in the host. When S. aureus invades the human body, one of the first line defenses is the complement system, which opsonizes the bacteria with C3b and attract neutrophils by release of chemotactic peptides. Neutrophils express Complement receptor-1 [CR1, CD35) that interacts with the C3b-opsonized particles and thereby plays an important role in pathogen recognition by phagocytic cells. In this study we observed that a fraction of S. aureus culture supernatant prevented binding of C3b to neutrophils. This fraction consisted of S. aureus leukocidins and Efb. The C-terminus of Efb is known to bind C3b and shares significant sequence homology to the extracellular complement binding protein [Ecb). Here we show that S. aureus Ecb displays various mechanisms to block bacterial recognition by neutrophils. The presence of Ecb blocked direct interaction between soluble CR1 and C3b and reduced the cofactor activity of CR1 in proteolytic inactivation of C3b. Furthermore, Ecb could dose-dependently prevent recognition of C3b by cell-bound CR1 that lead to impaired phagocytosis of NHS-opsonized S. aureus. Phagocytosis was furthermore reduced in the presence of soluble CR1 [sCR1). These data indicate that the staphylococcal protein Ecb prevents recognition of C3b opsonized bacteria by neutrophil CR1 leading to impaired killing by phagocytosis and thereby contribute to immune evasion of S. aureus.

Characterization of C3 in C3 glomerulopathy.

C3 glomerulopathy (C3G) is caused by overactivity of the alternative pathway of complement that results in bright glomerular C3 staining with minimal or no deposition of immunoglobulins on immunofluorescence microscopy. Laser microdissection and mass spectrometry of the two subtypes, C3 glomerulonephritis (C3GN) and dense deposit disease (DDD), have identified C3 as the predominant glomerular complement protein, although lesser amounts of C9, C5, C6, C7 and C8 are detectable. C3 plays a central role in complement activity, with its proteolytic cleavage first generating C3a and C3b, followed by inactivation of C3b generating iC3b (which includes C3α and C3β), which undergoes further breakdown yielding C3c and terminal breakdown fragment C3dg. The composition of C3 breakdown products in C3G is not known. In this study, we chose six cases each of C3GN and DDD to analyze the composition of C3 deposits. We analyzed the amino acid sequence of C3 spectra detected by mass spectrometry to determine the relative abundance of C3 fragments in C3G. Thus we were able to determine the amino acid sequences mapping to the various C3 activation products including C3dg, C3α (C3α1 and α2), and C3β that are part of C3b/iC3b/C3c. C3dg is the predominant cleavage product detected with the highest amino acid coverage. The remaining amino acids map to C3α (C3α1 and α2) and C3β. Amino acids mapping to C3a and C3f are absent. Taken together, the C3α and C3β amino acids represent iC3b prior to or after C3c cleavage of C3dg. The C3 spectra for both C3GN and DDD are surprisingly similar. The finding of large amounts of C3dg suggests that C3b deposition in the glomerulus is an active process triggered by thioester binding of C3b to the glycocalyx overlying the glomerular endothelial cells and glomerular basement membrane. Regulatory protein-mediated inactivation of C3b results in the generation of iC3b. After additional cleavages, mostly C3dg remains.

Interaction of uromodulin and complement factor H enhances C3b inactivation.

Recent studies suggest that uromodulin plays an important role in chronic kidney diseases. It can interact with several complement components, various cytokines and immune system cells. Complement factor H (CFH), as a regulator of the complement alternative pathway, is also associated with various renal diseases. Thus, we have been suggested that uromodulin regulates complement activation by interacting with CFH during tubulointerstitial injury. We detected co‐localization of uromodulin and CFH in the renal tubules by using immunofluorescence. Next, we confirmed the binding of uromodulin with CFH in vitro and found that the affinity constant (KD) of uromodulin binding to CFH was 4.07 × 10−6M based on surface plasmon resonance results. The binding sites on CFH were defined as the short consensus repeat (SCR) units SCR1–4, SCR7 and SCR19–20. The uromodulin‐CFH interaction enhanced the cofactor activity of CFH for factor I‐mediated cleavage of C3b to iC3b. These results indicate that uromodulin plays a role via binding and enhancing the function of CFH.

Streptococcus pyogenes Employs Strain-dependent Mechanisms of C3b Inactivation to Inhibit Phagocytosis and Killing of Bacteria

Evasion of complement-mediated opsonophagocytosis enables group A Streptococcus pyogenes (GAS) to establish infection. Different strain-dependent mechanisms are employed by the host to accomplish this goal. In general, GAS inhibits the amplification of the complement cascade on its cell surface by facilitating the degradation of C3b, an opsonin, to an inactive product, inactivated C3b (iC3b), in a step catalyzed by factor I (FI) and its cofactor, factor H (FH), with or without the participation of human host plasmin (hPm). GAS recruits FH to its cell surface via FH receptors, which are transcriptionally controlled by the two-component cluster of virulence responder-sensor system. The manner in which FI-FH and hPm function together on GAS cells is unknown. Using GAS strain AP53, which strongly binds host human plasminogen/plasmin (hPg/hPm) directly via an hPg/hPm surface receptor (PAM), we show that both FI-FH and hPm sequentially cleave C3b. Whereas FI-FH proteolytically cleaves C3b into iC3b, PAM-bound hPm catalyzes cleavage of iC3b into multiple smaller peptides. Unlike AP53, GAS strain M23ND weakly binds FH and recruits hPg/hPm to its cell surface indirectly via fibrinogen bound to M-protein, M23. In this case, FH-FI cleaves C3b into iC3b, with negligible degradation of iC3b by hPm that is bound to fibrinogen on the cells. AP53 and M23ND display similar resistance to human neutrophil-mediated phagocytosis, which results in a corresponding high lethality in mice after injection of these cells. These results suggest that GAS utilizes diverse mechanisms to degrade C3b and thus to protect bacterial cells from the complement response of the host.

A Novel Case of Direct Antiglobulin Test-Negative Intravascular Hemolysis in a Patient with Smoldering Myeloma

▪Objectives:1. Describe a case of severe DAT-negative intravascular hemolysis in plasma cell dyscrasia.2. Discuss a potential novel mechanism of light-chain mediated hemolysis.A 34-year old woman was admitted to hospital with fatigue and severe iron deficiency anemia (hemoglobin 47 g/dL, MCV 59 fL, ferritin 2 mcg/L). Her medical history included a presumptive diagnosis of paroxysmal nocturnal hemoglobinuria (PNH) from five years prior. She was transfused 2 units of red cells, started on oral iron and folate, and was discharged symptom-free with a hemoglobin of 71 g/dL. She returned three days later with abdominal pain, dark urine, and evidence of intravascular hemolysis. She was admitted for empiric treatment of PNH with high-dose glucocorticoids and therapeutic enoxaparin for presumed intra-abdominal thrombosis. Her flow cytometry, including granulocytes, was negative for PNH. Her direct antiglobulin test (DAT) was negative for IgG antibodies but positive for C3 complement. A thorough hemolysis workup was negative, including schistocytes and Donath Landsteiner testing. ADAMTS13 testing was uninterpretable due to high plasma free hemoglobin.Despite corticosteroids, brisk hemolysis continued with 10 units of RBCs required over 5 days to maintain a stable hemoglobin. Plasma free hemoglobin reached 1147 mg/L, prompting therapeutic plasmapheresis for renal protection by the end of day 5. She deteriorated clinically after her first plasmapheresis with acute confusion (GCS 10) and lactic acidosis. She was empirically treated for seizure with levetiracetam. CT and MRI scans of her brain and lumbar puncture were normal. Her consciousness improved with daily plasmapheresis. A bone marrow biopsy performed on day twelve of glucocorticoid therapy found monoclonal plasma cell proliferation of 15% with marked lambda light chain predominance (20:1) (Figure 1). Repeat bone marrow biopsy 3 months post-steroid therapy still revealed 10% clonal plasma cells.Hemolysis can be a rare presentation of plasma cell dyscrasia. Case reports of both autoimmune hemolytic anemia and microangiopathic hemolytic anemia associated with multiple myeloma exist. In our case, there was no evidence of a microangiopathic process, making thrombotic thrombocytopenic purpura (TTP) or atypical hemolytic-uremic syndrome (aHUS) unlikely. DAT was negative for IgG but did demonstrate C3 complement molecules bound to red cells. No previous case reports of complement-mediated hemolysis and multiple myeloma were found on literature review.We report the first in vivo association between complement-mediated hemolysis and plasma cell dyscrasia. Complement pathways bridge the innate and acquired immune systems by helping select cells to be targeted by the acquired immune system. The alternative complement pathway does not require an antigen-antibody interaction to become active; rather, it is controlled by direct binding of complement and regulated by cofactor molecules. Jokiranta et al. (J Immunol 1999) identified a monoclonal Ig-lambda dimer that efficiently activated the alternative pathway of complement, triggering complement molecules to enhance hemolysis of serum in vitro. This "miniautoantibody" specifically bound and blocked the function of complement factor H, inhibiting enzymatic inactivation of fluid-phase C3b with uncontrolled activation of the alternative pathway. It is possible that the relative immune dysfunction in this patient's plasma cell dyscrasia led to a disturbance in the alternate complement pathway, perhaps due to dimerization of abnormal lambda light chains, resulting in complement-mediated intravascular hemolysis. Glucocorticoids and plasmapheresis may have helped manage hemolysis in this case.By diagnostic criteria, this patient has smoldering myeloma, with urine monoclonal protein (1.2 g/24 hours), clonal bone marrow plasma cells (10-15%), and absence of myeloma-defining events. We have elected to manage her as such, with close observation. Further work-up performed for her plasma cell dyscrasia included a normal MRI of spine and pelvis. Over a year later, there has been no recurrence of hemolysis. Consideration will be given to treatment if she progresses to overt multiple myeloma. [Display omitted] DisclosuresSandhu:Novartis: Consultancy, Honoraria; Amgen: Consultancy, Honoraria; Celgene: Consultancy, Honoraria; Janssen: Consultancy, Honoraria.

Structural insights on complement activation.

The proteolytic cleavage of C3 to generate C3b is the central and most important step in the activation of complement, a major component of innate immunity. The comparison of the crystal structures of C3 and C3b illustrates large conformational changes during the transition from C3 to C3b. Exposure of a reactive thio-ester group allows C3b to bind covalently to surfaces such as pathogens or apoptotic cellular debris. The displacement of the thio-ester-containing domain (TED) exposes hidden surfaces that mediate the interaction with complement factor B to assemble the C3-convertase of the alternative pathway (AP). In addition, the displacement of the TED and its interaction with the macroglobulin 1 (MG1) domain generates an extended surface in C3b where the complement regulators factor H (FH), decay accelerating factor (DAF), membrane cofactor protein (MCP) and complement receptor 1 (CR1) can bind, mediating accelerated decay of the AP C3-convertase and proteolytic inactivation of C3b. In the last few years, evidence has accumulated revealing that the structure of C3b in solution is significantly more flexible than anticipated. We review our current knowledge on C3b structural flexibility to propose a general model where the TED can display a collection of conformations around the MG ring, as well as a few specialized positions where the TED is held in one of several fixed locations. Importantly, this conformational heterogeneity in C3b impacts complement regulation by affecting the interaction with regulators.

Complement regulator CD46: genetic variants and disease associations.

Membrane cofactor protein (MCP; CD46) is an ubiquitously expressed complement regulatory protein that protects host cells from injury by complement. This type-I membrane glycoprotein serves as a cofactor for the serine protease factor I to mediate inactivation of C3b and C4b deposited on host cells. More than 60 disease-associated mutations in MCP have now been identified. The majority of the mutations are linked to a rare thrombotic microangiopathic-based disease, atypical hemolytic uremic syndrome (aHUS), but new putative links to systemic lupus erythematosus, glomerulonephritis, and pregnancy-related disorders among others have also been identified. This review summarizes our current knowledge of disease-associated mutations in this complement inhibitor.

Mapping interactions between complement C3 and regulators using mutations in atypical hemolytic uremic syndrome

AbstractThe pathogenesis of atypical hemolytic uremic syndrome (aHUS) is strongly linked to dysregulation of the alternative pathway of the complement system. Mutations in complement genes have been identified in about two-thirds of cases, with 5% to 15% being in C3. In this study, 23 aHUS-associated genetic changes in C3 were characterized relative to their interaction with the control proteins factor H (FH), membrane cofactor protein (MCP; CD46), and complement receptor 1 (CR1; CD35). In surface plasmon resonance experiments, 17 mutant recombinant proteins demonstrated a defect in binding to FH and/or MCP, whereas 2 demonstrated reduced binding to CR1. In the majority of cases, decreased binding affinity translated to a decrease in proteolytic inactivation (known as cofactor activity) of C3b via FH and MCP. These results were used to map the putative binding regions of C3b involved in the interaction with MCP and CR1 and interrogated relative to known FH binding sites. Seventy-six percent of patients with C3 mutations had low C3 levels that correlated with disease severity. This study expands our knowledge of the functional consequences of aHUS-associated C3 mutations relative to the interaction of C3 with complement regulatory proteins mediating cofactor activity.

Anopheles midgut epithelium evades human complement activity by capturing factor H from the blood meal.

Hematophagous vectors strictly require ingesting blood from their hosts to complete their life cycles. Exposure of the alimentary canal of these vectors to the host immune effectors necessitates efficient counteractive measures by hematophagous vectors. The Anopheles mosquito transmitting the malaria parasite is an example of hematophagous vectors that within seconds can ingest human blood double its weight. The innate immune defense mechanisms, like the complement system, in the human blood should thereby immediately react against foreign cells in the mosquito midgut. A prerequisite for complement activation is that the target cells lack complement regulators on their surfaces. In this work, we analyzed whether human complement is active in the mosquito midgut, and how the mosquito midgut cells protect themselves against complement attack. We found that complement remained active for a considerable time and was able to kill microbes within the mosquito midgut. However, the Anopheles mosquito midgut cells were not injured. These cells were found to protect themselves by capturing factor H, the main soluble inhibitor of the alternative complement pathway. Factor H inhibited complement on the midgut cells by promoting inactivation of C3b to iC3b and preventing the activity of the alternative pathway amplification C3 convertase enzyme. An interference of the FH regulatory activity by monoclonal antibodies, carried to the midgut via blood, resulted in increased mosquito mortality and reduced fecundity. By using a ligand blotting assay, a putative mosquito midgut FH receptor could be detected. Thereby, we have identified a novel mechanism whereby mosquitoes can tolerate human blood.

The outer membrane protease PgtE of Salmonella enterica interferes with the alternative complement pathway by cleaving factors B and H.

The virulence factor PgtE is an outer membrane protease (omptin) of the zoonotic pathogen Salmonella enterica that causes diseases ranging from gastroenteritis to severe enteric fever. It is surface exposed in bacteria that have a short-chain, i.e., rough LPS, as observed e.g., in bacteria residing inside macrophages or just emerging from them. We investigated whether PgtE cleaves the complement factors B (B) and H (H), key proteins controlling formation and inactivation of the complement protein C3b and thereby the activity of the complement system. S. enterica serovar Typhimurium or omptin-expressing recombinant E. coli bacteria were incubated with purified human complement proteins or recombinant H fragments. PgtE cleaved both B and H, whereas its close homolog Pla of Yersinia pestis cleaved only H. H was cleaved at both N- and C-termini, while the central region resisted proteolysis. Because of multiple effects of PgtE on complement components (cleavage of C3, C3b, B, and H) we assessed its effect on the opsonophagocytosis of Salmonella. In human serum, C3 cleavage was dependent on proteolytically active PgtE. Human neutrophils interacted less with serum-opsonized FITC-stained S. enterica 14028R than with the isogenic ΔpgtE strain, as analyzed by flow cytometry. In conclusion, cleavage of B and H by PgtE, together with C3 cleavage, affects the C3-mediated recognition of S. enterica by human neutrophils, thus thwarting the immune protection against Salmonella.

The Serine Protease Pic From Enteroaggregative Escherichia coli Mediates Immune Evasion by the Direct Cleavage of Complement Proteins.

Enteroaggregative and uropathogenic Escherichia coli, Shigella flexneri 2a, and the hybrid enteroaggregative/Shiga toxin-producing E. coli strain (O104:H4) are important pathogens responsible for intestinal and urinary tract infections, as well as sepsis and hemolytic uremic syndrome. They have in common the production of a serine protease called Pic. Several biological roles for Pic have been described, including protection of E. coli DH5α from complement-mediated killing. Hereby we showed that Pic significantly reduces complement activation by all 3 pathways. Pic cleaves purified C3/C3b and other proteins from the classic and lectin pathways, such as C4 and C2. Cleavage fragments of C3, C4, and C2 were also observed with HB101(pPic1) culture supernatants, and C3 cleavage sites were mapped by fluorescence resonance energy transfer peptides. Experiments using human serum as a source of complement proteins confirmed Pic proteolytic activity on these proteins. Furthermore, Pic works synergistically with the human complement regulators factor I and factor H, promoting inactivation of C3b. In the presence of both regulators, further degradation of C3 α' chain was observed. Therefore, Pic may contribute to immune evasion of E. coli and S. flexneri, favoring invasiveness and increasing the severity of the disorders caused by these pathogens.

A novel factor I activity in Nipah virus inhibits human complement pathways through cleavage of C3b.

Complement is an innate immune system that most animal viruses must face during natural infections. Given that replication and dissemination of the highly pathogenic Nipah virus (NiV) include exposure to environments rich in complement factors, we tested the in vitro sensitivity of NiV to complement-mediated neutralization. Here we show that NiV was completely resistant to in vitro neutralization by normal human serum (NHS). Treatment of purified NiV with NHS activated complement pathways, but there was very little C3 deposition on virus particles. In in vitro reconstitution experiments, NiV particles provided time- and dose-dependent factor I-like protease activity capable of cleaving C3b into inactive C3b (iC3b). NiV-dependent inactivation of C3b only occurred with the cofactors factor H and soluble CR1 but not with CD46. Purified NiV particles did not support C4b cleavage. Electron microscopy of purified NiV particles showed immunogold labeling with anti-factor I antibodies. Our results suggest a novel mechanism by which NiV evades the human complement system through a unique factor I-like activity. Viruses have evolved mechanisms to limit complement-mediated neutralization, some of which involve hijacking cellular proteins involved in control of inappropriate complement activation. Here we report a previously unknown mechanism whereby NiV provides a novel protease activity capable of in vitro cleavage and inactivation of C3b, a key component of the complement cascade. These data help to explain how an enveloped virus such as NiV can infect and disseminate through body fluids that are rich in complement activity. Disruption of the ability of NiV to recruit complement inhibitors could form the basis for the development of effective therapies and safer vaccines to combat these highly pathogenic emerging viruses.

Factor H binds to Trypanosoma cruzi trypomastigotes via its C-terminus and contributes to parasite resistance against the alternative pathway of complement (MPF2P.809)

Abstract Trypanosoma cruzi (T. cruzi), the causal agent of devastating Chagas disease, possesses multifactorial resistance to the alternative pathway (AP) of complement. AP regulatory protein factor H (fH) uses its C-terminal 19-20 domains to recognize polyanions (i.e. sialic acid) and C3b on host cells, facilitating decay of C3 convertases and inactivation of C3b via the N-terminus of fH. We have shown that recombinant C-terminal fH domains 19-20 (rH19-20) compete with fH for binding to C3b and host cell polyanions, inhibiting protection from the AP. T. cruzi tripomastigotes resist the AP by capturing sialic acid from host cells. The specific role that fH plays in protecting T. cruzi from the AP and the fH domains involved remain unknown. Here we assessed the susceptibility of T. cruzi (MF, Y and Brenner strains) to AP-mediated killing, and the ability of fH to bind directly to the parasite by FACS. For determining the fH domains needed for the interaction with T. cruzi, trypomastigotes were incubated with normal human sera in the presence or absence of equimolar amounts of overlapping recombinant fH proteins (2 to 3 domains each) spanning domains 7 to 20. The ability of each protein to increase AP-mediated killing of T. cruzi was assessed. Only rH19-20 effectively competed with serum fH, inducing a significant decrease in parasite survival (p<0.0001), indicating that the C-terminus of fH is essential for fH-mediated recognition and protection of T. cruzi trypomastigotes from the AP.

A method of purifying intact complement factor H from human plasma

The aim of this study was to establish a method of purifying intact complement factor H (CFH) from human plasma. CFH was isolated from human plasma by polyethylene glycol (PEG) precipitation, following three sequential chromatographic columns, which consisted of l-lysine Sepharose column, Resource Q column and Sephacryl S-300 High Resolution HiPrep 16/60 column. All the above steps were performed at 4°C by Fast Protein Liquid Chromatography (FPLC) AKTA Purifier 10 with Frac-900. Identification of the purified CFH was confirmed by SDS–PAGE and Western blot. The following functions of the purified CFH were further analyzed compared with the commercial CFH in vitro: (1) binding ability with C3b; (2) binding ability with mCRP; (3) the protecting function of the hemolysis of sheep red blood cells; (4) the cofactor role for complement factor I-mediated proteolytic inactivation of C3b. Homogeneous CFH was purified from the plasma fraction through the above four steps. The purity and the functions of the purified CFH were comparable to the commercial CFH. The yield of CFH was 26±3% in our study. Compared with previous methods, our method was high yield with high purity. We established a stable and feasible system for purifying intact CFH, which could be used in the lab and clinical investigations.

Immune evasion in Leptospira: the secretion of proteases that cleave complement proteins

Event Abstract Back to Event Immune evasion in Leptospira: the secretion of proteases that cleave complement proteins Tatiana R. Fraga1, Daniella S. Courrol1, Mónica M. Castiblanco-Valencia1, Izaura Y. Hirata2, Luiz Juliano2, Sílvio A. Vasconcellos3, Angela S. Barbosa4 and Lourdes Isaac1* 1 University of São Paulo, Department of Immunology, Brazil 2 Federal University of São Paulo, Department of Biophysics, Brazil 3 University of São Paulo, Faculty of Veterinary Medicine, Brazil 4 Butantan Institute, Laboratory of Bacteriology, Brazil Leptospirosis is a neglected infectious disease of public health importance. The complement system is a major arm of innate immunity. However, pathogenic leptospires have evolved multiple strategies to escape complement attack. In this work we analyzed the ability of leptospires to secrete proteases that cleave and inactivate complement proteins, which could constitute an immune evasion strategy. We demonstrated that the culture supernatant of pathogenic, but not saprophytic leptospires, was able to inhibit the activation of the three complement pathways. Moreover, the supernatant of seven pathogenic strains presented proteolytic activity against C3, C3b and iC3b, proteins from the alternative (Factor B), classical and lectin pathways (C4b and C2). The cleavages were also observed when normal human serum was used. Interestingly, the proteases act together with the host regulators Factor I and Factor H, promoting an efficient inactivation of C3b. Proteolytic fragments of C3 had their N-terminal portion sequenced by Edman degradation. The α-chain cleavage produced 43 kDa (RSSKI-), 47 kDa (YGGGYG-) and 68 kDa bands and the β-chain degradation generates 28 kDa (HPSQKPL-) and 46 kDa fragments. Furthermore, the proteolytic activity was inhibited by 1,10-phenanthroline, indicating the participation of metalloproteases. A recombinant leptospiral metalloprotease from the thermolysin family was produced and cleaved C3 in human serum, suggesting that this protein could be one of the proteases responsible for the effect observed with the whole supernatant. In conclusion, pathogenic leptospiral proteases can deactivate immune effector molecules, and represent potential targets for the development of new therapies and prophylactic approaches in leptospirosis. Keywords: immune evasion of bacteria, complement system, secretion of proteases, Leptospira, Leptospirosis Conference: 15th International Congress of Immunology (ICI), Milan, Italy, 22 Aug - 27 Aug, 2013. Presentation Type: Abstract Topic: Host-pathogen interactions Citation: Fraga TR, Courrol DS, Castiblanco-Valencia MM, Hirata IY, Juliano L, Vasconcellos SA, Barbosa AS and Isaac L (2013). Immune evasion in Leptospira: the secretion of proteases that cleave complement proteins. Front. Immunol. Conference Abstract: 15th International Congress of Immunology (ICI). doi: 10.3389/conf.fimmu.2013.02.00733 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 16 Jun 2013; Published Online: 22 Aug 2013. * Correspondence: Dr. Lourdes Isaac, University of São Paulo, Department of Immunology, São Paulo, São Paulo, 05508-900, Brazil, louisaac@icb.usp.br Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Tatiana R Fraga Daniella S Courrol Mónica M Castiblanco-Valencia Izaura Y Hirata Luiz Juliano Sílvio A Vasconcellos Angela S Barbosa Lourdes Isaac Google Tatiana R Fraga Daniella S Courrol Mónica M Castiblanco-Valencia Izaura Y Hirata Luiz Juliano Sílvio A Vasconcellos Angela S Barbosa Lourdes Isaac Google Scholar Tatiana R Fraga Daniella S Courrol Mónica M Castiblanco-Valencia Izaura Y Hirata Luiz Juliano Sílvio A Vasconcellos Angela S Barbosa Lourdes Isaac PubMed Tatiana R Fraga Daniella S Courrol Mónica M Castiblanco-Valencia Izaura Y Hirata Luiz Juliano Sílvio A Vasconcellos Angela S Barbosa Lourdes Isaac Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Dissection of Functional Sites in Herpesvirus Saimiri Complement Control Protein Homolog

Herpesvirus saimiri is known to encode a homolog of human complement regulators named complement control protein homolog (CCPH). We have previously reported that this virally encoded inhibitor effectively inactivates complement by supporting factor I-mediated inactivation of complement proteins C3b and C4b (termed cofactor activity), as well as by accelerating the irreversible decay of the classical/lectin and alternative pathway C3 convertases (termed decay-accelerating activity). To fine map its functional sites, in the present study, we have generated a homology model of CCPH and performed substitution mutagenesis of its conserved residues. Functional analyses of 24 substitution mutants of CCPH indicated that (i) amino acids R118 and F144 play a critical role in imparting C3b and C4b cofactor activities, (ii) amino acids R35, K142, and K191 are required for efficient decay of the C3 convertases, (iii) positively charged amino acids of the linker regions, which are dubbed to be critical for functioning in other complement regulators, are not crucial for its function, and (iv) S100K and G110D mutations substantially enhance its decay-accelerating activities without affecting the cofactor activities. Overall, our data point out that ionic interactions form a major component of the binding interface between CCPH and its interacting partners.

The Yersinia pseudotuberculosis Outer Membrane Protein Ail Recruits the Human Complement Regulatory Protein Factor H

Previous investigations characterizing the mechanism(s) of complement resistance in Yersinia pseudotuberculosis showed that the outer membrane protein Ail can functionally recruit the regulator of the classical and lectin pathways of complement, C4b-binding protein. In this study, we extend these observations and show that Ail can also recruit the regulator of the alternative pathway (AP), factor H (fH). Binding to fH was dependent on Ail expression and observed in the context of full-length LPS. Inactivation of ail resulted in loss of fH binding. Ail expression conferred resistance to AP-mediated killing. Bound fH was functional as a cofactor for factor I-mediated cleavage and inactivation of C3b. Ail alone is sufficient to mediate fH binding and resistance to AP-mediated killing, because Ail expression in a laboratory Escherichia coli strain conferred both of these phenotypes. Binding was specific and inhibited by increasing heparin and NaCl concentrations. Using a panel of fH recombinant fragments, we observed that both short consensus repeats 5-7 and 19-20 regions are responsible for mediating the interaction with Ail. Collectively, these results suggest that fH recruitment is an additional mechanism of complement resistance of Ail. Recruitment of both fH and C4BP by Ail may confer Y. pseudotuberculosis with the ability to resist all pathways of complement activation.

Novel C3 mutation p.Lys65Gln in aHUS affects complement factor H binding

BackgroundAtypical hemolytic uremic syndrome (aHUS) is associated with mutations affecting complement proteins and regulators and with autoantibodies against complement factor H (CFH). Approximately half of the aHUS patients progress to end-stage renal disease. DNA analysis of the risk factor genes is important for prognosis of aHUS recurrence after renal transplantation.MethodsMutational screening of C3 encoding the central complement component was performed by Sanger sequencing in 70 aHUS patients. Mutated and wild type recombinant C3b proteins were produced and their affinity to CFH was analyzed by ELISA.ResultsA single novel missense change p.Lys65Gln in C3 was found in 3 aHUS patients. The alteration leads to decreased binding of C3b to CFH in vitro. All three patients acquired the illness as adults and had a first aHUS episode after renal transplantation or suffered recurrence of the disease after transplantation.ConclusionsThe novel C3 change was found in 3 aHUS patients. It results in decreased C3b binding to CFH and thus might lead to impaired C3b inactivation in vivo. The p.Lys65Gln is likely to be associated with aHUS after kidney transplantation and, therefore, might be an important prognostic factor.

Disabling complement regulatory activities of vaccinia virus complement control protein reduces vaccinia virus pathogenicity

Poxviruses encode a repertoire of immunomodulatory proteins to thwart the host immune system. One among this array is a homolog of the host complement regulatory proteins that is conserved in various poxviruses including vaccinia (VACV) and variola. The vaccinia virus complement control protein (VCP), which inhibits complement by decaying the classical pathway C3-convertase (decay-accelerating activity), and by supporting inactivation of C3b and C4b by serine protease factor I (cofactor activity), was shown to play a role in viral pathogenesis. However, the role its individual complement regulatory activities impart in pathogenesis, have not yet been elucidated. Here, we have generated monoclonal antibodies (mAbs) that block the VCP functions and utilized them to evaluate the relative contribution of complement regulatory activities of VCP in viral pathogenesis by employing a rabbit intradermal model for VACV infection. Targeting VCP by mAbs that inhibited the decay-accelerating activity as well as cofactor activity of VCP or primarily the cofactor activity of VCP, by injecting them at the site of infection, significantly reduced VACV lesion size. This reduction however was not pronounced when VCP was targeted by a mAb that inhibited only the decay-accelerating activity. Further, the reduction in lesion size by mAbs was reversed when host complement was depleted by injecting cobra venom factor. Thus, our results suggest that targeting VCP by antibodies reduces VACV pathogenicity and that principally the cofactor activity of VCP appears to contribute to the virulence.

Engineering complement activation on polypropylene sulfide vaccine nanoparticles

The complement system is an important regulator of both adaptive and innate immunity, implicating complement as a potential target for immunotherapeutics. We have recently presented lymph node-targeting, complement-activating nanoparticles (NPs) as a vaccine platform. Here we explore modulation of surface chemistry as a means to control complement deposition, in active or inactive forms, on polypropylene sulfide core, block copolymer Pluronic corona NPs. We found that nucleophile-containing NP surfaces activated complement and became functionalized in situ with C3 upon serum exposure via the alternative pathway. Carboxylated NPs displayed a higher degree of C3b deposition and retention relative to hydroxylated NPs, upon which deposited C3b was more substantially inactivated to iC3b. This in situ functionalization correlated with in vivo antigen-specific immune responses, including antibody production as well as T cell proliferation and IFN-γ cytokine production upon antigen restimulation. Interestingly, inactivation of C3b to iC3b on the NP surface did not correlate with NP affinity to factor H, a cofactor for protease factor I that degrades C3b into iC3b, indicating that control of complement protein C3 stability depends on architectural details in addition to factor H affinity. These data show that design of NP surface chemistry can be used to control biomaterials-associated complement activation for immunotherapeutic materials.