Killing of gram-negative bacteria by complement. Fractionation of cell membranes after complement C5b-9 deposition on to the surface of Salmonella minnesota Re595

Complement in breast milk modifies offspring gut microbiota to promote infant health

Breastfeeding offers demonstrable benefits to newborns and infants by providing nourishment and immune protection and by shaping the gut commensal microbiota. Although it has been appreciated for decades that breast milk contains complement components, the physiological relevance of complement in breast milk remains undefined. Here, we demonstrate that weanling mice fostered by complement-deficient dams rapidly succumb when exposed to murine pathogen Citrobacter rodentium (CR), whereas pups fostered on complement-containing milk from wild-type dams can tolerate CR challenge. The complement components in breast milk were shown to directly lyse specific members of gram-positive gut commensal microbiota via a C1-dependent, antibody-independent mechanism, resulting in the deposition of the membrane attack complex and subsequent bacterial lysis. By selectively eliminating members of the commensal gut community, complement components from breast milk shape neonate and infant gut microbial composition to be protective against environmental pathogens such as CR.

Structural basis for membrane attack complex inhibition by CD59

CD59 is an abundant immuno-regulatory receptor that protects human cells from damage during complement activation. Here we show how the receptor binds complement proteins C8 and C9 at the membrane to prevent insertion and polymerization of membrane attack complex (MAC) pores. We present cryo-electron microscopy structures of two inhibited MAC precursors known as C5b8 and C5b9. We discover that in both complexes, CD59 binds the pore-forming β-hairpins of C8 to form an intermolecular β-sheet that prevents membrane perforation. While bound to C8, CD59 deflects the cascading C9 β-hairpins, rerouting their trajectory into the membrane. Preventing insertion of C9 restricts structural transitions of subsequent monomers and indirectly halts MAC polymerization. We combine our structural data with cellular assays and molecular dynamics simulations to explain how the membrane environment impacts the dual roles of CD59 in controlling pore formation of MAC, and as a target of bacterial virulence factors which hijack CD59 to lyse human cells.

Evaluation of the immunogenic properties of the recombinant Histophilus somni outer membrane protein 40 kDa (rOMP40)

Background

Gram-negative bacterial infections are a serious problem in beef and dairy cattle. Bacterial outer membrane proteins (OMPs) play a pivotal role in cellular survival and the host-bacterium interaction. Histophilus somni OMP40 was identified as a porin with homology between its N-terminal amino acid sequence and the sequences of porins of other gram-negative bacteria The aim of this study was to produce recombinant H. somni OMP40 (rOMP40), optimize its production and evaluate its immunogenic properties in calves. The cross-reactivity of anti-rOMP40 antibodies were also checked.

Results

The highest overexpression of rOMP40 was demonstrated by Escherichia coli C41 using the autoinduction process. Double immunization of calves (20 μg rOMP40 per animal) induced a significant increase of anti-rOMP40 antibodies in the IgG1 (P ≤ 0.01) and IgG2 (P ≤ 0.01, after first immunization only) subclasses, but not IgM. ELISA revealed increased reactivity of the IgG against surface antigens of E. coli and Pasteurella multocida after the second immunization (P < 0.01). Cross reactivity of anti-rOMP40 antibodies with ~ 40 kDa antigens of most common gram-negative pathogens was shown by Western blotting.

Conclusion

Immunization with H. somni rOMP40 induced a humoral response in cattle with broad cross-reactivity with similar antigens of other species of Pasteurellaceae and Enterobacteriaceae families and the delayed-type hypersensitivity reaction. The obtained results encourage further study to evaluate the protective effect of the produced protein as a subunit vaccine in cattle.

Bispecific mAb2 Antibodies Targeting CD59 Enhance the Complement-Dependent Cytotoxicity Mediated by Rituximab

Inhibition of complement activation via the overexpression of complement-regulatory proteins (CRPs), most notably CD46, CD55 and CD59, is an efficient mechanism of disguise of cancer cells from a host immune system. This phenomenon extends to counteract the potency of therapeutic antibodies that could lyse target cells by eliciting complement cascade. The manifold functions and ubiquitous expression of CRPs preclude their systemic specific inhibition. We selected CD59-specific Fc fragments with a novel antigen binding site (Fcabs) from yeast display libraries using recombinant antigens expressed in bacterial or mammalian cells. To produce a bispecific antibody, we endowed rituximab, a clinically applied anti-CD20 antibody, used for therapy of various lymphoid malignancies, with an anti-CD59 Fcab. This bispecific antibody was able to induce more potent complement-dependent cytotoxicity for CD20 and CD59 expressing Raji cell line measured with lactate dehydrogenase-release assay, but had no effect on the cells with lower levels of the primary CD20 antigen or CD20-negative cells. Such molecules are promising candidates for future therapeutic development as they elicit a higher specific cytotoxicity at a lower concentration and hence cause a lower exhaustion of complement components.

Structural basis of soluble membrane attack complex packaging for clearance

Unregulated complement activation causes inflammatory and immunological pathologies with consequences for human disease. To prevent bystander damage during an immune response, extracellular chaperones (clusterin and vitronectin) capture and clear soluble precursors to the membrane attack complex (sMAC). However, how these chaperones block further polymerization of MAC and prevent the complex from binding target membranes remains unclear. Here, we address that question by combining cryo electron microscopy (cryoEM) and cross-linking mass spectrometry (XL-MS) to solve the structure of sMAC. Together our data reveal how clusterin recognizes and inhibits polymerizing complement proteins by binding a negatively charged surface of sMAC. Furthermore, we show that the pore-forming C9 protein is trapped in an intermediate conformation whereby only one of its two transmembrane β-hairpins has unfurled. This structure provides molecular details for immune pore formation and helps explain a complement control mechanism that has potential implications for how cell clearance pathways mediate immune homeostasis.

To prevent unregulated complement activation, extracellular chaperones capture soluble precursors to the membrane attack complex (sMAC). Here, structural analysis of sMAC reveals how clusterin recognizes heterogeneous sMAC complexes and inhibits polymerization of complement protein C9.

To Kill But Not Be Killed: Controlling the Activity of Mammalian Pore-Forming Proteins

Pore-forming proteins (PFPs) are present in all domains of life, and play an important role in host-pathogen warfare and in the elimination of cancers. They can be employed to deliver specific effectors across membranes, to disrupt membrane integrity interfering with cell homeostasis, and to lyse membranes either destroying intracellular organelles or entire cells. Considering the destructive potential of PFPs, it is perhaps not surprising that mechanisms controlling their activity are remarkably complex, especially in multicellular organisms. Mammalian PFPs discovered to date include the complement membrane attack complex (MAC), perforins, as well as gasdermins. While the primary function of perforin-1 and gasdermins is to eliminate infected or cancerous host cells, perforin-2 and MAC can target pathogens directly. Yet, all mammalian PFPs are in principle capable of generating pores in membranes of healthy host cells which-if uncontrolled-could have dire, and potentially lethal consequences. In this review, we will highlight the strategies employed to protect the host from destruction by endogenous PFPs, while enabling timely and efficient elimination of target cells.

Effects of elevated corticosterone on humoral innate and antibody-mediated immunity in southern leopard frog (Lithobates sphenocephalus) tadpoles

As a free-living larval stage of a vertebrate, tadpoles are good subjects for the study of the development of physiological systems and the study of evolutionarily conserved, context-dependent responses to variable environments. While the basic components of innate and adaptive immune defenses in tadpoles are known, the impact of glucocorticoids on immune defenses in tadpoles is not well-studied. We completed four experiments to assess effects of elevation of corticosterone on humoral innate defenses and antibody-mediated immunity in southern leopard frog tadpoles (Lithobates sphenocephalus). To test humoral innate defense within the tadpoles exposed to short-term and long-term elevation of glucocorticoids, we exposed tadpoles to exogenous corticosterone for different lengths of time in each experiment (0-84 days). We used bacterial killing assays to assess humoral innate immune defense. To test antibody-mediated immune responses, we again exposed tadpoles to exogenous corticosterone, while also exposing them to Aeromonas hydrophila. We used A. hydrophila ELISA comparing IgM and IgY responses among groups. Plasma from corticosterone-dosed tadpoles killed more A. hydrophila than control tadpoles each following a short-term (14 day) and long-term (56 day) exposure to exogenous corticosterone. Conversely, corticosterone-dosed tadpoles had significantly lower IgM and IgY against A. hydrophila after 12 weeks. Our fourth experiment revealed that the lower IgY response is a product of weaker, delayed isotype switching compared with controls. These results show that elevated corticosterone has differential effects on innate and acquired immunity in larval southern leopard frogs, consistent with patterns in more derived vertebrates and in adult frogs.

A robust fractionation method for protein subcellular localization studies in Escherichia coli

Fractionation in Gram-negative bacteria is used to identify the subcellular localization of proteins, in particular the localization of exported recombinant proteins. The process of cell fractionation can be fraught with cross-contamination issues and often lacks supporting data for fraction purity. Here, we compare three periplasm extraction and two cell disruption techniques in different combinations to investigate which process gives uncontaminated compartments from Escherichia coli. From these data, a robust method named PureFrac was compiled that gives pure periplasmic fractions and a superior recovery of soluble cytoplasmic proteins. The process extracts periplasm using cold osmotic shock with magnesium, prior to sonication and ultracentrifugation to separate the cytoplasm from insoluble material. This method handles cells cultivated in various conditions and allows preparation of active proteins in their respective compartments.

CryoEM reveals how the complement membrane attack complex ruptures lipid bilayers

The membrane attack complex (MAC) is one of the immune system's first responders. Complement proteins assemble on target membranes to form pores that lyse pathogens and impact tissue homeostasis of self-cells. How MAC disrupts the membrane barrier remains unclear. Here we use electron cryo-microscopy and flicker spectroscopy to show that MAC interacts with lipid bilayers in two distinct ways. Whereas C6 and C7 associate with the outer leaflet and reduce the energy for membrane bending, C8 and C9 traverse the bilayer increasing membrane rigidity. CryoEM reconstructions reveal plasticity of the MAC pore and demonstrate how C5b6 acts as a platform, directing assembly of a giant β-barrel whose structure is supported by a glycan scaffold. Our work provides a structural basis for understanding how β-pore forming proteins breach the membrane and reveals a mechanism for how MAC kills pathogens and regulates cell functions.

The mystery behind membrane insertion: a review of the complement membrane attack complex

The membrane attack complex (MAC) is an important innate immune effector of the complement terminal pathway that forms cytotoxic pores on the surface of microbes. Despite many years of research, MAC structure and mechanism of action have remained elusive, relying heavily on modelling and inference from biochemical experiments. Recent advances in structural biology, specifically cryo-electron microscopy, have provided new insights into the molecular mechanism of MAC assembly. Its unique 'split-washer' shape, coupled with an irregular giant β-barrel architecture, enable an atypical mechanism of hole punching and represent a novel system for which to study pore formation. This review will introduce the complement terminal pathway that leads to formation of the MAC. Moreover, it will discuss how structures of the pore and component proteins underpin a mechanism for MAC function, modulation and inhibition.This article is part of the themed issue 'Membrane pores: from structure and assembly, to medicine and technology'.

Can Pulp Fibroblasts Kill Cariogenic Bacteria? Role of Complement Activation

Complement system activation has been shown to be involved in inflammation and regeneration processes that can be observed within the dental pulp after moderate carious decay. Studies simulating carious injuries in vitro have shown that when human pulp fibroblasts are stimulated by lipoteichoic acid (LTA), they synthetize all complement components. Complement activation leads to the formation of the membrane attack complex (MAC), which is known for its bacterial lytic effect. This work was designed to find out whether human pulp fibroblasts can kill Streptococcus mutans and Streptococcus sanguinis via complement activation. First, histological staining of carious tooth sections showed that the presence of S. mutans correlated with an intense MAC staining. Next, to simulate bacterial infection in vitro, human pulp fibroblasts were incubated in serum-free medium with LTA. Quantification by an enzymatic assay showed a significant increase of MAC formation on bacteria grown in this LTA-conditioned medium. To determine whether the MAC produced by pulp fibroblasts was functional, bacteria sensitivity to LTA-conditioned medium was evaluated using agar well diffusion assay and succinyl dehydrogenase (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide [MTT]) assay. Both assays showed that S. mutans and S. sanguinis were sensitive to LTA-conditioned medium. Finally, to evaluate whether MAC formation on cariogenic bacteria, by pulp fibroblasts, can be directly induced by the presence of these bacteria, a specific coculture model of human pulp fibroblasts and bacteria was developed. Immunofluorescence revealed an intense MAC labeling on bacteria after direct contact with pulp fibroblasts. The observed MAC formation and its lethal effects were significantly reduced when CD59, an inhibitor of MAC formation, was added. Our findings demonstrate that the MAC produced by LTA-stimulated pulp fibroblasts is functional and can kill S. mutans and S. sanguinis. Taken together, these data clearly highlight the function of pulp fibroblasts in destroying cariogenic bacteria.

An engineered micropattern to reduce bacterial colonization, platelet adhesion and fibrin sheath formation for improved biocompatibility of central venous catheters

Background

Catheter-related bloodstream infections (CRBSIs) and catheter-related thrombosis (CRT) are common complications of central venous catheters (CVC), which are used to monitor patient health and deliver medications. CVCs are subject to protein adsorption and platelet adhesion as well as colonization by the natural skin flora (i.e. Staphylococcus aureus and Staphylococcus epidermidis). Antimicrobial and antithrombotic drugs can prevent infections and thrombosis-related complications, but have associated resistance and safety risks. Surface topographies have shown promise in limiting platelet and bacterial adhesion, so it was hypothesized that an engineered Sharklet micropattern, inspired by shark-skin, may provide a combined approach as it has wide reaching anti-fouling capabilities. To assess the feasibility for this micropattern to improve CVC-related healthcare outcomes, bacterial colonization and platelet interactions were analyzed in vitro on a material common for vascular access devices.

Methods

To evaluate bacterial inhibition after simulated vascular exposure, micropatterned thermoplastic polyurethane surfaces were preconditioned with blood proteins in vitro then subjected to a bacterial challenge for 1 and 18 h. Platelet adhesion was assessed with fluorescent microscopy after incubation of the surfaces with platelet-rich plasma (PRP) supplemented with calcium. Platelet activation was further assessed by monitoring fibrin formation with fluorescent microscopy after exposure of the surfaces to platelet-rich plasma (PRP) supplemented with calcium in a flow-cell. Results are reported as percent reductions and significance is based on t-tests and ANOVA models of log reductions. All experiments were replicated at least three times.

Results

Blood and serum conditioned micropatterned surfaces reduced 18 h S. aureus and S. epidermidis colonization by 70% (p ≤ 0.05) and 71% (p < 0.01), respectively, when compared to preconditioned unpatterned controls. Additionally, platelet adhesion and fibrin sheath formation were reduced by 86% and 80% (p < 0.05), respectively, on the micropattern, when compared to controls.

Conclusions

The Sharklet micropattern, in a CVC-relevant thermoplastic polyurethane, significantly reduced bacterial colonization and relevant platelet interactions after simulated vascular exposure. These results suggest that the incorporation of the Sharklet micropattern on the surface of a CVC may inhibit the initial events that lead to CRBSI and CRT.

Identification of the outer-membrane protein PagC required for the serum resistance phenotype in Salmonella enterica serovar Choleraesuis

Serum resistance is a crucial virulence factor for the development of systemic infections, including bacteraemia, by many pathogenic bacteria. Salmonella enterica serovar Choleraesuis is an important enteric pathogen that causes serious systemic infections in swine and humans. Here, it was found that, when introduced into Escherichia coli, a recombinant plasmid carrying the pagC gene from a plasmid-based genomic library of S. enterica serovar Choleraesuis conferred a high-level resistance to the bactericidal activity of pooled normal swine serum. The resistance was equal to the level conferred by rck, a gene encoding a 17 kDa outer-membrane protein which promotes the serum resistance phenotype in S. enterica serovar Typhimurium. Insertional mutagenesis of the cloned pagC gene generated a mutation that resulted in the loss of the serum resistance phenotype in E. coli. When this mutation was introduced into the chromosome of S. enterica serovar Choleraesuis by homology recombination with the wild-type allele, the resulting strain could not produce PagC, and it showed a decreased level of resistance to complement-mediated killing. The mutation could be restored by introduction of the intact pagC gene on a plasmid, but not by introduction of the point-mutated pagC gene. In addition, PagC was able to promote serum resistance in the S. enterica serovar Choleraesuis LPS mutant strain, which is highly sensitive to serum killing. Although PagC is not thought to confer serum resistance directly, these results strongly suggest that PagC is an important outer-membrane protein that plays an important role in the serum resistance of S. enterica serovar Choleraesuis.

Role of the human C8 subunits in complement-mediated bacterial killing: evidence that C8 gamma is not essential

Human C8 is one of five complement components (C5b, C6, C7, C8 and C9) that interact to form the cytolytic membrane attack complex (MAC) on bacterial cell membranes. It is an oligomeric protein composed of a disulfide-linked C8 alpha-gamma heterodimer and a non-covalently associated C8 beta chain. Previous studies revealed that C8 alpha and C8 beta have distinct roles in the formation of the MAC on simple cells such as erythrocytes and that both subunits are essential for cell lysis. These studies also determined that C8 gamma is not required for expression of MAC hemolytic activity. To determine if these conclusions are applicable to more biologically relevant systems, the C8 subunits were examined for their ability to support complement-mediated killing of Gram-negative bacteria. Results indicate: (1) C8 alpha-gamma, C8 alpha, C8 beta and C8 gamma have no independent bactericidal activity; (2) bacterial killing requires C8 beta and either C8 alpha-gamma or C8 alpha; (3) C8 alpha is an effective substitute for C8 alpha-gamma in bacterial killing; and (4) C8 gamma enhances, but is not required for C8 bactericidal activity. Together, these data suggest that C8 alpha and C8 beta have correspondingly similar roles in MAC-mediated lysis of erythrocytes and bacterial killing. Furthermore, they provide the first direct evidence that C8 gamma is not required for complement-mediated killing of Gram-negative bacteria.

Complement-resistance mechanisms of bacteria

Despite more than a century of parallel research on bacteria and the complement system, relatively little is known of the mechanisms whereby pathogenic bacteria can escape complement-related opsonophagocytosis and direct killing. It is likely that pathogenicity in bacteria has arisen more accidentally than in viruses, and on the basis of selection from natural mutants rather than by outright stealing or copying of genetic codes from the host. In this review we will discuss complement resistance as one of the features that makes a bacterium a pathogen.

Acquired resistance of Escherichia coli to complement lysis by binding of glycophosphoinositol-anchored protectin (CD59)

Protectin (CD59) is a glycophosphoinsitol (GPI)-anchored defender of human cells against lysis by the membrane attack complex of complement. In this study, we examined whether protectin released from human cell membranes can incorporate into the surface of gram-negative bacteria. Analysis by using radiolabeled protectin, immunofluorescence, flow cytometry, and whole-cell enzyme-linked immunosorbent assay demonstrated that protectin bound to nonencapsulated Escherichia coli EH237 (Re) and EH234 (Ra) in a calcium-dependent manner. The incorporation required the GPI-phospholipid moiety since no binding of a phospholipid-free soluble form of protectin was observed. Mg2+ did not enhance the binding, and a polysialic acid capsule prevented it (strain IH3080 [O18:K1:H8]). Bound protectin inhibited the C5b-9 neoantigen expression on complement-treated bacteria. Protection against complement lysis was observed in both a colony counting assay and a bioluminescence assay, where viable EH234 bacteria expressing the luciferase gene emitted green light in the presence of the luciferine substrate. In general, two- to four-times-higher serum concentrations were needed to obtain 50% lysis of protectin-coated versus noncoated bacteria. The results indicate that protectin can incorporate in a functionally active form into the cell membranes of the two nonencapsulated deep rough E. coli strains studied.

Mechanism of resistance to complement-mediated killing of bacteria encoded by the Salmonella typhimurium virulence plasmid gene rck

We find that pADEO16, a recombinant cosmid carrying the rck gene of the Salmonella typhimurium virulence plasmid, when cloned into either rough or smooth Escherichia coli and Salmonella strains, confers high level resistance to the bactericidal activity of pooled normal human serum. The rck gene encodes a 17-kD outer membrane protein that is homologous to a family of virulence-associated outer membrane proteins, including pagC and Ail. Complement depletion, C3 and C5 binding, and membrane-bound C3 cleavage products are similar in strains with and without rck. Although a large difference in C9 binding was not seen, trypsin cleaved 55.7% of bound 125I-C9 counts from rough S. typhimurium with pADEO16, whereas only 26.4% were released from S. typhimurium with K2011, containing a mutation in rck. The majority of C9 extracted from rck strain membranes sediments at a lower molecular weight than in strains without rck, suggesting less C9 polymerization. Furthermore, SDS-PAGE analysis of gradient peak fractions indicated that the slower sedimenting C9-containing complexes in rck strains did not contain polymerized C9 typical of the tubular membrane attack complex. These results indicate that complement resistance mediated by Rck is associated with a failure to form fully polymerized tubular membrane attack complexes.

Transfer of preformed terminal C5b-9 complement complexes into the outer membrane of viable gram-negative bacteria: effect on viability and integrity

An efficient fusion system between Gram-negative bacteria and liposomes incorporating detergent-extracted C5b-9 complexes has been developed that allows delivery of preformed terminal complexes to the cell envelope (Tomlinson et al., 1989b). Fusion of Salmonella minnesota Re595 and Escherichia coli 17 with C5b-9-incorporated liposomes resulted in the transfer of 1900 C5b-9 complexes to each target bacterial cell. No loss in viability of bacteria was observed following fusion, even though the deposotion of 900 complexes onto the envelope following exposure to lysozyme-free serum effected a greater than 99% loss of viability. Increased sensitivity to antibiotics normally excluded from the cell by an integral outer membrane (OM), as well as the ability of the chromogenic substrate PADAC to gain access to periplasmically located beta-lactamase, indicated that transferred C5b-9 complexes functioned as water-filled channels through the OM. A similar conclusion was drawn from measurements demonstrating the uptake by cells of the lipophilic cation tetraphenylphosphonium (bromide), a result further indicating that the membrane potential across the cytoplasmic membrane was maintained following C5b-9 transfer to the OM. Examination of S. minnesota Re595 by electron microscopy revealed no obvious difference between cells exposed to lethal concentrations of lysozyme-free serum and cells following fusion with C5b-9-incorporated liposomes. These data suggest either that there are critical sites in the OM to which liposome-delivered C5b-9 complexes are unable to gain access or that bacterial cell death is related to events occurring during polymerization of C9 on the cell surface.