Circular polymerization of the membranolytic ninth component of complement. Dependence on metal ions

Large and Stable Nanopores Formed by Complement Component 9 for Characterizing Single Folded Proteins

Biological nanopores offer a promising approach for single-molecule analysis of nucleic acids, peptides, and proteins. The work presented here introduces a biological nanopore formed by the self-assembly of complement component 9 (C9). This exceptionally large and cylindrical protein pore is composed of 20 ± 4 monomers of C9 resulting in a diameter of 10 ± 4 nm and an effective pore length of 13 nm. These poly(C9) pores remain stable for up to 30 min without indications of gating, flickering, or clogging across a range of transmembrane voltages (-150 to +150 mV) and ionic strengths (50 to 1000 mM). At physiologic pH, the ring-shaped distribution of negative and positive surface charges in the lumen of the pore enables capture of analyte proteins by electro-osmotic flow and leads to residence times of analyte proteins whose most probable values can exceed 300 μs. We used poly(C9) nanopores to determine the volume and shape of unlabeled folded proteins with molecular weights between 9 and 230 kDa with unprecedented accuracy in the context of resistive pulse recordings. Finally, poly(C9) pores made it possible to distinguish between the open and closed conformations of adenylate kinase based on differences in current modulations within resistive pulses and the corresponding differences in approximations of their shape. Thus, poly(C9) nanopores enable highly sensitive and accurate characterization of a wide range of natively folded proteins on a single molecule level.

Plasmid-encoded toxin of Escherichia coli cleaves complement system proteins and inhibits complement-mediated lysis in vitro

Plasmid-encoded toxin (Pet) is an autotransporter protein of the serine protease autotransporters of Enterobacteriaceae (SPATE) family, important in the pathogenicity of Escherichia coli. The pet gene was initially found in the enteroaggregative E. coli (EAEC) virulence plasmid, pAA2. Although this virulence factor was initially described in EAEC, an intestinal E. coli pathotype, pet may also be present in other pathotypes, including extraintestinal pathogenic strains (ExPEC). The complement system is an important defense mechanism of the immune system that can be activated by invading pathogens. Proteases produced by pathogenic bacteria, such as SPATEs, have proteolytic activity and can cleave components of the complement system, promoting bacterial resistance to human serum. Considering these factors, the proteolytic activity of Pet and its role in evading the complement system were investigated. Proteolytic assays were performed by incubating purified components of the complement system with Pet and Pet S260I (a catalytic site mutant) proteins. Pet, but not Pet S260I, could cleave C3, C5 and C9 components, and also inhibited the natural formation of C9 polymers. Furthermore, a dose-dependent inhibition of ZnCl2-induced C9 polymerization in vitro was observed. E. coli DH5α survived incubation with human serum pre-treated with Pet. Therefore, Pet can potentially interfere with the alternative and the terminal pathways of the complement system. In addition, by cleaving C9, Pet may inhibit membrane attack complex (MAC) formation on the bacterial outer membrane. Thus, our data are suggestive of a role of Pet in resistance of E. coli to human serum.

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 flanking731Leu–780 Leu 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.

Elucidating the Immune Evasion Mechanisms of Borrelia mayonii, the Causative Agent of Lyme Disease

Borrelia (B.) mayonii sp. nov. has recently been reported as a novel human pathogenic spirochete causing Lyme disease (LD) in North America. Previous data reveal a higher spirochaetemia in the blood compared to patients infected by LD spirochetes belonging to the B. burgdorferi sensu lato complex, suggesting that this novel genospecies must exploit strategies to overcome innate immunity, in particular complement. To elucidate the molecular mechanisms of immune evasion, we utilized various methodologies to phenotypically characterize B. mayonii and to identify determinants involved in the interaction with complement. Employing serum bactericidal assays, we demonstrated that B. mayonii resists complement-mediated killing. To further elucidate the role of the key regulators of the alternative pathway (AP), factor H (FH), and FH-like protein 1 (FHL-1) in immune evasion of B. mayonii, serum adsorption experiments were conducted. The data revealed that viable spirochetes recruit both regulators from human serum and FH retained its factor I-mediated C3b-inactivating activity when bound to the bacterial cells. In addition, two prominent FH-binding proteins of approximately 30 and 18 kDa were detected in B. mayonii strain MN14-1420. Bioinformatics identified a gene, exhibiting 60% identity at the DNA level to the cspA encoding gene of B. burgdorferi. Following PCR amplification, the gene product was produced as a His-tagged protein. The CspA-orthologous protein of B. mayonii interacted with FH and FHL-1, and both bound regulators promoted inactivation of C3b in the presence of factor I. Additionally, the CspA ortholog counteracted complement activation by inhibiting the alternative and terminal but not the classical and Lectin pathways, respectively. Increasing concentrations of CspA of B. mayonii also strongly affected C9 polymerization, terminating the formation of the membrane attack complex. To assess the role of CspA of B. mayonii in facilitating serum resistance, a gain-of-function strain was generated, harboring a shuttle vector allowing expression of the CspA encoding gene under its native promotor. Spirochetes producing the native protein on the cell surface overcame complement-mediated killing, indicating that CspA facilitates serum resistance of B. mayonii. In conclusion, here we describe the molecular mechanism utilized by B. mayonii to resists complement-mediated killing by capturing human immune regulators.

Cooperation between Hsp90 and mortalin/GRP75 in resistance to cell death induced by complement C5b-9

Cancer cells are commonly more resistant to cell death activated by the membranolytic protein complex C5b-9. Several surface-expressed and intracellular proteins that protect cells from complement-dependent cytotoxicity (CDC) have been identified. In this study, we investigated the function of heat shock protein 90 (Hsp90), an essential and ubiquitously expressed chaperone, overexpressed in cancer cells, in C5b-9-induced cell death. As shown, inhibition of Hsp90 with geldanamycin or radicicol is enhancing sensitivity of K562 erythroleukemia cells to CDC. Similarly, Hsp90 inhibition confers in Ramos B cell lymphoma cells elevated sensitivity to treatment with rituximab and complement. C5b-9 deposition is elevated on geldanamycin-treated cells. Purified Hsp90 binds directly to C9 and inhibits zinc-induced C9 polymerization, indicating that Hsp90 may act directly on the C5b-9 complex. Mortalin, also known as stress protein 70 or GRP75, is a mitochondrial chaperone that confers resistance to CDC. The postulated cooperation between Hsp90 and mortalin in protection from CDC was tested. Geldanamycin failed to sensitize toward CDC cells with knocked down mortalin. Direct binding of Hsp90 to mortalin was shown by co-immunoprecipitation in cell extracts after triggering with complement as well as by using purified recombinant proteins. These results provide an insight into the protective mechanisms utilized by cancer cells to evade CDC. They suggest that Hsp90 protects cells from CDC by inhibiting, together with mortalin, C5b-9 assembly and/or stability at the plasma membrane.

iTRAQ-Based Proteomics Analysis of Serum Proteins in Wistar Rats Treated with Sodium Fluoride: Insight into the Potential Mechanism and Candidate Biomarkers of Fluorosis

Fluorosis induced by exposure to high level fluoride is quite widespread in the world. The manifestations of fluorosis include dental mottling, bone damage, and impaired malfunction of soft tissues. However, the molecular mechanism of fluorosis has not been clarified until now. To explore the underlying mechanisms of fluorosis and screen out serum biomarkers, we carried out a quantitative proteomics study to identify differentially expressed serum proteins in Wistar rats treated with sodium fluoride (NaF) by using a proteomics approach of isobaric tagging for relative and absolute quantitation (iTRAQ). We fed Wistar rats drinking water that had 50, 150, and 250 mg/L of dissolved NaF for 24 weeks. For the experimental duration, each rat was given an examination of the lower incisors to check for the condition of dental fluorosis (DF). By the end of the treatment, fluoride ion concentration in serum and lower incisors were detected. The results showed that NaF treatment can induce rat fluorosis. By iTRAQ analysis, a total of 37 differentially expressed serum proteins were identified between NaF-treated and control rats. These proteins were further analyzed by bioinformatics, out of which two proteins were validated by enzyme-linked immunoadsorbent assays (ELISA). The major proteins were involved in complement and coagulation cascade, inflammatory response, complement activation, defense response, and wound response, suggesting that inflammation and immune reactions may play a key role in fluorosis pathogenesis. These proteins may contribute to the understanding of the mechanism of fluoride toxicity, and may serve as potential biomarkers for fluorosis.

BGA66 and BGA71 facilitate complement resistance of Borrelia bavariensis by inhibiting assembly of the membrane attack complex

Borrelia (B.) bavariensis exhibits a marked tropism for nervous tissues and frequently causes neurological manifestations in humans. The molecular mechanism by which B. bavariensis overcomes innate immunity, in particular, complement remains elusive. In contrast to other serum-resistant spirochetes, none of the B. bavariensis isolates investigated bound complement regulators of the alternative (AP) and classical pathway (CP) or proteolytically inactivated complement components. Focusing on outer surface proteins BGA66 and BGA71, we demonstrated that both molecules either inhibit AP, CP and terminal pathway (TP) activation, or block activation of the CP and TP respectively. Both molecules bind complement components C7, C8 and C9, and thereby prevent assembly of the terminal complement complex. This inhibitory activity was confirmed by the introduction of the BGA66 and BGA71 encoding genes into a serum-sensitive B. garinii strain. Transformed spirochetes producing either BGA66 or BGA71 overcome complement-mediated killing, thus indicating that both proteins independently facilitate serum resistance of B. bavariensis. The generation of C-terminally truncated proteins as well as a chimeric BGA71 protein lead to the localization of the complement-interacting binding site within the N-terminus. Collectively, our data reveal a novel immune evasion strategy of B. bavariensis that is directed against the activation of the TP.

Genetic Association of the Porcine C9 Complement Component with Hemolytic Complement Activity

The complement system is a part of the natural immune regulation mechanism against invading pathogens. Complement activation from three different pathways (classical, lectin, and alternative) leads to the formation of C5-convertase, an enzyme for cleavage of C5 into C5a and C5b, followed by C6, C7, C8, and C9 in membrane attack complex. The C9 is the last complement component of the terminal lytic pathway, which plays an important role in lysis of the target cells depending on its self-polymerization to form transmembrane channels. To address the association of C9 with traits related to disease resistance, the complete porcine C9 cDNA was comparatively sequenced to detect single nucleotide polymorphisms (SNPs) in pigs of the breeds Hampshire (HS), Duroc (DU), Berlin miniature pig (BMP), German Landrace (LR), Pietrain (PIE), and Muong Khuong (Vietnamese potbelly pig). Genotyping was performed in 417 F2 animals of a resource population (DUMI: DU×BMP) that were vaccinated with Mycoplasma hyopneumoniae, Aujeszky diseases virus and porcine respiratory and reproductive syndrome virus at 6, 14 and 16 weeks of age, respectively. Two SNPs were detected within the third exon. One of them has an amino acid substitution. The European porcine breeds (LR and PIE) show higher allele frequency of these SNPs than Vietnamese porcine breed (MK). Association of the substitution SNP with hemolytic complement activity indicated statistically significant differences between genotypes in the classical pathway but not in the alternative pathway. The interactions between eight time points of measurement of complement activity before and after vaccinations and genotypes were significantly different. The difference in hemolytic complement activity in the both pathways depends on genotype, kind of vaccine, age and the interaction to the other complement components. These results promote the porcine C9 (pC9) as a candidate gene to improve general animal health in the future.

Bacteria under stress by complement and coagulation

The complement and coagulation systems are two related protein cascades in plasma that serve important roles in host defense and hemostasis, respectively. Complement activation on bacteria supports cellular immune responses and leads to direct killing of bacteria via assembly of the Membrane Attack Complex (MAC). Recent studies have indicated that the coagulation system also contributes to mammalian innate defense since coagulation factors can entrap bacteria inside clots and generate small antibacterial peptides. In this review, we will provide detailed insights into the molecular interplay between these protein cascades and bacteria. We take a closer look at how these pathways are activated on bacterial surfaces and discuss the mechanisms by which they directly cause stress to bacterial cells. The poorly understood mechanism for bacterial killing by the MAC will be reevaluated in light of recent structural insights. Finally, we highlight the strategies used by pathogenic bacteria to modulate these protein networks. Overall, these insights will contribute to a better understanding of the host defense roles of complement and coagulation against bacteria.

Mortalin/GRP75 binds to complement C9 and plays a role in resistance to complement-dependent cytotoxicity

Mortalin/GRP75, the mitochondrial heat shock protein 70, plays a role in cell protection from complement-dependent cytotoxicity (CDC). As shown here, interference with mortalin synthesis enhances sensitivity of K562 erythroleukemia cells to CDC, whereas overexpression of mortalin leads to their resistance to CDC. Quantification of the binding of the C5b-9 membrane attack complex to cells during complement activation shows an inverse correlation between C5b-9 deposition and the level of mortalin in the cell. Following transfection, mortalin-enhanced GFP (EGFP) is located primarily in mitochondria, whereas mortalinΔ51-EGFP lacking the mitochondrial targeting sequence is distributed throughout the cytoplasm. Overexpressed cytosolic mortalinΔ51-EGFP has a reduced protective capacity against CDC relative to mitochondrial mortalin-EGFP. Mortalin was previously shown by us to bind to components of the C5b-9 complex. Two functional domains of mortalin, the N-terminal ATPase domain and the C-terminal substrate-binding domain, were purified after expression in bacteria. Similar to intact mortalin, the ATPase domain, but not the substrate-binding domain, was found to bind to complement proteins C8 and C9 and to inhibit zinc-induced polymerization of C9. Binding of mortalin to complement C9 and C8 occurs through an ionic interaction that is nucleotide-sensitive. We suggest that to express its full protective effect from CDC, mortalin must first reach the mitochondria. In addition, mortalin can potentially target the C8 and C9 complement components through its ATPase domain and inhibit C5b-9 assembly and stability.

Mapping of the complement C9 binding domain on Trichinella spiralis paramyosin

Background

Trichinellosis is an important foodborne zoonosis that is distributed worldwide. Trichinella spiralis may evade host complement-mediated attack by expressing complement inhibitory proteins, such as paramyosin (Pmy). Previous studies have shown that Trichinella spiralis paramyosin (Ts-Pmy) is able to bind to the human complement component C9 to inhibit the complement activation and protect the parasite from complement-mediated attack. Further determination of the complement-binding domain on Ts-pmy will enable us to better understand the Ts-Pmy’s biofunction in the immune evasion and provide feasible approach to develop epitope-based subunit vaccine against trichinellosis.

Methods

The complement C9 binding region on Ts-Pmy was determined by expression of overlapped fragments of Ts-Pmy and their binding activities to C9. The exact binding site was further narrowed-down to a 14-amino acid peptide at C-terminus using synthesized peptides with different size of amino acid sequence. The C9 complement-binding of the 14-amino acid peptide and its interference in the C9 polymerization and the complement-mediated lysis of rabbit erythrocytes was investigated.

Results

The protein interaction between human C9 and native Ts-Pmy was further confirmed by immunoprecipitation with T. spiralis lysates. The fragmental expression and C9 binding assays identified that the binding region of Ts-Pmy to C9 is located within 831–885 of Ts-Pmy C-terminus. The exact binding site on Ts-Pmy to C9 was narrowed down to 14 amino acid residues (⁸⁶⁶Val-⁸⁷⁹Met) by using different sizes of synthesized peptides. In the presence of the synthesized 14-amino acid peptide, human C9 polymerization and the hemolytic activity of the human complement was inhibited.

Conclusions

Our results revealed the precise molecular basis for T. spiralis to produce Ts-Pmy as an immunomodulator to evade the attack of the host complement system as a survival mechanism.

Trichinella spiralis paramyosin binds to C8 and C9 and protects the tissue-dwelling nematode from being attacked by host complement

Background

Paramyosin is a thick myofibrillar protein found exclusively in invertebrates. Evidence suggested that paramyosin from helminths serves not only as a structural protein but also as an immunomodulatory agent. We previously reported that recombinant Trichinella spiralis paramyosin (Ts-Pmy) elicited a partial protective immunity in mice. In this study, the ability of Ts-Pmy to bind host complement components and protect against host complement attack was investigated.

Methods and Findings

In this study, the transcriptional and protein expression levels of Ts-Pmy were determined in T. spiralis newborn larva (NBL), muscle larva (ML) and adult worm developmental stages by RT-PCR and western blot analysis. Expression of Ts-Pmy at the outer membrane was observed in NBL and adult worms using immunogold electron microscopy and immunofluorescence staining. Functional analysis revealed that recombinant Ts-Pmy(rTs-Pmy) strongly bound to complement components C8 and C9 and inhibited the polymerization of C9 during the formation of the membrane attack complex (MAC). rTs-Pmy also inhibited the lysis of rabbit erythrocytes (E_R) elicited by an alternative pathway-activated complement from guinea pig serum. Inhibition of native Ts-Pmy on the surface of NBL with a specific antiserum reduced larvae viability when under the attack of complement in vitro. In vivo passive transfer of anti-Ts-Pmy antiserum and complement-treated larvae into mice also significantly reduced the number of larvae that developed to ML.

Conclusion

These studies suggest that the outer membrane form of T. spiralis paramyosin plays an important role in the evasion of the host complement attack.

Trichinellosis is a serious food borne parasitic disease caused by the consumption of meat contaminated with the infective larvae of Trichinella spiralis. The ability of the tissue-dwelling parasite to evade the host complement attack is essential for its survival and for establishing infection in the host. This study describes the expression of paramyosin, a muscular protein in invertebrates, on the surface of Trichinella spiralis and its role in the defense against the host complement attack as a survival strategy. Using a specific antiserum, expression of Trichinella spiralis paramyosin was detected on the outer membrane of the adult worms and newborn larvae. Functional analysis revealed that recombinant Trichinella spiralis paramyosin protein strongly bound human complement components C8 and C9 and inhibited the formation of the complement membrane attack complex. Neutralization with a specific antiserum greatly impaired the protective effect of paramyosin on the viability and infectivity of Trichinella spiralis newborn larva when under attack by complement. These studies suggest that the outer membrane form of Trichinella spiralis paramyosin plays an important role in the evasion of the host complement attack and is therefore a good target for vaccine and pharmaceutical development.

The structure and function of mammalian membrane-attack complex/perforin-like proteins

The membrane-attack complex (MAC) of complement pathway and perforin (PF) are important tools deployed by the immune system to target pathogens. Both perforin and the C9 component of the MAC contain a common 'MACPF' domain and form pores in the cell membrane as part of their function. The MAC targets gram-negative bacteria and certain pathogenic parasites, while perforin, released by natural killer cells or cytotoxic T lymphocytes (CTLs), targets virus-infected and transformed host cells (1). Remarkably, recent structural studies show that the MACPF domain is homologous to the pore-forming portion of bacterial cholesterol-dependent cytolysins; these data have provided important insight into the mechanism of pore-forming MACPF proteins. In addition to their role in immunity, MACPF family members have been identified as animal venoms, factors required for pathogen migration across host cell membranes and factors that govern developmental processes such as embryonic patterning and neuronal guidance (2). While most MACPF proteins characterized to date either form pores or span lipid membranes, some do not (e.g. the C6 component of the MAC). A current challenge is thus to understand the role, pore forming or otherwise, of MACPF proteins in developmental biology. This review discusses structural and functional diversity of the mammalian MACPF proteins.

Mortalin inhibitors sensitize K562 leukemia cells to complement-dependent cytotoxicity

Mortalin, the mitochondrial hsp70, is a vital constitutively expressed heat shock protein. Its elevated expression has been correlated with malignant transformation and poor cancer prognosis. Cancer cells exhibit increased resistance to complement-dependent cytotoxicity, partly due to their capacity to eliminate the complement membrane attack complex (MAC) from their cell surface. As we have previously reported, mortalin and the complement membrane attack complexes are released in membrane vesicles from complement attacked cells. As shown here, knock down of mortalin with specific siRNA reduces MAC elimination and enhances cell sensitivity to MAC-induced cell death. Similar results were obtained with MKT-077, a cationic rhodacyanine dye that inhibits mortalin. Treatment of human erythroleukemia K562 and colorectal carcinoma HCT116 cells with MKT-077 sensitizes them to cell death mediated by MAC but not by streptolysin O. Pre-treatment of cells with MKT-077 also reduces the extent of MAC-mortalin vesiculation following a sublytic complement attack. In the presence of MKT-077, the direct binding of mortalin to complement C9, the major MAC component, is inhibited. The tumor suppressor protein p53 is a known mortalin client protein. The effect of MKT-077 on complement-mediated lysis of HCT116 p53(+/+) and p53(-/-) cells was found to be independent on the presence of p53. Our results also demonstrate that recombinant human mortain inhibits complement-mediated hemolysis of rabbit erythrocytes as well as zinc-induced C9 polymerization. We conclude that mortalin supports cancer cell resistance to complement-dependent cytotoxicity and propose consideration of mortalin as a novel target for cancer adjuvant immunotherapy.

Mapping of the complement C9 binding domain in paramyosin of the blood fluke Schistosoma mansoni

Schistosomes are believed to evade complement-mediated damage by expression of complement inhibitory proteins. Our previous results [Deng, J., Gold, D., LoVerde, P.T., Fishelson, Z., 2003. Inhibition of the complement membrane attack complex by Schistosoma mansoni paramyosin. Infect. Immun. 71, 6402-6410.] have demonstrated that paramyosin (Pmy) of the blood fluke S. mansoni binds to the human complement proteins C8 and C9, inhibits complement activation at the terminal stage and protects the parasite from complement-mediated damage. In order to locate the Pmy binding site to C8 and C9, various fragments of Pmy cDNA were PCR-cloned into a pET28a bacterial expression vector. Recombinant His-tagged Pmy fragments were expressed in BL21 Escherichia coli and purified over a nickel-nitrilotriacetic acid column. Binding assays by Western blotting with monoclonal anti-His antibody demonstrated that PmyCC (Pmy amino acids (744)Asp-(866)Met) was the only Pmy fragment that bound to human C8 and C9. Functional analyses demonstrated that PmyCC inhibited hemolysis of rabbit erythrocytes and of antibody-sensitized sheep erythrocytes by human complement. Importantly, PmyCC inhibited in vitro killing of trypsin-sensitized schistosomula of S. mansoni by human complement. In the presence of PmyCC, Zn(2+)-induced C9 polymerization was inhibited. Most of the immunodominant B-cell antigenic epitopes of Pmy are present in the PmyCC region, as antibodies collected from mice immunized with recombinant Pmy bound primarily to PmyCC. Taken together, this study has mapped the complement regulatory domain in Pmy, capable of binding to C8 and C9 and preventing polyC9 formation, to its C-terminal region.

Inhibition of the complement membrane attack complex by Schistosoma mansoni paramyosin

Larvae and adults of the parasitic blood fluke Schistosoma mansoni are resistant to killing by human complement. An earlier search by Parizade et al. for a schistosome complement inhibitor identified a 94-kDa surface protein which was named SCIP-1 (M. Parizade, R. Arnon, P. J. Lachmann, and Z. Fishelson, J. Exp. Med. 179:1625-1636, 1994). Following partial purification and analysis by mass spectrometry, we have determined SCIP-1 to be a surface-exposed form of the muscle protein paramyosin. As shown by immunofluorescence, anti-paramyosin antibodies label the surface of live schistosomula and adult worms. Like SCIP-1, purified native paramyosin reacts with a polyclonal rabbit anti-human CD59 antiserum, as shown by Western blot analysis. Also, the human complement components C8 and C9 bind to recombinant and native paramyosin. Analysis of paramyosin binding to fragments of C9 generated by thrombin or trypsin has demonstrated that paramyosin binds to C9 at a position located between Gly245 and Arg391. Paramyosin inhibited Zn(2+)-induced C9 polymerization and poly-C9 deposition onto rabbit erythrocytes (E(R)). In addition, paramyosin inhibited lysis of E(R) and of sensitized sheep erythrocytes by human complement. Finally, anti-paramyosin antibodies enhanced in vitro killing of schistosomula by normal and C4-depleted human complement. Taken together, these findings suggest that an exogenous form of S. mansoni paramyosin inhibits activation of the terminal pathway of complement and thus has an important immunomodulatory role in schistosomiasis.

Phosphorylation of the complement component, C9, by an ecto-protein kinase of human leukemic cells

Ecto-protein kinases (ecto-PK) are surface constituents of many, if not all, animal cell types; normal, transformed or malignant. The occurrence of ecto-PK on the surface of human leukemia cell lines was described [Paas, Y., Fishelson, Z., 1995. Shedding of tyrosine and serine/threonine ecto-PK from human leukemic cells. Arch. Biochem. Biophys. 316 780-788.]. These ecto-PKs have been shown to phosphorylate several exogenous substrates, including the complement C9 protein, an essential component of the terminal complement system. C9 is phosphorylated by ecto-PK of K562 cells on serine residue(s). Phosphorylation occurs in the N-terminal C9a portion produced by cleavage of phosphorylated C9 with human alpha-thrombin. C9 polymers generated upon incubation of C9 with ZnCl2 do not serve as substrates for the K562 ecto-PK. In contrast, unfolded C9, obtained by reduction and alkylation, serves as a superior substrate for the K562 ecto-PK. Native C9 phosphorylation produced a rather low stoichiometry of incorporated phosphate (around 3%) per C9. Despite that, the phosphorylated C9 expressed reduced hemolytic activity. The complement-sensitive variant of K562 (K562/S) did not express the C9 phosphorylating activity. Various PK inhibitors tested failed to block C9 phosphorylation. Only heparin and 2,3-diphosphoglycerate (dpGA) prevented C9 phosphorylation, indicating that the ecto-PK is related to the casein kinase CK2. C9 can be phosphorylated by ecto-PK from other tumor cells, including Jurkat, SK-OV-3 and BT-474. These results suggest that extracellular phosphorylation of C9 may serve as a protective mechanism against complement in tumor cells.

Complement C5b-9 increases plasminogen binding and activation on human endothelial cells

Deposition of the terminal complement proteins (C5b-9) on human endothelial cells can result in cell lysis or nonlytic alterations of cell function including procoagulant responses. Because regulation of fibrinolysis is a central endothelial function and because C9 contains a carboxyl-terminal lysine similar to other proteins that bind and facilitate activation of plasminogen (PG), the effects of complement injury on PG binding and activation on these cells were investigated. Activation of complement through deposition of C5b67 complexes on endothelial cells resulted in a small increase (approximately 20%) in PG binding. Incorporation of C8 into C5b-8 resulted in no further increase in binding; however, specific 125I-PG binding was increased by approximately 100% after C5b-9 deposition. Moreover, PG was found to bind specifically to C7 and C9. The PG bound to endothelial cells after C5b-9 deposition was readily activated by tissue-type plasminogen activator (TPA). In a cell-free system, complement C9 and a synthetic peptide composed of the 20 carboxyl-terminal amino acids of C9 enhanced PG activation by TPA. Removal of the carboxyl-terminal lysine of C9 abolished the enhancement of PG activation without diminishing PG binding. We conclude that membrane C9 may comprise a binding site for PG and serve to enhance activation of this zymogen by TPA. These findings suggest that immune injury to the endothelium may enhance both the fibrin-generating and fibrinolytic capacity of the vessel wall.

Affinity of the C9 molecule for the C5b-8 complex compared with that for the complex containing C9 molecules

Gram-negative bacterial cells exposed to a complement source may carry membrane attack complexes containing variable numbers of C9 molecules per C5b-8 site. In order to investigate the assembly of this complex, the ability of C9 molecules to bind to C5b-8 complexes was compared with the binding characteristics of C9 for C5b-8 complexes containing variable numbers of bound C9 molecules. The apparent dissociation constant (Kd) of the C9 molecule for the C5b-8 site on a complement-sensitive strain of Escherichia coli was 1.2 (+/- 0.15) nM at 0 degree C. These conditions allow the binding of one C9 molecule per C5b-8 site. The C5b-8 site containing one C9 molecule bound a second C9 molecule at 0 degree C only after incubation at 37 degrees C. The binding of C9 to a C5b-8 site containing one C9 molecule was found to be 1.3 (+/- 0.2) nM. Therefore, the presence of a C9 molecule did not significantly alter the binding capacity of the C5b-8 site for additional C9 molecules. A similar result was obtained by using rabbit erythrocytes bearing either C5b-8 sites or C5b-8 sites containing one molecule of C9 per complex at 0 degree C. The similarity of binding characteristics for the first and second C9 molecules argues that the initial C9 molecule in the complex does not affect the binding of subsequent C9 molecules. This suggests that a unique C9 binding site that does not involve previously bound C9 molecules may exist on the forming membrane attack complex.

The membrane attack complex of complement. Assembly, structure and cytotoxic activity

The membrane attack complex of complement is formed by the molecular fusion of the five terminal complement proteins, C5, C6, C7, C8, and C9. While the assembly process on a target membrane and its modulation by restriction factors present on host cells is now quite well understood the molecular details of the architecture of the complex still need much further clarification. This is especially true for the interaction of the last acting protein C9, which provides the cytotoxic action of the complex, with the precursor C5b-8 complex. Because of this lack of structural details the molecular mechanisms that lead to complement-mediated cell death remain cryptic, however, it is hoped that recent advances in controlling the assembly process and in site-specific modification of the terminal complement proteins by recombinant DNA techniques should change this predicament quickly.

A method for in vitro synthesis of unglycosylated recombinant complement component C9

A method for in vitro synthesis of human complement component C9 has been established in order to generate unglycosylated normal and mutant proteins without the need to sub-clone. One or two step polymerase chain reaction (PCR) was used to add the T7 RNA polymerase promoter and introduce multiple mutations within the cDNA. The cDNA was then transcribed by T7 RNA polymerase and the mRNA translated in a rabbit reticulocyte lysate or wheat germ system. Successful synthesis was confirmed by: the correct size of PCR product DNA on agarose gel electrophoresis, incorporation of [alpha-32P]UTP into mRNA, and formation of [35S]methionine-labelled protein of the correct molecular mass for full length C9. The wheat germ extract generated up to 1.5 micrograms of recombinant C9. This unglycosylated C9 had at least 10% of the haemolytic activity of native C9. Unglycosylated C9 polymerised more readily than the native protein. This spontaneous polymerisation was increased by removal of the first 23 amino acids or mutating two cysteines at positions 33 and 36. This therefore provides a rapid method for screening the effect of multiple mutations on the biological activity and polymerisation of pore forming proteins.

Ionic factors regulating the interaction of Gardnerella vaginalis hemolysin with red blood cells

We have studied the hemolytic properties of an exotoxin released by Gardnerella vaginalis (Gvh). We found that hemolysis induced by Gvh is modulated by the composition of the isotonic buffer in which the red cells are suspended. In particular, low pH enhances its lytic activity, whereas low ionic strength and divalent cations diminish it. The inhibitory effects of reduced salt concentration and divalent cations occur despite normal binding of the toxin to the cells. This suggests that some post-binding step is impaired. The toxin is also able to damage cholesterol-containing lipid vesicles, and even on these model membranes it is more active at low pH. From this point of view, Gvh has some similarity to Clostridium perfringens theta-toxin, a membrane-damaging toxin belonging to the family of 'thiol-activated' cytolysins produced by Gram-positive bacteria.

Structure of complement poly-C9 determined in projection by cryo-electron microscopy and single particle analysis

The ring-like complement 'lesions' found on membranes of complement lysed cells comprise a complex of components C5b through C9 that coalesce to form hollow cylinders which penetrate the membrane bilayer and create lytic pores. Walls of these C5b-9 membrane attack complex cylinders may consist primarily of the C9 component, since samples of purified, isolated C9 can polymerize into cylindrical structures which appear identical with the fully assembled C5b-9 complex. The structure of these poly-C9 molecules has been investigated using the techniques of cryo-electron microscopy and single particle analysis. Sets of single poly-C9 particles viewed as rings were selected from cryo-EM images, then particles were aligned and treated by correspondence analysis to identify the principle interparticle similarities and variations. The highest ranking variation found was the presence or absence of a dense inner ring of protein density. Other important variations were interpreted as different types of particle tilt. These results were used in selecting a subgroup of untilted particles for averaging and symmetry analysis. The rotational power spectrum of the initial average suggested 13-fold symmetry. The 13-fold symmetry was used to select and group particles for further analysis. Individual particles were 13-fold rotational averaged and those with enhanced peripheral features were placed into either a right-handed subgroup or into a left-handed subgroup based on orientation of the peripheral features. Particles within each group were aligned and averaged, and a poly-C9 structure was produced which shows important structural details and from which the C9 monomer structure can be deduced. The poly-C9 structure contains a dense inner ring of diameter between 113-181 A and which is modulated into 13 discrete peaks with peak-to-peak separation of approx. 35 A. The dense inner ring is surrounded by a less dense, concentric outer rim extending to 254 A diameter. The outer rim contains projections that are contiguous with the inner peaks but are skewed relative to the ring radius to produce the appearance of a pin-wheel. These projections correspond with the peripheral features picked up in the rotationally averaged individual particles; the left- or right-handed orientation of projections may result from the up/down orientation of individual particles in ice. The C9 monomer structure within the cylinder is suggested by the density distribution. The monomer would be a rod with diameter of 35 A, oriented parallel to the cylinder axis and would be roughly perpendicular to a membrane.(ABSTRACT TRUNCATED AT 400 WORDS)

The size, shape and stability of complement component C9

Electron microscopy of specimens of C9 tilted through 90 degrees visualized this protein to be a globular ellipsoid with dimensions of 77 x 70 x 52 A. To check the congruence of this observation with physical properties of the molecule, hydrodynamic parameters for C9 were determined. From this work a frictional ratio of 1.32 was calculated. C9 was compared with several other proteins of similar frictional ratios whose tertiary structures are known. All examples found of such proteins whose frictional ratios were between 1.26 and 1.37 are either heart-shaped or globular ellipsoids, but none are prolate ellipsoids. By comparison the size and shape of C9 determined by electron microscopy are congruent with its hydrodynamic parameters. Both electron microscopy and physical measurements suggest that the length (110-120 A) of C9 determined by neutron and X-ray scattering experiments is an overestimate. The source of the discrepancy was identified by the demonstration that the high concns of C9 employed in neutron and X-ray scattering work lead to aggregation of the protein. Thus, investigations involving neutron and X-ray scattering were measuring polydisperse solutions of C9. The deduced value of the radius of gyration from that work (33-35 A) is now recognized as being statistical and significantly higher than the correct value of monomeric C9 (26 A), which was calculated from electron microscopy measurements. Also high-resolution electron microscopy clearly visualized poly(C9) to be a barrel-stave construct. These results suggest that monomeric C9 must undergo a major conformational alteration to extend by 55-70 A in order to self-associate laterally in order to fashion the cylindrical poly(C9).

Assembly of macromolecular pores by immune defense systems

Immune defence systems (complement, cytolytic lymphocytes) make use of transmembrane pores assembled from up to 20 soluble monomers in a highly regulated process to induce cell death. Inhibitors of pore formation have been found which protect blood, endothelial and epithelial cells from the destructive effect of complement lesions. Recently, a pore-forming protein showing immunological crossreactivity to complement C9 has been found in the protozoan parasite Trypanosoma cruzi, thereby extending this protein family and generalizing its means of generating non-selective membrane permeability.

Binding properties of Paracentrotus lividus (Echinoidea) hemolysin

1. Paracentrotus lividus hemolysin binds erythrocytes, zymosan particles, lipopolysaccharide and laminarin surfaces but not auto and allogeneic cell membranes. 2. The binding could, at least for erythrocytes, involve phospholipids and cholesterol. 3. The protease activity of the coelomic fluid is not related to hemolysis. 4. The finding that very low concentrations of Zn2+ inactivate the hemolysin suggests a possible regulative function of the ion in the hemolytic reaction. 5. Ultrastructural observations on rabbit erythrocyte membranes indicate that most likely the transmembrane pores are induced by the lytic molecules.

Localization and molecular modelling of the membrane-inserted domain of the ninth component of human complement and perforin

Upon interaction with the membrane-bound C5b-8 complex, the ninth component of complement (C9) unfolds and inserts into the membrane of cells on which surface complement has been activated. Consequently C9 oligomerization occurs and transmembrane channels of varying sizes are formed. The domain of the unfolded protein interacting with the cell membrane has so far not been identified since, unlike many integral membrane proteins, the C9 sequence does not contain a continuous stretch of hydrophobic amino acids. We studied the interaction of C9 with the lipid bilayer using the membrane-restricted photoaffinity label 3-(trifluoromethyl)-3-(m[125I]iodophenyl)diazirine (125I-TID). C9 was assembled on liposomes and after photoactivation, several labeled and non-labeled peptides, obtained by chemical and enzymatic cleavage or the 125I-TID-labeled C9, were analyzed. The segment from 176 to 345 was identified as the region containing the membrane-interacting structure. By means of secondary structure predictions, we identified two amphipathic alpha-helices (292-308 and 313-333) separated by a turn (309-312). Based on these results, we constructed a molecular model for the membrane-spanning region of C9. By analogy, we also constructed a model for this domain in perforin/cytolysin, a pore-forming protein found in the cytoplasmic granules of cytotoxic T-lymphocytes.

Hemolysins: pore-forming proteins in invertebrates

Invertebrates possess lytic molecules which lyse vertebrate erythrocytes. In all the species studied so far, hemolytic activity depends on proteins which possess a wide range of reactivity. It is generally calcium-dependent and heat-labile, although calcium-independent and heat-stable hemolysins have also been detected. The molecules interact with sugars or lipids which could represent the membrane receptors by which circular lesions on target membranes are produced. On the basis of some analogies with vertebrate lytic molecules it is conceivable that the hemolysins evolved from a common ancestral gene which also led to vertebrate pore-forming proteins.

Detection of refolding conformers of complement protein C9 during insertion into membranes

Human complement protein C9 is a hydrophilic serum glycoprotein responsible for efficient expression of the cytotoxic and cytolytic functions of complement. It assembles on the surface of a target cell together with C5, C6, C7 and C8 to form the membrane attack complex (MAC) and therefore has to change structure to become an integral membrane protein. As the protein assumes a stable structure in an aqueous environment, the question arises as to how it can enter the hydrophobic interior of a membrane. During MAC assembly C9 polymerizes into a circular structure, termed poly(C9) (ref. 8), which is responsible for the cylindrical electron microscopic appearance of the MAC. The suggestion has been made that C9 must at least partly unfold in order to enter a membrane and also that polymerization of the molecule is intimately linked to insertion and cytotoxicity. The extent of unfolding and the mechanism of polymerization are not understood, nor is it known precisely which parts of the molecule participate in the proposed structural changes. We have been able to capture refolding C9 conformers during membrane insertion with the help of sequence-specific anti-peptide antibodies. Some of these antibodies inhibit C9-mediated haemolysis but not C9 polymerization, while others have the opposite effect. This suggests that the two processes are independent.

Interactions between earthworm hemolysins and sheep red blood cell membranes

The hemolytic activity exhibited by the coelomic fluid of the Annelid Eisenia fetida andrei is mediated by two lipoproteins of mass 40 and 45 kDa, each of them capable of hemolysis. Such an activity is not inhibited by zymosan, inulin or lipopolysaccharide (LPS), nor by hydrazine or methylamine, suggesting that earthworm hemolysins are not related to C3 or C3b complement components. Among the membrane lipids tested (phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol, sphingomyelin and cholesterol) only sphingomyelin inhibited hemolysis. The analysis of E.f. andrei proteins bound to sphingomyelin microvesicles, as well as to sheep red blood cell (SRBC) membranes, revealed a polymerization of E.f. andrei 40 kDa and/or 45 kDa hemolysins. Consequently, sphingomyelin appears a likely candidate for hemolytic complex receptor. Electron microscopy observations suggested that the polymerization causes an open channel through the lipid bilayer. As demonstrated using metal ions, heparin, chondroitin sulfate, poly(L-lysine) and protamine chloride, the mode of action of earthworm hemolytic complex is not analogous to that of C9 or perforine.

Immunohistochemical demonstration of age-related deposition of vitronectin (S-protein of complement) and terminal complement complex on dermal elastic fibers

Immunoreactivity of vitronectin was investigated in 100 skin specimens from different body regions in 87 individuals of different ages using monoclonal and polyclonal anti-vitronectin antibodies in an avidin-biotin-peroxidase complex technique. Vitronectin immunoreactivity was found in conjunction with dermal elastic fibers in all subjects older than 13 years. No vitronectin immunostaining was detected in subjects younger than six years, suggesting deposition of vitronectin during late childhood or early adolescence. Using an immunogold staining procedure, vitronectin immunoreactivity was ultrastructurally localized to the periphery of elastic fibers. The blood level of vitronectin in 20 healthy newborns was 67% of the adult level, suggesting active biosynthesis already in the fetus. To investigate whether vitronectin is deposited as part of the SC5b-9 complex or as uncomplexed protein, the immunoreactivity of vitronectin was compared with that of C9, using monoclonal and polyclonal antibodies against the C9 neoantigen. Distinct C9 neoantigen immunoreactivity was demonstrated in association with dermal elastic fibers in human skin in adults but only in subjects older than 30 years. The intensity of C9 neoantigen immunoreactivity appeared to increase with age and was found to be stronger in sun-exposed skin than in sun-protected skin. These findings indicate that uncomplexed vitronectin is deposited during childhood or early adolescence and that terminal complement complexes (C5b-9 and/or SC5b-9) are deposited on elastic fibers later on in life. Hypothetically, the tissue form of vitronectin may be involved in the prevention of tissue damage in proximity to local complement activation. In addition, it may be physiologically important as substratum for cells, stimulating cell migration and anchorage.

A biotin-avidin sandwich ELISA for quantification of intact complement component C9. The sera from hereditary C9 deficient individuals completely lack C9

A two-site sandwich ELISA method was developed for quantitating intact C9 protein using MoAb P40 (anti-C9b antibody). This antibody reacted with monomeric C9 but not with polymerized C9. MoAb P40 was used as a capture antibody and MoAb X195 (anti-C9a antibody) as a detection antibody. This method is highly sensitive and can detect approximately 0.5 ng/ml of native C9. No cross-reactivities of either C6, C7, or C8 were observed even at concentrations of 10 micrograms/ml per component. In addition, this method allows for measurement of only intact C9 molecules, eliminating the interference of polymerized C9 or inactivated C9. Using this assay, no C9 at all was detected in sera from inherited C9 deficient individuals, including both healthy blood donors and patients with meningococcal meningitis; although by hemolytic assay, C9 levels were reported to be less than 0.2% those of NHS. Therefore, this two-site sandwich ELISA method can replace the hemolytic assay, and is especially useful for measuring small amounts of C9 in serum.

The primary structure of the lymphocyte pore-forming protein perforin: partial amino acid sequencing and determination of isoelectric point

The murine lymphocyte pore-forming protein (PFP) was purified to apparent homogeneity by successive steps of liquid chromatography. Monospecific antibodies were raised against purified PFP that detect only one protein band in murine CTL lines. 25% of the primary sequence of PFP (134 amino acids) was determined by amino terminal analysis of the purified protein and of some of its enzymatic cleavage products. These primary sequences were identical to sequences deduced by cDNA cloning. By isoelectric focusing, PFP was found to have a pI of 6.4. On the chromatofocusing column Mono P, however, PFP was found to elute at pH 4.7. This suggests a tertiary structure for monomeric PFP that is enriched in surface acidic amino acids.

Inhibition of the lytic activity of perforin (cytolysin) and of late complement components by proteoglycans

The complement components (C6, C7, C8 and C9) implicated in the lysis of target cells and the pore-forming, lytic protein from cytotoxic T-lymphocytes and NK-cells, perforin, contain an amino acid sequence which is highly homologous to a repeat unit identified in the LDL-receptor (Tschopp et al., 1986, Nature, 322, 831-834). The domain of the LDL-receptor, which is thought to interact with a positively charged segment of its ligands apoprotein B and E, is rich in cysteine residues and contains a cluster of negative charges. We show that the negatively charged molecules suramin and glycosaminoglycans, the positively charged peptides protamine and polylysine, all of which are known to abolish binding of LDL to its receptor (Goldstein et al., 1985, A. Rev. cell. Biol., 1, 1-39) inhibit the lytic activities of C6, C7, C8, C9 and perforin. Moreover, these negatively charged molecules are potent inhibitors of cytolytic T-lymphocyte-mediated lysis of target cells, suggesting a functionally crucial role for perforin in cell-mediated cytolysis. We propose that the negatively charged, cysteine-rich domain of these complement proteins and perforin interacts with an as yet unidentified positively charged segment of its ligand in a manner analogous to the LDL-LDL receptor interaction. Homologous cysteine-rich domains in functionally unrelated proteins may therefore be functionally conserved as ideal rigid interaction domains with the conserved cysteine residues as framework. Specificity of the domain for its ligand would be conferred by the non-conserved amino acid residues.

Structural/functional similarity between proteins involved in complement- and cytotoxic T-lymphocyte-mediated cytolysis

Cytolysis mediated by complement or cytolytic lymphocytes results in the formation of morphology similar lesions in the target membrane. These lesions, formed by the polymerization of C9 or perforin respectively, contribute the major killing action by causing osmotic lysis of the target cell. Following the suggestion of Mayer that the mechanisms of humoral and cell-mediated cytotoxicity might be related, studies into the morphology of the membrane lesions formed, and the proteins responsible for causing the lesions, have shown several similarities. While the lesion caused by natural and T-killer cells is a little larger than that caused by complement, its overall shape is similar and in both cases the cylindrical pore is formed by polymerization of a monomeric subunit, C9 (relative molecular mass, Mr = 71,000) for complement, and perforin (Mr = 66,000) for cell-mediated cytotoxicity. C9 has an absolute requirement for a receptor in the target membrane formed by the earlier membrane attack complex components, C5b, C6, C7 and C8 (ref. 8). For perforin, polymerization in a target membrane requires no receptor, specificity being derived from the specific recognition between killer and target cell. Both proteins can be made to polymerize in vitro by the addition of divalent cations (Zn2+ for C9 (ref. 16) and Ca2+ for perforin) and the resultant complexes closely resemble their physiological counterparts. Antibodies raised against lymphocyte-killed targets have also been shown to cross-react with complement proteins, but the antigenically related proteins were not determined in these studies. We show here using purified proteins that perforin, C9 and complexes involving C7 and C8 share a common antigenic determinant which is probably involved in polymerization.

Increased hemolytic activity of the trypsin-cleaved ninth component of complement

Human C9 treated with trypsin is initially cleaved into two fragments with relative mol. wts of 53,000 and 20,000. This limited cleavage of C9 induces a 2.4-times increase in the hemolytic activity of C9 when compared to untreated C9. This difference diminishes when C9 activity is tested in an assay using a prolonged incubation time of C9 with C5b-8-bearing red blood cells. Trypsinization of C9 also promotes spontaneous C9 polymerization. SDS-resistant tubular C9 complexes are formed at a C9 concn of 1 mg/ml within 8 hr at 37 degrees C. Our data indicate that specific limited proteolysis of C9 not only induces spontaneous C9 polymerization but also increases the hemolytic activity of C9, suggesting that a similar molecular mechanism is involved in both processes.

Monoclonal antibodies recognizing a neoantigen of poly(C9) detect the human terminal complement complex in tissue and plasma

The terminal complement complex (TCC), consisting of C5b, C6, C7, C8, and C9, contains neoantigens that are absent from the individual native components. Neoantigens are present both in the membrane-bound (MAC) and the fluid-phase (SC5b-9) complex. The present study describes production of monoclonal antibodies against neoantigens of both forms of the TCC. A convenient screening and detection system, based mainly on enzyme-linked immunosorbent assays, crossed immunoelectrophoresis with autoradiography, and affinity chromatography with subsequent sodium dodecyl sulphate-polyacrylamide gel electrophoresis including immunoblotting, is described in detail. Two monoclonal antibodies were specific for a neoantigen located in the poly(C9) moiety of the TCC. One of these antibodies, MCaE11, was used for immunohistochemical detection of MAC in tissue and for quantification of the fluid-phase TCC in ethylenediaminetetraacetic acid plasma.

Monoclonal antibodies against neoantigens of the terminal C5b-9 complex of human complement

Assembly of the terminal C5b-C9 complement components into the cytolytic C5b-9 complex is accompanied by exposure of characteristic neoantigens on the macromolecule. We report the production and characterization of mouse monoclonal antibodies to C9-dependent neoantigens of human C5b-9. Binding-inhibition assays with EDTA-human plasma and micro-ELISA assays with purified C9 showed that the antibodies did not react with native complement components and thus confirmed the specificity of the antibodies for the neoantigens. The monoclonal antibodies did, however, cross-react with cytolytically inactive, fluid-phase C5b-9 complexes. Thus, expression of the neoantigenic determinants was not dependent on the formation of high molecular weight C9 polymers with the complex, since these are absent in fluid-phase C5b-9. Radioiodinated antibodies could be utilized in immunoradiometric assays for the detection and quantitation of C5b-9 on cell membranes. Cross-reactivities of the antibodies with C9-dependent neoantigens of several other animal species were examined and antibody clones cross-reacting with rabbit (clones 3B1, 3D8, and 2F3), sheep (clones 3D8 and 2F3) and guinea-pig (clone 3D8) neoantigens were identified. Three of four tested clones (3D8, 2F3, 1A12) precipitated C5b-9 complexes in double-diffusion assays, probably due to their interaction with multiple and repeating C9-epitopes on the terminal complexes. The monoclonal antibodies will be of value for definitive identification and quantitation of C5b-9 on cell membranes and in tissues, and for establishing immunoassays for detection and quantitation of terminal fluid-phase C5b-9 complexes in plasma.