Sensitive assay for detection of toxin-induced damage to the cytoplasmic membrane of human diploid fibroblasts

Staphylococcus aureus α-toxin: nearly a century of intrigue

Staphylococcus aureus secretes a number of host-injurious toxins, among the most prominent of which is the small β-barrel pore-forming toxin α-hemolysin. Initially named based on its properties as a red blood cell lytic toxin, early studies suggested a far greater complexity of α-hemolysin action as nucleated cells also exhibited distinct responses to intoxication. The hemolysin, most aptly referred to as α-toxin based on its broad range of cellular specificity, has long been recognized as an important cause of injury in the context of both skin necrosis and lethal infection. The recent identification of ADAM10 as a cellular receptor for α-toxin has provided keen insight on the biology of toxin action during disease pathogenesis, demonstrating the molecular mechanisms by which the toxin causes tissue barrier disruption at host interfaces lined by epithelial or endothelial cells. This review highlights both the historical studies that laid the groundwork for nearly a century of research on α-toxin and key findings on the structural and functional biology of the toxin, in addition to discussing emerging observations that have significantly expanded our understanding of this toxin in S. aureus disease. The identification of ADAM10 as a proteinaceous receptor for the toxin not only provides a greater appreciation of truths uncovered by many historic studies, but now affords the opportunity to more extensively probe and understand the role of α-toxin in modulation of the complex interaction of S. aureus with its human host.

Cellular effects of an anionic surfactant detected in V79 fibroblasts by different cytotoxicity tests

Several cytotoxicity tests were employed to detect the cellular effects of low concentrations of the anionic surfactant linear alkylbenzene sulphonate (LAS). When added to growth medium containing 5% foetal calf serum, LAS did not affect V79 cell growth, nor did it alter the permeability of cell membranes. The inactivity depended on the serum component of the medium. When treatments were carried out in serum-free saline, LAS inhibited cell proliferation, made the plasma membrane permeable to otherwise-undiffusible compounds, and reduced the uptake of tritiated thymidine. The alterations in membrane permeability were evaluated from the release of cytoplasmic molecules of different size (lactate dehydrogenase, adenine nucleotides, RNA) into the medium. The sensitivity of the spectrophotometric lactate dehydrogenase assay was inadequate for the conditions of treatment required to detect the cytotoxicity of LAS. In cultures pre-incubated with tritiated adenine instead, the release of labelled ATP pool components was time and dose dependent and allowed discrimination between levels of membrane damage causing the same degree of trypan blue staining. Also, macromolecular nucleic acids were detected outside the treated cells at doses of 4-6 mg LAS/litre, which indicated severe membrane damage.

Phenotypic characterization of enterotoxigenic Clostridium perfringens isolates from non-foodborne human gastrointestinal diseases

Clostridium perfringens enterotoxin (CPE) has been implicated as an important virulence factor inC. perfringens type A food poisoning and several non-foodborne human gastrointestinal (GI) illnesses, including antibiotic-associated diarrhea (AAD) and sporadic diarrhea (SPOR). Recent studies have revealed genotypic differences between cpe-positive isolates originating from different disease sources, with most, or all, food poisoning isolates carrying a chomosomal cpe and most, or all, non-foodborne human GI disease isolates carrying an episomal cpe. To evaluate whether these genotypic differences cause phenotypic effects that could influence the pathogenesis of CPE-associated non-foodborne human GI illnesses, a collection of SPOR and AAD isolates has been phenotypically characterized in the current study. All cpe-positive non-foodborne disease isolates examined were found to express CPE in a sporulation-associated manner. The CPE made by these AAD and SPOR isolates was shown to have the same deduced amino acid sequence and toxicity as the classical CPE made by food poisoning isolates. All of the surveyed non-foodborne human GI disease isolates were found to classify as type AC. perfringens, since they produce alpha toxin, but not beta, iota, or epsilon toxins. Finally, no consistent clonal relationships were detected between the surveyed non-foodborne human GI disease isolates. Since, by the criteria examined, all non-foodborne human GI disease isolates examined in this study appear to be phenotypically similar to food poisoning isolates, the current results confirm that the examined AAD and SPOR isolates have enteropathogenic potential. However, given the phenotypic similarities between food poisoning, AAD, and SPOR isolates that have been demonstrated in this study, it remains unclear why the symptomology of non-foodborne human GI diseases is typically more severe and longer-lasting than that of C. perfringens type A food poisoning.

A novel toxin homologous to large clostridial cytotoxins found in culture supernatant of Clostridium perfringens type C

An unknown cytotoxin was identified in the culture supernatant of Clostridium perfringens type C. The cytotoxin, named TpeL, which was purified using mAb-based affinity chromatography, had a lethal activity of 62 minimum lethal dose (MLD) mg(-1) in mice and a cytotoxic activity of 6.2x10(5) cytotoxic units (CU) mg(-1) in Vero cells. The nucleotide sequence of TpeL was determined. The entire ORF had a length of 4953 bases, and the same nucleotide sequence was not recorded in the GenBank/EMBL/DDBJ databases. The molecular mass calculated from the deduced amino acid sequence was 191 kDa, and a signal peptide region was not found within the ORF. The deduced amino acid sequence exhibited 30-39 % homology to Clostridium difficile toxins A (TcdA) and B (TcdB), Clostridium sordellii lethal toxin (TcsL) and Clostridium novyi alpha-toxin (TcnA). The amino acid sequence of TpeL is shorter than these toxins, and the homologous region was located at the N-terminal site. Eighteen strains of C. perfringens types A, B and C were surveyed for the presence of the tpeL gene by PCR. The tpeL gene was detected in all type B (one strain) and C strains (five strains), but not in any type A strains (12 strains). TpeL was detected in culture filtrates of the five type C strains by dot-blot analysis, but not in the type B strain. It was concluded that TpeL is a novel toxin similar to the known large clostridial cytotoxins. Furthermore, the data indicated that TpeL is produced by many C. perfringens type C strains.

An assay of mammalian cell micropermeabilization based on measurements of cellular lactate production

A method for the quantitative assay of mammalian cell micropermeabilization is described. The method is based on the permeabilization-induced loss of endogenous glycolytic cofactors and consequent discontinuation of cellular lactate production. Advantages of the method include sensitivity and precision similar to that of micropermeabilization assays based on the release of 86Rb+ from preloaded cells, avoidance of radioactivity, and simplicity of the measurements and equipment required.

Cloning, sequencing, and role in virulence of two phospholipases (A1 and C) from mesophilic Aeromonas sp. serogroup O:34

Two different representative recombinant clones encoding Aeromonas hydrophila lipases were found upon screening on tributyrin (phospholipase A1) and egg yolk agar (lecithinase-phospholipase C) plates of a cosmid-based genomic library of Aeromonas hydrophila AH-3 (serogroup O34) introduced into Escherichia coli DH5alpha. Subcloning, nucleotide sequencing, and in vitro-coupled transcription-translation experiments showed that the phospholipase A1 (pla) and C (plc) genes code for an 83-kDa putative lipoprotein and a 65-kDa protein, respectively. Defined insertion mutants of A. hydrophila AH-3 defective in either pla or plc genes were defective in phospholipase A1 and C activities, respectively. Lecithinase (phospholipase C) was shown to be cytotoxic but nonhemolytic or poorly hemolytic. A. hydrophila AH-3 plc mutants showed a more than 10-fold increase in their 50% lethal dose on fish and mice, and complementation of the plc single gene on these mutants abolished this effect, suggesting that Plc protein is a virulence factor in the mesophilic Aeromonas sp. serogroup O:34 infection process.

Cloning, characterization, and chromosomal mapping of a phospholipase (lecithinase) produced by Vibrio cholerae

Phospholipases are associated with virulence in bacterial diseases. Vibrio cholerae produces a phospholipase (lecithinase), with enzyme production visualized as a zone of clearing around colonies plated on egg yolk agar. The role of phospholipase in gut colonization or disease pathogenesis is unknown. We used the egg yolk agar assay to clone and characterize a gene encoding a phospholipase from V. cholerae El Tor strain E7946. Sequence analysis revealed a 1,254-bp open reading frame (lec) encoding a 418-amino-acid protein with a predicted molecular weight of 47,600. The predicted sequence exhibits DNA homology to other Vibrionaceae phospholipases. A potential signal sequence exists in the predicted amino acid sequence, as does a lipid binding motif found in prokaryotic and eukaryotic phospholipases and lipases. Polyacrylamide gel electrophoresis combined with an egg yolk agarose overlay demonstrated phospholipase activity migrating at a relative molecular weight of 45,000 in preparations of V. cholerae and the Escherichia coli clone. Restriction mapping and Southern blot analysis revealed that lec, hlyA (hemolysin), and hlyC (lipase) are adjacent on the V. cholerae chromosome, and chromosomal digests of several El Tor, classical, and O139 (Bengal) strains demonstrated conservation of this gene arrangement. An in-frame internal deletion of the lec gene was constructed and recombined into the chromosome of attenuated V. cholerae El Tor strain CVD 110. The resulting mutant lacked lecithinase activity on egg yolk agar but had undiminished reactivity in rabbit ligated ileal loop assays.

Inhibition of oxidative insult in cultured cells by a novel 6-chromanol-containing antioxidant

N18-RE-105 neuronal hybridoma cells were used in a cell culture system to evaluate the protective effects of a novel 6-chromanol-containing antioxidant, U78517F. First, the incorporation of the compound into the cells was evaluated, using a serum albumin carrier. Then the cells were exposed to peroxide-generating compounds, and the cell injury was estimated from the loss of alpha-aminoisobutyric acid (AIB) transport. We found that U78517F only protected the cells significantly when the degree of oxidative insult was below a certain limit; the measurable protection of cells by U78517F against either cumene hydroperoxide or H2O2 was limited to a narrow range of concentrations of the reactive oxygen species generator. Additionally, the protection provided by U78517F was largely localized to the cell membrane and did not extend to protection of mitochondrial function. The action of U78517 was fully consistent with a direct radical scavenging in the cells. The results indicate that the following factors must be taken into account for evaluation of antioxidants in cell culture: (a) the delivery of a compound to cells, especially when the compound is lipophilic; (b) the nature and extent of the oxidative insult used to evaluate protection; and (c) the location of the protective agent in the cells.

Role of alpha-toxin in Clostridium perfringens infection determined by using recombinants of C. perfringens and Bacillus subtilis

Clostridium perfringens type A strains which differed in alpha-toxin (phospholipase C [PLC]) productivity were inoculated intraperitoneally or intravenously into mice, and then their 50% mouse lethal doses (LD50) were determined. Strain NCTC 8237 produced ninefold higher PLC activity than strain 13. The mean LD50 for the former was 1 log unit lower than that for the latter. Two isogenic strains were constructed from strain 13: strain 13(pJIR418 alpha) (pJIR418 alpha contains the plc gene), which produced ninefold higher PLC activity than strain 13; and strain 13 PLC-, which showed no PLC productivity at all because of transformation-mediated gene disruption. The mean LD50 for strain 13(pJIR418 alpha) was 1 log unit lower than those for strain 13 PLC- and strain 13. These results indicate that PLC functions as a virulence-determining factor when it is produced in a sufficient amount. Such a difference in LD50 was also observed between Bacillus subtilis with and without the cloned plc gene. Inoculation of B. subtilis PLC+ intravenously into mice caused marked thrombocytopenia and leukocytosis. Mice inoculated with B. subtilis at 2 LD50 died because of circulatory collapse. Histological examination revealed that intravascular coagulation and vascular congestion occurred most prominently in the lungs. These results suggest that PLC plays a key role in the systemic intoxication of clostridial myonecrosis, probably by affecting the functions of platelets and phagocytes.

Cloning, nucleotide sequencing, and expression of the Clostridium perfringens enterotoxin gene in Escherichia coli

A complete copy of the gene (cpe) encoding Clostridium perfringens enterotoxin (CPE), an important virulence factor involved in C. perfringens food poisoning and other gastrointestinal illnesses, has been cloned, sequenced, and expressed in Escherichia coli. The cpe gene was shown to encode a 319-amino-acid polypeptide with a deduced molecular weight of 35,317. There was no consensus sequence for a typical signal peptide present in the 5' region of cpe. Cell lysates from recombinant cpe-positive E. coli were shown by quantitative immunoblot analysis to contain moderate amounts of CPE, and this recombinant CPE was equal to native CPE in cytotoxicity for mammalian Vero cells. CPE expression in recombinant E. coli appeared to be largely driven from a clostridial promoter. Immunoblot analysis also demonstrated very low levels of CPE in vegetative cell lysates of enterotoxin-positive C. perfringens. However, when the same C. perfringens strain was induced to sporulate, much stronger CPE expression was detected in these sporulating cells than in either vegetative C. perfringens cells or recombinant E. coli. Collectively, these results strongly suggest that sporulation is not essential for cpe expression, but sporulation does facilitate high-level cpe expression.

Bacterial phospholipases C

A variety of pathogenic bacteria produce phospholipases C, and since the discovery in 1944 that a bacterial toxin (Clostridium perfringens alpha-toxin) possessed an enzymatic activity, there has been considerable interest in this class of proteins. Initial speculation that all phospholipases C would have lethal properties has not been substantiated. Most of the characterized enzymes fall into one of four groups of structurally related proteins: the zinc-metallophospholipases C, the sphingomyelinases, the phosphatidylinositol-hydrolyzing enzymes, and the pseudomonad phospholipases C. The zinc-metallophospholipases C have been most intensively studied, and lethal toxins within this group possess an additional domain. The toxic phospholipases C can interact with eukaryotic cell membranes and hydrolyze phosphatidylcholine and sphingomyelin, leading to cell lysis. However, measurement of the cytolytic potential or lethality of phospholipases C may not accurately indicate their roles in the pathogenesis of disease. Subcytolytic concentrations of phospholipase C can perturb host cells by activating the arachidonic acid cascade or protein kinase C. Nonlethal phospholipases C, such as the Listeria monocytogenes PLC-A, appear to enhance the release of the organism from the host cell phagosome. Since some phospholipases C play important roles in the pathogenesis of disease, they could form components of vaccines. A greater understanding of the modes of action and structure-function relationships of phospholipases C will facilitate the interpretation of studies in which these enzymes are used as membrane probes and will enhance the use of these proteins as models for eukaryotic phospholipases C.

Biochemical and physiological changes induced by anthrax lethal toxin in J774 macrophage-like cells

Experiments were performed to probe the mechanism by which Bacillus anthracis Lethal Toxin (LeTx) causes lysis of J774 macrophage-like cells. After incubation of cells with saturating concentrations of the toxin, two categories of effects were found, which were distinguishable on the basis of chronology, Ca(2+)-dependence, and sensitivity to osmolarity. The earliest events (category I), beginning 45 min postchallenge, were an increase in permeability to 22Na and 86Rb and a rapid conversion of ATP to ADP and AMP. Later events (category II) included alterations in membrane permeability to 45Ca, 51Cr, 36Cl, 35SO4, 3H-amino acids, and 3H-uridine, beginning at 60 min; inhibition of macromolecular synthesis, leakage of cellular lactate dehydrogenase and onset of gross morphological changes, at approximately 75 min; and cell lysis, beginning at 90 min. Category II events exhibited an absolute requirement for extracellular Ca2+ and were blocked by addition of 0.3 M sucrose to the medium, whereas category I events were attenuated, but not blocked, by either of these conditions. On the other hand, both ATP depletion and the category II events were blocked in osmotically stabilized medium that was also isoionic for Na+ and K+. This suggests that permeabilization of the plasma membrane to monovalent cations and water may be the earliest of the physiological changes described here. The resulting influx of Na+ and efflux of K+ would be expected to cause depletion of ATP, via increased activity of the Na+/K+ pump. Subsequently the influx of Ca2+, induced by depletion of ATP, imbalances in monovalent cautions, and/or more dramatic changes in permeability due to influx of water, would be expected to trigger widespread changes leading ultimately to cytolysis.

A conjugated synthetic peptide corresponding to the C-terminal region of Clostridium perfringens type A enterotoxin elicits an enterotoxin-neutralizing antibody response in mice

A synthetic peptide homolog corresponding to the C-terminal 30 amino acids of Clostridium perfringens type A enterotoxin (CPE) was conjugated to a thyroglobulin carrier and used to immunize mice. Conjugate-immunized mice produced antibodies which neutralized native CPE cytotoxicity, at least in part, by blocking enterotoxin binding. This peptide may be useful for the development of a vaccine to protect against CPE-mediated disease.

Changes in membrane permeability during Semliki Forest virus induced cell fusion

The infection of Aedes albopictus cells by Semliki Forest virus (SFV) is a non lytic event. Exposure of infected cells to mildly acidic pH (less than 6.2) leads to syncytium formation. This polykaryon formation is accompanied by an influx of protons into the cells (Kempf et al. Biosci. Rep. 7, 761-769, 1987). We have further investigated this permeability change using various fluorescent or radiolabeled compounds. A significant, pH dependent increase of the membrane permeability to low molecular weight compounds (M(r) less than 1000) was observed when infected cells were exposed to a pH less than 6.2. The pH dependence of the permeability change was very similar to the pH dependence of cell-cell fusion. The permeability change was sensitive to divalent cations, protons and anionic antiviral drugs such as trypan blue. The nature of this virus induced, pH dependent permeability change is discussed.

Schistosoma mansoni: stage-dependent formation and repair of membrane pores induced by a cytotoxin from Pseudomonas aeruginosa

Various stages of Schistosoma mansoni were treated with a cytotoxin from Pseudomonas aeruginosa and their response to the damaging effect was studied in detail. Marker release and membrane potential measurements showed that the cytotoxin formed stable pores in all developmental stages. However, in juvenile 27-day-old worms, which are refractory to the killing effect of the cytotoxin, the pores had a smaller functional diameter as compared to other stages including 31-day-old worms. Furthermore, these resistant 27-day-old worms, but not susceptible older juvenile worms were able to repair the membrane lesions as shown by restoration of the resting membrane potential. In contrast, older juvenile and adult parasites were unable to cope with the breakdown of the resting potential induced by the cytotoxin. The results demonstrate the existence in 27-day-old schistosomes of effective repair mechanisms dealing with damage to the surface membrane.

Mapping of functional regions of Clostridium perfringens type A enterotoxin

Studies were conducted to allow construction of an initial map of the structure-versus-function relationship of the Clostridium perfringens type A enterotoxin (CPE). Removal of the N-terminal 25 amino acids of CPE increased the primary cytotoxic effect of CPE but did not affect binding. CPE sequences required for at least four epitopes were also identified.

Alpha-toxin of Staphylococcus aureus

Alpha-toxin, the major cytotoxic agent elaborated by Staphylococcus aureus, was the first bacterial exotoxin to be identified as a pore former. The protein is secreted as a single-chain, water-soluble molecule of Mr 33,000. At low concentrations (less than 100 nM), the toxin binds to as yet unidentified, high-affinity acceptor sites that have been detected on a variety of cells including rabbit erythrocytes, human platelets, monocytes and endothelial cells. At high concentrations, the toxin additionally binds via nonspecific absorption to lipid bilayers; it can thus damage both cells lacking significant numbers of the acceptor and protein-free artificial lipid bilayers. Membrane damage occurs in both cases after membrane-bound toxin molecules collide via lateral diffusion to form ring-structured hexamers. The latter insert spontaneously into the lipid bilayer to form discrete transmembrane pores of effective diameter 1 to 2 nm. A hypothetical model is advanced in which the pore is lined by amphiphilic beta-sheets, one surface of which interacts with lipids whereas the other repels apolar membrane constitutents to force open an aqueous passage. The detrimental effects of alpha-toxin are due not only to the death of susceptible targets, but also to the presence of secondary cellular reactions that can be triggered via Ca2+ influx through the pores. Well-studied phenomena include the stimulation of arachidonic acid metabolism, triggering of granule exocytosis, and contractile dysfunction. Such processes cause profound long-range disturbances such as development of pulmonary edema and promotion of blood coagulation.(ABSTRACT TRUNCATED AT 250 WORDS)

A recombinant C-terminal toxin fragment provides evidence that membrane insertion is important for Clostridium perfringens enterotoxin cytotoxicity

Clostridium perfringens enterotoxin (CPE) is believed to be involved in several important gastrointestinal illnesses. Recent studies have identified a number of distinct molecular events which occur after CPE treatment of eukaryotic cells or isolated membranes. Additional studies are underway to determine the temporal order and intrinsic importance of each CPE event for cytotoxicity. We now demonstrate that a truncated CPE fragment binds to membranes, but is unable to insert into membranes or cause any other subsequent post-insertion event. This is the first experimental evidence supporting the importance of membrane insertion for CPE cytotoxicity. Binding of the CPE fragment is also shown to be irreversible, strongly suggesting that the irreversible binding of wild-type CPE is not due solely to insertion of CPE into membranes.

Studies of Clostridium perfringens enterotoxin action at different temperatures demonstrate a correlation between complex formation and cytotoxicity

The cytotoxicity of Clostridium perfringens enterotoxin (CPE) was completely blocked in Vero cells continuously CPE treated at 4 degrees C. [125I]CPE-specific binding to either Vero cells or isolated rabbit intestinal brush border membranes (BBMs) was lower at 4 degrees C than at 24 or 37 degrees C, but reduced enterotoxin binding could not totally explain the loss of cytotoxicity at low temperature. Insertion of enterotoxin into Vero cell membranes or BBMs was temperature independent. However, CPE complex formation (A. P. Wnek and B. A. McClane, Infect. Immun. 57:574-581, 1989) in BBMs and Vero cells was blocked at 4 degrees C. When Vero cells were CPE treated at 4 degrees C, washed to remove unbound toxin, and then shifted to 37 degrees C, complex formation and cytotoxicity were rapidly detected. When CPE binding and complex formation were permitted for 2 min at 37 degrees C, and the Vero cells were then shifted to 4 degrees C, cytotoxicity was detectable at 4 degrees C. These results are consistent with complex formation, rather than complex activity, being the temperature-sensitive step in CPE action which is blocked at 4 degrees C. These studies demonstrate a strong correlation between complex formation and cytotoxicity and are consistent with complex involvement in CPE cytotoxicity. These studies also strongly suggest that CPE insertion precedes both complex formation and induction of small-molecule permeability changes.

Molecular cloning of the 3' half of the Clostridium perfringens enterotoxin gene and demonstration that this region encodes receptor-binding activity

Clostridium perfringens type A enterotoxin (CPE) causes the symptoms associated with C. perfringens food poisoning. To determine whether the C-terminal half of CPE contains receptor-binding activity, the 3' half of the cpe structural gene was cloned with an Escherichia coli expression vector system. E. coli lysates containing the expressed C-terminal CPE fragment (CPEfrag) were then assayed for CPE-like serologic, receptor-binding, and cytotoxic activities. CPEfrag was shown to contain an epitope located at or near the receptor-binding domain of the CPE molecule. Competitive-binding studies showed specific competition for CPE receptors between CPE and CPEfrag lysates. CPEfrag lysates did not cause cytotoxicity in Vero (African green monkey kidney) cells. However, preincubation with CPEfrag lysates specifically protected Vero cells from subsequent CPE challenge. This indicates that CPEfrag recognizes the physiologic receptor which mediates CPE cytotoxicity. Collectively, these studies indicate that the C-terminal half of CPE contains a receptor-binding domain but additional amino acid sequences appear to be required for CPE cytotoxicity.

Human hyperimmune globulin protects against the cytotoxic action of staphylococcal alpha-toxin in vitro and in vivo

Alpha-toxin, the major cytolysin of Staphylococcus aureus, preferentially attacks human platelets and cultured monocytes, thereby promoting coagulation and the release of interleukin-1 and tumor necrosis factor. Titers of naturally occurring antibodies in human blood are not high enough to substantially inhibit these pathological reactions. In the present study, F(ab')2 fragment preparations from hyperimmune globulin obtained from immunized volunteers were tested for their capacity to inhibit the cytotoxic action of alpha-toxin in vitro and in vivo. These antibody preparations exhibited neutralizing anti-alpha-toxin titers of 80 to 120 IU/ml, whereas titers in commercial immunoglobulin preparations were 1 to 4 IU/ml. In vitro, the presence of 2 to 4 mg of hyperimmune globulin per ml protected human platelets against the action of 1 to 2 micrograms of alpha-toxin per ml. Similarly, these antibodies fully protected human monocytes against the ATP-depleting and cytokine-liberating effects of 0.1 to 1 microgram of alpha-toxin per ml. Intravenous application of 0.5 mg (85 to 120 micrograms/kg of body weight) of alpha-toxin in cynomolgus monkeys elicited acute pathophysiological reactions which were heralded by a selective drop in blood platelet counts. Toxin doses of 1 to 2 mg (170 to 425 micrograms/kg) had a rapid lethal effect, the animals presenting with signs of cardiovascular collapse and pulmonary edema. Prior intravenous application of 4 ml of hyperimmune globulins per kg inhibited the systemic toxic and lethal effects of 1 mg (200 micrograms/kg) of alpha-toxin. In contrast, normal human immunoglobulins exhibited no substantial protective efficacy in vitro and only marginal effects in vivo. It is concluded that high-titered anti-alpha-toxin antibodies effectively protect against the cytotoxic actions of alpha-toxin.

Evidence that alterations in small molecule permeability are involved in the Clostridium perfringens type A enterotoxin-induced inhibition of macromolecular synthesis in Vero cells

The mechanism by which Clostridium perfringens enterotoxin (CPE) simultaneously inhibits RNA, DNA, and protein synthesis is unknown. In the current study the possible involvement of small molecule permeability alterations in CPE-induced inhibition of macromolecular synthesis was examined. Vero cells CPE-treated in minimal essential medium (MEM) completely ceased net precursor incorporation into RNA and protein within 15 minutes of CPE treatment. However, RNA and protein synthesis continued for at least 30 minutes in Vero cells CPE-treated in buffer (ICIB) approximating intracellular concentrations of most ions. Addition of intracellular concentrations of amino acids to ICIB (ICIB-AA) caused a further small but detectable increase in protein synthesis in CPE-treated cells. ICIB did not affect CPE-specific binding levels or rates. Similar small molecule permeability changes (i.e., 86Rb-release) were observed in cells CPE-treated in either ICIB or in Hanks' balanced salt solution. Collectively these findings suggest that CPE-treatment of cells in ICIB-AA ameliorates CPE-induced changes in intracellular concentrations of ions and amino acids and permits the continuation of RNA and protein synthesis. These results are consistent with and support the hypothesis that permeability alterations for small molecules are involved in the CPE-induced inhibition of precursor incorporation into macromolecules in Vero cells.

Characterization of calcium involvement in the Clostridium perfringens type A enterotoxin-induced release of 3H-nucleotides from Vero cells

This report characterizes the involvement of Ca2+ in the release of nucleotides from Vero cells caused by Clostridium perfringens enterotoxin (CPE). A positive linear correlation was observed between increased CPE-induced nucleotide-release and increased extracellular calcium over the range 0.01 to 10 mM calcium. Above 5 mM Ca2+, CPE-specific lysis (i.e. disintegration of cells as monitored by light microscopy) was observed. Addition of 1.7 mM Ca2+ to Vero cells previously CPE-treated in Ca2+-free buffer rapidly increased nucleotide-release, even when cells had been previously incubated for 1 h at 37 degrees C in Ca2+-free buffer. Withdrawal of Ca2+, even after the onset of nucleotide-release, halted further CPE-induced nucleotide-release. These results indicate that Ca2+ must be continuously present for significant CPE-induced nucleotide-release. However, withdrawal of Ca2+ did not reverse membrane bleb formation by CPE. This differentiates bleb formation and nucleotide-release (both Ca2+-dependent CPE effects) and suggests that nucleotide-release does not result simply from bleb formation. Lastly, it was shown that other ions besides physiologic Ca2+ (1.7 mM) are required for CPE-induced nucleotide-release. Interestingly, a role for other ions (but not physiologic Ca2+) is also shown for 86Rb-release by CPE (an early Ca2+-independent CPE effect). This indicates that extracellular ions other than physiologic Ca2+ can be required for both Ca2+-independent and Ca2+-dependent CPE effects.

The effects of Clostridium perfringens enterotoxin on intracellular levels or transport of uridine, thymidine and leucine do not fully explain enterotoxin-induced inhibition of macromolecular synthesis in Vero cells

Clostridium perfringens type A enterotoxin (CPE) has been shown previously to inhibit the incorporation of radiolabeled precursors into acid-insoluble material but the mechanism of inhibition is unknown. It has also been shown that extracellular calcium is required for some CPE effects. In this report, it is shown that CPE completely and virtually simultaneously inhibits incorporation of precursors into RNA, DNA and protein in either the presence or absence of extracellular divalent cations and that changes in intracellular precursor levels did not consistently correlate with this CPE-induced inhibition of incorporation. These results strongly suggest that CPE can inhibit macromolecular synthesis, not just inhibit precursor transport. It is inferred from this that CPE can affect DNA and RNA synthesis, and possibly protein synthesis, by altering other cellular processes besides, or in addition to, precursor transport and these effects then lead to a shutdown of macromolecular synthesis.

Quantitative analysis of the binding and oligomerization of staphylococcal alpha-toxin in target erythrocyte membranes

The binding of staphylococcal alpha-toxin to rabbit and human erythrocytes was quantitated over a wide range of toxin concentrations (3 x 10(-11) to 3 x 10(-6) M) with the use of an enzyme-linked immunosorbent assay that permitted simultaneous quantitation of monomeric and oligomeric toxin forms. Three basic observations were made. First, in no range of concentrations did the binding of alpha-toxin to rabbit erythrocytes display characteristics of a receptor-ligand interaction. Net binding to rabbit cells was nil at sublytic concentrations (10(-10) M or 3 ng/ml). The onset of binding occurred at around 10 ng/ml and remained fairly constant and ineffective (5 to 8% of toxin offered) over a wide concentration range (up to 10 micrograms/ml). Second, hemolysis of rabbit and human erythrocytes at 37 degrees C was always accompanied by the formation of toxin oligomers in the membrane. Third, overall toxin binding at 0 degree C followed a pattern similar to that at 37 degrees C. However, oligomer formation and cell lysis were retarded (but not totally inhibited) at 0 degree C. When rabbit erythrocytes were incubated with low levels of toxin at 0 degree C (0.5 microgram/ml) for 30 min, the toxin became bound exclusively in monomer form, and no lysis occurred. When cells thus treated were washed and suspended at 37 degrees C, lysis rapidly ensued, and native monomeric toxin was replaced by oligomeric toxin. The collective results directly support the oligomer pore concept of toxin action and also indicate that toxin oligomers form by lateral aggregation of bound monomers in the bilayer. They speak against the existence of specific binding sites for alpha-toxin on rabbit erythrocytes.

Quantitation of monomeric and oligomeric forms of membrane-bound staphylococcal alpha-toxin by enzyme-linked immunosorbent assay with a neutralizing monoclonal antibody

A murine monoclonal antibody generated against staphylococcal alpha-toxin was shown to react only with the monomeric (native), 3S form of the toxin. A sensitive sandwich enzyme-linked immunosorbent assay (ELISA) constructed with this antibody permitted detection of 0.25 to 0.5 ng of native toxin per ml. Toxin oligomers formed either by heat aggregation in solution, on target erythrocyte membranes, or on phosphatidylcholine-cholesterol liposomes were unreactive in the ELISA when membranes were solubilized with the nondenaturing detergent Triton X-100. After dissociation of the oligomers by boiling in sodium dodecyl sulfate, however, the ELISA reactivity of the liberated 3S toxin was fully restored. Parallel determinations of membrane-bound toxin with sodium dodecyl sulfate and Triton X-100 solubilization thus permitted direct quantitation of total and monomeric toxin, respectively; the difference between these two values was represented by toxin oligomers. The detection limits for membrane-bound oligomeric and monomeric toxin on erythrocyte membranes are in the order of 100 molecules and 1 molecule per cell, respectively. Using this ELISA, we show that over 90% of alpha-toxin molecules bound to target membranes at 37 degrees C are in oligomeric form. Evidence is given that the monoclonal antibody neutralizes alpha-toxin by inhibiting its binding to both rabbit and human erythrocytes. This ELISA is the first assay that quantitatively discriminates between mono- and oligomeric forms of a pore-forming protein on target cell membranes.

Differential effects of Clostridium difficile toxins A and B on rabbit ileum

The pathogenesis of Clostridium difficile enterocolitis appears to involve colonization of the bowel followed by release of toxin A, an enterotoxin, and toxin B, a cytotoxin. The purpose of this study was to determine the effect of purified toxins A and B on intestinal secretion, epithelial permeability, and morphology in perfused rabbit ileal loops. Intestinal permeability after toxin exposure was assessed by blood-to-lumen clearance of [3H]mannitol. Toxin A at doses of 5-100 micrograms/10 cm ileal loop caused a threefold to fivefold increase in [3H]mannitol permeability (p less than 0.001) vs. equal concentrations of toxin B or buffer control. In addition, perfusate from toxin A-exposed loops contained significantly more neutrophils (p less than 0.001) than toxin B or control loops. Toxin A caused severe epithelial cell necrosis with destruction of villi and polymorphonuclear infiltration. Electron microscopy of mucosa subjected to a low dose of toxin revealed widespread nonspecific dilatation of endoplasmic reticulum and mitochondrial swelling. In contrast to these effects of toxin A in ileal loops, in vitro experiments with ileal explants in short-term organ culture revealed that toxin A had no effect on epithelial cell permeability, protein synthesis, release of alkaline phosphatase, or morphology. Our results show that purified toxin A but not toxin B causes severe inflammatory enteritis in rabbit ileal loops, but has no discernable effect on rabbit ileum in vitro. We speculate that toxin A may contribute significantly to intestinal damage in C. difficile-associated colitis and diarrhea.

Nuclear magnetic resonance investigation of the conformation of delta-haemolysin bound to dodecylphosphocholine micelles

delta-Haemolysin in mixed micelles with perdeuterated dodecylphosphocholine was investigated with two-dimensional proton nuclear magnetic resonance experiments at 500 MHz. A single set of resonance lines was observed for the micelle-bound polypeptide, indicating that delta-haemolysin adopts a single conformation in this environment. Nearly complete, sequence-specific assignments were obtained for the segment 5-23 of this 26-residue polypeptide chain. From the sequential connectivities and numerous medium-range nuclear Overhauser effects this central portion of the molecule was found to form an extended helix with pronounced amphipathic distribution of polar and nonpolar amino acid side-chains.

Crystallization of the delta toxin of Staphylococcus aureus

Delta toxin is a small cytolytic polypeptide produced and secreted by the organism Staphylococcus aureus and belongs to a family of surface-active toxins that exhibit pronounced effects on a wide variety of cellular membranes. Although this class of proteins has been much studied by a wide variety of physical techniques, no consensus has been reached on their mode of action. Therefore, in order to investigate their role in causing membrane damage, a structural analysis of the delta toxin has been initiated. Crystals of this protein have been grown by dialysis against mixtures of 2-methylpentan-2,4-diol and water. These crystals are relatively insensitive to radiation damage and diffract to high resolution. The results of this study should provide a valuable insight into the cytolytic properties of these molecules.

Early events in the action of staphylococcal alpha-toxin on the plasma membrane of adrenocortical Y1 tumor cells

The early events in staphylococcal alpha-toxin action on mouse adrenocortical (Y1) tumor cells were studied. Cell-bound toxin could be partially neutralized by anti-alpha-toxin and inactivated by trypsin added within 10 min at 37 degrees C after the end of the binding step. Likewise, cell-bound toxin was capable of lysing rabbit erythrocytes (RRBC) added to the cells within 10 min after binding at 37 degrees C. After this time, the Y1 cells could not be rescued from intoxication by antibodies or trypsin, and the toxin was not accessible for lysis of RRBC. However, at 0 to 4 degrees C, the cell-bound toxin remained accessible to antibodies for at least 4 h. CaCl2 (30 mM) did not affect binding of the toxin to Y1 cells but completely prevented the intoxication if added within 10 min at 37 degrees C after the end of the binding step. The intoxication was independent of metabolic energy, active receptor clustering on the cell surface, and endocytosis of the toxin. Therefore, alpha-toxin interacted with the Y1 cell membrane in at least three separable steps: binding, a conformational change at the cell surface, and membrane damage. These early events appear to be similar to those occurring on RRBC treated with alpha-toxin.

Production and characterization of monoclonal antibodies against Clostridium perfringens type A enterotoxin

Hybridomas secreting monoclonal antibodies (MABs) specific for Clostridium perfringens type A enterotoxin were produced by fusion of P3X63Ag8.653 myeloma cells with spleen cells from BALB/c mice immunized with purified enterotoxin. Wells containing hybridomas secreting immunoglobulin G (IgG) antibodies against enterotoxin were specifically identified by an indirect enzyme-linked immunosorbent assay (ELISA), and 10 ELISA-positive hybridomas were selected and cloned twice by limiting dilution. All 10 hybridomas produced MABs containing immunoglobulin G1 heavy chains and kappa (kappa) light chains. These hybridomas were then grown as ascitic tumors in mice, and MABs were purified from the ascites fluids with DEAE Affi-gel blue. The specificity of the MABs for enterotoxin was demonstrated by immunoblotting and ELISA. Competitive radioimmunoassay with 125I-MABs suggests that these MABs recognized at least four epitopes on the enterotoxin molecule. The enterotoxin-neutralizing ability of MABs from both hybridoma culture supernatants and ascites fluids was assessed by using a 3H-nucleotide-release Vero (African green monkey kidney) cell assay. Only 2 of the 10 hybridomas produced MABs which completely (greater than 90%) neutralized the biologic activity of enterotoxin. Preincubation of 125I-enterotoxin with MABs demonstrated that MAB neutralizing ability correlated with MAB-specific inhibition of specific binding of enterotoxin to intestinal brush border membranes.

Correlation between toxin binding and hemolytic activity in membrane damage by staphylococcal alpha-toxin

The binding of Staphylococcus aureus alpha-toxin to rabbit and human erythrocytes was studied by hemolytic assays and sodium dodecyl sulfate-polyacrylamide gel electrophoresis immunoblotting. Hemolytic assays showed that toxin binding to 10% cell suspensions at neutral pH was very ineffective in the concentration range 3 X 10(-8) to 3 X 10(-7) M (1 to 10 micrograms/ml), and less than 5% of added toxin became cell bound. However, binding was augmented as toxin levels were raised, abruptly increasing to 50 to 60% at 2 X 10(-6) to 3 X 10(-6) M (60 to 100 micrograms/ml). When rabbit erythrocytes were lysed with 1 to 5 micrograms of toxin per ml, both monomeric and hexameric forms of the toxin could be detected on the membranes by sodium dodecyl sulfate-polyacrylamide gel electrophoresis immunoblotting. In contrast, human erythrocytes treated with 1 to 6 micrograms of toxin per ml did not lyse, and membrane-bound toxin was not detectable. When toxin concentrations were raised to 30 to 100 micrograms/ml, human erythrocytes also lysed and toxin hexamers became membrane bound in comparable amounts as on rabbit cell membranes. Lowering the pH led to a marked increase in susceptibility of human, but not rabbit erythrocytes towards alpha-toxin. When human cells were lysed at pH 5.0 with 5 micrograms of toxin per ml, membrane-bound hexameric toxin became detectable. The demonstrated correlation between the presence of hexameric, cell-bound toxin and hemolytic activity supports the channel concept of toxin-mediated cytolysis. The results also show that toxin binding does not exhibit overall characteristics of a simple receptor-ligand interaction.

Osmotic stabilizers differentially inhibit permeability alterations induced in Vero cells by Clostridium perfringens enterotoxin

Using a sensitive Vero (African green monkey kidney) cell model system, studies were performed to further investigate whether Clostridium perfringens enterotoxin acts via disruption of the colloid-osmotic equilibrium of sensitive cells. Enterotoxin was shown to cause a rapid loss of intracellular 86Rb+ (Mr approx. 100) with time- and dose-dependent kinetics. The enterotoxin-induced release of intracellular 86Rb+ preceded the loss of two larger labels, 51Cr label (Mr approx. 3500) and 3H-labeled nucleotides (Mr less than 1000). The osmotic stabilizers, sucrose and poly(ethylene glycol), differentially inhibited enterotoxin-induced larger label loss versus 86Rb+ loss. Further, enterotoxin was shown to cause a rapid influx of 24Na+ that was not significantly inhibited by osmotic stabilizers. Additional studies demonstrated that lysosomotropic agents were not protective against characteristic enterotoxin-induced membrane permeability alterations or morphological damage. Taken collectively, these results are consistent with an action for enterotoxin which involves a disruption of the osmotic equilibrium.

Generation of leukotrienes from human granulocytes by alveolysin from Bacillus alvei

We investigated the effect of alveolysin on human granulocytes. Alveolysin is an exoprotein produced by Bacillus alvei and belongs to the group of sulfhydryl-activated cytolysins. Other members of this group are streptolysin O and theta-toxin from Clostridium perfringens. It is demonstrated that alveolysin leads to leukotriene generation from human granulocytes, which exert chemotactic (leukotriene B4) and slow-reacting substance (leukotriene C4, D4, and E4) activity under sublytic concentrations.