Targeted small molecule inhibitors blocking the cytolytic effects of pneumolysin and homologous toxins.

Pneumolysin (PLY) is a cholesterol-dependent cytolysin (CDC) from Streptococcus pneumoniae, the main cause for bacterial pneumonia. Liberation of PLY during infection leads to compromised immune system and cytolytic cell death. Here, we report discovery, development, and validation of targeted small molecule inhibitors of PLY (pore-blockers, PB). PB-1 is a virtual screening hit inhibiting PLY-mediated hemolysis. Structural optimization provides PB-2 with improved efficacy. Cryo-electron tomography reveals that PB-2 blocks PLY-binding to cholesterol-containing membranes and subsequent pore formation. Scaffold-hopping delivers PB-3 with superior chemical stability and solubility. PB-3, formed in a protein-templated reaction, binds to Cys428 adjacent to the cholesterol recognition domain of PLY with a KD of 256 nM and a residence time of 2000 s. It acts as anti-virulence factor preventing human lung epithelial cells from PLY-mediated cytolysis and cell death during infection with Streptococcus pneumoniae and is active against the homologous Cys-containing CDC perfringolysin (PFO) as well.

In Silico Identification and Experimental Validation of Peptide-Based Inhibitors Targeting Clostridium difficile Toxin A.

Clostridium difficile infection is mediated by two major exotoxins: toxins A (TcdA) and B (TcdB). Inhibiting the biocatalytic activities of these toxins with targeted peptide-based drugs can reduce the risk of C. difficile infection. In this work, we used a computational strategy that integrates a peptide binding design (PepBD) algorithm and explicit-solvent atomistic molecular dynamics simulation to determine promising toxin A-targeting peptides that can recognize and bind to the catalytic site of the TcdA glucosyltransferase domain (GTD). Our simulation results revealed that two out of three in silico discovered peptides, viz. the neutralizing peptides A (NPA) and B (NPB), exhibit lower binding free energies when bound to the TcdA GTD than the phage-display discovered peptide, viz. the reference peptide (RP). These peptides may serve as potential inhibitors against C. difficile infection. The efficacy of the peptides RP, NPA, and NPB to neutralize the cytopathic effects of TcdA was tested in vitro in human jejunum cells. Both phage-display peptide RP and in silico peptide NPA were found to exhibit strong toxin-neutralizing properties, thereby preventing the TcdA toxicity. However, the in silico peptide NPB demonstrates a relatively low efficacy against TcdA.

Polycyclic polyprenylated acylphloroglucinol congeners from Garcinia yunnanensis Hu with inhibitory effect on α-hemolysin production in Staphylococcus aureus.

α-Hemolysin (Hla) is an extracellular protein secreted by methicillin-resistant Staphylococcus aureus (MRSA) strains that plays a critical role in the pathogenesis of pulmonary, intraperitoneal, intramammary, and corneal infections, rendering Hla a potential therapeutic target. In this study, 10 unreported polycyclic polyprenylated acylphloroglucinol (PPAP) derivatives, garciyunnanins C-L (1-10), with diverse skeletons, were isolated from Garcinia yunnanensis Hu. The structures of these new compounds were determined by HRMS, NMR, electronic circular dichroism (ECD) calculations, single-crystal X-ray diffraction, and biomimetic transformation. Garciyunnanins C and D (1 and 2) were found to be potent Hla inhibitors in the anti-virulence efficacy evaluation against MRSA strain.

Novel GUCY2C variant causing familial diarrhea in a Mennonite kindred and a potential therapeutic approach.

Guanylate cyclase 2C (GC-C), encoded by the GUCY2C gene, is implicated in hereditary early onset chronic diarrhea. Several families with chronic diarrhea symptoms have been identified with autosomal dominant, gain-of-function mutations in GUCY2C. We have identified a Mennonite patient with a novel GUCY2C variant (c.2381A > T; p.Asp794Val) with chronic diarrhea and an extensive maternal family history of chronic diarrhea and bowel dilatation. Functional studies including co-segregation analysis showed that all family members who were heterozygous for this variant had GI-related symptoms. HEK-293 T cells expressing the Asp794Val GC-C variant showed increased cGMP production when stimulated with Escherichia coli heat-stable enterotoxin STp (HST), which was reversed when 5-(3-Bromophenyl)-5,11-dihydro-1,3-dimethyl-1H-indeno[2',1':5,6]pyrido[2,3-d]pyrimidine-2,4,6(3H)-trione (BPIPP; a GC-C inhibitor) was used. In addition, cystic fibrosis transmembrane conductance regulator (CFTR) activity measured with SPQ fluorescence assay was increased in these cells after treatment with HST, indicating a crucial role for CFTR activity in the pathogenesis of this disorder. These results support pathogenicity of the GC-C Asp794Val variant as a cause of chronic diarrhea in this family. Furthermore, this work identifies potential candidate drug, GC-C inhibitor BPIPP, to treat diarrhea caused by this syndrome.

The Importance of Therapeutically Targeting the Binary Toxin from Clostridioides difficile.

Novel therapeutics are needed to treat pathologies associated with the Clostridioides difficile binary toxin (CDT), particularly when C. difficile infection (CDI) occurs in the elderly or in hospitalized patients having illnesses, in addition to CDI, such as cancer. While therapies are available to block toxicities associated with the large clostridial toxins (TcdA and TcdB) in this nosocomial disease, nothing is available yet to treat toxicities arising from strains of CDI having the binary toxin. Like other binary toxins, the active CDTa catalytic subunit of CDT is delivered into host cells together with an oligomeric assembly of CDTb subunits via host cell receptor-mediated endocytosis. Once CDT arrives in the host cell's cytoplasm, CDTa catalyzes the ADP-ribosylation of G-actin leading to degradation of the cytoskeleton and rapid cell death. Although a detailed molecular mechanism for CDT entry and host cell toxicity is not yet fully established, structural and functional resemblances to other binary toxins are described. Additionally, unique conformational assemblies of individual CDT components are highlighted herein to refine our mechanistic understanding of this deadly toxin as is needed to develop effective new therapeutic strategies for treating some of the most hypervirulent and lethal strains of CDT-containing strains of CDI.

Characterization of a novel mCH3 conjugated anti-PcrV scFv molecule.

Pseudomonas aeruginosa (PA) is a leading cause of nosocomial infections and death in cystic fibrosis patients. The study was conducted to evaluate the physicochemical structure, biological activity and serum stability of a recombinant anti-PcrV single chain variable antibody fragment genetically attached to the mCH3cc domain. The stereochemical properties of scFv-mCH3 (YFL001) and scFv (YFL002) proteins as well as molecular interactions towards Pseudomonas aeruginosa PcrV were evaluated computationally. The subcloned fragments encoding YFL001 and YFL002 in pET28a were expressed within the E. coli BL21-DE3 strain. After Ni-NTA affinity chromatography, the biological activity of the proteins in inhibition of PA induced hemolysis as well as cellular cytotoxicity was assessed. In silico analysis revealed the satisfactory stereochemical quality of the models as well as common residues in their interface with PcrV. The structural differences of proteins through circular dichroism spectroscopy were confirmed by NMR analysis. Both proteins indicated inhibition of ExoU positive PA strains in hemolysis of red blood cells compared to ExoU negative strains as well as cytotoxicity effect on lung epithelial cells. The ELISA test showed the longer serum stability of the YFL001 molecule than YFL002. The results were encouraging to further evaluation of these two scFv molecules in animal models.

Targeting NLRP3 and Staphylococcal pore-forming toxin receptors in human-induced pluripotent stem cell-derived macrophages.

Staphylococcus aureus causes necrotizing pneumonia by secreting toxins such as leukocidins that target front-line immune cells. The mechanism by which leukocidins kill innate immune cells and trigger inflammation during S. aureus lung infection, however, remains unresolved. Here, we explored human-induced pluripotent stem cell-derived macrophages (hiPSC-dMs) to study the interaction of the leukocidins Panton-Valentine leukocidin (PVL) and LukAB with lung macrophages, which are the initial leukocidin targets during S. aureus lung invasion. hiPSC-dMs were susceptible to the leukocidins PVL and LukAB and both leukocidins triggered NLPR3 inflammasome activation resulting in IL-1ß secretion. hiPSC-dM cell death after LukAB exposure, however, was only temporarily dependent of NLRP3, although NLRP3 triggered marked cell death after PVL treatment. CRISPR/Cas9-mediated deletion of the PVL receptor, C5aR1, protected hiPSC-dMs from PVL cytotoxicity, despite the expression of other leukocidin receptors, such as CD45. PVL-deficient S. aureus had reduced ability to induce lung IL-1ß levels in human C5aR1 knock-in mice. Unexpectedly, inhibiting NLRP3 activity resulted in increased wild-type S. aureus lung burdens. Our findings suggest that NLRP3 induces macrophage death and IL-1ß secretion after PVL exposure and controls S. aureus lung burdens.

Anti-inflammatory effects of a p38 MAP kinase inhibitor, doramapimod, against bacterial cell wall toxins in equine whole blood.

Doramapimod (BIRB-796-BS), is an anti-inflammatory compound, acting through p38 MAPK inhibition, but its anti-inflammatory effects have not previously been studied in the horse. Whole blood aliquots from healthy horses diluted 1:1 with cell culture medium were incubated for 21 h with 1 µg/ml of lipopolysaccharide (LPS), lipoteichoic acid (LTA) or peptidoglycan (PGN) in the presence of increasing concentrations of doramapimod (3 × 10-8 M to 10-5 M). Cell bioassays were used to measure TNF-α and IL-1ß activity. Doramapimod significantly and potently inhibited TNF-α and IL-1ß activity induced by all three bacterial toxins. There was no significant difference in IC50 or maximum inhibition of TNF-α or IL-1ß production between any of the toxins. Maximum inhibition of IL-1ß was higher than that of TNF-α for all toxins, and this difference was significant for LPS (P = 0.04). Doramapimod was a potent inhibitor of TNF-α and IL-1ß for inflammation induced by LPS, LTA and PGN, with potency much greater than that of other drugs previously tested using similar methods.

Betulin efficiently suppresses the process of an experimental Listeria monocytogenes infection as an antagonist against listeriolysin O.

Listeria monocytogenes (Lm) is a widespread foodborne intracellular pathogen that invades a variety of cells, causing abortions and severe human diseases. After internalization into host cells, pore-forming cytolysin listeriolysin O (LLO) disrupts the phagosome, which allows the bacterium to survive and colonize the cytoplasm, providing the bacterium the chance to infect neighboring cells. Betulin is an extracted natural compound from birch bark with diverse pharmacological activities. Here, we showed that LLO-induced rabbit red blood cell lysis in vitro was inhibited by preincubation with betulin, which suppressed the oligomerization process. Infectious assays performed with human monocyte macrophages indicated that betulin significantly protected cells against Lm-induced cell injury. In addition, Balb/c mice were used to perform a general infection, and betulin administration obviously inhibited organ damage and bacterial burden in livers and spleens of infected mice. In conclusion, betulin obviously inhibited Lm-induced cell injury in vitro and protected against infection in vivo through an antivirulence effect. Our results showed betulin as a new candidate against listeriosis by targeting LLO and highlight the potential of natural product-based medicine to be applied in the treatment of pathogenic infections.

Lead Optimization Yields High Affinity Frizzled 7-Targeting Peptides That Modulate Clostridium difficile Toxin B Pathogenicity in Epithelial Cells.

Frizzled 7 (FZD7) receptors have been shown to play a central role in intestinal stem cell regeneration and, more recently, in Clostridium difficile pathogenesis. Yet, targeting FZD7 receptors with small ligands has not been explored as an approach to block C. difficile pathogenesis. Here, we report the discovery of high affinity peptides that selectively bind to FZD7 receptors. We describe an integrated approach for lead optimization, utilizing structure-based rational design and directed evolution, to enhance the peptide binding affinity while still maintaining FZD7 receptor selectivity. This work yielded new peptide leads with picomolar binding constants to FZD7 as measured by biophysical methods. The new peptides block the interaction between C. difficile toxin B (TcdB) and FZD receptors and perturb C. difficile pathogenesis in epithelial cells. As such, our findings provide a proof of concept that targeting FZD receptors could be a viable pharmacological approach to protect epithelial cells from TcdB pathogenicity.

Fosfomycin Protects Mice From Staphylococcus aureus Pneumonia Caused by α-Hemolysin in Extracellular Vesicles by Inhibiting MAPK-Regulated NLRP3 Inflammasomes.

α-Hemolysin (Hla) is a significant virulence factor in Staphylococcus aureus (S. aureus)-caused infectious diseases such as pneumonia. Thus, to prevent the production of Hla when treating S. aureus infection, it is necessary to choose an antibiotic with good antibacterial activity and effect. In our study, we observed that Fosfomycin (FOM) at a sub-inhibitory concentration inhibited expression of Hla. Molecular dynamics demonstrated that FOM bound to the binding sites LYS 154 and ASP 108 of Hla, potentially inhibiting Hla. Furthermore, we verified that staphylococcal membrane-derived vesicles (SMVs) contain Hla and that FOM treatment significantly reduced the production of SMVs and Hla. Based on our pharmacological inhibition analysis, ERK and p38 activated NLRP3 inflammasomes. Moreover, FOM inhibited expression of MAPKs and NLRP3 inflammasome-related proteins in S. aureus as well as SMV-infected human macrophages (MΦ) and alveolar epithelial cells. In vivo, SMVs isolated from S. aureus DU1090 (an isogenic Hla deletion mutant) or the strain itself caused weaker inflammation than that of its parent strain 8325-4. FOM also significantly reduced the phosphorylation levels of ERK and P38 and expression of NLRP3 inflammasome-related proteins. In addition, FOM decreased MPO activity, pulmonary vascular permeability and edema formation in the lungs of mice with S. aureus-caused pneumonia. Taken together, these data indicate that FOM exerts protective effects against S. aureus infection in vitro and in vivo by inhibiting Hla in SMVs and blocking ERK/P38-mediated NLRP3 inflammasome activation by Hla.

Rapid Discovery and Characterization of Synthetic Neutralizing Antibodies against Anthrax Edema Toxin.

Anthrax, a lethal, weaponizable disease caused by Bacillus anthracis, acts through exotoxins that are primary mediators of systemic toxicity and also targets for neutralization by passive immunotherapy. The ease of engineering B. anthracis strains resistant to established therapy and the historic use of the microbe in bioterrorism present a compelling test case for platforms that permit the rapid and modular development of neutralizing agents. In vitro antigen-binding fragment (Fab) selection offers the advantages of speed, sequence level molecular control, and engineering flexibility compared to traditional monoclonal antibody pipelines. By screening an unbiased, chemically synthetic phage Fab library and characterizing hits in cell-based assays, we identified two high-affinity neutralizing Fabs, A4 and B7, against anthrax edema factor (EF), a key mediator of anthrax pathogenesis. Engineered homodimers of these Fabs exhibited potency comparable to that of the best reported neutralizing monoclonal antibody against EF at preventing EF-induced cyclic AMP production. Using internalization assays in COS cells, B7 was found to block steps prior to EF internalization. This work demonstrates the efficacy of synthetic alternatives to traditional antibody therapeutics against anthrax while also demonstrating a broadly generalizable, rapid, and modular screening pipeline for neutralizing antibody generation.

Protective effect of lycopene against chemical and natural toxins: A review.

People are exposed to a number of environmental, occupational, and therapeutic toxic agents which may be natural or man made. These hazardous substances may manifest as direct side effects on the function of organs or indirectly induced alteration of gene expression, cancer-associated metabolic pathways, and/or alter homeostasis. Lycopene, as a one of the most potent antioxidant, is found in fruits and vegetables. High-intake of lycopene has been shown to be effective in decreasing the risk of both natural toxins including mycotoxins, bacterial toxins, and chemical toxins including heavy metals, pesticides as well as herbicides. Recently, there is growing attention in understanding the mechanisms of the phytochemicals and carotenoids as antioxidative, antiapoptotic, radical scavenging, and chelating agents and their roles in the modulation of inflammatory pathways. This review summarizes available data from several recent studies about lycopene and its role against chemical and natural toxicants. © 2018 BioFactors, 45(1):5-23, 2019.

Photorhabdus luminescens Tc toxin is inhibited by the protease inhibitor MG132 and activated by protease cleavage resulting in increased binding to target cells.

Photorhabdus luminescens Tc toxins consist of the cell-binding component TcA, the linker component TcB, and the enzyme component TcC. TccC3, a specific isoform of TcC, ADP-ribosylates actin and causes redistribution of the actin cytoskeleton. TccC5, another isoform of TcC, ADP-ribosylates and activates Rho proteins. Here, we report that the proteasome inhibitor MG132 blocks the intoxication of cells by Tc toxin. The inhibitory effect of MG132 was not observed, when the ADP-ribosyltransferase domain of the TcC component was introduced into target cells by protective antigen, which is the binding and delivery component of anthrax toxin. Additionally, MG132 affected neither pore formation by TcA in artificial membranes nor binding of the toxin to cells. Furthermore, the in vitro ADP-ribosylation of actin by the enzyme domain of TccC3 was not affected by MG132. Similar to MG132, several calpain inhibitors blocked the action of the Tc toxin. Proteolytic cleavage of the binding component TcA induced by P. luminescens protease PrtA1 or by collagenase largely increased the toxicity of the Tc toxin. MG132 exhibited no inhibitory effect on the cleaved TcA component. Moreover, binding of TcA to target cells was largely increased after cleavage. The data indicate that Tc toxin is activated by proteolytic processing of the TcA component, resulting in increased receptor binding. Toxin processing is probably inhibited by MG132.

A potential bio-control agent from baical skullcap root against listeriosis via the inhibition of sortase A and listeriolysin O.

Listeria monocytogenes (LM) is a classical model intracellular pathogen and the leading cause of listeriosis, which has long been a global public health issue. The successful infection of LM is related to a series of virulence factors, such as the transpeptidase enzyme sortase A (SrtA) and listeriolysin O (LLO), which are crucial for bacterial internalization and escape from phagosomes respectively. It is speculated that targeting multiple virulence factors may be due to a synergistic effect on listeriosis therapy. In this study, an active flavonoids component of Scutellaria baicalensis Georgi, baicalein, was found to potently block both listerial SrtA catalyzed activity and LLO hemolytic activity within 16 µg/mL. After pretreatment with baicalein, 86.30 (±11.35) % of LM failed to associate with Caco-2 cells compared to the LM without preincubation (regarded as 100% internalization). Furthermore, baicalein addition may aid in bacterial degradation and clearance in macrophagocytes. During a 5 h observation, LM in cells incubated with baicalein showed significantly decreased vacuole escapes and sluggish endocellular growth. In addition, baicalein directly prevented LM-induced cells injury and mice fatality (survival rate from 10.00% to 54.55% in 4 days post-intraperitoneal injection). Taken together, as an antagonist against SrtA and LLO, baicalein can be further developed into a biotherapeutic agent for listeriosis.

Natural Compounds and Their Analogues as Potent Antidotes against the Most Poisonous Bacterial Toxin.

Botulinum neurotoxins (BoNTs), the most poisonous proteins known to humankind, are a family of seven (serotype A to G) immunologically distinct proteins synthesized primarily by different strains of the anaerobic bacterium Clostridium botulinum Being the causative agents of botulism, the toxins block neurotransmitter release by specifically cleaving one of the three soluble N-ethylmaleimide-sensitive factor attachment receptor (SNARE) proteins, thereby inducing flaccid paralysis. The development of countermeasures and therapeutics against BoNTs is a high-priority research area for public health because of their extreme toxicity and potential for use as biowarfare agents. Extensive research has focused on designing antagonists that block the catalytic activity of BoNTs. In this study, we screened 300 small natural compounds and their analogues extracted from Indian plants for their activity against BoNT serotype A (BoNT/A) as well as its light chain (LCA) using biochemical and cellular assays. One natural compound, a nitrophenyl psoralen (NPP), was identified to be a specific inhibitor of LCA with an in vitro 50% inhibitory concentration (IC50) value of 4.74 ± 0.03 µM. NPP was able to rescue endogenous synaptosome-associated protein 25 (SNAP-25) from cleavage by BoNT/A in human neuroblastoma cells with an IC50 of 12.2 ± 1.7 µM, as well as to prolong the time to the blocking of neutrally elicited twitch tensions in isolated mouse phrenic nerve-hemidiaphragm preparations.IMPORTANCE The long-lasting endopeptidase activity of BoNT is a critical biological activity inside the nerve cell, as it prompts proteolysis of the SNARE proteins, involved in the exocytosis of the neurotransmitter acetylcholine. Thus, the BoNT endopeptidase activity is an appropriate clinical target for designing new small-molecule antidotes against BoNT with the potential to reverse the paralysis syndrome of botulism. In principle, small-molecule inhibitors (SMIs) can gain entry into BoNT-intoxicated cells if they have a suitable octanol-water partition coefficient (log P) value and other favorable characteristics (P. Leeson, Nature 481:455-456, 2012, https://doi.org/10.1038/481455a). Several efforts have been made in the past to develop SMIs, but inhibitors effective under in vitro conditions have not in general been effective in vivo or in cellular models (L. M. Eubanks, M. S. Hixon, W. Jin, S. Hong, et al., Proc Natl Acad Sci U S A 104:2602-2607, 2007, https://doi.org/10.1073/pnas.0611213104). The difference between the in vitro and cellular efficacy presumably results from difficulties experienced by the compounds in crossing the cell membrane, in conjunction with poor bioavailability and high cytotoxicity. The screened nitrophenyl psoralen (NPP) effectively antagonized BoNT/A in both in vitro and ex vivo assays. Importantly, NPP inhibited the BoNT/A light chain but not other general zinc endopeptidases, such as thermolysin, suggesting high selectivity for its target. Small-molecule (nonpeptidic) inhibitors have better oral bioavailability, better stability, and better tissue and cell permeation than antitoxins or peptide inhibitors.

Role of a Small Molecule in the Modulation of Cell Death Signal Transduction Pathways.

Inflammasomes activate caspase-1 in response to molecular signals from pathogens and other dangerous stimuli as a part of the innate immune response. A previous study discovered a small-molecule, 4-fluoro- N'-[1-(2-pyridinyl)ethylidene]benzohydrazide, which we named DN1, that reduces the cytotoxicity of anthrax lethal toxin (LT). We determined that DN1 protected cells irrespectively of LT concentration and reduced the pathogenicity of an additional bacterial exotoxin and several viruses. Using the LT cytotoxicity pathway, we show that DN1 does not prevent LT internalization and catalytic activity or caspase-1 activation. Moreover, DN1 does not affect the proteolytic activity of host cathepsin B, which facilitates the cytoplasmic entry of toxins. PubChem Bioactivities lists two G protein-coupled receptors (GPCR), type-1 angiotensin II receptor and apelin receptor, as targets of DN1. The inhibition of phosphatidylinositol 3-kinase, phospholipase C, and protein kinase B, which are downstream of GPCR signaling, synergized with DN1 in protecting cells from LT. We hypothesize that DN1-mediated antagonism of GPCRs modulates signal transduction pathways to induce a cellular state that reduces LT-induced pyroptosis downstream of caspase-1 activation. DN1 also reduced the susceptibility of Drosophila melanogaster to toxin-associated bacterial infections. Future experiments will aim to further characterize how DN1 modulates signal transduction pathways to inhibit pyroptotic cell death in LT-sensitive macrophages. DN1 represents a novel chemical probe to investigate host cellular mechanisms that mediate cell death in response to pathogenic agents.

Historical links between toxinology and immunology.

Research on bacterial toxins is closely linked to the birth of immunology. Our understanding of the interaction of bacterial protein toxins with immune cells has helped to decipher immunopathology, develop preventive and curative treatments for infections, and propose anti-cancer immunotherapies. The link started when Behring and Kitasato demonstrated that serotherapy was effective against 'the strangling angel', namely diphtheria, and its dreadful toxin discovered by Roux and Yersin. The antitoxin treatment helped to save thousands of children. Glenny demonstrated the efficacy of the secondary immune response compared to the primary one. Ramon described anatoxins that allowed the elaboration of effective vaccines and discovered the use of adjuvant to boost the antibody response. Similar approaches were later made for the tetanus toxin. Studying antitoxin antibodies Ehrlich demonstrated, for the first time, the transfer of immunity from mother to newborns. In 1989 Marrack and Kappler coined the concept of 'superantigens' to characterize protein toxins that induce T-lymphocyte proliferation, and cytokine release by both T-lymphocytes and antigen presenting cells. More recently, immunotoxins have been designed to kill cancer cells targeted by either specific antibodies or cytokines. Finally, the action of IgE antibodies against toxins may explain their persistence through evolution despite their side effect in allergy.

Human alpha-defensin-1 protects cells from intoxication with Clostridium perfringens iota toxin.

Iota toxin is produced by Clostridium perfringens type E strains and associated with diarrhea in cattle and lambs. This binary protein toxin comprises the enzyme component iota a (Ia), which ADP-ribosylates G-actin, and the separate transport component iota b (Ib), which delivers Ia into the cytosol of target cells. Ib binds to cell receptors and forms biologically active toxin complexes with Ia, which cause rounding of adherent cells due to the destruction of the actin cytoskeleton. Here, we report that the human peptide α-defensin-1 protects cultured cells including human colon cells from intoxication with iota toxin. In contrast, the related ß-defensin-1 had no effect, indicating a specific mode of action. The α-defensin-1 did not inhibit ADP-ribosylation of actin by Ia in vitro. Pretreatment of Ib with α-defensin-1 prior to addition of Ia prevented intoxication. Additionally, α-defensin-1 protected cells from cytotoxic effects mediated by Ib in the absence of Ia, implicating that α-defensin-1 interacts with Ib to prevent the formation of biologically active iota toxin on cells. In conclusion, the findings contribute to a better understanding of the functions of α-defensin-1 and suggest that this human peptide might be an attractive starting point to develop novel pharmacological options to treat/prevent diseases associated with iota toxin-producing Clostridium perfringens strains.

Neutralizing Alpha-Toxin Accelerates Healing of Staphylococcus aureus-Infected Wounds in Nondiabetic and Diabetic Mice.

Staphylococcus aureus wound infections delay healing and result in invasive complications such as osteomyelitis, especially in the setting of diabetic foot ulcers. In preclinical animal models of S. aureus skin infection, antibody neutralization of alpha-toxin (AT), an S. aureus-secreted pore-forming cytolytic toxin, reduces disease severity by inhibiting skin necrosis and restoring effective host immune responses. However, whether therapeutic neutralization of alpha-toxin is effective against S. aureus-infected wounds is unclear. Herein, the efficacy of prophylactic treatment with a human neutralizing anti-AT monoclonal antibody (MAb) was evaluated in an S. aureus skin wound infection model in nondiabetic and diabetic mice. In both nondiabetic and diabetic mice, anti-AT MAb treatment decreased wound size and bacterial burden and enhanced reepithelialization and wound resolution compared to control MAb treatment. Anti-AT MAb had distinctive effects on the host immune response, including decreased neutrophil and increased monocyte and macrophage infiltrates in nondiabetic mice and decreased neutrophil extracellular traps (NETs) in diabetic mice. Similar therapeutic efficacy was achieved with an active vaccine targeting AT. Taken together, neutralization of AT had a therapeutic effect against S. aureus-infected wounds in both nondiabetic and diabetic mice that was associated with differential effects on the host immune response.