Analysis of the functional domains of Arcanobacterium pyogenes pyolysin using monoclonal antibodies

Revealing the Mechanism of NLRP3 Inflammatory Pathway Activation through K+ Efflux Induced by PLO via Signal Point Mutations

Trueperella pyogenes is an important opportunistic pathogenic bacterium widely distributed in the environment. Pyolysin (PLO) is a primary virulence factor of T. pyogenes and capable of lysing many different cells. PLO is a member of the cholesterol-dependent cytolysin (CDC) family of which the primary structure only presents a low level of homology with other members from 31% to 45%. By deeply studying PLO, we can understand the overall pathogenic mechanism of CDC family proteins. This study established a mouse muscle tissue model infected with recombinant PLO (rPLO) and its single-point mutations, rPLO N139K and rPLO F240A, and explored its mechanism of causing inflammatory damage. The inflammatory injury abilities of rPLO N139K and rPLO F240A are significantly reduced compared to rPLO. This study elaborated on the inflammatory mechanism of PLO by examining its unit point mutations in detail. Our data also provide a theoretical basis and practical significance for future research on toxins and bacteria.

Single Point Mutation and Its Role in Specific Pathogenicity to Reveal the Mechanism of Related Protein Families

Pyolysin (PLO) is secreted by Trueperella pyogenes as a water-soluble monomer after forming transmembrane β-barrel channels in the cell membrane by binding cholesterol. Two significantly conserved residues at domain 1 of PLO are mutated, which provides novel evidence of a relationship between conformational change and interaction with the cell membrane and uncovers the pore formation mechanism of the cholesterol-dependent cytolysin (CDC) family. Moreover, PLO is a special member of the CDCs, which the percentage of sequence identities between PLO and other CDC members is from 31% to 45%, while others are usually from 40% to 70%. It is important to understand that at very low sequence identities, models can be different in the pathogenic mechanisms of these CDC members, which are dedicated to a large number of Gram-positive bacterial pathogens. Our studies, for the first time, located and mutated two different highly conserved structural sites in the primary structure critical for PLO structure and function that proved the importance of these sites. Together, novel and repeatable observations into the pore formation mechanism of CDCs are provided by our findings. IMPORTANCE Postpartum disease of dairy cows caused by persistent bacterial infection is a global disease, which has a serious impact on the development of the dairy industry and brings huge economic losses. As one of the most relevant pathogenic bacteria for postpartum diseases in dairy cows, Trueperella pyogenes can secrete pyolysin (PLO), a member of the cholesterol-dependent cytolysin (CDC) family and recognized as the most important toxin of T. pyogenes. However, the current research work on PLO is still insufficient. The pathogenic mechanism of this toxin can be fully explored by changing the local structure and overall function of the toxin by a previously unidentified single point mutation. These studies lay the groundwork for future studies that will explore the contribution of this large family of CDC proteins to microbial survival and human disease.

Mapping of Recognition Sites of Monoclonal Antibodies Responsible for the Inhibition of Pneumolysin Functional Activity

The pathogenicity of many bacteria, including Streptococcus pneumoniae, depends on pore-forming toxins (PFTs) that cause host cell lysis by forming large pores in cholesterol-containing cell membranes. Therefore, PFTs-neutralising antibodies may provide useful tools for reducing S. pneumoniae pathogenic effects. This study aimed at the development and characterisation of monoclonal antibodies (MAbs) with neutralising activity to S. pneumoniae PFT pneumolysin (PLY). Five out of 10 produced MAbs were able to neutralise the cytolytic activity of PLY on a lung epithelial cell line. Epitope mapping with a series of recombinant overlapping PLY fragments revealed that neutralising MAbs are directed against PLY loops L1 and L3 within domain 4. The epitopes of MAbs 3A9, 6E5 and 12F11 located at L1 loop (aa 454–471) were crucial for PLY binding to the immobilised cholesterol. In contrast, the MAb 12D10 recognising L3 (aa 403–423) and the MAb 3F3 against the conformational epitope did not interfere with PLY-cholesterol interaction. Due to conformation-dependent binding, the approach to use overlapping peptides for fine epitope mapping of the neutralising MAbs was unsuccessful. Therefore, the epitopes recognised by the MAbs were analysed using computational methods. This study provides new data on PLY sites involved in functional activity.

Evaluation of the Potency of Two Pyolysin-Derived Recombinant Proteins as Vaccine Candidates of Trueperella Pyogenes in a Mouse Model: Pyolysin Oligomerization and Structural Change Affect the Efficacy of Pyolysin-Based Vaccines

Trueperella pyogenes (T. pyogenes) is an important opportunistic pathogen in livestock and wild animals. However, only one commercial T. pyogenes vaccine is currently available, and its immunoprotective effect is not ideal. Pyolysin (PLO) is one of the important virulence factors expressed by T. pyogenes and one of the targets for the development of new T. pyogenes vaccines. In this study, we constructed two recombinant antigens, tPLOA1 (contains amino acids 1–110 and domain 4 of the PLO molecule) and tPLOA2 (contains amino acids 190–296 and domain 4 of the PLO molecule). Vaccines were prepared by mixing the two recombinant antigens with incomplete Freund’s adjuvant or sheep red blood cell membrane and provided partial immune protection to immunized mice against the lethal challenge of T. pyogenes. Analysis of the PLO-specific IgG levels of immunized mice indicated that the antibody-inducing potency and immunoprotective efficacy of PLO-based vaccines are affected by the oligomerization and structural changes of PLO after binding to a cholesterol-containing membrane. In addition, the titer of anti-hemolysis antibodies is not a suitable indicator of the immunoprotective effect of these vaccines in PLO-based vaccine-immunized animals. The results provide new insights into the development of T. pyogenes vaccines.

Pathogenicity and Virulence of Trueperella pyogenes: A Review

Bacteria from the species Trueperella pyogenes are a part of the biota of skin and mucous membranes of the upper respiratory, gastrointestinal, or urogenital tracts of animals, but also, opportunistic pathogens. T. pyogenes causes a variety of purulent infections, such as metritis, mastitis, pneumonia, and abscesses, which, in livestock breeding, generate significant economic losses. Although this species has been known for a long time, many questions concerning the mechanisms of infection pathogenesis, as well as reservoirs and routes of transmission of bacteria, remain poorly understood. Pyolysin is a major known virulence factor of T. pyogenes that belongs to the family of cholesterol-dependent cytolysins. Its cytolytic activity is associated with transmembrane pore formation. Other putative virulence factors, including neuraminidases, extracellular matrix-binding proteins, fimbriae, and biofilm formation ability, contribute to the adhesion and colonization of the host tissues. However, data about the pathogen–host interactions that may be involved in the development of T. pyogenes infection are still limited. The aim of this review is to present the current knowledge about the pathogenic potential and virulence of T. pyogenes.

Replacing the 238th aspartic acid with an arginine impaired the oligomerization activity and inflammation-inducing property of pyolysin

Trueperella pyogenes (T. pyogenes) is an important opportunistic pathogen. Pyolysin (PLO) importantly contributes to the pathogenicity of T. pyogenes. However, the relationship between the structure and function and the virulence of PLO is not well documented. In the current study, recombinant PLO (rPLO) and three rPLO mutants were prepared. rPLO D238R, a mutant with the 238th aspartic acid replaced with an arginine, showed impairment in oligomerization activity on cholesterol-containing liposome and pore-forming activity on sheep red blood cell membrane. Further study employing the prepared mutants confirmed that the pore-forming activity of PLO is essential for inducing excessive inflammation responses in mice by upregulating the expression levels of IL-1β, TNF-α, and IL-6. By contrast, rPLO P499F, another mutant with impaired cell membrane binding capacity, elicited an inflammation response that was dependent on pathogen-associated molecular pattern (PAMP) activity, given that the mutant significantly upregulated the expression of IL-10 in macrophages and in mice, whereas rPLO did not. Results indicated that domain 1 of the PLO molecule plays an important role in maintaining pore-forming activity. Moreover, the PLO pore-forming activity and not PAMP activity is responsible for the inflammation-inducing effect of PLO. The results of this study provided new information for research field on the structure, function, and virulence of PLO.

Abbreviations: T. pyogenes: Trueperella pyogenes; PLO: Pyolysin; rPLO: recombinant PLO; PAMP: pathogen-associated molecular pattern; CDCs: cholesterol-dependent cytolysins; PLY: pneumolysin; NLRP3: NLR family pyrin domain containing protein 3; PRRs: pattern recognition receptors; Asp: aspartic acid; TLR4: Toll-like receptor 4; Arg: arginine; Asn: asparagine; IPTG: Isopropyl-β-d-thiogalactoside; PBS: phosphate-buffered saline; sRBCs: sheep red blood cells; TEM: Transmission electron microscopy; RBCM: red blood cell membrane; SDS-PAGE: sodium dodecyl sulfate–polyacrylamide gel electrophoresis; NC membrane: nitrocellulose membrane; SDS-AGE: dodecyl sulfate agarose gel electrophoresis; MDBK cells: Madin–Darby bovine kidney cells; RPMI-1640 medium: Roswell Park Memorial Institute-1640 medium; FBS: fetal bovine serum; BMDMs: bone marrow-derived macrophages; TNF-α: tumor necrosis factor α; IL-1β: interleukin-1β; IFN-γ: interferon-γ; TGF-β: transforming growth factor-β; ELISA: enzyme-linked immunosorbent assay

Chitosan-DNA nanoparticles enhanced the immunogenicity of multivalent DNA vaccination on mice against Trueperella pyogenes infection

BACKGROUND

Trueperella pyogenes is a commensal and opportunistic pathogen that normally causes mastitis, liver abscesses and pneumonia of economically important livestock. To develop efficacious and potent vaccine against T. pyogenes, chimeric gene DNA vaccines were constructed and encapsulated in chitosan nanoparticles (pPCFN-CpG-CS-NPs).

RESULTS

The pPCFN-CpG-CS-NPs consists of the plo, cbpA, fimA, and nanH gene of T. pyogenes and CpG ODN1826. It was produced with good morphology, high stability, a mean diameter of 93.58 nm, and a zeta potential of + 5.27 mV. Additionally, chitosan encapsulation was confirmed to protect the DNA plasmid from DNase I digestion. The immunofluorescence assay indicated that the four-chimeric gene could synchronously express in HEK293T cells and maintain good bioactivity. Compared to the mice immunized with the control plasmid, in vivo immunization showed that mice immunized with the pPCFN-CpG-CS-NPs had better immune responses, and release of the plasmid DNA was prolonged. Importantly, immunization with pPCFN-CpG-CS-NPs could significantly protect mice from highly virulent T. pyogenes TP7 infection.

CONCLUSIONS

This study indicates that chitosan-DNA nanoparticles are potent immunization candidates against T. pyogenes infection and provides strategies for the further development of novel vaccines encapsulated in chitosan nanoparticles.

Identification of B-cell linear epitopes in domains 1-3 of pyolysin of Trueperella pyogenes using polyclonal antibodies

Trueperella pyogenes is an important opportunistic pathogen. Pyolysin (PLO) makes important contributions to the pathogenicity of T. pyogenes. However, the structure and function of PLO has not been well documented. In the current study, epitopes in domain 1-3 of PLO have been mapped using rabbit anti-recombinant PLO (rPLO) polyclonal antibodies, and then the results were re-checked by using mouse and chicken anti-rPLO polyclonal antibodies, respectively. The results indicated that the region of aa 281-393 in PLO could not elicit antibodies against linear epitopes. A total of six B cell linear epitopes have been found in domain 1 of PLO. Two of the six epitopes (EP1 and EP2) were used to immunize mice and chicken. Chicken anti-EP1 and anti-EP2 serum and mouse anti-EP2 serum could react with rPLO and corresponding epitope polypeptide in western blot assay; however, only mouse anti-EP2 serum shows weak anti-hemolysis effect in the rPLO and sheep red blood system. Our results provide some new information to the research field of PLO structure and function.

A chimeric protein composed of the binding domains of Clostridium perfringens phospholipase C and Trueperella pyogenes pyolysin induces partial immunoprotection in a mouse model

Trueperella pyogenes and Clostridium perfringens are two kinds of conditional pathogens frequently associated with wound infections and succeeding lethal complications in various economic livestock. Pyolysin (PLO) and phospholipase C (PLC) are the key virulence factors of these two pathogens, respectively. In our study, a chimeric protein called rPC-PD4, which is composed of the binding regions of PLO and PLC, was synthesized. The toxicity of rPC-PD4 was evaluated. Results revealed that rPC-PD4 is a safe chimeric molecule that can be used to develop vaccines. Immunizing BALB/c mice with rPC-PD4 induced high titers of serum antibodies that could efficiently neutralize the hemolytic activity of recombinant PLO and PLC. After the challenge with T. pyogenes or C. perfringens was performed through the intraperitoneal route, we observed that rPC-PD4 immunization could provide partial immunoprotection and reduce lung, intestine, and liver tissue damage to mice. This work demonstrated the efficacy of the rationally designed rPC-PD4 chimeric protein as a potential vaccine candidate against C. perfringens and T. pyogenes.

DNA vaccination based on pyolysin co-immunized with IL-1β enhances host antibacterial immunity against Trueperella pyogenes infection

Trueperella pyogenes is a commensal and opportunistic pathogen normally causes mastitis, liver abscesses and pneumonia of economically important livestock. To date, no specific control measure was reported to prevent T. pyogenes infections. In this study, we first constructed a recombinant plasmid pVAX1-PLO based on the main virulent factor pyolysin gene as DNA vaccine against T. pyogenes infection. Subsequently, transient expression of pVAX1-PLO and pcDNA3.1/V5-fIL-1β were identified in Human embryonic kidney cells (HEK293T) by immunofluorescence assay. Humoral and cellular immune responses were evaluated in mice to compare the immunogenicity between different immunized groups. The results showed that the successful expression of PLO or fIL-1β protein was detected by confocal microscopy for cells transfected with plasmid pVAX1-PLO and/or pcDNA3.1/V5-fIL-1β. The mice immunized with pVAX1-PLO elicited a higher titer of PLO-specific antibody than the control group. The levels of IFN-γ and IL-2 were significantly increased in the pVAX1-PLO immunized mice, while the expression level of IL-4 was slightly increased but not significant. These findings suggested that the DNA vaccine pVAX1-PLO can primarily induce Th1 immune response. The residual Colony-Forming Units (CFUs) from the liver and peritoneal fluid were decreased obviously in the pVAX1-PLO treated mice compared with the control. Importantly, co-immunization with pcDNA3.1/V5-fIL-1β and pVAX1-PLO could enhance host humoral and cellular immune responses and significantly protect mouse from T. pyogenes infection. In conclusion, our study provides a promising strategy against T. pyogenes infections and implies the potential clinical application of combined DNA vaccines in diseases control.

Isoleucine 61 is important for the hemolytic activity of pyolysin of Trueperella pyogenes

Pyolysin (PLO) is a hemolysin secreted by Trueperella pyogenes (T. pyogenes) and is important for the pathogenicity of T. pyogenes. Oligomerization of PLO monomers is a critical step in the process of hemolysis. However, the mechanisms of intermolecular interaction of PLO monomers are still not clearly illuminated. In this study, two monoclonal antibodies (mAbs) against PLO, named AP-1A3 and AP-4F12, respectively, were generated firstly, of which AP-1A3 showed no or undetectable hemolysis inhibition activity against recombinant PLO (rPLO), whereas AP-4F12 could markedly inhibit the hemolytic activity of rPLO. Epitope mapping revealed that AP-1A3 recognized amino acid residues ranging from 64 to 79 of mature PLO (91-106 including the signal peptide), whereas AP-4F12 recognized amino acid residues ranging from 58 to 75 (85-102 including the signal peptide). Comparison of the amino acid sequence of two epitopes revealed that six amino acid residues ranging from 58 to 63 of PLO were associated with the hemolytic activity of PLO. Alanine scan showed that substitution of each amino acid ranging from 58 to 62 with alanine had apparent impact on the hemolytic activity of rPLO, especially for the substitution of isoleucine 61 which caused almost complete loss of hemolytic activity of rPLO. Our findings identified a region in PLO and an amino acid in that region might play important role in the process of oligomerization of PLO monomers.

Arcanobacterium pyogenes: molecular pathogenesis of an animal opportunist

Arcanobacterium pyogenes is a commensal and an opportunistic pathogen of economically important livestock, causing diseases as diverse as mastitis, liver abscessation and pneumonia. This organism possesses a number of virulence factors that contribute to its pathogenic potential. A. pyogenes expresses a cholesterol-dependent cytolysin, pyolysin, which is a haemolysin and is cytolytic for immune cells, including macrophages. Expression of pyolysin is required for virulence and this molecule is the most promising vaccine candidate identified to date. A. pyogenes also possesses a number of adherence mechanisms, including two neuraminidases, the action of which are required for full adhesion to epithelial cells, and several extracellular matrix-binding proteins, including a collagen-binding protein, which may be required for adhesion to collagen-rich tissue. A. pyogenes also expresses fimbriae, which are similar to the type 2 fimbriae of Actinomyces naeslundii, and forms biofilms. However, the role of these factors in the pathogenesis of A. pyogenes infections remains to be elucidated. A. pyogenes also invades and survives within epithelial cells and can survive within J774A.1 macrophages for up to 72 h, suggesting an important role for A. pyogenes interaction with host cells during pathogenesis. The two component regulatory system, PloSR, up-regulates pyolysin expression and biofilm formation but down-regulates expression of proteases, suggesting that it may act as a global regulator of A. pyogenes virulence. A. pyogenes is a versatile pathogen, with an arsenal of virulence determinants. However, most aspects of the pathogenesis of infection caused by this important opportunistic pathogen remain poorly characterized.

Effect of amino acid substitutions in the epitope regions of pyolysin from Arcanobacterium pyogenes

Pyolysin (PLO), secreted by Arcanobacterium pyogenes, is a novel member of the thiol-activated cytolysin (TACY, cholesterol-dependent cytolysin) family of bacterial toxins. Recently, we demonstrated that the epitopes of monoclonal antibodies (mAbs) S, H, C, and G lie in the regions of amino acids regions 55-73, 123-166, 482-506, and 482-506 of PLO, respectively, by the reaction of mAbs with truncated PLOs. In this study, we substituted the amino acids in these epitope regions of PLO by site-directed mutagenesis and examined the effect of these amino acid substitutions. Mutants I70S/R71A/L73S, Y131S/P132S, and L163S/P164S for mAbs H or S completely lost the hemolytic activity of the proteins, but these mutants still bound to erythrocyte membranes. Mutants L495S/W497S and W500S/W501S for mAbs C and G also completely lost their hemolytic activity, but still bound to erythrocyte membranes. In the undecapeptide region of PLO, the cysteine residue required for thiol activation is replaced with alanine. Therefore, we substituted Ala-492 of the undecapeptide region for Cys. The hemolytic activity of this mutant A492C decreased by adding hydrogen peroxide or storing at 4 degrees C, and the decreased hemolytic activity was restored by adding L-cysteine.

Structural gene and strain specificity of a novel cysteine protease produced by Staphylococcus aureus isolated from a diseased chicken

Recently whole genome sequencing of Staphylococcus aureus has revealed the genes encoding cysteine proteases such as staphopain and SspB. In this study, we cloned and sequenced the structural gene (ScpA) encoding a cysteine (thiol) protease of S. aureus strain CH-91 from a chicken with dermatitis using polymerase chain reaction (PCR) and inverse PCR methods. The sequence information revealed a coding sequence (CDS) of 1200 nucleotides encoding the ScpA preproenzyme of 399 amino acids with a molecular mass of 45,071 Da. The deduced amino acid sequence of the ScpA differed at many positions from those of staphopain and SspB with identities of 64 and 42%, respectively. In the Southern blot analysis with a total DNA of S. aureus strain CH-91, the ScpA probe hybridized with a single 7.7 kb XbaI fragment or 2.8 and 0.8 kb EcoRI fragments, whereas the staphopain and SspB probes did not hybridize with these DNA fragments. These results suggest that this ScpA gene is a single-copy gene and is a novel gene, which is not found in the published whole genome sequences of S. aureus. In immunoblot, PCR, and Southern blot assays, the ScpA or its gene was detected in high protease-producing strains from chickens, but was not recognized in bovine and porcine strains or low protease-producing avian strains. These results indicate that the ScpA of CH-91 type may be specific to the high protease-producing strains of S. aureus from chickens, namely, there is a strain specificity of the ScpA.