Secretion of Pertussis Toxin from Bordetella pertussis

Production and secretion of pertussis toxin (PT) is essential for the virulence of Bordetella pertussis. Due to the large oligomeric structure of PT, transport of the toxin across bacterial membrane barriers represents a significant hurdle that the bacteria must overcome in order to maintain pathogenicity. During the secretion process, PT undergoes a two-step transport process. The first step involves transport of the individual polypeptide chains of PT across the inner membrane utilizing a generalized secretion pathway, most likely the bacterial Sec system. The second step involves the use of a specialized apparatus to transport the toxin across the outer membrane of the bacterial cell. This apparatus, which has been termed the Ptl transporter and which is unique to the PT secretion pathway, is a member of the type IV family of bacterial transporters. Here, the current understanding of the PT secretion process is reviewed including a description of the Ptl proteins that assemble to form the transporter, the general structure of type IV transporters, the known similarities and differences between canonical type IV substrate transport and Ptl-mediated transport of PT, as well as the known sequence of events in the assembly and secretion of PT.

Multiple weak interactions between BvgA~P and ptx promoter DNA strongly activate transcription of pertussis toxin genes in Bordetella pertussis

Pertussis toxin is the preeminent virulence factor and major protective antigen produced by Bordetella pertussis, the human respiratory pathogen and etiologic agent of whooping cough. Genes for its synthesis and export are encoded by the 12 kb ptx-ptl operon, which is under the control of the pertussis promoter, Pptx. Expression of this operon, like that of all other known protein virulence factors, is regulated by the BvgAS two-component global regulatory system. Although Pptx has been studied for years, characterization of its promoter architecture vis-à-vis BvgA-binding has lagged behind that of other promoters, mainly due to its lower affinity for BvgA~P. Here we take advantage of a mutant BvgA protein (Δ127-129), which enhances ptx transcription in B. pertussis and also demonstrates enhanced binding affinity to Pptx. By using this mutant protein labeled with FeBABE, binding of six head-to-head dimers of BvgA~P was observed, with a spacing of 22 bp, revealing a binding geometry similar to that of other BvgA-activated promoters carrying at least one strong binding site. All of these six BvgA-binding sites lack sequence features associated with strong binding. A genetic analysis indicated the degree to which each contributes to Pptx activity. Thus the weak/medium binding affinity of Pptx revealed in this study explains its lower responsiveness to phosphorylated BvgA, relative to other promoters containing a high affinity binding site, such as that of the fha operon.

[Production of pertussis toxin by Bordetella pertussis strains isolated from patients with whooping cough]

AIM

To assess level of pertussin toxin (PT) production by vaccine strains of Bordetella pertussis and strains isolated from patients with whooping cough.

MATERIALS AND METHODS

Concentration of PT in supernatants of microbial cultures of 3 vaccine strains and 25 strains of B. pertussis isolated from patients with pertussis in 2001 - 2005 was measured with enzyme immunoassay using gamma-globulin fractions of rabbit antiserum to PT as immunosorbent or included in peroxidase conjugates.

RESULTS

Level of PT production by strains isolated from infected persons varied from 3 +/- 0.5 to 64.8 +/- 12.2 ng/MFU/ml: in 9 strains--from 3 +/- 0.5 to 9.4 +/- 2.1 ng/MFU/ml, in 7--10.5 +/- 1.8 to 18.4 +/- 2.6 ng/MFU/ml, and in 9--23.6 +/- 4.5 to 64.8 +/- 12.2 ng/MFU/ml.

CONCLUSION

B. pertussis strains isolated from patients were heterogeneous on level of PT production. Difference in expression of PT between strains were as high as 20-fold. Conditionally low, moderate and high levels of PT production had 9 (36%), 7 (28%), and 9 (36%) of 25 studied strains. Three vaccine strains had levels of toxin production similar to recently isolated strains with moderate level of its production.

Generation of plant-derived recombinant DTP subunit vaccine

The current diphtheria-tetanus-pertussis (DTP) pediatric vaccine is produced from the corresponding pathogenic bacteria Corynebacterium diphtheriae, Clostridium tetani and Bordetella pertussis; five injected doses of DTaP (acellular) vaccine are required for every child in the standard US vaccination schedule. Because the vaccine is derived from native live sources, adverse effects are possible and production is complex and costly. To address issues of safety, ease of renewability and expense, we used recombinant technology in an effort to develop a subunit DPT vaccine derived in non-pathogenic plant expression systems. Expression of diphtheria toxin (DT), tetanus fragment-C (TetC) and the non-toxic S1 subunit of pertussis toxin (PTX S1) antigenic proteins in soluble form in low-alkaloid tobacco plants and carrot cell cultures allowed efficient downstream purification to levels suitable for intramuscular injection in BALB/c mice. At working concentrations of 5mug per dose, these preparations induced high levels of antigen-specific IgGs in mouse sera. Our results clearly support the feasibility of producing recombinant pediatric vaccine components in plants.

Fed-batch cultivation of Bordetella pertussis: Metabolism and Pertussis Toxin production

The production of acellular pertussis in comparison with whole cell pertussis vaccines demands 5-25 times the amount of Bordetella pertussis' virulence factors, such as Pertussis Toxin (PT), to produce the same number of vaccine doses. An increase in the volumetric productivity by employing fed-batch rather than the currently used batch cultivations of B. pertussis could reduce the cost price of acellular pertussis vaccines. This study defined the conditions that enable fed-batch cultivations at high specific PT production. A solution containing lactate and glutamate was fed to the cultures at various rates. The feed rate and whether or not the fed substrates were completely consumed, significantly influenced cellular metabolism. If lactate was detectable in the culture broth while glutamate was not, poly-hydroxy-butyrate (PHB) was formed. Any PHB present was metabolized when glutamate became detectable again in the culture liquid. At higher lactate and glutamate concentrations, free fatty acids were produced. Though toxic, free fatty acids were not the reason the cultures stopped growing. By choosing appropriate conditions, a cell density of 6.5 g/L dry weight was reached, i.e. a 7-fold increase compared to batch culture. The metabolic mechanisms behind the formation of PHB and fatty acids are discussed, as well as how to increase the cell density further. The PT production stopped at 12 mg/L, well before growth stopped, indicating that regulatory mechanisms of PT production may be involved.

Development of safer pertussis DNA vaccine expressing non-toxic C180 polypeptide of pertussis toxin S1 subunit

A toxic N-terminal 180-amino-acid fragment (C180) of pertussis toxin S1 subunit has the most potent ability to induce protective immunity against pertussis toxin (PT) following DNA-based immunization [Kamachi K, Arakawa Y. Infect Immun 2004;72:4293-6]. For the development of a safer pertussis DNA vaccine, three plasmids encoding mutant C180 (C180-R9K, C180-E129G and C180-R9K/E129G) were constructed and tested for their protective immunogenicity and cytotoxicity. All of the gene gun delivery of the plasmid, performed by inserting the mutant C180 gene into a mammalian expression vector pcDNA3.1, successfully induced anti-PT IgG antibody production without the loss of immunogenicity in mice. The immunizations of mice with the plasmids significantly inhibited leukocytosis-promoting activity by PT. Among stably transfected Chinese hamster ovary (CHO) cells expressing mutant C180, the expression of C180-R9K and C180-R9K/E129G was non-toxic to the transfectants, confirming that these mutant C180s have no cytotoxicity to mammalian cells. These results indicate that C180-R9K and C180-R9K/E129G genes, especially C180-R9K/E129G, are candidates for safe and effective antigen DNAs in the development of pertussis DNA vaccine.

Effect of relevant culture parameters on Pertussis Toxin expression by Bordetella pertussis

Whooping cough vaccines are produced using different ranges of cultivation conditions and medium compositions, which are known to influence growth rate, virulence factor production and degradation, as well as the virulence factors' association to the cell. This study quantifies the impact of individual parameters on Pertussis Toxin (PT) production, using an optimized chemically defined medium as starting point, rather than a complex medium. A number of chemicals that are identified affect both growth rate and virulence factor production, which occur at similar levels in various commonly used production media. Also, degradation by proteolytic activity is shown to be an important parameter to monitor, since it significantly affects the PT yield. Low sodium concentrations, i.e. 50-75 mM rather than the conventional 100-140 mM, significantly increase the growth rate of the organism, the final optical density, as well as the association of PT to the cells. The absolute amount of biomass produced measured as dry weight, is similar for all sodium concentrations tested, contrary to earlier work. While it is known that high iron concentrations inhibit virulence factor production, it is shown here that iron-limited growth results in very high specific PT production. This finding may be used to produce a whole-cell vaccine with little biomass per dose, reducing whole-cell vaccine toxicity. The Bordetella pertussis strain 509 used here produces 30% more PT at 34 than at 37 degrees C, a commonly used cultivation temperature. The data in this study show that existing production processes for cellular and acellular vaccines can in principle be optimised considerably by taking simple measures.

Expression and purification of a trivalent pertussis toxin-diphtheria toxin-tetanus toxin fusion protein in Escherichia coli

Pertussis toxoid, diphtheria toxoid, and tetanus toxoid are key components of diphtheria-tetanus-acellular pertussis vaccines. The efficacy of the vaccines is well documented, however, the vaccines are expensive partly because the antigens are derived from three different bacteria. In this study, a fusion protein (PDT) composed of the immunoprotective S1 fragment of pertussis toxin, the full-length non-toxic diphtheria toxin, and fragment C of tetanus toxin was constructed via genetic means. The correct fusion was verified by restriction endonuclease analysis and Western immunoblotting. Escherichia coli carrying the recombinant plasmid (pCoPDT) produced a 161kDa protein that was recognized by antibodies specific to the three toxins. The expression of the PDT protein was inducible by isopropyl-beta-d-thio-galactoside but the total amount of protein produced was relatively low. Attempts to improve the protein yield by expression in an E. coli strain (Rosetta-gami 2) that could alleviate rare-codon usage bias and by supplementation of the growth media with amino acids deemed to be a limiting factor in translation were not successful. The PDT protein remained in the insoluble fraction when the recombinant E. coli was grown at 37 degrees C but the protein became soluble when the bacteria were grown at 22 degrees C. The PDT protein was isolated via affinity chromatography on a NiCAM column. The protein was associated with five other proteins via disulfide bonds and non-covalent interactions. Following treatment with beta-mercaptoethanol, the PDT fusion was purified to homogeneity by preparative polyacrylamide gel electrophoresis with a yield of 45 microg/L of culture. Antisera generated against the purified PDT protein recognized the native toxins indicating that some, if not all, of the native epitopes were conserved.

Expression of a C terminally truncated form of pertussis toxin S1 subunit effectively induces protection against pertussis toxin following DNA-based immunization

Four plasmids encoding different C terminally and N terminally truncated pertussis toxin S1 subunits of Bordetella pertussis were constructed and tested for inducibility of protection against pertussis toxin in mice after DNA-based immunization. The region encoding an N-terminal 180-amino-acid fragment of the S1 subunit had the most potent ability to induce protective immunity.

Analysis of relative levels of production of pertussis toxin subunits and Ptl proteins in Bordetella pertussis

Pertussis toxin is transported across the outer membrane of Bordetella pertussis by the type IV secretion system known as the Ptl transporter, which is composed of nine different proteins. In order to determine the relative levels of production of pertussis toxin subunits and Ptl proteins in B. pertussis, we constructed translational fusions of the gene for alkaline phosphatase, phoA, with various ptx and ptl genes. Comparison of the alkaline phosphatase activity of strains containing ptx'- or ptl'-phoA fusions indicated that pertussis toxin subunits are produced at higher levels than Ptl proteins, which are encoded by genes located toward the 3' end of the ptx-ptl operon. We also engineered strains of B. pertussis by introducing multiple copies of the ptl genes or subsets of these genes and then examined the ability of each of these strains to secrete pertussis toxin. From these studies, we determined that certain Ptl proteins appear to be limiting in the secretion of pertussis toxin from the bacteria. These results represent an important first step in assessing the stoichiometric relationship of pertussis toxin and its transporter within the bacterial cell.