Genomic analysis of the adenylate cyclase-hemolysin C-terminal region of Bordetella pertussis, Bordetella parapertussis and Bordetella bronchiseptica

The Eukaryotic Host Factor 14-3-3 Inactivates Adenylate Cyclase Toxins of Bordetella bronchiseptica and B. parapertussis, but Not B. pertussis

Bordetella pertussis, Bordetella bronchiseptica, and Bordetella parapertussis share highly homologous virulence factors and commonly cause respiratory infections in mammals; however, their host specificities and disease severities differ, and the reasons for this remain largely unknown. Adenylate cyclase toxin (CyaA) is a homologous virulence factor that is thought to play crucial roles in Bordetella infections. We herein demonstrate that CyaAs function as virulence factors differently between B. bronchiseptica/B. parapertussis and B. pertussis. B. bronchiseptica CyaA bound to target cells, and its enzyme domain was translocated into the cytosol similarly to B. pertussis CyaA. The hemolytic activity of B. bronchiseptica CyaA on sheep erythrocytes was also preserved. However, in nucleated target cells, B. bronchiseptica CyaA was phosphorylated at Ser³⁷⁵, which constitutes a motif (RSXpSXP [pS is phosphoserine]) recognized by the host factor 14-3-3, resulting in the abrogation of adenylate cyclase activity. Consequently, the cytotoxic effects of B. bronchiseptica CyaA based on its enzyme activity were markedly attenuated. B. parapertussis CyaA carries the 14-3-3 motif, indicating that its intracellular enzyme activity is abrogated similarly to B. bronchiseptica CyaA; however, B. pertussis CyaA has Phe³⁷⁵ instead of Ser, and thus, was not affected by 14-3-3. In addition, B. pertussis CyaA impaired the barrier function of epithelial cells, whereas B. bronchiseptica CyaA did not. Rat infection experiments suggested that functional differences in CyaA are related to differences in pathogenicity between B. bronchiseptica/B. parapertussis and B. pertussis.

Bordetella pertussis, B. bronchiseptica, and B. parapertussis are bacterial respiratory pathogens that are genetically close to each other and produce many homologous virulence factors; however, their host specificities and disease severities differ, and the reasons for this remain unknown. Previous studies attempted to explain these differences by the distinct virulence factors produced by each Bordetella species. In contrast, we indicated functional differences in adenylate cyclase toxin, a homologous virulence factor of Bordetella. The toxins of B. bronchiseptica and presumably B. parapertussis were inactivated by the host factor 14-3-3 after phosphorylation in target cells, whereas the B. pertussis toxin was not inactivated because of the lack of the phosphorylation site. This is the first study to show that 14-3-3 inactivates the virulence factors of pathogens. The present results suggest that pathogenic differences in Bordetella are attributed to the different activities of adenylate cyclase toxins.

Characterization of Post-Translational Modifications and Cytotoxic Properties of the Adenylate-Cyclase Hemolysin Produced by Various Bordetella pertussis and Bordetella parapertussis Isolates

Bordetella pertussis and Bordetella parapertussis are the causal agents of whooping cough in humans. They produce diverse virulence factors, including adenylate cyclase-hemolysin (AC-Hly), a secreted toxin of the repeat in toxins (RTX) family with cyclase, pore-forming, and hemolytic activities. Post-translational modifications (PTMs) are essential for the biological activities of the toxin produced by B. pertussis. In this study, we compared AC-Hly toxins from various clinical isolates of B. pertussis and B. parapertussis, focusing on (i) the genomic sequences of cyaA genes, (ii) the PTMs of partially purified AC-Hly, and (iii) the cytotoxic activity of the various AC-Hly toxins. The genes encoding the AC-Hly toxins of B. pertussis and B. parapertussis displayed very limited polymorphism in each species. Most of the sequence differences between the two species were found in the C-terminal part of the protein. Both toxins harbored PTMs, mostly corresponding to palmitoylations of the lysine 860 residue and palmoylations and myristoylations of lysine 983 for B. pertussis and AC-Hly and palmitoylations of lysine 894 and myristoylations of lysine 1017 for B. parapertussis AC-Hly. Purified AC-Hly from B. pertussis was cytotoxic to macrophages, whereas that from B. parapertussis was not.

Characterization of Post-Translational Modifications and Cytotoxic Properties of the Adenylate-Cyclase Hemolysin Produced by Various Bordetella pertussis and Bordetella parapertussis Isolates

Bordetella pertussis and Bordetella parapertussis are the causal agents of whooping cough in humans. They produce diverse virulence factors, including adenylate cyclase-hemolysin (AC-Hly), a secreted toxin of the repeat in toxins (RTX) family with cyclase, pore-forming, and hemolytic activities. Post-translational modifications (PTMs) are essential for the biological activities of the toxin produced by B. pertussis. In this study, we compared AC-Hly toxins from various clinical isolates of B. pertussis and B. parapertussis, focusing on (i) the genomic sequences of cyaA genes, (ii) the PTMs of partially purified AC-Hly, and (iii) the cytotoxic activity of the various AC-Hly toxins. The genes encoding the AC-Hly toxins of B. pertussis and B. parapertussis displayed very limited polymorphism in each species. Most of the sequence differences between the two species were found in the C-terminal part of the protein. Both toxins harbored PTMs, mostly corresponding to palmitoylations of the lysine 860 residue and palmoylations and myristoylations of lysine 983 for B. pertussis and AC-Hly and palmitoylations of lysine 894 and myristoylations of lysine 1017 for B. parapertussis AC-Hly. Purified AC-Hly from B. pertussis was cytotoxic to macrophages, whereas that from B. parapertussis was not.

Structure-Function Relationships Underlying the Capacity of Bordetella Adenylate Cyclase Toxin to Disarm Host Phagocytes

Bordetellae, pathogenic to mammals, produce an immunomodulatory adenylate cyclase toxin-hemolysin (CyaA, ACT or AC-Hly) that enables them to overcome the innate immune defense of the host. CyaA subverts host phagocytic cells by an orchestrated action of its functional domains, where an extremely catalytically active adenylyl cyclase enzyme is delivered into phagocyte cytosol by a pore-forming repeat-in-toxin (RTX) cytolysin moiety. By targeting sentinel cells expressing the complement receptor 3, known as the CD11b/CD18 (α_Mβ₂) integrin, CyaA compromises the bactericidal functions of host phagocytes and supports infection of host airways by Bordetellae. Here, we review the state of knowledge on structural and functional aspects of CyaA toxin action, placing particular emphasis on signaling mechanisms by which the toxin-produced 3',5'-cyclic adenosine monophosphate (cAMP) subverts the physiology of phagocytic cells.

Bordetella Adenylate Cyclase-Hemolysin Toxins

Adenylate cyclase-hemolysin toxin is secreted and produced by three classical species of the genus Bordetella: Bordetella pertussis, B. parapertussis and B. bronchiseptica. This toxin has several properties such as: (i) adenylate cyclase activity, enhanced after interaction with the eukaryotic protein, calmodulin; (ii) a pore-forming activity; (iii) an invasive activity. It plays an important role in the pathogenesis of these Bordetella species responsible for whooping cough in humans or persistent respiratory infections in mammals, by modulating host immune responses. In contrast with other Bordetella toxins or adhesins, lack of (or very low polymorphism) is observed in the structural gene encoding this toxin, supporting its importance as well as a potential role as a vaccine antigen against whooping cough. In this article, an overview of the investigations undertaken on this toxin is presented.

New Data on Vaccine Antigen Deficient Bordetella pertussis Isolates

Evolution of Bordetella pertussis is driven by natural and vaccine pressures. Isolates circulating in regions with high vaccination coverage present multiple allelic and antigenic variations as compared to isolates collected before introduction of vaccination. Furthermore, during the last epidemics reported in regions using pertussis acellular vaccines, isolates deficient for vaccine antigens, such as pertactin (PRN), were reported to reach high proportions of circulating isolates. More sporadic filamentous hemagglutinin (FHA) or pertussis toxin (PT) deficient isolates were also collected. The whole genome of some recent French isolates, deficient or non-deficient in vaccine antigens, were analyzed. Transcription profiles of the expression of the main virulence factors were also compared. The invasive phenotype in an in vitro human tracheal epithelial (HTE) cell model of infection was evaluated. Our genomic analysis focused on SNPs related to virulence genes known to be more likely to present allelic polymorphism. Transcriptomic data indicated that isolates circulating since the introduction of pertussis vaccines present lower transcription levels of the main virulence genes than the isolates of the pre-vaccine era. Furthermore, isolates not producing FHA present significantly higher expression levels of the entire set of genes tested. Finally, we observed that recent isolates are more invasive in HTE cells when compared to the reference strain, but no multiplication occurs within cells.

The Bordetella adenylate cyclase repeat-in-toxin (RTX) domain is immunodominant and elicits neutralizing antibodies

The adenylate cyclase toxin (ACT) is a multifunctional virulence factor secreted by Bordetella species. Upon interaction of its C-terminal hemolysin moiety with the cell surface receptor αMβ2 integrin, the N-terminal cyclase domain translocates into the host cell cytosol where it rapidly generates supraphysiological cAMP concentrations, which inhibit host cell anti-bacterial activities. Although ACT has been shown to induce protective immunity in mice, it is not included in any current acellular pertussis vaccines due to protein stability issues and a poor understanding of its role as a protective antigen. Here, we aimed to determine whether any single domain could recapitulate the antibody responses induced by the holo-toxin and to characterize the dominant neutralizing antibody response. We first immunized mice with ACT and screened antibody phage display libraries for binding to purified ACT. The vast majority of unique antibodies identified bound the C-terminal repeat-in-toxin (RTX) domain. Representative antibodies binding two nonoverlapping, neutralizing epitopes in the RTX domain prevented ACT association with J774A.1 macrophages and soluble αMβ2 integrin, suggesting that these antibodies inhibit the ACT-receptor interaction. Sera from mice immunized with the RTX domain showed similar neutralizing activity as ACT-immunized mice, indicating that this domain induced an antibody response similar to that induced by ACT. These data demonstrate that RTX can elicit neutralizing antibodies and suggest it may present an alternative to ACT.

Heterogeneity of Bordetella bronchiseptica adenylate cyclase (cyaA) RTX domain

Bordetella bronchiseptica is a widespread pathogen, with a broad host range, occasionally including humans. Diverse virulence factors (adhesins, toxins) allow its adaptation to its host, but this property of the adenylate cyclase (cyaA) toxin is not well understood. In this study, we analyzed the repeats-in-toxin domain of B. bronchiseptica cyaA with PCR, followed by restriction fragment length analysis. Of ninety-two B. bronchiseptica strains collected from different hosts and geographic regions, 72 (78.3 %) carried cyaA and four RFLP types (A-D) were established using NarI and SalI. However, in 20 strains, cyaA was replaced with a peptide transport protein operon. A phylogenetic tree based on partial nucleotide sequences of cyaA revealed that group 2 contains strains of specifically human origin, whereas subgroup 1a contains all but one of the strains from pigs. The human strains showed many PCR-RFLP and sequence variants, confirming the clonal population structure of B. bronchiseptica.

Antibody-mediated inhibition of Bordetella pertussis adenylate cyclase-haemolysin-induced macrophage cytotoxicity is influenced by variations in the bacterial population

Whooping cough is a vaccine-preventable disease presenting with epidemic cycles linked to natural and/or vaccine-driven evolution of the aetiological agent of the disease, Bordetella pertussis. Adenylate cyclase-haemolysin (AC-Hly) is a major toxin produced by this pathogen, which mediates macrophage apoptosis in vitro and in vivo. While current acellular pertussis vaccine (APV) formulations do not include AC-Hly, they all contain pertussis toxin and can comprise filamentous haemagglutinin (FHA), which interacts with AC-Hly, and pertactin (PRN), which has been hypothesized also to interact with AC-Hly. We aimed to study the capacity of specific antibodies to inhibit the in vitro B. pertussis AC-Hly-mediated cytotoxicity of J774A.1 murine macrophages in a background of a changing bacterial population. We demonstrate that: (i) clinical isolates of different types or PRN phenotype are all cytotoxic and lethal in the mouse model of respiratory infection; (ii) lack of PRN production does not impact AC-Hly-related phenotypes; (iii) anti-AC-Hly antibodies inhibit cell lysis whatever the phenotype of the isolate, while anti-PRN antibodies significantly inhibit cell lysis provided the isolate produces this antigen, which might be relevant in vivo for APV-induced immunity; and (iv) anti-FHA antibodies only inhibit lysis induced by isolates collected in 2012, maybe indicating specific characteristics of epidemic lineages of B. pertussis.

Epidemiology of whooping cough & typing of Bordetella pertussis

Bordetella pertussis is a Gram-negative human-restricted bacterium that evolved from the broad-range mammalian pathogen, Bordetella bronchiseptica. It causes whooping cough or pertussis in humans, which is the most prevalent vaccine-preventable disease worldwide. The introduction of the pertussis whole-cell vaccination for young children, followed by the introduction of the pertussis acellular vaccination (along with booster vaccination) for older age groups, has affected the bacterial population and epidemiology of the disease. B. pertussis is relatively monomorphic worldwide, but nevertheless, different countries are facing different epidemiological evolutions of the disease. Although it is tempting to link vaccine-driven phenotypic and genotypic evolution of the bacterium to epidemiology, many other factors should be considered and surveillance needs to continue, in addition to studies investigating the impact of current clinical isolates on vaccine efficacy.

Comparison of lipopolysaccharide structures of Bordetella pertussis clinical isolates from pre- and post-vaccine era

Endotoxins are lipopolysaccharides (LPS), and major constituents of the outer membrane of Gram-negative bacteria. Bordetella pertussis LPS were the only major antigens, of this agent of whooping-cough, that were not yet analyzed on isolates from the pre- and post-vaccination era. We compared here the LPS structures of four clinical isolates with that of the vaccine strain BP 1414. All physico-chemical analyses, including SDS-PAGE, TLC, and different MALDI mass spectrometry approaches were convergent. They helped demonstrating that, on the contrary to some other B. pertussis major antigens, no modification occurred in the dodecasaccharide core structure, as well as in the whole LPS molecules. These results are rendering these major antigens good potential vaccine components. Molecular modeling of this conserved LPS structure also confirmed the conclusions of previous experiments leading to the production of anti-LPS monoclonal antibodies and defining the main epitopes of these major antigens.

Monitoring of Bordetella isolates circulating in Saint Petersburg, Russia between 2001 and 2009

Bordetella isolates in the Saint Petersburg region have been monitored since 1998. Over the past ten years, concomitant with the increase in pertussis whole-cell vaccine coverage, the incidence of whooping cough has decreased. However, this decrease exists only for Bordetella pertussis infections, as the incidence of Bordetella parapertussis confirmed cases has remained stable, suggesting that pertussis-vaccine-induced immunity is not protective against parapertussis, as expected. B. pertussis and B. parapertussis clinical isolates were analyzed using serotyping, immunoblotting, pulsed-field gel electrophoresis of chromosomal DNA (after digestion with XbaI) and sequencing of virulence genes. The bacterial population is now similar to that observed in other European countries.