Role of adhesins and toxins in invasion of human tracheal epithelial cells by Bordetella pertussis

Eosinophils as drivers of bacterial immunomodulation and persistence

Traditionally, eosinophils have been linked to parasitic infections and pathological disease states. However, emerging literature has unveiled a more nuanced and intricate role for these cells, demonstrating their key functions in maintaining mucosal homeostasis. Eosinophils exhibit diverse phenotypes and exert multifaceted effects during infections, ranging from promoting pathogen persistence to triggering allergic reactions. Our investigations primarily focus on Bordetella spp., with particular emphasis on Bordetella bronchiseptica, a natural murine pathogen that induces diseases in mice akin to pertussis in humans. Recent findings from our published work have unveiled a striking interaction between B. bronchiseptica and eosinophils, facilitated by the btrS-mediated mechanism. This interaction serves to enhance pathogen persistence while concurrently delaying adaptive immune responses. Notably, this role of eosinophils is only noted in the absence of a functional btrS signaling pathway, indicating that wild-type B. bronchiseptica, and possibly other Bordetella spp., possess such adeptness in manipulating eosinophils that the true function of these cells remains obscured during infection. In this review, we present the mounting evidence pointing toward eosinophils as targets of bacterial exploitation, facilitating pathogen persistence and fostering chronic infections in diverse mucosal sites, including the lungs, gut, and skin. We underscore the pivotal role of the master regulator of Bordetella pathogenesis, the sigma factor BtrS, in orchestrating eosinophil-dependent immunomodulation within the context of pulmonary infection. These putative convergent strategies of targeting eosinophils offer promising avenues for the development of novel therapeutics targeting respiratory and other mucosal pathogens.

Pasteurian Contributions to the Study of Bordetella pertussis Toxins

As a tribute to Louis Pasteur on the occasion of the 200th anniversary of his birth, this article summarizes the main contributions of scientists from Pasteur Institutes to the current knowledge of toxins produced by Bordetella pertussis. The article therefore focuses on publications authored by researchers from Pasteur Institutes and is not intended as a systematic review of B. pertussis toxins. Besides identifying B. pertussis as the causative agent of whooping cough, Pasteurians have made several major contributions with respect to the structure–function relationship of the Bordetella lipo-oligosaccharide, adenylyl cyclase toxin and pertussis toxin. In addition to contributing to the understanding of these toxins’ mechanisms at the molecular and cellular levels and their role in pathogenesis, scientists at Pasteur Institutes have also exploited potential applications of the gathered knowledge of these toxins. These applications range from the development of novel tools to study protein–protein interactions over the design of novel antigen delivery tools, such as prophylactic or therapeutic vaccine candidates against cancer and viral infection, to the development of a live attenuated nasal pertussis vaccine. This scientific journey from basic science to applications in the field of human health matches perfectly with the overall scientific objectives outlined by Louis Pasteur himself.

Bordetella pertussis targets the basolateral membrane of polarized respiratory epithelial cells, gets internalized, and survives in intracellular locations

The airway epithelial barrier is a continuous highly organized cell layer that separates the exterior from the underlying mucosal tissue, preventing pathogen invasion. Several respiratory pathogens have evolved mechanisms to compromise this barrier, invade and even reside alive within the epithelium. Bordetella pertussis is a persistent pathogen that infects the human airway epithelium, causing whooping cough. Previous studies have shown that B. pertussis survives inside phagocytic and nonphagocytic cells, suggesting that there might be an intracellular stage involved in the bacterial infectious process and/or in the pathogen persistence inside the host. In this study we found evidence that B. pertussis is able to survive inside respiratory epithelial cells. According to our results, this pathogen preferentially attaches near or on top of the tight junctions in polarized human bronchial epithelial cells and disrupts these structures in an adenylate cyclase-dependent manner, exposing their basolateral membrane. We further found that the bacterial internalization is significantly higher in cells exposing this membrane compared with cells only exposing the apical membrane. Once internalized, B. pertussis mainly remains in nondegradative phagosomes with access to nutrients. Taken together, these results point at the respiratory epithelial cells as a potential niche of persistence.

Filamentous Hemagglutinin of Bordetella pertussis Does Not Interact with the β2 Integrin CD11b/CD18

The pertussis agent Bordetella pertussis produces a number of virulence factors, of which the filamentous hemagglutinin (FhaB) plays a role in B. pertussis adhesion to epithelial and phagocytic cells. Moreover, FhaB was recently found to play a crucial role in nasal cavity infection and B. pertussis transmission to new hosts. The 367 kDa FhaB protein translocates through an FhaC pore to the outer bacterial surface and is eventually processed to a ~220 kDa N-terminal FHA fragment by the SphB1 protease. A fraction of the mature FHA then remains associated with bacterial cell surface, while most of FHA is shed into the bacterial environment. Previously reported indirect evidence suggested that FHA, or its precursor FhaB, may bind the β2 integrin CD11b/CD18 of human macrophages. Therefore, we assessed FHA binding to various cells producing or lacking the integrin and show that purified mature FHA does not bind CD11b/CD18. Further results then revealed that the adhesion of B. pertussis to cells does not involve an interaction between the bacterial surface-associated FhaB and/or mature FHA and the β2 integrin CD11b/CD18. In contrast, FHA binding was strongly inhibited at micromolar concentrations of heparin, corroborating that the cell binding of FHA is ruled by the interaction of its heparin-binding domain with sulfated glycosaminoglycans on the cell surface.

Susceptibility of Human Airway Tissue Models Derived From Different Anatomical Sites to Bordetella pertussis and Its Virulence Factor Adenylate Cyclase Toxin

To study the interaction of human pathogens with their host target structures, human tissue models based on primary cells are considered suitable. Complex tissue models of the human airways have been used as infection models for various viral and bacterial pathogens. The Gram-negative bacterium Bordetella pertussis is of relevant clinical interest since whooping cough has developed into a resurgent infectious disease. In the present study, we created three-dimensional tissue models of the human ciliated nasal and tracheo-bronchial mucosa. We compared the innate immune response of these models towards the B. pertussis virulence factor adenylate cyclase toxin (CyaA) and its enzymatically inactive but fully pore-forming toxoid CyaA-AC^-. Applying molecular biological, histological, and microbiological assays, we found that 1 µg/ml CyaA elevated the intracellular cAMP level but did not disturb the epithelial barrier integrity of nasal and tracheo-bronchial airway mucosa tissue models. Interestingly, CyaA significantly increased interleukin 6, interleukin 8, and human beta defensin 2 secretion in nasal tissue models, whereas tracheo-bronchial tissue models were not significantly affected compared to the controls. Subsequently, we investigated the interaction of B. pertussis with both differentiated primary nasal and tracheo-bronchial tissue models and demonstrated bacterial adherence and invasion without observing host cell type-specific significant differences. Even though the nasal and the tracheo-bronchial mucosa appear similar from a histological perspective, they are differentially susceptible to B. pertussis CyaA in vitro. Our finding that nasal tissue models showed an increased innate immune response towards the B. pertussis virulence factor CyaA compared to tracheo-bronchial tissue models may reflect the key role of the nasal airway mucosa as the first line of defense against airborne pathogens.

Selective Enhancement of the Cell-Permeabilizing Activity of Adenylate Cyclase Toxin Does Not Increase Virulence of Bordetella pertussis

The whooping cough agent, Bordetella pertussis, secretes an adenylate cyclase toxin–hemolysin (CyaA, ACT, or AC-Hly) that catalyzes the conversion of intracellular ATP to cAMP and through its signaling annihilates the bactericidal activities of host sentinel phagocytes. In parallel, CyaA permeabilizes host cells by the formation of cation-selective membrane pores that account for the hemolytic activity of CyaA. The pore-forming activity contributes to the overall cytotoxic effect of CyaA in vitro, and it has previously been proposed to synergize with the cAMP-elevating activity in conferring full virulence on B. pertussis in the mouse model of pneumonic infection. CyaA primarily targets myeloid phagocytes through binding of their complement receptor 3 (CR3, integrin α_Mβ₂, or CD11b/CD18). However, with a reduced efficacy, the toxin can promiscuously penetrate and permeabilize the cell membrane of a variety of non-myeloid cells that lack CR3 on the cell surface, including airway epithelial cells or erythrocytes, and detectably intoxicates them by cAMP. Here, we used CyaA variants with strongly and selectively enhanced or reduced pore-forming activity that, at the same time, exhibited a full capacity to elevate cAMP concentrations in both CR3-expressing and CR3-non-expressing target cells. Using B. pertussis mutants secreting such CyaA variants, we show that a selective enhancement of the cell-permeabilizing activity of CyaA does not increase the overall virulence and lethality of pneumonic B. pertussis infection of mice any further. In turn, a reduction of the cell-permeabilizing activity of CyaA did not reduce B. pertussis virulence any importantly. These results suggest that the phagocyte-paralyzing cAMP-elevating capacity of CyaA prevails over the cell-permeabilizing activity of CyaA that appears to play an auxiliary role in the biological activity of the CyaA toxin in the course of B. pertussis infections in vivo.

Naturally circulating pertactin-deficient Bordetella pertussis strains induce distinct gene expression and inflammatory signatures in human dendritic cells

Respiratory infections caused by Bordetella pertussis are reemerging despite high pertussis vaccination coverage. Since the introduction of the acellular pertussis vaccine in the late twentieth century, circulating B. pertussis strains increasingly lack expression of the vaccine component pertactin (Prn). In some countries, up to 90% of the circulating B. pertussis strains are deficient in Prn. To better understand the resurgence of pertussis, we investigated the response of human monocyte-derived dendritic cells (moDCs) to naturally circulating Prn-expressing (Prn-Pos) and Prn-deficient (Prn-Neg) B. pertussis strains from 2016 in the Netherlands. Transcriptome analysis of moDC showed enriched IFNα response-associated gene expression after exposure to Prn-Pos B. pertussis strains, whereas the Prn-Neg strains induced enriched expression of interleukin- and TNF-signaling genes, as well as other genes involved in immune activation. Multiplex immune assays confirmed enhanced proinflammatory cytokine secretion by Prn-Neg stimulated moDC. Comparison of the proteomes from the Prn-Pos and Prn-Neg strains revealed, next to the difference in Prn, differential expression of a number of other proteins including several proteins involved in metabolic processes. Our findings indicate that Prn-deficient B. pertussis strains induce a distinct and stronger immune activation of moDCs than the Prn-Pos strains. These findings highlight the role of pathogen adaptation in the resurgence of pertussis as well as the effects that vaccine pressure can have on a bacterial population.

A Mutation Upstream of the rplN-rpsD Ribosomal Operon Downregulates Bordetella pertussis Virulence Factor Production without Compromising Bacterial Survival within Human Macrophages

The BvgS/BvgA two-component system controls expression of ∼550 genes of Bordetella pertussis, of which, ∼245 virulence-related genes are positively regulated by the BvgS-phosphorylated transcriptional regulator protein BvgA (BvgA∼P). We found that a single G-to-T nucleotide transversion in the 5'-untranslated region (5'-UTR) of the rplN gene enhanced transcription of the ribosomal protein operon and of the rpoA gene and provoked global dysregulation of B. pertussis genome expression. This comprised overproduction of the alpha subunit (RpoA) of the DNA-dependent RNA polymerase, downregulated BvgA and BvgS protein production, and impaired production and secretion of virulence factors by the mutant. Nonetheless, the mutant survived like the parental bacteria for >2 weeks inside infected primary human macrophages and persisted within infected mouse lungs for a longer period than wild-type B. pertussis These observations suggest that downregulation of virulence factor production by bacteria internalized into host cells may enable persistence of the whooping cough agent in the airways.IMPORTANCE We show that a spontaneous mutation that upregulates transcription of an operon encoding ribosomal proteins and causes overproduction of the downstream-encoded α subunit (RpoA) of RNA polymerase causes global effects on gene expression levels and proteome composition of Bordetella pertussis Nevertheless, the resulting important downregulation of the BvgAS-controlled expression of virulence factors of the whooping cough agent did not compromise its capacity to persist for prolonged periods inside primary human macrophage cells, and it even enhanced its capacity to persist in infected mouse lungs. These observations suggest that the modulation of BvgAS-controlled expression of virulence factors may occur also during natural infections of human airways by Bordetella pertussis and may possibly account for long-term persistence of the pathogen within infected cells of the airways.

Bordetella pertussis induces IFN-γ production by NK cells resulting in chemo-attraction by respiratory epithelial cells

BACKGROUND

Whooping cough is caused by infection of the airways with Bordetella pertussis (Bp). As IFN-γ is essential for protective immunity against Bp we investigated how IFN-γ is induced by Bp or the virulence antigens FHA, Prn or PT, and how IFN-γ contributes to local immune responses in humans.

METHODS

PBMCs from healthy donors and/or respiratory epithelial cells were stimulated with soluble antigens or inactivated intact Bp and the presence or absence of blocking antibodies or chemokines. Supernatants and cells were analyzed for IFN-γ and chemokine production and lymphocyte migration tested using epithelial supernatants.

RESULTS

The soluble antigens failed to induce IFN-γ production, whereas inactivated Bp induced IFN-γ production. NK cells were the main source of IFN-γ production, which was enhanced by IL-15. Epithelial-PBMC co-cultures showed robust IFN-γ-dependent CXCL9 and CXCL10 production by the epithelial cells following stimulation with IFN-γ and Bp. The epithelial-derived chemokines resulted in CXCR3-dependent recruitment of NK and T cells.

CONCLUSIONS

Inactivated Bp, but not antigens, induced potent IFN-γ production by NK cells, resulting in chemo-attraction of lymphocytes towards the respiratory epithelium. These data provide insight into the requirements for IFN-γ production and how IFN-γ enhances local immune responses to prevent Bp-mediated disease.

Role of Major Toxin Virulence Factors in Pertussis Infection and Disease Pathogenesis

Bordetella pertussis produces several toxins that affect host-pathogen interactions. Of these, the major toxins that contribute to pertussis infection and disease are pertussis toxin, adenylate cyclase toxin-hemolysin and tracheal cytotoxin. Pertussis toxin is a multi-subunit protein toxin that inhibits host G protein-coupled receptor signaling, causing a wide array of effects on the host. Adenylate cyclase toxin-hemolysin is a single polypeptide, containing an adenylate cyclase enzymatic domain coupled to a hemolysin domain, that primarily targets phagocytic cells to inhibit their antibacterial activities. Tracheal cytotoxin is a fragment of peptidoglycan released by B. pertussis that elicits damaging inflammatory responses in host cells. This chapter describes these three virulence factors of B. pertussis, summarizing background information and focusing on the role of each toxin in infection and disease pathogenesis, as well as their role in pertussis vaccination.

A Bordetella pertussis MgtC homolog plays a role in the intracellular survival

Bordetella pertussis, the causative agent of whooping cough, has the capability to survive inside the host cells. This process requires efficient adaptation of the pathogen to the intracellular environment and the associated stress. Among the proteins produced by the intracellular B. pertussis we identified a protein (BP0414) that shares homology with MgtC, a protein which was previously shown to be involved in the intracellular survival of other pathogens. To explore if BP0414 plays a role in B. pertussis intracellular survival a mutant strain defective in the production of this protein was constructed. Using standard in vitro growth conditions we found that BP0414 is required for B. pertussis growth under low magnesium availability or low pH, two environmental conditions that this pathogen might face within the host cell. Intracellular survival studies showed that MgtC is indeed involved in B. pertussis viability inside the macrophages. The use of bafilomycin A1, which inhibits phagosome acidification, abolished the survival defect of the mgtC deficient mutant strain suggesting that in intracellular B. pertussis the role of MgtC protein is mainly related to the bacterial adaptation to the acidic conditions found inside the of phagosomes. Overall, this work provides an insight into the importance of MgtC in B. pertussis pathogenesis and its contribution to bacterial survival within immune cells.

Bordetella pertussis Adenylate Cyclase Toxin Disrupts Functional Integrity of Bronchial Epithelial Layers

The airway epithelium restricts the penetration of inhaled pathogens into the underlying tissue and plays a crucial role in the innate immune defense against respiratory infections. The whooping cough agent, Bordetella pertussis, adheres to ciliated cells of the human airway epithelium and subverts its defense functions through the action of secreted toxins and other virulence factors. We examined the impact of B. pertussis infection and of adenylate cyclase toxin-hemolysin (CyaA) action on the functional integrity of human bronchial epithelial cells cultured at the air-liquid interface (ALI). B. pertussis adhesion to the apical surface of polarized pseudostratified VA10 cell layers provoked a disruption of tight junctions and caused a drop in transepithelial electrical resistance (TEER). The reduction of TEER depended on the capacity of the secreted CyaA toxin to elicit cAMP signaling in epithelial cells through its adenylyl cyclase enzyme activity. Both purified CyaA and cAMP-signaling drugs triggered a decrease in the TEER of VA10 cell layers. Toxin-produced cAMP signaling caused actin cytoskeleton rearrangement and induced mucin 5AC production and interleukin-6 (IL-6) secretion, while it inhibited the IL-17A-induced secretion of the IL-8 chemokine and of the antimicrobial peptide beta-defensin 2. These results indicate that CyaA toxin activity compromises the barrier and innate immune functions of Bordetella-infected airway epithelia.

Hyperbiofilm Formation by Bordetella pertussis Strains Correlates with Enhanced Virulence Traits

Pertussis, or whooping cough, caused by the obligate human pathogen Bordetella pertussis is undergoing a worldwide resurgence. The majority of studies of this pathogen are conducted with laboratory-adapted strains which may not be representative of the species as a whole. Biofilm formation by B. pertussis plays an important role in pathogenesis. We conducted a side-by-side comparison of the biofilm-forming abilities of the prototype laboratory strains and the currently circulating isolates from two countries with different vaccination programs. Compared to the reference strain, all strains examined herein formed biofilms at high levels. Biofilm structural analyses revealed country-specific differences, with strains from the United States forming more structured biofilms. Bacterial hyperaggregation and reciprocal expression of biofilm-promoting and -inhibitory factors were observed in clinical isolates. An association of increased biofilm formation with augmented epithelial cell adhesion and higher levels of bacterial colonization in the mouse nose and trachea was detected. To our knowledge, this work links for the first time increased biofilm formation in bacteria with a colonization advantage in an animal model. We propose that the enhanced biofilm-forming capacity of currently circulating strains contributes to their persistence, transmission, and continued circulation.

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.

Cyclic AMP-Elevating Capacity of Adenylate Cyclase Toxin-Hemolysin Is Sufficient for Lung Infection but Not for Full Virulence of Bordetella pertussis

The adenylate cyclase toxin-hemolysin (CyaA, ACT, or AC-Hly) of Bordetella pertussis targets phagocytic cells expressing the complement receptor 3 (CR3, Mac-1, α_Mβ₂ integrin, or CD11b/CD18). CyaA delivers into cells an N-terminal adenylyl cyclase (AC) enzyme domain that is activated by cytosolic calmodulin and catalyzes unregulated conversion of cellular ATP into cyclic AMP (cAMP), a key second messenger subverting bactericidal activities of phagocytes. In parallel, the hemolysin (Hly) moiety of CyaA forms cation-selective hemolytic pores that permeabilize target cell membranes. We constructed the first B. pertussis mutant secreting a CyaA toxin having an intact capacity to deliver the AC enzyme into CD11b-expressing (CD11b⁺) host phagocytes but impaired in formation of cell-permeabilizing pores and defective in cAMP elevation in CD11b^- cells. The nonhemolytic AC⁺ Hly^- bacteria inhibited the antigen-presenting capacities of coincubated mouse dendritic cells in vitro and skewed their Toll-like receptor (TLR)-triggered maturation toward a tolerogenic phenotype. The AC⁺ Hly^- mutant also infected mouse lungs as efficiently as the parental AC⁺ Hly⁺ strain. Hence, elevation of cAMP in CD11b^- cells and/or the pore-forming capacity of CyaA were not required for infection of mouse airways. The latter activities were, however, involved in bacterial penetration across the epithelial layer, enhanced neutrophil influx into lung parenchyma during sublethal infections, and the exacerbated lung pathology and lethality of B. pertussis infections at higher inoculation doses (>10⁷ CFU/mouse). The pore-forming activity of CyaA further synergized with the cAMP-elevating activity in downregulation of major histocompatibility complex class II (MHC-II) molecules on infiltrating myeloid cells, likely contributing to immune subversion of host defenses by the whooping cough agent.

Bacterial Toxins as Pathogen Weapons Against Phagocytes

Bacterial toxins are virulence factors that manipulate host cell functions and take over the control of vital processes of living organisms to favor microbial infection. Some toxins directly target innate immune cells, thereby annihilating a major branch of the host immune response. In this review we will focus on bacterial toxins that act from the extracellular milieu and hinder the function of macrophages and neutrophils. In particular, we will concentrate on toxins from Gram-positive and Gram-negative bacteria that manipulate cell signaling or induce cell death by either imposing direct damage to the host cells cytoplasmic membrane or enzymatically modifying key eukaryotic targets. Outcomes regarding pathogen dissemination, host damage and disease progression will be discussed.

The relationship between Bordetella pertussis genotype and clinical severity in Australian children with pertussis

OBJECTIVES

Changes in circulating Bordetella pertussis genotypes, including a novel pertussis toxin promoter ptxP3 allele and absence of pertactin (Prn) antigen, have been reported from several countries but limited data on relative severity are available. We compared markers of disease severity in children with B. pertussis infection due to strains of differing genotype.

METHODS

Culture confirmed cases presenting to tertiary paediatric hospitals in three Australian states between 2008 and 2012 were classified as severe if they required a hospital stay greater than seven days, were admitted to intensive care, or if death occurred. Associations between age, vaccination, genotype and severity were assessed.

RESULTS

Of 199 pertussis cases, 81 (41%) were <3 months, including 32/39 (82%) of severe cases. The proportion of isolates from these cases that were Prn deficient increased markedly between 2008 and 2012. Of B. pertussis isolates, the proportion considered severe was similar for Prn positive (27/128, 21%) and Prn deficient (12/71, 17%) cases but only 1/22 (4.5%) of non ptxP3 cases were severe versus 38/177 (21.4%) ptxP3 positive. Adjusting for ptxP type, vaccination status and age, disease severity was not significantly associated with Prn status (RRA: 0.95, [0.57-1.56]; p = 0.83).

CONCLUSIONS

In children, we found no relationship between Prn status and markers of severe pertussis. An increased proportion of severe disease in isolates with the ptxP3 allele was observed.

Bordetella pertussis Isolates from Argentinean Whooping Cough Patients Display Enhanced Biofilm Formation Capacity Compared to Tohama I Reference Strain

Pertussis is a highly contagious disease mainly caused by Bordetella pertussis. Despite the massive use of vaccines, since the 1950s the disease has become re-emergent in 2000 with a shift in incidence from infants to adolescents and adults. Clearly, the efficacy of current cellular or acellular vaccines, formulated from bacteria grown in stirred bioreactors is limited, presenting a challenge for future vaccine development. For gaining insights into the role of B. pertussis biofilm development for host colonization and persistence within the host, we examined the biofilm forming capacity of eight argentinean clinical isolates recovered from 2001 to 2007. All clinical isolates showed an enhanced potential for biofilm formation compared to the reference strain Tohama I. We further selected the clinical isolate B. pertussis 2723, exhibiting the highest biofilm biomass production, for quantitative proteomic profiling by means of two-dimensional fluorescence difference gel electrophoresis (2D-DIGE) coupled with mass spectrometry, which was accompanied by targeted transcriptional analysis. Results revealed an elevated expression of several virulence factors, including adhesins involved in biofilm development. In addition, we observed a higher expression of energy metabolism enzymes in the clinical isolate compared to the Tohama I strain. Furthermore, all clinical isolates carried a polymorphism in the bvgS gene. This mutation was associated to an increased sensitivity to modulation and a faster rate of adhesion to abiotic surfaces. Thus, the phenotypic biofilm characteristics shown by the clinical isolates might represent an important, hitherto underestimated, adaptive strategy for host colonization and long time persistence within the host.

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.

Adenylate Cyclase Toxin promotes bacterial internalisation into non phagocytic cells

Bordetella pertussis causes whooping cough, a respiratory infectious disease that is the fifth largest cause of vaccine-preventable death in infants. Though historically considered an extracellular pathogen, this bacterium has been detected both in vitro and in vivo inside phagocytic and non-phagocytic cells. However the precise mechanism used by B. pertussis for cell entry, or the putative bacterial factors involved, are not fully elucidated. Here we find that adenylate cyclase toxin (ACT), one of the important toxins of B. pertussis, is sufficient to promote bacterial internalisation into non-phagocytic cells. After characterization of the entry route we show that uptake of "toxin-coated bacteria" proceeds via a clathrin-independent, caveolae-dependent entry pathway, allowing the internalised bacteria to survive within the cells. Intracellular bacteria were found inside non-acidic endosomes with high sphingomyelin and cholesterol content, or "free" in the cytosol of the invaded cells, suggesting that the ACT-induced bacterial uptake may not proceed through formation of late endolysosomes. Activation of Tyr kinases and toxin-induced Ca(2+)-influx are essential for the entry process. We hypothesize that B. pertussis might use ACT to activate the endocytic machinery of non-phagocytic cells and gain entry into these cells, in this way evading the host immune system.

Filamentous hemagglutinin of Bordetella pertussis: a key adhesin with immunomodulatory properties?

The filamentous hemagglutinin of pathogenic Bordetellae is a prototype of a large two-partner-system-secreted and β-structure-rich bacterial adhesin. It exhibits several binding activities that may facilitate bacterial adherence to airway mucosa and host phagocytes in the initial phases of infection. Despite three decades of research on filamentous hemagglutinin, there remain many questions on its structure-function relationships, integrin interactions and possible immunomodulatory signaling capacity. Here we review the state of knowledge on this important virulence factor and acellular pertussis vaccine component. Specific emphasis is placed on outstanding questions that are yet to be answered.

T-cell immune responses to Bordetella pertussis infection and vaccination

The recent immunological investigations, stemming from the studies performed in the nineties within the clinical trials of the acellular pertussis vaccines, have highlighted the important role played by T-cell immunity to pertussis in humans. These studies largely confirmed earlier investigations in the murine respiratory infection models that humoral immunity alone is not sufficient to confer protection against Bordetella pertussis infection and that T-cell immunity is required. Over the last years, knowledge of T-cell immune response to B. pertussis has expanded broadly, taking advantage of the general progress in the understanding of anti-bacterial immunity and of refinements in methods to approach immunological investigations. In particular, experimental models of B. pertussis infection highlighted the cooperative role played by T-helper (Th)1 and Th17 cells for protection. Furthermore, the new baboon experimental model suggested a plausible explanation for the differences observed in the strength and persistence of protective immunity induced by the acellular or whole-cell pertussis vaccines and natural infection in humans, contributing to explain the upsurge of recent pertussis outbreaks. Despite the progress, open questions remain, the answer to them will possibly provide better tools to fight one of the hardest-to-control vaccine preventable disease.

Rapid increase in pertactin-deficient Bordetella pertussis isolates, Australia

Acellular vaccines against Bordetella pertussis were introduced in Australia in 1997. By 2000, these vaccines had replaced whole-cell vaccines. During 2008–2012, a large outbreak of pertussis occurred. During this period, 30% (96/320) of B. pertussis isolates did not express the vaccine antigen pertactin (prn). Multiple mechanisms of prn inactivation were documented, including IS481 and IS1002 disruptions, a variation within a homopolymeric tract, and deletion of the prn gene. The mechanism of lack of expression of prn in 16 (17%) isolates could not be determined at the sequence level. These findings suggest that B. pertussis not expressing prn arose independently multiple times since 2008, rather than by expansion of a single prn-negative clone. All but 1 isolate had ptxA1, prn2, and ptxP3, the alleles representative of currently circulating strains in Australia. This pattern is consistent with continuing evolution of B. pertussis in response to vaccine selection pressure.

Other Bordetellas, lessons for and from pertussis vaccines

The Bordetella genus comprises nine species of which Bordetella pertussis and B. parapertussis are isolated from humans and are the most studied Bordetella species since they cause whooping cough. They both originate from B. bronchiseptica, which infects several mammals and immune compromised humans, but the intensive use of pertussis vaccines induced changes in B. pertussis and B. parapertussis populations. B. petrii and B. holmesii are other species of unknown reservoir and transmission pattern that have been described in humans. It is still unknown whether these species are pathogens for humans or only opportunistic bacteria but biological diagnosis has confirmed the presence of B. holmesii in human respiratory samples while B. petrii and the four other species have little implications for public health.

Acellular pertussis vaccines and the role of pertactin and fimbriae

The introduction of acellular pertussis (Pa) vaccines in countries with a low uptake of whole-cell pertussis (Pw) vaccines has led to a dramatic reduction in pertussis disease. Diphtheria-tetanus-acellular pertussis (DTPa) vaccines have also ensured continued high level disease protection in these countries following the shift from Pw- to Pa-containing vaccines, and allowed pertussis booster programs to be implemented. Vaccines containing between one and five components have been licensed and implemented. Those with three or more components consisting of filamentous hemagglutinin (FHA), pertussis toxin (PT) and pertactin (PRN) are considered to be more effective than one/two-component Pa vaccines that contain only PT or both PT and FHA. Changes in circulating Bordetella pertussis strains may impact vaccine efficacy and, thus, incidence and transmission of pertussis and deserve to be followed carefully. To date, vaccine-induced shifts among fimbriae (FIM) are reported and this could impact the efficacy of FIM-containing vaccines. Currently, FIM3 appears to be dominant in most European countries, Canada and Australia. Data obtained from a DTPa5 vaccine containing FIM2 and FIM3 have indicated a shift towards an increase in FIM3-expressing B. pertussis clinical breakthrough cases when compared with control vaccine. By contrast, relatively minor PT and PRN sequence polymorphisms have been identified without demonstrable association with vaccination programs. Adsorption of PRN to aluminum salt appears critical for optimal protective capacity in murine pertussis lung challenge. In addition, clinical studies have shown anti-PRN antibody levels to be higher when PRN is adsorbed at a 8-microg dosage versus non-adsorbed PRN at a 3-microg dosage. The available data, therefore, demonstrate that appropriately formulated acellular vaccines containing PT and PRN are the preferred option for pertussis immunization.

Are vaccination programs and isolate polymorphism linked to pertussis re-emergence?

Whooping cough remains an endemic disease, and the re-emergence of pertussis in older children and adolescents has been reported in several countries, despite high vaccine coverage. Polymorphism of Bordetella pertussis has been observed over time, and some characteristics of pertussis isolates have gradually diverged from the vaccine strains. The present review summarizes the current knowledge on B. pertussis variability in countries with different vaccination programs and discusses its potential impact on the recently observed increased incidence of whooping cough. No direct association between B. pertussis isolate variability and vaccination programs has been observed to date, except for shifts from fimbriae Fim2 to Fim3. More likely explanations for the re-emergence of pertussis include the change in the epidemiology and transmission patterns of pertussis in highly vaccinated populations, and a shift of disease from young children to adolescents and adults due to waning protective immunity.

Protective role of adenylate cyclase in the context of a live pertussis vaccine candidate

Despite high vaccination coverage, pertussis remains an important respiratory infectious disease and the least-controlled vaccine-preventable infectious disease in children. Natural infection with Bordetella pertussis is known to induce strong and long-lasting immunity that wanes later than vaccine-mediated immunity. Therefore, a live attenuated B. pertussis vaccine, named BPZE1, has been developed and has recently completed a phase I clinical trial in adult human volunteers. In this study, we investigated the contribution of adenylate cyclase (CyaA) in BPZE1-mediated protection against pertussis. A CyaA-deficient BPZE1 mutant was thus constructed. Absence of CyaA did not compromise the adherence properties of the bacteria onto mammalian cells. However, the CyaA-deficient mutant displayed a slight impairment in the ability to survive within macrophages compared to the parental BPZE1 strain. In vivo, whereas the protective efficacy of the CyaA-deficient mutant was comparable to the parental strain at a vaccine dose of 5 × 10(5) colony forming units (CFU), it was significantly impaired at a vaccine dose of 5 × 10(3) CFU. This impairment correlated with impaired lung colonization ability, and impaired IFN-γ production in the animal immunized with the CyaA-deficient BPZE1 mutant while the pertussis-specific antibody profile and Th17 response were comparable to those observed in BPZE1-immunized mice. Our findings thus support a role of CyaA in BPZE1-mediated protection through induction of cellular mediated immunity.

Combined hexavalent diphtheria-tetanus-acellular pertussis-hepatitis B-inactivated poliovirus-Haemophilus influenzae type B vaccine; Infanrix™ hexa: twelve years of experience in Italy

Infant vaccination using 2-dose priming at 3 and 5 mo of age with a booster at 11–12 mo of age was pioneered in Italy. The 3-5-11 schedule is now used in a growing number of European countries. Infanrix™ hexa (DTPa-HBV-IPV/Hib, GlaxoSmithKline Vaccines) was first licensed for use in 2000 and has been the only pediatric hexavalent vaccine available since 2005. We reviewed available clinical trial data describing the immunogenicity of DTPa-HBV-IPV/Hib when administered at 3, 5, and 11 mo of age, and conducted an analysis of safety using global and Italian post-marketing surveillance data. In Italy, DTPa-HBV-IPV/Hib has a demonstrated safety record extending over a decade of use, it has been associated with record levels of vaccine coverage, and with sustained disease control in vaccinated cohorts. Hexavalent vaccines will continue to contribute to high vaccine coverage in Italy and across Europe.

The virulence factors of Bordetella pertussis: talented modulators of host immune response

Approximately 40 million whooping cough cases and between 200,000 and 400,000 pertussis-linked deaths are recorded each year. Although several types of vaccines are licensed and widely used, Bordetella pertussis continues to circulate in populations with high vaccine coverage of infants and children due to the waning of protection induced by the vaccination. B. pertussis typically expresses a wide array of virulence factors which promote bacterial adhesion and invasion by altering the local environment, including pertussis toxin, tracheal cytotoxin, adenylate cyclase toxin, filamentous hemagglutinin, and the lipooligosaccharide. The virulence factors of B. pertussis also possess immunomodulatory properties, exerted through their enzymatic and receptor-binding activities. Both pro- and anti-inflammatory effects are mediated, that can subvert host innate and adaptive immunity and favor the onset of a long-term infection. This review describes the capacities of B. pertussis virulence factors to modulate host immune responses and the mechanisms employed, which have been the subject of extensive research in the recent years, both in murine and human experimental systems. Knowledge of these mechanisms is gaining increasing importance, since it could provide in the near future the basis for the identification of therapeutic agents for modulating the immune system as well as novel molecular targets to treat pertussis.

Bordetella pertussis entry into respiratory epithelial cells and intracellular survival

Bordetella pertussis is the causative agent of pertussis, aka whooping cough. Although generally considered an extracellular pathogen, this bacterium has been found inside respiratory epithelial cells, which might represent a survival strategy inside the host. Relatively little is known, however, about the mechanism of internalization and the fate of B. pertussis inside the epithelia. We show here that B. pertussis is able to enter those cells by a mechanism dependent on microtubule assembly, lipid raft integrity, and the activation of a tyrosine-kinase-mediated signaling. Once inside the cell, a significant proportion of the intracellular bacteria evade phagolysosomal fusion and remain viable in nonacidic lysosome-associated membrane-protein-1-negative compartments. In addition, intracellular B. pertussis was found able to repopulate the extracellular environment after complete elimination of the extracellular bacteria with polymyxin B. Taken together, these data suggest that B. pertussis is able to survive within respiratory epithelial cells and by this means potentially contribute to host immune system evasion.

Anti-adhesion methods as novel therapeutics for bacterial infections

Anti-adhesion therapies for bacterial infections offer an alternative to antibiotics, with those therapies bacteria are not killed but are prevented from causing harm to a host by inhibiting adherence to host cells and tissues, a prerequisite for the majority of infectious diseases. The mechanisms of these potential therapeutic agents include inhibition of adhesins and their host receptors, vaccination with adhesins or analogs, use of probiotics and dietary supplements that interfere with receptor-adhesin interactions, subminimal inhibitory concentrations of antibiotics and manipulation of hydrophobic interactions. Once developed, these drugs will contribute to the arsenal for fighting infectious disease in the future, potentially subverting antibiotic resistance.

Contribution of Bordetella filamentous hemagglutinin and adenylate cyclase toxin to suppression and evasion of interleukin-17-mediated inflammation

Bordetella pertussis and Bordetella bronchiseptica establish respiratory infections with notorious efficiency. Our previous studies showed that the fhaB genes of B. pertussis and B. bronchiseptica, which encode filamentous hemagglutinin (FHA), are functionally interchangeable and provided evidence that FHA-deficient B. bronchiseptica induces more inflammation in the lungs of mice than wild-type B. bronchiseptica. We show here that the robust inflammatory response to FHA-deficient B. bronchiseptica is characterized by the early and sustained influx of interleukin-17 (IL-17)-positive neutrophils and macrophages and, at 72 h postinoculation, IL-17-positive CD4(+) T cells, suggesting that FHA allows the bacteria to suppress the development of an IL-17-mediated inflammatory response. We also show that the cyaA genes of B. pertussis and B. bronchiseptica, which encode adenylate cyclase toxin (ACT), are functionally interchangeable and that ACT, specifically its catalytic activity, is required for B. bronchiseptica to resist phagocytic clearance but is neither required for nor inhibitory of the induction of inflammation if bacteria are present in numbers sufficient to persist during the first 3 days postinoculation. Incubation of bone marrow-derived macrophages with a ΔcyaA strain caused decreased production of IL-1β and increased production of tumor necrosis factor alpha (TNF-α) and IL-12, while incubation with a ΔcyaA ΔfhaB strain caused increased production of IL-23. These data suggest that FHA and ACT both contribute to suppress the recruitment of neutrophils and the development of an IL-17-mediated immune response. To our knowledge, this is the first demonstration of a microbial pathogen suppressing IL-17-mediated inflammation in vivo as a strategy to evade innate immunity.

Prevalence and genetic characterization of pertactin-deficient Bordetella pertussis in Japan

The adhesin pertactin (Prn) is one of the major virulence factors of Bordetella pertussis, the etiological agent of whooping cough. However, a significant prevalence of Prn-deficient (Prn⁻) B. pertussis was observed in Japan. The Prn⁻ isolate was first discovered in 1997, and 33 (27%) Prn⁻ isolates were identified among 121 B. pertussis isolates collected from 1990 to 2009. Sequence analysis revealed that all the Prn⁻ isolates harbor exclusively the vaccine-type prn1 allele and that loss of Prn expression is caused by 2 different mutations: an 84-bp deletion of the prn signal sequence (prn1ΔSS, n = 24) and an IS481 insertion in prn1 (prn1::IS481, n = 9). The frequency of Prn⁻ isolates, notably those harboring prn1ΔSS, significantly increased since the early 2000s, and Prn⁻ isolates were subsequently found nationwide. Multilocus variable-number tandem repeat analysis (MLVA) revealed that 24 (73%) of 33 Prn⁻ isolates belong to MLVA-186, and 6 and 3 Prn⁻ isolates belong to MLVA-194 and MLVA-226, respectively. The 3 MLVA types are phylogenetically closely related, suggesting that the 2 Prn⁻ clinical strains (harboring prn1ΔSS and prn1::IS481) have clonally expanded in Japan. Growth competition assays in vitro also demonstrated that Prn⁻ isolates have a higher growth potential than the Prn⁺ back-mutants from which they were derived. Our observations suggested that human host factors (genetic factors and immune status) that select for Prn⁻ strains have arisen and that Prn expression is not essential for fitness under these conditions.

Bordetella petrii infection with long-lasting persistence in human

B. petrii infection can persist in persons with chronic obstructive pulmonary disease.

We report the repeated isolation of Bordetella petrii in the sputum of a 79-year-old female patient with diffuse bronchiectasis and persistence of the bacterium for >1 year. The patient was first hospitalized due to dyspnea, which developed into severe cough with purulent sputum that yielded B. petrii on culture. After this first episode, the patient was hospitalized an additional 4 times with bronchorrhea symptoms. The isolates collected were analyzed by using biochemical, genotypic, and proteomic tools. Expression of specific proteins was analyzed by using serum samples from the patient. The B. petrii isolates were compared with other B. petrii isolates collected from humans or the environment and with isolates of B. pertussis, B. parapertussis, B. bronchiseptica, and B. holmesii, obtained from human respiratory tract infections. Our observations indicate that B. petrii can persist in persons with chronic pulmonary obstructive disease as has been previously demonstrated for B. bronchiseptica.

Bordetella pertussis commits human dendritic cells to promote a Th1/Th17 response through the activity of adenylate cyclase toxin and MAPK-pathways

The complex pathology of B. pertussis infection is due to multiple virulence factors having disparate effects on different cell types. We focused our investigation on the ability of B. pertussis to modulate host immunity, in particular on the role played by adenylate cyclase toxin (CyaA), an important virulence factor of B. pertussis. As a tool, we used human monocyte derived dendritic cells (MDDC), an ex vivo model useful for the evaluation of the regulatory potential of DC on T cell immune responses. The work compared MDDC functions after encounter with wild-type B. pertussis (BpWT) or a mutant lacking CyaA (BpCyaA-), or the BpCyaA- strain supplemented with either the fully functional CyaA or a derivative, CyaA*, lacking adenylate cyclase activity. As a first step, MDDC maturation, cytokine production, and modulation of T helper cell polarization were evaluated. As a second step, engagement of Toll-like receptors (TLR) 2 and TLR4 by B. pertussis and the signaling events connected to this were analyzed. These approaches allowed us to demonstrate that CyaA expressed by B. pertussis strongly interferes with DC functions, by reducing the expression of phenotypic markers and immunomodulatory cytokines, and blocking IL-12p70 production. B. pertussis-treated MDDC promoted a mixed Th1/Th17 polarization, and the activity of CyaA altered the Th1/Th17 balance, enhancing Th17 and limiting Th1 expansion. We also demonstrated that Th1 effectors are induced by B. pertussis-MDDC in the absence of IL-12p70 through an ERK1/2 dependent mechanism, and that p38 MAPK is essential for MDDC-driven Th17 expansion. The data suggest that CyaA mediates an escape strategy for the bacterium, since it reduces Th1 immunity and increases Th17 responses thought to be responsible, when the response is exacerbated, for enhanced lung inflammation and injury.

Intracellular trafficking of Bordetella pertussis in human macrophages

Although Bordetella pertussis has been observed to survive inside macrophages, its ability to resist or evade degradation in phagolysosomes has not been defined. We here investigated the trafficking of B. pertussis upon entry into human macrophages. During the first hours following phagocytosis, a high percentage of bacteria were destroyed within acidic compartments positive for the lysosome-associated membrane proteins (LAMP). However, roughly one-fourth of the bacteria taken up evade this initial killing event, remaining in nonacidic compartments. Forty-eight hours after infection, the number of intracellular bacteria per cell increased, suggesting that B. pertussis is capable of replicating in this type of compartment. Viable bacteria accumulated within phagosomal compartments positive for the early endosomal marker Rab5 but not the late endosomal marker LAMP. Moreover, B. pertussis-containing phagosomes acquired exogenously added transferrin, indicating that intracellular bacteria have access to extracellular components and essential nutrients via the host cell recycling pathway. Overall, these results suggest that B. pertussis survives and eventually replicates in compartments with characteristics of early endosomes, potentially contributing to its extraordinary ability to persist within hosts and populations.

First report and detailed characterization of B. pertussis isolates not expressing Pertussis Toxin or Pertactin

Bordetella pertussis isolates not expressing Pertussis Toxin (PT) or Pertactin (PRN) have been collected, for the first time in 2007, in France, a highly vaccinated country with acellular vaccines. Non-expression was due to deletion of the entire ptx locus, to IS481 insertion in the prn gene or deletion of a part of this gene. Genome sequencing does not indicate any regions of differences when compared to other circulating isolates. It nevertheless shows some sequence differences and an increased number of repeated sequences. The infant infected by the isolate not expressing pertussis toxin, did not present hyperlymphocytosis. All isolates were found less pathogen in animal or cellular models; their circulation raises the problem of clinical and biological diagnoses.

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

Adenylate cyclase-hemolysin plays an important role in the virulence of Bordetella pertussis, Bordetella parapertussis and Bordetella bronchiseptica species. Its C-terminal region carries protective epitopes and receptor binding site for human cells. Genomic analyses of this region indicate no polymorphism in B. pertussis and B. parapertussis regions, but substantial variability in B. bronchiseptica that might be linked to the various niches of this species.

International Bordetella pertussis assay standardization and harmonization meeting report. Centers for Disease Control and Prevention, Atlanta, Georgia, United States, 19-20 July 2007

An international meeting on Bordetella pertussis assay standardization and harmonization was held at the Centers for Disease Control and Prevention (CDC), Atlanta, GA, 19–20 July 2007. The goal of the meeting was to harmonize the immunoassays used for pertussis diagnostics and vaccine evaluation, as agreed upon by academic and government researchers, regulatory authorities, vaccine manufacturers, and the World Health Organization (WHO).

The primary objectives were (1) to provide epidemiologic, laboratory, and statistical background for support of global harmonization; (2) to overview the current status of global epidemiology, pathogenesis and immunology of pertussis; (3) to develop a consensus opinion on existing gaps in understanding standardization of pertussis assays used for serodiagnosis and vaccine evaluation; and (4) to search for a multicenter process for addressing these priority gaps. Presentations and discussions by content experts addressed these objectives. A prioritized list of action items to improve standardization and harmonization of pertussis assays was identified during a group discussion at the end of the meeting. The major items included: (1) to identify a group that will organize, prepare, maintain, and distribute proficiency panels and key reagents such as reference and control sera; (2) to encourage the development and identification of one or more reference laboratories that can serve as an anchor and resource for other laboratories; (3) to define a performance-based assay method that can serve as a reference point for evaluating laboratory differences; (4) to develop guidance on quality of other reagents, e.g., pertussis toxin and other antigens, and methods to demonstrate their suitability; (5) to establish an international working group to harmonize the criteria to evaluate the results obtained on reference and proficiency panel sera; (6) to create an inventory to determine the amount of appropriate and well-characterized sera that are available globally to be used as bridging reagents for vaccine licensure; and (7) to seek specific guidance from regulatory authorities regarding the expectations and requirements for the licensure of new multicomponent pertussis vaccines.

Adenylate cyclase toxin subverts phagocyte function by RhoA inhibition and unproductive ruffling

Adenylate cyclase toxin (CyaA or ACT) is a key virulence factor of pathogenic Bordetellae. It penetrates phagocytes expressing the alpha(M)beta(2) integrin (CD11b/CD18, Mac-1 or CR3) and paralyzes their bactericidal capacities by uncontrolled conversion of ATP into a key signaling molecule, cAMP. Using pull-down activity assays and transfections with mutant Rho family GTPases, we show that cAMP signaling of CyaA causes transient and selective inactivation of RhoA in mouse macrophages in the absence of detectable activation of Rac1, Rac2, or RhoG. This CyaA/cAMP-induced drop of RhoA activity yielded dephosphorylation of the actin filament severing protein cofilin and massive actin cytoskeleton rearrangements, which were paralleled by rapidly manifested macrophage ruffling and a rapid and unexpected loss of macropinocytic fluid phase uptake. As shown in this study for the first time, CyaA/cAMP signaling further caused a rapid and near-complete block of complement-mediated phagocytosis. Induction of unproductive membrane ruffling, hence, represents a novel sophisticated mechanism of down-modulation of bactericidal activities of macrophages and a new paradigm for action of bacterial toxins that hijack host cell signaling by manipulating cellular cAMP levels.

Pathology and pathogenesis of fatal Bordetella pertussis infection in infants

BACKGROUND

Each year, Bordetella pertussis infection causes an estimated 294,000 deaths worldwide, primarily among young, nonvaccinated children. Approximately 90% of all deaths due to pertussis in the Unites States occur in young infants. These children often develop intractable pulmonary hypertension; however, the pathophysiologic mechanism responsible for this complication has not been well characterized, and there have been no detailed descriptions of the pathology of this disease since the 1940s.

METHODS

Respiratory tissue samples obtained at autopsy from 15 infants aged <or=4 months who had polymerase chain reaction- or culture-confirmed B. pertussis pneumonia were evaluated by multiple histochemical stains, immunohistochemical evaluation, and electron microscopic examination.

RESULTS

The pulmonary histopathologic examination of the samples revealed a descending infection dominated by necrotizing bronchiolitis, intra-alveolar hemorrhage, and fibrinous edema. All samples had marked leukocytosis, and most showed luminal aggregates of abundant leukocytes in small pulmonary arteries, veins, and lymphatics. A novel immunohistochemical stain for B. pertussis revealed abundant extracellular bordetellae in cilia of the trachea, bronchi, and bronchioles, as well as intracellular bacteria and antigens in alveolar macrophages and ciliated epithelium.

CONCLUSIONS

Pertussis should be suspected in any infant death associated with marked leukocytosis, bronchopneumonia, or refractory pulmonary hypertension, particularly in children aged <or=4 months. The pathologic findings identified in the respiratory tracts of these children, in addition to recognized physiologic responses of the infant lung to hypoxia, suggest that B. pertussis pneumonia triggers a cascade of events that includes acute pulmonary vasoconstriction and pertussis toxin-mediated increases in circulating leukocyte mass. These responses ultimately compromise pulmonary blood flow, exacerbate hypoxemia, and create a vicious cycle of refractory pulmonary hypertension.

Replacement of adenylate cyclase toxin in a lineage of Bordetella bronchiseptica

Bordetella bronchiseptica is a gram-negative respiratory pathogen that infects a wide range of hosts and causes a diverse spectrum of disease. This diversity is likely affected by multiple factors, such as host immune status, polymicrobial infection, and strain diversity. In a murine model of infection, we found that the virulence of B. bronchiseptica strains, as measured by the mean lethal dose, varied widely. Strain 253 was less virulent than the typically studied strain, RB50. Transcriptome analysis showed that cyaA, the gene encoding adenylate cyclase toxin (CyaA), was the most downregulated transcript identified in strain 253 compared to that in strain RB50. Comparative genomic hybridization and genome sequencing of strain 253 revealed that the cya locus, which encodes, activates, and secretes CyaA, was replaced by an operon (ptp) predicted to encode peptide transport proteins. Other B. bronchiseptica strains from the same phylogenetic lineage as that of strain 253 also lacked the cya locus, contained the ptp genes, and were less virulent than strain RB50. Although the loss of CyaA would be expected to be counterselected since it is conserved among the classical bordetellae and believed to be important to their success, our data indicate that the loss of this toxin and the gain of the ptp genes occurred in an ancestral strain that then expanded into a lineage. This suggests that there may be ecological niches in which CyaA is not critical for the success of B. bronchiseptica.

Bordetella pertussis inhibition of interleukin-12 (IL-12) p70 in human monocyte-derived dendritic cells blocks IL-12 p35 through adenylate cyclase toxin-dependent cyclic AMP induction

Bordetella pertussis, the causative agent of whooping cough, possesses an array of virulence factors, including adenylate cyclase toxin (ACT), relevant in the establishment of infection. Here we better define the impact of cyclic AMP (cAMP) intoxication due to the action of ACT on dendritic cell (DC)-driven immune response, by infecting monocyte-derived DC (MDDC) with an ACT-deficient B. pertussis mutant (ACT- 18HS19) or its parental strain (WT18323). Both strains induced MDDC maturation and antigen-presenting cell functions; however, only ACT- 18HS19 infected MDDC-induced production of interleukin-12 (IL-12) p70. Gene expression analysis of the IL-12 cytokine family subunits revealed that both strains induced high levels of p40 (protein chain communal to IL-12 p70 and IL-23) as well as p19, a subunit of IL-23. Conversely only ACT- 18HS19 infection induced consistent transcription of IL-12 p35, a subunit of IL-12 p70. Addition of the cAMP analogous D-butyril-cAMP (D-cAMP) abolished IL-12 p70 production and IL-12 p35 expression in ACT- 18HS19-infected MDDC. ACT- 18HS19 infection induced the expression of the transcription factors interferon regulatory factor 1 (IRF-1) and IRF-8 and of beta interferon, involved in IL-12 p35 regulation, and the expression of these genes was inhibited by D-cAMP addition and in WT18323-infected MDDC. The concomitant expression of IL-12 p70 and IL-23 allowed ACT- 18HS19 to trigger a more pronounced T helper 1 polarization compared to WT18323. The present study suggests that ACT-dependent cAMP induction leads to the inhibition of pathways ultimately leading to IL-12 p35 production, thus representing a mechanism for B. pertussis to escape the host immune response.