Phagocytosed Bordetella pertussis fails to survive in human neutrophils

Antibody and B-cell Immune Responses Against Bordetella Pertussis Following Infection and Immunization

Neither immunization nor recovery from natural infection provides life-long protection against Bordetella pertussis. Replacement of a whole-cell pertussis (wP) vaccine with an acellular pertussis (aP) vaccine, mutations in B. pertussis strains, and better diagnostic techniques, contribute to resurgence of number of cases especially in young infants. Development of new immunization strategies relies on a comprehensive understanding of immune system responses to infection and immunization and how triggering these immune components would ensure protective immunity. In this review, we assess how B cells, and their secretory products, antibodies, respond to B. pertussis infection, current and novel vaccines and highlight similarities and differences in these responses. We first focus on antibody-mediated immunity. We discuss antibody (sub)classes, elaborate on antibody avidity, ability to neutralize pertussis toxin, and summarize different effector functions, i.e. ability to activate complement, promote phagocytosis and activate NK cells. We then discuss challenges and opportunities in studying B-cell immunity. We highlight shared and unique aspects of B-cell and plasma cell responses to infection and immunization, and discuss how responses to novel immunization strategies better resemble those triggered by a natural infection (i.e., by triggering responses in mucosa and production of IgA). With this comprehensive review, we aim to shed some new light on the role of B cells and antibodies in the pertussis immunity to guide new vaccine development.

Avirulent phenotype promotes Bordetella pertussis adaptation to the intramacrophage environment

Bordetella pertussis, the causative agent of whooping cough, is an extracellular, strictly human pathogen. However, it has been shown that B. pertussis cells can escape phagocytic killing and survive in macrophages upon internalization. Our time-resolved RNA-seq data suggest that B. pertussis efficiently adapts to the intramacrophage environment and responds to host bactericidal activities. We show that this adaptive response is multifaceted and, surprisingly, related to the BvgAS two-component system, a master regulator of virulence. Our results show that the expression of this regulatory circuit is downregulated upon internalization. Moreover, we demonstrate that the switch to the avirulent Bvg^- phase augments a very complex process based on the adjustment of central and energy metabolism, cell wall reinforcement, maintenance of appropriate redox and metal homeostasis, and repair of damaged macromolecules. Nevertheless, not all observed effects could be simply attributed to the transition to Bvg^- phase, suggesting that additional regulators are involved in the adaptation to the intramacrophage environment. Interestingly, a large number of genes required for the metabolism of sulphur were strongly modulated within macrophages. In particular, the mutant lacking two genes encoding cysteine dioxygenases displayed strongly attenuated cytotoxicity toward THP-1 cells. Collectively, our results suggest that intracellular B. pertussis cells have adopted the Bvg^- mode to acclimate to the intramacrophage environment and respond to antimicrobial activities elicited by THP-1 cells. Therefore, we hypothesize that the avirulent phase represents an authentic phenotype of internalized B. pertussis cells.

Blockade of the Adenylate Cyclase Toxin Synergizes with Opsonizing Antibodies to Protect Mice against Bordetella pertussis

Bordetella produces an array of virulence factors, including the adenylate cyclase toxin (ACT), which is essential, immunogenic in humans, and highly conserved. Despite mediating immune-evasive functions as a leukotoxin, ACT’s potential role as a protective antigen is unclear. To better understand the contributions of humoral anti-ACT immunity, we evaluated protection against Bordetella pertussis by antibodies binding structurally defined ACT epitopes in a mouse pneumonia model. An ACT-neutralizing antibody, but not a nonneutralizing antibody or an isotype control, significantly increased mouse survival after lethal challenge with B. pertussis. When modified to impair Fc effector functions, the neutralizing antibody retained protective capabilities, indicating that protection was mediated by the blockade of the interactions of ACT with its α_Mβ₂ integrin receptor. After infection with a lower bacterial dose, ACT neutralization synergistically reduced lung bacterial colonization levels when combined with an opsonic antibody binding the surface antigen pertactin. Notably, protection was significantly enhanced when antibodies were administered intranasally as opposed to systemically, indicating that local immune responses are key to antibody-mediated protection against ACT and pertactin. These data reconcile previous conflicting reports to indicate that neutralizing anti-ACT antibodies support the phagocytosis of opsonized B. pertussis and thereby contribute to pertussis protection in vivo.

Long Lasting Antibodies From Convalescent Pertussis Patients Induce ROS Production and Bacterial Killing by Human Neutrophils

Pertussis is a respiratory infection caused by the Gram-negative bacterium Bordetella pertussis. Despite high vaccination coverage this disease remains a public health concern worldwide. A better understanding of the protective immune responses to B. pertussis is required for the development of improved vaccines. The aim of this study was to determine the production of reactive oxygen species (ROS) by human neutrophils in response to B. pertussis and to determine the contribution of opsonizing antibodies from convalescent pertussis patients in this response. The serum samples from convalescent patients were taken at <3, 9, 18 and 36 months after diagnosis of pertussis. Also included were sera from healthy age-matched controls. We show that neutrophils produced high levels of ROS in response to opsonized, compared to non-opsonized, B. pertussis and that this effect was independent of the time the convalescent serum samples were taken. This indicates the presence of functional opsonizing antibodies up to 3 years after B. pertussis infection. While opsonization of B. pertussis with serum samples from uninfected controls also induced ROS production, sera from infected individuals induced significantly higher ROS levels. Spearman correlations analysis showed that IgG antibodies targeting fimbriae3 followed by pertactin, and BrkA correlate with ROS production. Additionally, we observed that neutrophils killed opsonized B. pertussis in a ROS-dependent manner. Searching for other antigen-specific antibodies from convalescent pertussis patients involved in ROS production by neutrophils may assist in the identification of novel antigens to improve the current pertussis vaccines.

Transcriptional profiling of human macrophages during infection with Bordetella pertussis

Bordetella pertussis, a strictly human re-emerging pathogen and the causative agent of whooping cough, exploits a broad variety of virulence factors to establish efficient infection. Here, we used RNA sequencing to analyse the changes in gene expression profiles of human THP-1 macrophages resulting from B. pertussis infection. In parallel, we attempted to determine the changes in intracellular B. pertussis-specific transcriptomic profiles resulting from interaction with macrophages. Our analysis revealed that global gene expression profiles in THP-1 macrophages are extensively rewired 6 h post-infection. Among the highly expressed genes, we identified those encoding cytokines, chemokines, and transcription regulators involved in the induction of the M1 and M2 macrophage polarization programmes. Notably, several host genes involved in the control of apoptosis and inflammation which are known to be hijacked by intracellular bacterial pathogens were overexpressed upon infection. Furthermore, in silico analyses identified large temporal changes in expression of specific gene subsets involved in signalling and metabolic pathways. Despite limited numbers of the bacterial reads, we observed reduced expression of majority of virulence factors and upregulation of several transcriptional regulators during infection suggesting that intracellular B. pertussis cells switch from virulent to avirulent phase and actively adapt to intracellular environment, respectively.

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 toxin is a unique ligand of the integrin complement receptor 3

Integrins are heterodimeric cell surface adhesion and signaling receptors that are essential for metazoan existence. Some integrins contain an I-domain that is a major ligand binding site. The ligands preferentially engage the active forms of the integrins and trigger signaling cascades that alter numerous cell functions. Here we found that the adenylate cyclase toxin (CyaA), a key virulence factor of the whooping cough agent Bordetella pertussis, preferentially binds an inactive form of the integrin complement receptor 3 (CR3), using a site outside of its I-domain. CyaA binding did not trigger downstream signaling of CR3 in human monocytes and CyaA-catalyzed elevation of cAMP effectively blocked CR3 signaling initiated by a natural ligand. This unprecedented type of integrin-ligand interaction distinguishes CyaA from all other known ligands of the I-domain-containing integrins and provides a mechanistic insight into the previously observed central role of CyaA in the pathogenesis of B. pertussis.

Review of the neutrophil response to Bordetella pertussis infection

The nature and timing of the neutrophil response to infection with Bordetella pertussis is influenced by multiple virulence factors expressed by the bacterium. After inoculation of the host airway, the recruitment of neutrophils signaled by B. pertussis lipooligosaccharide (LOS) is suppressed by pertussis toxin (PTX). Over the next week, the combined activities of PTX, LOS and adenylate cyclase toxin (ACT) result in production of cytokines that generate an IL-17 response, promoting neutrophil recruitment which peaks at 10-14 days after inoculation in mice. Arriving at the site of infection, neutrophils encounter the powerful local inhibitory activity of ACT, in conjunction with filamentous hemagglutinin. With the help of antibodies, neutrophils contribute to clearance of B. pertussis, but only after 28-35 days in a naïve mouse. Studies of the lasting, antigen-specific IL-17 response to infection in mice and baboons has led to progress in vaccine development and understanding of pathogenesis. Questions remain about the mediators that coordinate neutrophil recruitment and the mechanisms by which neutrophils overcome B. pertussis virulence factors.

Improved pertussis vaccines based on adjuvants that induce cell-mediated immunity

Bordetella pertussis is a Gram-negative bacterium that causes the severe and sometimes lethal respiratory disease whooping cough in infants and children. There has been a recent resurgence in the number of cases of pertussis in several countries with high vaccine coverage. This has been linked with waning or ineffective immunity induced by current acellular pertussis vaccines. These acellular pertussis vaccines are formulated with alum as the adjuvant, which promotes strong antibody responses but is less effective at inducing Th1-type responses crucial for effective bacterial clearance. Studies in animal models have demonstrated that replacing alum with alternative adjuvants, such as toll-like receptor agonists, can promote more robust cell-mediated immunity and confer a high level of protection against infection following respiratory challenge.

A novel investigation of a blister-like syndrome in aquarium Echinopora lamellosa

This study investigates potential causes of a novel blister-like syndrome in the plating coral Echinopora lamellosa. Visual inspections of this novel coral syndrome showed no obvious signs of macroparasites and the blisters themselves manifested as fluid-filled sacs on the surface of the coral, which rose from the coenosarc between the coral polyps. Histological analysis of the blisters showed that there was no associated necrosis with the epidermal or gastrodermal tissues. The only difference between blistered areas and apparently healthy tissues was the presence of proliferated growth (possible mucosal cell hyperplasia) directly at the blister interface (area between where the edge of the blister joined apparently healthy tissue). No bacterial aggregates were identified in any histological samples, nor any sign of tissue necrosis identified. We conclude, that the blister formations are not apparently caused by a specific microbial infection, but instead may be the result of irritation following growth anomalies of the epidermis. However, future work should be conducted to search for other potential casual agents, including viruses.

rptA, a novel gene from Ensifer (Sinorhizobium) meliloti involved in conjugal transfer

We approached the identification of Ensifer (Sinorhizobium) meliloti conjugal functions by random Tn5-B13 mutagenesis of the pSmeLPU88a plasmid of E. meliloti strain LPU88 and the subsequent selection of those mutants that had lost the ability to mobilize the small plasmid pSmeLPU88b. The Tn5-B13-insertion site of one of the mutants was cloned as an EcoRI-restricted DNA fragment that after subsequent isolation and sequencing demonstrated that a small open reading frame of 522 bp (designated rptA, for rhizobium plasmid transfer A) had been disrupted. The predicted gene product encoded by the rptA sequence shows a significant similarity to two hypothetical proteins of the plasmid pSmed03 of Ensifer medicae WSM419 and other rhizobia plasmids. No significant similarity was found to any protein sequence of known function registered in the databases. Although the rptA gene was required for pSmeLPU88b-plasmid mobilization in the strain 2011 background, it was not required in the original strain LPU88 background.

FHA-mediated cell-substrate and cell-cell adhesions are critical for Bordetella pertussis biofilm formation on abiotic surfaces and in the mouse nose and the trachea

Bordetella spp. form biofilms in the mouse nasopharynx, thereby providing a potential mechanism for establishing chronic infections in humans and animals. Filamentous hemagglutinin (FHA) is a major virulence factor of B. pertussis, the causative agent of the highly transmissible and infectious disease, pertussis. In this study, we dissected the role of FHA in the distinct biofilm developmental stages of B. pertussis on abiotic substrates and in the respiratory tract by employing a murine model of respiratory biofilms. Our results show that the lack of FHA reduced attachment and decreased accumulation of biofilm biomass on artificial surfaces. FHA contributes to biofilm development by promoting the formation of microcolonies. Absence of FHA from B. pertussis or antibody-mediated blockade of surface-associated FHA impaired the attachment of bacteria to the biofilm community. Exogenous addition of FHA resulted in a dose-dependent inhibitory effect on bacterial association with the biofilms. Furthermore, we show that FHA is important for the structural integrity of biofilms formed on the mouse nose and trachea. Together, these results strongly support the hypothesis that FHA promotes the formation and maintenance of biofilms by mediating cell-substrate and inter-bacterial adhesions. These discoveries highlight FHA as a key factor in establishing structured biofilm communities in the respiratory tract.

Immunization of teenagers with a fifth dose of reduced DTaP-IPV induces high levels of pertussis antibodies with a significant increase in opsonophagocytic activity

Waning vaccine-induced immunity against Bordetella pertussis is observed among adolescents and adults. A high incidence of pertussis has been reported in this population, which serves as a reservoir for B. pertussis. A fifth dose of reduced antigen of diphtheria-tetanus-acellular-pertussis and inactivated polio vaccine was given as a booster dose to healthy teenagers. The antibody activity against B. pertussis antigens was measured prior to and 4 to 8 weeks after the booster by different assays: enzyme-linked immunosorbent assays (ELISAs) of IgG and IgA against pertussis toxin (PT) and filamentous hemagglutinin (FHA), IgG against pertactin (PRN), opsonophagocytic activity (OPA), and IgG binding to live B. pertussis. There was a significant increase in the IgG activity against PT, FHA, and PRN following the booster immunization (P < 0.001). The prebooster sera showed a geometric mean OPA titer of 65.1 and IgG binding to live bacteria at a geometric mean concentration of 164.9 arbitrary units (AU)/ml. Following the fifth dose, the OPA increased to a titer of 360.4, and the IgG concentration against live bacteria increased to 833.4 AU/ml (P < 0.001 for both). The correlation analyses between the different assays suggest that antibodies against FHA and PRN contribute the most to the OPA and IgG binding.

Decreased leukocyte accumulation and delayed Bordetella pertussis clearance in IL-6-/- mice

IL-6, a pleiotropic cytokine primarily produced by the innate immune system, has been implicated in the development of acquired immune responses, though its roles are largely undefined and may vary in the context of different diseases. Using a murine model of infection, we established that IL-6 influences the adaptive immune responses against the endemic human respiratory pathogen Bordetella pertussis. IL-6 was induced in the lungs of C57BL/6 mice by B. pertussis. IL-6(-/-) mice showed a protracted infectious course and were less efficiently protected by B. pertussis vaccination than wild-type mice. Abs from IL-6(-/-) mice, though lower in titer, efficiently reduced B. pertussis numbers in IL-6-sufficient mice. Pulmonary leukocyte recruitment and splenic or pulmonary T cell cytokine responses to B. pertussis, including Th1 and Th17 cytokine production, were lower in IL-6(-/-) mice than in wild-type mice. Adoptive transfer of immune wild-type CD4(+) cells ameliorated the defect of IL-6(-/-) mice in the control of B. pertussis numbers. Together, these results reveal the dysregulation of multiple aspects of adaptive immune responses in B. pertussis-infected IL-6(-/-) mice and suggest that IL-6 is involved in regulating Ab generation, pulmonary leukocyte accumulation, and T cell cytokine production in response to B. pertussis as well as the generation of effective vaccine-induced immunity against this pathogen.

Bordetella evades the host immune system by inducing IL-10 through a type III effector, BopN

The inflammatory response is one of several host alert mechanisms that recruit neutrophils from the circulation to the area of infection. We demonstrate that Bordetella, a bacterial pathogen, exploits an antiinflammatory cytokine, interleukin-10 (IL-10), to evade the host immune system. We identified a Bordetella effector, BopN, that is translocated into the host cell via the type III secretion system, where it induces enhanced production of IL-10. Interestingly, the BopN effector translocates itself into the nucleus and is involved in the down-regulation of mitogen-activated protein kinases. Using pharmacological blockade, we demonstrated that BopN-induced IL-10 production is mediated, at least in part, by its ability to block the extracellular signal-regulated kinase pathway. We also showed that BopN blocks nuclear translocation of nuclear factor κB p65 (NF-κBp65) but, in contrast, promotes nuclear translocation of NF-κBp50. A BopN-deficient strain was unable to induce IL-10 production in mice, resulting in the elimination of bacteria via neutrophil infiltration into the pulmonary alveoli. Furthermore, IL-10–deficient mice effectively eliminated wild-type as well as BopN mutant bacteria. Thus, Bordetella exploits BopN as a stealth strategy to shut off the host inflammatory reaction. These results explain the ability of Bordetella species to avoid induction of the inflammatory response.

Phagocytosis of heat-killed Staphylococcus aureus by eosinophils: comparison with neutrophils

Eosinophils are characterized by several functional properties, such as chemotaxis, adhesion, superoxide anion production, and degranulation. In this article, we have studied the role of bacterial ingestion by eosinophils in comparison with that by neutrophils. Eosinophils and neutrophils were purified by using the Percoll gradient method followed by selection with CD16-coated immunomagnetic beads and centrifugation through a Ficoll-Hypaque gradient combined with dextran sedimentation, respectively. Both cells were preincubated with anti-FcgammaRIIa mAb (CD32 mAb), anti-FcgammaRIIIb mAb (CD16 mAb), anti-CR3 (CD11b mAb), or anti-CR1 (CD35 mAb) before being examined for phagocytosis of opsonized heat-killed Staphylococcus aureus (S. aureus). Phagocytosis and production of hydrogen peroxide were simultaneously measured by flow cytometry using S. aureus labeled with propidium iodide and stained with 2',7'-dichlorofluorescein diacetate. Eosinophils showed significantly lower activity than neutrophils in both phagocytosis and hydrogen peroxide production. Phagocytosis by both cells was decreased by heat-inactivated serum. Phagocytosis by neutrophils was significantly inhibited by CD16 mAb and CD32 mAb, whereas that by eosinophils was only inhibited by CD35 mAb. Whereas the mechanism of phagocytosis by neutrophils was mediated by CD16 and CD32, that of eosinophils was modulated by complement receptor 1 (CD35).

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.

Pertussis toxin inhibits early chemokine production to delay neutrophil recruitment in response to Bordetella pertussis respiratory tract infection in mice

Pertussis is an acute respiratory disease of humans caused by the bacterium Bordetella pertussis. Pertussis toxin (PT) plays a major role in the virulence of this pathogen, including important effects that it has soon after inoculation. Studies in our laboratory and other laboratories have indicated that PT inhibits early neutrophil influx to the lungs and airways in response to B. pertussis respiratory tract infection in mice. Previous in vitro and in vivo studies have shown that PT can affect neutrophils directly by ADP ribosylating G(i) proteins associated with surface chemokine receptors, thereby inhibiting neutrophil migration in response to chemokines. However, in this study, by comparing responses to wild-type (WT) and PT-deficient strains, we found that PT has an indirect inhibitory effect on neutrophil recruitment to the airways in response to infection. Analysis of lung chemokine expression indicated that PT suppresses early neutrophil recruitment by inhibiting chemokine upregulation in alveolar macrophages and other lung cells in response to B. pertussis infection. Enhancement of early neutrophil recruitment to the airways in response to WT infection by addition of exogenous keratinocyte-derived chemokine, one of the dominant neutrophil-attracting chemokines in mice, further revealed an indirect effect of PT on neutrophil chemotaxis. Additionally, we showed that intranasal administration of PT inhibits lipopolysaccharide-induced chemokine gene expression and neutrophil recruitment to the airways, presumably by modulation of signaling through Toll-like receptor 4. Collectively, these results demonstrate how PT inhibits early inflammatory responses in the respiratory tract, which reduces neutrophil influx in response to B. pertussis infection, potentially providing an advantage to the pathogen in this interaction.

Opsonophagocytic activity and other serological indications of Bordetella pertussis infection in military recruits in Norway

Bordetella pertussis is the causative agent of pertussis (whooping cough). Despite high vaccination coverage, pertussis remains a significant disease in many countries. Besides vaccination, transient carriage of Bordetella spp. or other cross-reacting organisms adds to the immunity against pertussis. However, the various immunological mechanisms conferring protection remain largely unknown. In this study, paired serum samples from 464 healthy Norwegian military recruits were collected, the first at enrolment and the second about 8 months later. The prevalence of pertussis during military service was examined by comparing the paired serum samples for immunoglobulin G (IgG) antibodies against pertussis toxin (PT) by enzyme-linked immunosorbent assay (ELISA). Seventy-eight percent of the recruits had low levels of IgG antibodies against PT in both samples. Conversely, 8.4% of the recruits demonstrated high anti-PT IgG levels in the first sample, indicative of recent pertussis prior to enrolment. One recruit experienced seroconversion, indicating pertussis during service. A subset of 248 serum samples with low, medium, and high anti-PT IgG titers were analyzed by a different ELISA kit for IgG and IgA antibodies against PT and filamentous hemagglutinin (FHA) and for opsonophagocytic activity (OPA), for induction of C3b deposition products, and for IgG binding with live B. pertussis as the antigen. We observed high correlations between OPA and IgG against live bacteria (r = 0.83), between OPA and IgG anti-FHA (r = 0.79), between OPA and anti-PT IgG (r = 0.68), and between OPA and C3b binding (r = 0.70) (P < 0.0001 for all). Anti-PT IgA did not correlate closely with the other assays.

A dual fluorescence flow cytometric analysis of bacterial adherence to mammalian host cells

Flow cytometry has provided a powerful tool for analyzing bacteria-host cell associations. Established approaches have used bacteria, labeled either directly with fluorochromes or indirectly with fluorescently conjugated antibodies, to detect these associations. Although useful, these techniques are consistently unable to include all host cells in the analysis while excluding free, aggregated bacteria. This study describes a new flow cytometry method of assessing bacterial adherence to host cells based on direct fluorescent labeling of both bacteria and host cells. Eukaryotic host cells were labeled with PKH-26, a red fluorescent dye, and bacteria were labeled with fluorescein isothiocyanate, a green fluorescent dye. The red host cells were gated and the mean green fluorescence intensity (MFI) of these red cells was determined. We used MFI values obtained from control samples (unlabeled and labeled host cells with unlabeled bacteria) to eliminate contributions due to autofluorescence. The final MFI values represent fluorescence of host cells resulting from the adherent bacteria. Because all red fluorescent cells are analyzed, this method includes all the eukaryotic cells for analysis but excludes all free or aggregated bacteria that are not bound to target cells.

Adjuvanticity of native and detoxified adenylate cyclase toxin of Bordetella pertussis towards co-administered antigens

The cell-invasive adenylate cyclase toxin (CyaA) of Bordetella pertussis was shown to be highly antigenic in mice, stimulating serum anti-CyaA IgG antibody responses which were able to neutralise the cytotoxic effect of CyaA on J774.2 macrophage-like cells. The effect of co-administration to mice of the fully functional CyaA toxin or a toxin lacking adenylate cyclase enzymic activity (CyaA*) with other antigens from B. pertussis, namely pertussis toxin (PT) or pertussis toxoid (PTd), filamentous haemagglutinin (FHA) and pertactin (PRN), was investigated. CyaA* enhanced the serum IgG antibody responses to each of these antigens whereas, with CyaA, only anti-PRN antibody titres showed a modest increase. Peritoneal macrophages and spleen cells, collected at 2 weeks post-immunisation, were cultured and tested for nitric oxide (NO) and IFNgamma production, respectively, after stimulation in vitro with heat-killed B. pertussis cells or CyaA proteins. NO and IFNgamma production were higher in cells collected from mice immunised with CyaA or CyaA* in combination with a PT, FHA and PRN antigen mixture than from those taken from mice injected with antigen mixture alone, again with CyaA* acting as a better adjuvant than CyaA. The apparent enhancement of immune responses to the antigen mixture by CyaA* in particular was not paralleled by increased protection of mice against aerosol challenge with B. pertussis, but a statistically significant increase in protection was seen after intranasal challenge with B. parapertussis.

Association of Bordetella pertussis with host immune cells in the mouse lung

Mouse models are frequently used to study immunity and pathogenesis to Bordetella pertussis infection. To improve the understanding of the mouse infection model, the influx of host cells and B. pertussis localisation in the lungs were evaluated. Furthermore, the roles of filamentous hemagglutinin (FHA) and fimbriae (Fim) in these processes were determined. B. pertussis infection stimulated the recruitment of polymorphonuclear granulocytes (PMN), alveolar macrophages, and lymphocytes. As determined by double immunofluorescence staining, 2 hr after infection most B. pertussis were free in the alveolar space, some were attached to alveolar epithelia, and some were associated with and phagocytosed by PMN. After 3 days, most bacteria were associated with and phagocytosed by macrophages, some by PMN. B. pertussis was shown not to be ingested by epithelial cells or associated with interstitial macrophages. B. pertussis mutants lacking expression of FHA or Fim were associated with and phagocytosed by the same cell types as parental bacteria. The Fim mutant, however, induced a more severe inflammation, and was cleared faster from the lungs compared to the parental strain and the FHA mutant. These results suggest that Fim does not affect bacterial localisation in the mouse lung, but does influence host immune mechanisms. Possibly, Fim may exert an anti-inflammatory function and thereby inhibit killing by macrophages.

Role of immune sera in the in-vitro phagocytosis of Bordetella pertussis strains

In this study, phagocytosis of Bordetella pertussis was assessed using a human monocyte-derived macrophage line (THP-1) and immune sera from children who had received primary vaccination during the Italian clinical trial on the efficacy of two acellular three-component (PT-FHA-PRN) and one whole-cell pertussis vaccines. The results demonstrate that phagocytosis of opsonized bacteria with specific immune sera is not significantly enhanced compared with that of non-opsonized bacteria or bacteria opsonized with non-immune sera. A similar result was obtained also using B. pertussis strains showing variants of the pertactin antigen suggesting that those variations do not reduce the capability of the bacterium to invade the monocytes.

Pertussis toxin and lipopolysaccharide influence phagocytosis of Bordetella pertussis by human monocytes

The potential of human monocytes to mediate the clearance of Bordetella pertussis infection was examined. Bacteria expressing green fluorescent protein were incubated with adherent peripheral blood monocytes, and phagocytosis was quantified by using fluorescence microscopy. Monocytes internalized only a small percentage of the adherent bacteria. Surface-associated Bvg-regulated virulence factors, including adenylate cyclase toxin and filamentous hemagglutinin, did not affect attachment or phagocytosis. However, 1-h pretreatment with purified pertussis toxin inhibited the ability of monocytes to internalize wild-type bacteria. Mutations affecting the terminal trisaccharide of lipopolysaccharide resulted in reduced internalization without affecting adherence of bacteria to monocytes. Opsonization with human serum played only a modest role in promoting phagocytosis. The viability of internalized bacteria was determined by colony counts following treatment with polymyxin B and gentamicin. Less than 1% of internalized bacteria remained viable. These results suggest that pertussis toxin plays a role in the evasion of monocyte phagocytosis and that these cells represent a potential mediator of the clearance of B. pertussis infection.

Fc receptor-mediated immunity against Bordetella pertussis

The relevance of specific Abs for the induction of cellular effector functions against Bordetella pertussis was studied. IgG-opsonized B. pertussis was efficiently phagocytosed by human polymorphonuclear leukocytes (PMN). This process was mediated by the PMN IgG receptors, FcgammaRIIa (CD32) and FcgammaRIIIb (CD16), working synergistically. Furthermore, these FcgammaR triggered efficient PMN respiratory burst activity and mediated transfer of B. pertussis to lysosomal compartments, ultimately resulting in reduced bacterial viability. Bacteria opsonized with IgA triggered similar PMN activation via FcalphaR (CD89). Simultaneous engagement of FcalphaRI and FcgammaR by B. pertussis resulted in increased phagocytosis rates, compared with responses induced by either isotype alone. These data provide new insights into host immune mechanisms against B. pertussis and document a crucial role for Ig-FcR interactions in immunity to this human pathogen.

Environmental sensing mechanisms in Bordetella

The success of a bacterial pathogen may depend on its ability to sense and respond to different environments. This is particularly true of those pathogens whose survival depends on adaptation to different niches both within and outside the host. Members of the genus Bordetella cause infections in humans, other animals and birds. Two closely related species, B. pertussis and B. bronchiseptica, cause respiratory disease and express a similar range of virulence factors during infection, but exhibit different host ranges and responses to environmental change. B. pertussis has no known reservoir other than humans and is assumed to be transmitted directly via aerosol droplets between hosts. B. bronchiseptica, on the other hand, has the potential to survive and grow in the natural environment. Comparison of the manner in which these two organisms respond to external signals has provided important insights into the co-ordinate regulation of gene expression as a response to a changing environment. During infection, both species produce a range of virulence factors whose expression is co-ordinated by two members of the two-component family of signal transduction proteins, the bvg (bordetella virulence gene) and ris (regulator of intracellular stress response) loci. When active, the bvg locus directs the activity of a number of virulence determinants in both species whose products, such as adhesins and toxins, establish colonization of the host by the bacteria, although each organism has evolved a slightly different strategy during pathogenesis. B. pertussis, the causative agent of whooping cough, promotes an acute disease and tends to be more virulent than B. bronchiseptica which generally causes chronic and persistent asymptomatic colonization of the respiratory tract. The recently identified ris locus appears to control the expression of factors important for intracellular survival of B. bronchiseptica, but a role for this regulatory locus in B. pertussis infection has not been established. Expression of the virulence determinants controlled by the bvg and ris loci is subject to modulation by different environmental signals, such as low temperature, which act through these two-component systems. Evidence indicates that, for B. bronchiseptica, bvg-controlled determinants expressed under modulating conditions, such as motility, facilitate adaptation and survival in environments outside the host. With B. pertussis, however, there is no apparent requirement for prolonged survival outside the host and this difference is reflected in the expression of different, as yet uncharacterized, determinants as a response to modulating signals. The nature of the gene products involved and their assumed role in the life cycle of B. pertussis remains to be determined. Thus, comparative analysis of these species provides an excellent model for understanding the genetic requirements for pathogenesis of respiratory infection and adaptation to changing environments, both within and outside the host.

Immunity to Bordetella pertussis

Bordetella pertussis exploits extracellular and intracellular niches in the respiratory tract and a variety of immune evasion strategies to prolong its survival in the host. This article reviews evidence of complementary roles for cellular and humoral immunity in protection. It discusses the effector mechanisms of bacterial elimination, the strategies employed by the bacteria to subvert protective immune responses and the immunological basis for systemic and neurological responses to infection and vaccination.

Pertussis: an old disease that is still with us

Our knowledge of pathogenesis, clinical presentation and prevention of pertussis has improved substantially over recent years. We now better understand the function of long-known virulence factors for Bordetella pertussis, and genome sequencing has identified a multitude of new proteins; their functions are yet to be elucidated. Furthermore, improved diagnostic tools have revealed the broad spectrum of disease, and new insights into the host's immune response have been gained. Finally, the development, evaluation, licensing and implementation of several new acellular pertussis vaccines with high acceptance have changed the epidemiology of pertussis in many countries (i.e. a shift towards an increasing burden of disease in adolescent persons and adults). These developments are likely to have great impact on the daily practice not only of paediatricians, but also of general practitioners, internists, gynaecologists and many other specialists in adolescent and adult medicine. The present review provides an update on recent progress.

Characterization of bactericidal immune responses following vaccination with acellular pertussis vaccines in adults

Sera from six adults, collected before and after acellular pertussis vaccination, and from a placebo control were examined for the ability to elicit two bactericidal immune defenses, (i) antibody-dependent complement-mediated bacterial lysis and (ii) opsonization and phagocytosis by human neutrophils. The samples were chosen based on low preimmunization titers and strong postimmunization responses to various combinations of vaccine antigens. All but two prevaccination samples demonstrated activity indicative of complement-mediated lysis. Preimmunization activity could have been due to prior infection or childhood immunization. Immunization did not result in improved bactericidal activity for any of the individuals, and in two cases immunization caused a statistically significant decrease in complement-mediated lysis. Similarly, opsonization with the postimmunization sera failed to enhance attachment or phagocytosis of bacteria by neutrophils, and one postimmunization sample with a strong response to filamentous hemagglutinin caused an inhibition of phagocytosis that was statistically significant compared to that observed for the no-serum control. In summary, booster immunization of adults with acellular pertussis vaccines was not found to increase bactericidal activity over preimmunization levels. Identifying ways to promote bactericidal immune responses might improve the efficacy of acellular pertussis vaccines.

Neutralizing antibodies to adenylate cyclase toxin promote phagocytosis of Bordetella pertussis by human neutrophils

A previous study showed that opsonization with human immune serum could either promote or antagonize phagocytosis of Bordetella pertussis by human neutrophils depending on whether the bacteria expressed adenylate cyclase toxin. Opsonization of the wild-type strain inhibited phagocytosis relative to unopsonized controls. In contrast, mutants lacking adenylate cyclase toxin were efficiently phagocytosed when opsonized with human immune serum. In this study, we examined opsonization in the presence or absence of monoclonal antibodies to adenylate cyclase toxin. Addition of neutralizing monoclonal antibodies to adenylate cyclase toxin converted a serum that previously inhibited both attachment and phagocytosis of the wild-type strain to one that increased both attachment and phagocytosis compared to the no-serum control. Monoclonal antibodies that recognize the adenylate cyclase toxin but fail to neutralize activity were without effect. These results suggest that adenylate cyclase toxin inhibits both Fc receptor-mediated attachment and phagocytosis of B. pertussis by neutrophils.

Bordetella pertussis virulence factors affect phagocytosis by human neutrophils

The interaction between human neutrophils and wild-type Bordetella pertussis or mutants expressing altered lipopolysaccharide or lacking virulence factors-pertussis toxin, adenylate cyclase toxin, dermonecrotic toxin, filamentous hemagglutinin (FHA), pertactin, or BrkA-was examined. In the absence of antibodies, the wild-type strain and the mutants, with the exception of mutants lacking FHA, attached efficiently to neutrophils. The addition of opsonizing antibodies caused a significant reduction (approximately 50%) in attachment of the wild-type strain and most of the mutants expressing FHA, suggesting that bacterium-mediated attachment is more efficient than Fc-mediated attachment. Phagocytosis was also examined. In the absence of antibodies, about 12% of the wild-type bacteria were phagocytosed. Opsonization caused a statistically significant reduction in phagocytosis (to 3%), possibly a consequence of reduced attachment. Phagocytosis of most of the mutants was similar to that of the wild type, with the exception of the mutants lacking adenylate cyclase toxin. About 70% of the adenylate cyclase toxin mutants were phagocytosed, but only in the presence of opsonizing antibody, suggesting that Fc receptor-mediated signaling may be needed for phagocytosis. These studies indicate that FHA mediates attachment of B. pertussis to neutrophils, but adenylate cyclase toxin blocks phagocytosis.