Development of a carbohydrate binding assay for the B-oligomer of pertussis toxin and toxoid

Structural Basis for Antibody Neutralization of Pertussis Toxin

Pertussis toxin (PT) is a key protective antigen in vaccine- and natural immunity-mediated protection from Bordetella pertussis infection. Despite its importance, no PT-neutralizing epitopes have been characterized structurally. To define neutralizing epitopes and identify key structural elements to preserve during PT antigen design, we determined a 3.6 Å cryo-electron microscopy structure of genetically detoxified PT (PTg) bound to hu11E6 and hu1B7, two potently neutralizing anti-PT antibodies with complementary mechanisms: disruption of toxin adhesion to cells and intracellular activities, respectively. Hu11E6 bound the paralogous S2 and S3 subunits of PTg via a conserved epitope, but surprisingly did not span the sialic acid binding site implicated in toxin adhesion. High-throughput glycan array analysis showed that hu11E6 specifically prevents PTg binding to sialylated N-glycans, while a T cell activation assay showed that hu11E6 blocks PTg mitogenic activities to define the neutralizing mechanism. Hu1B7 bound a quaternary epitope spanning the S1 and S5 subunits, although functional studies of hu1B7 variants suggested that S5 binding is not involved in its PT neutralization mechanism. These results are the first to structurally define neutralizing epitopes on PT, improving our molecular understanding of immune protection from B. pertussis and providing key information for the future development of PT immunogens.

In Vivo Models and In Vitro Assays for the Assessment of Pertussis Toxin Activity

One of the main virulence factors produced by Bordetella pertussis is pertussis toxin (PTx) which, in its inactivated form, is the major component of all marketed acellular pertussis vaccines. PTx ADP ribosylates Gα_i proteins, thereby affecting the inhibition of adenylate cyclases and resulting in the accumulation of cAMP. Apart from this classical model, PTx also activates some receptors and can affect various ADP ribosylation- and adenylate cyclase-independent signalling pathways. Due to its potent ADP-ribosylation properties, PTx has been used in many research areas. Initially the research primarily focussed on the in vivo effects of the toxin, including histamine sensitization, insulin secretion and leukocytosis. Nowadays, PTx is also used in toxicology research, cell signalling, research involving the blood–brain barrier, and testing of neutralizing antibodies. However, the most important area of use is testing of acellular pertussis vaccines for the presence of residual PTx. In vivo models and in vitro assays for PTx often reflect one of the toxin’s properties or details of its mechanism. Here, the established and novel in vivo and in vitro methods used to evaluate PTx are reviewed, their mechanisms, characteristics and limitations are described, and their application for regulatory and research purposes are considered.

iGIST-A Kinetic Bioassay for Pertussis Toxin Based on Its Effect on Inhibitory GPCR Signaling

Detection of pertussis toxin (PTX) activity is instrumental for the development and manufacturing of pertussis vaccines. These quality and safety measures require thousands of mice annually. Here, we describe Interference in Gαi-mediated Signal Transduction (iGIST), an animal-free kinetic bioassay for detection of PTX, by measuring its effect on inhibitory G protein-coupled receptor (GPCR) signaling. PTX ADP-ribosylates inhibitory α-subunits of the heterotrimeric G proteins, thereby perturbing the inhibitory GPCR signaling. iGIST is based on HEK293 cells coexpressing a somatostatin receptor 2 (SSTR2), which is an inhibitory GPCR controllable by a high-affinity agonist octreotide; and a luminescent 3'5'-cyclic adenosine monophosphate (cAMP) probe. iGIST has a low sensitivity threshold in the pg/mL range of PTX, surpassing by 100-fold in a parallel analysis the currently used in vitro end-point technique to detect PTX, the cluster formation assay (CFA) in Chinese hamster ovary cells. iGIST also detects PTX in complex samples, i.e., a commercial PTX-toxoid-containing pertussis vaccine that was spiked with an active PTX. iGIST has an objective digital readout and is observer independent, offering prospects for automation. iGIST emerges as a promising animal-free alternative to detect PTX activity in the development and manufacturing of pertussis vaccines. iGIST is also expected to facilitate basic PTX research, including identification and characterization of novel compounds interfering with PTX.

Assays for Determining Pertussis Toxin Activity in Acellular Pertussis Vaccines

Whooping cough is caused by the bacterium Bordetella pertussis. There are currently two types of vaccines that can prevent the disease; whole cell vaccines (WCV) and acellular vaccines (ACV). The main virulence factor produced by the organism is pertussis toxin (PTx). This toxin is responsible for many physiological effects on the host, but it is also immunogenic and in its detoxified form is the main component of all ACVs. In producing toxoid for vaccines, it is vital to achieve a balance between sufficiently detoxifying PTx to render it safe while maintaining enough molecular structure that it retains its protective immunogenicity. To ensure that the first part of this balancing act has been successfully achieved, assays are required to accurately measure residual PTx activity in ACV products accurately. Quality control assays are also required to ensure that the detoxification procedures are robust and stable. This manuscript reviews the methods that have been used to achieve this aim, or may have the potential to replace them, and highlights their continuing requirement as vaccines that induce a longer lasting immunity are developed to prevent the re-occurrence of outbreaks that have been observed recently.

Safety testing of acellular pertussis vaccines: Use of animals and 3Rs alternatives

The current test of acellular Bordetella pertussis (aP) vaccines for residual pertussis toxin (PTx) is the Histamine Sensitization test (HIST), based on the empirical finding that PTx sensitizes mice to histamine. Although HIST has ensured the safety of aP vaccines for years, it is criticized for the limited understanding of how it works, its technical difficulty, and for animal welfare reasons. To estimate the number of mice used worldwide for HIST, we surveyed major aP manufacturers and organizations performing, requiring, or recommending the test. The survey revealed marked regional differences in regulatory guidelines, including the number of animals used for a single test. Based on information provided by the parties surveyed, we estimated the worldwide number of mice used for testing to be 65,000 per year: ∼48,000 by manufacturers and ∼17,000 by national control laboratories, although the latter number is more affected by uncertainty, due to confidentiality policies. These animals covered the release of approximately 850 final lots and 250 in-process lots of aP vaccines yearly. Although there are several approaches for HIST refinement and reduction, we discuss why the efforts needed for validation and implementation of these interim alternatives may not be worthwhile, when there are several in vitro alternatives in various stages of development, some already fairly advanced. Upon implementation, one or more of these replacement alternatives can substantially reduce the number of animals currently used for the HIST, although careful evaluation of each alternative's mechanism and its suitable validation will be necessary in the path to implementation.

Investigation in a murine model of possible mechanisms of enhanced local reactions to post-primary diphtheria-tetanus toxoid boosters in recipients of acellular pertussis-diphtheria-tetanus vaccine

In recipients primed with acellular pertussis diphtheria-tetanus combined vaccine (DTaP) an increased incidence of severe local reactions with extensive redness/swelling has been reported for each subsequent dose of diphtheria-tetanus based combination vaccine given as a booster. This has been attributed to residual active pertussis toxin (PT) in the primary vaccine. In this study, we investigated the possible contribution of the A-subunit enzymatic activity and the B-oligomer carbohydrate binding activity of residual PT in DTaP to local reactions in a murine model using Japanese DTaP batches produced before and after the introduction of a test for reversion of pertussis toxoid to toxin. Residual PT activity was correlated with the B-oligomer carbohydrate binding activity. The in vivo mouse footpad swelling model assay indicated that the B-oligomer carbohydrate binding activity and possibly other factors were associated with intensified sensitization to local reaction following diphtheria toxoid booster.

A global regulatory science agenda for vaccines

The Decade of Vaccines Collaboration and development of the Global Vaccine Action Plan provides a catalyst and unique opportunity for regulators worldwide to develop and propose a global regulatory science agenda for vaccines. Regulatory oversight is critical to allow access to vaccines that are safe, effective, and of assured quality. Methods used by regulators need to constantly evolve so that scientific and technological advances are applied to address challenges such as new products and technologies, and also to provide an increased understanding of benefits and risks of existing products. Regulatory science builds on high-quality basic research, and encompasses at least two broad categories. First, there is laboratory-based regulatory science. Illustrative examples include development of correlates of immunity; or correlates of safety; or of improved product characterization and potency assays. Included in such science would be tools to standardize assays used for regulatory purposes. Second, there is science to develop regulatory processes. Illustrative examples include adaptive clinical trial designs; or tools to analyze the benefit-risk decision-making process of regulators; or novel pharmacovigilance methodologies. Included in such science would be initiatives to standardize regulatory processes (e.g., definitions of terms for adverse events [AEs] following immunization). The aim of a global regulatory science agenda is to transform current national efforts, mainly by well-resourced regulatory agencies, into a coordinated action plan to support global immunization goals. This article provides examples of how regulatory science has, in the past, contributed to improved access to vaccines, and identifies gaps that could be addressed through a global regulatory science agenda. The article also identifies challenges to implementing a regulatory science agenda and proposes strategies and actions to fill these gaps. A global regulatory science agenda will enable regulators, academics, and other stakeholders to converge around transformative actions for innovation in the regulatory process to support global immunization goals.

Report on the international workshop on alternatives to the murine histamine sensitization test (HIST) for acellular pertussis vaccines: state of the science and the path forward

Regulatory authorities require safety and potency testing prior to the release of each production lot of acellular pertussis (aP)-containing vaccines. Currently, the murine histamine sensitization test (HIST) is used to evaluate the presence of residual pertussis toxin in aP containing vaccines. However, the testing requires the use of a significant number of mice and results in unrelieved pain and distress. NICEATM, ICCVAM, their partners in the International Cooperation on Alternative Test Methods, and the International Working Group for Alternatives to HIST organized a workshop to discuss recent developments in alternative assays to the HIST, review data from an international collaborative study on non-animal alternative tests that might replace the HIST, and address the path toward global acceptance of this type of method. Currently, there are three potential alternative methods to HIST. Participants agreed that no single in vitro method was sufficiently developed for harmonized validation studies at this time. It is unlikely that any single in vitro method would be applicable to all aP vaccines without modification, due to differences between vaccines. Workshop participants recommended further optimization of cell-based assays under development. Participants agreed that the next international collaborative studies should commence in 2013 based on discussions during this workshop.

Confocal microscopy study of pertussis toxin and toxoids on CHO-cells

Pertussis toxin in its detoxified form is a major component of all current acellular pertussis vaccines. Here we report the membrane translocation and internalization activities of pertussis toxin and various pertussis toxoids using Chinese hamster ovary cells and confocal microscopy based on indirect immunofluorescence labeling. Chemically detoxified pertussis toxoids were able to translocate/internalize into cells at the concentration about 1,000 times higher than the native toxin. Pertussis toxoids detoxified with different procedures (glutaraldehyde, glutaraldehyde plus formaldehyde, hydrogen peroxide or genetic mutation) showed differences in fluorescence intensity under the same condition, indicating toxoids from different detoxification methods may have different translocation/internalization activities on cells.

Improved protocols for histamine sensitization testing of acellular pertussis vaccines

The histamine sensitization test is a widely used method for measuring the residual toxicity of pertussis toxin in acellular pertussis vaccines. Although it has been used as a routine assay for decades, the current protocols are difficult to standardize because the test results vary considerably and are based on several factors, including mouse strain, age and sex. In this study, we observed that mice of strains CD1, ddY and C57/BL6 were sufficiently sensitive to pertussis toxin among six mice strains tested and that aged male mice were more sensitive to pertussis toxin than younger or female mice. Using this animal model, we showed pertussis toxin dose-dependent responses in the two histamine sensitization test protocols based on either lethal end-point determination or mouse rectal temperature measurement. Sensitivity to pertussis toxin was further enhanced by the addition of lipopolysaccharide in both methods. With these improvements, pertussis toxin activity can be estimated more accurately and reproducibly using a reduced number of animals.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for the period 2005-2006

This review is the fourth update of the original review, published in 1999, on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2006. The review covers fundamental studies, fragmentation of carbohydrate ions, method developments, and applications of the technique to the analysis of different types of carbohydrate. Specific compound classes that are covered include carbohydrate polymers from plants, N- and O-linked glycans from glycoproteins, glycated proteins, glycolipids from bacteria, glycosides, and various other natural products. There is a short section on the use of MALDI-TOF mass spectrometry for the study of enzymes involved in glycan processing, a section on industrial processes, particularly the development of biopharmaceuticals and a section on the use of MALDI-MS to monitor products of chemical synthesis of carbohydrates. Large carbohydrate-protein complexes and glycodendrimers are highlighted in this final section.

Identification and characterization of the carbohydrate ligands recognized by pertussis toxin via a glycan microarray and surface plasmon resonance

Binding of pertussis toxin (PTx) was examined by a glycan microarray; 53 positive hits fell into four general groups. One group represents sialylated biantennary compounds with an N-glycan core terminating in alpha2-6-linked sialic acid. The second group consists of multiantennary compounds with a canonical N-glycan core, but lacking terminal sialic acids, which represents a departure from the previous understanding of PTx binding to N-glycans. The third group consists of Neu5Acalpha2-3(lactose or N-acetyllactosamine) forms that lack the branched mannose core found in N-glycans; thus, their presentation is more similar to that of O-linked glycans and glycolipids. The fourth group of compounds consists of Neu5Acalpha2-8Neu5Acalpha2-8Neu5Ac, which is seen in the c series gangliosides and some N-glycans. Quantitative analysis by surface plasmon resonance of the relative affinities of PTx for terminal Neu5Acalpha2-3 versus Neu5Acalpha2-6, as well as the affinities for the trisaccharide Neu5Acalpha2-8Neu5Acalpha2-8Neu5Ac versus disaccharide, revealed identical global affinities, even though the amount of bound glycan varied by 4-5-fold. These studies suggest that the conformational space occupied by a glycan can play an important role in binding, independent of affinity. Characterization of N-terminal and C-terminal binding sites on the S2 and S3 subunits by mutational analysis revealed that binding to all sialylated compounds was mediated by the C-terminal binding sites, and binding to nonsialylated N-linked glycans is mediated by the N-terminal sites present on both the S2 and S3 subunits. A detailed understanding of the glycans recognized by pertussis toxin is essential to understanding which cells are targeted in clinical disease.

ADP-ribosylation activity in pertussis vaccines and its relationship to the in vivo histamine-sensitisation test

Pertussis toxin (PTx) is a major virulence factor produced by Bordetella pertussis. In its detoxified form (PTd), it is an important component of acellular pertussis vaccines although some residual PTx activity may likely be present because of the limitations of the detoxification processes used. Furthermore, different detoxification procedures have been shown to result in different amino acid side-chain modifications for the resulting PTd. The histamine-sensitisation test (HIST) in mice is currently used for the safety testing of these vaccines. However, an alternative test is needed because of large assay variability and ethical concerns. The ADP-ribosylation enzyme activity of PTx is thought to be the major factor responsible for the histamine-sensitising activity detected in vivo. In the present study, the ADP-ribosylation activity in different acellular pertussis-based combination vaccine formulations was measured and compared with reactivity in the HIST. The results indicated that different products showed differences in ADP-ribosylation activity and a level which would be significant in relation to the reactivity seen in the HIST could not be defined, except for vaccines that contain genetically detoxified PTx, which do not have enzymatic activity nor in vivo toxicity. Different detoxification procedures as well as formulation factors could contribute to this variation. Relying solely on the residual enzyme activity of PTx in vaccines containing chemically detoxified PTd may not fully reflect the in vivo reactivity observed by the HIST. Refinement of the in vitro test to include a step which monitors the B-subunit activity of PTx may provide a better correlation with the in vivo HIST.