An in vitro assay system as a potential replacement for the histamine sensitisation test for acellular pertussis based combination vaccines

Safety assessments of recombinant DTaP vaccines developed in South Korea

Purpose

Pertussis bacteria have many pathogenic and virulent antigens and severe adverse reactions have occurred when using inactivated whole-cell pertussis vaccines. Therefore, inactivated acellular pertussis (aP) vaccines and genetically detoxified recombinant pertussis (rP) vaccines are being developed. The aim of this study was to assess the safety profile of a novel rP vaccine under development in comparison to commercial diphtheria-tetanus-acellular pertussis (DTaP) vaccines.

Materials and Methods

The two positive control DTaP vaccines (two- and tri-components aP vaccines) and two experimental recombinant DTaP (rDTaP) vaccine (two- and tri-components aP vaccines adsorbed to either aluminum hydroxide or purified oat beta-glucan) were used. Temperature histamine sensitization test (HIST), indirect Chinese hamster ovary (CHO) cell cluster assay, mouse-weight-gain (MWG) test, leukocytosis promoting (LP) test, and intramuscular inflammatory cytokine assay of the injection site performed for safety assessments.

Results

HIST results showed absence of residual pertussis toxin (PTx) in both control and experimental DTaP vaccine groups, whereas in groups immunized with tri-components vaccines, the experimental tri-components rDTaP absorbed to alum showed an ultra-small amount of 0.0066 IU/mL. CHO cell clustering was observed from 4 IU/mL in all groups. LP tests showed that neutrophils and lymphocytes were in the normal range in all groups immunized with the two components vaccine. However, in the tri-components control DTaP vaccine group, as well as two- and tri-components rDTaP with beta-glucan group, a higher monocyte count was observed 3 days after vaccination, although less than 2 times the normal range. In the MWG test, both groups showed changes less than 20% in body temperature and body weight before the after the final immunizations. Inflammatory cytokines within the muscle at the injection site on day 3 after intramuscular injection revealed no significant response in all groups.

Conclusion

There were no findings associated with residual PTx, and no significant differences in both local and systemic adverse reactions in the novel rDTaP vaccine compared to existing available DTaP vaccines. The results suggest that the novel rDTaP vaccine is safe.

Collaborative study for the calibration of the replacement International Standard for pertussis toxin for use in histamine sensitisation and CHO cell clustering assays

Pertussis toxin (PT) in its detoxified form is one of the major protective antigens in vaccines against Bordetella pertussis (whooping cough). Reference preparations of native PT are required for the quality control of pertussis vaccines. Stocks of the first WHO International Standard (IS) for PT (JNIH-5) were low and a replacement was required. One candidate material was donated by a vaccine manufacturer to NIBSC. It was formulated, lyophilised into sealed glass ampoules and coded 15/126. An international collaborative study assessed the suitability of this material to replace JNIH-5. Fourteen laboratories from 12 countries took part in the study. Eleven laboratories performed lethal murine histamine sensitisation assay (HIST), 14 performed Chinese Hamster Ovary (CHO) cell clustering assay. International Units (IU) were assigned to the material using these assays as they were used to assign units to JNIH-5. It was found that, unlike JNIH-5, the activities of 15/126 in HIST and CHO cell assays did not agree and therefore different unitage for each assay was assigned. The preparation 15/126 was established as the Second WHO IS for PT for HIST and CHO cell assays. It was assigned a unitage of 1,881 IU/ampoule in HIST and 680 IU/ampoule in the CHO cell clustering assay.

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.

An ELISA method to estimate the mono ADP-ribosyltransferase activities: e.g in pertussis toxin and vaccines

ADP-ribosyltransferase activities have been observed in many prokaryotic and eukaryotic species and viruses and are involved in many cellular processes, including cell signalling, DNA repair, gene regulation and apoptosis. In a number of bacterial toxins, mono ADP-ribosyltransferase is the main cause of host cell cytotoxicity. Several approaches have been used to analyse this biological system from measuring its enzyme products to its functions. By using a mono ADP-ribose binding protein we have now developed an ELISA method to estimate native pertussis toxin mono ADP-ribosyltransferase activity and its residual activities in pertussis vaccines as an example. This new approach is easy to perform and adaptable in most laboratories. In theory, this assay system is also very versatile and could measure the enzyme activity in other bacteria such as Cholera, Clostridium, E. coli, Diphtheria, Pertussis, Pseudomonas, Salmonella and Staphylococcus by just switching to their respective peptide substrates. Furthermore, this mono ADP-ribose binding protein could also be used for staining mono ADP-ribosyl products resolved on gels or membranes.

Reporter cell lines for detection of pertussis toxin in acellular pertussis vaccines as a functional animal-free alternative to the in vivo histamine sensitization test

Detoxified pertussis toxin (pertussis toxoid) is a major antigen in acellular pertussis vaccines. Testing these vaccines on the presence of residual pertussis toxin (PTx) and reversion to toxicity is performed by the regulatory required in vivo Histamine Sensitization test (HIST). Lack of mechanistic understanding of the HIST, technical handicaps and animal welfare concerns, have promoted the development of alternative methods. As the majority of the cellular effects of PTx depend on its ability to activate intracellular pathways involving cAMP, the in vitro cAMP-PTx assay was developed. Although this assay could be used to detect PTx activity, it lacked sensitivity and robustness for use in a quality control setting. In the present study, novel reporter cell lines (CHO-CRE and A10-CRE) were generated that stably express a reporter construct responsive to changes in intracellular cAMP levels. These reporter cell lines were able to detect PTx in a concentration-dependent manner when combined with fixed amounts of forskolin. The CHO-CRE cell line enabled detection of PTx in the context of a multivalent vaccine containing aP, with a sensitivity equal to the HIST. However, the sensitivity of the A10-CRE cells was insufficient for this purpose. The experiments also suggest that the CHO-CRE reporter cell line might be suitable for assessment of cellular effects of PTd reverted to PTx. The CHO-CRE reporter cell line provides a platform that meets the criteria for specificity and sensitivity and is a promising in vitro model with potential to replace the HIST.

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.

Identification of biomarkers to detect residual pertussis toxin using microarray analysis of dendritic cells

In this study we aimed to identify genes that are responsive to pertussis toxin (PTx) and might eventually be used as biological markers in a testing strategy to detect residual PTx in vaccines. By microarray analysis we screened six human cell types (bronchial epithelial cell line BEAS-2B, fetal lung fibroblast cell line MRC-5, primary cardiac microvascular endothelial cells, primary pulmonary artery smooth muscle cells, hybrid cell line EA.Hy926 of umbilical vein endothelial cells and epithelial cell line A549 and immature monocyte-derived dendritic cells) for differential gene expression induced by PTx. Immature monocyte-derived dendritic cells (iMoDCs) were the only cells in which PTx induced significant differential expression of genes. Results were confirmed using different donors and further extended by showing specificity for PTx in comparison to Escherichia coli lipopolysaccharide (LPS) and Bordetella pertussis lipo-oligosaccharide (LOS). Statistical analysis indicated 6 genes, namely IFNG, IL2, XCL1, CD69, CSF2 and CXCL10, as significantly upregulated by PTx which was also demonstrated at the protein level for genes encoding secreted proteins. IL-2 and IFN-γ gave the strongest response. The minimal PTx concentrations that induced production of IL-2 and IFN-γ in iMoDCs were 12.5 and 25IU/ml, respectively. High concentrations of LPS slightly induced IFN-γ but not IL-2, while LOS and detoxified pertussis toxin did not induce production of either cytokine. In conclusion, using microarray analysis we evaluated six human cell lines/types for their responsiveness to PTx and found 6 PTx-responsive genes in iMoDCs of which IL2 is the most promising candidate to be used as a biomarker for the detection of residual PTx.

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.

Bioassays for evaluation of medical products derived from bacterial toxins

Bioassays play central role in evaluation of biological products and those derived from bacterial toxins often rely exclusively on in vivo models for assurance of safety and potency. This chapter reviews existing regulatory approved methods designed to provide information on potency and safety of complex biological medicines with an insight into strategies considered for alternative procedures.