In vitro innate immune cell based models to assess whole cell Bordetella pertussis vaccine quality: a proof of principle

Overcoming scientific barriers in the transition from in vivo to non-animal batch testing of human and veterinary vaccines

INTRODUCTION

Before release, vaccine batches are assessed for quality to evaluate whether they meet the product specifications. Vaccine batch tests, in particular of inactivated and toxoid vaccines, still largely rely on in vivo methods. Improved vaccine production processes, ethical concerns, and suboptimal performance of some in vivo tests have led to the development of in vitro alternatives.

AREAS COVERED

This review describes the scientific constraints that need to be overcome for replacement of in vivo batch tests, as well as potential solutions. Topics include the critical quality attributes of vaccines that require testing, the use of cell-based assays to mimic aspects of in vivo vaccine-induced immune responses, how difficulties with testing adjuvanted vaccines in vitro can be overcome, the use of altered batches to validate new in vitro test methods, and how cooperation between different stakeholders is key to moving the transition forward.

EXPERT OPINION

For safety testing, many in vitro alternatives are already available or at an advanced level of development. For potency testing, in vitro alternatives largely comprise immunochemical methods that assess several, but not all critical vaccine properties. One-to-one replacement by in vitro alternatives is not always possible and a combination of methods may be required.

Nitric Oxide Production and Fc Receptor-Mediated Phagocytosis as Functional Readouts of Macrophage Activity upon Stimulation with Inactivated Poultry Vaccines In Vitro

Vaccine batches must pass routine quality control to confirm that their ability to induce protection against disease is consistent with batches of proven efficacy from development studies. For poultry vaccines, these tests are often performed in laboratory chickens by vaccination-challenge trials or serological assays. The aim of this study was to investigate innate immune responses against inactivated poultry vaccines and identify candidate immune parameters for in vitro quality tests as alternatives for animal-based quality tests. For this purpose, we set up assays to measure nitric oxide production and phagocytosis by the macrophage-like cell line HD11, upon stimulation with inactivated poultry vaccines for infectious bronchitis virus (IBV), Newcastle disease virus (NDV), and egg drop syndrome virus (EDSV). In both assays, macrophages became activated after stimulation with various toll-like receptor agonists. Inactivated poultry vaccines stimulated HD11 cells to produce nitric oxide due to the presence of mineral oil adjuvant. Moreover, inactivated poultry vaccines were found to enhance Fc receptor-mediated phagocytosis due to the presence of allantoic fluid in the vaccine antigen preparations. We showed that inactivated poultry vaccines stimulated nitric oxide production and Fc receptor-mediated phagocytosis by chicken macrophages. Similar to antigen quantification methods, the cell-based assays described here can be used for future assessment of vaccine batch-to-batch consistency. The ability of the assays to determine the immunopotentiating properties of inactivated poultry vaccines provides an additional step in the replacement of current in vivo batch-release quality tests.

A PBMC-Based System to Assess Human T Cell Responses to Influenza Vaccine Candidates In Vitro

Vaccine development is an expensive and time-consuming process that heavily relies on animal models. Yet, vaccine candidates that have previously succeeded in animal experiments often fail in clinical trials questioning the predictive value of animal models. Alternative assay systems that can add to the screening and evaluation of functional characteristics of vaccines in a human context before embarking on costly clinical trials are therefore urgently needed. In this study, we have established an in vitro system consisting of long-term cultures of unfractionated peripheral blood mononuclear cells (PBMCs) from healthy volunteers to assess (recall) T cell responses to vaccine candidates. We observed that different types of influenza vaccines (whole inactivated virus (WIV), split, and peptide vaccines) were all able to stimulate CD4 and CD8 T cell responses but to different extents in line with their reported in vivo properties. In-depth analyses of different T cell subsets revealed that the tested vaccines evoked mainly recall responses as indicated by the fact that the vast majority of the responding T cells had a memory phenotype. Furthermore, we observed vaccine-induced activation of T follicular helper cells, which are associated with the induction of humoral immune responses. Our results demonstrate the suitability of the established PBMC-based system for the in vitro evaluation of memory T cell responses to vaccines and the comparison of vaccine candidates in a human immune cell context. As such, it can help to bridge the gap between animal experiments and clinical trials and assist in the selection of promising vaccine candidates, at least for recall antigens.

Functional Programming of Innate Immune Cells in Response to Bordetella pertussis Infection and Vaccination

Despite widespread vaccination, B. pertussis remains one of the least controlled vaccine-preventable diseases. Although it is well known that acellular and whole cell pertussis vaccines induce distinct immune functionalities in memory cells, much less is known about the role of innate immunity in this process. In this review, we provide an overview of the known differences and similarities in innate receptors, innate immune cells and inflammatory signalling pathways induced by the pertussis vaccines either licensed or in development and compare this to primary infection with B. pertussis. Despite the crucial role of innate immunity in driving memory responses to B. pertussis, it is clear that a significant knowledge gap remains in our understanding of the early innate immune response to vaccination and infection. Such knowledge is essential to develop the next generation of pertussis vaccines with improved host defense against B. pertussis.

Distinctive Responses in an In Vitro Human Dendritic Cell-Based System upon Stimulation with Different Influenza Vaccine Formulations

Vaccine development relies on testing vaccine candidates in animal models. However, results from animals cannot always be translated to humans. Alternative ways to screen vaccine candidates before clinical trials are therefore desirable. Dendritic cells (DCs) are the main orchestrators of the immune system and the link between innate and adaptive responses. Their activation by vaccines is an essential step in vaccine-induced immune responses. We have systematically evaluated the suitability of two different human DC-based systems, namely the DC-cell line MUTZ-3 and primary monocyte-derived DCs (Mo-DCs) to screen immunopotentiating properties of vaccine candidates. Two different influenza vaccine formulations, whole inactivated virus (WIV) and subunit (SU), were used as model antigens as they represent a high immunogenic and low immunogenic vaccine, respectively. MUTZ-3 cells were restricted in their ability to respond to different stimuli. In contrast, Mo-DCs readily responded to WIV and SU in a vaccine-specific way. WIV stimulation elicited a more vigorous induction of activation markers, immune response-related genes and secretion of cytokines involved in antiviral responses than the SU vaccine. Furthermore, Mo-DCs differentiated from freshly isolated and freeze/thawed peripheral blood mononuclear cells (PBMCs) showed a similar capacity to respond to different vaccines. Taken together, we identified human PBMC-derived Mo-DCs as a suitable platform to evaluate vaccine-induced immune responses. Importantly, we show that fresh and frozen PBMCs can be used indistinctly, which strongly facilitates the routine use of this system. In vitro vaccine pre-screening using human Mo-DCs is thus a promising approach for evaluating the immunopotentiating capacities of new vaccine formulations that have not yet been tested in humans.

Proteome Analysis Is a Valuable Tool to Monitor Antigen Expression during Upstream Processing of Whole-Cell Pertussis Vaccines

Physicochemical and immunochemical assays were applied to substantiate the relation between upstream processing and the quality of whole-cell pertussis vaccines. Bordetella pertussis bacteria were cultured on a chemically defined medium using a continuous cultivation process in stirred tank reactors to obtain uniform protein expression. Continuous culture favors the consistent production of proteins known as virulence factors. Magnesium sulfate was added during the steady state of the culture in order to diminish the expression of virulence proteins. Changes in gene expression and antigen composition were measured by microarrays, mass spectrometry and ELISA. Transcriptome and proteome data revealed high similarity between the biological triplicates demonstrating consistent cultivation of B. pertussis. The addition of magnesium sulfate resulted in an instant downregulation of the virulence genes in B. pertussis, but a gradual decrease of virulence proteins. The quantity of virulence proteins concurred highly with the potency of the corresponding whole-cell pertussis vaccines, which were determined by the Kendrick test. In conclusion, proteome analysis provided detailed information on the composition and proportion of virulence proteins present in the whole-cell preparations of B. pertussis. Moreover, proteome analysis is a valuable method to monitor the production process of whole-cell biomass and predict the product quality of whole-cell pertussis vaccines.

Vaccine-Mediated Activation of Human TLR4 Is Affected by Modulation of Culture Conditions during Whole-Cell Pertussis Vaccine Preparation

The potency of whole-cell pertussis (wP) vaccines is still determined by an intracerebral mouse protection test. To allow development of suitable in vitro alternatives to this test, insight into relevant parameters to monitor the consistency of vaccine quality is essential. To this end, a panel of experimental wP vaccines of varying quality was prepared by sulfate-mediated suppression of the BvgASR master virulence regulatory system of Bordetella pertussis during cultivation. This system regulates the transcription of a range of virulence proteins, many of which are considered important for the induction of effective host immunity. The protein compositions and in vivo potencies of the vaccines were BvgASR dependent, with the vaccine containing the highest amount of virulence proteins having the highest in vivo potency. Here, the capacities of these vaccines to stimulate human Toll-like receptors (hTLR) 2 and 4 and the role these receptors play in wP vaccine-mediated activation of antigen-presenting cells in vitro were studied. Prolonged BvgASR suppression was associated with a decreased capacity of vaccines to activate hTLR4. In contrast, no significant differences in hTLR2 activation were observed. Similarly, vaccine-induced activation of MonoMac-6 and monocyte-derived dendritic cells was strongest with the highest potency vaccine. Blocking of TLR2 and TLR4 showed that differences in antigen-presenting cell activation could be largely attributed to vaccine-dependent variation in hTLR4 signalling. Interestingly, this BvgASR-dependent decrease in hTLR4 activation coincided with a reduction in GlcN-modified lipopolysaccharides in these vaccines. Accordingly, expression of the lgmA-C genes, required for this glucosamine modification, was significantly reduced in bacteria exposed to sulfate. Together, these findings demonstrate that the BvgASR status of bacteria during wP vaccine preparation is critical for their hTLR4 activation capacity and suggest that including such parameters to assess consistency of newly produced vaccines could bring in vitro testing of vaccine quality a step closer.

Adaptive immune response to whole cell pertussis vaccine reflects vaccine quality: A possible complementation to the Pertussis Serological Potency test

Whole cell Bordetella pertussis (wP) vaccines are still used in many countries to protect against the respiratory disease pertussis. The potency of whole-cell pertussis vaccine lots is determined by an intracerebral challenge test (the Kendrick test). This test is criticized due to lack of immunological relevance of the read-out after an intracerebral challenge with B. pertussis. The alternative in vivo test, which assesses specific antibody levels in serum after wP vaccination, is the Pertussis Serological Potency test (PSPT). Although the PSPT focuses on a parameter that contributes to protection, the protective immune mechanisms after wP vaccination includes more elements than specific antibody responses only. In this study, additional parameters were investigated, i.e. circulating pro-inflammatory cytokines, antibody specificity and T helper cell responses and it was evaluated whether they can be used as complementary readout parameters in the PSPT to assess wP lot quality. By deliberate manipulation of the vaccine preparation procedure, a panel of high, intermediate and low quality wP vaccines were made. The results revealed that these vaccines induced similar IL-6 and IP10 levels in serum 4h after vaccination (innate responses) and similar antibody levels directed against the entire bacterium. In contrast, the induced antibody specificity to distinct wP antigens differed after vaccination with high, intermediate and low quality wP vaccines. In addition, the magnitude of wP-induced Th cell responses (Th17, Th1 and Th2) was reduced after vaccination with a wP vaccine of low quality. T cell responses and antibody specificity are therefore correlates of qualitative differences in the investigated vaccines, while the current parameter of the PSPT alone was not sensitive enough to distinguish between vaccines of different qualities. This study demonstrates that assessment of the magnitude of Th cell responses and the antigen specificity of antibodies induced by wP vaccination could form valuable complementary parameters to the PSPT.

Sampling From the Proteome to the Human Leukocyte Antigen-DR (HLA-DR) Ligandome Proceeds Via High Specificity

Comprehensive analysis of the complex nature of the Human Leukocyte Antigen (HLA) class II ligandome is of utmost importance to understand the basis for CD4⁺ T cell mediated immunity and tolerance. Here, we implemented important improvements in the analysis of the repertoire of HLA-DR-presented peptides, using hybrid mass spectrometry-based peptide fragmentation techniques on a ligandome sample isolated from matured human monocyte-derived dendritic cells (DC). The reported data set constitutes nearly 14 thousand unique high-confident peptides, i.e. the largest single inventory of human DC derived HLA-DR ligands to date. From a technical viewpoint the most prominent finding is that no single peptide fragmentation technique could elucidate the majority of HLA-DR ligands, because of the wide range of physical chemical properties displayed by the HLA-DR ligandome. Our in-depth profiling allowed us to reveal a strikingly poor correlation between the source proteins identified in the HLA class II ligandome and the DC cellular proteome. Important selective sieving from the sampled proteome to the ligandome was evidenced by specificity in the sequences of the core regions both at their N- and C- termini, hence not only reflecting binding motifs but also dominant protease activity associated to the endolysosomal compartments. Moreover, we demonstrate that the HLA-DR ligandome reflects a surface representation of cell-compartments specific for biological events linked to the maturation of monocytes into antigen presenting cells. Our results present new perspectives into the complex nature of the HLA class II system and will aid future immunological studies in characterizing the full breadth of potential CD4⁺ T cell epitopes relevant in health and disease.