Introduction
The aim of this function is to conduct detailed immunogenicity studies, even in the early stages of development, in order to guide decisions for example on the optimal doses or routes of vaccination. A range of doses should be evaluated in small animal models of immunogenicity and protection in order to understand the relationship between the two and inform dose selection in nonhuman primates (NHPs) challenge studies where it is not feasible to test multiple doses. An understanding of the immunology and protection associated with a range of doses and routes should help guide dose selection in later preclinical and early clinical development. In addition, in-depth mechanistic studies should be conducted which aim to identify specific and quantifiable immune responses that correlate with protection. In the later pre-clinical stages, consideration should be given to the immunogenicity assays, which are transferable to human trials. However, it is important to understand the limitations of the animal data caused by the absence of validated immune correlates of protection and insufficient clinical efficacy data to identify which animal model best predicts vaccine efficacy in humans.
In this TB vaccine development pathway, animal studies are referred to as Pre-clinical because of an emphasis on providing guidance for animal testing performed before entering the clinical stages i.e. up to Stage C. References are made to formal, published documents some of which use the terminology of non-clinical testing because they describe both pre- and post-clinical animal studies.
Vaccine technology specific considerations
Viral vectored vaccines: Pre-existing immunity due to cross-reactivity with related or wild type versions of the vector and/or the source of the heterologous antigen insert needs consideration. If observed, full characterisation of such anti-vector immunity is needed to determine mitigation strategies such as changing the interval between prime and boost. Data on the immune responses to the antigens of the vector are important in estimating the possibility of re-use of the vector virus in another vaccine.
Live vaccines: These are composed of multiple antigens and therefore a ‘representative’ antigen or antigen sub-set e.g. PPD needs to be selected with which to consistently monitor the immune response to the vaccine.
Subunit vaccines: Because of the potential large number of antigen-adjuvant variations – considering adjuvant type, quantity (dose) and formulation, early immunogenicity studies, such as dose range studies, generally conducted in mice are important for optimisation. See the guidance for stages A and B for examples of assays and readouts.
RNA/DNA based vaccines: Various techniques and devices are used to deliver DNA or RNA vaccines. The success of antigen delivery and subsequent appropriate immune response would be first identified by immunogenicity studies in appropriate species e.g mice.
TB vaccine target population considerations
For neonatal vaccines, consider immunogenicity studies in neonatal animal, if feasible, in order to confirm that a relevant immune response is obtained.
- Evaluate immunogenicity
- Compare to benchmark, if applicable
- Evidence of relevant immunogenicity to antigens in at least one animal species
- Above baseline and/or benchmark (if applicable) responses to antigens preferred
In Stage A, immunogenicity studies should demonstrate that a clear immune response to the target antigens can be induced in the same animal model used to demonstrate protection. Mice are often used in early studies, but may not be appropriate for all candidates (for example, it would not be appropriate to evaluate vaccines based on CD1-associated lipids, as mice lack most CD1 molecules). Balb/C, C57Bl/6 and CB6F1 are the most commonly used strains of mice. There is a large body of literature describing disease and vaccine-induced protection in these strains (Orme and Ordway, 2016, Stylianou et al., 2018 and Aagaard et al., 2011). In the absence of a correlate of protection, the immune parameters measured should be relevant to immunity to Mtb and to the proposed mechanism of action of the vaccine. Due to the costs of Mtb protection studies, early-stage decisions e.g. on doses, formulations etc, would be most likely based upon immunogenicity, such as the magnitude of antigen-specific T-cells. A response above baseline is considered a minimum requirement but, if similar to others in development, the candidate should show an obvious differentiating characteristic – qualitative or quantitative.
- Confirm and characterise immunogenicity
- Develop hypothesis for immunological mechanism of action; design the Non-Human Primate (NHP) (or other model) study with appropriate outcomes, sample size, read-outs, data analysis plan, etc.
- Confirmed consistent immune response to antigens in at least one animal species
- Hypothesis for immunological mechanism of action defined; synopsis of NHP study (or another advanced model) available
Immunogenicity studies in Stage B should demonstrate that the candidate is immunogenic in the animal model chosen to confirm protection with the aim to link or correlate specific immune responses to protection. A more detailed characterisation of the immune response should therefore be performed, for example identifying specific T-cell subsets or cytokine secretion profiles and, as with Stage A the types of immune response should support the proposed mechanism of action of the candidate. The immune response profile identified in Stages A and B should also be used to assist the design of studies in the NHP (or other advanced) model in Stage C. This would include informing the optimal immunisation protocol (e.g. the timing of prime and boost vaccinations) and to identify the most appropriate assays and samples to be taken. Immunogenicity studies in Stage B might also include dose-finding in the context of live-attenuated mycobacteria in order to demonstrate that a relevant immune response is achieved with doses of vaccine that are safe and feasible (for example, for manufacture). The immunogenicity data generated in Stage B would be used as part of the data package to select between the candidates chosen to advance to Stage C.
- Confirm immunogenicity against relevant benchmark
- Expand immunogenicity (Th1, Th17, γδ T cells etc.) to explore mechanism in same species used to demonstrate efficacy
- Immune response against relevant benchmark established
- Immune mechanisms and breadth of immune response explored
Pre-clinical studies in Stage C are typically conducted in NHP or another highly relevant (and often resource-intensive) model. As the results of these studies are used as key gating criteria for advancement into clinical trials, they should be conducted as carefully and thoroughly as possible. Importantly, before proceeding to a challenge study, the vaccine dose and regimen should be optimised for the model selected and based upon the proposed mechanism of action (for example, dose or route selection for an adenovirus-based candidate may be based upon CD8+ T-cell responses characterised by intracellular cytokine staining, see Hokey et al., 2014). Immunogenicity evaluations should then be expanded in the challenge study. In addition to the inclusion of assays to confirm vaccine “take”, additional exploratory assays should be included to explore the mechanism of action and to identify potential correlates of protection. These may include the use of multiple or more extensive intracellular cytokine staining panels (to characterise Th1, Th17, gamma delta T-cells, etc.), antibody assays, and systems biology approaches. In order to conserve resources, samples for such exploratory assays can be collected and stored for analysis pending the challenge outcome. A recommendation is to consider the use of technologies provided by providers of platforms such as transcript- or other omics, e.g., GH-VAP (ghvap.org), and TRANSVAC (transvac.org), which offer unbiased and specialised expert assessments. Finally, consideration should be given to how key assays might bridge to the clinic.