Navigating the immune system: Improving CD8+ T cell responses for vaccine design

Abstract

Most vaccines rely on the protective effect of the humoral response. In case of intracellular- or rapidly mutating pathogens, humoral responses are less protective and the cellular response, mainly CD8+ T cells, can convey protection. However, vaccine efficacy is hampered by insufficient knowledge of vaccine-antigen processing and subsequent activation of the cellular compartment. Additionally, and most importantly, vaccine design in the future cannot be empirical; it needs to be rational, based on scientific knowledge of underlying mechanisms of immune responses targeted to vaccine-antigens. Therefore, the goal of this research was to have a better understanding of antigen processing and subsequent initiation of a CD8+ T cell response. We start with a new way of antigen processing, proteasome-catalyzed peptide splicing (PCPS). We have developed a novel reverse immunology-based multi-level approach to identify spliced epitopes that are targeted by CD8+ T cells in Listeria monocytogenes infection. Increasing evidence of spliced epitopes is available but due to insufficient tools to predict them, the identification remains challenging. We have not only shown that the prediction is highly accurate, we also demonstrate the immunological relevance of PCPS in infection by identifying CD8+ T cells that could specifically recognize the spliced epitopes. We found an additional purpose of this phenomenon because CD8+ T cells that are activated by the linear epitope, can be cross reactive to spliced epitopes that share sequence similarity. Such a mechanism might reduce the changes of immune evasion. Modulation of antigen processing can have major consequences for CD8+ T cells and we have studied ways to perform this modulation in order to enhance the CD8+ T cell response. We have shown a complete strategy to enhance vaccine’ immunogenicity in which the multistage cDNA vaccine H56, encoding three secreted Mycobacterium tuberculosis antigens, was used as a model vaccine and delivered using dermal DNA tattoo immunization. We started by prediction of the epitopes and subsequently optimized the antigen processing using flanking residue modulation and linkage of the cDNA of the antigen to cDNA of proteins with immunogenicity enhancing properties. We continued with the H56 cDNA as a model vaccine and studied the effectiveness of a heterologous prime-boost strategy using dermal H56 DNA tattoo immunization in combination with BCG::H56 subcutaneous injection. BCG is the only licenced vaccine against TB but it lacks efficacy in certain situations. We therefore tested whether we could successfully boost the effect of BCG by incorporating it in a heterologous prime boost regimen and concluded that heterologous prime-boost was most optimal in the case of a combination of a dermal H56 cDNA prime and a subcutaneously given BCG::H56 boost. Thus, in the research described in this thesis we have proven the immunological relevance of PCPS in bacterial infection and have shown a complete vaccination strategy that is effective in Mycobacterium tuberculosis infection. We can conclude that these results give more insight in how modulation of antigen processing influences immune responses, mainly CD8+ T cells, which will contribute to better vaccine design.