Immune mechanisms that determine the quality of antibody responses to vaccines

Abstract

Effective vaccines for many dangerous infectious diseases are still lacking. An improved understanding of the mechanisms that determine the quality of vaccine responses is essential to guide future attempts to design effective vaccines. This thesis aimed to characterize the early and late immune responses of different clinically relevant vaccine platforms. All studies were performed in the non-human primate (NHP) model due to their physiological and immunological similarities to humans.

Protein subunit vaccines are commonly used when live attenuated vaccines are unsafe. Adjuvants are often needed in protein subunit vaccines to stimulate sufficiently potent immune responses. In paper I we developed an in vivo model to examine the initial immunological events after vaccine administration into the muscle. Specifically, we studied the mechanisms of the adjuvants Alum, MF59 and Alum-TLR7. We found that all adjuvants induced rapid infiltration of immune cells at the site of vaccine injection. Alum-TLR7 and MF59 showed stronger adaptive immune profiles compared to Alum, including higher antibody titers, better neutralization and a stronger germinal center (GC) response. However, their innate immune activation was very different. Alum-TLR7 exclusively induced a type I interferon response and excelled at dendritic cell maturation while MF59 promoted neutrophil recruitment to the vaccine draining lymph nodes (LNs).

In paper II-III we studied mRNA-based vaccines which have emerged as promising candidates against several diseases including Zika and influenza. In paper II we used fluorescently labeled lipid nanoparticles (LNP) encapsulating mRNA encoding for the fluorescent protein mCitrine, for tracking of vaccine uptake and mRNA translation separately. We found a rapid infiltration of immune cells to the site of injection as well as uptake and translation of the mRNA into antigen with both intradermal and intramuscular immunizations. A strong type I interferon response was induced at the site of injection and the draining LNs. Priming of T cells occurred in the vaccine-draining LNs. In paper III we investigated the adaptive immune profile of a mRNA vaccine encoding influenza hemagglutinin. Immunization induced a quick B cell response including vaccine-specific memory B cells in blood and plasma cells in the bone marrow. Formation of GCs was detected in the vaccine draining LNs. Circulating vaccine-specific CXCR3+ T follicular helper cells, associated with a Th1 response, transiently appeared in the blood and correlated with antibody avidity in serum.

Finally, in paper IV, we focused on the methodological advancements of GC analysis. LNs were divided in two and each half was analyzed by either immunohistology or flow cytometry. We propose a method to analyze and present GC data to make a direct comparison possible between studies regardless of the technique. Altogether, the data presented in the thesis will add to the understanding of how vaccine responses are initiated and regulated. Ultimately this will help in the development of new or improved vaccines.