Fine-specificity of flavivirus antibodies in humans and mice

The induction of virus neutralizing antibodies by natural infection or vaccination is a cornerstone of the immune response that contributes to long-lasting protection against viral diseases. In the case of enveloped viruses, these protective antibodies are primarily directed to viral surface proteins involved in attachment and/or membrane fusion and - by inhibiting these functions interfere with virus entry into cells. Through the mechanisms of germinal center formation and affinity maturation in lymph nodes, the initial diversity of antibody specificities becomes restricted in the course of the immune response and a limited number of antibody subsets will usually dominate an individual`s antibody pattern to a given protein. Since both the extent and mechanism of virus neutralization are influenced by the location of the antibody binding site in the three-dimensional structure of viral surface proteins, the process of restricting antibody specificities - resulting in the phenomenon of immunodominance - can affect the functional potency of immune responses. In the course of this project, we want to investigate the individual variability of antibody fine-specificities and its impact on virus neutralizing activity in humans after natural infection with tick-borne encephalitis (TBE) virus and vaccination with a formalin-inactivated TBE vaccine. In parallel, controlled immunization studies will be conducted in mice to assess the influence of species-specific and genetic factors as well as quaternary structure and adjuvants on fine-specificity patterns and functional activity of antibody responses. TBE virus is one of the major human pathogenic flaviviruses and closely related to mosquitoe-borne viruses causing yellow fever, dengue, Japanese encephalitis, and West Nile fever. The structure of the major envelope protein (E) as well as its quaternary arrangement at the viral surface are known at high resolution, forming the basis for a structure-specific analyis of antibody responses. The modular organization of E allows the production of recombinant protein fragments as antigens for quantitative immunoassays that will provide fingerprints of individual antibody patterns after infection and vaccination and information on the contribution of certain antibody subpopulations to virus neutralization. The results of this study should reveal important new insights into the variability of antibody immunodominance as well as its impact on virus neutralization and may thus provide leads for the rational structure-based design of new immunogens.

Antibodies are a primary component of antiviral immune responses and in many instances provide long-lasting immunity after natural infection or vaccination. These molecules are capable of neutralizing the infectivity of viruses by binding to the surface of viral particles and thereby inhibiting their entry into cells. Virus surfaces are complex structures, displaying an array of so-called antigenic determinants that can potentially induce a multitude of antibodies with different specificities and varying capacities to neutralize the virus and to provide protection from disease. However, in the course of immune responses, some antibodies will be induced preferentially and thus dominate the composition of antibody populations found in the blood. In our project, we focused on this phenomenon of immunodominance and developed technologies for dissecting the antibody populations in the blood of humans and mice after tick-borne encephalitis infection and/or vaccination. Specifically, we investigated the extent of variation of antibody responses among individuals (in humans and in mice), the influence of immunological adjuvants as well as genetic background on immunodominance (in mice) and in which way previous vaccination against the related yellow fever (YF) virus influences the antibody response to TBE vaccination (in humans). It was a key finding of our studies that the specificities of antibodies induced and their dominance in the blood differed strongly among individuals (both in mice and humans), although each had been immunized with physically precisely the same immunogen. In addition, as revealed in mouse studies, the specificity patterns of antibody populations were strongly shaped by genetic background and the use of immunological adjuvants. We also discovered that the binding of antibodies can induce structural changes in the virus and thus modulate immunodominance patterns induced by virus antibody complexes. Such mechanisms are relevant in the course of sequential vaccinations or infections with antigenically related viruses when the new virus encounters pre-existing antibodies that are cross-reactive and capable of forming immune complexes. The practical importance of this aspect was specifically addressed in our project by analyzing the effect of previous YF vaccination on the antibody response to TBE vaccination. Importantly, in individuals who were pre-immune to YF virus, the neutralizing activity of antibodies induced by TBE vaccination was diminished. Together, the results of our project shed new light on the variability of antibody responses among individuals and highlight the practical importance of the phenomenon of immunodominance in the context of vaccine-induced immunity.