Immunodominance, diversity and longevity in immune responses to the Plasmodium falciparum circumsporozoite protein

Abstract

Successful vaccines induce sustained antibody responses directed at critical pathogen antigens. Thus for vaccine development it is important to identify protective epitopes, and develop strategies to induce persistent serological memory targeted against those epitopes. One obstacle to vaccine development has been the presence of non-protective, immunodominant B cell responses. An immunodominant B cell response is defined when a set of epitope(s) drives a disproportionately high response, in comparison to other epitopes which are termed as subdominant epitopes. In the context of malaria, B cell responses to sporozoite stage are predominantly directed to the central multivalent repeat region in the Plasmodium falciparum circumsporozoite protein (PfCSP). This immunodominant repeat specific antibodies do possess anti-parasitic activities, and is thus included in RTS/S, AS01 the only WHO approved vaccine. However, the vaccine fails to induce long lasting protection, due to the rapid decline of repeat specific antibody titers. As a large amounts of repeat specific antibody is required for protection, it has also been hypothesised that that the repeat region is a decoy to prevent the immune system from generating responses against other targets of protective immunity. In this thesis we establish that the central repeat region of PfCSP primes a B cell response that is immunodominant over responses to the flanking N and the C terminus region. This immunodominance was not driven by a larger pool of naive B cells specific to the central repeat region of PfCSP. We found that immunodominance was driven by the valency of the central repeat region, crosslinking multiple BCRs on repeat specific B cells. Importantly, truncating the number of central repeats allowed the expansion of responses to subdominant epitopes of PfCSP. This diversification of antibody response within the PfCSP region translated into an improved vaccine efficacy during malaria challenge. We explored the mechanisms of immunodominance using a model tool to investigate and manipulate the immunodominance hierarchy of central repeat, by conjugating different molar amounts of the hapten: NP (4-Hydroxy-3-nitrophenyl acetyl) to PfCSP molecule. Using this system, we found that the immunodominance hierarchy is established early in the response. Imaging of the germinal centers (GC) responses revealed that B cells with different specificities are found in the same GC, and thus could potentially be in competition with each other for CD4 T cell help. Yet, we found that reducing the ability of one B cells to obtain CD4 T cell help did not affect responses to other epitope contradicting this hypothesis. Finally, the longevity of vaccine efficacy is maintained by the plasma cells (PC) that secrete antibodies long after antigen encounter without need for antigen re-stimulation. We sought to understand the requirements for the emergence of these PC, and observed that waning GC are enriched for B cells differentiating into pre PC population. While others have reported that CD4 T cells drive PC formation we observed that late stage GC response have fewer CD4 T cells per GC B cell. Moreover, reducing the amount of MHC-II expression on the antigen specific B cells in an attempt to limit the ability of cells to receive CD4 T cell help did not result in a decrease in PC output. However, provision of additional CD4 T cell antigen did result in enhanced pre PC formation, but provision of antigen that was only able to stimulate B cell but not CD4 T cells did not result in expansion of pre PC. These results can aid in developing better vaccination and booster regimen by de-hyphenation of the role of B cell and CD4 T cells at the various stages of GC response. Show more