Guiding clinical malaria vaccine development using immune cell monitoring and controlled human malaria infection

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Dokumentart: PhDThesis
Date: 2023-06-27
Language: English
Faculty: 4 Medizinische Fakultät
Department: Medizin
Advisor: Mordmüller, Benjamin (Prof. Dr.)
Day of Oral Examination: 2023-05-19
DDC Classifikation: 610 - Medicine and health
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Despite being treatable and preventable, malaria kills about half a million people each year, mostly in Africa. Notwithstanding many attempts to develop a malaria vaccine, only the vaccine RTS,S/AS01 (Mosquirix), has been fully developed and was recommended by the World Health Organization for use in children in malaria endemic areas on October 6th in 2021. RTS,S is a subunit vaccine and provides only partial protection. Manufacturing of pharmaceutical grade Plasmodium falciparum (Pf) sporozoites (SPZ) has been achieved in the past decade, making the development of plasmodial whole cell vaccines possible. This may be a novel and better platform to design malaria vaccines. In addition, the availability of cryopreserved infectious sporozoites has boosted possibilities to assess vaccine efficacy by using controlled human malaria infection (CHMI) trials. The most effective vaccination strategy to date is to inoculate PfSPZ while using chemoprophylactic antimalarials such as chloroquine – the PfSPZ Chemoprophylaxis Vaccine (PfSPZ-CVac). Although CHMI can test vaccine efficacy (VE), the immune mechanisms underneath are not fully understood. Particularly, the cellular immune response is poorly characterised compared to the humoral response (i.e. antibody titers). Thus, the systematic monitoring of cellular immune responses during immunization in combination with CHMI might be a valuable step to guide the clinical development of malaria vaccine candidates and identify immune mechanisms causally related to protection. In Chapter 1, I investigated the influence of two accelerated regimens of PfSPZ-CVac on the generation of pro-inflammatory Pf-specific CD4+ T helper cells and their suitability as a surrogate of protection. The trial was conducted in healthy, malaria-naïve adults in Tübingen. To detect antigen-specific cells in peripheral blood with increased sensitivity, Plasmodium specific T cells were enriched by magnetic-activated cell sorting followed by staining and multiparameter flow cytometry (MFC). Measurements were done using the flow cytometer FACS Canto II and stimulations were done with infected red blood cells (iRBCs). Uninfected red blood cells and Staphylococcal enterotoxin B (SEB) were also as negative and positive controls, respectively. Unexpectedly, I found that a condensed vaccination schedule, where the three vaccinations were given within 10 days induced higher frequencies of Plasmodium specific CD40L+CD4+ TNF-α+/IFN-γ+ cells than a 28-day regimen. The response was also qualitatively different. The shorter regimen led to a polarized Th1 response with more CD45RO+CCR7- effector memory T cells. This may lead to higher numbers of memory cells in the liver. A tendency towards higher frequencies of specific CD40L+CD4+ TNF-α+/IFN-γ+ effector memory T cells was present in protected volunteers but no reliable correlate of protection could be identified. Future research will be needed to identify effector and regulatory responses that predict vaccine efficacy. It will be particularly important to include the characterization of Plasmodium-specific cytotoxic T cells. More than 94% of the estimated malaria cases globally occur in Africa. Therefore, every malaria vaccine must be tested in malaria exposed volunteers to be sure that it has a significant public health impact. Unfortunately, for many vaccines, VE is lower in Africa. This effect is particularly strong in malaria vaccines. The GMZ2 vaccine was developed to prevent malaria and its complications by mimicking naturally acquired immunity. The vaccine antigen consists of a fusion protein between fragments of the merozoite surface protein-3 and the glutamate rich protein. In Chapter 2, I investigated the immunogenicity of GMZ2 adjuvanted with two different immune modulators: Alhydrogel or CAF01. The study was performed in healthy, adult, lifelong malaria-exposed volunteers from Lambaréné, Gabon. MFC was used to systematically measure the T and B cell response, and to compare immune response patterns before and after immunization. Peripheral blood mononuclear cells were cryopreserved and measured upon completion of the trial using a Sony SP6800 Spectral Analyzer. Monitoring of GMZ2-stimulated CD4+ T, and CD20+ B cells showed that GMZ2 did not induce significant immune responses beyond the baseline. The low response to vaccination was unexpected as was the similar performance of the two adjuvants. VE in CHMI was similar between the groups, hence these findings may be expected. Notwithstanding the negative result, this study will help guiding the development of the next generation of blood stage malaria vaccines in malaria-exposed volunteers, where baseline responses play a major role in generating successful immune responses following vaccination. In summary, the work presented as part of the thesis shows that systematic monitoring of the cellular immune responses by MFC, in combination with CHMI studies is a valid, and stringent approach to measure VE and identify correlates of protection, surrogate markers, and the effect of schedule and pre-existing immunity on vaccine responses. More work will be required to replace CHMI with immunological surrogate endpoints and understand vaccine-induced antimalarial immunity.

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