Characterization of Humoral Immune Responses to Two Subunit Malaria Vaccine Candidates in Humans

Abstract

Malaria remains a major public health scourge affecting millions of people worldwide. An effective antimalarial vaccine is currently lacking and if available would add to other malaria control strategies. Although antibodies (Abs) are thought to mediate protective immunity to malaria, the exact Ab-mediated mechanisms or immunological correlates of protection against the complex plasmodial parasite remain unclear. Standardized assays for Ab measurement are also lacking and therefore no consensus exists on the best approach to quantitate, report and interpret antimalarial Abs. In an attempt to address some of these hurdles, this dissertation aims to implement robust standardized assays for measurements of Plasmodium-specific Abs and to investigate Ab avidity as a potential surrogate marker of vaccine-mediated protection against malaria.

In the first study, a novel cytometric based immunofluorescence assay technique is described that improves the detection of anti-plasmodial Abs using whole parasites and may be suitable for investigating Ab-based correlates of protection. An overlap subtraction algorithm (OSA) developed in parallel eliminates the investigator-dependent effects and thus facilitates the data analysis process. The workflow was applied to pre-(D0) and post-vaccination (D84) clinical samples from children and adult participants of Phase 1 trials of the malaria vaccine GMZ2. The results demonstrate that children vaccinated with the highest GMZ2 dose (100µg) showed a 1.33-fold increase in percent positive fluorescent cells (PPFC; p=0.003) on D84 compared to D0. Meanwhile, on D84, a vaccine-induced boosting effect of pre-existing anti-parasitic immunity (1.23-fold increase in mean fluorescent intensity; MFI, p=0.03) was observed in semi-immune adults.

In a second study, a modified ELISA-based method to assess the avidity index (AI) of anti-circumsporozoite protein (CSP) Abs elicited by two immunization (0-1-2 month and 0-1-7 month) schedules with the malaria vaccine RTS,S in a cohort of healthy African infants was used. The analyses revealed that the avidity maturation of anti-CSP Abs following RTS,S vaccination occurred as expected, although absolute AI did not predict vaccine efficacy. The AIs of Abs were found to be similar in both immunization schemes. Interestingly, the change in anti-CSP Ab titer (dCSP) and avidity index (dAI) between second and third immunization was associated with 77% and 54% riskreduction to develop clinical disease, respectively. Avidity maturation of vaccine-specific Abs deserves further investigation as surrogate marker of protective efficacy.

Together, standardized new tools for investigating parasite-specific Ab responses were developed and the detailed investigation of anti-CSP Ab avidity expands contemporary understanding of Ab-mediated indicators of protective immunity against malaria. These studies might serve as a basis for further work on Ab-based immunity to malaria and contribute to the development and evaluation of functional second-generation antimalarial vaccines.