Background: The protective effect of polyclonal immunoglobulins following passive transfer from malaria-immune adults to patients hospitalised with malaria provides strong motivation for successful vaccination. Antibodies against the merozoite stage of the parasite have been associated with clinical protection in immuno-epidemiological studies and controlled human malaria infection (CHMI) trials. Unfortunately, this knowledge has yet to be translated into highly effective malaria vaccines. This failure is partly attributed to the lack of robust functional correlates of vaccine-induced or naturally acquired immunity.
This thesis contributes to this knowledge gap by proposing an antibody-mediated natural killer cell (ab-NK) assay as a novel Fc-mediated functional correlate of clinical protection against Plasmodium falciparum (P. falciparum) merozoites.
Methods: I used multiparameter flow cytometry to quantify Fc receptor-mediated natural killer (ab-NK) cell degranulation (CD107a) and IFNγ production following co-incubation with anti-merozoite antibodies. I evaluated the utility of this assay as a functional correlate of protection from clinical malaria using samples from two independent prospective studies. The first involved 142 semi-immune Kenyan adults from a controlled human malaria infection study (CHMI-SIKA). In the second, 293 children from Junju village in Kilifi, Kenya, were monitored weekly for clinical malaria episodes over six months. I developed an antigen- specific ab-NK assay to identify potential merozoite targets.
Results: The ab-NK cell assay was specific, showed minimal day-to-day variation, was effective against multiple P. falciparum strains and enhanced antibody-mediated invasion inhibition activity in vitro. In CHMI-SIKA, high ab-NK degranulation and IFNγ secretion were strongly correlated with reduced in vivo parasite growth (Hazard Ratio 0.25 and 0.22, respectively, P<0.000). In Junju, ab-NK activity increased with age and was boosted by a recent infection. High ab-NK degranulation and IFNγ secretion were associated with clinical protection (Hazard Ratio 0.27, P<0.000 and 0.54, P<0.040, respectively). Using the antigen- specific approach, I identified eight of the fourteen merozoite antigens tested as potential targets. These included merozoite surface antigens, MSP11, MSP2, and well-characterized malaria vaccine candidates like AMA-1 and MSP3, supporting their interest as potential vaccine candidates. Interestingly, poorly studied antigens like P41 and Pf113 also induced comparable ab-NK responses.
Conclusions: These findings identify ab-NK cell responses targeting merozoites as a strong predictor of naturally acquired immunity against P. falciparum malaria. Our antigen-specific ab-NK assay facilitated faster evaluation of ab-NK responses against multiple antigens, enabling the functional analysis of potential vaccine candidates that may be considered in future sub-unit vaccine design
