Breadth of Fc-mediated effector function correlates with clinical immunity following human malaria challenge

Abstract

Malaria is a life-threatening disease of global health importance, particularly in sub-Saharan Africa. The growth inhibition assay (GIA) is routinely used to evaluate, prioritize, and quantify the efficacy of malaria blood-stage vaccine candidates but does not reliably predict either naturally acquired or vaccine-induced protection. Controlled human malaria challenge studies in semi-immune volunteers provide an unparalleled opportunity to robustly identify mechanistic correlates of protection. We leveraged this platform to undertake a head-to-head comparison of seven functional antibody assays that are relevant to immunity against the erythrocytic merozoite stage of Plasmodium falciparum. Fc-mediated effector functions were strongly associated with protection from clinical symptoms of malaria and exponential parasite multiplication, while the gold standard GIA was not. The breadth of Fc-mediated effector function discriminated clinical immunity following the challenge. These findings present a shift in the understanding of the mechanisms that underpin immunity to malaria and have important implications for vaccine development.

Keywords: CHMI; Fc-mediated effector function; antibodies; growth inhibition activity; malaria; merozoite; naturally acquired immunity; plasmodium falciparum.

Graphical abstract

Figure 1. Controlled human challenge provides clear endpoints for clinical immunity

(A) Study design. Kenyan adult volunteers (N = 64) were infected with 3,200 live P. falciparum sporozoites via direct venous injection. Parasitaemia was quantified by qPCR from day 7 to 21. (B) NI volunteers; parasitaemia > 500/μL of blood and/or developed fever (≥37.5°C) with any parasitaemia, n = 28. (C) CI volunteers; remained afebrile with parasitaemia < 500/μL, n = 36. The dotted line indicates the treatment threshold. See also Table S1.

Figure 2. Cytophilic antibodies and Fc functions targeting merozoites are correlated with clinical immunity

(A) The overall prevalence of IgG, IgM, and IgG1−4 antibodies against merozoites measured by ELISA in plasma samples collected 1 day before the challenge (C-1). (B) The prevalence of IgG, IgM, and IgG1−4 against merozoites and total IgG against tetanus toxoid extract measured by ELISA in C-1 plasma samples. (C) Quantities of IgG, IgM, and IgG1−4 antibodies against merozoites and total IgG against tetanus toxoid measured by ELISA in C-1 plasma samples. Error bars: median and 95% confidence intervals. Analysis using the Mann-Whitney U test. Seropositivity cutoff: dotted black horizontal line. (D) A comparison of plasma GIA (neutralization) and Fc-mediated effector functions in NI and CI. GIA and Fc-mediated activity measured using functional assays in C-1 plasma samples. Error bars: median and 95% confidence intervals. Analysis using the Mann-Whitney U test. Seropositivity cutoff: dotted black horizontal line. RLU, relative light units; RPI, relative phagocytosis index. (E) ROC curves for effector functions and area under the curve (AUC). (F) Forest plot showing the adjusted hazard ratios for each effector function using the Cox regression hazard model, comparing the time to treatment between volunteers with high versus low Fc-mediated function. Error bars: 95% confidence intervals. The red line indicates no protection (hazard ratio = 1.0). (G) Heatmap showing Spearman’s correlation matrix for antibody effector functions targeting merozoites and IgG, IgG subtypes (1−4), and IgM binding to merozoites (M). The color intensity indicates the strength of the correlation, while numbers indicate pairwise Spearman’s rho. See also Table S2.

Figure 3. The breadth of Fc-mediated function is correlated with clinical immunity

(A) PCA for the six Fc-mediated effector functions with colors representing clinical outcome. (B) The distribution of breadth scores within CI versus NI. (C) PCA for the six Fc-mediated effector functions with colors representing breadth scores. (D) ROC curve for the breadth score with AUC. (E) Survival analysis using the breadth score, showing the percentage of volunteers who were CI over time post-challenge. Each line represents the breadth of Fc function. Analysis using the log-rank (Mantel-Cox) test. (F) The quantity of anti-merozoite IgG measured in C-1 plasma samples is compared between volunteers with varying Fc breadth scores. Each dot represents an individual. Error bars: median and 95% confidence intervals. Analysis using the Kruskal-Wallis test. Seropositivity cutoff: dotted black horizontal line. (G) Heatmap of the six Fc-mediated effector functions in NI and CI volunteers. The quantity of Fc function increases from 0 to 1 and is demonstrated by darker shades of red as shown in the scale. Columns represent specific Fc-mediated functions, while rows represent individual volunteers. See also Figure S1.