PvDBPII elicits multiple antibody-mediated mechanisms that reduce growth in a Plasmodium vivax challenge trial

Abstract

The receptor-binding domain, region II, of the Plasmodium vivax Duffy binding protein (PvDBPII) binds the Duffy antigen on the reticulocyte surface to mediate invasion. A heterologous vaccine challenge trial recently showed that a delayed dosing regimen with recombinant PvDBPII SalI variant formulated with adjuvant Matrix-M^TM reduced the in vivo parasite multiplication rate (PMR) in immunized volunteers challenged with the Thai P. vivax isolate PvW1. Here, we describe extensive analysis of the polyfunctional antibody responses elicited by PvDBPII immunization and identify immune correlates for PMR reduction. A classification algorithm identified antibody features that significantly contribute to PMR reduction. These included antibody titre, receptor-binding inhibitory titre, dissociation constant of the PvDBPII-antibody interaction, complement C1q and Fc gamma receptor binding and specific IgG subclasses. These data suggest that multiple immune mechanisms elicited by PvDBPII immunization are likely to be associated with protection and the immune correlates identified could guide the development of an effective vaccine for P. vivax malaria. Importantly, all the polyfunctional antibody features that correlated with protection cross-reacted with both PvDBPII SalI and PvW1 variants, suggesting that immunization with PvDBPII should protect against diverse P. vivax isolates.

Fig. 1. ELISA reactivity and binding inhibitory activity of sera from volunteers after immunization with protein-in-adjuvant or viral-vectored PvDBPII vaccines.

a ELISA titres for recognition of PvDBPII SalI by sera from volunteers at different timepoints over the course of the trial reported as median and range for each vaccine group are shown. Black arrows show the time points of the immunizations. Shaded grey areas represent the period after blood stage P. vivax challenge. b ELISA recognition titres and binding inhibition titres for PvDBPII variants SalI and PvW1 for all volunteers at C-2. Medians are shown in horizontal bars, **p < 0.01, Kruskal–Wallis test with Dunn’s correction for multiple comparisons.

Fig. 2. Reactivity of sera from immunized volunteers with P. vivax schizonts.

a Representative images of P. vivax schizonts incubated with sera from an individual of the PvDBPII/M-M (0-1-14 months) group at dilution 1:5000. Sera at time point C-2 showed apical staining to PvDBP (green) in P. vivax schizonts compared to sera collected prior to initial immunization (D0). b Signal intensity in IFA for day C-2 and D0 sera from different groups tested at different dilutions. The fluorescence intensity of each sample was scored as: +++, for high intensity; ++, medium intensity; +, low intensity; and -, no signal. Some IFA samples that were not done are designated as ND.

Fig. 3. Variable correlations and feature selection of the antibody functions measured in the study.

a Correlations between the anti-PvDBPII SalI antibody functions are shown. Correlation coefficients (shown in a double red-blue gradient) for each comparison were calculated using Spearman’s rank correlation tests. p values for each significant correlation are indicated, *p < 0.05 **p < 0.01 ***p < 0.001 ****p < 0.0001. b Importance plot for classification of variables specific to PvDBPII SalI that significantly contribute to the PMR reduction using the Boruta algorithm. The importance of each variable is defined as the Z-score of the mean decrease accuracy (normalised permutation importance). Blue boxes correspond to the minimal, average, and maximum Z-scores of shadow features. The variables that contribute significantly (green) or not (red) to the PMR reduction are shown. Boxplots show median Z-score (horizontal bar), interquartile range (boxes), range (whiskers), and outliers (open circles). c Correlations between the PvDBPII PvW1 antibody functions are depicted as in a. d Importance plot for classification of variables specific to PvDBPII PvW1 are depicted as in b.

Fig. 4. Correlation of binding inhibition titre and PMR reduction.

a Individual correlations of the PMR reduction and binding inhibition titres for PvDBPII SalI or PvW1. (b) Correlation between binding inhibition titres for PvDBPII SalI and PvW1. Correlations shown in a and b were calculated using Spearman’s rank correlation tests. Correlation coefficients and p values for each comparison are shown. c Importance plot for the binding inhibition titres specific to PvDBPII SalI and PvW1 using Random Forest regression of the PMR reduction.