Magnitude and durability of ProC6C-AlOH/Matrix-Mtm vaccine-induced malaria transmission-blocking antibodies in Burkinabe adults from a Phase 1 randomized trial

Abstract

ProC6C is a multi-stage malaria vaccine designed to disrupt parasite transmission and prevent infection by incorporating three parasite proteins (Pfs230-Pro, Pfs48/45-6C, and CSP) in a single vaccine antigen. The Phase 1 clinical trial (PACTR202201848463189) conducted in Burkina Faso, showed ProC6C-AlOH/Matrix-M was safe, well tolerated, immunogenic and generated a functional antibody response to all three constituent antigens at the primary output (D70). As magnitude and durability are central to an efficacious malaria vaccine, analysis was expanded past the initial endpoint, to determine transmission-blocking antibodies (anti-Pfs230 and anti-Pfs48/45-6C) present through D180. Analysis of transmission-reducing activity (TRA) showed 7/20 samples remained biologically active at D180. To identify immune biomarkers for high levels of TRA, the Pfs48/45-6C IgG concentration (calculated relative to the transmission-blocking mAb TB31F) was compared among TRA positive and negative individuals. The magnitude of anti-Pfs48/45-6C IgG had an excellent predictive accuracy (area under the receiver operating curve [ROC AUC] >0.8) with a threshold of 8.7 μg/ml for significant TRA. Additionally, there was significant correlation of TRA and anti-Pfs48/45 epitope I IgG concentration but not significant correlation for anti-Pfs230-Pro IgG, suggesting that vaccine-induced anti-Pfs48/45-6C IgG is the main predictor of TRA. This finding was corroborated by the observation that complement had no effect on TRA in the standard membrane feeding assay (SMFA). Collectively, these efforts confirm the transmission-blocking attributes of ProC6C and suggest that an alternative dosing regimen be evaluated in future clinical trials to improve longevity of functional transmission-reducing antibodies.

Keywords: Burkina Faso; Malaria; Matrix-M; Pfs230; Pfs48/45; TBV; antibodies; clinical trial; durability; transmission-blocking vaccine; vaccine.

Figure 1.

Dynamics of antibody responses to ProC6C constituent antigens. Antibody levels to the ProC6C constituent antigens, Pfs48/45-6C (a & b) and Pfs230-pro (c & d), in individuals immunized with ProC6C-AlOH/Matrix-M (G2D, geometric mean titer, GMT-blue line) or a control vaccine (G2E, GMT-black line) at each study time point (D0, D14, D42, D70, D140, and D180). GMT is indicated in supplementary table S1.

Figure 2.

Transmission-reducing activity of antibodies. Significant TRA denoted by a closed circles and non-significant TRA by open circles. (a) The TRA for the ProC6C-AlOH/Matrix-M (G2D) and control group (G2E) is plotted for each volunteer at D70 and D180 from purified IgG at 15 mg/mL in the SMFA (with human complement) as TRA. The median for each group is indicated by a line and p values were determined by Mann Whitney test. Paired aligned dot plots showing the (b) anti-Pfs48/45-6C and (c) anti-Pfs230-pro antibody levels at D70 and D180. Individuals were categorized into 3 subgroups based on their TRA levels: “a” significant TRA at both time points; “b” significant TRA at D70 only; and “c” insignificant TRA at both timepoints. Pairwise comparisons were performed using the paired t test (log-transformed ELISA titers were used for all analyses) (d) Pfs48/45-6C antibody levels (μg/mL) for each subgroup (“+” significant TRA and “-“ non-significant TRA). Both D70 and D180 data were combined in this analysis. GMT indicated by horizontal line. Unpaired t-tests were utilized. (e) Pfs230-pro antibody levels (a.U.) for each subgroup (“+” significant TRA and “-“ non-significant TRA). Both D70 and D180 data were combined. GMT indicated by horizontal line. Unpaired t-tests were utilized; ****p < .0001, ***p < .001, ** p < .01, *p < .05, ns p > .05.

Figure 3.

Correlation between transmission-reducing activity and Pfs48/45-6C and Pfs230-pro specific antibodies. G2D individuals indicated in blue and G2E individuals indicated in black circles. (a) Correlation of anti-Pfs48/45-6C antibody levels with TRA at D70 and D180 in ProC6C-AlOH/Matrix-M vaccinated individuals (G2D). (b) Correlation of Pfs230-pro antibody levels with TRA at D70 and D180 in ProC6C-AlOH/Matrix-M vaccinated individuals (G2D). (c) Correlation of anti-Pfs48/45-6C antibody levels with TRA at D70 and D180 in control vaccinated individuals (G2E). (d) Correlation of Pfs230-pro antibody levels with TRA at D70 and D180 in control vaccinated individuals. The TRA is plotted in log of mean oocyst ratio (LMR) between control and test IgGs (y axis) respective to square root (sqrt) of antibody levels of Pfs48/45-6C or Pfs230-pro (x axis). For ease of comprehension, the y axis shows corresponding percent TRA values instead of LMR. Correlations were assessed using Spearman’s rank correlation. The p value and correlation coefficient (r) in each panel are shown.

Figure 4.

Correlation between antibodies against epitope I (competition) and total Pfs48/45-6C IgG and TRA. Using the TB31F competition ELISA, D70 and D180 epitope I antibody reactivity was determined in volunteers vaccinated with ProC6C-AlOH/Matrix-M (G2D). The square root (sqrt) of D70 antibody levels of Pfs48/45-6C (y axis) is plotted against the sqrt of epitope I antibody levels (x axis). Antibody levels are given as TB31F equivalence (μg/mL) for Pfs48/45-6C IgG. The coefficient of correlation and p value using Spearman’s rank test are provided. The red dotted line shows a y=x.

Figure 5.

Receiver operating characteristic (ROC) curves. Receiver operating characteristic curves were generated for the square root of IgG: anti-Pfs48/45-6C (a) and anti-epitope I (b) relative to significant TRA. (a) Anti-Pfs48/45-6C demonstrated an AUC of 0.8933 and a threshold of 8.7 μg/mL. (b) Anti-epitope I demonstrated an AUC of 0.8267 a threshold of 5.5 μg/mL. In contrast Pfs230-pro IgG did not demonstrate a high level of specificity and sensitivity (supplementary figure S1). Youden’s topleft analysis was utilized to determine the optimal threshold on the ROC curve as indicated by dot and light dotted lines. Black dotted line y=x.

Figure 6.

Transmission-reducing activity of ProC6C IgG is independent of complement. The TRA of ProC6C-AlOH/Matrix-M (G2D) sera is plotted for each volunteer at D70 with (+C) and without (−C) human complement. p values determined by Wilcoxon matched-pairs signed rank test; ns, p > .05.