New antigens for a multicomponent blood-stage malaria vaccine

Abstract

An effective blood-stage vaccine against Plasmodium falciparum remains a research priority, but the number of antigens that have been translated into multicomponent vaccines for testing in clinical trials remains limited. Investigating the large number of potential targets found in the parasite proteome has been constrained by an inability to produce natively folded recombinant antigens for immunological studies. We overcame these constraints by generating a large library of biochemically active merozoite surface and secreted full-length ectodomain proteins. We then systematically examined the antibody reactivity against these proteins in a cohort of Kenyan children (n = 286) who were sampled at the start of a malaria transmission season and prospectively monitored for clinical episodes of malaria over the ensuing 6 months. We found that antibodies to previously untested or little-studied proteins had superior or equivalent potential protective efficacy to the handful of current leading malaria vaccine candidates. Moreover, cumulative responses to combinations comprising 5 of the 10 top-ranked antigens, including PF3D7_1136200, MSP2, RhopH3, P41, MSP11, MSP3, PF3D7_0606800, AMA1, Pf113, and MSRP1, were associated with 100% protection against clinical episodes of malaria. These data suggest not only that there are many more potential antigen candidates for the malaria vaccine development pipeline but also that effective vaccination may be achieved by combining a selection of these antigens.

Fig. 1.

Sero-prevalence of each of the 36 immunoreactive antigens within the recombinant merozoite protein library, grouped by antigen localization within the parasite. The threshold for positivity was defined as an absorbance reading greater than the mean + 3 SD of 20 non-malaria-exposed control sera. Antibodies to AARP, SPATR and P12p were not included.

Fig. 2.

Unique antibody response profiles in protected children. (A) Each row represents an antigen ranked by individual protective efficacy (n = 36, Table 1); each column a parasite-positive child (n = 121). Children with no clinical episode of malaria (n = 80) and those with (n = 41) are sorted in increasing age from left to right within each category except for the first four non-protected children who did not have antibodies to the protective antigens studied. Protective responses are indicated in blue and non-protective in green. The breadth score was compared for (B) protective (blue) and (C) non-protective (green) antibodies in children with and without malaria. Normal density curves are shown in red.

Fig. 3.

Increasing breadth of responses to combinations of the most protective antigens is strongly associated with protection from malaria. (A) Normalized distributions of protective efficacies for all five-way combinations of 36 antibodies (n = 376,992) when all five (black) or only one (dark blue) responses were present. All combinations restricted to the ten top-ranked antibodies (n = 252) in which all five responses were present were significantly (P < 0.05) more protective than random combinations (light blue, top 5% marked by vertical line). (B) In this example, “Combo1” contains the five individually most protective antibodies, “Combo2” the subsequent five etc. Combo7 contains the five least protective antigens, TLP was excluded. Numbers on bars indicate the percentage of individuals with corresponding antibody responses. **P <0.001; *P <0.05. (C) Representation of antibodies to individual antigens amongst combinations that showed significant breadth effects on protection. Antibodies were ranked (x-axis) by their individual protective efficacy. Horizontal line indicates expected representation by chance alone.