Generation of Humoral Immune Responses to Multi-Allele PfAMA1 Vaccines; Effect of Adjuvant and Number of Component Alleles on the Breadth of Response

Abstract

There is increasing interest in multi-allele vaccines to overcome strain-specificity against polymorphic vaccine targets such as Apical Membrane Antigen 1 (AMA1). These have been shown to induce broad inhibitory antibodies in vitro and formed the basis for the design of three Diversity-Covering (DiCo) proteins with similar immunological effects. The antibodies produced are to epitopes that are shared between vaccine alleles and theoretically, increasing the number of component AMA1 alleles is expected to broaden the antibody response. A plateau effect could however impose a limit on the number of alleles needed to achieve the broadest specificity. Moreover, production cost and the vaccine formulation process would limit the number of component alleles. In this paper, we compare rabbit antibody responses elicited with multi-allele vaccines incorporating seven (three DiCos and four natural AMA1 alleles) and three (DiCo mix) antigens for gains in broadened specificity. We also investigate the effect of three adjuvant platforms on antigen specificity and antibody functionality. Our data confirms a broadened response after immunisation with DiCo mix in all three adjuvants. Higher antibody titres were elicited with either CoVaccine HT™ or Montanide ISA 51, resulting in similar in vitro inhibition (65–82%) of five out of six culture-adapted P. falciparum strains. The antigen binding specificities of elicited antibodies were also similar and independent of the adjuvant used or the number of vaccine component alleles. Thus neither the four extra antigens nor adjuvant had any observable benefits with respect to specificity broadening, although adjuvant choice influenced the absolute antibody levels and thus the extent of parasite inhibition. Our data confirms the feasibility and potential of multi-allele PfAMA1 formulations, and highlights the need for adjuvants with improved antibody potentiation properties for AMA1-based vaccines.

Simulation of B Cell Affinity Maturation Explains Enhanced Antibody Cross-Reactivity Induced by the Polyvalent Malaria Vaccine AMA1

“…Researchers have explored two approaches to tackle the issue of antigenic variation in AMA1: polyvalent formulations using multiple strains of AMA1 (47,48) and chimeric variants of AMA1 (51,68), which incorporate a subset of high-frequency polymorphisms into a small set of constructs. In both cases, allelic coverage is the guiding principle, meaning that the vaccine should incorporate as broad a range of polymorphic variation as is possible (43).…”

“…The main parameters that were set to model AMA1 in this simulation were the number of epitopes in the AMA1 Ag, the relative immunogenicity of each epitope, and the antigenic distance of each epitope between different AMA1 strains. These parameters were determined using previous experimental data from monovalent AMA1 vaccination studies (47,48,51). We coded the algorithm in Python, and the source code is freely available from the authors by request.…”

EDiP: the Epitope Dilution Phenomenon. Lessons learnt from a malaria vaccine antigen and its applicability to polymorphic antigens

“…Rabbits immunized with a mixture of the three AMA1 DiCo proteins (D1, D2, and D3 in Figure 2) produced antibodies that recognized a panel of three natural AMA1 variants in ELISA and functional assays similar to the FVO antibodies titers obtained using a homologous (FVO) immunization regime [16]. Rabbit antibodies to the DiCo mixture significantly inhibited red blood cell (RBC) invasion by five different parasite strains (FCR3, 3D7, HB3, CAMP, and 7G8) [28], and a mixture of three AMA1 DiCo proteins formulated in potent adjuvants was subsequently shown to induce high (functional) responses to a panel of three laboratory strains in nonhuman primates [29]. Of note here is that none of the DiCo proteins had a glutamic acid at the previously defined immunodominant position 197 [19], yet growth inhibition against the 3D7 strain harboring glutamic acid at position 197 was comparable to inhibition of assay strains with a homologous amino acid at position 197 (e.g.…”

“…Sera obtained after mixed antigen vaccination also more efficiently neutralized an outgroup strain (CAMP; Figure 2) not contained in the vaccine formulation [31]. Using competition ELISA and affinity purification experiments, it was shown that immunization with a mixture of three antigens yielded an increased fraction of cross-reactive antibodies, rather than a focus of the antibody response on strain-specific epitopes [28,31]. The increase in cross-reactive antibody responses following vaccination with a mixture of AMA1 variants was later elegantly confirmed using P. berghei AMA1 orthologue scaffolds expressing conserved or variable residues in the correct conformation [31].…”

Using Population Genetics to Guide Malaria Vaccine Design

“…High levels of polymorphism in AMA1 [8,91,92] due to strong balancing selection [93] has resulted in hundreds of distinct AMA1 haplotypes; this might indicate that the development of a broadly effective AMA1-based malaria vaccine will be difficult. However, little is known about the antigenic diversity of AMA1 and recent studies suggest that immunization with a small number of different alleles might give broad reactivity [94,95]. The availability of a 3D structural model for AMA1 has greatly advanced our understanding of antibody inhibition of AMA1 function by demonstrating that several polymorphisms are found on the edge of a hydrophobic pocket within which it is thought the receptor binds [96,97].…”