A conserved multi-gene family induces cross-reactive antibodies effective in defense against Plasmodium falciparum

Limited variation in vaccine candidate Plasmodium falciparum Merozoite Surface Protein-6 over multiple transmission seasons

BACKGROUND

Plasmodium falciparum Merozoite Surface Protein-6 (PfMSP6) is a component of the complex proteinacious coat that surrounds P. falciparum merozoites. This location, and the presence of anti-PfMSP6 antibodies in P. falciparum-exposed individuals, makes PfMSP6 a potential blood stage vaccine target. However, genetic diversity has proven to be a major hurdle for vaccines targeting other blood stage P. falciparum antigens, and few endemic field studies assessing PfMSP6 gene diversity have been conducted. This study follows PfMSP6 diversity in the Peruvian Amazon from 2003 to 2006 and is the first longitudinal assessment of PfMSP6 sequence dynamics.

METHODS

Parasite DNA was extracted from 506 distinct P. falciparum infections spanning the transmission seasons from 2003 to 2006 as part of the Malaria Immunology and Genetics in the Amazon (MIGIA) cohort study near Iquitos, Peru. PfMSP6 was amplified from each sample using a nested PCR protocol, genotyped for allele class by agarose gel electrophoresis, and sequenced to detect diversity. Allele frequencies were analysed using JMP v.8.0.1.0 and correlated with clinical and epidemiological data collected as part of the MIGIA project.

RESULTS

Both PfMSP6 allele classes, K1-like and 3D7-like, were detected at the study site, confirming that both are globally distributed. Allele frequencies varied significantly between transmission seasons, with 3D7-class alleles dominating and K1-class alleles nearly disappearing in 2005 and 2006. There was a significant association between allele class and village location (p-value = 0.0008), but no statistically significant association between allele class and age, sex, or symptom status. No intra-allele class sequence diversity was detected.

CONCLUSIONS

Both PfMSP6 allele classes are globally distributed, and this study shows that allele frequencies can fluctuate significantly between communities separated by only a few kilometres, and over time in the same community. By contrast, PfMSP6 was highly stable at the sequence level, with no SNPs detected in the 506 samples analysed. This limited diversity supports further investigation of PfMSP6 as a blood stage vaccine candidate, with the clear caveat that any such vaccine must either contain both alleles or generate cross-protective responses that react against both allele classes. Detailed immunoepidemiology studies are needed to establish the viability of these approaches before PfMSP6 advances further down the vaccine development pipeline.

Toward the rational design of a malaria vaccine construct using the MSP3 family as an example: contribution of antigenicity studies in humans

Plasmodium falciparum merozoite surface protein (MSP3) is a main target of protective immunity against malaria that is currently undergoing vaccine development. It was shown recently to belong, together with MSP6, to a new multigene family whose C-terminal regions have a similar organization, contain both homologous and divergent regions, and are highly conserved across isolates. In an attempt to rationally design novel vaccine constructs, we extended the analysis of antigenicity and function of region-specific antibodies, previously performed with MSP3 and MSP6, to the remaining four proteins of the MSP3 family using four recombinant proteins and 24 synthetic peptides. Antibodies to each MSP3 family antigen were found to be highly prevalent among malaria-exposed individuals from the village of Dielmo (Senegal). Each of the 24 peptides was antigenic, defining at least one epitope mimicking that of the native proteins, with a distinct IgG isotype pattern for each, although with an overall predominance of the IgG3 subclass. Human antibodies affinity purified upon each of the 24 peptides exerted an antiparasite antibody-dependent cellular inhibition effect, which in most cases was as strong as that of IgG from protected African adults. The two regions with high homology were found to generate a broad network of cross-reactive antibodies with various avidities. A first multigenic construct was designed using these findings and those from related immunogenicity studies in mice and demonstrated valuable immunological properties. These results indicate that numerous regions from the MSP3 family play a role in protection and provide a rationale for the tailoring of new MSP3-derived malaria vaccines.

Toward the rational design of a malaria vaccine construct using the MSP3 family as an example: contribution of immunogenicity studies in models

Plasmodium falciparum merozoite surface protein 3 (MSP3), the target of antibodies that mediate parasite killing in cooperation with blood monocytes and are associated with protection in exposed populations, is a vaccine candidate under development. It belongs to a family of six structurally related genes. To optimize immunogenicity, we attempted to improve its design based on knowledge of antigenicity of various regions from the conserved C terminus of the six proteins and an analysis of the immunogenicity of "tailored" constructs. The immunogenicity studies were conducted in BALB/c and C57BL/6J mice, using MSP3 (referred to here as MSP3-1) as a model. Four constructs were designed in order to assess the effect of sequences flanking the 69-amino-acid region of MSP3-1 previously shown to be the target of biologically active antibodies. The results indicate major beneficial effects of removing (i) the subregion downstream from the 69-amino-acid sequence, since antibody titers increased by 2 orders of magnitude, and (ii) the upstream subregion which, although it defines a T-helper cell epitope, is not the target of antibodies. The construct, excluding both flanking sequences, was able to induce Th1-like responses, with a dominance of cytophilic antibodies. This led to design a multigenic construct based on these results, combining the six members of the MSP3 family. This new construction was immunogenic in mice, induced antibodies that recognized the parasite native proteins, and inhibited parasite growth in the functional antibody-dependent cellular inhibition assay, thus satisfying the preclinical criteria for a valuable vaccine candidate.