A direct and rapid sequencing strategy for the Plasmodium falciparum antigen gene gp190/MSA1

Induction of multi-antigen multi-stage immune responses against Plasmodium falciparum in rhesus monkeys, in the absence of antigen interference, with heterologous DNA prime/poxvirus boost immunization

The present study has evaluated the immunogenicity of single or multiple Plasmodium falciparum (Pf) antigens administered in a DNA prime/poxvirus boost regimen with or without the poloxamer CRL1005 in rhesus monkeys. Animals were primed with PfCSP plasmid DNA or a mixture of PfCSP, PfSSP2/TRAP, PfLSA1, PfAMA1 and PfMSP1-42 (CSLAM) DNA vaccines in PBS or formulated with CRL1005, and subsequently boosted with ALVAC-Pf7, a canarypox virus expressing the CSLAM antigens. Cell-mediated immune responses were evaluated by IFN-γ ELIspot and intracellular cytokine staining, using recombinant proteins and overlapping synthetic peptides. Antigen-specific and parasite-specific antibody responses were evaluated by ELISA and IFAT, respectively. Immune responses to all components of the multi-antigen mixture were demonstrated following immunization with either DNA/PBS or DNA/CRL1005, and no antigen interference was observed in animals receiving CSLAM as compared to PfCSP alone. These data support the down-selection of the CSLAM antigen combination. CRL1005 formulation had no apparent effect on vaccine-induced T cell or antibody responses, either before or after viral boost. In high responder monkeys, CD4+IL-2+ responses were more predominant than CD8+ T cell responses. Furthermore, CD8+ IFN-γ responses were detected only in the presence of detectable CD4+ T cell responses. Overall, this study demonstrates the potential for multivalent Pf vaccines based on rational antigen selection and combination, and suggests that further formulation development to increase the immunogenicity of DNA encoded antigens is warranted.

AA. A comparative study of the genetic diversity of the 42kDa fragment of the merozoite surface protein 1 in Plasmodium falciparum and P. vivax

We investigated the genetic diversity of the 42kDa fragment of the merozoite surface protein 1 (MSP-1) antigen in Plasmodium falciparum and P. vivax, as well as in non-human primate malarial parasites. This fragment undergoes a proteolytic cleavage generating two fragments of 19kDa (MSP-1(19)) and 33kDa (MSP-1(33)) that are critical in erythrocyte invasion. We found that overall the MSP-1(33) fragment exhibits greater genetic diversity than the MSP-1(19) regardless of the species. We have found evidence for positive natural selection only in the human malaria parasites by comparing the rate of non-synonymous versus synonymous substitutions. In addition, we found clear differences between the two major human malaria parasites. In the case of P. falciparum, positive natural selection is acting on the MSP-1(19) region while the MSP-1(33) is neutral or under purifying selection. The opposite pattern was observed in P. vivax. Our results suggest different roles of this antigen in the host-parasite immune interaction in each of the major human malarial parasites.

A multigene family that interacts with the amino terminus of plasmodium MSP-1 identified using the yeast two-hybrid system

Merozoite surface protein 1 (MSP-1) is a high-molecular-weight protein expressed on the surface of the malaria merozoite in a noncovalent complex with other protein molecules. MSP-1 undergoes a series of proteolytic processing events, but no precise biological role for the various proteolytic fragments of MSP-1 or for the additional proteins present in the complex is known. Through the use of the yeast two-hybrid system, we have isolated genes encoding proteins that interact with a region of the amino-terminal proteolytic fragment of MSP-1 from the mouse parasite Plasmodium yoelii. This analysis has led to the isolation of two sequence-related molecules, one of which is the P. yoelii homologue of MSP-7 originally described in Plasmodium falciparum. BLAST analysis of the P. falciparum database has revealed that there are six related protein molecules present in this species encoded near each other on chromosome 13. In P. falciparum, we designated these molecules MSRP-1 to -5. Analysis of the P. yoelii database indicates a similar chromosomal organization for the two genes in the mouse parasite species. The three P. falciparum sequences with the highest degree of homology to the P. yoelii sequences isolated in the two-hybrid screen have been characterized at the molecular level (MSRP-1 to -3). Expression analysis indicated that the mRNAs are expressed at various levels in the different asexual stages. Immunofluorescence studies colocalized the expression of the MSRP molecules and the amino-terminal portion of MSP-1 to the surfaces of trophozoites. In vitro binding experiments confirmed the interaction between MSRP-1, MSRP-2, and the amino-terminal region of P. falciparum MSP-1.

Sequence diversity of the merozoite surface protein 1 of Plasmodium falciparum in clinical isolates from the Kilombero District, Tanzania

Merozoite surface protein 1 of Plasmodium falciparum (PfMSP-1) is regarded as a key candidate antigen for malaria vaccine development. It exhibits significant antigenic polymorphism and has been divided into 17 building blocks based on the analysis of sequence diversity. Differences in the antigenic composition of PfMSP-1 in local P. falciparum populations may result in differences in the efficacy of vaccines, which contain sequences of particular allelic variant(s) of PfMSP-1. To contribute to the required knowledge of genetic diversity of malaria parasites in geographically diverse regions, we have used the polymerase chain reaction (PCR) to analyze the sequence diversity of blocks 1-4 of PfMSP-1 in disease isolates from the Kilombero District in Tanzania. In the semi-conserved block 1, in which dimorphic amino acid variances have been described at three positions, we found three of the five previously described combinations of these three pairs of amino acids. In addition one combination was found, which has not been reported before in parasite isolates from different locations worldwide. Of the two sequence variants, which were dominating, one (S44-Q47-V52) corresponded to the 83.1 sequence incorporated into the SPf66 malaria peptide vaccine, while the other one (G44-H47-I52) differed from the previous in all three dimorphic amino acids. The partial protection observed in a phase III SPf66 trial conducted in the Kilombero District in children aged 1-5, thus does not seem to be associated with a clear dominance of favourable variants of block 1 of PfMSP-1 in this area. All three different principle types of block 2, the major polymorphic region of PfMSP-1, were found in the Tanzanian isolates. Most of the sequences contained K1-type tripeptide repeats, but clones with MAD20-type repeats or no repetitive sequence (RO33-type block 2) were also present. K1- and MAD20-type tripeptide repeat motifs were never mixed within one parasite clone. In one sequence a hexapeptide repeat was found at the end of block 2, which has not been reported before. Dimorphism in 13 of the 17 previously described variable positions of the semi-conserved block 3 and three of four recombination types of block 4 (K/K, M/K and M/M) were found among the Tanzanian isolates. Apart from previously described dimorphic amino acid positions, polymorphism was rare in the non-repeated building blocks. Selection and spreading of parasite variants, which contain amino acid exchanges at other than the dimorphic positions thus, is not a common event. Parasite isolates frequently harboured more than one PfMSP-1 allele. Three of the four heterogeneous isolates analysed contained two different general types of sequences. One isolate contained at least four distinct clones, demonstrating the high endemicity of malaria in the Kilombero District, which is a well-established site for malaria vaccine field trials.

Genetic polymorphism and natural selection in the malaria parasite Plasmodium falciparum

We have studied the genetic polymorphism at 10 Plasmodium falciparum loci that are considered potential targets for specific antimalarial vaccines. The polymorphism is unevenly distributed among the loci; loci encoding proteins expressed on the surface of the sporozoite or the merozoite (AMA-1, CSP, LSA-1, MSP-1, MSP-2, and MSP-3) are more polymorphic than those expressed during the sexual stages or inside the parasite (EBA-175, Pfs25, PF48/45, and RAP-1). Comparison of synonymous and nonsynonymous substitutions indicates that natural selection may account for the polymorphism observed at seven of the 10 loci studied. This inference depends on the assumption that synonymous substitutions are neutral, which we test by analyzing codon bias and G+C content in a set of 92 gene loci. We find evidence for an overall trend towards increasing A+T richness, but no evidence for mutation bias. Although the neutrality of synonymous substitutions is not definitely established, this trend towards an A+T rich genome cannot explain the accumulation of substitutions at least in the case of four genes (AMA-1, CSP, LSA-1, and PF48/45) because the Gleft and right arrow C transversions are more frequent than expected. Moreover, the Tajima test manifests positive natural selection for the MSP-1 and, less strongly, MSP-3 polymorphisms; the McDonald-Kreitman test manifests natural selection at LSA-1 and PF48/45. We conclude that there is definite evidence for positive natural selection in the genes encoding AMA-1, CSP, LSA-1, MSP-1, and Pfs48/45. For four other loci, EBA-175, MSP-2, MSP-3, and RAP-1, the evidence is limited. No evidence for natural selection is found for Pfs25.