The major merozoite surface antigen (MSAI) of plasmodium falciparum

Plasmodium falciparum: allelic variation in the merozoite surface protein 1 gene in wild isolates from southern Vietnam

Allelic variation in the Plasmodium falciparum merozoite surface protein 1 (MSP1) gene is expressed as an association of allelic types in variable blocks. In this study, a PCR strategy that can detect 24 different MSP1 association types was used to investigate allelic variation in the MSP1 gene. We identified 236 distinct association type clones in 136 wild isolates collected from southern Vietnam, analysis of which revealed that (1) recombination between two representative allelic types in the central part of the MSP1 gene did not exist, (2) frequency distribution of MSP1 association types did not differ in different population groups, and (3) particular MSP1 association types were predominant. Statistical analysis for the association of allelic types indicated significant, nonrandom associations between blocks 4 and 6 but not between blocks 2 and 4, and 2 and 6. These results suggest that selection operates in favor of particular MSP1 association types. In addition, direct sequencing of 31 isolates confirmed reported sequence substitutions in the C-terminal 19-kDa Cys-rich region of MSP1, supporting a notion of limited variations in this region, a strong vaccine candidate molecule.

Plasmodium falciparum: the repetitive MSA-1 surface protein of the RO-71 isolate is recognized by mouse antibody against the nonrepetitive repeat block of RO-33

We have expressed in Escherichia coli the nonrepetitive repeat zone of the MSA-1 surface protein of the RO-33 isolate of Plasmodium falciparum. The recombinant protein was used to immunize mice and the resulting RO-33 monospecific serum was used to screen our P. falciparum strain collection in order to recover additional alleles lacking tripeptide repeats in block 2 of MSA-1. Only 1 (RO-71) out of 30 isolates tested reacted strongly with the serum by indirect immunofluorescence assay. Surprisingly, block 2 of the RO-71 MSA-1 allele contains tripeptide repeats resembling those of the K1 isolate of P. falciparum. Additional sequence analysis of the entire DNA coding for the 80-kDa MSA-1-derived surface component did not reveal any amino acid stretches similar to block 2 of RO-33 which could rationalize the immunological cross-reactivity. We eliminated the possibility that the RO-71 culture was contaminated with RO-33 type alleles of MSA-1 by Southern blotting and PCR analysis. The RO-33-specific mouse serum used for the initial selection of RO-71 did not react with the antigen in the denatured state (Western blot). This and the sequence analysis suggest that the cross-reactive epitope in the MSA-1 protein of RO-71 is conformational. The possibility that a truncated frame-shift protein encoded by mutated MSA-1 mRNA is recognized by the serum is discussed.

Polymorphism of the alleles of the merozoite surface antigens MSA1 and MSA2 in Plasmodium falciparum wild isolates from Colombia

The degree of polymorphism and the allelic distribution of 2 major Plasmodium falciparum merozoite surface antigens (MSA1 and MSA2) have been analysed in clinical isolates from Colombia. DNA was prepared directly from patients' blood and used in PCR reactions to amplify block 2 of MSA1 and the central region from MSA2. Thirty one samples were analysed and a marked degree of length polymorphism was detected, especially for MSA2. A high proportion of multiple bands was also observed, most probably resulting from mixed infections. Allele-specific oligonucleotides were used to type both alleles. For MSA1, 26 out of 31 clinical isolates were of the RO33 type, 15 were MAD20 and three were typed as KI. When the MSA2 allele was analysed, 7 isolates hybridised with a CAMP specific probe and 6 hybridised strongly with an FC27-derived oligonucleotide. Two samples, which showed multiple bands, hybridised with both probes. Interestingly, in 14 out of 27 isolates the MSA2 allele remained unassigned by the specific probes. Five of these were cloned and their DNA sequenced; these sequences are discussed.

A study of the genomic diversity of Plasmodium falciparum in Senegal. 1. Typing by Southern blot analysis

The genomic polymorphism of Plasmodium falciparum was investigated in a series of samples collected in Senegal during one transmission season. Restriction site polymorphism was studied by Southern blot analysis using six different probes. The patterns of the ribosomal RNA genes and of the gene coding for antigen 2L indicated a limited genomic polymorphism. Sequences hybridizing to the repeats of the Palo Alto/Wellcome serotype of S-antigen were found in one out of twelve isolates examined. This strain was shown to express the Palo Alto serotype. Restriction fragment length polymorphism was observed for the 332 gene and the 11.1 locus. The hybridization patterns showed that each sample had a distinct 11.1 locus. A comparison of three probes (332, 11.1 and rep20) detecting fragment length polymorphism indicated that maximum sensitivity was obtained using the subtelomeric repeats rep20; less sensitive patterns were observed using the 11.1 27 bp repeat probe. By using these three probes it was found that all samples were genetically distinct.