Genetic diversity in the major merozoite surface antigen of Plasmodium falciparum: high prevalence of a third polymorphic form detected in strains derived from malaria patients

Analysis of sequence diversity in the Plasmodium falciparum merozoite surface protein-1 (MSP-1)

Immunization with the first identified Plasmodium falciparum merozoite surface protein (MSP-1) protected monkeys from an otherwise fatal infection. The question of whether the high degree of diversity in MSP-1 among parasite clones will be an impediment to its development as a vaccine candidate needs to be resolved. We have aligned all published sequences, identifying errors, resequencing a portion of one parasite clone, and identifying probable duplicate sequences of four pairs of parasite clones. The sequences are displayed in a fashion that facilitates the study of variation and its potentially diverse origins. The original dimorphic sequences described by Tanabe et al. have been modified to include only common sequences throughout the entire gene. The extension of the dimorphic region to the 5' end of block 3 brings into question the involvement of intragenic crossover as the major mechanism generating allelic diversity. Additional diversity developed from point mutations and recombination in certain regions of the gene. The regions of variability and conservation should serve as a data base for planning vaccine trials.

Conservation of antigen components from two recombinant hybrid proteins protective against malaria

Recently, we have shown that two hybrid proteins carrying partial sequences of the blood-stage antigens SERP, HRPII, and MSAI from Plasmodium falciparum confer protective immunity on Aotus monkeys against an experimental parasite infection (B. Knapp, E. Hundt, B. Enders, and H. A. Küpper, Infect. Immun. 60:2397-2401, 1992). The malarial components of the hybrid proteins consist of amino acid residues 630 to 892 of SERP, amino acid residues 146 to 260 of MSAI, and the 189 C-terminal residues of HRPII. We have studied the diversity of these protein regions in field isolates of P. falciparum. Genomic DNA was extracted from the blood of six donors from two different areas where malaria is endemic. The gene regions of SERP and MSAI coding for the corresponding sequences of the protective hybrid proteins and the exon II region of the HRPII gene were amplified by polymerase chain reaction and sequenced. All three regions were found to be highly conserved. In the 262-amino-acid fragment of SERP, one single conservative amino acid substitution was found. The exon II region of HRPII showed only a slight variability in number and arrangement of the repeat units. The 115-amino-acid fragment of MSAI which is located within an N-terminal region known to be conserved among different parasite strains was shown to be the most variable among the vaccine components: amino acid substitutions were found in 14 different positions of this MSAI region when both laboratory strains and field isolates were compared.

RNA virus-like particles in pathogenic plant trypanosomatids

A double-stranded RNA (ds RNA) with an approximate size of 4.7 kb was found in 6 Phytomonas isolates specifically associated with plant pathogenicity in coconut trees ("Hartrot" disease) and oil palm ("Marchitez sorpressiva" disease). This ds RNA was not detected in 10 non-pathogenic Phytomonas isolates from different lactiferous plants or in the insect trypanosomatids Crithidia and Herpetomonas. Analysis by electron microscopy of a sucrose gradient fraction containing this ds RNA revealed virus-like particles.

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.

Roles of conserved and allelic regions of the major merozoite surface protein (gp195) in immunity against Plasmodium falciparum

The Plasmodium falciparum major merozoite surface protein gp195 is a candidate antigen for a vaccine against human malaria. The significance of allelism and polymorphism in vaccine-induced immunity to gp195 was investigated in this study. Rabbits were immunized with each of two allelic forms of gp195 that were affinity purified from the FUP and FVO parasite isolates. gp195-specific antibodies raised against one allelic form of gp195 cross-reacted extensively with the gp195 of the heterologous allele in enzyme-linked immunosorbent assays (ELISAs) and immunofluorescence assays. Competitive binding ELISAs with homologous and heterologous gp195s confirmed that a majority of the anti-gp195 antibodies produced against either allelic protein were cross-reactive. Moreover, the biological activities of the gp195 antibody responses were also highly cross-reactive, since anti-gp195 sera inhibited the in vitro growth of the homologous and heterologous parasites with equal efficiency. The degree of cross-reactivity with strain-specific and allele-specific determinants of gp195 in the ELISA was low. These results suggest that the immunological cross-reactivity between the two gp195 proteins is due to recognition of conserved determinants. They also suggest that a gp195-based vaccine may be effective against blood-stage infection with a diverse array of parasite isolates.

Sequence variation in the tripeptide repeats and T cell epitopes in P190 (MSA-1) of Plasmodium falciparum from field isolates

The N-terminal part of p190, the precursor to the major merozoite surface antigens of Plasmodium falciparum, contains the T and B cell epitopes and tripeptide repeats. The p190 gene exhibits allelic dimorphism, but the tripeptide repeat-encoding region is the only exception to the dimorphic variations of the gene. To date, sequences available to document variations in the epitopes are very limited. Thus, in this study, the extent of the variation in these regions was analyzed using the polymerase chain reaction to amplify the DNA fragment encompassing these regions, followed by sequencing. Twenty-five gene clones were obtained from 19 isolates from the Mae Sod district in Thailand and their sequences were compared with those reported elsewhere. Results reveal 3 sequence types in the tripeptide-encoding region, and each contains a novel repetitive consensus nucleotide sequence. On the other hand, almost all nucleotide substitutions in block III are dimorphic. Identification of linkages of the dimorphic substitutions has led us to postulate 6 potential crossover sites, where intragenic recombinations of p190 alleles could occur. Sequences of T and B cell epitopes are highly conserved among these wild isolates and those from geographically diverse culture-adapted parasites.

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.

Detection of polymerase chain reaction-amplified malarial DNA in infected blood and individual mosquitoes

Chelex treatment of Plasmodium falciparum and P. berghei infected tissues, in lieu of organic extraction, was followed directly by polymerase chain reaction amplification of primed circumsporozoite gene sequences. The amplified DNA products were detected in stained gels and hybridization blots of extracts from individual infected mosquitoes and dissected mosquito tissues as well as small volumes of infected blood. Parasite development, within the mosquito midgut and salivary gland, was also monitored as a function of time post infectious blood meal. The temporal presence of amplifiable circumsporozoite gene sequences in the infected mosquito midgut lumen, midgut endothelium, and salivary glands corresponded directly to the visual identification of ookinetes, oocysts, and salivary gland sporozoites, respectively.

A study of the genomic diversity of Plasmodium falciparum in Senegal. 2. Typing by the use of the polymerase chain reaction

The genomic polymorphism of Plasmodium falciparum was investigated in a series of samples collected in Senegal during one transmission season. PCR analysis was performed on several genes coding for blood-stage antigens: the gene for the major merozoite surface antigen P190, the gene for the second merozoite surface antigen MSA2 and the gene coding for antigen 96tR/GBP130. In each case, several distinct forms of the genes studied were observed. Both the MAD20 and K1 allelic families of P190 genes were observed. PCR analysis of a single variable region did not differentiate each isolate. However, when the data obtained for several markers are combined, each isolate had a specific genotype. Thus, using PCR to study in parallel several loci is a useful tool to genetically type strains.

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.

Determination of genetic variation within Plasmodium falciparum by using enzymatically amplified DNA from filter paper disks impregnated with whole blood

A new method which allows the enzymatic amplification of DNA extracted from whole blood dried on filter paper disks is presented. The method was used to study heterogeneity within an erythrocyte-binding antigen (EBA-175) of Plasmodium falciparum. Blood specimens from malaria-infected patients in Southeast Asia and Africa were spotted onto filter paper, dried, and transported for processing. P. falciparum DNA was extracted by boiling the filter paper disks in the presence of Chelex-100 ion-chelating resin, amplified by the polymerase chain reaction, and analyzed for the presence of genetic variation. In all cases examined, plasmodial DNA was successfully amplified and characterized from filter paper disks. Hybridization of the polymerase chain reaction products with internal probes demonstrated simultaneous infection with two strains of P. falciparum in two patients. This technique represents a sensitive and practical field method for the determination of genetic variation within P. falciparum and the study of molecular epidemiology.

Primary structure of the merozoite surface antigen 1 of Plasmodium vivax reveals sequences conserved between different Plasmodium species

Merozoite surface antigen 1 (MSA1) of several species of plasmodia has been shown to be a promising candidate for a vaccine directed against the asexual blood stages of malaria. We report the cloning and characterization of the MSA1 gene of the human malaria parasite Plasmodium vivax. This gene, which we call Pv200, encodes a polypeptide of 1726 amino acids and displays features described for MSA1 genes of other species, such as signal peptide and anchoring sequences, conserved cysteine residues, number of potential N-glycosylation sites, and repeats consisting here of 23 glutamine residues in a row. When the nucleotide and deduced amino acid sequences of the MSA1 of P. vivax are compared to those of another human malaria parasite, Plasmodium falciparum, and to those of the rodent parasite Plasmodium yoelii, 10 regions of high amino acid similarity are observed despite the very different dG + dC contents of the corresponding genes. All of the interspecies conserved regions reside within the conserved or semiconserved blocks delimited by the sequences of different alleles of the MSA1 gene of P. falciparum.

Antigenic polymorphism in malaria: is it an important mechanism for immune evasion?

Malarial infections do not readily evoke an effective protective immunity against re-infection. Possible reasons for this include the ability of the parasites to interfere with the host's immune response and to evade the response in an immune host, by, for example, exploiting antigenic polymorphism or variation. Antigenic polymorphism undoubtedly exists in malaria parasite populations but does this polymorphism actually contribute to immune evasion by the parasite? Here, Kamini Mendis and colleagues examine the evidence for this and its implications for future malaria vaccines.