Primary structure of the precursor to the three major surface antigens of Plasmodium falciparum merozoites

Primary structure and localization of a conserved immunogenic Plasmodium falciparum glutamate rich protein (GLURP) expressed in both the preerythrocytic and erythrocytic stages of the vertebrate life cycle

A gene coding for a 220-kDa glutamate rich protein (GLURP), an exoantigen of Plasmodium falciparum, was isolated and its nucleotide sequence was determined. The deduced amino acid sequence contains 2 repeat regions. The sequence of one of these was shown to be conserved among geographically dispersed isolates, and a fusion protein containing that sequence was able to stimulate B- and T-cells. Antibodies against GLURP stained erythrocytic stages of the parasite as well as the hepatic stage as detected by electron microscopy.

Proteolytic processing of the Plasmodium falciparum merozoite surface protein-1 produces a membrane-bound fragment containing two epidermal growth factor-like domains

The amino-terminal sequence has been obtained for 2 fragments of the Plasmodium falciparum T9/94 merozoite surface protein precursor (PfMSP1) and these have been compared with the sequence predicted from the gene. These data define the position of these fragments in the precursor and indicate that the C-terminal sequence which is carried into the red cell during invasion consists of 2 epidermal growth factor (EGF)-like domains. A homologous cleavage sequence and domain structure can be identified in the MSP1 molecules of other malarial species. In addition the results suggest that the smaller fragment is not N-glycosylated.

Ability of recombinant or native proteins to protect monkeys against heterologous challenge with Plasmodium falciparum

To circumvent problems associated with polymorphic vaccine candidates for Plasmodium falciparum malaria, we evaluated recombinant proteins representing sequences from relatively high conserved regions of the precursor to the major merozoite surface proteins, gp190, for their ability to protect Saimiri monkeys against malaria challenge. Recombinant proteins represented amino acid residues 147 to 321 (p190-1) or 147 to 321 and 1060 to 1195 (p190-3), and their efficacy was compared with that of native gp190 and its processed products. All antigens were derived from P. falciparum K1, a Thai isolate, while the challenge strain was Palo Alto (from Uganda, Africa), which contains, with the exception of the N-terminal 375 amino acids, which are almost identical to the K1 sequence, essentially the MAD-20 allelic form of gp190. By 12 days following challenge, each control monkey required drug treatment. Three monkeys injected with p190-3 required therapy, while one cleared the parasites without therapy. Two monkeys injected with p190-1 received therapy on day 14, while the remaining two cleared the parasites without therapy. Of four animals injected with native gp190, because of health reasons unrelated to malaria, one was not challenged with parasites and one was removed from the study 8 days after challenge when its parasitemia was 1.1% (parasitemias in control animals ranged from 4.3 to 9%); the remaining two cleared the parasites after maximum parasitemias of 0.45 and 0.53%. The highest levels of antiparasite antibody were produced by animals immunized with native gp190. There was a significant correlation between monkeys which did not require drug treatment and antiparasite antibody. These results may suggest that native gp190 and/or its processed products can provide excellent protection against heterologous challenge and that antibody is important for protection. The challenge for vaccine development is to identify the protective sequence(s).

In vivo and in vitro derived Palo Alto lines of Plasmodium falciparum are genetically unrelated

The Uganda Palo Alto strain of Plasmodium falciparum (FUP) is routinely used as a reference isolate in a number of laboratories. It is one of the few P. falciparum strains that can both be propagated in vivo in monkeys and maintained in culture. The Palo Alto parasites have been characterized for several biochemical and molecular markers, but many of the data reported so far are contradictory. We have analyzed and compared by Southern blotting, PCR and DNA sequencing, several DNA preparations obtained from different FUP lines and from the FCR3 strain. We show here that FUP lines propagated in Saimiri monkeys (FUP/S) and those maintained in culture (FUP/C) for many years in our laboratory differ in the various genetic markers investigated (P190, MSA2, S-Ag, KAHRP, 96 tR, pPFPA rep 20 and pPF 11.1). Therefore, at the present, two genetically unrelated strains of P. falciparum widely distributed over numerous laboratories are designated FUP/Palo Alto. When the Saimiri-propagated FUP/S line was used to initiate an in vitro culture in human red blood cells, no evidence of instability or genetic drift was obtained. The growth rate and genomic characteristics remained constant for several months. Likewise, the FUP/C line was found unchanged after three transfers in Saimiri monkeys. FUP/CP parasites were shown to be genetically closely related to FCR3. In addition, a subline of FUP/C strain selected by repeated flotation on gelatin was found to differ in several characters such as KHARP, P190 and S-antigen genes, which are known to be located on different chromosomes.

Population genetics of Plasmodium falciparum within a malaria hyperendemic area

Serotyping with monoclonal antibodies was used to estimate the number and frequencies of allelic variants of two merozoite surface proteins, MSP1 and MSP2, and an exported protein Exp-1, in a sample of 344 clinical isolates of Plasmodium falciparum from an urban region of The Gambia. Represented among the isolates were 36, 8 and 2 alleles of the MSP1, MSP2 and Exp-1 loci respectively. Relative frequencies of these alleles remained stable in the parasite population over the 2 years of the study. A computer program was used to calculate from the frequencies of individual alleles at the three loci, the probable number of different genotypes in samples from the population, assuming random assortment among the loci. No significant difference was found between the expected and the observed genotype diversity. It is concluded that recombination among unlinked loci is a common consequence of sexual reproduction of P. falciparum in The Gambia. Slightly lower genotype diversity was observed in each of two villages, which may be a consequence of smaller population size compared with the urban region.

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.

The structure of the calmodulin gene of Plasmodium falciparum

The calmodulin gene and its flanking sequences from the malaria parasite, Plasmodium falciparum, have been analysed. The structure of this gene is unique amongst other known calmodulin genes. It exists as a single copy on chromosome 14 and has a single intron. The nucleotide sequence of this 4-kb region suggests the existence of three transcriptional units, each separated by a highly A+T-rich sequence. Sequences controlling gene expression might be expected to occur in these intergenic regions. The predicted protein sequences suggest that these other genes are transcribed in different orientations. Primer extension studies suggest that calmodulin mRNA has a major start site 62 bases upstream of the initial ATG. The calmodulin gene possesses consensus eukaryotic TATA, CAAT box, polyadenylation and splice junction sequences. This is the first detailed report of the DNA sequence surrounding a housekeeping gene in P. falciparum.

Plasmodium falciparum: genetic stability of the Uganda Palo Alto strain propagated in the squirrel monkey (Saimiri sciureus)

The Uganda Palo Alto strain of Plasmodium falciparum has been extensively used in several laboratories to infect Saimiri monkeys. In the study reported here three distinct lines derived by parallel serial blood transfers from a single 10-year-old passage of the Palo Alto strain were examined for genetic diversity using the polymerase chain reaction and restriction mapping techniques. A comparison of the PF 11.1, P190, MSA2, S-Ag, and KAHRP genes indicates that these parasite lines are apparently homogeneous and stable. Nevertheless, microheterogeneity was observed with molecular probes which are known to easily detect restriction fragment length polymorphisms (rep 20 and telomeric probes). These data show that the genetic characters of the strain can be considered monomorphic and are conserved after multiple passages in the squirrel monkey.

Development of natural immunity in Plasmodium falciparum malaria: study of antibody response by Western immunoblotting

A longitudinal study was carried out in Burkina Faso to investigate the natural development of the immune response to Plasmodium falciparum malaria. Three bleedings were carried out before, during, and after the seasonal peak of transmission. Detailed antigen mapping and antibody prevalence of the 248 collected serum samples were established by immunoblotting on the basis of several epidemiological and biological parameters. An improved Western immunoblotting system was used to analyze up to 67 serum samples on each nitrocellulose sheet. This system allowed us to perform the entire study with strictly comparable conditions. Two different blood-stage antigens (exoantigens and somatic antigens) were used to analyze the distribution of different classes and subclasses of immunoglobulins according to the age of the individuals, the presence or absence of a malarial attack, the transmission period, the origin of parasite isolates, and the response to intraerythrocytic stages. Although this analysis emphasizes strong individual variations, reactions with two major antigens of 115 and 103 kDa were especially noted. These antigens induced high antibody levels and prevalences but were probably not involved in protection. The prevalence of immunoglobulin G (IgG) antibodies differed by isotype. Most of antigens stimulating IgG production were also responsible for the IgM antibody response. The role played by these antibodies in the development of natural immunity against malaria is discussed.

Malaria vaccine

Among infectious diseases caused by protozoa, malaria is still the greatest killer of children. Mortality in adults living in endemic areas is significantly lower because they frequently acquire partial or complete immunity to the major pathogen, Plasmodium falciparum. This natural protection indicates that vaccination may be possible, and the first candidate antigens were cloned with the use of human immune sera as probes. Genetic and biochemical analysis of the parasite proteins revealed that they are polymorphic, and frequently gene sequences were discovered which were specific for a particular parasite isolate, which eliminated most antigens for purposes of vaccine development. The most promising candidate antigens today are the major surface proteins of sporozoites and blood stage parasites. However, the immune response against those is not sufficient for complete protection, and additional, intensive research is necessary to identify new molecules to be included in a vaccine cocktail against malaria. The current spread of the disease due to increasing drug resistance of parasites and mosquito vectors emphasizes the urgent need for a vaccine.

Molecular cloning and sequence analysis of the gene encoding the major merozoite surface antigen of Plasmodium chabaudi chabaudi IP-PC1

The complete nucleotide sequence of the gene encoding the precursor to the major merozoite surface antigens of Plasmodium chabaudi chabaudi strain IP-PC1 has been determined. A single open reading frame was detected, that coded for a protein of 199 kDa. The encoded protein (p199) contains putative signal and membrane anchor sequences and shows a clustering of Cys residues in the last 120 amino acids. Incompletely conserved tandem repeat oligopeptides are present at different positions in the molecule. P199 shows 69% overall homology to the analogous antigen in Plasmodium yoelii yoelii strain YM. The divergence between these antigens is largely confined to 4 areas where a number of insertions and/or deletions have occurred. All repeats occur in these divergent regions. The overall homology with both alleles of Plasmodium falciparum PMMSA is 33%.

Glycolipid anchorage of Plasmodium falciparum surface antigens

Human red blood cells (RBC) were infected with the malarial parasite Plasmodium falciparum, the anchoring of schizont proteins to RBC membranes by glycoinositol phospholipids was demonstrated by three criteria: (1) metabolic incorporation of 3H-ethanolamine and 3H-myristate into the protein; (2) release of 35S-methionine-labelled protein into the supernatant after incubation with phosphatidylinositol-specific phospholipase C; and (3) the exposure of a glycoinositol phosphate epitope on the methionine-labelled protein following phospholipase C cleavage. Labelled proteins were analysed by immunoprecipitation, polyacrylamide gel electrophoresis in sodium dodecylsulphate and gel fluorography. Several candidate proteins were observed when each criteria was investigated. Among these, 3 proteins which met all three criteria were identified by immunoprecipitation with monospecific sera or monoclonal antibodies. These included 3 possible vaccine candidates, the p190 major surface antigen, the p76 serine protease and the p71 protein which is thought to be a member of the family of heat-shock Hsp70 proteins.

Analysis of the sequences flanking the translational start sites of Plasmodium falciparum

The 5' and 3' regions adjacent to the initiation codon in 22 Plasmodium falciparum sequences were examined. A 5' consensus sequence (AAAA/ATG) was found. Although P. falciparum non-translated DNA is A-rich, A occurred significantly more frequently in the 4 positions preceding the initiation ATG than in adjacent non-translated DNA, suggesting that this consensus sequence has functional significance in the initiation of translation. This region has similarities with the equivalent sequences in yeast and Drosophila but differs markedly from that in vertebrates. No significant bias in nucleotide frequencies was found 3' to the initiation codon.

Measuring cellular immune responses to malaria antigens in endemic populations: epidemiological, parasitological and physiological factors which influence in vitro assays

Measurements of in vitro cellular immune responses to malaria antigens are influenced by a variety of external factors. The physiological status of the donor, which is affected by, for example, malaria infection, intercurrent illness and pregnancy, can influence the lymphoproliferative response to specific antigens. Prior exposure to malaria antigens, determined by malaria endemicity, seasonal variations in transmission and the degree of polymorphism of the particular antigen, will also affect the prevalence and intensity of responses. Malaria-related immunosuppression may be both generalised and antigen specific. Although in vitro responses to malaria antigens are profoundly suppressed in acutely infected individuals, there is evidence that lymphocyte activation does occur in vivo. We conclude that longitudinal studies, correlating specific immune responses with subsequent malaria morbidity are required, to identify potentially protective antigens and appropriate effector mechanisms.

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

We studied the diversity of the polymorphic 195-kDa antigen (p190) of Plasmodium from infected individuals. Genomic parasite DNA was extracted from the blood of 30 donors from different endemic areas of Brazil. The 5' region, encoding the polymorphic N-terminal part of p190 was analysed following polymerase chain reaction (PCR). Multiple infections of genetically distinct parasites could be detected within infected malaria patients. Sequence analysis and oligodeoxyribonucleotide typing of the PCR products demonstrated the prevalence of a third polymorphic form of p190.

Chromosome 9 from independent clones and isolates of Plasmodium falciparum undergoes subtelomeric deletions with similar breakpoints in vitro

We show that chromosome 9 in all isolates and clones of Plasmodium falciparum examined so far exists as one of two distinctly different forms, a large form about 1.9 megabases long or a smaller form about 25% shorter. Physical maps of chromosome 9 from independent clones with large and small forms of chromosome 9, and from an isolate with the large form and 3 derived clones with the small form reveal the underlying structural basis of this size polymorphism. The small form differs from the large only in that there are subtelomeric deletions at each end, one of these deletions involving about 0.45 megabases. Remarkably, the breakpoints map within about +/- 1% of the total chromosome length for each of these populations. We discuss some possible mechanisms for this.

Expression of hybrid malaria antigens in insect cells and their engineering for correct folding and secretion

Hybrid proteins containing selected regions of the major surface antigens of the sporozoite and merozoite stages of Plasmodium falciparum were expressed in insect cells using baculovirus vectors. A recombinant protein containing the signal peptide from the precursor to the major merozoite surface antigens (PMMSA) fused to a fragment from the carboxy (C) terminus of the same gene was recognized by monoclonal antibodies specific for reduction-sensitive conformational epitopes within the C-terminal fragment, suggesting that correct disulphide cross-linking of cysteine residues within this region had occurred. Addition of 26 copies of the tetrapeptide repeat from the circumsporozoite protein (CSP) resulted in a protein recognized by anti-CSP antiserum as well as the conformation specific monoclonal antibodies. Deletion of the C-terminal putative anchor sequence from both proteins resulted in secretion of protein in a fully soluble form antigenically indistinguishable from the anchor containing products. Correct conformation was not observed when the proteins were expressed as polyhedrin fusions without the signal peptide. These data indicate that the PMMSA signal peptide is recognized in insect cells and that correct assembly of disulphide cross-links is dependent upon targeting the protein to the endoplasmic reticulum.

Polyclonal in vitro proliferative responses from nonimmune donors to Plasmodium falciparum malaria antigens require UCHL1+ (memory) T cells

The in vitro polyclonal proliferative responses of peripheral blood mononuclear cells to whole blood stage parasites or fractionated antigens from the human malaria parasite Plasmodium falciparum were studied. Cells from healthy laboratory donors who had never been exposed to malaria antigens in vivo consistently proliferated to P. falciparum antigens, as did cord blood mononuclear cells. This response was only observed in sheep rosette-positive cells in the presence of adherent cells and was inhibited by NH4Cl, indicating a requirement for antigen processing. The proliferative response was strongest at day 6 and was dependent on the presence of cells expressing high levels of CD45 180-kD isomer (UCHL1 monoclonal antibody), a marker for activated or memory cells, but not for CD45R (SN130 monoclonal antibody) a marker for naive or unprimed T cells. This suggests a similarity to the recall response to tuberculin antigen. These results suggest that the proliferative response to malaria antigens observed previously and described as a nonspecific mitogenic response may be a cross-reactive response to epitopes shared between P. falciparum and other common immunogens. This would explain the establishment of T cell clones to malaria antigens from such donors, but might suggest that the epitopes to which such clones are specific may be of questionable protective or diagnostic use.

Cross-reactivity of antibody against an epitope of the Plasmodium falciparum second merozoite surface antigen

Monoclonal antibodies directed against the 51 kD merozoite surface antigen of Plasmodium falciparum also bind to other antigens within the infected cell. The sizes of these cross-reacting antigens have been characterized. Immunofluorescence due to the reaction of one of the monoclonal antibodies with these cross-reacting antigens was localized in the intra-erythrocytic parasite and in granules in the infected red cell cytoplasm. This immunofluorescence could be distinguished from the merozoite surface antigen in parasite lines with a variant serotype of the merozoite surface antigen which fails to react with the monoclonal antibodies. It was found that the in-vitro growth inhibition caused by the presence of one of the monoclonal antibodies, 8G10/48, was dependent on the expression of the corresponding serotype of merozoite surface antigen, a finding consistent with the inhibitory effect of this antibody being primarily directed against the merozoite surface antigen and not the cross-reacting antigens. Analysis of the frequency at which epitopes occur suggests that such cross-reacting proteins will be commonly seen in malaria, without the need to postulate a selective advantage for such cross-reacting specificities.

Cloning and analysis of the gene encoding the 230-kilodalton merozoite surface antigen of Plasmodium yoelii

The complete nucleotide sequence of the gene for the 230-kDa precursor to the major merozoite surface antigens (PMMSA) of Plasmodium yoelii YM has been determined. A single open reading frame of 5316 bp encodes a protein of calculated molecular mass 197,000. The deduced amino acid sequence contains potential signal peptide and membrane anchor sequences of 19 and 18 residues, respectively, and a region of six tandemly repeated tetrapeptides, Gly-Ala-Val-Pro. Comparison with the 195-kDa Plasmodium falciparum analogue (Pf195) at the amino acid level indicated an overall homology of approximately 30%, with areas of as high as 60% conservation. The tripeptide repeats present near the N-terminus of the Pf195 antigen are absent from the Py230 sequence. The PMMSA can be divided into 22 blocks based upon interspecies amino acid conservation.

Analysis of human T cell response to two Plasmodium falciparum merozoite surface antigens

Eight novel human T cell epitopes were identified within the two major merozoite surface antigens (MSA1 and MSA2) of Plasmodium falciparum using synthetic peptides. All except one of the peptides conformed structurally to an amphipathic alpha helix and three out of the four MSA1 peptides also contained sequences containing the Rothbard motif. Peptide MSA2/2, which fitted none of these criteria, was recognized by our donors to a similar degree as the other peptides. This peptide also contains a B cell epitope. Proliferative responses were obtained in both immune and nonimmune donors, however, the number of responses in the immune donor group was significantly higher. There was no correlation between the level of proliferation and antibody titers to these antigens. No peptides were preferentially recognized in association with specific HLA class II antigens.

Plasmodium falciparum: an intervening sequence in the GBP 130/96 tR gene

The 96 tR antigen is a heat stable protein produced during the late stages of the intraerythrocytic development of the malaria parasite Plasmodium falciparum and is released into the culture supernatant or the sera of infected patients at the time of schizont rupture. This antigen, identified as a putative protective antigen, was shown to be identical to the glycophorin-binding protein GBP 130 (Perkins 1988, Bonnefoy et al. 1988). We report here that the gene contains a small undescribed intervening sequence located immediately after the sequence coding for the signal sequence. This shows that in P. falciparum, all the genes described so far coding for proteins exported outside the parasitophorous vacuole share a common organization.

A protective monoclonal antibody recognizes a linear epitope in the precursor to the major merozoite antigens of Plasmodium chabaudi adami

The monoclonal antibody 5C10/66 was shown to afford strong protection in mice against fulminating Plasmodium chabaudi adami infection. This was remarkable, as immunity to this organism is regarded to be mainly T-cell mediated. This antibody identified a 250-kDa molecule in schizonts and an 83-kDa fragment in merozoites. A cDNA clone selected by 5C10/66 was the homologue of the Plasmodium falciparum precursor to the major merozoite surface antigen (PMMSA). Comparison with the P. falciparum sequence showed that the P. chabaudi adami clone encoded the middle portion of the gene and that it can also be divided into variable and conserved blocks. Screening of a set of all possible octamer peptides predicted by the cDNA clone revealed that the core epitope of 5C10/66 was Glu-Thr-Thr-Glu-Thr. This region resides in a variable block of PMMSA.

Polymorphism of a 35-48 kDa Plasmodium falciparum merozoite surface antigen

Glycoproteins of the asexual blood stages of Plasmodium falciparum were labelled with radioactive glucosamine and analysed by two-dimensional electrophoresis. Four major glycoproteins were detected in all eight parasite isolates studied. Two of the glycoproteins, designated GP2 and GP4, were invariant among the isolates, while the other two GP1 and GP3 were found to be polymorphic in both their biochemical and antigenic properties. By immunoblotting and immunoprecipitation with specific monoclonal antibodies, the two polymorphic glycoproteins were identified as surface antigens of merozoites.

Age-related prevalence of antibody response against three different, defined Plasmodium falciparum antigens in children from the Haut-Ogooué province in Gabon

The kinetics of the humoral response to defined Plasmodium falciparum antigens was studied in 543 children, 1 month to 15 years old, living in an area endemic for malaria. The antigens used for enzyme-linked immunosorbent assay were (i) the synthetic peptide (NANP)40 representing the immunodominant repeated region of the circumsporozoite protein, and (ii) the fusion peptide 31.1, representing the N-terminal portion of the 83 kDa polypeptide expressed at the surface of merozoites which is a processed product of the 190-200 kDa glycoprotein. In addition, glutaraldehyde-fixed infected red blood cells (RBC) were used to detect ring-infected erythrocyte surface antigen (RESA) and unfixed infected RBC to detect intra-erythrocytic asexual form (IEF) antigens by immunofluorescence. In the 1 to 2 months age group, 50%, 26% and 21% of the children had antibodies for IEF, (NANP)40 and 31.1 respectively, but none had anti-RESA antibodies. The proportions of positive subjects decreased until 3 to 6 months and then increased progressively for the 4 antigens, approaching, but not reaching, adult values by the age of 15 years. Antibodies against specific antigens were acquired concomitantly. Children born from (NANP)40-positive mothers showed enhanced anti-(NANP)40 IgG responses.

Cross-reactive antigenic determinants present on different Plasmodium falciparum blood-stage antigens

A gene encoding a previously undescribed antigen of Plasmodium falciparum has been isolated from a genomic expression library by use of a pool of human immune sera. Northern blot analysis indicated that the gene is expressed at the late stages of the intra-erythrocytic cycle. This antigen, 332, contains a series of degenerated amino acid repeats. Human antibodies affinity-purified on the 332 recombinant antigen reacted with a family of parasite proteins that are products of different genes. We identified antigens 11.1 and Pf155-RESA as members of this family and confirmed, using a human monoclonal antibody, the presence of cross-reacting determinants. The sequences of these antigens also share some structural homologies. The significance of this family of blood-stage antigens is discussed.

An update on candidate malaria vaccines

The purpose of this work is to review the progress towards malaria vaccination that has been made over the last four or so years, and the prospects and difficulties as they now appear. Although some of the older literature will be referred to as necessary background, it is not treated here in any detail. The reader who wishes for a fuller historical perspective should see, for instance, Brown (1969), Cohen & Mitchell (1978), Desowitz & Miller (1980), Mitchell (1984), Miller, David & Hadley (1984), Heidrich (1986) and, specifically for a consideration of sporozoite vaccination, Nussenzweig & Nussenzweig (1984, 1986). Naturally, any summary of vaccination will lean on the immunology of the disease, but malaria immunology is not reviewed here in its own right. The reader requiring the most recent attempt to cover this field should see the work edited by Perlmann & Wigzell (1988); some individual chapters of that volume will be referred to below.

Molecular cloning, genomic structure and localization in a blood stage antigen of Plasmodium falciparum characterized by a serine stretch

Two short DNA segments were isolated by screening of a lambda gt11 library from Plasmodium falciparum schizont cDNA with an antiserum against the 140 kDa protein, which confers protective immunity to monkeys. The segments were used to identify a genomic fragment which carries the entire coding sequence for a protein of 113 kDa characterized by a stretch of serine residues (SERP I). We present the complete nucleotide and deduced amino acid sequence as well as the structure of the SERP I gene. The gene consists of four exons interrupted by three short introns located at the amino-terminal half. Exon 1 and the first part of exon 2 code for hydrophobic amino acids of a putative signal sequence. Exon 2 contains two repetitive segments, the first encoding six glycine rich octapeptides and a second region coding for 37 consecutive serine residues. Southern blot analysis demonstrated the conservation of the SERP I gene in four different parasite strains. SERP I could be localized in the parasitophorous vacuole and in the surrounding membranes. We discuss the relationship of this protein to the recently described P126 polypeptide and the possible role of this antigen as a vaccine candidate.

Plasmodium antigens external to the parasite but with the infected erythrocyte

Three Plasmodium berghei exoantigens with apparent mol. wt. of 120, 31, and 13 kDa, found in infected erythrocytes by immunofluorescence, are further characterized. These antigens, synthesized in the late trophozoite and schizont stages, were released into the culture medium after schizont-infected erythrocytes were placed in culture; however, they were not found in the sera of infected animals. The 120-kDa antigen proved to be somewhat heat-stable, whereas the other two did not. A monoclonal antibody (MAb) recognizing the 13-kDa antigen cross-reacted with proteins from P. chabaudi and P. yoelii, whereas MAbs against the other two antigens reacted only with proteins from strains of P. berghei.

Plasmodium vivax: cloning and expression of a major blood-stage surface antigen

Plasmodium vivax is a highly prevalent malaria pathogen of man; the following report is the first to describe the cloning and expression of a major asexual erythrocytic stage antigen of this species. The screening of a genomic DNA expression library with a monoclonal antibody directed against a 200-kDa surface component (Pv200) of the more mature schizonts of P. vivax led to the selection of a recombinant bacterial clone which produced a fusion protein. Mouse and rabbit immune sera raised against the purified fusion protein recognized the 200-kDa parasite antigen on Western blots and reacted with the surface of segmenters by immunofluorescence. Sequencing of the 1.9-kb P. vivax DNA insert coding for this fusion protein revealed a 45-47% homology at the nucleotide level with the P. falciparum gene of a parasite surface antigen, Pf195, which has been shown to be a promising candidate for a malaria vaccine in primates and in man.

A hybrid gene to express protein epitopes from both sporozoite and merozoite surface antigens of Plasmodium falciparum

The DNA coding for parts of the repetitive amino acid sequence of Plasmodium falciparum circumsporozoite protein has been spliced to a sequence encoding part of the precursor to the major merozoite surface antigens, to produce a hybrid gene. Expression in Escherichia coli produces a protein with antigenic determinants from both malaria proteins. Antibodies raised against the expressed material react with both a peptide derived from the circumsporozoite repeat sequence, and the merozoite surface molecule. Hybrid molecules of this type may be the basis of a malaria vaccine.

Immunization against Plasmodium falciparum with recombinant polypeptides produced in Escherichia coli

Two proteins produced in recombinant Escherichia coli and containing amino acid sequences from the Plasmodium falciparum precursor to major merozoite surface antigens (PMMSA) have been partially purified. These proteins, together with a preparation of merozoites, have been used to immunize animals. The antibody response and the degree of protection were compared. Animals immunized with merozoites produced antibodies reacting with many P. falciparum proteins, whereas a response specific for PMMSA was detected in those receiving the recombinant material. Incomplete protection was conferred to both groups and there was no apparent correlation between antibody levels and protection.

Plasmodium falciparum: gene structure and hydropathy profile of the major merozoite surface antigen (gp195) of the Uganda-Palo Alto isolate

The gene encoding the 195,000-Da major merozoite surface antigen (gp195) of the FUP (Uganda-Palo Alto) isolate of Plasmodium falciparum, a strain widely used for monkey vaccination experiments, has been cloned and sequenced. The translated amino acid sequence of the FUP gp195 protein is closely related to the sequences of corresponding proteins of the CAMP (Malaysia) and MAD-20 (Papua New Guinea) isolates and more distantly related to those of the Wellcome (West Africa) and K1 (Thailand) isolates, supporting the proposed allelic dimorphism of gp195 within the parasite population. The prevalence of dimorphic sequences within the gp195 protein suggests that many gp195 epitopes would be group-specific. Despite the extensive differences in amino acid sequence between gp195 proteins of these two groups, the hydropathy profiles of proteins representative of both groups are very similar. The conservation of overall secondary structure shown by the hydropathy profile comparison indicates that gp195 proteins of the various P. falciparum isolates are functionally equivalent. This information on the primary structure of the FUP gp195 protein will enable us to evaluate the possible roles of conserved, group-specific and variable epitopes in immunity to the blood stage of the malaria parasite.

Third form of the precursor to the major merozoite surface antigens of Plasmodium falciparum

The precursor to the major merozoite surface antigens of Plasmodium falciparum appears to be encoded by two distinctly different (dimorphic) alleles able to undergo limited recombination. We analyzed 18 previously uncharacterized P. falciparum isolates to test the dimorphic model. All but one, a Thailand isolate, conformed to the dimorphic model, and this isolate conformed to the dimorphic model in all but variable block 2. Sequence analysis revealed that block 2 of isolate CSL2 was a third form. Hence, the dimorphic model is not strictly correct. Recombination between alleles was found only within two conserved blocks near the 5' end of the gene.

Mung bean nuclease exhibits a generalized gene-excision activity upon purified Plasmodium falciparum genomic DNA

A novel set of reaction conditions for mung bean nuclease has been described in which Plasmodium genes were specifically excised as intact fragments from purified DNA. We have now determined that under the new conditions mung bean nuclease cleaves precisely at sites outside of the coding region of every P. falciparum gene for which the extent of the protein coding region in genomic DNA is known. We conclude that this enzyme activity is probably a general one for P. falciparum genes. Introns are not specifically cleaved, although one gene contained a cleavage site within an intron. There is no direct relationship between dA.dT-richness and sites of cleavage under these conditions. Also contrary to the expectations of a model based on cleavage at denaturation bubbles, there was no general relationship between the concentration of the DNA denaturant, formamide, and the size of the resulting gene-containing fragments. Thus, the data strongly suggest the involvement of an altered DNA structure near gene boundaries in determining the recognition sites for this enzyme activity.

Identification of two integral membrane proteins of Plasmodium falciparum

We describe the isolation and cloning of two integral membrane protein antigens of Plasmodium falciparum. The antigens were isolated by Triton X-114 temperature-dependent phase separation, electrophoretically transferred to nitrocellulose, and used to affinity-purify monospecific human antibodies. These antibodies were used to isolate the corresponding cDNA clones from a phage lambda gt11-Amp3 cDNA expression library. Clone Ag512 corresponds to a Mr 55,000 merozoite rhoptry antigen, and clone Ag513 corresponds to a Mr 45,000 merozoite surface antigen. Both proteins can be biosynthetically labeled with [3H]glucosamine and [3H]myristic acid, suggesting that they may be anchored in membranes via a glycosylphosphatidylinositol moiety. Similarities in the C-terminal sequences of the Mr 45,000 merozoite surface antigen and the Trypanosoma brucei variant surface glycoproteins provides further evidence that this antigen has a glycosylphosphatidylinositol anchor.

Multiple Trypanosoma cruzi antigens containing tandemly repeated amino acid sequence motifs

Chromosomal DNA from Trypanosoma cruzi, the agent of the American trypanosomiasis (Chagas' disease), was used for construction of a DNA library, employing the expression vector lambda gt11. Nine clones encoding different parasite antigens were isolated from this library by screening with an antiserum from a Chagasic patient. Nucleotide sequence analysis showed that seven out of the nine isolated clones code for antigens which contain tandemly repeated amino acid sequence motifs. Each of the seven antigens contains a unique repeat, ranging in length between 5 and 68 amino acids. The length of the repeats is highly conserved within each clone. Fusion proteins, expressed from two of the clones, reacted with a large proportion of sera collected from Chagasic patients in Argentina, Brazil and Chile. These clones appear thus to encode antigens which are shared between different strains of T. cruzi. Immunofluorescence experiments with live parasites showed that three of the antigens were detectable on the surface of trypanosomes.

Third form of the precursor to the major merozoite surface antigens of Plasmodium falciparum

The precursor to the major merozoite surface antigens of Plasmodium falciparum appears to be encoded by two distinctly different (dimorphic) alleles able to undergo limited recombination. We analyzed 18 previously uncharacterized P. falciparum isolates to test the dimorphic model. All but one, a Thailand isolate, conformed to the dimorphic model, and this isolate conformed to the dimorphic model in all but variable block 2. Sequence analysis revealed that block 2 of isolate CSL2 was a third form. Hence, the dimorphic model is not strictly correct. Recombination between alleles was found only within two conserved blocks near the 5' end of the gene.

An epitope recognised by inhibitory monoclonal antibodies that react with a 51 kilodalton merozoite surface antigen in Plasmodium falciparum

Monoclonal antibodies designated 8G10/48 and 9E3/48 raised against mature asexual blood stages of Plasmodium falciparum inhibit parasite growth in vitro. Both antibodies bind to an epitope which includes the linear sequence Ser Thr Asn Ser and which is present in a cDNA clone from a P. falciparum expression library. These antibodies recognise a glycosylated antigen of approximately 51 kDa which is located on the merozoite surface membrane.

Variation in the precursor to the major merozoite surface antigens of Plasmodium falciparum

The major antigens on the surface of Plasmodium falciparum merozoites are derived from a single high molecular weight polypeptide. The precursor to the major merozoite surface antigen (PMMSA) has conserved and variable antigenic determinants and varies in size in different isolates. Since the protein is a candidate for a malaria vaccine, it is important to understand the molecular basis of this variation. We present the complete sequence of the PMMSA of the Papua New Guinea isolate FC27 and the partial sequence of the West African isolate NF7. The gene consists of blocks of sequence which are either conserved or variable between different isolates. Variable sequences fall into two distinct types, indicating that the PMMSA is encoded by dimorphic alleles that undergo recombination within conserved blocks at the 5' end of the gene. Genetic exchange is not apparent within the other two-thirds of the gene in 12 isolates, suggesting strong selection against such recombinants. The most variable block located near the 5' end contains repeats that are different in independent cloned isolates. This variation presumably accounts for much of the size and antigenic variability.

Malaria parasite invasion: interactions with the red cell membrane

The capacity to invade red cells is central to the biology of malaria parasites; both asexual multiplication and reinfection of the definitive mosquito host depend upon intraerythrocytic stages. The invasion process is complex. The briefly free merozoite specifically recognizes and adheres to ligands on the red cell surface, then alters the red cell membrane to produce an invagination into which it moves, and so becomes enclosed in a membrane-bound parasitophorous vacuole. Here we assess new evidence that bears on our understanding of this process. This has come from sources including biochemical and ultrastructural studies of the specialized surface and organelles of merozoites, from in vitro invasion studies using naturally refractory or artificially modified red cells, and from structural, chemical, and immunological analyses of the newly parasitized cell.