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

Gene-specific signatures of elevated non-synonymous substitution rates correlate poorly across the Plasmodium genus

BACKGROUND

Comparative genome analyses of parasites allow large scale investigation of selective pressures shaping their evolution. An acute limitation to such analysis of Plasmodium falciparum is that there is only very partial low-coverage genome sequence of the most closely related species, the chimpanzee parasite P. reichenowi. However, if orthologous genes have been under similar selective pressures throughout the Plasmodium genus then positive selection on the P. falciparum lineage might be predicted to some extent by analysis of other lineages.

PRINCIPAL FINDINGS

Here, three independent pairs of closely related species in different sub-generic clades (P. falciparum and P. reichenowi; P. vivax and P. knowlesi; P. yoelii and P. berghei) were compared for a set of 43 candidate ligand genes considered likely to be under positive directional selection and a set of 102 control genes for which there was no selective hypothesis. The ratios of non-synonymous to synonymous substitutions (dN/dS) were significantly elevated in the candidate ligand genes compared to control genes in each of the three clades. However, the rank order correlation of dN/dS ratios for individual candidate genes was very low, less than the correlation for the control genes.

SIGNIFICANCE

The inability to predict positive selection on a gene in one lineage by identifying elevated dN/dS ratios in the orthologue within another lineage needs to be noted, as it reflects that adaptive mutations are generally rare events that lead to fixation in individual lineages. Thus it is essential to complete the genome sequences of particular species of phylogenetic importance, such as P. reichenowi.

Genetic fingerprints of parasites causing severe malaria in a setting of low transmission in Sudan

In this study we intended to examine the extent of genetic diversity of Plasmodium falciparum parasites causing severe malaria (SM). For this purpose, 100 parasite isolates were obtained from patients with SM and uncomplicated malaria, from an area of low and unstable malaria transmission in Sudan. The diversity of infection (DOI) was estimated by relating the number of the different parasite genotypes that were detected to the total number of parasites that were genotyped (parasite population/subpopulation). We used different molecular markers individually (pfcrt-76, pfmr1-86, GLURP size and MSP2 family and size) and as a group to set a multilocus genetic profile for each parasite isolate. The DOI as estimated by MSP2 and GLURP was 0.553 and 0.435, respectively. However, combination of all four molecular markers (multilocus genetic profile) revealed a fingerprint pattern of genetic diversity with a DOI of 0.936, indicating that in SM infection, diversity is the rule and homogeny is the exception. Furthermore, our clinical data suggest that the virulence markers might also be more diverse than expected. In conclusion, the results are unexpected and overturn the assumption that parasites causing SM are a limited subpopulation of virulent parasites or of a clonal nature. However, it was more likely that there was a genetically unique parasite in each infection.

Glutamate-rich protein (GLURP) induces antibodies that inhibit in vitro growth of Plasmodium falciparum in a phase 1 malaria vaccine trial

The glutamate-rich protein (GLURP) of P. falciparum is the target of cytophilic antibodies which are significantly associated with protection against clinical malaria. A phase 1 clinical trial was conducted in healthy adult volunteers with the long synthetic peptide (LSP) GLURP(85-213) combined with either Aluminum Hydroxide (Alum, 18 volunteers) or Montanide ISA 720 (ISA, 18 volunteers) as adjuvants. Immunizations with 10, 30 or 100 microg GLURP(85-213) were administered subcutaneously at days 0, 30, and 120. Adverse events occurred more frequently with increasing dosage of GLURP(85-213) LSP and were more prevalent in the ISA group. Serious vaccine-related adverse events were not observed. The vaccine induced dose-dependent cellular and humoral immune responses, with high levels of (mainly cytophilic IgG1) antibodies that recognize parasites by immunofluorescence (IFA). Plasma samples collected 30 days after the last immunization induced a dose-dependent inhibition of parasite growth in vitro in the presence of monocytes. In conclusion, immunizations with GLURP(85-213) LSP formulations induce adverse events but can be administered safely, generating antibodies with capacity to mediate growth-inhibitory activity against P. falciparum in vitro.

Immunogenicity of synthetic peptides derived from Plasmodium falciparum proteins

To obtain antibodies suitable to be used in an antigen-capture assay, we have identified, synthesized, and evaluated a series of peptides from different Plasmodium falciparum excretory-secretory proteins: glutamate-rich protein (GLURP); histidine-rich protein 2; histidine-rich protein 3; Falciparum interspersed repeat antigen and, serine-rich antigen homologous. Conformational as well as antigenic predictions were performed using the ANTHEPROT package. Chemical synthesis was carried out by the multiple manual synthesis using the t-boc strategy. The peptides were used as antigens for the preparation of polyclonal antibodies in rabbits. Out of the 14 peptide constructs, eight by ELISA and, six by MABA elicited antibodies that showed correspondence between the predictive study and the immunogenicity obtained in rabbits. All antipeptide (GLURP, HRP2, and FIRA) antisera were found to bind to the corresponding synthetic sequence in an ELISA assay. The binding activity and specificity of antibodies were determined by Western blot with supernatant culture from P. falciparum. Anti-GLURP (IMT-94 and IMT-200) antisera bound to five molecules present in supernatant with molecular weight of 73, 82, 116, 124, and 128 kDa. Anti-HRP2 (IMT-192) antisera recognized a band of 58 kDa. In both cases, the specific molecules were inhibited by preincubation with the homologous peptide. Anti-HRP3, anti-FIRA neither anti-SERPH antisera showed reactivity. Anti-peptides HRP2 antibodies recognized the recombinant protein present in Parasight-F test. The same way, synthetic peptides from HRPII molecule were recognized by monoclonal antibody present in the Parasight-F assay. Our results confirm the potential value of synthetic peptides when inducing monospecific polyclonal antibodies for the development of diagnostic tests based on the capture of antigens.

Distribution of IgG subclass antibodies specific for Plasmodium falciparum glutamate-rich-protein molecule in sickle cell trait children with asymptomatic infections

Polymorphism in the beta-globin gene (hemoglobin S) has been associated with protection against severe forms of malaria. In a cross-sectional study, 180 young Gabonese children with and without sickle cell trait and harboring asymptomatic Plasmodium falciparum infections, were assessed for the responses to recombinant protein containing the conserved region of glutamate-rich protein (GLURP). We reported increased age-dependence of antibody prevalence and levels of total IgG (p<0.0001), IgG1 (p=0.009), and IgG3 (p<0.03) antibodies to GLURP with a cut-off at 5 years of age. Whatever the hemoglobin type, cytophilic antibodies (IgG1 and IgG3) were prevalent, but GLURP-specific IgG4 antibodies were detected at significantly (p<0.05) lower levels in HbAS children. We showed that the distribution of non-cytophilic IgG antibodies differs according to the hemoglobin type and to the malaria antigens tested. This may have possible implication for the clearance of malaria parasites and for protection against severe malaria.

Immunization of Saimiri sciureus monkeys with a recombinant hybrid protein derived from the Plasmodium falciparum antigen glutamate-rich protein and merozoite surface protein 3 can induce partial protection with Freund and Montanide ISA720 adjuvants

The immunogenicity and efficacy of a hybrid recombinant protein derived from the N-terminal end of the glutamate-rich protein (GLURP) and the C-terminal portion of the merozoite surface protein 3 (MSP3) of Plasmodium falciparum was evaluated in Saimiri sciureus monkeys. The GLURP/MSP3 hybrid protein, expressed in Lactococcus lactis, was administered in association with alum, Montanide ISA720, or complete or incomplete Freund adjuvant (CFA/IFA) in groups of five animals each. The three formulations were shown to be immunogenic, but the one with alum was shown to be weak compared to the other two, particularly CFA/IFA, which provided very high antibody titers (enzyme-linked immunosorbent assay titers of >3,000,000 and immunofluorescence antibody test titers of 6,400). After a challenge infection with P. falciparum FUP strain, all five monkeys from the GLURP/MSP3-alum group showed a rapid increase in parasitemia, reaching 10% and were treated early. The two monkeys with the highest antibody titers in group GLURP/MSP3-Montanide ISA720 had a delay in the course of parasitemia and were treated late due to a low hematocrit. In the GLURP/MSP3-CFA/IFA group, parasitemia remained below this threshold in four of the five animals and, after it reached a peak, parasitemia started to decrease and monkeys were treated late. When all animals were grouped according to the outcome, a statistically significant association between high antibody titers and partial protection was observed. The challenge infection boosted the antibody titers, and the importance of this event for vaccine efficacy in areas where this parasite is endemic is discussed. In conclusion, these data suggest that GLURP and MSP3 can induce protection against malaria infection if antibodies are induced at properly high titers.

Characterisation of two novel proteins from the asexual stage of Plasmodium falciparum, H101 and H103

The merozoite surface of the pathogenic malaria parasite Plasmodium falciparum is comprised of proteins that are important for the identification and invasion of human red cells. Merozoite surface protein (MSP)3 is a polymorphic protein associated with the surface of merozoites and is also a vaccine candidate. A distinct feature of the MSP3 sequence is three blocks of alanine-rich heptad repeats that are predicted to form an intramolecular coiled-coil. Three orthologues of MSP3 that also contain alanine-rich heptad repeats have been described in P. vivax and we therefore searched the P. falciparum genome database for MSP3 paralogues. We have identified two genes, H101 and H103 related to MSP3, however like another MSP3 paralogue, MSP6, H101 and H103 do not contain heptad repeats. H101 and H103 are expressed during the asexual cycle and immunofluorescence indicates H103 localises to the merozoite surface as a peripheral membrane protein. Transfected parasite lines that express truncated forms of H101 or H103 were viable and grew at the same rate as the parental parasite line. This result may reflect redundancy in function among members of the MSP3/MSP6 gene family as has been described for other families of paralogue genes in P. falciparum.

Protein trafficking in Plasmodium falciparum-infected red blood cells

Plasmodium falciparum inhabits a niche within the most highly terminally differentiated cell in the human body--the mature red blood cell. Life inside this normally quiescent cell offers the parasite protection from the host's immune system, but provides little in the way of cellular infrastructure. To survive and replicate in the red blood cell, the parasite exports proteins that interact with and dramatically modify the properties of the host red blood cell. As part of this process, the parasite appears to establish a system within the red blood cell cytosol that allows the correct trafficking of parasite proteins to their final cellular destinations. In this review, we examine recent developments in our understanding of the pathways and components involved in the delivery of important parasite-encoded proteins to their final destination in the host red blood cell. These complex processes are not only fundamental to the survival of malaria parasites in vivo, but are also major determinants of the unique pathogenicity of this parasite.

Plasmodium falciparum merozoite surface protein 6 is a dimorphic antigen

Merozoite surface protein 1 (MSP1) is a highly polymorphic Plasmodium falciparum merozoite surface protein implicated in the invasion of human erythrocytes during the asexual cycle. It forms a complex with MSP6 and MSP7 on the merozoite surface, and this complex is released from the parasite around the time of erythrocyte invasion. MSP1 and many other merozoite surface proteins contain dimorphic elements in their protein structures, and here we show that MSP6 is also dimorphic. The sequences of eight MSP6 genes indicate that the alleles of each dimorphic form of MSP6 are highly conserved. The smaller 3D7-type MSP6 alleles are detected in parasites from all malarious regions of the world, whereas K1-type MSP6 alleles have only been detected in parasites from mainland Southeast Asia. Cleavage of MSP6, which produces the p36 fragment in 3D7-type MSP6 and associates with MSP1, also occurs in K1-type MSP6 but at a different site in the protein. Anti-3D7 MSP6 antibodies weakly inhibited erythrocyte invasion by homologous 3D7 merozoites but did not inhibit a parasite line expressing the K1-type MSP6 allele. Antibodies from hyperimmune individuals affinity purified on an MSP3 peptide cross-reacted with MSP6; therefore, MSP6 may also be a target of antibody-dependent cellular inhibition.

Pre-erythrocytic antigens of Plasmodium falciparum: from rags to riches?

A growing number of Plasmodium genomes have joined the sequencing treadmill, and the genome of Plasmodium falciparum has recently been published. Most malaria vaccinologists will soon be confronted by a bewildering array of new potential antigens from the recently completed genome of this parasite. However, for those aiming to target the pre-erythrocytic stages of the hepatic parasite, the wait might be long. In the absence of readily available materials and specific reagents, the selection of pre-erythrocytic antigens from raw sequence data is likely to prove difficult. Here, current knowledge of pre-erythrocytic antigens is updated in the light of recent results, and the post-genomic prospects of completing the antigenic repertoire of these immunologically important and intriguing stages is discussed.

Diversity of Plasmodium falciparum clones infecting children living in a holoendemic area in north-eastern Tanzania

The diversity of Plasmodium falciparum clones and their role in progression from asymptomatic to symptomatic condition in children have been investigated. Attempts to identify whether particular parasite genotypes were associated with the development of clinical symptoms have been made. A cohort of 34 initially asymptomatic parasitaemic children aged 1-5 years were followed daily for 31 days. Clinical examinations were made each day for signs and symptoms of clinical malaria, followed by parasitological investigation. Nineteen children developed symptoms suggestive of clinical malaria during this period. Daily blood parasite samples from 13 children who developed clinical malaria symptoms and 7 who remained asymptomatic were genotyped by PCR-amplification of the polymorphic regions of the merozoite surface proteins 1 and 2 (MSP1 and MSP2) and the glutamate rich protein (GLURP) genes. Infections were found to be highly complex in both groups of children. Every isolate examined from both groups had a mixture of parasite clones. Daily changes were observed in both parasite density and genotypic pattern. The mean number of genotypes per individual was estimated at 4.9 and 2.7 for asymptomatic and symptomatic groups of children, respectively. Analysis of allele frequency distributions showed that these differed significantly for the MSP1 locus only.

The Plasmodium falciparum genome--a blueprint for erythrocyte invasion

Erythrocyte invasion by Plasmodium falciparum involves multiple ligand-receptor interactions and numerous apparent redundancies. The genome sequence of this parasite reveals new gene families encoding proteins that appear to mediate erythrocyte invasion.

Malaria vaccine: candidate antigens, mechanisms, constraints and prospects

More than 30 years after the first report of successful vaccination against malaria using radiation-attenuated sporozoites, an effective malaria vaccine is not yet available. However, field and experimental data indicate that it can be developed. An astonishing amount of data has accumulated concerning parasite biology, host-parasite interactions, immunity and escape mechanisms, targets and modulators of immune responses. Nevertheless, so far this knowledge has not been enough to make us understand how to properly manipulate the whole system to build an effective vaccine. In this article, we describe candidate antigens, mechanisms, targets and trials performed with potential malaria vaccines and discuss the approaches, in vivo and in vitro models, constraints and how technologies such as DNA vaccination, genomics/proteomics and reverse immunogenetics are providing exciting results and opening new doors to make malaria vaccine a reality.

Genetic polymorphism of Plasmodium falciparum isolates from Loreto, Peru

Eight genotypes of Plasmodium falciparum were detected after analysing blood samples obtained from 30 Peruvian jungle-dwelling patients in Loreto, a high transmission area for P. falciparum, using amplification of the polymorphic marker gene GLURP (glutamate-rich protein). Genotypes I (GLURP450) and VIII (GLURP800) were the most common (15/30 and 13/30, respectively). This single copy gene showed 15 patients to be infected with a single genotype of P. falciparum; the other 15 were infected with mixed genotypes, one of them with 4 genotypes. These findings are compatible with a high genetic complexity of P. falciparum. Further investigations are needed, using this and other markers, in order to design malaria control measures in Peru.

Population structure of Plasmodium falciparum isolates during an epidemic in southern Mauritania

While the population structure of Plasmodium falciparum is well analysed in selected areas with high malaria endemicity in East and West Africa, only limited data are available for low endemicity regions bordering the Saharan desert. This is one of the first studies for the Sahel, where atypically strong rainfalls in 1998 and 1999 led to a severe outbreak of falciparum malaria in south-east Mauritania. During a study on in vivo-drug resistance against chloroquine we collected blood samples of patients with fever in two medical centres located in non-endemic and hypoendemic areas. We analysed 386 samples by polymerase chain reaction for infection with P. falciparum, and 173 (45%) tested positive. The isolates were genotyped for three polymorphic genetic markers: merozoite surface protein 1 (MSP1), MSP2 and glutamate-rich protein (GLURP). Differences between the two regions could be shown in either number of clones per infection or in their distribution on the different allelic groups. While the mean minimal number of clones in the non-endemic region around Aioun was 1.57, blood samples collected in the hypoendemic region around Kobeni showed multiple infections with an average of 2.34 clones (P < 0.001). In addition, clear differences between endemic regions were apparent in three of the investigated allelic groups: RO33 of the MSP1 gene and FC and Indochina of the MSP2 gene.

Selection of glutamate-rich protein long synthetic peptides for vaccine development: antigenicity and relationship with clinical protection and immunogenicity

Antibodies against three long synthetic peptides (LSPs) derived from the glutamate-rich protein (GLURP) of Plasmodium falciparum were analyzed in three cohorts from Liberia, Ghana, and Senegal. Two overlapping LSPs, LR67 and LR68, are derived from the relatively conserved N-terminal nonrepeat region (R0), and the third, LR70, is derived from the R2 repeat region. A high prevalence of antibody responses to each LSP was observed in all three areas of endemic infection. Levels of cytophilic immunoglobulin G (IgG) antibodies against both GLURP regions were significantly correlated with protection from clinical P. falciparum malaria. Protected children from the Ghana cohort possessed predominantly IgG1 antibodies against the nonrepeat epitope and IgG3 antibodies against the repeat epitope. T-cell proliferation responses, studied in the cohort from Senegal, revealed that T-helper-cell epitopes were confined to the nonrepeat region. When used as immunogens, the LR67 and LR68 peptides elicited strong IgG responses in outbred mice and LR67 also induced antibodies in mice of different H-2 haplotypes, confirming the presence of T-helper-cell epitopes in these constructs. Mouse antipeptide antisera recognized parasite proteins as determined by immunofluorescence and immunoblotting. This indicates that synthetic peptides derived from relatively conserved epitopes of GLURP might serve as useful immunogens for vaccination against P. falciparum malaria.

Genotyping of Plasmodium falciparum infections by PCR: a comparative multicentre study

Genetic diversity of malaria parasites represents a major issue in understanding several aspects of malaria infection and disease. Genotyping of Plasmodium falciparum infections with polymerase chain reaction (PCR)-based methods has therefore been introduced in epidemiological studies. Polymorphic regions of the msp1, msp2 and glurp genes are the most frequently used markers for genotyping, but methods may differ. A multicentre study was therefore conducted to evaluate the comparability of results from different laboratories when the same samples were analysed. Analyses of laboratory-cloned lines revealed high specificity but varying sensitivity. Detection of low-density clones was hampered in multiclonal infections. Analyses of isolates from Tanzania and Papua New Guinea revealed similar positivity rates with the same allelic types identified. The number of alleles detected per isolate, however, varied systematically between the laboratories especially at high parasite densities. When the analyses were repeated within the laboratories, high agreement was found in getting positive or negative results but with a random variation in the number of alleles detected. The msp2 locus appeared to be the most informative single marker for analyses of multiplicity of infection. Genotyping by PCR is a powerful tool for studies on genetic diversity of P. falciparum but this study has revealed limitations in comparing results on multiplicity of infection derived from different laboratories and emphasizes the need for highly standardized laboratory protocols.

The merozoite surface protein 6 gene codes for a 36 kDa protein associated with the Plasmodium falciparum merozoite surface protein-1 complex

A complex of non-covalently bound polypeptides is located on the surface of the merozoite form of the human malaria parasite Plasmodium falciparum. Four of these polypeptides are derived by proteolytic processing of the merozoite surface protein 1 (MSP-1) precursor. Two components, a 22 and a 36 kDa polypeptide are not derived from MSP-1. The N-terminal sequence of the 36 kDa polypeptide has been determined, the corresponding gene cloned, and the protein characterised. The 36 kDa protein consists of 211 amino acids and is derived from a larger precursor of 371 amino acids. The precursor merozoite surface protein 6 (MSP-6) has been designated, and the 36 kDa protein, MSP-6(36). Mass spectrometric analysis of peptides released from the polypeptide by tryptic digestion confirmed that the gene identified codes for MSP-6(36). Antibodies were produced to a recombinant protein containing the C-terminal 45 amino acid residues of MSP-6(36). In immunofluorescence studies these antibodies bound to antigen at the parasite surface or in the parasitophorous vacuole within schizonts, with a pattern indistinguishable from that of antibodies to MSP-1. MSP-6(36) was present in the MSP-1 complex immunoprecipitated from the supernatant of in vitro parasite cultures, but was also immunoprecipitated from this supernatant in a form not bound to MSP-1. Examination of the MSP-6 gene in three parasite lines detected no sequence variation. The sequence of MSP-6(36) is related to that of the previously described merozoite surface protein 3 (MSP-3). The MSP-6(36) amino acid sequence has 50% identity and 85% similarity with the C-terminal region of MSP-3. The proteins share a specific sequence pattern (ILGWEFGGG-[AV]-P) and a glutamic acid-rich region. The remainder of MSP-6 and MSP-3 are unrelated, except at the N-terminus. Both MSP-6(36) and MSP-3 are partially associated with the parasite surface and partially released as soluble proteins on merozoite release. MSP-6(36) is a hydrophilic negatively charged polypeptide, but there are two clusters of hydrophobic amino acids at the C-terminus, located in two amphipathic helical structures identified from secondary structure predictions. It was suggested that this 35 residue C-terminal region may be involved in MSP-6(36) binding to MSP-1 or other molecules; alternatively, based on the secondary structure and coil formation predictions, the region may form an intramolecular anti-parallel coiled-coil structure.

Conservation and heterogeneity of the glutamate-rich protein (GLURP) among field isolates and laboratory lines of Plasmodium falciparum

Genetic variation of the glutamate-rich protein (GLURP) of Plasmodium falciparum was analysed in 29 field isolates and 15 laboratory lines of diverse geographical origin, by DNA sequencing of the non-repetitive 5'-region (R0) of the glurp gene. Polymorphism with respect to the length of the GLURP R2 repeat region was also analysed by a polymerase chain reaction method. As reference, the nucleotide sequence of the highly polymorphic 3'-region of the circumsporozoite protein gene was determined in the same isolates. It was found that even in the presence of variations in the GLURP R2 repeat region, immunodominant parts of the GLURP R0 region are surprisingly well conserved and the conservation is most pronounced in isolates from locations with very high malaria transmission. This might indicate that the R0 structure plays an important role in the parasite.

Identification of a major B-cell epitope of the Plasmodium falciparum glutamate-rich protein (GLURP), targeted by human antibodies mediating parasite killing

The antigenicity of the glutamate-rich protein (GLURP) of Plasmodium falciparum was comprehensively evaluated in epitope-mapping studies utilizing a phage display library, synthetic peptides and anti-GLURP IgG preparations previously shown to promote strong antibody-dependent cellular inhibition (ADCI) effects. We identified six major B-cell epitopes within the nonrepetitive region R0, corresponding to amino acid residues 173 to 187 (P1), 193 to 207 (P3), 216 to 229 (P4), 264 to 288 (P11), 343 to 357 (P10), and 407 to 434 (S3). Of these, four (P1, P3, P4, and S3) were frequently recognized by high-titered IgG antibodies in plasma samples from immune Liberian adults (prevalence: 29.1-45.0%). The three epitopes P1, P3, and P4 contained a common motif (seven out of nine positions are identical) and may thus constitute a family of structurally related epitopes. This leaves two distinct epitopes, one (P3) representing this new epitope family and S3 as targets for biologically active antibodies. Human IgG antibodies from single plasma samples were affinity-purified against these peptides. P3-specific IgG preparations were consistently more effective in ADCI than S3-specific IgG. Among the different GLURP epitopes, we therefore suggest that the P3 epitope is potentially the most important epitope in GLURP for the development of clinical immunity to malaria in man.

Cytophilic immunoglobulin responses to Plasmodium falciparum glutamate-rich protein are correlated with protection against clinical malaria in Dielmo, Senegal

The goal of this study was to analyze antibody responses to Plasmodium falciparum glutamate-rich protein (GLURP) using clinical data and plasma samples obtained from villagers of Dielmo, Senegal. This molecule was chosen because it is targeted by human antibodies which induce parasite growth inhibition in antibody-dependent cellular inhibition (ADCI) assays. The results showed a strong correlation between protection against malaria attacks and levels of immunoglobulin G2 (IgG2) and IgG3 against GLURP(94-489) (R0) and IgG3 against GLURP(705-1178) (R2) when corrected for the confounding effect of age-related exposure to malaria. Thus, GLURP may play a role in the induction of protective immunity against P. falciparum malaria.

Immunogenicity of malaria transmission-blocking vaccine candidate, y230.CA14 following crosslinking in the presence of tetanus toxoid

Proteolytically processed 310 kDa form of Plasmodium falciparum gamete surface antigen, Pfs230, is the target of malaria transmission-blocking monoclonal antibodies. To design a recombinant malaria transmission-blocking subunit vaccine, the amino terminus of the 310 kDa surface-exposed form of Pfs230 was mapped to amino acids (aa) 522 and 584 using a series of peptides and recombinant proteins encoding distinct regions of Pfs230. Antiserum generated against an Escherichia coli-produced recombinant protein, spanning the Pfs230 processing site and extending into the cysteine domains, r230/MBP.C (aa 443-1132), reduced parasite infectivity by 71.2-89.8%. To determine if the region spanning the cleavage site blocked malaria transmission when produced as a secreted protein by Saccharomyces cerevisiae, y230.CA14 (aa 467-584) was generated, purified, emulsified in adjuvant and used to vaccinate mice. In contrast to E. coli-produced r230/MBP.C, the immune response generated against y230. CA14 was very weak. To enhance the response, y230.CA14 was mixed with tetanus toxoid, chemically crosslinked, repurifed, and its immunogenicty compared with unconjugated y230.CA14. Conjugated-y230. CA14/TT required fewer booster injections to induce an immune response against Pfs230 and the antibodies generated reacted with the surface of intact gametes and immunoprecipitated radiolabelled Pfs230 extracted from 125I surface-labelled gametes to a greater extent. After seven injections, all y230.CA14 vaccinated mice developed anti-Pfs230 antibodies and the isotype profile was the same. In addition to enhancing the initial immune response generated against y230.CA14, conjugation focuses the immune response toward epitopes within the region of Pfs230 present on the surface of the gamete.

Biased distribution of msp1 and msp2 allelic variants in Plasmodium falciparum populations in Thailand

Plasmodium falciparum isolates were obtained from Thai patients attending a malaria clinic on the Thai-Kampuchean border over 4 cross-sectional surveys carried out at 3-monthly intervals. The genetic structure of the parasite populations was determined by nested polymerase chain reaction (PCR) amplification of polymorphic regions of 3 P. falciparum antigen genes: msp1, msp2 and glurp. Although a high degree of diversity characterized these isolates, the overall population structure of the parasites associated with patent malaria infections was observed to remain relatively stable over time. The highest degree of polymorphism was observed with msp2, and the mean number of lines per infection (multiplicity of infection) calculated with this marker was higher than that obtained using msp1 or glurp alone, or combined. Infections with > or = 2 parasite lines were seen in 76% of the samples, and were proportionally more numerous at the start and end of the rainy season. Two interesting exceptions to the random distribution were observed and involved 2 allelic variants which in one case were found dissociated (msp1 MAD20-family) and in the other were associated (msp2 FC27-family). The epidemiological significance of these types of data is discussed.

Helicobacter pylori detection in human biopsies: a competitive PCR assay with internal control reveals false results

A polymerase chain reaction assay (PCR) for the diagnosis of Helicobacter pylori in human gastric biopsies was developed. To prevent false-negative results while performing PCR on human tissues, an internal control is necessary. Primer set ACT1-ACT2 which specifically amplifies a 542-bp fragment of the 16S rRNA gene of H. pylori was used. dUTP and hot-start were used to prevent false-positives from carryover of previous products and avoid non-specific extension products. A competitive internal control DNA fragment was constructed to detect the presence of inhibitors. Biopsies from 101 unselected patients with gastric symptoms were tested. PCR results were compared with results from microscopy of histological sections and conventional culturing for H. pylori. Forty-two percent of the biopsies were found to contain compounds inhibiting the PCR. The addition of the internal control assures the performance of the PCR assay and is an important quality control parameter.

Inflammatory reactions in placental blood of Plasmodium falciparum-infected women and high concentrations of soluble E-selectin and a circulating P. falciparum protein in the cord sera

To better understand reasons for increased susceptibility to malaria in pregnancy; and the interrelationships between maternal malaria, local immune reactions and the development of the fetus, concentrations of soluble interleukin-10 (IL-10), cytokine receptors, adhesion molecules, a Plasmodium falciparum protein, glutamate-rich protein (GLURP) and antibodies to P. falciparum rhoptry-associated protein-1 were measured among 105 Gambian women and their neonates. Peripheral blood concentrations of IL-10, soluble cytokine receptors and soluble adhesion molecules were found to be different from those concentrations measured in the placenta. Markers of inflammatory reactions: IL-10, sIL-2R, sIL-4R, and soluble tumour necrosis factor receptor I (sTNF-RI) were found in high concentrations in the placenta, indicating that inflammatory reactions take place in the placenta which has been regarded as an immunoprivileged site. Concentrations of soluble vascular cell adhesion molecule-1 (sVCAM-1) and soluble intracellular adhesion molecule-1 (sICAM-1), potential adhesion receptors for malaria parasites, were associated with an active P. falciparum infection in the placenta although the associations did not reach significance. P. falciparum exoantigen, GLURP, was detected in cord blood indicating transplacental passage of malarial antigens. Concentrations of E-selectin were higher in cord blood samples compared with peripheral blood samples. This appeared to be associated with development of cord endothelial cells and not with P. falciparum infection.

The glutamate-rich protein (GLURP) of Plasmodium falciparum is a target for antibody-dependent monocyte-mediated inhibition of parasite growth in vitro

Monocyte-dependent as well as direct inhibitory effects of antimalarial antibodies point toward antigens accessible at the time of merozoite release as targets for biologically active antibodies capable of mediating protection against Plasmodium falciparum. The glutamate-rich protein (GLURP), being an antigen associated with mature schizont-infected erythrocytes, was therefore the object of the present investigation, in which we analyzed whether anti-GLURP antibodies can either interfere directly with merozoite invasion or act indirectly by promoting a monocyte-dependent growth inhibition, antibody-dependent cellular inhibition. GLURP-specific human immunoglobulin G (IgG) antibodies, from pooled IgG of healthy Liberian adults who were clinically immune to malaria, were purified by affinity chromatography on columns containing R0 (N-terminal nonrepetitive region of GLURP) or R2 (C-terminal repetitive region of GLURP) recombinant protein or synthetic peptides as ligands. Analysis of the pattern of reactivity of highly purified anti-GLURP antibodies led to the definition of at least four B-cell epitopes. One epitope was specific for R0, two were specific for R2, and the fourth displayed cross-reactivity between R0 and R2. None of the purified IgG antibodies had direct invasion-inhibitory effects, even at high concentrations. In contrast, when allowed to cooperate with monocytes, all anti-GLURP IgG preparations mediated a strong monocyte-dependent parasite growth inhibition in a dose-dependent manner.

Molecular analysis of recrudescent parasites in a Plasmodium falciparum drug efficacy trial in Gabon

Recrudescent Plasmodium falciparum parasites were sampled from 108 children taking part in a drug efficacy trial in Gabon. A finger-prick blood sample was taken from each child before treatment, and a post-treatment sample taken of the recrudescent parasites. Sample deoxyribonucleic acid was amplified by the polymerase chain reaction using primers specific to the P. falciparum antigen genes MSP-1, MSP-2 and GLURP. Seventy-seven children had identical parasites in their pre- and post-treatment samples, indicating genuine recrudescences of resistant parasites. Fourteen children had completely different parasites in their pre- and post-treatment samples, indicating either a fresh infection from a mosquito or growth of a population of parasites not detected in the pre-treatment sample, perhaps due to sequestration. The remaining 17 children had a mixture of pre-treatment and new parasites in their post-treatment samples. This study demonstrated the use of polymorphic markers to confirm whether parasites in patients with clinical recrudescences after drug treatment are genuinely resistant.

Plasmodium chabaudi: immunogenicity of a highly antigenic glutamate-rich protein

The immunogenicity of a 93-kDa Plasmodium chabaudi protein that contains glutamate-rich tandem repeats was investigated in this study. Immunoblotting with various monoclonal antibodies indicates that this 93-kDa protein is equivalent to a potential P. chabaudi RESA analogue. However, the sequence of the P. chabaudi protein does not exhibit any significant homology to Pf155/RESA. Antibodies against the 93-kDa protein appear early during P. chabaudi infection and reach high titers. The highest antibody titers are found when the parasitemia is descending, suggesting that this protein may play some role in immunity. Immunization of mice with the recombinant protein also results in high antibody titers, indicating that the protein is quite immunogenic. However, mice immunized with recombinant protein and challenged with P. chabaudi do not exhibit a delayed appearance of parasitemia, a reduced parasitemia, or a shortened duration of parasitemia. Glutamate-rich P. falciparum proteins such as Pf155/RESA, are being considered as vaccine candidates. The studies with P. chabaudi suggest that interpretation of serological data using glutamate-rich proteins should proceed with caution. The glutamate-rich repeats, although highly immunogenic, may not be important in host immunity against malaria. However, antibodies that appear late in the P. chabaudi infection do appear to play a role in anti-malarial immunity.

Extensive genetic diversity of Plasmodium falciparum isolates collected from patients with severe malaria in Dakar, Senegal

While some genetic host factors are known to protect against severe Plasmodium falciparum malaria, little is known about parasite virulence factors. We have compared the genetic characteristics of P. falciparum isolates collected from 56 severe malaria patients and from 30 mild malaria patients recruited in Hôpital Principal, Dakar, Senegal. All isolates were typed using polymerase chain reaction amplification of polymorphic genetic loci (MSP-1, MSP-2, HRP1, GLURP, CSP, RESA, and the multigene family Pf60). The complexity of infections was lower in severe than in mild malaria and the parasite genetic diversity in both groups was very large. No specific genetic make-up was associated with severity; there were, however, marked differences in allele frequencies in both groups, with a prevalence up to 60% of MSP-2 alleles specifically observed in the severe malaria isolates. In addition, the presence of MSP-1/RO33 alleles was significantly associated with a higher plasma level of tumour necrosis factor alpha receptor 1 (P < 0.05), a reported indicator of severity in human malaria. These results point to potential differences in the genetic characteristics of parasites inducing severe versus mild pathology.

Antibody reactivities to glutamate-rich peptides of Plasmodium falciparum parasites in humans from areas of different malaria endemicity

Synthetic P. falciparum peptides were evaluated as tools in epidemiological investigations of malaria. Plasma IgM and IgG antibody reactivities against synthetic peptides covering sequences of glutamate-rich protein (GLURP) and acidic-basic repeat antigen (ABRA) were measured by ELISA in individuals from malaria-endemic areas of Sudan, Indonesia and The Gambia to study antibody responses to these peptides in donors living in areas of different malaria endemicity. IgG and IgM reactivities to the peptides increased with malaria endemicity, although there were no differences in reactivities to the GLURP peptide between non-exposed donors and donors living in areas of low malaria endemicity. IgG reactivities to the GLURP peptide in Sudanese adults were high one month after treatment in all adults tested, while IgG reactivities to the ABRA peptide were infrequent. IgM responses to the peptides tested were shortlived in most patients. In Gambian children with malaria, IgM reactivities but not IgG antibody reactivities against the ABRA peptide were higher in those with mild malaria than in those with severe malaria. The peptides may be useful in future epidemiological studies, especially in areas of low malaria endemicity.

Mating patterns in malaria parasite populations of Papua New Guinea

Description of the genetic structure of malaria parasite populations is central to an understanding of the spread of multiple-locus drug and vaccine resistance. The Plasmodium falciparum mating patterns from madang, Papua New Guinea, where intense transmission of malaria occurs, are described here. A high degree of inbreeding occurs in the absence of detectable linkage disequilibrium. This contrasts with other studies, indicating that the genetic structure of malaria parasite populations is neither clonal nor panmictic but will vary according to the transmission characteristics of the region.

Antigenicity and immunogenicity of recombinant glutamate-rich protein of Plasmodium falciparum expressed in Escherichia coli

A recombinant Plasmodium falciparum glutamate-rich protein (GLURP) was produced in Escherichia coli as a nearly full-length protein. In order to map immunodominant regions on GLURP, the nonrepetitive amino-terminal region (R0) as well as the central repeat region (R1) and the carboxy-terminal repeat region (R2) were also produced as separate products. All four purified gene products reacted specifically with serum samples from adults living in an area of Liberia where malaria is holoendemic. It appears that the human immune response against GLURP is primarily directed against the R2 region because 94% of the serum samples reacted with this region in an immunoassay. Antibody reactivity against the R0 region was also observed in 75% of the serum samples, while the R1 region showed only weak antibody-binding activity. When the nearly full-length GLURP molecule was adsorbed to Al(OH)3 it was found to be immunogenic in mice. In these experiments, the antibody response was almost exclusively directed against the R2 region. When anti-GLURP sera were obtained from rabbits immunized with the three regions, R0, R1, and R2, respectively, they recognized in immunoprecipitation experiments authentic GLURP from P. falciparum grown in vitro. These results demonstrate that GLURP produced in E. coli can induce a humoral immune response against GLURP derived from blood-stage parasites.

Immune responses to band 3 neoantigens on Plasmodium falciparum-infected erythrocytes in subjects living in an area of intense malaria transmission are associated with low parasite density and high hematocrit value

During the intracellular development of the human malarial parasite, Plasmodium falciparum, cryptic regions of the erythrocyte band 3 protein are exposed. Antibodies against these band 3-related neoantigens block cytoadherence, and peptides based on amino acid sequences of putative exofacial loops of band 3 protein block the in vitro and in vivo adherence of P. falciparum-infected erythrocytes. At present, it is not known whether reactivity to these antigens is related to exposure to the malaria parasite or is correlated with protective immunity. The reactivities of plasma to peptides containing amino acid sequences of putative exofacial loops 3 and 7 of human band 3 protein were determined for children and adults living in an area of perennial malaria transmission (Liberia) and for donors who had never been exposed to malaria (Denmark). Plasma samples from children and adults living in an area of intense malaria transmission showed a much higher reactivity with the band 3 peptides than did those from nonimmune individuals. High reactivity to the loop 3 peptide (amino acids 546 to 555) was correlated with lower mean parasite density in children in the 5- to 9-year-old age group. The presence of antibodies against loop 3 and 7 peptides was not associated with a low packed erythrocyte volume (hematocrit); in fact, higher-than-average reactivities to both peptides were positively correlated with high hematocrit values, indicating that antibodies which specifically recognize the band 3-related neoantigens are not involved in hemolysis (autoimmunity).

The influence of Maloprim chemoprophylaxis on cellular and humoral immune responses to Plasmodium falciparum asexual blood stage antigens in schoolchildren living in a malaria endemic area of Mozambique

We examined the impact of chemoprophylaxis on the cellular and humoral immune responses to polypeptides of the asexual Plasmodium falciparum blood stage antigens, the glutamate rich protein GLURP and Pf155/RESA, both of which in previous field studies have been identified as potentially protective antigens. The study was carried out in the Escola Primária de Lingamo, a primary school in a suburban area of Maputo, Mozambique. A cohort of 392 schoolchildren (aged 7-12 years) was randomly allocated to two equal groups, one receiving chemoprophylaxis with dapsone/pyrimethamine (Maloprim), the other receiving placebo every week from December 1989 to November 1990. The groups were then followed until November 1991 without chemoprophylaxis. Cellular responses to immunodominant epitopes from Pf155/RESA and GLURP, and to non malaria antigens C. albicans and PPD, were assessed by lymphocyte proliferation assays in vitro. Anti-GLURP and anti-Pf155/RESA antibodies were detected by enzyme-linked immunosorbent assay (ELISA) and erythrocyte membrane immunofluorescence (EMIF), and total anti-P. falciparum antibodies were measured by indirect fluorescent antibody test (IFAT). Immunological reactivities were evaluated every six months, at the end of the rainy season and at the end of the dry season, both during the period of chemoprophylaxis and during the follow-up. The antibody response rate to the GLURP was lower in the Maloprim group than in the placebo group during the intervention phase. The lymphoproliferative response rate to the malaria antigens was significantly lower at the end of the rainy season than at the end of the dry season, but the difference between the experimental group and the control group of schoolchildren was not statistically significant. These results suggest that the antibody responses to the GLURP molecule and partly to the Pf155/RESA antigen in this study population were shortlived and dependent on frequent boostering, but whether these antigens play a role in the development of natural clinical immunity remains open. In the experimental group of schoolchildren weekly chemoprophylaxis successfully reduced the parasite rate during the rainy season from 43% to 4%, and during the dry season from 18% to 0%. Chemoprophylaxis may therefore have a useful role in combination with another partially effective malaria control measure such as insecticide-impregnated bed nets or a malaria vaccine.

Immunogenicity of the Plasmodium falciparum glutamate-rich protein expressed by vaccinia virus

The glurp gene of Plasmodium falciparum F32 has been inserted into a vaccinia virus, and the recombinant virus was designated VVG4. Expression of glurp in VVG4-infected Vero cells was analyzed by immunoprecipitation and revealed a primary GLURP product of approximately 220,000 Da; GLURP was detected both intracellularly and in culture supernatants. To study the immunogenicity of vaccinia virus-expressed GLURP, mice were immunized with VVG4 and serum samples were analyzed for antibody reactivity with three polypeptides, covering almost the entire GLURP molecule; these three polypeptides were produced in recombinant form in Escherichia coli. The immune response was primarily directed against a carboxy-terminal repeat region. The mouse anti-GLURP serum recognized authentic GLURP by immunoprecipitation analysis from P. falciparum grown in vitro. These results demonstrate that vaccinia virus-expressed glurp product can induce a humoral immune response against GLURP derived from blood-stage parasites.

The multifactorial and multistage character of protective immunity to Plasmodium falciparum, naturally acquired by an indigenous population in Burkina Faso

In malaria-endemic areas, protective immunity is acquired gradually. Some authors have proposed that different stages can be distinguished during development. To test this hypothesis, several in vitro assays of the host immune response to P. falciparum were performed in three groups of individuals: 'unprotected' children with clinical attacks, 'semi-immune' children, without clinical attacks but with transient high parasitaemias during the transmission period, and 'protected' adults with low residual parasitaemias. By comparison of immune responses in these groups and multifactorial analyses, discriminant factors and potential protective mechanisms were identified. Anti-RESA antibody levels were lower in 'unprotected' than in 'semi-immune' children, while specific cellular responses, TNF levels and percentage of activated T lymphocytes were higher. Low humoral immunity and high cellular activation in children were followed by high humoral immunity and low cellular activation in adults. Therefore, protective immunity seems to pass through different stages and to result from the association of different immune mechanisms according to the level and duration of the individual experience of malaria.

Identification and characterisation of a novel repetitive antigen from Onchocerca spp

A novel repetitive antigen from the cattle parasite Onchocerca gibsoni was shown to be recognised by sera from humans infected with Onchocerca volvulus, Wuchereria bancroftii or Brugia malayi. The O. gibsoni protein was produced in a recombinant form, and antibodies raised to this protein used to screen cDNA libraries for O. volvulus. A series of clones were isolated which encoded repetitive regions very similar to those in O. gibsoni, but interspersed between these were longer repeating units which we have not so far found in O. gibsoni. The repetitive antigen was shown to be of high molecular weight and present only in the insoluble (membrane) fraction of O. gibsoni microfilariae. Immunofluorescence techniques demonstrated that the antigen was associated both with muscle and with specific membrane layers, including a peripheral layer which corresponds to either the outer hypodermis or an inner region of the cuticle in adult female O. gibsoni. In many respects, the proteins encoded by the O. gibsoni and O. volvulus cDNA clones resembled repetitive antigens from several distantly related eukaryotic parasites, and a possible common role in immune evasion is discussed.

Immunoglobulin M and G antibody responses to Plasmodium falciparum glutamate-rich protein: correlation with clinical immunity in Gambian children

The aims of the present study were to describe the age-related immunoglobulin M (IgM) and IgG response to part of a 220-kDa glutamate-rich protein (GLURP) from Plasmodium falciparum and to determine possible correlations of possession of these antibodies with malaria morbidity. IgM and IgG levels were measured with a recombinant fusion protein consisting of the carboxy-terminal 783 amino acids of the GLURP. Samples for the study were obtained during a longitudinal malaria morbidity survey performed in The Gambia; cross-sectional surveys were performed at the beginning of the transmission season in May and in October. Seropositivity rates increased with age to a maximum of 77% for IgM and 95% for IgG in adults. High prevalences of seropositivity were associated with certain human leukocyte antigen class II alleles (DRw8, DR9, DR7, DR4, DQw7, and DQw2) or haplotypes. The relationship between anti-GLURP489-1271 antibodies and clinical immunity is not clear; asymptomatically infected children aged 5 to 8 years had significantly higher levels of IgG than clinically ill children of the same age, suggesting that antibodies to the carboxy-terminal part of the GLURP may contribute to immunity to P. falciparum. However, this was not significant for younger children.

Capture ELISA for IgM antibodies against Plasmodium falciparum glutamate rich protein

This report describes a novel mu chain capture ELISA for the detection of IgM antibodies against a Plasmodium falciparum antigen. A fragment of the 220 kDa P. falciparum glutamate rich protein containing amino acid residues 489-1271 was expressed in E. coli as a recombinant chimeric beta-galactosidase fusion protein and used as antigen after purification and biotinylation. Specific IgM antibodies were found in 51% (39/77) of sera from adult Liberians immune to malaria. The binding of IgM antibodies was specific for the malaria portion of the fusion protein and no cross-reactivity was found in sera from patients with IgM antibodies due to other diseases. Inhibition studies with a fusion protein containing amino acid residues 816-1134 (GLURP816-1134) representing the carboxy-terminal repeat region suggested a different use of epitopes for IgM antibodies in different individuals.

The antibody response to well-defined malaria antigens after acute malaria in individuals living under continuous malaria transmission

The IgG and IgM antibody responses to the C-terminal 783 amino acids of the P. falciparum glutamate-rich protein, GLURP489-1271, expressed as an E. coli fusion protein, the IgG response to a 18-mer synthetic peptide EDKNEKGQHEIVEVEEIL (GLURP899-916) representing the C-terminal repeats of GLURP, and a synthetic peptide (EENV)6 representing the C-terminal repeats from Pf155/RESA, were investigated longitudinally in 13 children and 7 adults living under conditions of continuous, intense malaria transmission. Some subjects did not recognize the antigens after malaria infection, and in subjects recognizing the antigens, the responses were often short-lived. In adults, the antibody responses to the GLURP489-1271 fusion protein and the (EENV)6 peptide peaked after 2 weeks, and not all individuals responded to all antigens. The antibody response, even against large fragments of conserved antigens, is not uniformly elicited by natural malaria infection in previously primed donors.

Crossreactive antigens between life cycle stages ofplasmodium faiciparum

The high mortality and morbidity induced by falciparum malaria has motivated research to find an efficient antimalarial vaccine. The parasite has a complex life cycle, both in the mosquito and human hosts, and presents a number of potential targets for vaccine-induced immune attack. Here, Inge Moelans and John Schoenmakers discuss how the search for protective antigens has been complicated by the discovery of multiple crossreactivities between different parasite proteins.

T-cell responses in malaria

Malaria is caused by infection with protozoan parasites of the genus Plasmodium. It remains one of the most severe health problems in tropical regions of the world, and the rapid spread of resistance to drugs and insecticides has stimulated intensive research aimed at the development of a malaria vaccine. Despite this, no efficient operative vaccine is currently available. A large amount of information on T-cell responses to malaria antigens has been accumulated, concerning antigens derived from all stages of the parasite life cycle. The present review summarizes some of that information, and discusses factors affecting the responses of T cells to malaria antigens.