Allelic diversity of merozoite surface protein 2 gene of P falciparum among children in Osogbo, Nigeria

The genetic diversity of Plasmodium falciparum (P falciparum) infections in humans is implicated in the pathogenesis of malaria. This study provides the first estimate of the genetic diversity and genotype multiplicity of Plasmodium falciparum infection in children with uncomplicated P falciparum malaria in Osogbo, Nigeria. One hundred and one isolates were used for analysis of parasite population polymorphism and genotyped by nested-PCR of merozoite surface protein 2 (MSP2) block 3. Amplicons were obtained for all the 101 genotyped samples in MSP2 PCR with 9 alleles varying in size between 300 and 800 base pair. Thirty-three (31.7%) samples had FC27 allele while 27 (26.7%) had 3D7 allele and 35 (34.7%) had mixed alleles (3D7+FC27). The Multiplicity of Infection (MOI) in the population was 1.6. Children in the age group of > 4-8 years had the highest number of different genotypes in their samples (1.8). The number of MSP2 bands per isolate was lower in the older age group (1.3) but the difference was not statistically significant. Children with parasite density range 5001-10 000 had the highest MOI of 2 while those with parasite density range 1000-5000 had the lowest of 1.5. In conclusion, the present study shows that the field isolates are highly diverse in respect of MSP2 and multiplicity of infection was neither age nor parasite density dependent in the study population.

High prevalence of human antibodies to recombinant Duffy binding-like alpha domains of the Plasmodium falciparum-infected erythrocyte membrane protein 1 in semi-immune adults compared to that in nonimmune children

We used a panel of nine fusion proteins that contain different Duffy binding-like alpha (DBL-alpha) domains of Plasmodium falciparum-infected erythrocyte membrane protein 1 to assess the levels of antibody activity in serum samples obtained from semi-immune or nonimmune individuals from Lambaréné, Gabon. Recognition was measured in terms of either the prevalence or the magnitude of the response. A strong correlation between the immune status of the patients and reactivity with recombinant proteins was observed, which was interpreted as a reflection of the number of infections acquired over time. The antibody responses were predominantly directed toward variable epitopes of the DBL-alpha domain. Antibody responses could be reduced by preincubation of the sera with various fusion proteins. A portion of individuals who exhibited high-level responses to all fusion proteins also had antibodies which recognized conserved epitopes. The possibility that a synergizing effect of anti-DBL-alpha domain antibodies could support chemotherapy is discussed.

Change in Plasmodium falciparum genotype during successive malaria episodes in Gabonese children

Extensive polymorphism in the malaria parasite Plasmodium falciparum is one of the major obstacles to controlling the disease. With the aim of analysing the dynamics of P. falciparum inoculations, we investigated the parasite genotypes of successive malaria episodes. Polymerase chain reaction was performed on blood samples collected longitudinally from 31 children in Lambaréné, Gabon. The polymorphic regions of the merozoite surface antigens 1 and 2 were used as genetic markers. The data show that children in this area are exposed to many different P. falciparum strains. In a few cases, the same parasite genotypic pattern was observed in samples from two consecutive clinical attacks indicating probable recrudescences after therapy. In six cases the first successive infections with a particular merozoite surface antigen (MSA)-2 strain (3D7) were followed by infections with the other MSA-2 genotype (FC27). In all other cases the genetic characteristics of the parasite were different from one infection to the next, indicating that reinfection was caused by a new parasite strain.

Properties of the Plasmodium falciparum homologue of a protective vaccine candidate of Plasmodium yoelii

We describe an unusual tryptophan-rich protein of Plasmodium falciparum that contains threonine-rich repeats. The protein is encoded by a 2.5 kb gene with a two-exon structure including a short AT-rich intron that is spliced out of the mature message. The 5' end of the gene encodes a hydrophobic region, which is assumed to be a signal peptide. The peptide sequence is characterised by a tryptophan-rich region and a block of degenerate threonine repeats. The protein is synthesised throughout the asexual life cycle and has an apparent molecular weight of approximately 94 kDa. It has a variable molecular weight in different strains of P. falciparum. Length polymorphisms can be found in the intron region and the second exon. Four single nucleotide mutations are localised in the tryptophan-rich region and two were found in the threonine-repeat block. Homology searches based on gene structure and amino acid sequence revealed a relationship with a P. yoelii antigen that has been used successfully in vaccine studies. Thus, this P. falciparum antigen should be considered an additional candidate for assessment in vaccination against the asexual blood-stages of P. falciparum.

Nitric oxide synthase 2(Lambaréné) (G-954C), increased nitric oxide production, and protection against malaria

A point mutation in the promoter of the nitric oxide synthase 2 gene (NOS2), termed NOS2(Lambaréné) (NOS2-G954C), protects heterozygous carriers against severe malaria as effectively as the sickle cell trait. In a prospective longitudinal study, 841 individual infections of initially 200 children (151 wild-type vs. 49 NOS2(Lambaréné) carriers) were monitored for 4 years, to assess the rates of malarial attacks in the 2 groups; carriers of the NOS2(Lambaréné) polymorphism were significantly less likely to experience malarial attacks than were others (P=.002). The distribution of the NOS2(Lambaréné) polymorphism was investigated in malaria-endemic areas. It was found to be present with the highest frequency in Africa and at a lower frequency in Asia. Ex vivo studies showed that cells isolated from people with this polymorphism have a 7-fold higher baseline NOS activity, compared with the levels detected in cells from subjects with the wild-type gene (P=.003).

Plasmodium falciparum transmission intensity and infection rates in children in Gabon

Several factors can determine the outcome of a malarial infection. Studies on susceptibility or resistance to malarial infection can be confounded by differences in transmission. In the present study, the relationship between vector abundance and Plasmodium falciparum infection rate of Gabonese children was studied. Indoor human bait catches were conducted in the houses of two groups of children, those who had been found earlier to be either frequently (> 3 infections per year) or rarely (< 0.5 infections per year) infected with P. falciparum. The human biting rate was 12 and 31 bites per person per night during the dry and the rainy season, with 3% and 16% Anopheles, respectively. Anopheles gambiae and A. moucheti were found to be the only vectors involved in the transmission of malaria in this area. No significant difference in the abundance and the rate of P. falciparum infection of the Anopheles mosquitoes was found among children rarely or frequently infected. Differences in transmission cannot account for differences in infection rates in our study group. Hereditary and immunological factors seem to be the primary determinants for the outcome of malarial infection.

Mosquito distribution and entomological inoculation rates in three malaria-endemic areas in Gabon

Mosquitoes were collected during 3 separate periods in 3 areas of different malaria transmission rates in the province of Moyen Ogooué, Gabon, within 1 year (July 1996-May 1997). The campus of the Albert Schweitzer Hospital (HAS) and 2 villages, Bellevue and Tchad, were investigated. A total of 19,836 specimens were collected: 13,122 Mansonia, 3944 Anopheles, 2755 Culex and 15 Aedes were captured. The number of mosquitoes was 7896 and 7995 in July to August and from April to May respectively, and dropped to approximately half in November to December. The individual species showed a different distribution pattern in the 3 study areas. In Tchad we found the lowest number of mosquitoes and also the fewest Anopheles, but when we investigated the number of Plasmodium falciparum-infected Anopheles sp. we observed the highest entomological inoculation rate (EIR) there. The EIRs were 23 in HAS, 53 in Bellevue and 61 in Tchad. The method used to determine the number of infected mosquitoes was an enzyme-linked immunosorbent assay (ELISA), confirmed by a polymerase chain reaction-based approach. The ELISA alone revealed too many false-positive mosquitoes.

Promoter variants of the human mannose-binding lectin gene show different binding

Mannose-binding lectin (MBL) levels in the plasma of humans are highly variable. The level is influenced by gene mutations in exon1 and the promoter. Here we describe the distribution of three point mutations linked with a deletion in the MBL gene promoter in populations of Central Africa, Thailand, and Papua New Guinea. Among African children we find 20% with the wild-type allele, 53% are heterozygous, and 27% are homozygous for the mutation. In Thailand we find 65% with the wild-type allele, 33% are heterozygous, and 2% are homozygous for the variant. In Papua New Guinea the polymorphism is not found. The occurrence of the mutation was associated with MBL levels in the plasma (P = 0.043). Oligonucleotides derived from the variant promoter regions bind proteins differently according to their DNA sequence. The binding of proteins can be influenced by induction with interleukin-6.

Schistosoma mansoni: adhesion of mannan-binding lectin to surface glycoproteins of cercariae and adult worms

Schistosoma mansoni is a blood-dwelling trematode which can persist for several years in the vessels of the human host. The schistosomal surface has been extensively characterized by lectin binding studies, revealing the carbohydrate composition of the worm's tegument. Using fluorescent and scanning electron microscopy we demonstrate that the surface carbohydrates of cercariae and adult worms are the binding ligands for mannanbinding lectin (MBL), a serum protein that is part of the innate immune system. An in vitro complement activation assay with C1q-deficient complement suggests that MBL, in association with the serine proteases MASP-1 and MASP-2, is capable of fixing complement components on the schistosomal tegument and activating the complement cascade via the "MBL pathway." MBL is constitutively expressed by hepatocytes and present in the blood at a stable level. Since it is also a weak acute-phase protein and therefore upregulated in an acute-phase response we investigated the serum MBL levels in patients infected with Schistosoma sp. and in healthy control persons. An enzyme-linked immunosorbent assay indicated no differences between the two groups. Although our results suggest an involvement of MBL activated complement in vitro, its role in vivo remains to be clarified.

Variants of NOS1, NOS2, and NOS3 genes in asthmatics

Nitric oxide (NO) gas concentrations are higher in expired air in asthmatics. NO is synthesized by three isoforms of NO synthase (NOS) encoded by three distinct genes, NOS1, NOS2, and NOS3. Genome-wide searches have identified linkages to asthma on chromosomes 7, 12, and 17 where these three genes are localized. No association study, however, has been reported to date. To test whether variants of NOS1, NOS2, and NOS3 relate to asthma, a genetic association study was conducted in a British population (n = 300). Intragenic microsatellite variants of NOS1 were significantly associated with asthma [odds ratio (OR) = 2.08, 95% CI: 1.20-3.57 (95% CI), P = 0.008 (Pc = 0.048)], but not with IgE levels. Neither NOS2 nor NOS3 variants showed any association with asthma nor IgE levels. These findings suggest that NOS1 variants may be a significant contributor to asthma in a British population.

Association of the ICAM-1Kilifi mutation with protection against severe malaria in Lambaréné, Gabon

The intercellular adhesion molecule-1 (ICAM-1) is thought to be a receptor that mediates binding of Plasmodium falciparum-infected erythrocytes. Especially in vital organs, the binding of parasitized cells to the endothelium via ICAM-1 may lead to severe disease and death. Recently, a mutation in the coding region of ICAM-1, termed ICAM-1Kilifi, was described, causing a change from Lys to Met in the loop that interacts with rhinoviruses, lymphocytes, and parasitized red blood cells. Surprisingly, this mutation was shown to increase susceptibility of Kenyan children to severe malaria in one study. When we compared the distribution of ICAM-1Kilifi in two groups of Gabonese children enrolled in a case-control, matched-pair study who presented with either mild or severe malaria, we found that 55% of the patients with mild malaria were carriers whereas only 39% of those with severe malaria were carriers. The difference in the distribution of ICAM-1Kilifi homozygous pairs between the groups, as well as the distribution of ICAM-1Kilifi carriers, was statistically highly significant (P = 0.027 and P = 0.012, by the McNemar test). In a group of healthy school children from the same region, a distribution of 52% ICAM-1Kilifi carriers to 48% wild-type individuals was found. In a survey for the ICAM-1Kilifi in other malaria-endemic regions, this allele was also found in Nigeria and Papua New Guinea, but not in Thailand.

Low-dose treatment with sulfadoxine-pyrimethamine combinations selects for drug-resistant Plasmodium falciparum strains

A total of 252 children were enrolled in a drug trial to assess the effect of minimal doses of sulfadoxine (Sdx) and pyrimethamine (Pyr). Parasite samples isolated from these patients were analyzed before and after treatment to investigate the level of drug-resistant strains. The parasite genes encoding dihydrofolate reductase (DHFR) and dihydropteroate synthase (DHPS) were assayed for point mutations that are associated with resistance against drugs. Before treatment, Pyr(r) genotypes of the DHFR gene were found in 42% of all samples, 8% of the patients harbored a mixed parasite population and 50% had a sensitive DHFR genotype. In terms of the DHPS gene, we found mutations in 45% of the parasites. Twenty-four percent had a Ser(436) mutation, and 26% had a Gly(437) mutation. Recrudescent parasites were highly enriched for both Pyr(r) and Sdx(r) strains after treatment (P < 0.001 and P = 0.029, respectively).

HLA class II factors associated with Plasmodium falciparum merozoite surface antigen allele families

In Plasmodium falciparum malaria, certain human leukocyte antigens (HLA) and the parasite's merozoite surface antigens 1 and 2 (MSA-1, MSA-2) have been shown to influence the course of the infection. This report is on associations of distinct HLA factors with the occurrence of particular MSA families in a group of patients with either severe or mild P. falciparum malaria in Gabon. Different distributions of HLA-DPB1 alleles were found in the 2 groups. DR *04 alleles were observed more frequently among patients with severe malaria. Several alleles of different loci were associated with distinct MSA allele families. In addition, carriers of the amino acid methionine at position 11 of the DPA1 allele were more often infected by MSA-1 K1 parasites and less frequently by MSA-1 RO33 parasites. Furthermore, associations of HLA factors with polyclonal infections were found.

Plasmodium falciparum: structural and functional domains of the mature-parasite-infected erythrocyte surface antigen

The mature parasite-infected erythrocyte surface antigen (MESA) is a protein exported to the membrane skeleton of the infected red cell, where it forms a strong noncovalent interaction with the host red cell protein, protein 4.1. The complete gene structure of MESA from the Ugandan isolate Palo Alto is described. Comparison to the previously reported MESA sequence from the Papua New Guinean cloned line D10 reveals strong conservation of the general gene structure of a short first exon and a long second exon. The exact exon/intron boundaries were determined by the generation and sequencing of a cDNA from this region. The MESA gene from both isolates consists of seven blocks of repeats that are identical in order. Repeat blocks are conserved to a high degree; however, differences are noted in most blocks in the form of scattered mutations or differences in repeat numbers. Previous work had shown that synthetic peptides spanning a 19-residue region could inhibit the binding of MESA to protein 4.1. Removal of this region from MESA almost completely abolished the binding of MESA to IOVs. Sequencing of this region from a number of laboratory and field isolates demonstrates complete conservation of the cytoskeletal binding domain and flanking sequences.

Mannose-binding lectin plasma levels and gene polymorphisms in Plasmodium falciparum malaria

The contribution of mannose-binding lectin (MBL) to protection from malaria was assessed by comparing plasma concentrations of MBL and the frequency of MBL gene polymorphisms in groups of Gabonese children participating in a prospective study of severe and mild malaria due to infection with Plasmodium falciparum. At admission, a higher proportion of patients with severe malaria had a low level of MBL compared with subjects with mild malaria (0.35 vs. 0.19, P = .02). Two mutations in codons 54 and 57 of the MBL gene were detected. They were present at higher frequency in those with severe malaria (0.45 vs. 0.31, P = .04). These results suggest that deficient innate immune responses, in the form of low MBL levels, may be a risk factor for severe malaria in some young children who lack well-developed, clinically protective acquired immune responses.

Merozoite surface antigen 1 and 2 genotypes and rosetting of Plasmodium falciparum in severe and mild malaria in Lambaréné, Gabon

We present a case-control study to investigate the distribution of Plasmodium falciparum genotypes in patients with severe and mild malaria. We compared clinical and parasitological data with the parasites' genotype and rosetting. The study group consisted of 100 children suffering severe malaria, defined as severe anaemia and hyperparasitaemia. These children were matched by age, sex and provenance with 100 children with mild malaria. For characterization of the parasites we used the polymerase chain reaction to determine merozoite surface antigen (MSA) 1 and 2 genotypes and the phenomenon of rosette formation. We found a significant association between rosette formation and disease severity, and a significant association of severe anaemia with the presence of the MSA-1 allele K1. Infections with 2 genotypes in the severely affected group were significantly associated with severe anaemia and the presence of MSA-1 allele K1. Comparison with the findings of other groups led to the conclusion that the occurrence of P. falciparum genotypes seems to differ geographically.

A putative Plasmodium falciparum exported serine/threonine protein kinase

An 8kb gene coding for a putative serine/threonine protein kinase from Plasmodium falciparum has been cloned and sequenced. It is arranged in two exons: exon I is 2 kb and exon II is 5.6 kb. The gene codes for a large protein of 2510 amino acids. Antibodies raised against a fusion protein were used to localize the putative kinase. By immunofluorescence microscopy, it was found in the cytoplasm of infected red cells. By immunoelectron microscopy it was associated with membranous structures in the red cell and with the red cell membrane, particularly at parasite-induced knobs. This is the first putative protein kinase of P. falciparum to be exported from the parasite into its host cell.

Molecular variation in a novel polymorphic antigen associated with Plasmodium falciparum merozoites

A cDNA clone encoding part of a novel polymorphic merozoite antigen from Plasmodium falciparum was isolated by screening a cDNA library with human immune serum from Papua New Guinea. Immunofluorescence microscopy and immunoblotting with affinity-purified antibodies recognized a highly polymorphic antigen, Ag956, present in schizonts and merozoites. Biosynthetic labeling and immunoprecipitation experiments demonstrated that Ag956 is proteolytically cleaved during merozoite maturation. The complete genomic sequence of Ag956 from the D10 clone of P. falciparum isolate FC27 encodes a secreted protein of calculated molecular mass 43,243 that is very hydrophilic and contains a region of unusual heptad repeats of the general structure AXXAXXX. This antigen has been named the secreted polymorphic antigen associated with merozoites (SPAM). The sequence of a second SPAM allele from the 3D7 clone of isolate NF54 reveals that the alanine heptad repeats and the hydrophilic C-terminal half of the protein are conserved. Variation among SPAM alleles is the result of deletions and amino acid substitutions in non-repetitive sequences within and flanking the alanine heptad-repeat domain. Heptad repeats in which the a and d position contain hydrophobic residues generate amphipathic alpha-helices which give rise to helical bundles or coiled-coil structures in proteins. Thus, SPAM is the first example of a P. falciparum antigen in which a repetitive sequence has features characteristic of a well-defined structural element.

Plasmodium falciparum: a region of polymorphism in the 3' end of the gene for the ring-infected erythrocyte surface antigen

A clone expressing the 3' end of the RESA gene of the Palo Alto isolate of Plasmodium falciparum was isolated by screening a lambda expression library of Mung bean nuclease-digested genomic DNA fragments with antibodies from malaria patients. The DNA sequence of the RESA gene of this isolate has seven single-base changes compared to that seen in the Papua New Guinean isolate FC27. Six of the seven mutations cause amino acid substitutions. The same amino acid sequence as the Palo Alto sequence is found in the Ghanaian isolate NF7. To determine whether these are the only changes in this region of the molecule, we performed PCR analyses on 12 blood samples from malaria patients from Indonesia. The Palo Alto form of the RESA gene was detected in 10 of the samples and the FC27 form of the gene was detected in the other 2 samples. No other forms of the RESA gene were found, suggesting that RESA exists in two alleles for the examined region of the gene. These results indicate that there are two major allelic forms of RESA present in this part of Indonesia.