Biomedical subjects

Spencer D Polley

Publications and source records attributed to Spencer D Polley.

10 recordsLinked to original sources

PubMed2006

Plasmodium falciparum merozoite surface protein 3 is a target of allele-specific immunity and alleles are maintained by natural selection.

BACKGROUND: Plasmodium falciparum merozoite surface protein (MSP) 3 is an asexual blood-stage malaria vaccine candidate antigen. Sequence polymorphisms divide alleles into 2 major types, but the adaptive and immunological significance of the types has not been defined. METHODS: One hundred one msp3 allele sequences were sampled from 2 populations living in areas where malaria is endemic and were analyzed for evidence of natural selection. Recombinant antigens representing full-length sequences of different allelic types and a relatively conserved C-terminal region were produced, to evaluate immunization-induced antibody responses in mice and protective associations for naturally acquired antibodies in a cohort of 319 Gambian children under surveillance for malaria. RESULTS: Frequency-based statistical analyses indicated that polymorphisms are maintained by balancing selection in each of the 2 populations studied. Immunization of mice with full-length MSP3 antigens induced predominantly type-specific antibodies, and a large proportion of naturally acquired antibodies to MSP3 in humans also discriminated between the alleles. Among Gambian children, antibodies to allele-specific and conserved epitopes in MSP3 were associated prospectively with protection from clinical malaria, even after adjustment for age and for the presence of antibodies to other merozoite antigens. CONCLUSIONS: A vaccine incorporating both major allelic types of this promising candidate antigen could be particularly useful for induction of protective immunity in infants and young children.

Alleles↗

PubMed2006

Contrasting signatures of selection on the Plasmodium falciparum erythrocyte binding antigen gene family.

Erythrocyte binding antigens of Plasmodium falciparum are involved in erythrocyte invasion, and may be targets of acquired immunity. Of the five eba genes, protein products have been detected for eba-175, eba-181 and eba-140, but not for psieba-165 or ebl-1, providing opportunity for comparative analysis of genetic variation to identify selection. Region II of each of these genes was sequenced from a cross-sectional sample of parasites in an endemic Kenyan population, and the frequency distributions of polymorphisms analysed. A positive value of Tajima's D was observed for eba-175 (D=1.13) indicating an excess of intermediate frequency polymorphisms, while all other genes had negative values, the most negative being ebl-1 (D=-2.35) followed by psieba-165 (D=-1.79). The eba-175 and ebl-1 genes were then studied in a sample of parasites from Thailand, for which a positive Tajima's D value was again observed for eba-175 (D=1.79), and a negative value for ebl-1 (D=-1.85). This indicates that eba-175 is under balancing selection in each population, in strong contrast to the other members of the gene family, particularly ebl-1 and psieba-165 that may have been under recent directional selection. Population expansion simulations were performed under a neutral model, further supporting the departures from neutrality of these genes.

Animals↗

PubMed2005

High levels of serum antibodies to merozoite surface protein 2 of Plasmodium falciparum are associated with reduced risk of clinical malaria in coastal Kenya.

The merozoite surface protein (MSP) 2 is a vaccine candidate antigen of Plasmodium falciparum that is polymorphic in natural populations. In a prospective cohort study in two coastal populations of Kenya using recombinant proteins derived from the two major allelic types of MSP2, high serum levels of IgG to MSP2 were associated with protection from clinical malaria. This protection was independent of that associated with antibodies to another vaccine candidate antigen (AMA1) in these populations. However, low antibody levels to MSP2 appeared to be associated with increased susceptibility to malaria within people who were parasite negative at the time of serum collection. These data suggest that an MSP2 based vaccine should be designed to induce high level antibody responses against the different MSP2 types present globally in P. falciparum populations and that MSP2 could be combined with other P. falciparum antigens to form a multi-component malaria vaccine.

Adolescent↗

PubMed2005

Orthologous gene sequences of merozoite surface protein 1 (MSP1) from Plasmodium reichenowi and P. gallinaceum confirm an ancient divergence of P. falciparum alleles.

Merozoite surface protein 1 (MSP 1) of Plasmodium falciparum has a major allelic dimorphism in the majority of its sequence, the origin and significance of which is obscure. Here, the cloning and sequencing of the msp1 gene from P. reichenowi (a chimpanzee parasite that is the nearest relative of P. falciparum) and P. gallinaceum (a malaria parasite of birds) is reported. P. reichenowi msp1 is most closely related to one allelic type (K1) of P. falciparum. The other P. falciparum major allelic type (MAD20) is very divergent from these sequences, although not as divergent as msp1 of P. gallinaceum. Assuming a date of 6 million years ago (mya) for the divergence of the P. falciparum K1 and the P. reichenowi msp1 genes (on the basis of previous estimates for these parasite species as well as host divergence times), the most recent common ancestor of the dimorphic region of msp1 would date to approximately 27mya. Thus, the P. falciparum msp1 dimorphism is confirmed as one of the oldest polymorphisms known with the exception of self-incompatibility S genes in Solanaceae. In contrast with the major allelic dimorphism, the polymorphisms present in the relatively conserved C terminus of P. falciparum msp1 appear to have arisen since the divergence of the P. falciparum and P. reichenowi msp1 genes.

Alleles↗

PubMed2004

Human antibodies to recombinant protein constructs of Plasmodium falciparum Apical Membrane Antigen 1 (AMA1) and their associations with protection from malaria.

Serum antibodies from 1071 people in two Kenyan villages were assayed using eight different recombinant Apical Membrane Antigen 1 (AMA1) protein constructs to investigate their role in naturally acquired immunity. In both communities, antibodies against the full-length ectodomain (both FVO and 3D7 allele constructs) prior to a malaria transmission season were significantly associated with protection from malaria in the following 6 months, even after adjusting for age and antibody reactivity to whole parasite (schizont) extract. However, these protective associations of antibodies were only seen among subjects that were parasite slide positive at the time of pre-season serum sampling. Competition ELISAs with the FVO and 3D7 allele constructs showed that antibodies can recognise either conserved or allele-specific epitopes in AMA1. Results encourage the development of an AMA1 vaccine based on the full-length ectodomain, and indicate that the function of human antibodies to allele-specific and conserved epitopes in AMA1 should be studied further.

Adolescent↗

PubMed2004

Vaccination for vivax malaria: targeting the invaders.

Among the surface-exposed antigens of the malaria parasite, those with known essential functions that can be disrupted by antibodies represent the most promising candidates for development as malaria vaccines. Two recombinant protein subunits of the Plasmodium vivax merozoite surface protein 1 have been shown to bind to reticulocytes in enzyme-linked immunosorbent assays. This article discusses the importance of such pre-clinical analyses in the validation of candidate vaccine molecules for P. vivax, given the constraints imposed by the use of primate models and the cost of producing suitable material for human trials.

Animals↗

PubMed2004

Cross-sectional study of specific antibodies to a polymorphic Plasmodium falciparum antigen and of parasite antigen genotypes in school children on the slope of Mount Cameroon.

To investigate relationships between Plasmodium falciparum parasitaemia, parasite genotypes, and specific anti-parasite antibodies, 244 school children (aged 4 to 16 years) were studied in April/May 2002, the peak malaria transmission season in Buea, Cameroon. Antibody reactivities were analysed by ELISA using an array of recombinant antigens representing different sequences from the polymorphic block 2 region of the merozoite surface protein 1 (MSP1), and the blood samples that were slide-positive for P. falciparum were genotyped for msp1 block 2 alleles. The prevalence of antibodies to the specific MSP1 block 2 antigens was significantly higher in children at one particular school (situated at the lowest altitude) compared to the others, although the prevalence of infection or particular parasite genotypes did not differ. Thus, at a population level, the prevalence of these antibodies does not simply reflect prevalence of parasites, but rather may be due to differences in the incidence of past infections. However, there were weak positive associations between specific antibody reactivity and the presence of the corresponding allele in the blood of individuals (statistically significant for the MAD20-type allele of block 2), indicating that antibody specificities are to some extent determined by current parasite infections.

Adolescent↗

PubMed2003

Multiple splice variants encode a novel adenylyl cyclase of possible plastid origin expressed in the sexual stage of the malaria parasite Plasmodium falciparum.

We report the characterization of an unusual adenylyl cyclase gene from Plasmodium falciparum, here designated PfACalpha. The level of mRNA expression is maximum during development of gametocytes (the sexual blood stage of the parasite life cycle). The gene is highly interrupted by 22 introns, and reverse transcriptase-PCR analysis revealed that there are multiple mRNA splice variants. One intron has three alternative 3'-splice sites that confer the potential to encode distinct forms of the enzyme using alternative start codons. Deduced amino acid sequences predict membrane-spanning regions, the number of which can vary between two and six depending on the splice variant. Expression of a synthetic form of two of these variants in Xenopus oocytes and in Dictyostelium adenylyl cyclase-deficient mutants, confirms that PfACalpha is a functional adenylyl cyclase. These results identify a novel mechanism in P. falciparum for the generation of multiple isoforms of a key, membrane-bound signaling molecule from a single genomic copy. Comparisons of the catalytic domains of PfACalpha and a second putative P. falciparum adenylyl cyclase (PfACbeta) with those from other species reveal an unexpected similarity with adenylyl cyclases from certain prokaryotes including the cyanobacteria (blue green algae). In addition, the presence of an unusual active site substitution in a position that determines substrate specificity, also characteristic of these prokaryotic forms of the enzyme, further suggests a plastid origin for the Plasmodium cyclases.

Adenylyl Cyclases↗

PubMed2003

Allele frequency-based analyses robustly map sequence sites under balancing selection in a malaria vaccine candidate antigen.

The Plasmodium falciparum apical membrane antigen 1 (AMA1) is a leading candidate for a malaria vaccine. Here, within-population analyses of alleles from 50 Thai P. falciparum isolates yield significant evidence for balancing selection on polymorphisms within the disulfide-bonded domains I and III of the surface accessible ectodomain of AMA1, a result very similar to that seen previously in a Nigerian population. Studying the frequency of nucleotide polymorphisms in both populations shows that the between-population component of variance (F(ST)) is significantly lower in domains I and III compared to the intervening domain II and compared to 11 unlinked microsatellite loci. A nucleotide site-by-site analysis shows that sites with exceptionally high or low F(ST) values cluster significantly into serial runs, with four runs of low values in domain I and one in domain III. These runs may map the sequences that are consistently under the strongest balancing selection from naturally acquired immune responses.

Animals↗

PubMed2003

Repeat sequences in block 2 of Plasmodium falciparum merozoite surface protein 1 are targets of antibodies associated with protection from malaria.

Human antibodies to the block 2 region of Plasmodium falciparum merozoite surface protein 1 (MSP1) are associated with a reduced prospective risk of clinical malaria. Block 2 is highly polymorphic, but all known alleles can be grouped into three major types. Two of these types (the K1-like and MAD20-like types) contain type-specific sequences (found in all alleles of a particular type) that flank polymorphic tripeptide repeats. These repeats contain both type-specific and subtype-specific sequences. To evaluate the antibody recognition of these parts of block 2, a new panel of six recombinant proteins was used (fused type-specific flanking sequences and two representative repeat sequences for each of the K1-like and MAD20-like types separately). Extensive testing of these antigens and full-length block 2 antigens showed that human serum immunoglobulin G antibodies induced by infection can recognize (i) type-specific epitopes in the repeats, (ii) subtype-specific epitopes in the repeats, or (iii) type-specific epitopes in flanking sequences. A large prospective study in The Gambia showed that antibodies to the repeats are strongly associated with protection from clinical malaria. The results are important for design of a vaccine to induce protective antibodies, and they address hypotheses about repeat sequences in malaria antigens.

Adult↗