Skip to main page content
Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 2015 Jan;83(1):276-85.
doi: 10.1128/IAI.01979-14. Epub 2014 Nov 3.

Immune Characterization of Plasmodium Falciparum Parasites With a Shared Genetic Signature in a Region of Decreasing Transmission

Affiliations
Free PMC article

Immune Characterization of Plasmodium Falciparum Parasites With a Shared Genetic Signature in a Region of Decreasing Transmission

Amy K Bei et al. Infect Immun. .
Free PMC article

Abstract

As the intensity of malaria transmission has declined, Plasmodium falciparum parasite populations have displayed decreased clonal diversity resulting from the emergence of many parasites with common genetic signatures (CGS). We have monitored such CGS parasite clusters from 2006 to 2013 in Thiès, Senegal, using the molecular barcode. The first, and one of the largest observed clusters of CGS parasites, was present in 24% of clinical isolates in 2008, declined to 3.4% of clinical isolates in 2009, and then disappeared. To begin to explore the relationship between the immune responses of the population and the emergence and decline of specific parasite genotypes, we have determined whether antibodies to CGS parasites correlate with their prevalence. We measured (i) antibodies capable of inhibiting parasite growth in culture and (ii) antibodies recognizing the surfaces of infected erythrocytes (RBCs). IgG obtained from volunteers in 2009 showed increased reactivity to the surfaces of CGS-parasitized erythrocytes over IgG from 2008. Since P. falciparum EMP-1 (PfEMP-1) is a major variant surface antigen, we used var Ups quantitative reverse transcription-PCR (qRT-PCR) and sequencing with degenerate DBL1α domain primers to characterize the var genes expressed by CGS parasites after short-term in vitro culture. CGS parasites show upregulation of UpsA var genes and 2-cysteine-containing PfEMP-1 molecules and express the same dominant var transcript. Our work indicates that the CGS parasites in this cluster express similar var genes, more than would be expected by chance in the population, and that there is year-to-year variation in immune recognition of surface antigens on CGS parasite-infected erythrocytes. This study lays the groundwork for detailed investigations of the mechanisms driving the expansion or contraction of specific parasite clones in the population.

Figures

FIG 1
FIG 1
Equivalent inhibition of CGS parasites by GIA. (A) Association between levels of inhibition of two CGS parasite strains by IgG. The percentages of inhibition by GIA are shown for the CGS-1 and CGS-2 strains. Inhibition by IgG from 2008 (n = 8) and 2009 (n = 9) is shown. The Spearman rank correlation coefficient and P value are shown. (B through D) No significant difference in inhibition between IgG from 2008 and IgG from 2009 was observed for the CGS parasites combined (CGS-1 and CGS-2) (B), for CGS-1 parasites (C), or for CGS-2 parasites (D). A nonparametric Mann-Whitney U test was performed. Wide horizontal bars represent medians; error bars, interquartile ranges. ns, not significant.
FIG 2
FIG 2
Differential recognition of CGS parasite variant surface antigens over time. (A) A significant positive correlation was observed for VSA staining of the two CGS parasites by using plasma samples from 2008 and 2009 combined. The data shown are from the largest experiment (n, 49 plasma samples). The Spearman rank correlation coefficient and P value are shown, as well as a line representing the best linear fit. (B) When the surface staining levels of all 109 IgGs were compared by using a 2σ cutoff, no difference between years was observed in VSA surface staining levels for the non-CGS parasite; however, significant differences in VSA surface staining levels were observed for both CGS parasites.
FIG 3
FIG 3
UpsA var expression in CGS parasites. (A) Quantitative PCR of var Ups classes. Data are displayed as ΔCT values relative to that of seryl-tRNA synthetase. Error bars represent means for triplicates (or, in the case of fructose biphosphate aldolase, for sextuplicates); error bars, standard errors of the means. Asterisks indicate significant differences by one-way analysis of variance (*, P < 0.05; **, P < 0.01; ***, P < 0.001). (B) The fold change in var Ups expression by CGS parasites from that by non-CGS parasites was calculated as 2−ΔΔCT. Dotted lines indicate 2-fold differences in either direction, which were selected as the cutoffs for biologically significant differences in transcription. Significant upregulation of UpsA (specifically UpsA3) is observed for CGS parasites relative to expression by non-CGS parasites. Bars represent mean fold changes; error bars, ranges. FBPA, fructose-bisphosphate aldolase.
FIG 4
FIG 4
Characteristics of DBL1α domains of CGS parasites. (A) Schematic illustrating the DBL sequencing strategy as well as DBL1α subdomains and PCR segments. (B) Alignment of representative sequences from VARseg2 for CGS and non-CGS parasites. CGS parasites show a predominance of 2-Cys var genes, whereas non-CGS parasites show a predominance of 4-Cys var genes. Cysteines are highlighted in yellow. (C) Motifs identified in CGS and non-CGS parasites based on previous studies by Normark et al. (31). (D) Network showing the relationship of CGS and non-CGS sequences to the global background. Each var gene is represented by a vertex, and vertices are linked if they share an exact match of significant length at the amino acid level. Colored vertices correspond to Senegalese isolates and are slightly enlarged for clarity. Gray vertices correspond to the seven global isolates creating the global background of var DBL1α sequence diversity. (E) Overlap diagrams for the genomic var repertoire (gDNA) (left) and expressed var transcripts (cDNA) (right). Each vertex in the diagram represents a unique var DBL1α sequence tag, and links indicate 80% or more sequence identity at the amino acid level. Vertex sizes correspond to the numbers of corroborating sequences, with larger vertices indicating higher confidence.

Similar articles

See all similar articles

Cited by 4 articles

Publication types

MeSH terms

Substances

LinkOut - more resources

Feedback