Erythrocyte invasion phenotypes of Plasmodium falciparum in The Gambia

Abstract

In vitro experimentation with Plasmodium falciparum has determined that a number of different receptor-ligand interactions are involved in the invasion of erythrocytes. Most culture-adapted parasite isolates use a mechanism of invasion that depends primarily on the erythrocyte sialoglycoprotein glycophorin A (GYPA) and erythrocyte-binding antigen 175 (EBA-175) of the parasite blood-stage merozoite. However, a minority of culture-adapted parasites and a majority of Indian field isolates can apparently invade by other means. Here, erythrocyte invasion phenotypes of P. falciparum field isolates in Africa were studied. For 38 Gambian isolates, invasion of neuraminidase-treated and trypsin-treated erythrocytes was inhibited, on average, by more than 60 and 85%, respectively, indicating a high level of dependence on sialic acid and trypsin-sensitive proteins on the erythrocyte surface. These results support the hypothesis that African P. falciparum parasites use GYPA as a primary receptor for invasion. However, the considerable variation among isolates confirms the idea that alternative receptors are also used by many parasites. Three amino acid polymorphisms in the GYPA-binding region of EBA-175 (region II) were not significantly associated with invasion phenotype. There was variation among isolates in the selectivity index (i.e., a statistical tendency toward aggregation or multiple invasions of host erythrocytes), but this variation did not correlate with enzyme-determined invasion phenotype or with eba-175 alleles. Overall, these invasion phenotypes in Africa support a vaccine strategy of inhibiting EBA-175 binding to GYPA but suggest that parasites with alternative phenotypes would be selected for if this strategy were used alone.

FIG. 1.

Histogram of percent inhibition of invasion due to neuraminidase (a) and trypsin (b) treatments of target erythrocytes for 38 P. falciparum field isolates from The Gambia.

FIG. 2.

Comparison of in vitro inhibition of invasion due to neuraminidase treatment (x axis) and trypsin treatment (y axis) of target erythrocytes for 38 P. falciparum field isolates from The Gambia (closed squares) and 11 laboratory-maintained isolates from broad geographical origins (open squares) (4). Lines indicate 50% inhibition by enzyme treatment for visual clarity.

FIG. 3.

Scatter plots of enzyme-determined invasion phenotypes of all 38 Gambian isolates. Data show multiplicity of P. falciparum infection (a), alleles at codon 577 of eba-175 (b), alleles at codon 584 of eba-175 (c), and alleles at codon 592 of eba-175 (d). Isolates with mixed eba-175 genotypes were omitted.

FIG. 4.

Test for association between in vivo SI in a peripheral blood smear and percent inhibition by neuraminidase treatment (a) and trypsin treatment (b) of target erythrocytes in cultures.