Plasmodium falciparum var gene expression is modified by host immunity

Abstract

Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) is a potentially important family of immune targets, which play a central role in the host-parasite interaction by binding to various host molecules. They are encoded by a diverse family of genes called var, of which there are approximately 60 copies in each parasite genome. In sub-Saharan Africa, although P. falciparum infection occurs throughout life, severe malarial disease tends to occur only in childhood. This could potentially be explained if (i) PfEMP1 variants differ in their capacity to support pathogenesis of severe malaria and (ii) this capacity is linked to the likelihood of each molecule being recognized and cleared by naturally acquired antibodies. Here, in a study of 217 Kenyan children with malaria, we show that expression of a group of var genes "cys2," containing a distinct pattern of cysteine residues, is associated with low host immunity. Expression of cys2 genes was associated with parasites from young children, those with severe malaria, and those with a poorly developed antibody response to parasite-infected erythrocyte surface antigens. Cys-2 var genes form a minor component of all genomic var repertoires analyzed to date. Therefore, the results are compatible with the hypothesis that the genomic var gene repertoire is organized such that PfEMP1 molecules that confer the most virulence to the parasite tend also to be those that are most susceptible to the development of host immunity. This may help the parasite to adapt effectively to the development of host antibodies through modification of the host-parasite relationship.

Fig. 1.

Relationship between cys2 var expression, host age, and malaria syndromes. For both severe (A) and nonsevere (B) malaria, the proportion of expressed cys2 tag sequences is plotted against age of the child from whom the isolate was obtained. Spearman's rank correlation ρ and p values are indicated at the bottom of each plot. (C–F) Results of four multiple regression models with one of the cys2 subgroups (MFK⁺ REY⁻, MFK⁻ REY⁺, MFK⁻ REY⁻, and group A-like) as the dependent variable and each with host age and the three severe malaria syndromes as explanatory variables. For each of the four subgroup-specific expression profiles the respective regression coefficients, 95% confidence intervals, and p values for their relationship with host age (C), severe malarial anemia (D), impaired consciousness (E), and severe respiratory distress (F) are shown. These are compared to relationships with total cys2 expression (black arrows). Statistically significant relationships are indicated (*). Parasitemia at presentation to hospital was not associated with cys2 expression (Fig. S2) and hence was not included in the models.

Fig. 2.

A hypothetical model for immunity-dependent expression of PfEMP1 variants. Different shapes represent variants encoded by a hypothetical parasite genome (A) or relative numbers of parasites expressing each variant at four stages in the development of immunity to this genome (B–E). Within a naïve host (B), the cytoadherence phenotype of the variant expressed by each parasite lineage determines the relative growth rate of that lineage. The variant that best maximizes parasite growth (black triangles) will become dominant. This initial benefit has a cost because this variant will also be first to stimulate a protective antibody response (black Y shapes, C) and is cleared from individuals who already carry antibodies to it (D). In these individuals the next most effective variant will dominate (blue circles). If similar cytoadherence properties confer a growth advantage in all naïve hosts this may limit the number of variants associated with infections of young children. In older children with a well-developed repertoire of anti-PfEMP1 immune responses (different colored Y shapes, E), antigenic novelty has an advantage over optimal cytoadherence. (F) Age-corrected regression coefficients, 95% confidence intervals, and p values for the association between mean fluorescence intensity (MFI) of IgG binding to the surface of parasite-infected erythrocytes and cys2 expression. The eight isolates used for flow cytometry together with their cys2 expression profiles are shown. See Fig. S6 for raw MFI values.