Acquisition of naturally occurring antibody responses to recombinant protein domains of Plasmodium falciparum erythrocyte membrane protein 1

Abstract

Background: Antibodies targeting variant antigens expressed on the surface of Plasmodium falciparum infected erythrocytes have been associated with protection from clinical malaria. The precise target for these antibodies is unknown. The best characterized and most likely target is the erythrocyte surface-expressed variant protein family Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1).

Methods: Using recombinant proteins corresponding to five domains of the expressed A4 var gene, A4 PfEMP1, the naturally occurring antibody response was assessed, by ELISA, to each domain in serum samples obtained from individuals resident in two communities of differing malaria transmission intensity on the Kenyan coast. Using flow cytometry, the correlation in individual responses to each domain with responses to intact A4-infected erythrocytes expressing A4 PfEMP1 on their surface as well as responses to two alternative parasite clones and one clinical isolate was assessed.

Results: Marked variability in the prevalence of responses between each domain and between each transmission area was observed, as wasa strong correlation between age and reactivity with some but not all domains. Individual responses to each domain varied strikingly, with some individuals showing reactivity to all domains and others with no reactivity to any, this was apparent at all age groups. Evidence for possible cross-reactivity in responses to the domain DBL4gamma was found.

Conclusion: Individuals acquire antibodies to surface expressed domains of a highly variant protein. The finding of potential cross-reactivity in responses to one of these domains is an important initial finding in the consideration of potential vaccine targets.

Figure 1

Proportion of individuals in each age group recognizing recombinant domains. Sera from 1222 individuals older than six months, was tested for reactivity against each recombinant protein in turn using ELISA. Responses were scored as positive if the mean OD obtained was greater than the mean OD plus 3 standard deviations of a panel of 20 non-malaria exposed donors. The solid grey bars refer to individuals resident in Chonyi and the hatched bars to individuals resident in Ngerenya. a) DBL1α, b) CIDR1α, c) DBL2β, d) DBL4γ, e) DBL5β. P-values given are chi-squared for trend, C = Chonyi N = Ngerenya.

Figure 2

Mean number of domains recognised with age. Graph shows mean number of domains, plus upper 95% confidence limits, recognized by individuals in each age group. The dark grey bars refer to individuals in Chonyi and the hatched bars to individuals resident in Ngerenya. *=P-value <0.05 (Wilcoxon Ranksum).

Figure 3

Proportion of individuals in each age group recognizing parasite line A4u. Sera from 272 individuals older than six months, was tested for reactivity against the parasite line A4U using flow cytometry. The proportion of individuals in each age category, with upper 95% confidence interval, scoring positive for antibody recognition are shown. Positivity was scored as defined in the text. The light grey bars represent individuals resident in Chonyi and the dark grey bars represent individuals resident in Ngerenya.