Allele-specific antibodies to Plasmodium vivax merozoite surface protein-1: prevalence and inverse relationship to haemoglobin levels during infection

Abstract

Background: Antigenic polymorphisms are considered as one of the main strategies employed by malaria parasites to escape from the host immune responses after infections. Merozoite surface protein-1 (MSP-1) of Plasmodium vivax, a promising vaccine candidate, is a highly polymorphic protein whose immune recognition is not well understood.

Methods and results: The IgG responses to conserved (MSP-1₁₉) and polymorphic (block 2 and block 10) epitopes of PvMSP-1 were evaluated in 141 P. vivax infected patients. Ten recombinant proteins corresponding to block 2 (variants BR07, BP29, BP39, BP30, BEL) and block 10 (BR07, BP29, BP39, BP01, BP13) often observed in Brazilian P. vivax isolates were assessed by ELISA in order to determine levels of specific antibodies and their respective seroprevalence. The magnitude and the frequency of variant-specific responses were very low, except for BR07 variant (>40%), which was the predominant haplotype as revealed by block 10 PvMSP-1 gene sequencing. By contrast, 89% of patients had IgG against the C-terminal conserved domain (PvMSP-1₁₉), confirming the high antigenicity of this protein. Using multiple linear and logistic regression models, there was evidence for a negative association between levels of haemoglobin and several IgG antibodies against block 2 variant antigens, with the strongest association being observed for BP39 allelic version. This variant was also found to increase the odds of anaemia in these patients.

Conclusions: These findings may have implications for vaccine development and represent an important step towards a better understanding of the polymorphic PvMSP-1 domain as potential targets of vaccine development. These data highlight the importance of extending the study of these polymorphic epitopes of PvMSP-1 to different epidemiological settings.

Keywords: Anaemia; Antibodies; MSP-1; Plasmodium vivax; Polymorphism.

Fig. 1

Antibody data for P. vivax patients. a Box plots of reactivity index of IgG antibodies to variable (block 2 and block 10) and conserved (block 13) recombinant antigens of PvMSP-1. Antibody reactivity of PvMSP-1₁₉ antigen is statistically higher than that of each variant PvMSP-1 antigen (Wilcoxon signed-rank test, all P values <0.001). b Prevalence of seropositivity to each IgG responses and respective 95% confidence intervals, where seropositivity was determined as the reactivity index greater than 1. Seroprevalence of PvMSP-1₁₉ antigen is statistically higher than that for each variant PvMSP-1 antigen (McNemar test, all P values <0.001)

Fig. 2

Frequencies of allele-specific IgG responses to PvMSP-1. a Frequency distribution of the total number of seropositive responses to conserved (PvMSP-1₁₉) and variant antigens PvMSP-1 antigens. b Frequency distribution of different joint seropositivity profiles for block 2 variant antigens: BELÉM (BEL), BP30, BP39, BP29, and BR07 antigens. c Frequency distribution of different joint seropositivity profiles for block 10 variant antigens: BP13, BP01, BP39, BP29, and BR07 antigens

Fig. 3

Pairwise correlation based on Spearman’s non-parametric coefficient between levels of IgG antibodies (reactivity index) to variant recombinant antigens of block 2 and block 10 of PvMSP-1 among overall population (n = 141 patients) and among responders to specific proteins (green correlation <−0.5, yellow—(−0.5) ≤ correlation ≤0.5, red—correlation >0.5)

Fig. 4

Associations between each IgG level and haemoglobin concentration (linear regression analysis) and anaemia (logistic regression model) where −log₁₀(P value) is a measure of the strength of association and the dashed line refers to the threshold ensuring a 5% global statistical significance using the Bonferroni multiple testing correction