Naturally acquired antibodies to polymorphic and conserved epitopes of Plasmodium falciparum merozoite surface protein 3

Abstract

Many studies on the role of merozoite surface protein 3 (MSP3) in immunity against malaria have focused on a conserved section of MSP3. New evidence suggests that polymorphic sequences within MSP3 are under immune selection. We report a detailed analysis of naturally-acquired antibodies to allele-specific and conserved parts of MSP3 in a Kenyan cohort. Indirect and competition ELISA to heterologous recombinant MSP3 proteins were used for antibody assays, and parasites were genotyped for msp3 alleles. Antibody reactivity to allele-specific and conserved epitopes of MSP3 was heterogeneous between individuals. Overall, the prevalence of allele-specific antibody reactivity was significantly higher (3D7-specific 54%, K1-specific 41%) than that to a recombinant protein representing a conserved portion of C-terminal MSP3 (24%, P < 0.01). The most abundant IgG subclass was IgG3, followed by IgG1. Allele-specific reactivity to the K1-type of MSP3 was associated with a lower risk of clinical malaria episodes during a 6-month follow-up in individuals who were parasitized at the start of the malaria transmission season (Relative risk 0.41 with 95% confidence interval 0.20-0.81, P = 0.011). The potential importance of allele-specific immunity to MSP3 should be considered in addition to immunity to conserved epitopes, in the development of an MSP3 malaria vaccine.

Figure 1

Correlation between antibody reactivity to K1 MSP3 and 3D7 MSP3 in the sera of 536 individuals from Chonyi village. Dashed lines indicate cut-off values for positive reactivity for 3D7 and K1 MSP3, respectively, as determined by mean + 3 SD of reactivity of 20 non-malaria exposed controls. Pair wise correlation coefficient was 0·6752, P = 0·0000, n = 536.

Figure 2

Competition ELISA using titred amounts of competing antigen on six representative sera. Data on remaining 14 sera are shown in supplementary figures.

Figure 3

Age-specific prevalence of anti-MSP3 antibodies. Numbers above each group of bars indicate the number of individuals in each age category. The presence of allele-specific reactivity to 3D7 and K1 MSP3 was determined as follows: samples that were sero-positive (serum IgG OD level above the mean + 3 SD of that of 20 non-malaria exposed sera) for 3D7 MSP3 and sero-negative for K1 MSP3 were considered to be allele-specific for 3D7-MSP3, and likewise for K1 MSP3; samples that were sero-positive for both 3D7 and K1 MSP3 were tested by competition ELISA and differences of > 0·3 OD units between the heterologous and homologous competition assays were counted as evidence of allele-specific reactivity. Samples were considered to be positive for C-terminal MSP3 if the serum IgG OD level was above the mean + 3 SD of that of 20 non-malaria exposed sera.

Figure 4

(a, b) IgG subclass reactivity to MSP3 recombinant antigens, (a) 3D7 MSP3 and (b) K1 MSP3, respectively. Horizontal bars indicate means (n = 96).