Acquisition of antibodies against Plasmodium falciparum merozoites and malaria immunity in young children and the influence of age, force of infection, and magnitude of response

Abstract

Individuals in areas of Plasmodium falciparum endemicity develop immunity to malaria after repeated exposure. Knowledge of the acquisition and nature of protective immune responses to P. falciparum is presently limited, particularly for young children. We examined antibodies (IgM, IgG, and IgG subclasses) to merozoite antigens and their relationship to the prospective risk of malaria in children 1 to 4 years of age in a region of malaria endemicity in Papua New Guinea. IgG, IgG1, and IgG3 responses generally increased with age, were higher in children with active infection, and reflected geographic heterogeneity in malaria transmission. Antigenic properties, rather than host factors, appeared to be the main determinant of the type of IgG subclass produced. High antibody levels were not associated with protection from malaria; in contrast, they were typically associated with an increased risk of malaria. Adjustment for malaria exposure, using a novel molecular measure of the force of infection by P. falciparum, accounted for much of the increased risk, suggesting that the antibodies were markers of higher exposure to P. falciparum. Comparisons between antibodies in this cohort of young children and in a longitudinal cohort of older children suggested that the lack of protective association was explained by lower antibody levels among young children and that there is a threshold level of antibodies required for protection from malaria. Our results suggest that in populations with low immunity, such as young children, antibodies to merozoite antigens may act as biomarkers of malaria exposure and that, with increasing exposure and responses of higher magnitude, antibodies may act as biomarkers of protective immunity.

FIG 1

IgM responses to merozoite antigens in relation to age and P. falciparum infection status. Children (n = 183) were divided into 4 age groups to examine associations with age. As indicated, the presence of P. falciparum was determined by PCR. Data are plotted as box-and-whisker plots (boxes show medians and interquartile ranges; error bars show 95% confidence intervals). m, months. Filled circles represent outlier values.

FIG 2

IgG subclass responses in relation to age and P. falciparum infection status. Children (n = 183) were divided into 4 age groups to examine associations with age. As indicated, the presence of P. falciparum was determined by PCR. Data are presented as box-and-whisker plots (boxes show medians and interquartile ranges; error bars show 95% confidence intervals).

FIG 3

IgG responses to merozoite antigens by village. (A) Schematic map of study area, including houses of participants (dots) and health centers (crosses) (modified from reference under the Creative Commons Attribution License). (B to E) IgG responses to MSP1-19, MSP2 FC27, MSP2 3D7, and AMA-1 categorized by village. The dashed line in each panel indicates the overall median response for a given antigen. Data are shown as box-and-whisker plots (boxes show medians and interquartile ranges; error bars show minimums and maximums). Antibody levels were significantly different between villages for all antigens (P = <0.001).

FIG 4

IgG responses to merozoite antigens in relation to _molFOI and P. falciparum infection status. Children were divided into 3 groups (those with low, medium, and high _molFOI levels) to examine associations between antibody levels (OD) at baseline and _molFOI as a marker of exposure. As indicated, the presence of P. falciparum at baseline was determined by PCR. Data are presented as box-and-whisker plots (boxes show medians and interquartile ranges; error bars show 95% confidence intervals). Differences categorized by level of _molFOI are significant for AMA-1 and MSP2 responses among the PCR-positive children (P < 0.01).

FIG 5

Comparison of IgG levels to merozoite antigens of P. falciparum in two cohorts of children of different ages. Total IgG levels specific for MSP1-19, AMA-1, MSP2 3D7, and MSP2 FC27 in the top quartile of responders for each antigen in the group of children 1 to 4 years of age (1-3 yr HR group; age at enrollment) were compared with those of low, medium, and high responders in the group of children 5 to 14 years of age. Horizontal bars indicate median antibody levels, and P values refer to comparisons between the top quartile of responders from the group of 1- to 4-year-old children and each of the low, medium, and high responder groups of the 5- to 14-year-old children. LR, low responders; MR, medium responders; HR, high responders.

FIG 6

Models of the evolving role of antibodies to P. falciparum merozoite antigens with changing malaria exposure and antibody levels. (A) Low antibody levels are not protective against malaria, but as antibody levels increase (with age and/or exposure) and reach a theoretical threshold, antibodies contribute to protective immunity and serve as biomarkers of malaria immunity. (B) Antibody levels may be valuable as biomarkers to predict malaria risk or protective immunity because they identify individuals who are being exposed to infection. In young children, or those with limited exposure, antibodies have a high predictive value for an increased risk of malaria and have a poor predictive value for protective immunity. As age or cumulative exposure or both increase, the predictive value of antibodies for increased risk of malaria declines to a point at which antibodies become better markers of protection from malaria.