The Breadth, but Not the Magnitude, of Circulating Memory B Cell Responses to P. falciparum Increases with Age/Exposure in an Area of Low Transmission

Medical Research Council Laboratories, Fajara, Banjul, The Gambia.
PLoS ONE (Impact Factor: 3.23). 10/2011; 6(10):e25582. DOI: 10.1371/journal.pone.0025582
Source: PubMed


Malaria caused by Plasmodium falciparum remains a major cause of death in sub-Saharan Africa. Immunity against symptoms of malaria requires repeated exposure, suggesting either that the parasite is poorly immunogenic or that the development of effective immune responses to malaria may be impaired.
We carried out two age-stratified cross-sectional surveys of anti-malarial humoral immune responses in a Gambian village where P. falciparum malaria transmission is low and sporadic. Circulating antibodies and memory B cells (MBC) to four malarial antigens were measured using ELISA and cultured B cell ELISpot.
The proportion of individuals with malaria-specific MBC and antibodies, and the average number of antigens recognised by each individual, increased with age but the magnitude of these responses did not. Malaria-specific antibody levels did not correlate with either the prevalence or median number of MBC, indicating that these two assays are measuring different aspects of the humoral immune response. Among those with immunological evidence of malaria exposure (defined as a positive response to at least one malarial antigen either by ELISA or ELISPOT), the median number of malaria-specific MBC was similar to median numbers of diphtheria-specific MBC, suggesting that the circulating memory cell pool for malaria antigens is of similar size to that for other antigens.

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    • "The intensity of antibody response to P. falciparum is significantly lower amongst residents of the low transmission hilltop site compared to the valley bottom where transmission is more stable. Although those previous studies did not compare the breadth of antibody responses between the sites, we found that there was significant difference in the number of antigens recognized by sera from the different sites but only amongst the youngest age group, confirming previous findings that repeated parasite exposure expands the repertoire of antibodies as children grow older [14,16], and is associated with protection from disease in adults [64]. "
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    ABSTRACT: Malaria represents a major public health problem in Africa. In the East African highlands, the high-altitude areas were previously considered too cold to support vector population and parasite transmission, rendering the region particularly prone to epidemic malaria due to the lack of protective immunity of the population. Since the 1980's, frequent malaria epidemics have been reported and these successive outbreaks may have generated some immunity against Plasmodium falciparum amongst the highland residents. Serological studies reveal indirect evidence of human exposure to the parasite, and can reliably assess prevalence of exposure and transmission intensity in an endemic area. However, the vast majority of serological studies of malaria have been, hereto, limited to a small number of the parasite's antigens. We surveyed and compared the antibody response profiles of age-stratified sera from residents of two endemic areas in the western Kenyan highlands with differing malaria transmission intensities, during two distinct seasons, against 854 polypeptides of P. falciparum using high-throughput proteomic microarray technology. We identified 107 proteins as serum antibody targets, which were then characterized for their gene ontology biological process and cellular component of the parasite, and showed significant enrichment for categories related to immune evasion, pathogenesis and expression on the host's cell and parasite's surface. Additionally, we calculated age-fitted annual seroconversion rates for the immunogenic proteins, and contrasted the age-dependent antibody acquisition for those antigens between the two sampling sites. We observed highly immunogenic antigens that produce stable antibody responses from early age in both sites, as well as less immunogenic proteins that require repeated exposure for stable responses to develop and produce different seroconversion rates between sites. We propose that a combination of highly and less immunogenic proteins could be used in serological surveys to detect differences in malaria transmission levels, distinguishing sites of unstable and stable transmission.
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    • "It appears that frequent reinfection is required to maintain high levels of circulating Ab, thus in highly endemic areas Ab levels are stable [36,37], but in low or unstable transmission areas Ab levels diminish quickly after an infection [28,38], showing seasonal variation [26,27,39–41]. Conversely, memory B cells (MBC) can persist with reduced transmission [42–45]. However, a study reported the presence of Ab, but only very low frequencies of malaria-specific MBC in children, suggesting a low induction of malaria-specific circulating MBC as a reason for short-lived anti-malarial Ab responses [46]. "
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    • "This could partly explain the high prevalence of antibodies to MSP-1 in this study relative to the finding in the coastal Kenya population with high malaria endemicity [26]. Similarly, a study in the Gambia during a period of low malaria transmission recorded high prevalence of antibodies to AMA-1, MSP-2 and MSP-3 [32]. Persistent malaria infection evidently intereferes with establishmment of long-lived B cell memory, probably by deleting antigen specific B cells [33] or disrupting of germinal cell architecture hence disrupting transformation of centrocytes [34]. "
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