Measurement of Antibody Levels against Region II of the Erythrocyte-Binding Antigen 175 of Plasmodium falciparum in an Area of Malaria Holoendemicity in Western Kenya

Abstract

Region II of the 175-kDa erythrocyte-binding antigen (EBA-175RII) of Plasmodium falciparum is functionally important in sialic acid-dependent erythrocyte invasion and is considered a prime target for an invasionblocking vaccine. The objectives of this study were to (i) determine the prevalence of anti-EBA-175RII antibodies in a naturally exposed population, (ii) determine whether naturally acquired antibodies have a functional role by inhibiting binding of EBA-175RII to erythrocytes, and (iii) determine whether antibodies against EBA-175RII correlate with immunity to clinical malaria. We treated 301 lifelong residents of an area of malaria holoendemicity in western Kenya for malaria, monitored them during a high-transmission season, and identified 33 individuals who were asymptomatic despite parasitemia (clinically immune). We also identified 50 clinically susceptible individuals to serve as controls. These 83 individuals were treated and monitored again during the subsequent low-transmission season. Anti-EBA-175RII antibodies were present in 98.7% of the individuals studied. The antibody levels were relatively stable between the beginning and end of the high-transmission season and correlated with the plasma EBA-175RII erythrocyte-binding-inhibitory activity. There was no difference in anti-EBA-175RII levels or plasma EBA-175RII erythrocyte-binding-inhibitory activity between clinically immune and clinically susceptible groups. However, these parameters were higher in nonparasitemic than in parasitemic individuals at enrollment. These results suggest that although antibodies against EBA-175RII may be effective in suppressing some of the wild parasite strains, EBA-175RII is unlikely to be effective as a monovalent vaccine against malaria, perhaps due to allelic heterogeneity and/or presence of sialic acid-independent strains.A vaccine against Plasmodium falciparum that can prevent the morbidity and mortality (8, 9, 25) from this parasite is greatly needed. Among the antigens being considered for vaccine development is the erythrocyte-binding antigen 175 . This is a 175-kDa protein that is expressed in the micronemes of merozoites (24), the stage of the parasite that invades erythrocytes. Its potential as a malaria vaccine antigen is based on the fact that the majority of P. falciparum isolates use EBA-175 as a ligand for the invasion of erythrocytes (6, 18). EBA-175 binds to sialic acid-dependent epitopes on erythrocyte glycophorin A (11, 23) and is probably involved in the formation of a junction between the erythrocyte and the apical portion of the merozoite just before invagination (10). This step is a key part of the erythrocyte invasion process and provides a logical target for vaccine-mediated immunity (23).EBA-175 is structurally divided into seven regions (1), and the cysteine-rich region II functions as the erythrocyte-binding ligand domain (23). Region II contains epitopes recognized by antibodies that block erythrocyte invasion (15,19,22) and by antibodies eluted from immune clusters of merozoites (21...

Breadth and Magnitude of Antibody Responses to MultiplePlasmodium falciparumMerozoite Antigens Are Associated with Protection from Clinical Malaria

“…A separate study found that individuals with high-titer anti-MSP-1 19 -specific invasion-inhibitory antibodies were protected from infection (33) and underscored the importance of developing robust functional assays for malaria. Antibodies to the F2 subdomain of EBA-175 were not associated with protection from clinical disease in our studies, as has been found in other parts of Africa where this (49) and other subdomains of EBA-175 (32,47,49) have been studied. To date, only one study has reported significantly higher antibody levels to EBA-175 peptide 4 (residues 1062 to 1103, within region V) in children protected from clinical attacks of malaria compared to susceptible children (68).…”

Safety and Immunogenicity of a Recombinant Nonglycosylated Erythrocyte Binding Antigen 175 Region II Malaria Vaccine in Healthy Adults Living in an Area Where Malaria Is Not Endemic

“…Antibodies directed against EBA-175-RII are capable of blocking the merozoite invasion of RBCs (39). Interestingly, anti-EBA-175-RII antibodies not only were capable of blocking the sialic acid-dependent invasion pathways but also inhibited alternative, sialic acid-independent pathways (24, 30).Taken together, the aforementioned data suggest that EBA-175-RII is a plausible vaccine candidate, especially when incorporated into a multicomponent vaccine to overcome allelic heterogeneity and/or the use of sialic acid-independent pathways by the parasite (2,3,27,29). Sim and colleagues evaluated the immunogenicity of a DNA vaccine that expressed EBA-175-RII in mice, rabbits, and monkeys and found the vaccine to be safe and immunogenic (38).…”

“…A threshold anti-EBA-175 level at which one expects a disease-modifying effect is currently not known. However, current evidence suggests that anti-EBA-175 increases in prevalence with age in areas where malaria is endemic, and higher levels may be associated with clinical protection (27,28).…”

“…Taken together, the aforementioned data suggest that EBA-175-RII is a plausible vaccine candidate, especially when incorporated into a multicomponent vaccine to overcome allelic heterogeneity and/or the use of sialic acid-independent pathways by the parasite (2,3,27,29). Sim and colleagues evaluated the immunogenicity of a DNA vaccine that expressed EBA-175-RII in mice, rabbits, and monkeys and found the vaccine to be safe and immunogenic (38).…”

“…The cysteine-rich second region of EBA-175 (EBA-175-RII) is the erythrocyte binding domain of the protein and is highly conserved among laboratory and clinical P. falciparum strains (19,37). Antibodies against EBA-175-RII have been detected among individuals living in areas where malaria is endemic, with an increasing prevalence with age and a trend of disease attenuation at high antibody levels (27,28). Antibodies directed against EBA-175-RII are capable of blocking the merozoite invasion of RBCs (39).…”