Interaction of HLA and Age on Levels of Antibody toPlasmodium falciparumRhoptry-Associated Proteins 1 and 2

Abstract

The Plasmodium falciparum rhoptry-associated proteins 1 and 2 (RAP1 and RAP2) are candidate antigens for a subunit malaria vaccine. The design of the study, which looks at the acquisition of immunity to malaria from childhood to old age, has allowed us to document the interaction of HLA and age on levels of antibody to specific malarial antigens. Antibodies reach maximum levels to RAP1 after the age of 15 but to RAP2 only after the age of 30. The effect of HLA-DRB1 and -DQB1 and age on levels of antibody to rRAP1 and rRAP2 was analyzed with a multiple regression model in which all HLA alleles and age were independent variables. DQB1*0301 and -*03032 showed an age-dependent association with levels of antibody to rRAP1, being significant in children 5 to 15 years (P < 0.001) but not in individuals over 15 years of age. DRB1*03011 showed an age-dependent association with antibody levels to rRAP2; however, this association was in adults over the age of 30 years (P < 0.01) but not in individuals under the age of 30 years. No associations were detected between DRB1 alleles and RAP1 antibody levels or between DQB1 alleles and RAP2 antibody levels. Thus, not only the HLA allele but also the age at which an interaction is manifested varies for different malarial antigens. The interaction may influence either the rate of acquisition of antibody or the final level of antibody acquired by adults.The rhoptry-associated proteins 1 and 2 (RAP1 and RAP2) form a low-molecular-mass complex located in the rhoptries of Plasmodium falciparum (5, 9). The rhoptries are a pair of organelles at the apical end of the parasite that are involved in the invasion of erythrocytes; thus, molecules in the rhoptries were among the first components identified as potential candidates for a subunit vaccine. Subsequent studies have borne out this potential. Monoclonal antibodies (MAbs) directed against rhoptry-associated proteins have been shown to provide substantial inhibition of parasite invasion in vitro (7,17,19,22,29,37). In addition, protein preparations containing both RAP1 and RAP2 have been used to immunize Saimiri monkeys. In these studies, the immunized monkeys developed a lower peak parasitemia than nonimmunized controls and were protected from subsequent lethal blood-stage challenge with P. falciparum (9,30,32,34) in a way similar to the protection acquired by immunization with whole merozoite (27,39,46). Two rhoptry-associated proteins, RAP1 and RAP2, have been sequenced (20,36). Unlike many other P. falciparum antigens, RAP1 and RAP2 exhibit a high degree of sequence similarity among P. falciparum isolates. Isolates from Honduras,

Human Leukocyte Antigen Class II Alleles Influence Levels of Antibodies to thePlasmodium falciparumAsexual-Stage Apical Membrane Antigen 1 but Not to Merozoite Surface Antigen 2 and Merozoite Surface Protein 1

“…Single-allele analysis of the DQB1 alleles with RAP1 antibody level resulted in confirmation of the previously reported association between RAP1 and DQB1*03 (23). Detailed results are not given since these analyses are not a replication on a new or additional sample but rather a replication on an enriched sample from the same group as the original study.…”

“…Results show that, in addition to the previously reported influence of HLA on levels of antibodies to RAP1 and RAP2 (23), HLA class II allelic products influence the level of antibodies to the variant of rAMA1 tested. No HLA influence was observed for the variant of MSA2 and MSP1-190L tested or for any of the MSP1 19 variants used in the study.…”

“…In an earlier report, high levels of antibodies to RAP1 and RAP2 were associated with an HLA class II allele (23). Because the previously analyzed data set for RAP1 and RAP2 included individuals living in the same household, we have subsequently reanalyzed the data using only one individual per household.…”

“…In fact, the genes that encode class II alleles were originally identified as immune response genes because of their influence on antibody levels (26). It has been reported that specific HLA-DR and -DQ alleles influence levels of antibodies to rhoptry-associated protein 1 (RAP1) and RAP2 (23). Other investigators have reported an association between an HLA class II allele and the acquisition of antibodies to a B-cell epitope in the ring-erythrocyte-stage antigen (RESA) (38), the subunit vaccine antigen SPf66 (3), and a malaria sporozoite antigen (44).…”

Associations between Responses to the Rhoptry-Associated Membrane Antigen of Plasmodium falciparum and Immunity to Malaria Infection

“…Although their contents are only transiently accessible to antibodies, seroepidemiological studies have demonstrated the development of antibody responses to the rhoptry proteins RhopH3, RAP1, and RAP2 following infection (8,12,20,23). In vitro growth inhibition assays have indicated that antibodies directed against the RAP1 and RAP2 proteins have inhibitory effects on P. falciparum growth in in vitro culture (6,9,14,18).…”