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

Abstract

Rhoptry proteins participate in the invasion of red blood cells by merozoites during the malaria parasite's asexual-stage cycle. Interference with the rhoptry protein function has been shown to prevent invasion, and three rhoptry proteins have been suggested as potential components of a vaccine against malaria. Rhoptryassociated membrane antigen (RAMA) is a 170-kDa protein of Plasmodium falciparum which is processed to a 60-kDa mature form in the rhoptries. p60/RAMA is discharged from rhoptries of free merozoites and binds to the red-cell membrane before being internalized to form part of the parasitophorous vacuole of the newly developing ring. We examined the range of anti-RAMA responses in individuals living in an area of endemicity for malaria and determined its association with clinical immunity. RAMA is immunogenic during infections, and at least three epitopes within RAMA are recognized by hyperimmune sera in immunoblots. Sera from individuals living in a region of Vietnam where malaria is endemic possessed strong antibody responses toward two C-terminal regions of RAMA. Cytophilic antibody isotypes (immunoglobulin G1 [IgG1] and IgG3) predominated in humoral responses to both C-terminal epitopes. Acute episodes of P. falciparum infection result in significant boosting of levels of antibody to an epitope at the extreme C terminus of RAMA that harbors the red-cell-binding domain. Immunity to P. falciparum infection was linked to elevated levels of IgG3 responses to this functional domain of RAMA, suggesting that the region may contain a protective epitope useful for inclusion in a multiepitope vaccine against malaria.Plasmodium falciparum malaria is responsible for Ͼ2 million deaths each year. With the increasing resistance of Plasmodium parasites to antimalarial drugs and of the Anopheles mosquito vector to commonly available insecticides, an effective vaccine that would protect nonimmune individuals from death would be valuable. The life cycle of malaria parasites is quite complex and provides a number of potential vaccine targets. Several proteins expressed by P. falciparum during the erythrocytic stages are being investigated as candidates for a subunit vaccine, among them rhoptry-associated proteins. The most important immune response for immunity to asexual-stage infection is believed to be humoral, although cell-mediated responses may also contribute to immunity.Rhoptries are intracellular organelles of malaria parasites involved in the invasion of red blood cells (RBCs) by Plasmodium merozoites. 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). Moreover, immunization of Saimiri monkeys with RAP1 protected the animals from a lethal...

Target Antigen, Age, and Duration of Antigen Exposure Independently Regulate Immunoglobulin G Subclass Switching in Malaria

“…In humans, both IgG1 and IgG3 have high affinities for Fc receptors, fix complement, and are preferentially induced during Th1-type responses, but IgG3 appears to be a more efficient mediator of FcR crosslinking and phagocyte activation (8,22). Consistent with this, IgG3 antibodies to various blood-stage antigens are particularly effective mediators of antibody-dependent cellular inhi- (5) and have been repeatedly associated with the acquisition of clinical immunity to malaria (12,14,29,42,45,48). These data make a strong case for trying to engineer antimalarial vaccines to induce high titers of IgG3 antibodies, but this will only be feasible when we have a better understanding of the requirements for induction of IgG3 class switching in human B cells.…”

Rhop-3 protein conservation among Plasmodium species and induced protection against lethal P. yoelii and P. berghei challenge

“…These results suggest that mice were protected. Plasmodium merozoite rhoptry proteins contribute to blood stage immunity and are important targets for a blood stage vaccine (Holder and Freeman 1981;Jakobsen et al 1993Jakobsen et al , 1996Facer and Tanner 1997;Collins et al 2000;Toploska et al 2004;Nixon et al 2005). The Rhop-3 protein, along with other merozoite rhoptry proteins participates in membrane raft formation and parasitophorous vacuole formation during erythrocyte invasion (Hiller et al 2003;Toploska et al 2003).…”

Plasmodium rhoptry proteins: why order is important

“…Intriguingly, however, no functional equivalent of RAMA has to date been found in other Apicomplexan spp. It should also be noted that because the C-terminus of RAMA is embedded within the lipid bilayer [30], an unidentified TM escorter protein would have to be present in vesicles at the Golgi exit face, interacting with both RAMA and the cytosolic sorting machinery to escort these protein complexes to nascent rhoptries.…”