Plasmodium falciparum 19-Kilodalton Merozoite Surface Protein 1 (MSP1)-Specific Antibodies That Interfere with Parasite GrowthIn VitroCan Inhibit MSP1 Processing, Merozoite Invasion, and Intracellular Parasite Development

Abstract

cMerozoite surface protein 1 (MSP1) is a target for malaria vaccine development. Antibodies to the 19-kDa carboxy-terminal region referred to as MSP1 19 inhibit erythrocyte invasion and parasite growth, with some MSP1-specific antibodies shown to inhibit the proteolytic processing of MSP1 that occurs at invasion. We investigated a series of antibodies purified from rabbits immunized with MSP1 19 and AMA1 recombinant proteins for their ability to inhibit parasite growth, initially looking at MSP1 processing. Although significant inhibition of processing was mediated by several of the antibody samples, there was no clear relationship with overall growth inhibition by the same antibodies. However, no antibody samples inhibited processing but not invasion, suggesting that inhibition of MSP1 processing contributes to but is not the only mechanism of antibody-mediated inhibition of invasion and growth. Examining other mechanisms by which MSP1-specific antibodies inhibit parasite growth, we show that MSP1 19 -specific antibodies are taken up into invaded erythrocytes, where they persist for significant periods and result in delayed intracellular parasite development. This delay may result from antibody interference with coalescence of MSP1 19 -containing vesicles with the food vacuole. Antibodies raised against a modified recombinant MSP1 19 sequence were more efficient at delaying intracellular growth than those to the wild-type protein. We propose that antibodies specific for MSP1 19 can mediate inhibition of parasite growth by at least three mechanisms: inhibition of MSP1 processing, direct inhibition of invasion, and inhibition of parasite development following invasion. The balance between mechanisms may be modulated by modifying the immunogen used to induce the antibodies.

Sequential Processing of Merozoite Surface Proteins during and after Erythrocyte Invasion by Plasmodium falciparum

“…6). This is in contrast to previous reports showing that antibodies to MSP1-19 can be internalized and result in delayed or inhibited intraerythrocytic parasite development in some cases (70).…”

“…The ability of antibodies to be carried into invaded RBCs was observed previously for MSP1-19, with internalization of noninhibitory monoclonal antibodies (24,25) as well as inhibitory polyclonal rabbit antibodies, which can result in a specific growth delay (70). Other studies have reported that antibodies to MSP1 are able to disrupt intraerythrocytic development as well as inhibit invasion (81), a process that may be due to the internalization of antibodies during invasion.…”

“…However, it is also possible that internalized MSP2 antibodies have no role in inhibiting intraerythrocytic development, as MSP2 is rapidly degraded and presumably has no role in the subsequent development of the parasite postinvasion. This is in contrast to MSP1-19, which has been hypothesized to have a role in food vacuole formation, with internalized antibodies to MSP1-19 having the potential to disrupt function postinvasion (24,70). It is interesting to speculate that the ability of the merozoite to carry antibodies bound to surface proteins, such as MSP2, into the invaded RBC without apparent disruption of invasion or growth may represent an adaption of the parasite to render antibodies to the merozoite surface nonfunctional, thereby contributing to immune evasion.…”

“…Parasites were analyzed by flow cytometry at 40 h postinvasion as described above for GIAs, with parasites being gated as EtBr-and green fluorescent protein (GFP)-positive cells. To investigate the ability of MAbs to induce a growth delay, parasites were analyzed for mean fluorescence intensity (MFI) of GFP and EtBr at 30 and 40 h postinvasion (70).…”

“…MSP2 MAbs but not rabbit antibodies to MSP4 are internalized during merozoite invasion and maintained for approximately 24 h. It was previously reported that antibodies to MSP1-19 are able to be internalized during invasion along with the MSP1-19 protein (24,25,70). To assess whether this was also the case for the internalized proteins MSP2 and MSP4, MSP2 MAbs and MSP4 rabbit antibodies were incubated with merozoites and RBCs in invasion assays.…”