Morbidity and mortality due to Plasmodium falciparum and Plasmodium vivax, the two predominant human malaria parasites, result during the asexual development and replication of these protozoan parasites within erythrocytes (RBCs) (44). To reduce this burden on nearly half of the world's population, several malaria vaccine strategies are being pursued (28,40,53). Blood-stage vaccines are being developed to reduce parasite load and/or prevent life-threatening complications of malaria once parasites are replicating within RBCs. The single most feasible strategy for blood-stage malaria is to immunize with subunit vaccines that induce high titers of antibodies that neutralize extracellular merozoites and prevent the invasion of erythrocytes (2,25,31,40). The multiple receptor-ligand interactions and alternate redundant pathways involved in the merozoite invasion of RBCs combined with the polymorphism of vaccine candidate antigens present a challenge for vaccine design (2,25,26).P. falciparum merozoite surface protein 1 (MSP-1)

Immunization with Plasmodium yoelii merozoite surface protein-8 (PyMSP-8) has been shown to protect mice against lethal P. yoelii 17XL malaria. Here we demonstrate that PyMSP-8-specific antibodies preferentially suppress P. yoelii 17XL growth in mature erythrocytes compared to growth in reticulocytes and do not suppress the growth of nonlethal P. yoelii 17X, a parasite that primarily replicates in reticulocytes. The protection against normocyte-associated P. yoelii malaria parasites is mediated by antibodies that recognize conformational epitopes of PyMSP-8 that are nonpolymorphic. We examined changes in gene expression in reticulocyterestricted P. yoelii 17XL parasites that escaped neutralization by PyMSP-8-specific antibodies using P. yoelii DNA microarrays. Of interest, Pymsp-8 gene expression decreased, while the expression of msp-1, msp-7, and several rhoptry protein genes increased. Breakthrough parasites also exhibited increases in the expression of a subset of yir and Pyst-a genes that are predicted to encode polymorphic antigens expressed on the surface of infected erythrocytes. These data suggest that changes in the expression of parasite proteins expressed on the merozoite surface, as well as the surface of infected erythrocytes, may alter host cell tropism and contribute to the ability of malaria parasites to evade merozoite-specific, neutralizing antibodies.

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