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For the development of blood-stage malaria vaccines, there is a clear need to establish in vitro measures of the antibody-mediated and the cell-mediated immune responses that correlate with protection. In this study, we focused on establishing correlates of antibody-mediated immunity induced by immunization with apical membrane antigen 1 (AMA1) and merozoite surface protein 1 42 (MSP1 42 ) subunit vaccines. To do so, we exploited the Plasmodium chabaudi rodent model, with which we can immunize animals with both protective and nonprotective vaccine formulations and allow the parasitemia in the challenged animals to peak. Vaccine formulations were varied with regard to the antigen dose, the antigen conformation, and the adjuvant used. Prechallenge antibody responses were evaluated by enzyme-linked immunosorbent assay and were tested for a correlation with protection against nonlethal P. chabaudi malaria, as measured by a reduction in the peak level of parasitemia. The analysis showed that neither the isotype profile nor the avidity of vaccine-induced antibodies correlated with protective efficacy. However, high titers of antibodies directed against conformationindependent epitopes were associated with poor vaccine performance and may limit the effectiveness of protective antibodies that recognize conformation-dependent epitopes. We were able to predict the efficacies of the P. chabaudi AMA1 (PcAMA1) Infection with the protozoan parasites Plasmodium falciparum and Plasmodium vivax causes 300 million to 500 million clinical episodes of malaria annually (21). With at least 40% of the world's population at risk for malaria, multiple strategies are being explored to reduce this global public health problem. Progress continues to be made in the development of malaria vaccines for potential use in areas where malaria is endemic (18,28). It is encouraging that in a recent trial, the rate of severe malaria was significantly reduced in young children in Mozambique immunized with RTS,S, a P. falciparum preerythrocytic-stage vaccine (1); and it was found that RTS,S was safe, immunogenic, and efficacious in infants (1 to 3 months of age) (3). For blood-stage vaccines, the testing of vaccine safety, immunogenicity, and efficacy in human subjects has moved forward for two candidate antigens, namely, apical membrane antigen 1 (AMA1) (15,31,42) and merozoite surface protein 1 (MSP1) (30,35,47,49,56).Both AMA1 and MSP1 are expressed on the surface of extracellular, invasive merozoites and are essential for bloodstage parasite growth (17,36,50). We do not fully understand the precise functions of AMA1 and MSP1 in this invasion process, but their roles do appear to be distinct and nonoverlapping. The basic strategy for AMA1-and MSP1-based vaccines is the induction of antibodies that neutralize the merozoites released upon schizont rupture. The mechanisms of action of such neutralizing antibodies may include the blocking of key receptor-ligand interactions, inhibition of the proteolytic processing steps required for the invasion of e...

BackgroundMicroarray studies using in vitro cultures of synchronized, blood-stage Plasmodium falciparum malaria parasites have revealed a ‘just-in-time’ cascade of gene expression with some indication that these transcriptional patterns remain stable even in the presence of external stressors. However, direct analysis of transcription in P. falciparum blood-stage parasites obtained from the blood of infected patients suggests that parasite gene expression may be modulated by factors present in the in vivo environment of the host. The aim of this study was to examine changes in gene expression of the rodent malaria parasite, Plasmodium yoelii 17X, while varying the in vivo setting of replication.MethodsUsing P. yoelii 17X parasites replicating in vivo, differential gene expression in parasites isolated from individual mice, from independent infections, during ascending, peak and descending parasitaemia and in the presence and absence of host antibody responses was examined using P. yoelii DNA microarrays. A genome-wide analysis to identify coordinated changes in groups of genes associated with specific biological pathways was a primary focus, although an analysis of the expression patterns of two multi-gene families in P. yoelii, the yir and pyst-a families, was also completed.ResultsAcross experimental conditions, transcription was surprisingly stable with little evidence for distinct transcriptional states or for consistent changes in specific pathways. Differential gene expression was greatest when comparing differences due to parasite load and/or host cell availability. However, the number of differentially expressed genes was generally low. Of genes that were differentially expressed, many involved biologically diverse pathways. There was little to no differential expression of members of the yir and pyst-a multigene families that encode polymorphic proteins associated with the membrane of infected erythrocytes. However, a relatively large number of these genes were expressed during blood-stage infection regardless of experimental condition.ConclusionsTaken together, these results indicate that 1) P. yoelii gene expression remains stable in the presence of a changing host environment, and 2) concurrent expression of a large number of the polymorphic yir and pyst-a genes, rather than differential expression in response to specific host factors, may in itself limit the effectiveness of host immune responses.