Progress and prospects for blood-stage malaria vaccines

Evidence from a natural experiment that malaria parasitemia is pathogenic in retinopathy-negative cerebral malaria

Cerebral malaria (CM) can be classified as retinopathy-positive or retinopathy-negative, based on the presence or absence of characteristic retinal features. While malaria parasites are considered central to the pathogenesis of retinopathy-positive CM, their contribution to retinopathy-negative CM is largely unknown. One theory is that malaria parasites are innocent bystanders in retinopathy-negative CM and the etiology of the coma is entirely non-malarial. Because hospitals in malaria-endemic areas often lack diagnostic facilities to identify non-malarial causes of coma, it has not been possible to evaluate the contribution of malaria infection to retinopathy-negative CM. To overcome this barrier, we studied a natural experiment involving genetically inherited traits, and find evidence that malaria parasitemia does contribute to the pathogenesis of retinopathy-negative CM. A lower bound for the fraction of retinopathy-negative CM that would be prevented if malaria parasitemia were to be eliminated is estimated to be 0.93 (95% confidence interval: 0.68, 1).

Immunoscreening of Plasmodium falciparum proteins expressed in a wheat germ cell-free system reveals a novel malaria vaccine candidate

The number of malaria vaccine candidates in preclinical and clinical development is limited. To identify novel blood-stage malaria vaccine candidates, we constructed a library of 1,827P. falciparum proteins prepared using the wheat germ cell-free system (WGCFS). Also, a high-throughput AlphaScreen procedure was developed to measure antibody reactivity to the recombinant products. Purified IgGs from residents in malaria endemic areas have shown functional activity against blood-stage parasites as judged by an in vitro parasite Growth Inhibition Assay (GIA). Therefore, we evaluated the GIA activity of 51 plasma samples prepared from Malian adults living in a malaria endemic area against the WGCFS library. Using the AlphaScreen-based immunoreactivity measurements, antibody reactivity against 3 proteins was positively associated with GIA activity. Since anti-LSA3-C responses showed the strongest correlation with GIA activity, this protein was investigated further. Anti-LSA3-C-specific antibody purified from Malian adult plasmas showed GIA activity, and expression of LSA3 in blood-stage parasites was confirmed by western blotting. Taken together, we identified LSA3 as a novel blood-stage vaccine candidate, and we propose that this system will be useful for future vaccine candidate discovery.

Antibody profiles to wheat germ cell-free system synthesized Plasmodium falciparum proteins correlate with protection from symptomatic malaria in Uganda

The key targets of protective antibodies against Plasmodium falciparum remain largely unknown. In this study, we determined immunoreactivity to 1827 recombinant proteins derived from 1565 genes representing ∼30% of the entire P. falciparum genome, for identification of novel malaria vaccine candidates. The recombinant proteins were expressed by wheat germ cell-free system, a platform that can synthesize quality plasmodial proteins that elicit biologically active antibodies in animals. Sera were obtained from indigenous residents of a malaria endemic region in Northern Uganda who were enrolled at the start of a rainy season and prospectively monitored for symptomatic malaria episodes for a year. Immunoreactivity to sera was determined by AlphaScreen; a homogeneous high-throughput system that detects protein interactions. Our analysis revealed antibody responses to 128 proteins that significantly associated with protection from symptomatic malaria. From 128 proteins, 53 were down-selected as the most plausible targets of host protective immune response by virtue of having a predicted signal peptide and/or transmembrane domain(s), or confirmed localization on the parasite surface. The 53 proteins comprised of not only previously characterized vaccine candidates but also uncharacterized proteins. Proteins involved in erythrocyte invasion; RON4, RON2 and CLAG3.1 and pre-erythrocytic proteins; SIAP-2, TRAP and CelTOS, were recommended for prioritization for further evaluation as vaccine candidates. The findings clearly demonstrate that generation of the protein library using the wheat germ cell-free system coupled with high throughput immunoscreening with AlphaScreen offers new options for rational discovery and selection of potential malaria vaccine candidates.

Differential miRNA Expression in the Liver of Balb/c Mice Protected by Vaccination during Crisis of Plasmodium chabaudi Blood-Stage Malaria

MicroRNAs are increasingly recognized as epigenetic regulators for outcome of diverse infectious diseases and vaccination efficacy, but little information referring to this exists for malaria. This study investigates possible effects of both protective vaccination and P. chabaudi malaria on the miRNome of the liver as an effector against blood-stage malaria using miRNA microarrays and quantitative PCR. Plasmodium chabaudi blood-stage malaria takes a lethal outcome in female Balb/c mice, but a self-healing course after immunization with a non-infectious blood-stage vaccine. The liver robustly expresses 71 miRNA species at varying levels, among which 65 miRNA species respond to malaria evidenced as steadily increasing or decreasing expressions reaching highest or lowest levels toward the end of the crisis phase on day 11 p.i. in lethal malaria. Protective vaccination does not affect constitutive miRNA expression, but leads to significant (p < 0.05) changes in the expression of 41 miRNA species, however evidenced only during crisis. In vaccination-induced self-healing infections, 18 miRNA-species are up- and 14 miRNA-species are down-regulated by more than 50% during crisis in relation to non-vaccinated mice. Vaccination-induced self-healing and survival of otherwise lethal infections of P. chabaudi activate epigenetic miRNA-regulated remodeling processes in the liver manifesting themselves during crisis. Especially, liver regeneration is accelerated as suggested by upregulation of let-7a-5p, let-7b-5p, let-7c-5p, let-7d-5p, let-7f-5p, let-7g-5p, let-7i-5p, miR-26a, miR-122-5p, miR30a, miR27a, and mir-29a, whereas the up-regulated expression of miR-142-3p by more than 100% is compatible with the view of enhanced hepatic erythropoiesis, possibly at expense of megakaryopoiesis, during crisis of P. chabaudi blood-stage malaria.

Antibody titres and boosting after natural malaria infection in BK-SE36 vaccine responders during a follow-up study in Uganda

The malaria vaccine BK-SE36 is a recombinant protein (SE36) based on the Honduras 1 serine repeat antigen-5 of Plasmodium falciparum, adsorbed to aluminium hydroxide gel. The phase Ib trial in Uganda demonstrated the safety and immunogenicity of BK-SE36. Ancillary analysis in the follow-up study of 6–20 year-old volunteers suggest significant differences in time to first episodes of clinical malaria in vaccinees compared to placebo/control group. Here, we aimed to get further insights into the association of anti-SE36 antibody titres and natural P. falciparum infection. Children who received BK-SE36 and whose antibody titres against SE36 increased by ≥1.92-fold after vaccination were categorised as responders. Most responders did not have or only had a single episode of natural P. falciparum infection. Notably, responders who did not experience infection had relatively high anti-SE36 antibody titres post-second vaccination compared to those who were infected. The anti-SE36 antibody titres of the responders who experienced malaria were boosted after infection and they had lower risk of reinfection. These findings show that anti-SE36 antibody titres induced by BK-SE36 vaccination offered protection against malaria. The vaccine is now being evaluated in a phase Ib trial in children less than 5 years old.

Protective Vaccination against Blood-Stage Malaria of Plasmodium chabaudi: Differential Gene Expression in the Liver of Balb/c Mice toward the End of Crisis Phase

Protective vaccination induces self-healing of otherwise fatal blood-stage malaria of Plasmodium chabaudi in female Balb/c mice. To trace processes critically involved in self-healing, the liver, an effector against blood-stage malaria, is analyzed for possible changes of its transcriptome in vaccination-protected in comparison to non-protected mice toward the end of the crisis phase. Gene expression microarray analyses reveal that vaccination does not affect constitutive expression of mRNA and lincRNA. However, malaria induces significant (p < 0.01) differences in hepatic gene and lincRNA expression in vaccination-protected vs. non-vaccinated mice toward the end of crisis phase. In vaccination-protected mice, infections induce up-regulations of 276 genes and 40 lincRNAs and down-regulations of 200 genes and 43 lincRNAs, respectively, by >3-fold as compared to the corresponding constitutive expressions. Massive up-regulations, partly by >100-fold, are found for genes as RhD, Add2, Ank1, Ermap, and Slc4a, which encode proteins of erythrocytic surface membranes, and as Gata1 and Gfi1b, which encode transcription factors involved in erythrocytic development. Also, Cldn13 previously predicted to be expressed on erythroblast surfaces is up-regulated by >200-fold, though claudins are known as main constituents of tight junctions acting as paracellular barriers between epithelial cells. Other genes are up-regulated by <100- and >10-fold, which can be subgrouped in genes encoding proteins known to be involved in mitosis, in cell cycle regulation, and in DNA repair. Our data suggest that protective vaccination enables the liver to respond to P. chabaudi infections with accelerated regeneration and extramedullary erythropoiesis during crisis, which contributes to survival of otherwise lethal blood-stage malaria.