Objective: The antigen, falciparum malaria protein 1 (FMP1), represents the 42-kDa C-terminal fragment of merozoite surface protein-1 (MSP-1) of the 3D7 clone of P. falciparum. Formulated with AS02 (a proprietary Adjuvant System), it constitutes the FMP1/AS02 candidate malaria vaccine. We evaluated this vaccine's safety, immunogenicity, and efficacy in African children. Methods: A randomised, double-blind, Phase IIb, comparator-controlled trial.The trial was conducted in 13 field stations of one mile radii within Kombewa Division, Nyanza Province, Western Kenya, an area of holoendemic transmission of P. falciparum. We enrolled 400 children aged 12–47 months in general good health.Children were randomised in a 1:1 fashion to receive either FMP1/AS02 (50 µg) or Rabipur® rabies vaccine. Vaccinations were administered on a 0, 1, and 2 month schedule. The primary study endpoint was time to first clinical episode of P. falciparum malaria (temperature ≥37.5°C with asexual parasitaemia of ≥50,000 parasites/µL of blood) occurring between 14 days and six months after a third dose. Case detection was both active and passive. Safety and immunogenicity were evaluated for eight months after first immunisations; vaccine efficacy (VE) was measured over a six-month period following third vaccinations. Results: 374 of 400 children received all three doses and completed six months of follow-up. FMP1/AS02 had a good safety profile and was well-tolerated but more reactogenic than the comparator. Geometric mean anti-MSP-142 antibody concentrations increased from1.3 µg/mL to 27.3 µg/mL in the FMP1/AS02 recipients, but were unchanged in controls. 97 children in the FMP1/AS02 group and 98 controls had a primary endpoint episode. Overall VE was 5.1% (95% CI: −26% to +28%; p-value = 0.7). Conclusions: FMP1/AS02 is not a promising candidate for further development as a monovalent malaria vaccine. Future MSP-142 vaccine development should focus on other formulations and antigen constructs.

/pdf/blood-stage-malaria-vaccine-eliciting-high-antigen-specific-10elbfiytn.pdf

Blood stage malaria vaccine eliciting high antigen-specific antibody concentrations confers no protection to young children in Western Kenya.

Merozoite surface protein–1 (MSP-1) of the human malaria parasite Plasmodium falciparum undergoes at least two endoproteolytic cleavage events during merozoite maturation and release, and erythrocyte invasion. We have previously demonstrated that mAbs which inhibit erythrocyte invasion and are specific for epitopes within a membrane-proximal, COOH-terminal domain of MSP-1 (MSP-119) prevent the critical secondary processing step which occurs on the surface of the extracellular merozoite at around the time of erythrocyte invasion. Certain other anti–MSP-119 mAbs, which themselves inhibit neither erythrocyte invasion nor MSP-1 secondary processing, block the processing-inhibitory activity of the first group of antibodies and are termed blocking antibodies. We have now directly quantitated antibody-mediated inhibition of MSP-1 secondary processing and invasion, and the effects on this of blocking antibodies. We show that blocking antibodies function by competing with the binding of processing-inhibitory antibodies to their epitopes on the merozoite. Polyclonal rabbit antibodies specific for certain MSP-1 sequences outside of MSP-119 also act as blocking antibodies. Most significantly, affinity-purified, naturally acquired human antibodies specific for epitopes within the NH2-terminal 83-kD domain of MSP-1 very effectively block the processing-inhibitory activity of the anti-MSP-119 mAb 12.8. The presence of these blocking antibodies also completely abrogates the inhibitory effect of mAb 12.8 on erythrocyte invasion by the parasite in vitro. Blocking antibodies therefore (a) are part of the human response to malarial infection; (b) can be induced by MSP-1 structures unrelated to the MSP-119 target of processing-inhibitory antibodies; and (c) have the potential to abolish protection mediated by anti–MSP-119 antibodies. Our results suggest that an effective MSP-119–based falciparum malaria vaccine should aim to induce an antibody response that prevents MSP-1 processing on the merozoite surface.

/pdf/antibodies-that-inhibit-malaria-merozoite-surface-protein-1-323fx9pzw4.pdf

Antibodies that Inhibit Malaria Merozoite Surface Protein–1 Processing and Erythrocyte Invasion Are Blocked by Naturally Acquired Human Antibodies

A major challenge facing malaria vaccine development programs is identifying efficacious combinations of antigens. To date, merozoite surface protein 1 (MSP1) is regarded as the leading asexual vaccine candidate. Apical membrane antigen 1 (AMA1) has been identified as another leading candidate for an asexual malaria vaccine, but without any direct in vivo evidence that a recombinant form of Plasmodium falciparum AMA1 would have efficacy. We evaluated the efficacy of a form of P. falciparum AMA1, produced in Pichia pastoris, by vaccinating Aotus vociferans monkeys and then challenging them with P. falciparum parasites. Significant protection from this otherwise lethal challenge with P. falciparum was observed. Five of six animals had delayed patency; two of these remained subpatent for the course of the infection, and two controlled parasite growth at <0.75% of red blood cells parasitized. The protection induced by AMA1 was superior to that obtained with a form of MSP1 used in the same trial. The protection induced by a combination vaccine of AMA1 and MSP1 was not superior to the protection obtained with AMA1 alone, although the immunity generated appeared to operate against both vaccine components.

https://iai.asm.org/content/iai/70/12/6961.full.pdf

Vaccination of Monkeys with Recombinant Plasmodium falciparum Apical Membrane Antigen 1 Confers Protection against Blood-Stage Malaria

Synthetic oligodeoxynucleotides containing CpG motifs enhance immunogenicity of a peptide malaria vaccine in Aotus monkeys.

The C-terminal region of Plasmodium falciparum merozoite surface protein 1 (MSP-119) is at present a leading malaria vaccine candidate. Antibodies against the epidermal growth factor-like domains of MSP-119are associated with immunity to P. falciparum1,2,3,4 and active immunization with recombinant forms of the molecule protect against malaria challenge in various experimental systems5,6,7,8. These findings, with the knowledge that epidermal growth factor-like domains in other molecules have essential binding functions, indicate the importance of this protein in merozoite invasion of red blood cells. Despite extensive molecular epidemiological investigations, only limited sequence polymorphism has been identified in P. falciparum MSP-119 (refs. 9–11). This indicates its sequence is functionally constrained, and is used in support of the use of MSP-119 as a vaccine. Here, we have successfully complemented the function of most of P. falciparum MSP-119 with the corresponding but highly divergent sequence from the rodent parasite P. chabaudi. The results indicate that the role of MSP-119 in red blood cell invasion is conserved across distantly related Plasmodium species and show that the sequence of P. falciparum MSP-119 is not constrained by function.

Functional conservation of the malaria vaccine antigen MSP-119across distantly related Plasmodium species.

The immunogenicity and protective efficacy of baculovirus recombinant polypeptide based on the Plasmodium falciparum merozoite surface protein 1 (MSP-1) has been evaluated in Aotus lemurinus griseimembra monkeys. The MSP-1-based polypeptide, BVp42, corresponds to the 42-kDa C-terminal processing fragment of the precursor molecule. Immunization of Aotus monkeys with BVp42 in complete Freund's adjuvant resulted in high antibody titers against the immunogen as well as parasite MSP-1. Fine specificity studies indicated that major epitopes recognized by these antibodies localize to conserved determinants of the 19-kDa C-terminal fragment derived from cleavage of the 42-kDa processing fragment. Effective priming of MSP-1-specific T cells was also demonstrated in lymphocyte proliferation assays. All three Aotus monkeys immunized with BVp42 in complete Freund's adjuvant showed evidence of protection of protection against blood-stage challenge with P. falciparum. Two animals were completely protected, with only one parasite being detected in thick blood films on a single days after injection. The third animal had a modified course of infection, controlling its parasite infection to levels below detection by thick blood smears for an extended period in comparison with adjuvant control animals. All vaccinated, protected Aotus monkeys produced antibodies which inhibited in vitro parasite growth, indicating that this assay may be a useful correlate of protective immunity and that immunity induced by BVp42 immunization is mediated, at least in part, by a direct effect of antibodies against the MSP-1 C-terminal region. The high level of protection obtained in these studies supports further development of BVp42 as a candidate malaria vaccine.

A recombinant baculovirus 42-kilodalton C-terminal fragment of Plasmodium falciparum merozoite surface protein 1 protects Aotus monkeys against malaria.

An effective malaria vaccine must be capable of eliciting a protective immune response in individuals of diverse genetic makeup In this report, we describe the co-regulation of immune responsiveness to growth-inhibitory Plasmodium falciparum merozoite surface protein-1 (MSP-1) epitopes by MHC-linked immune response genes and by the adjuvant used in MSP-1 vaccine formulations When congenic mice differing in MHC haplotype were immunized with MSP-1 either in CFA or incorporated into a synthetic monophosphoryl lipid A (LA-15-PH)-liposome formulation, mice of different haplotypes produced anti-MSP-1 Abs capable of inhibiting P falciparum growth Mice of H-2b and H-2ja haplotypes produced Abs possessing high levels of inhibitory activity upon immunization with MSP-1 in LA-15-PH/liposomes whereas these haplotypes produced noninhibitory Abs when immunized with MSP-1 in CFA Conversely, H-2d haplotype mice produced inhibitory Abs when immunized with MSP-1 in CFA but not when immunized with MSP-1 in LA-15-PH/liposomes The LA-15-PH/liposome adjuvant was more effective than CFA in inducing growth-inhibitory Abs The level of parasite growth inhibition observed for a particular mouse strain correlated with Ab titers against conserved, C-terminal MSP-1 epitopes, which appear to be important targets for Ab-mediated inhibition in mice immunized with both adjuvant formulations Our results suggest that adjuvant formulation and MHC genes act in a reciprocal manner to control immune responsiveness to specific epitopes, and raise the possibility of manipulating genetically-controlled responsiveness to vaccine Ags by utilizing alternative adjuvants in vaccine formulations

C. M. Nikaido

Papers

A recombinant baculovirus 42-kilodalton C-terminal fragment of Plasmodium falciparum merozoite surface protein 1 protects Aotus monkeys against malaria.

Regulation of antibody specificity to Plasmodium falciparum merozoite surface protein-1 by adjuvant and MHC haplotype.