Clinical trials of malaria vaccines: progress and prospects

Malaria vaccine development using synthetic peptides as a technical platform

The review covers the development of synthetic peptides as vaccine candidates for Plasmodium falciparum- and Plasmodium vivax-induced malaria from its beginning up to date and the concomitant progress of solid phase peptide synthesis (SPPS) that enables the production of long peptides in a routine fashion. The review also stresses the development of other complementary tools and actions in order to achieve the long sought goal of an efficacious malaria vaccine.

Vaccination with a DNA vaccine based on human PSCA and HSP70 adjuvant enhances the antigen-specific CD8+ T-cell response and inhibits the PSCA+ tumors growth in mice

BACKGROUND

DNA vaccines have been shown to be an effective approach to induce antigen-specific cellular and humoral immunity. However, the lower immune intensity in clinical trials limits the application of DNA vaccine. Here we intend to develop a new DNA vaccine based on prostate stem-cell antigen (PSCA), which has been suggested as a potential target for prostate cancer therapy, and enhance the DNA vaccine potency with heat shock proteins (HSPs) as adjuvant.

METHODS

A series of DNA plasmids encoding human PSCA, human HSP70 and their conjugates was constructed and injected into male mice intramuscularly (i.m.). To evaluate the immune responses and therapeutic efficacy of these plasmids, major histocompatibility complex (MHC)-restricted PSCA and HSP70-specific epitopes were predicted and a mouse model with a human PSCA-expressing tumor was constructed.

RESULTS

The result showed that mice vaccinated with PSCA-HSP plasmids generated the strongest PSCA-specific CD8+ T-cell immune response, but the CD4+ TH1 and TH2 cell immune responses were similar with those vaccinated with other HSP-adjuvant PSCA plasmids or only PSCA DNA. The immunity of HSP70 was also observed and the mice i.m. injected with PSCA+ HSP mixed plasmids generated the lowest anti-HSP antibodies. Furthermore, these vaccinations inhibited the growth of PSCA-expressing tumors and prolonged mouse survival.

CONCLUSIONS

These observations emphasize and extend the potential of the human HSP70 gene as adjuvant for DNA vaccines, and the vaccine based on PSCA and HSP70 is of potential value for treating prostate cancer.

Long-lasting protective immune response to the 19-kilodalton carboxy-terminal fragment of Plasmodium yoelii merozoite surface protein 1 in mice

Merozoite surface protein 1 (MSP1) is the major protein on the surface of the plasmodial merozoite, and its carboxy terminus, the 19-kDa fragment (MSP1(19)), is highly conserved and effective in induction of a protective immune response against malaria parasite infection in mice and monkeys. However, the duration of the immune response has not been elucidated. As such, we immunized BALB/c mice with a standard four-dose injection of recombinant Plasmodium yoelii MSP1(19) formulated with Montanide ISA51 and CpG oligodeoxynucleotide (ODN) and monitored the MSP1(19)-specific antibody levels for up to 12 months. The antibody titers persisted constantly over the period of time without significant waning, in contrast to the antibody levels induced by immunization with Freund's adjuvant, where the antibody levels gradually declined to significantly lower levels 12 months after immunization. Investigation of immunoglobulin G (IgG) subclass longevity revealed that only the IgG1 antibody level (Th2 type-driven response) decreased significantly by 6 months, while the IgG2a antibody level (Th1 type-driven response) did not change over the 12 months after immunization, but the boosting effect was seen in the IgG1 antibody responses but not in the IgG2a antibody responses. After challenge infection, all immunized mice survived with negligibly patent parasitemia. These findings suggest that protective immune responses to MSP1(19) following immunization using oil-based Montanide ISA51 and CpG ODN as an adjuvant are very long-lasting and encourage clinical trials for malaria vaccine development.

Rhop-3 protein conservation among Plasmodium species and induced protection against lethal P. yoelii and P. berghei challenge

In the present study, Rhop-3 polymorphism among Plasmodium falciparum field and laboratory isolates and among rodent Plasmodium species was investigated and identified. The Rhop-3 gene was found in all Plasmodium species so far tested. The overall structure of the Rhop-3 protein was found conserved among P. falciparum, Plasmodium yoelii, and Plasmodium berghei. However, it was more conserved among rodent Plasmodium species than between P. falciparum and Plasmodium vivax. The most conserved regions of Rhop-3 are the second half of exon 6 (amino acid #548 to #665) and the beginning of exon 3 (amino acid #59 to #210). Recombinant C-terminal partial and full-length Rhop-3 proteins of P. yoelii and P. berghei were expressed in Escherichia coli and purified. Immunization-challenge experiments in mice using recombinant Rhop-3 proteins led to a delay in parasite development and protected mice from a homologous lethal challenge infection. In a group of eight outbred Carworth Farm White (CFW) mice immunized with P. yoelii C-terminal recombinant His-Y1412 protein, three mice (37.5%) were protected from a lethal P. yoelii challenge. In BALB/cJ mice one mouse (20%) survived the infection. Immunization of mice with P. berghei recombinant full-length Rhop-3 protein in BALB/cJ mice led to a 40% survival from lethal P. berghei challenge. CFW mice immunized with P. berghei recombinant full-length Rhop-3 protein showed a significant delay in parasite development with a heterologous P. yoelii challenge. The Rhop-3 protein is a promising candidate for an asexual stage malaria vaccine.

Vaccination with novel immunostimulatory adjuvants against blood-stage malaria in mice

An important aspect of malaria vaccine development is the identification of an appropriate adjuvant which is both capable of stimulating a protective immune response and safe for use by humans. Here, we investigated the feasibility of using novel immunostimulatory molecules as adjuvants combined with a crude antigen preparation and coadsorbed to aluminum hydroxide (alum) as a vaccine against blood-stage Plasmodium chabaudi AS malaria. Prior to challenge infection, immunization of genetically susceptible A/J mice with the combination of malaria antigen plus recombinant interleukin-12 (IL-12) in alum induced a Th1 immune response with production of high levels of gamma interferon (IFN-gamma) and diminished IL-4 levels by spleen cells stimulated in vitro with parasite antigen compared to mice immunized with antigen alone, antigen in alum, or antigen plus IL-12. Mice immunized with malaria antigen plus recombinant IL-12 in alum had high levels of total malaria-specific antibody and immunoglobulin G2a. Compared to unimmunized mice, immunization with antigen plus IL-12 in alum induced the highest level of protective immunity against challenge infection with P. chabaudi AS, which was evident as a significantly decreased peak parasitemia level and 100% survival. Protective immunity was dependent on CD4(+) T cells, IFN-gamma, and B cells and was long-lasting. Replacement of IL-12 as an adjuvant by synthetic oligodeoxynucleotides (ODN) containing CpG motifs induced a similar level of vaccine-induced protection against challenge infection with P. chabaudi AS. These results illustrate that it is possible to enhance the potency of a crude malaria antigen preparation delivered in alum by inclusion of immunostimulatory molecules, such as IL-12 or CpG-ODN.

Does infection with Human Immunodeficiency Virus affect the antibody responses to Plasmodium falciparum antigenic determinants in asymptomatic pregnant women?

OBJECTIVES

HIV-seropositive pregnant women are more susceptible to malaria than HIV-seronegative women. We assessed whether HIV infection alters maternal and cord plasma malarial antibody responses and the mother-to-infant transfer of malaria antibodies.

METHODS

We determined plasma levels of maternal and cord antibodies [Immunoglobulin (IgG)] to recombinant malarial proteins [merozoite surface protein 1 (MSP-1(19kD)), the erythrocyte binding antigen (EBA-175)], the synthetic peptides [MSP-2, MSP-3, rhoptry associated protein 1 (RAP-1), and the pre-erythrocytic stage, circumsporozoite protein (NANP)(5)] antigenic determinants of Plasmodium falciparum; and tetanus toxoid (TT) by ELISA among samples of 99 HIV-seropositive mothers, 69 of their infants, 102 HIV-seronegative mothers and 62 of their infants.

RESULTS

The prevalence of maternal antibodies to the malarial antigenic determinants ranged from 18% on MSP3 to 91% on EBA-175; in cord plasma it ranged from 13% to 91%, respectively. More than 97% of maternal and cord samples had antibodies to TT. In multivariate analysis, HIV infection was only associated with reduced antibodies to (NANP)(5) in maternal (P=0.001) and cord plasma (P=0.001); and reduced mother-to-infant antibody transfer to (NANP)(5) (P=0.012). This effect of HIV was independent of maternal age, gravidity and placental malaria. No consistent HIV-associated differences were observed for other antigenic determinants.

CONCLUSION

An effect of HIV infection was only observed on one malarial antigenic determinant, suggesting that the increased susceptibility to malaria among HIV-infected pregnant women may not be explained on the basis of their reduced antibody response to malaria antigens.

The importance of models of infection in the study of disease resistance

Models currently occupy the crucial first step in the research flow for the development of new drugs and vaccines. Some animal models are better at reflecting the host-pathogen interaction in humans than others; this depends on the pathogen and its host specificity. Data gathered from what are often poorly adapted models provide a mosaic of sometimes contradictory information, yet there is little incentive to better delineate the relevance of models or to exploit recent advances to develop improved ones. This review reports on three particularly intractable human pathogens - Mycobacterium, Plasmodium and Schistosoma - and reflects that the extent to which these model systems mimic infection and protection processes in humans might not be sufficiently well defined.

Malaria: current status of control, diagnosis, treatment, and a proposed agenda for research and development

Rolling back malaria is possible. Tools are available but they are not used. Several countries deploy, as their national malaria control treatment policy, drugs that are no longer effective. New and innovative methods of vector control, diagnosis, and treatment should be developed, and work towards development of new drugs and a vaccine should receive much greater support. But the pressing need, in the face of increasing global mortality and general lack of progress in malaria control, is research into the best methods of deploying and using existing approaches, particularly insecticide-treated mosquito nets, rapid methods of diagnosis, and artemisinin-based combination treatments. Evidence on these approaches should provide national governments and international donors with the cost-benefit information that would justify much-needed increases in global support for appropriate and effective malaria control.

Modulation of host responses to blood-stage malaria by interleukin-12: from therapy to adjuvant activity

This review focuses on the role of interleukin (IL)-12, a proinflammatory cytokine with pleiotropic effects as a potent immunoregulatory molecule and hematopoietic growth factor, in infection with Plasmodium parasites, the causative agents of malaria. IL-12 has been demonstrated to have profound effects on the immune response to blood-stage malaria, to induce protection, and to alleviate malarial anemia. In combination with an anti-malarial drug, IL-12 is effective in an established malaria infection. This cytokine also has potent immune effects as a malaria vaccine adjuvant. However, IL-12 can also mediate pathology during blood-stage malaria.

Protection against Plasmodium falciparum malaria in chimpanzees by immunization with the conserved pre-erythrocytic liver-stage antigen 3

In humans, sterile immunity against malaria can be consistently induced through exposure to the bites of thousands of irradiated infected mosquitoes. The same level of protection has yet to be achieved using subunit vaccines. Recent studies have indicated an essential function for intrahepatic parasites, the stage after the mosquito bite, and thus for antigens expressed during this stage. We report here the identification of liver-stage antigen 3, which is expressed both in the mosquito and liver-stage parasites. This Plasmodium falciparum 200-kilodalton protein is highly conserved, and showed promising antigenic and immunogenic properties. In chimpanzees (Pan troglodytes), the primates most closely related to humans and that share a similar susceptibility to P. falciparum liver-stage infection, immunization with LSA-3 induced protection against successive heterologous challenges with large numbers of P. falciparum sporozoites.

Safety and immunogenicity of a three-component blood-stage malaria vaccine in adults living in an endemic area of Papua New Guinea

A Phase I safety and immunogenicity study with a three-component blood-stage malaria vaccine was conducted in adult male subjects living in an endemic area of Papua New Guinea. The preparations were recombinant proteins which corresponded to parts of the two merozoite surface proteins of Plasmodium falciparum (MSP1 and 2), and of the ring-infected erythrocyte surface antigen (RESA). The three proteins were emulsified with the adjuvant Montanide ISA720. Ten subjects were injected twice (four weeks apart) with the vaccine formulation and two with the adjuvant alone. Mild pain at the site of injection was reported by about half of the subjects but no systemic reaction related to the formulation occurred. There was a sharp rise in geometric mean stimulation index after the second dose compared to baseline for MSP1 and RESA, while the rise was small for MSP2. Geometric mean antibody titres increased for MSP1 during the study, whereas they hardly changed for MSP2 and RESA. The vaccine formulation was safe when used in an already immune population. The vaccine induced good cellular responses, especially for MSP1 and RESA. Boosting of humoral responses was weak, probably because of high baseline antibody levels.

Sequence analysis of the Rhop-3 gene of Plasmodium yoelii

The 110 kDa/Rhop-3 rhoptry protein of Plasmodium falciparum is non-covalently associated with two other proteins, the 140 kDa Rhop-1 and the 130 kDa Rhop-2. cDNAs encoding Rhop-3 from Plasmodium yoelii were isolated using rhoptry-specific antisera from Plasmodium falciparum, P. yoelii, and Plasmodium chabaudi. The cDNAs encoded peptides with partial homology to the C-terminal region (residues 541-861) of P. falciparum Rhop-3. Core regions of homology to the P. falciparum gene will be useful in determining the biological role of Rhop-3 and its potential as a vaccine candidate for malaria.

Sequence diversity of the merozoite surface protein 1 of Plasmodium falciparum in clinical isolates from the Kilombero District, Tanzania

Merozoite surface protein 1 of Plasmodium falciparum (PfMSP-1) is regarded as a key candidate antigen for malaria vaccine development. It exhibits significant antigenic polymorphism and has been divided into 17 building blocks based on the analysis of sequence diversity. Differences in the antigenic composition of PfMSP-1 in local P. falciparum populations may result in differences in the efficacy of vaccines, which contain sequences of particular allelic variant(s) of PfMSP-1. To contribute to the required knowledge of genetic diversity of malaria parasites in geographically diverse regions, we have used the polymerase chain reaction (PCR) to analyze the sequence diversity of blocks 1-4 of PfMSP-1 in disease isolates from the Kilombero District in Tanzania. In the semi-conserved block 1, in which dimorphic amino acid variances have been described at three positions, we found three of the five previously described combinations of these three pairs of amino acids. In addition one combination was found, which has not been reported before in parasite isolates from different locations worldwide. Of the two sequence variants, which were dominating, one (S44-Q47-V52) corresponded to the 83.1 sequence incorporated into the SPf66 malaria peptide vaccine, while the other one (G44-H47-I52) differed from the previous in all three dimorphic amino acids. The partial protection observed in a phase III SPf66 trial conducted in the Kilombero District in children aged 1-5, thus does not seem to be associated with a clear dominance of favourable variants of block 1 of PfMSP-1 in this area. All three different principle types of block 2, the major polymorphic region of PfMSP-1, were found in the Tanzanian isolates. Most of the sequences contained K1-type tripeptide repeats, but clones with MAD20-type repeats or no repetitive sequence (RO33-type block 2) were also present. K1- and MAD20-type tripeptide repeat motifs were never mixed within one parasite clone. In one sequence a hexapeptide repeat was found at the end of block 2, which has not been reported before. Dimorphism in 13 of the 17 previously described variable positions of the semi-conserved block 3 and three of four recombination types of block 4 (K/K, M/K and M/M) were found among the Tanzanian isolates. Apart from previously described dimorphic amino acid positions, polymorphism was rare in the non-repeated building blocks. Selection and spreading of parasite variants, which contain amino acid exchanges at other than the dimorphic positions thus, is not a common event. Parasite isolates frequently harboured more than one PfMSP-1 allele. Three of the four heterogeneous isolates analysed contained two different general types of sequences. One isolate contained at least four distinct clones, demonstrating the high endemicity of malaria in the Kilombero District, which is a well-established site for malaria vaccine field trials.

Use of reconstituted influenza virus virosomes as an immunopotentiating delivery system for a peptide-based vaccine

Immunopotentiating reconstituted influenza virosomes (IRIV) were used as a delivery system for the synthetic peptide-based malaria vaccine SPf66. The reduced SPf66 peptide molecules containing terminal cysteine residues were covalently attached to phosphatidylethanolamine with the heterobifunctional crosslinker gamma-maleimidobutyric acid N-hydroxysuccinimide ester. The SPf66-phosphatidylethanolamine was incorporated into IRIV and BALB/c mice were immunized twice by intramuscular injection with peptide-loaded virosomes. Titres of elicited anti-SPf66 IgG were determined by ELISA. These titres were significantly higher and the required doses of antigen were lower, when mice had been preimmunized with a commercial whole virus influenza vaccine. After preimmunization with the influenza vaccine, SPf66-IRIV elicited far more consistently anti-SPf66 antibody responses than SPf(66)n adsorbed to alum. MoAb produced by four B cell hybridoma clones derived from a SPf66-IRIV-immunized mouse cross-reacted with Plasmodium falciparum blood stage parasites in immunofluorescence assays. All four MoAbs were specific for the merozoite surface protein-1 (MSP-1)-derived 83.1 portion of SPf66. Sequencing of their functionally rearranged kappa light chain variable region genes demonstrated that the four hybridomas were generated from clonally related splenic B cells. Biomolecular interaction analyses (BIA) together with these sequencing data provided evidence for the selection of somatically mutated affinity-matured B cells upon repeated immunization with SPf66-IRIV. The results indicate that IRIV are a suitable delivery system for synthetic peptide vaccines and thus have a great potential for the design of molecularly defined combined vaccines targeted against multiple antigens and development stages of one parasite, as well as against multiple pathogens.

Antibodies against Plasmodium falciparum vaccine candidates in infants in an area of intense and perennial transmission: relationships with clinical malaria and with entomological inoculation rates

Serum immunoglobulin (Ig)G1, IgG3 and total IgG were assessed by immunoabsorbent assay in 198 infants from a Tanzanian village highly endemic for Plasmodium falciparum. Antibodies were measured against epitopes of the circumsporozoite protein (the repetitive epitope (NANP)50 and a construct of the flanking regions (CS27IC)), the malaria vaccine SPf66, and two constructs of the merozoite surface protein-1 (MSP-1), a 19-kDa fragment from the C-terminal domain (MSP-119) and an N-terminal fragment spanning blocks 1-6 (H6-p190 M-1/6-H6). IgG1 and total IgG titres showed similar age profiles, all decreasing for the first 2 months of life. Anti-(NANP)50 titres remained very low throughout the first year of life, while anti-CS27IC antibody appeared to peak around 7 months of age. Only a slight tendency to increase with age was observed for levels of the other antibodies studied. IgG3 titres except for H6-p190(1/6), were very low initially and remained very low throughout the first year of life. Clinical malaria incidence at the village dispensary was analysed prospectively in relation to antibody. No IgG1 or total IgG titre showed protective effects, but low IgG3 against p190(1/6) appeared to be a risk factor in some age groups. Given the large number of antibodies tested, this single indication of possible protection could merely be chance. There were no strong associations between antibody titres and entomologically assessed sporozoite exposure suggesting that transmission-reducing interventions may have little effect on antibody levels in such children.

Therapy with a combination of low doses of interleukin 12 and chloroquine completely cures blood-stage malaria, prevents severe anemia, and induces immunity to reinfection

The immunoregulatory cytokine interleukin 12 (IL-12) induces host resistance against experimental malaria. In this study, we tested the feasibility of using IL-12 in combination with chloroquine (CQ) to rescue susceptible A/J mice from lethal blood-stage Plasmodium chabaudi AS infection. Combined treatment with low doses of CQ and IL-12 resulted in a >15-fold reduction in the parasite load and 100% survival of A/J mice with established infections. Compared to control mice, which succumbed to severe anemia, CQ-plus-IL-12-treated mice had significantly higher early- and late-stage erythroid-cell progenitors in the bone marrow and spleen, resulting in significantly higher hematocrits, erythrocyte counts, and percentages of reticulocytes. Production of parasite-specific gamma interferon (IFN-gamma) by splenocytes from these mice was upregulated >20-fold relative to controls in parallel with enhanced IFN-gamma mRNA expression. Further, enhanced responsiveness to IL-12 and increased downstream IFN-gamma production in CQ-plus-IL-12-treated mice was evident from increased mRNA expression for the beta1 and beta2 subunits of IL-12 receptor in the splenocytes. Moreover, this combined therapy induced higher levels of anti-malaria antibodies than did CQ alone as well as sterile immunity against reinfection. Because IL-12 can be used at low doses and is effective even in established infections, it may be feasible to use this immunochemotherapeutic approach in human malaria.

Immune responses to Plasmodium falciparum antigens during a malaria vaccine trial in Tanzanian children

Among Tanzanian children living in an area of intense and perennial malaria transmission, prevalence of naturally acquired IgG antibodies that recognize SPf66, NANP, p190 and a 19 kDa fragment of the merozoite surface protein-1 (MSP-1) is high and increases with age. This possibly reflects the high level of natural exposure of the children to P. falciparum. The prevalences of IgG antibodies that recognize the three putative merozoite derived sequences contained in the malaria vaccine SPf66 (83.1, 55.1 and 35.1) is low but also show some age dependence. Three doses of the SPf66 vaccine induce a strong IgG antibody response against both the SPf66 construct, NANP and the three individual peptides. Vaccination with SPf66 did not result in an increase of anti19 kDa fragment antibodies. This reflects the specificity of the humoral immune response induced by the SPf66 construct. Among vaccinated children, antibody titres against SPf66 decreased over time following the third dose. However, 18 months after the third dose, SPf66 recipients still had significantly higher IgG titres and stimulation indices of peripheral blood mononuclear cells (PBMC) than placebo recipients. Within the vaccine group, there is a trend for increasing anti-SPf66 IgG titre to be associated with decreasing risk of clinical malaria but this was not statistically significant. Results also show the difficulties of establishing whether antibody responses are related to protection in field trials in endemic areas.