Studies on the humoral immune response to a synthetic vaccine against Plasmodium falciparum malaria

Progress in the Development of Subunit Vaccines against Malaria

Malaria is a life-threatening disease and one of the main causes of morbidity and mortality in the human population. The disease also results in a major socio-economic burden. The rapid spread of malaria epidemics in developing countries is exacerbated by the rise in drug-resistant parasites and insecticide-resistant mosquitoes. At present, malaria research is focused mainly on the development of drugs with increased therapeutic effects against Plasmodium parasites. However, a vaccine against the disease is preferable over treatment to achieve long-term control. Trials to develop a safe and effective immunization protocol for the control of malaria have been occurring for decades, and continue on today; still, no effective vaccines are available on the market. Recently, peptide-based vaccines have become an attractive alternative approach. These vaccines utilize short protein fragments to induce immune responses against malaria parasites. Peptide-based vaccines are safer than traditional vaccines, relatively inexpensive to produce, and can be composed of multiple T- and B-cell epitopes integrated into one antigenic formulation. Various combinations, based on antigen choice, peptide epitope modification and delivery mechanism, have resulted in numerous potential malaria vaccines candidates; these are presently being studied in both preclinical and clinical trials. This review describes the current landscape of peptide-based vaccines, and addresses obstacles and opportunities in the production of malaria vaccines.

Conserved Binding Regions Provide the Clue for Peptide-Based Vaccine Development: A Chemical Perspective

Synthetic peptides have become invaluable biomedical research and medicinal chemistry tools for studying functional roles, i.e., binding or proteolytic activity, naturally-occurring regions’ immunogenicity in proteins and developing therapeutic agents and vaccines. Synthetic peptides can mimic protein sites; their structure and function can be easily modulated by specific amino acid replacement. They have major advantages, i.e., they are cheap, easily-produced and chemically stable, lack infectious and secondary adverse reactions and can induce immune responses via T- and B-cell epitopes. Our group has previously shown that using synthetic peptides and adopting a functional approach has led to identifying Plasmodium falciparumconserved regions binding to host cells. Conserved high activity binding peptides’ (cHABPs) physicochemical, structural and immunological characteristics have been taken into account for properly modifying and converting them into highly immunogenic, protection-inducing peptides (mHABPs) in the experimental Aotus monkey model. This article describes stereo–electron and topochemical characteristics regarding major histocompatibility complex (MHC)-mHABP-T-cell receptor (TCR) complex formation. Some mHABPs in this complex inducing long-lasting, protective immunity have been named immune protection-inducing protein structures (IMPIPS), forming the subunit components in chemically synthesized vaccines. This manuscript summarizes this particular field and adds our recent findings concerning intramolecular interactions (H-bonds or π-interactions) enabling proper IMPIPS structure as well as the peripheral flanking residues (PFR) to stabilize the MHCII-IMPIPS-TCR interaction, aimed at inducing long-lasting, protective immunological memory.

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.

Diagnosis of parasitic diseases: old and new approaches

Methods for the diagnosis of infectious diseases have stagnated in the last 20–30 years. Few major advances in clinical diagnostic testing have been made since the introduction of PCR, although new technologies are being investigated. Many tests that form the backbone of the “modern” microbiology laboratory are based on very old and labour-intensive technologies such as microscopy for malaria. Pressing needs include more rapid tests without sacrificing sensitivity, value-added tests, and point-of-care tests for both high- and low-resource settings. In recent years, research has been focused on alternative methods to improve the diagnosis of parasitic diseases. These include immunoassays, molecular-based approaches, and proteomics using mass spectrometry platforms technology. This review summarizes the progress in new approaches in parasite diagnosis and discusses some of the merits and disadvantages of these tests.

Induction of mucosal and systemic humoral immune responses in murine system by intranasal immunization with peptide antigens of P. vivax and CpG oligodeoxynucleotide (ODN) in microparticle delivery

In the present study we have investigated the immunomodulatory effects of two adjuvants, CpG 1826 (two copies of CpG motifs) and CpG 2006 (three copies of CpG motifs) to the five peptide antigens of Plasmodium vivax derived from circumsporozoite protein (CSP), merozoite surface protein-1 (MSP1#1, MSP1#23), apical membrane antigen-1 (AMA-1) and gametocyte surface antigen (Pvs24) in alum and microparticle formulations, using intramuscular and intranasal routes of immunization. Alum formulation without CpG ODN generated low serum IgG and IgA antibody titers and the predominant IgG isotypes were IgG1 but with the addition of CpG ODN (1826 or 2006), the antibody titers were increased by four fold with the predominance of IgG2a/2b isotypes. The SIgA peak titers in lung and intestinal washes were significantly increased with the intranasal mode of administration. Specific activity measurement was done to calculate for the accurate amounts of total serum IgG, IgA and SIgA in washes and showed direct correlation between antibody titer and its concentration. High titer anti-Pvs24 antibodies have significant inhibitory effects on parasite development in the mosquito midgut when tested in membrane feeding assays. The immunofluorescence results show that the peptide specific antisera reacted with the air-dried parasite antigens isolated from P. vivax patients. The present study demonstrates that intranasal route of immunization appears to be an alternate mode of inducing protective immunity in P. vivax malaria.

Synthetic vaccine update: Applying lessons learned from recent SPf66 malarial vaccine physicochemical, structural and immunological characterization

The SPf66 synthetic malaria vaccine, developed and obtained almost 2 decades ago, represents the first approach towards developing a multi-antigenic, multi-stage synthetic malarial vaccine composed of subunits derived from different Plasmodium falciparum stage proteins. It is shown here that batches 03, 04, 05, 06, 07, 08, 09, 10, 11, 12, 13, 14, 15 and 16 produced from a few milligrams to kilogram amounts and used in assays on monkeys and humans showed high reproducibility in physicochemical analysis. (1)H NMR two-dimensional studies also revealed high similarity, even in non-oxidized batches. Reproducibility was also high, especially in preclinical studies carried out on Aotus, clinical trials Phase I, IIa and IIb and field-studies carried out in La Tola, Rio Rosario (Colombia), Majadas (Venezuela), La Te (Ecuador), Ifakara (Tanzania) in which there was high antibody titer production, having similar population distribution when done with different batches. These results provide great support for peptide-synthesized vaccines containing minimal epitopes from protection-inducing antigens which have several advantages, such as low cost, safety, reproducibility, stability, being straightforwardly scaled-up from milligram to kilogram amounts; make them the vaccines of choice for the future in a worldwide attempt to scourge diseases such as malaria.

Sharing of antigens between Plasmodium falciparum and Anopheles albimanus

The presence of common antigens between Plasmodium falciparum and Anopheles albimanus was demonstrated. Different groups of rabbits were immunized with: crude extract from female An. albimanus (EAaF), red blood cells infected with Plasmodium falciparum (EPfs), and the SPf66 synthetic malaria vaccine. The rabbit's polyclonal antibodies were evaluated by ELISA, Multiple Antigen Blot Assay (MABA), and immunoblotting. All extracts were immunogenic in rabbits according to these three techniques, when they were evaluated against the homologous antigens. Ten molecules were identified in female mosquitoes and also in P. falciparum antigens by the autologous sera. The electrophoretic pattern by SDS-PAGE was different for the three antigens evaluated. Cross-reactions between An. albimanus and P. falciparum were found by ELISA, MABA, and immunoblotting. Anti-P. falciparum and anti-SPf66 antibodies recognized ten and five components in the EAaF crude extract, respectively. Likewise, immune sera against female An. albimanus identified four molecules in the P. falciparum extract antigen. As far as we know, this is the first work that demonstrates shared antigens between anophelines and malaria parasites. This finding could be useful for diagnosis, vaccines, and the study of physiology of the immune response to malaria.

Potent, long lasting systemic antibody levels and mixed Th1/Th2 immune response after nasal immunization with malaria antigen loaded PLGA microparticles

The immunogenicity of the synthetic malaria vaccine SPf66 has been recently improved by the application of new adjuvants as QS-21 saponin or poly-D,L-lactide-co-glycolide (PLGA) polymers. The search for less invasive administration routes made us test the immunogenicity of SPf66-loaded microparticles by the nasal route in Balb/c mice. We report here that the intranasal administration of the adequate PLGA vaccine formulations greatly improves and maintains higher antibody levels compared to the conventional alum adjuvant and to the administration of the particles by other routes (subcutaneous, oral). Systemic immune responses were characterized as mixed Th1/Th2-type: IFN-gamma and IgG2a isotype were found as signs of Th1 activation, whilst IgE and IgG1 secretions indicate Th2 response. Since both types of response have been associated to protective immunity in malaria, we postulate that this new approach supposes an advantage over the traditional adjuvants and routes.

Immune response after oral administration of the encapsulated malaria synthetic peptide SPf66

The synthetic peptide SPf66 adsorbed on alum is one of the few Plasmodium falciparum vaccines which have been tested in field trials. We previously reported that subcutaneous administration of SPf66 loaded PLGA microparticles (MP) enhances the antibody response to this antigen compared to the conventional alum formulation. We now evaluate the suitability of polymeric formulations to obtain systemic immune responses by gastric intubation of Balb/c mice. Formulations composed of 1:1 mixtures of PLGA 50:50 and 75:25 (lactic:glycolic) microparticles were administered by the oral route, and when animals were boosted 3 weeks later significant systemic IgG antibody responses were elicited, comparable to alum triple shot and superior to the aqueous vaccine given by the oral route. The finding of IgG2a isotype for PLGA-vaccinated mice compared to the absent levels of this isotype for the alum-vaccinated group could be interpreted as a sign of Th1-like immune response and cellular immune response activation. Our results confirm that using the appropriate schedule the oral administration of PLGA particles is suitable to obtain systemic immune responses to the carried antigen.

Immunization of cattle with synthetic peptides derived from the Boophilus microplus gut protein (Bm86)

Three synthetic peptides (SBm4912, SBm7462 and SBm19733), derived from the Bm86 glycoprotein from Boophilus microplus gut, were constructed and used to immunize cattle from a tick-free area. The immunized animals received three subcutaneous doses of the peptides, with saponin as adjuvant, at 30-day intervals. The immune response was evaluated by IgG elicited against the peptides by the detection of anti-Bm86 specific antibodies in situ and by Western blotting analysis. After tick challenge, reduction in the number, weight and oviposition capacity of engorged females was observed in the tick population that had fed on immunized animals. The results pointed a high efficacy (81.05%) for the SBm7462 synthetic peptide in relation to the others (p<0.01), demonstrating the efficiency of the immune response elicited by synthetic peptides to control the cattle tick B. microplus.

Safety, tolerability and immunogenicity of new formulations of the Plasmodium falciparum malaria peptide vaccine SPf66 combined with the immunological adjuvant QS-21

SPf66 is a synthetic malaria peptide vaccine, which has been widely tested in combination with aluminium hydroxide (alum) as the adjuvant. Since this formulation is weakly immunogenic, we sought to improve its immunogenicity by using the saponin adjuvant QS-21. SPf66/QS-21 vaccines were evaluated for safety, tolerability and immunogenicity in healthy adults. The vaccines were found to be safe in 87/89 (97.8%) volunteers studied. However, two individuals developed severe vaccine allergy following the third dose of 1/3 SPf66/QS-21 formulations tested. Vaccine formulations containing QS-21 induced a 45- to over 200-fold increase in anti-SPf66 IgG titres over the alum formulation after the second and third doses, respectively. Anti-SPf66 antibody from some subjects reacted against asexual blood stage parasites, as demonstrated by immunofluorescence and immunoblotting. Antibody responses generated by the QS-21 formulations were of longer duration compared to those evoked by the alum formulation. While SPf66/alum has been found to induce only CD4+ T cell response, the QS-21 formulations exhibited the potential to also elicit SPf66-specific CD8+ responses. These observations demonstrate that the use of QS-21 can substantially enhance the immunogenicity of peptide vaccines, such as SPf66.

Remarkably high antibody levels and protection against P. falciparum malaria in Aotus monkeys after a single immunisation of SPf66 encapsulated in PLGA microspheres

Single dose immunisation is a major goal in vaccine design. The purpose of this study was the development of a single dose delivery system for the SPf66 malaria vaccine, based on this antigen's microencapsulation in PLGA microspheres by double emulsion method. Results indicate that a single immunisation in mice and monkeys with the SPf66 malaria vaccine, encapsulated in a mixture of two formulations of PLGA microspheres, induced a remarkably high and long-lasting immune response as assessed by ELISA and Western Blott. This immune response was associated with a good protective capacity in Aotus monkeys, after experimental challenge, indicating that antigen integrity lasted following the microencapsulation process. PLGA biodegradable microspheres thus serve as an effective delivery system for the design of a single dose immunisation vaccine, such as the SPf66 synthetic malaria vaccine.

Double dimer peptide constructs are immunogenic and protective against Plasmodium falciparum in the experimental Aotus monkey model

Multiple antigen peptide constructs (MAPs) have been used to obtain defined multimeric peptide molecules useful in the development of possible synthetic malaria vaccines. In this context, a method was developed, named double dimer constructs (DDCs), involving the direct synthesis of a dimeric peptide with a C-terminal cysteine. A tetrameric molecule was then obtained by oxidation of sulfhydryl groups. Dimer synthesis was optimized using a Fmoc/tBu strategy, dimers were purified by HPLC, oxidized with DMSO and characterized by HPLC and MALDI-TOF-MS. The tetramers or DDCs obtained by this method were used as immunogens in the search for a possible malaria vaccine. It was found that they were immunogenic in the experimental Aotus monkey model, and were able to induce protective immunity when challenged experimentally with a highly infective Plasmodium falciparum malaria strain.

Humoral immune responses during a malaria vaccine trial in Tanzanian infants

The development of a malaria vaccine is a priority for improved and sustained malaria control. Optimal use of a vaccine in Africa will only be achieved if it can be delivered through the Expanded Programme of Immunization (EPI). We have completed a trial of the peptide vaccine SPf66 in Tanzanian infants, given alongside the EPI vaccines. This paper describes the humoral responses to SPf66 and the EPI vaccines. A total of 1207 infants were recruited into a two-arm, double-blind, individually randomized placebo-controlled trial of SPf66. The objectives of the trial were to determine the safety, immunogenicity and efficacy of SPf66 and to assess interactions with EPI vaccines when three doses of SPf66 were delivered alongside the EPI vaccines. Cross-sectional surveys were carried out to asses seroconversion rates to the EPI vaccines and the antibody response to SPf66 (NANP)50 and Plasmodium falciparum lysates. Seroconversion rates to EPI vaccines were high and no statistically significant differences in prevalence or titres were found between SPf66 and placebo recipients. IgG antibodies against SPf66 (NANP)50 and whole P. falciparum lysate were present in high titres in mothers of recruited children at the time of birth. Vaccination with SPf66 stimulated a good anti-SPf66 IgG response which declined to preimmunization levels by 2 years of age and which was not associated with protection against clinical malaria. The vaccine induced no IgG antibodies against (NANP)50 or P. falciparum lysates. SPf66 stimulated a humoral immune response when given to very young infants and did not interfere with seroconversion to EPI vaccines. The response to SPf66 was qualitatively different from that seen in older children, in whom SPf66 has been shown to be moderately efficacious. This difference raises concerns about the difficulties of immunizing very young infants who need to be targeted by antimalarial vaccination programs.

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.

Model multiple antigenic and homopolymeric peptides from non-repetitive sequences of malaria merozoite proteins elicit biologically irrelevant antibodies

Three model peptides containing B-epitopes from conserved, non-repetitive regions of the merozoite surface antigens, MSA2 and MSA1, and the erythrocyte binding protein EBP of Plasmodium falciparum were synthesised. The peptides incorporated GPG spacers and C residues at the N and C termini, and were polymerised by oxidation to form cystine bridges. Multiple copies of essentially the same peptide sequences were also synthesised on a branching lysyl matrix to form a tetrameric multiple antigen peptide. Rabbits were immunised with the polymerised and multiple antigen peptides, in alum followed by Freund's adjuvant, and the antibody responses examined by IFA and ELISA. Reproducible antibody responses were obtained against the MSA1 and EBP but not MSA2 peptides. IgG antibody levels detected by ELISA after three injections of antigen in alum, increased significantly after further immunisation in Freund's adjuvant. IgG levels were largely maintained for at least 23 weeks after the final immunisation. IgM antibodies, generally detectable only after immunisation in Freund's adjuvant, were absent 23 weeks later. Antibody titres against the native protein on fixed parasites, assayed by IFA, were three to five orders of magnitude lower than the corresponding ELISA titres against the peptides. Antibody-dependent inhibition of P. falciparum growth in vitro could not be demonstrated with the immune rabbit sera. The MSA1 and EBP peptides elicited cross-reactive antibodies. The results suggest that the selected non-repetitive sequences are conformationally constrained in the native proteins and only a small proportion of the anti-peptide antibodies bind to the native proteins. The significance of the findings for the development of peptide vaccines and the use of peptides in immunoassays is discussed.

Humoral immune response to the anti-malaria vaccine SPf66 in the Colombian Atrato River region

The immunogenicity of anti-malaria synthetic vaccine SPf66 was tested in a region of the Colombian middle Atrato river. The specific serum antibodies against SPf66 were quantified in vaccinees and placebo injected controls for a two-years period post-immunization. The frequency of individuals showing seroconversion of anti-SPf66 antibodies three months after completion of the immunization schedule was higher in vaccinees than in controls (52.7% and 25.5%, respectively, p < 0.01). However, an over than four-fold increase of the specific anti-SPf66 antibody titers was observed only in 1.4% of vaccinees and 0.2% of the controls (p < 0.01). The anti-SPf66 antibody titers augmented in vaccinees from first dose application to three months after the third dose, continuously decreasing thereafter to reach below baseline values two years after completion of the immunization schedule. The results show that SPf66 has very low immunogenicity and induces a short term humoral immune response (six months).

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.

SPf66 malaria vaccine is safe and immunogenic in malaria naive adults in Thailand

In preparation for an efficacy trial of malaria vaccine SPf66 in Thailand, a series of overlapping Phase I trials were conducted of US-manufactured SPf66. Here, two clinical lots were evaluated for safety and immunogenicity in a combined open-label trial. Eleven healthy, malaria naive, 18-44 year-old Thai men and women received three doses by subcutaneous injection in alternate arms at 0, 1 and 6 months. Safety was assessed by monitoring local and systemic reactogenicity and laboratory parameters. Common side effects were mild erythema, induration and tenderness at the site of injection which resolved within 24-48 h. At third immunization, two volunteers developed acute bilateral reactions with induration, erythema and pruritus limited to the sites of the second and third immunizations. Eight of 11 volunteers sero-converted by ELISA, six of whom would be classified as high responders by Colombian standards. Eight of 11 volunteers developed a lymphoproliferative response to the SPf66 antigen. Side effects were more common and antibody and lymphoproliferative responses greatest, among the four female volunteers. This initial study of SPf66 malaria vaccine in Asia constitutes an essential link between the initial Phase I study in the US and subsequent field studies in a semi-immune population in a malaria endemic area of Thailand. This study further establishes comparability of US-manufactured SPf66 with that of Colombian provenance and substantiates the validity of the subsequent negative efficacy results of SPf66 in a field trial in Thailand.

Randomised double-blind placebo-controlled trial of SPf66 malaria vaccine in children in northwestern Thailand. Shoklo SPf66 Malaria Vaccine Trial Group

BACKGROUND

Previous efficacy trials of SPf66 malaria vaccine have produced conflicting results in different populations. We report a randomised double-blind trial of the SPf66 vaccine conducted in Karen children aged 2-15 living in a malarious region of northwestern Thailand. Recombinant hepatitis B vaccine was used as a comparator.

METHODS

The study had a power of 90% to detect an efficacy of 30%, defined as a reduction in the incidence of first cases of symptomatic falciparum malaria after three doses of vaccine. 1221 children received three immunisations and were eligible for the primary efficacy analysis. Intense active and passive case detection continued over 15 months of follow-up.

FINDINGS

The SPf66 vaccine was well tolerated, although 26 children had mild or moderately severe local or systemic allergic reactions, compared with none in the comparator group. The vaccine was immunogenic; after three doses, 73% of recipients had seroconverted. There were no deaths due to malaria during the study. During the 15-month period of evaluation there were 379 first cases of symptomatic falciparum malaria (195 in the SPf66 recipients, 184 in the comparator group); an SPf66 efficacy of -9% (95% CI -33 to 14, p = 0.41). No significant differences between the two study groups in parasite density or any other measure of malaria-related morbidity were detected.

INTERPRETATION

These findings are consistent with a recent study showing lack of efficacy of SPf66 among Gambian infants and differ from earlier positive reports from South America and evidence of borderline efficacy from Tanzania. We conclude that SPf66 does not protect against clinical falciparum malaria and that further efficacy trials are not warranted.

Malaria vaccine trials: the missing qualitative data

Recent population-based efficacy trials of the synthetic malaria vaccine SPf66 have shown restricted, if any, clinical protection against Plasmodium falciparum infection. Despite the well-established role of antibodies in effector responses against asexual blood-stage malaria parasites, the titres of anti-SPf66 IgG antibodies do not correlate with the ability of sera from vaccine recipients to inhibit parasite growth in vitro nor with partial clinical protection which could be detected in some trials. Qualitative or functional parameters of SP66-induced antibody responses, such as IgG subclass composition and affinity, may be more predictive of clinical protection against malaria than quantitative estimates of antibody concentration or titre. Since these parameters are readily estimated by laboratory techniques currently available, and may be modulated by changes in vaccination protocols and by the use of different adjuvants, a better understanding of qualitative antibody responses induced by SPf66 and other asexual blood-stage malaria vaccine candidates, and of their relationship with clinical protection in vivo, is urgently needed for the improvement of currently used immunization schedules.

Malaria vaccines

Significant progress has been made in the development of the malaria vaccine during the last 20 years. Ninety percent of the 300-500 million clinical cases of malaria per year worldwide occur in Africa. Thus, research must be directed toward the 1 million African children under 5 years of age who die every year of malaria. An asexual blood-stage vaccine, capable of reducing severe and complicated malaria and malaria-related mortality, is therefore an important public health tool in these countries. Although knowledge of the parasite's biology is incomplete, research has allowed insight into some of the mechanisms that the parasite uses to evade host immunity. This is the basis for adopting an "antigenic cocktail" approach toward obtaining a synthetic or recombinant subunit vaccine such as the synthetic Colombian Malaria vaccine SPf 66. During the development of Spf66, field trials under both low and high malaria endemicity areas in Latin America and Africa have been carried out. The results from these studies showed a protective efficacy ranging between 38.8 and 60.2% against Plasmodium falciparum malaria. Given the characteristics of the normal immune response to malaria (relatively short-lived and not completely effective), it is understandable that the main goal is to try to increase the host's natural immunity. The best candidates for designing a malaria vaccine are the proteins required for parasite survival, those with low mutation rates and conserved epitopes. Because these proteins play an important role in multiple or alternative steps during the invasion process, they should be the targets against which a protective immune response should be elicited. The interaction between the malaria parasite and its host is complex. It is therefore crucial to define new ways of improving the immune response-such as directly modifying the chemical structure of epitopes or using new adjuvants or DNA immunization techniques-to produce novel vaccines against this disease.

The effect of temperature on the stability of the synthetic SPf(66)n malaria vaccine

We hereby report on the effect of temperature on the physico-chemical and immunogenic characteristics and stability of the SPf(66)n synthetic malaria vaccine. The pattern of polymerization, the adsorption to aluminium hydroxide and the immunological properties of these vaccine groups were not significantly affected.

Assessment of the role of the humoral response to Plasmodium falciparum MSP2 compared to RESA and SPf66 in protecting Papua New Guinean children from clinical malaria

The prevalence and concentration of naturally acquired humoral response (IgG) to merozoite surface protein 2 (MSP2), RESA, SPf66 and crude schizont extract were measured in a population living in a malaria highly endemic area of Papua New Guinea. A prospective longitudinal study in 0.5-15 year old children was conducted for one year in order to examine the relationship between the humoral response to these antigens and subsequent susceptibility to clinical malaria using a series of clinical definitions. The prevalence and concentration of antibodies to all antigens increased with age. Such correlation with age was most marked for MSP2 recombinant proteins. When age and previous exposure were controlled for, only antibody levels to MSP2 recombinant proteins (3D7 and d3D7) and to RESA predicted a reduction in incidence rate of episodes of clinical malaria. Our results support the inclusion of the recombinant proteins of the 3D7 allelic family of merozoite surface antigen 2 and RESA into a subunit vaccine against malaria.

Field trials of an asexual blood stage malaria vaccine: studies of the synthetic peptide polymer SPf66 in Thailand and the analytic plan for a phase IIb efficacy study

Several years ago the Walter Reed Army Institute of Research (WRAIR) initiated an independent analysis of the candidate malaria blood stage vaccine SPf66. WRAIR contracted for the synthesis and formulation of SPf66 in United States Food and Drug Administration (FDA) inspected laboratories within the U.S., and in 1992, filed an Investigational New Drug (IND) application with the FDA. Preclinical studies indicated that the vaccine could be synthesized to meet its release specifications, and when adjuvanted with alum, was essentially equivalent to Colombian produced SPf66 in regards to immunogenicity in preclinical studies of rodents and primates, and in human volunteers in Phase I studies. The goal of these efforts was ultimately to conduct a Phase IIb field trial to determine the safety and efficacy of SPf66 produced under current Good Manufacturing Practices (cGMP). Such a trial is currently underway in a malaria endemic refugee camp along the Thai-Burmese border. Here we briefly describe the study and present the formal analytic plan that was submitted to regulatory authorities in the United States for analysis of the study results. We believe such independent confirmatory studies are an essential part of the vaccine development process and are required to provide important data regarding the safety and efficacy of candidate vaccines in diverse geographical regions, and as a means to assess their role in the context of broader malaria control programmes.

Antibody and clinical responses in volunteers to immunization with malaria peptide-diptheria toxoid conjugates

Twenty residue peptides from the 185-200-kD and 45-kD merozoite surface antigens of the malaria parasite Plasmodium falciparum were covalently linked to diphtheria toxoid as a carrier and used to immunize human volunteers with aluminium hydroxide as an adjuvant. Significant antibody levels were elicited by two boosting injections. The antibodies reacted with acetone-methanol fixed merozoite membranes in an immunofluorescence assay, but no inhibition of merozoite reinvasion could be detected in in vitro cultures containing the antibodies. Antibody levels against the immunizing peptides declined markedly within 77 days after the third injection. No hypersensitivity was observed against the peptides. However, the volunteers developed hypersensitivity against diphteria toxoid, and in particular a pronounced type III (Arthus) hypersensitivity after three injections with the toxoid. This effect might appear to limit the use of peptide-diphtheria toxoid conjugates for human immunization. Several biochemical, haematological and immunological tests done on the volunteers showed no other adverse effects from the immunizations.

Randomised trial of efficacy of SPf66 vaccine against Plasmodium falciparum malaria in children in southern Tanzania

Effective, safe antimalarial vaccines have proved elusive. The synthetic polypeptide SPf66 vaccine is based on preerythrocytic and asexual blood-stage proteins of Plasmodium falciparum. We report here a randomised double-blind placebo-controlled trial of the efficacy of the SPf66 vaccine against clinical P falciparum malaria in idete, southern Tanzania, an area of intense perennial malaria transmission. 586 children aged 1-5 years received three doses of vaccine (n = 274) or placebo (n = 312). The incidence and density of parasitaemia were assessed through repeated cross-sectional surveys on subgroups of children. Morbidity was monitored over a 1 year period through passive case detection in all children plus active case detection in a subgroup of 191. An episode of clinical malaria was defined as measured fever (> or = 37.5 degrees C) and parasite density > 20,000/microL. No severe side-effects were seen and the frequency of mild side-effects after the third dose was less than 6%. The vaccine was highly immunogenic and after three doses all vaccine recipients had detectable anti-SPf66 antibodies: the geometric mean index of response was 8.3 in the vaccine group and 0.7 in the placebo group. The incidence of parasitaemia was similar in both groups. 123 children had at least one episode of clinical malaria during the follow-up period after the third dose and annual incidence rates were 0.25 in the vaccine group and 0.35 in the placebo group. Estimated vaccine efficacy was 31% (95% confidence interval 0-52%; p = 0.046). After the third dose there were 6 deaths among the study cohort (1 vaccine, 5 placebo). This study confirms that SPf66 is safe, immunogenic and reduces the risk of clinical malaria among children exposed to intense P falciparum transmission.

Immunoglobulin G2a isotype may have a protective role in Plasmodium berghei NK65 infection in immunised mice

All CBA mice that had been immunised by means of four successive inoculations of Plasmodium berghei NK65, each inoculation being followed by chemotherapy, survived an intravenous challenge inoculation of parasite, with 4/12 mice developing patent parasitaemia that resolved within 2 weeks. In contrast, all non-immunised control mice died before the 10th day post-challenge. Examination of sera for antibodies revealed that the immunised mice, all of which survived the challenge, had significantly high anti-plasmodial whole IgG, IgG1, and IgG2a titres before the challenge. A 16-fold rise in IgG2a titre alone was recorded on the 5th day post-challenge, with a further boosting of the titre to 4096 being observed on day 10. In comparison, the titres of Ig isotypes in the non-immunised control mice that succumbed to the challenge remained below 4. Specific IgG subclasses, in particular IgG2a, could be involved in the humoral immune protection against this rodent parasite.

Characterization of SPf(66)n: a chimeric molecule used as a malaria vaccine

SPf66 is a chemically synthesized 45 amino acid peptide derived from fractions of four different proteins of Plasmodium falciparum (83, 55 and 35 kDa and CS, the circumsporozoite protein) that elicits a protective immune response against malaria. In this paper we show the characterization of the SPf(66)n in batch 9 to be used in a field trial in young children at Ifakara in Tanzania. The analysis of SPf(66)n indicates that it is highly soluble in water and that the amino acid composition and sequence corresponds to that designed for the synthesis of the polypeptide. The packed product has a molecular weight ranging from 10 to 25 kDa. It is pure, free of metallic contaminants, atoxic and stable at 4 degrees C. The antibodies raised against this product in rabbits recognize the individual antigenic determinants of the molecule and the native epitopes of merozoites.

Safety, immunogenicity and protective effect of the SPf66 malaria synthetic vaccine against Plasmodium falciparum infection in a randomized double-blind placebo-controlled field trial in an endemic area of Ecuador

A total of 537 subjects were randomized to receive either SPf66 malaria vaccine against Plasmodium falciparum or placebo in three doses (days 0, 30 and 180). Subjects completing the course of vaccination (230 in the vaccine and 238 in the placebo group) were followed up for a further 12 months. Case detection surveillance was implemented by parasitological cross-sectional surveys every 2 months and by monthly household visits to each participant. Symptomatic subjects were also diagnosed in a local health centre. Minor local side-effects were observed mainly after the second dose in about 19% of the vaccinated subjects and in 3.7% of the placebo group. Thirty days after the third dose the prevalence of anti-SPf66 antibodies was 57% in the vaccine and 8.8% in the placebo groups. The prevaccination prevalence of antibodies measured by indirect immunofluorescence assay increased with age and seemed to be inversely related to anti-SPf66 antibody production. Immune response to SPf66 was independent of age. Vaccine efficacy was calculated based on person-time of exposure. The protective effect considering any malaria episode was 66.8% (95% confidence interval = -2.7-89.3%) and considering only one episode per individual was 60.2% (95% confidence interval = -26-87.5%).

SPf66, a chemically synthesized subunit malaria vaccine, is safe and immunogenic in Tanzanians exposed to intense malaria transmission

As part of the first trial of the SPf66 malaria vaccine in Africa, three randomized double-blind placebo-controlled studies of SPf66 have been conducted in a highly endemic area of Tanzania. The objectives were to confirm that the product is immunogenic and safe in highly exposed individuals. Results from ten male adult expatriates indicated that the product used in Tanzania is at least as immunogenic as that used in Colombia. No major side-effects were observed in indigenous SPf66 recipients (18 adults, and 25 children aged 1-4 years). Anti-SPf66 antibody titres in all groups showed clear responses to three doses of the vaccine.

A trial of the synthetic malaria vaccine SPf66 in Tanzania: rationale and design

The development of a safe, affordable and effective malaria vaccine to form part of control schemes in malaria endemic countries is a priority for researchers and public health officials. SPf66 is the first malaria vaccine to have shown partial protection against natural challenge in a phase III trial carried out in a hypoendemic area of Colombia. This paper describes the rationale and design of the first field trial of SPf66 outside South America, and the first to be conducted in an area of high perennial transmission.

Vaccination with SPf66, a chemically synthesised vaccine, against Plasmodium falciparum malaria in Colombia

Preclinical and clinical studies have established the safety and immunogenicity of the chemically synthesised SPf66 malaria vaccine. The present study is a phase III randomised, double-blind, placebo-controlled, efficacy trial completed in La Tola, Colombia. 1548 volunteers over one year of age received three doses of either the vaccine (n = 738) or placebo (n = 810). Active and passive case detection methods were used to document clinical episodes of malaria among the study population. The follow-up period began one month after the third dose and lasted for one year. 168 and 297 episodes of Plasmodium falciparum malaria were documented in the SPf66 group and the placebo group, respectively; this corresponds to a crude protective efficacy of 38.8%. Incidence rates for first or only P falciparum malarial episodes were 22.3% per annum among the vaccinee group and 33.5% among the placebo group (RR = 1.5; 95% Cl 1.23, 1.84). Therefore, the protective efficacy of SPf66 against first or only episodes was 33.6% (95% Cl 18.8, 45.7), being highest in children aged 1-4 years (77%) and adults older than 45 years (67%). The estimated protective efficacy against second episodes was 50.5% (95% Cl 12.9-71.9). Our study shows that the chemically synthesised SPf66 malaria vaccine is safe, immunogenic, and protective against P falciparum malaria in semi-immune populations subject to natural challenge.

Malaria vaccines

The development of an effective malaria vaccine is a feasible goal. Most of the vaccines being developed today are subunit vaccines derived from selected parasite antigens or their immunologically active fragments. The precise characterization of protective immune responses against Plasmodium parasites remains a fundamental part of present research aimed at obtaining a malaria vaccine(s).

Determination of the immunization schedule for field trials with the synthetic malaria vaccine SPf 66

The synthetic malaria vaccine SPf 66 has been shown to be safe, immunogenic and effective in trials performed with controlled groups naturally and experimentally exposed to the disease. In order to continue the trials in open populations, it was necessary to standardize the vaccination characteristics. We have performed four field trials with soldier volunteers with the aim, among others, of defining the number of doses required, the intervals between applications, the protein concentration, and the adjuvant to be used. In these trials, the vaccinated individuals' immune responses were evaluated by assaying anti-SPf 66 antibody titres, in vitro growth inhibition of the P. falciparum parasite, and the vaccinees' capacity to recognize P. falciparum native proteins. From these results we conclude that the best vaccination schedule, for adults, is three doses administered subcutaneously on days 0, 30 and 180, each containing 2 mg of the synthetic polymerized petide SPf 66 adsorbed to alum hydroxide.

A specific T-cell receptor genotype preference in the immune response to a synthetic Plasmodium falciparum malaria vaccine

In recent studies with 63 and 122 volunteers vaccinated with the SPf 66 synthetic malaria vaccine, specific antibody patterns were classified as high or low responders. Using the Polymerase Chain Reaction (PCR), a specific and selective preference was shown for the V beta arrangement of the T-cell receptor in the high responder group involving the V beta-8 gene. The low responder group showed the rearrangement of a different set of genes, and a particular association with V beta-10.

In human malaria protective antibodies are directed mainly against the Lys-Glu ion pair within the Lys-Glu-Lys motif of the synthetic vaccine SPf 66

The KEK (Lysine-Glutamic acid-Lysine) motif is frequently found in the primary structure of certain malaria proteins involved in invasion, and plays an important role in the interaction of these proteins with the erythrocyte. This motif is contained in a peptide which forms part of the polymeric synthetic malaria vaccine SPf 66, currently undergoing extensive human trials. Analysis of the antibody titres against the subunit peptides that comprise this vaccine has shown that protection is associated with high titres to the KEK-containing peptide. In this paper we examine the fine recognition of this motif by polyclonal sera from protected vaccinated individuals, demonstrating the critical role played by the interacting ion pair formed between the amino terminal lysine (K) and glutamic acid (E), which act as contact residues for an important proportion of the antibody population directed against this vaccine. This ion pair in the KEK motif constitutes perhaps one of the most important malaria epitopes involved in protection, and could explain the mechanism through which protective immunity is acquired.

Study of the safety and immunogenicity of the synthetic malaria SPf66 vaccine in children aged 1-14 years

Safety and immunogenicity tests of the SPf66 malaria vaccine have been carried out on a population of children, aged 1 to 14 years, in the town of Tumaco, Colombia. Adverse reactions measured after each vaccination were local and minimal, and observed in only a small percentage of the vaccinated children. One year later, no delayed reaction was evident. The majority of the child population developed high antibody titres against SPf66 and the degree of response did not vary with age. These induced antibodies recognize the native parasite proteins, in particular the molecules from which the amino acid sequence of this vaccine was deduced. These studies demonstrate that the SPf66 vaccine is safe and highly immunogenic for use in children greater than 1 year old.

The first field trials of the chemically synthesized malaria vaccine SPf66: safety, immunogenicity and protectivity

This paper reports the results of the first field study performed to assess the safety, immunogenicity and protectivity of the synthetic malaria vaccine SPf66 directed against the asexual blood stages of Plasmodium falciparum. Clinical and laboratory tests were performed on all volunteers prior to and after each immunization, demonstrating that no detectable alteration was induced by the immunization process. The vaccines were grouped as high, intermediate or low responders according to their antibody titres directed against the SPf66 molecule. Two of the 185 (1.08%) SPf66-vaccinated and nine of the 214 (4.20%) placebo-vaccinated volunteers developed P. falciparum malaria. The efficacy of the vaccine was calculated as 82.3% against P. falciparum and 60.6% against Plasmodium vivax.

Genetic control of the immune response to a synthetic vaccine against Plasmodium falciparum

Two independent vaccination trials using a hybrid synthetic polypeptide containing epitopes from four proteins of Plasmodium falciparum were performed. In the first trial 63 and in the second 122 volunteers were vaccinated, using different immunization schedules. The analysis of the humoral response to the vaccine, measured by IgG antibody titres to the polypeptide showed a bimodal distribution in both cases suggesting genetic control of the immune response to this protein. There was a small group of low or non-responders and a large group of good responders. HLA phenotyping of the two groups disclosed an association of the low responders to HLA-DR4 antigens with chi-square P value of 0.00039 when compared with the good responders group. These findings provide evidence for the genetic control of the immune response to the synthetic vaccine by the association of this response with particular alleles of the HLA class II antigens; such findings may lead to an explanation of the mechanism involved in disease susceptibility and need to be used in the design of a totally effective vaccine.