Anti-apical-membrane-antigen-1 antibody is more effective than anti-42-kilodalton-merozoite-surface-protein-1 antibody in inhibiting plasmodium falciparum growth, as determined by the in vitro growth inhibition assay

T cell responses induced by adenoviral vectored vaccines can be adjuvanted by fusion of antigen to the oligomerization domain of C4b-binding protein

Viral vectored vaccines have been shown to induce both T cell and antibody responses in animals and humans. However, the induction of even higher level T cell responses may be crucial in achieving vaccine efficacy against difficult disease targets, especially in humans. Here we investigate the oligomerization domain of the α-chain of C4b-binding protein (C4 bp) as a candidate T cell "molecular adjuvant" when fused to malaria antigens expressed by human adenovirus serotype 5 (AdHu5) vectored vaccines in BALB/c mice. We demonstrate that i) C-terminal fusion of an oligomerization domain can enhance the quantity of antigen-specific CD4(+) and CD8(+) T cell responses induced in mice after only a single immunization of recombinant AdHu5, and that the T cells maintain similar functional cytokine profiles; ii) an adjuvant effect is observed for AdHu5 vectors expressing either the 42 kDa C-terminal domain of Plasmodium yoelii merozoite surface protein 1 (PyMSP1(42)) or the 83 kDa ectodomain of P. falciparum strain 3D7 apical membrane antigen 1 (PfAMA1), but not a candidate 128kDa P. falciparum MSP1 biallelic fusion antigen; iii) following two homologous immunizations of AdHu5 vaccines, antigen-specific T cell responses are further enhanced, however, in both BALB/c mice and New Zealand White rabbits no enhancement of functional antibody responses is observed; and iv) that the T cell adjuvant activity of C4 bp is not dependent on a functional Fc-receptor γ-chain in the host, but is associated with the oligomerization of small (<80 kDa) antigens expressed by recombinant AdHu5. The oligomerization domain of C4 bp can thus adjuvant T cell responses induced by AdHu5 vectors against selected antigens and its clinical utility as well as mechanism of action warrant further investigation.

Can growth inhibition assays (GIA) predict blood-stage malaria vaccine efficacy?

An effective vaccine against P. falciparum malaria remains a global health priority. Blood-stage vaccines are an important component of this effort, with some indications of recent progress. However only a fraction of potential blood-stage antigens have been tested, highlighting a critical need for efficient down-selection strategies. Functional in vitro assays such as the growth/invasion inhibition assays (GIA) are widely used, but it is unclear whether GIA activity correlates with protection or predicts vaccine efficacy. While preliminary data in controlled human malaria infection (CHMI) studies indicate a possible association between in vitro and in vivo parasite growth rates, there have been conflicting results of immunoepidemiology studies, where associations with exposure rather than protection have been observed. In addition, GIA-interfering antibodies in vaccinated individuals from endemic regions may limit assay sensitivity in heavily malaria-exposed populations. More work is needed to establish the utility of GIA for blood-stage vaccine development.

Phase Ia clinical evaluation of the safety and immunogenicity of the Plasmodium falciparum blood-stage antigen AMA1 in ChAd63 and MVA vaccine vectors

Background

Traditionally, vaccine development against the blood-stage of Plasmodium falciparum infection has focused on recombinant protein-adjuvant formulations in order to induce high-titer growth-inhibitory antibody responses. However, to date no such vaccine encoding a blood-stage antigen(s) alone has induced significant protective efficacy against erythrocytic-stage infection in a pre-specified primary endpoint of a Phase IIa/b clinical trial designed to assess vaccine efficacy. Cell-mediated responses, acting in conjunction with functional antibodies, may be necessary for immunity against blood-stage P. falciparum. The development of a vaccine that could induce both cell-mediated and humoral immune responses would enable important proof-of-concept efficacy studies to be undertaken to address this question.

Methodology

We conducted a Phase Ia, non-randomized clinical trial in 16 healthy, malaria-naïve adults of the chimpanzee adenovirus 63 (ChAd63) and modified vaccinia virus Ankara (MVA) replication-deficient viral vectored vaccines encoding two alleles (3D7 and FVO) of the P. falciparum blood-stage malaria antigen; apical membrane antigen 1 (AMA1). ChAd63-MVA AMA1 administered in a heterologous prime-boost regime was shown to be safe and immunogenic, inducing high-level T cell responses to both alleles 3D7 (median 2036 SFU/million PBMC) and FVO (median 1539 SFU/million PBMC), with a mixed CD4⁺/CD8⁺ phenotype, as well as substantial AMA1-specific serum IgG responses (medians of 49 µg/mL and 41 µg/mL for 3D7 and FVO AMA1 respectively) that demonstrated growth inhibitory activity in vitro.

Conclusions

ChAd63-MVA is a safe and highly immunogenic delivery platform for both alleles of the AMA1 antigen in humans which warrants further efficacy testing. ChAd63-MVA is a promising heterologous prime-boost vaccine strategy that could be applied to numerous other diseases where strong cellular and humoral immune responses are required for protection.

Trial Registration

ClinicalTrials.gov NCT01095055

A review of malaria vaccine clinical projects based on the WHO rainbow table

Development and Phase 3 testing of the most advanced malaria vaccine, RTS,S/AS01, indicates that malaria vaccine R&D is moving into a new phase. Field trials of several research malaria vaccines have also confirmed that it is possible to impact the host-parasite relationship through vaccine-induced immune responses to multiple antigenic targets using different platforms. Other approaches have been appropriately tested but turned out to be disappointing after clinical evaluation.

As the malaria community considers the potential role of a first-generation malaria vaccine in malaria control efforts, it is an apposite time to carefully document terminated and ongoing malaria vaccine research projects so that lessons learned can be applied to increase the chances of success for second-generation malaria vaccines over the next 10 years.

The most comprehensive resource of malaria vaccine projects is a spreadsheet compiled by WHO thanks to the input from funding agencies, sponsors and investigators worldwide. This spreadsheet, available from WHO's website, is known as "the rainbow table". By summarizing the published and some unpublished information available for each project on the rainbow table, the most comprehensive review of malaria vaccine projects to be published in the last several years is provided below.

The blood-stage malaria antigen PfRH5 is susceptible to vaccine-inducible cross-strain neutralizing antibody

Current vaccine strategies against the asexual blood stage of Plasmodium falciparum are mostly focused on well-studied merozoite antigens that induce immune responses after natural exposure, but have yet to induce robust protection in any clinical trial. Here we compare human-compatible viral-vectored vaccines targeting ten different blood-stage antigens. We show that the full-length P. falciparum reticulocyte-binding protein homologue 5 (PfRH5) is highly susceptible to cross-strain neutralizing vaccine-induced antibodies, out-performing all other antigens delivered by the same vaccine platform. We find that, despite being susceptible to antibody, PfRH5 is unlikely to be under substantial immune selection pressure; there is minimal acquisition of anti-PfRH5 IgG antibodies in malaria-exposed Kenyans. These data challenge the widespread beliefs that any merozoite antigen that is highly susceptible to immune attack would be subject to significant levels of antigenic polymorphism, and that erythrocyte invasion by P. falciparum is a degenerate process involving a series of parallel redundant pathways.

Juxtamembrane shedding of Plasmodium falciparum AMA1 is sequence independent and essential, and helps evade invasion-inhibitory antibodies

The malarial life cycle involves repeated rounds of intraerythrocytic replication interspersed by host cell rupture which releases merozoites that rapidly invade fresh erythrocytes. Apical membrane antigen-1 (AMA1) is a merozoite protein that plays a critical role in invasion. Antibodies against AMA1 prevent invasion and can protect against malaria in vivo, so AMA1 is of interest as a malaria vaccine candidate. AMA1 is efficiently shed from the invading parasite surface, predominantly through juxtamembrane cleavage by a membrane-bound protease called SUB2, but also by limited intramembrane cleavage. We have investigated the structural requirements for shedding of Plasmodium falciparum AMA1 (PfAMA1), and the consequences of its inhibition. Mutagenesis of the intramembrane cleavage site by targeted homologous recombination abolished intramembrane cleavage with no effect on parasite viability in vitro. Examination of PfSUB2-mediated shedding of episomally-expressed PfAMA1 revealed that the position of cleavage is determined primarily by its distance from the parasite membrane. Certain mutations at the PfSUB2 cleavage site block shedding, and parasites expressing these non-cleavable forms of PfAMA1 on a background of expression of the wild type gene invade and replicate normally in vitro. The non-cleavable PfAMA1 is also functional in invasion. However – in contrast to the intramembrane cleavage site - mutations that block PfSUB2-mediated shedding could not be stably introduced into the genomic pfama1 locus, indicating that some shedding of PfAMA1 by PfSUB2 is essential. Remarkably, parasites expressing shedding-resistant forms of PfAMA1 exhibit enhanced sensitivity to antibody-mediated inhibition of invasion. Drugs that inhibit PfSUB2 activity should block parasite replication and may also enhance the efficacy of vaccines based on AMA1 and other merozoite surface proteins.

The malaria parasite invades red blood cells. During invasion several parasite proteins, including a vaccine candidate called PfAMA1, are clipped from the parasite surface. Most of this clipping is performed by an enzyme called PfSUB2, but some also occurs through intramembrane cleavage. The function of this shedding is unknown. We have examined the requirements for shedding of PfAMA1, and the effects of mutations that block shedding. Mutations that block intramembrane cleavage have no effect on the parasite. We then show that PfSUB2 does not recognise a specific amino acid sequence in PfAMA1, but rather cleaves it at a position determined primarily by its distance from the parasite membrane. Certain mutations at the PfSUB2 cleavage site prevent shedding, and parasites expressing non-cleavable PfAMA1 along with unmodified PfAMA1 grow normally. However, these mutations cannot be introduced into the parasite's genome, showing that some shedding by PfSUB2 is essential for parasite survival. Parasites expressing shedding-resistant mutants of PfAMA1 show enhanced sensitivity to invasion-inhibitory antibodies, suggesting that shedding of surface proteins during invasion helps the parasite to evade potentially protective antibodies. Drugs that inhibit PfSUB2 may prevent disease and enhance the efficacy of vaccines based on PfAMA1.

Phase Ia clinical evaluation of the Plasmodium falciparum blood-stage antigen MSP1 in ChAd63 and MVA vaccine vectors

Efficacy trials of antibody-inducing protein-in-adjuvant vaccines targeting the blood-stage Plasmodium falciparum malaria parasite have so far shown disappointing results. The induction of cell-mediated responses in conjunction with antibody responses is thought to be one alternative strategy that could achieve protective efficacy in humans. Here, we prepared chimpanzee adenovirus 63 (ChAd63) and modified vaccinia virus Ankara (MVA) replication-deficient vectors encoding the well-studied P. falciparum blood-stage malaria antigen merozoite surface protein 1 (MSP1). A phase Ia clinical trial was conducted in healthy adults of a ChAd63-MVA MSP1 heterologous prime-boost immunization regime. The vaccine was safe and generally well tolerated. Fewer systemic adverse events (AEs) were observed following ChAd63 MSP1 than MVA MSP1 administration. Exceptionally strong T-cell responses were induced, and these displayed a mixed of CD4(+) and CD8(+) phenotype. Substantial MSP1-specific serum immunoglobulin G (IgG) antibody responses were also induced, which were capable of recognizing native parasite antigen, but these did not reach titers sufficient to neutralize P. falciparum parasites in vitro. This viral vectored vaccine regime is thus a leading approach for the induction of strong cellular and humoral immunogenicity against difficult disease targets in humans. Further studies are required to assess whether this strategy can achieve protective efficacy against blood-stage malaria infection.

Impact on malaria parasite multiplication rates in infected volunteers of the protein-in-adjuvant vaccine AMA1-C1/Alhydrogel+CPG 7909

Background

Inhibition of parasite growth is a major objective of blood-stage malaria vaccines. The in vitro assay of parasite growth inhibitory activity (GIA) is widely used as a surrogate marker for malaria vaccine efficacy in the down-selection of candidate blood-stage vaccines. Here we report the first study to examine the relationship between in vivo Plasmodium falciparum growth rates and in vitro GIA in humans experimentally infected with blood-stage malaria.

Methods

In this phase I/IIa open-label clinical trial five healthy malaria-naive volunteers were immunised with AMA1/C1-Alhydrogel+CPG 7909, and together with three unvaccinated controls were challenged by intravenous inoculation of P. falciparum infected erythrocytes.

Results

A significant correlation was observed between parasite multiplication rate in 48 hours (PMR) and both vaccine-induced growth-inhibitory activity (Pearson r = −0.93 [95% CI: −1.0, −0.27] P = 0.02) and AMA1 antibody titres in the vaccine group (Pearson r = −0.93 [95% CI: −0.99, −0.25] P = 0.02). However immunisation failed to reduce overall mean PMR in the vaccine group in comparison to the controls (vaccinee 16 fold [95% CI: 12, 22], control 17 fold [CI: 0, 65] P = 0.70). Therefore no impact on pre-patent period was observed (vaccine group median 8.5 days [range 7.5–9], control group median 9 days [range 7–9]).

Conclusions

Despite the first observation in human experimental malaria infection of a significant association between vaccine-induced in vitro growth inhibitory activity and in vivo parasite multiplication rate, this did not translate into any observable clinically relevant vaccine effect in this small group of volunteers.

Trial Registration

ClinicalTrials.gov [NCT00984763]

Transgene optimization, immunogenicity and in vitro efficacy of viral vectored vaccines expressing two alleles of Plasmodium falciparum AMA1

BACKGROUND

Apical membrane antigen 1 (AMA1) is a leading candidate vaccine antigen against blood-stage malaria, although to date numerous clinical trials using mainly protein-in-adjuvant vaccines have shown limited success. Here we describe the pre-clinical development and optimization of recombinant human and simian adenoviral (AdHu5 and ChAd63) and orthopoxviral (MVA) vectors encoding transgene inserts for Plasmodium falciparum AMA1 (PfAMA1).

METHODOLOGY/PRINCIPAL FINDINGS

AdHu5-MVA prime-boost vaccination in mice and rabbits using these vectors encoding the 3D7 allele of PfAMA1 induced cellular immune responses as well as high-titer antibodies that showed growth inhibitory activity (GIA) against the homologous but not heterologous parasite strains. In an effort to overcome the issues of PfAMA1 antigenic polymorphism and pre-existing immunity to AdHu5, a simian adenoviral (ChAd63) vector and MVA encoding two alleles of PfAMA1 were developed. This antigen, composed of the 3D7 and FVO alleles of PfAMA1 fused in tandem and with expression driven by a single promoter, was optimized for antigen secretion and transmembrane expression. These bi-allelic PfAMA1 vaccines, when administered to mice and rabbits, demonstrated comparable immunogenicity to the mono-allelic vaccines and purified serum IgG now showed GIA against the two divergent strains of P. falciparum encoded in the vaccine. CD8(+) and CD4(+) T cell responses against epitopes that were both common and unique to the two alleles of PfAMA1 were also measured in mice.

CONCLUSIONS/SIGNIFICANCE

Optimized transgene inserts encoding two divergent alleles of the same antigen can be successfully inserted into adeno- and pox-viral vaccine vectors. Adenovirus-MVA immunization leads to the induction of T cell responses common to both alleles, as well as functional antibody responses that are effective against both of the encoded strains of P. falciparum in vitro. These data support the further clinical development of these vaccine candidates in Phase I/IIa clinical trials.

Non-apical membrane antigen 1 (AMA1) IgGs from Malian children interfere with functional activity of AMA1 IgGs as judged by growth inhibition assay

BACKGROUND

Apical membrane antigen 1 (AMA1) is one of the best-studied blood-stage malaria vaccine candidates. When an AMA1 vaccine was tested in a malaria naïve population, it induced functionally active antibodies judged by Growth Inhibition Assay (GIA). However, the same vaccine failed to induce higher growth-inhibitory activity in adults living in a malaria endemic area. Vaccination did induce functionally active antibodies in malaria-exposed children with less than 20% inhibition in GIA at baseline, but not in children with more than that level of baseline inhibition.

METHODS

Total IgGs were purified from plasmas collected from the pediatric trial before and after immunization and pools of total IgGs were made. Another set of total IgGs was purified from U.S. adults immunized with AMA1 (US-total IgG). From these total IgGs, AMA1-specific and non-AMA1 IgGs were affinity purified and the functional activity of these IgGs was evaluated by GIA. Competition ELISA was performed with the U.S.-total IgG and non-AMA1 IgGs from malaria-exposed children.

RESULTS

AMA1-specific IgGs from malaria-exposed children and U.S. vaccinees showed similar growth-inhibitory activity at the same concentrations. When mixed with U.S.-total IgG, non-AMA1 IgGs from children showed an interference effect in GIA. Interestingly, the interference effect was higher with non-AMA1 IgGs from higher titer pools. The non-AMA1 IgGs did not compete with anti-AMA1 antibody in U.S.-total IgG in the competition ELISA.

CONCLUSION

Children living in a malaria endemic area have a fraction of IgGs that interferes with the biological activity of anti-AMA1 antibody as judged by GIA. While the mechanism of interference is not resolved in this study, these results suggest it is not caused by direct competition between non-AMA1 IgG and AMA1 protein. This study indicates that anti-malaria IgGs induced by natural exposure may interfere with the biological effect of antibody induced by an AMA1-based vaccine in the target population.

Serologic imprint of dengue virus in urban Haiti: characterization of humoral immunity to dengue in infants and young children

Dengue is endemic to Haiti but not recognized as an important illness in the autochthonous population. To evaluate the prevalence of antibodies to dengue virus (DENV), serum samples from infants and young children 7-36 months of age (n = 166) were assayed by plaque reduction neutralization assays to each DENV serotype. Dengue virus serotype 1 had infected 40% of this study population, followed by serotype 2 (12%), serotype 3 (11%), and serotype 4 (2%). Fifty-three percent of infants and young children less than 12 months of age had already experienced DENV infection, and the seroprevalence of antibody to DENV increased to 65% by 36 months. Heterotypic antibody responses were an important component of the total dengue immunity profile.

Immunological responses against Plasmodium falciparum Apical Membrane Antigen 1 vaccines vary depending on the population immunized

Clinical development of malaria vaccines progresses from trials in malaria naïve adults to malaria exposed adults followed by malaria exposed children. It is not well known whether immune responses in non-target populations are predictive of those in target populations, particularly in African children. Therefore humoral responses in three different populations (U.S. adults, Malian adults and Malian children) were compared in this study. They were immunized with 80 μg of Apical Membrane Antigen 1 (AMA1)/alhydrogel on days 0 and 28. Sera were collected on days 0 and 42; antibody levels were determined by ELISA and the functionality of antibodies was evaluated by Growth Inhibition Assay. After immunization, there was no significant difference in antibody levels between the Malian children and the Malian adults, but U.S. adults showed lower antibody levels. Vaccination did not significantly change growth-inhibitory activity in Malian adults, but inhibition increased significantly in both U.S. adults and Malian children. Vaccine-induced inhibitory activity was reversed by pre-incubation with AMA1 protein, but pre-existing infection-induced inhibition was not. This study shows that humoral responses elicited by the AMA1 vaccine varied depending on the population, most likely reflecting different levels of previous malaria exposure. Thus predicting immune responses from non-target populations is not desirable.

Malaria vaccine design: immunological considerations

The concept of a malaria vaccine has sparked great interest for decades; however, the challenge is proving to be a difficult one. Immune dysregulation by Plasmodium and the ability of the parasite to mutate critical epitopes in surface antigens have proved to be strong defense weapons. This has led to reconsideration of polyvalent and whole parasite strategies and ways to enhance cellular immunity to malaria that may be more likely to target conserved antigens and an expanded repertoire of antigens. These and other concepts will be discussed in this review.

Adenovectors induce functional antibodies capable of potent inhibition of blood stage malaria parasite growth

An effective malaria vaccine remains a global health priority. Recombinant adenoviruses are a promising vaccine platform, and Plasmodium falciparum apical membrane antigen 1 (AMA1) and merozoite surface protein 1-42 (MSP1(42)) are leading blood stage vaccine candidates. We evaluated the importance of surface antigen localization and glycosylation on the immunogenicity of adenovector delivered AMA1 and MSP1(42) and assessed the ability of these vaccines to induce functional antibody responses capable of inhibiting parasite growth in vitro. Adenovector delivery induced unprecedented levels of biologically active antibodies in rabbits as indicated by the parasite growth inhibition assay. These responses were as potent as published results using any other vaccine system, including recombinant protein in adjuvant. The cell surface associated and glycosylated forms of AMA1 and MSP1(42) elicited 99% and 60% inhibition of parasite growth, respectively. Antigens that were expressed at the cell surface and glycosylated were much better than intracellular antigens at inducing antibody responses. Good T cell responses were observed for all forms of AMA1 and MSP1(42). Antigen-specific antibody responses, but typically not T cell responses, were boosted by a second administration of adenovector. These data highlight the importance of rational vaccine design and support the advancement of adenovector delivery technology for a malaria vaccine.

Phase 1 trial of the Plasmodium falciparum blood stage vaccine MSP1(42)-C1/Alhydrogel with and without CPG 7909 in malaria naïve adults

BACKGROUND

Merozoite surface protein 1(42) (MSP1(42)) is a leading blood stage malaria vaccine candidate. In order to induce immune responses that cover the major antigenic polymorphisms, FVO and 3D7 recombinant proteins of MSP1(42) were mixed (MSP1(42)-C1). To improve the level of antibody response, MSP1(42)-C1 was formulated with Alhydrogel plus the novel adjuvant CPG 7909.

METHODS

A Phase 1 clinical trial was conducted in healthy malaria-naïve adults at the Center for Immunization Research in Washington, D.C., to evaluate the safety and immunogenicity of MSP1(42)-C1/Alhydrogel +/- CPG 7909. Sixty volunteers were enrolled in dose escalating cohorts and randomized to receive three vaccinations of either 40 or 160 microg protein adsorbed to Alhydrogel +/- 560 microg CPG 7909 at 0, 1 and 2 months.

RESULTS

Vaccinations were well tolerated, with only one related adverse event graded as severe (Grade 3 injection site erythema) and all other vaccine related adverse events graded as either mild or moderate. Local adverse events were more frequent and severe in the groups receiving CPG. The addition of CPG enhanced anti-MSP1(42) antibody responses following vaccination by up to 49-fold two weeks after second immunization and 8-fold two weeks after the third immunization when compared to MSP1(42)-C1/Alhydrogel alone (p<0.0001). After the third immunization, functionality of the antibody was tested by an in vitro growth inhibition assay. Inhibition was a function of antibody titer, with an average of 3% (range -2 to 10%) in the non CPG groups versus 14% (3 to 32%) in the CPG groups.

CONCLUSION/SIGNIFICANCE

The favorable safety profile and high antibody responses induced with MSP1(42)-C1/Alhydrogel + CPG 7909 are encouraging. MSP1(42)-C1/Alhydrogel is being combined with other blood stage antigens and will be taken forward in a formulation adjuvanted with CPG 7909.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT00320658.

In vitro growth-inhibitory activity and malaria risk in a cohort study in mali

Immunity to the asexual blood stage of Plasmodium falciparum is complex and likely involves several effector mechanisms. Antibodies are thought to play a critical role in malaria immunity, and a corresponding in vitro correlate of antibody-mediated immunity has long been sought to facilitate malaria vaccine development. The growth inhibition assay (GIA) measures the capacity of antibodies to limit red blood cell (RBC) invasion and/or growth of P. falciparum in vitro. In humans, naturally acquired and vaccine-induced P. falciparum-specific antibodies have growth-inhibitory activity, but it is unclear if growth-inhibitory activity correlates with protection from clinical disease. In a longitudinal study in Mali, purified IgGs, obtained from plasmas collected before the malaria season from 220 individuals aged 2 to 10 and 18 to 25 years, were assayed for growth-inhibitory activity. Malaria episodes were recorded by passive surveillance over the subsequent 6-month malaria season. Logistic regression showed that greater age (odds ratio [OR], 0.78; 95% confidence interval [95% CI], 0.63 to 0.95; P = 0.02) and growth-inhibitory activity (OR, 0.50; 95% CI, 0.30 to 0.85; P = 0.01) were significantly associated with decreased malaria risk in children. A growth-inhibitory activity level of 40% was determined to be the optimal cutoff for discriminating malaria-immune and susceptible individuals in this cohort, with a sensitivity of 97.0%, but a low specificity of 24.3%, which limited the assay's ability to accurately predict protective immunity and to serve as an in vitro correlate of antibody-mediated immunity. These data suggest that antibodies which block merozoite invasion of RBC and/or inhibit the intra-RBC growth of the parasite contribute to but are not sufficient for the acquisition of malaria immunity.