Recombinant viral-vectored vaccines expressing Plasmodium chabaudi AS apical membrane antigen 1: mechanisms of vaccine-induced blood-stage protection

Protective Immunity Induced by Immunization with Baculovirus, Virus-like Particle, and Vaccinia Virus Expressing the AMA1 of Plasmodium berghei

Heterologous prime–boost immunization regimens using various vaccine platforms demonstrated promising results against infectious diseases. Here, mice were sequentially immunized with the recombinant baculovirus (rBV), virus-like particle (VLP), and recombinant vaccinia virus (rVV) vaccines expressing the Plasmodium berghei apical membrane antigen 1 (AMA1) for protective efficacy evaluation. The rBV_V_rVV heterologous immunization regimen elicited high levels of parasite-specific IgG, IgG2a, and IgG2b antibody responses in sera. Upon P. berghei challenge infection, proliferations of germinal center B cells in the inguinal lymph nodes, as well as blood CD4+ and CD8+ T cells were induced. More importantly, rBV_V_rVV immunization significantly diminished the parasitemia and prevented drastic bodyweight loss in mice post-challenge infection with P. berghei. Our findings revealed that immunization with rBV, VLP, and rVV expressing the AMA1 conferred protection against P. berghei infection, providing evidence for the potential implementation of this strategy.

Immunogenicity analysis of conserved fragments in Plasmodium ovale species merozoite surface protein 4

Background

There is an urgent need for an effective vaccine to control and eradicate malaria, one of the most serious global infectious diseases. Plasmodium merozoite surface protein 4 (MSP4) has been listed as a blood-stage subunit vaccine candidate for malaria. Infection with Plasmodium ovale species including P. ovale wallikeri and P. ovale curtisi, is also a source of malaria burden in tropical regions where it is sometimes mixed with other Plasmodium species. However, little is known about P. ovale MSP4.

Methods

The msp4 gene was amplified through polymerase chain reaction using genomic DNA extracted from blood samples of 46 patients infected with P. ovale spp. and amplified products were sequenced. Open reading frames predicted as immunogenic peptides consisting of 119 and 97 amino acids of P. ovale curtisi MSP4 (PocMSP4) and P. ovale wallikeri MSP4 (PowMSP4), respectively, were selected for protein expression. Recombinant proteins (rPoMSP4) were expressed in Escherichia coli, purified, analysed, and immunized in BALB/c mice. The specificity of anti-MSP4-immunoglobulin (Ig) G antibodies was evaluated by Western blot and enzyme-linked immunosorbent assays, and cellular immune responses were analysed via lymphocyte proliferation assays.

Results

Full peptide sequences of PocMSP4 and PowMSP4 were completely conserved in all clinical isolates, except in the epidermal growth factor-like domain at the carboxyl terminus where only one mutation was observed in one P. o. wallikeri isolate. Further, truncated PoMSP4 segments were successfully expressed and purified as ~ 32 kDa proteins. Importantly, high antibody responses with end-point titres ranging from 1:10,000 to 1:2,560,000 in all immunized mouse groups were observed, with high IgG avidity to PocMSP4 (80.5%) and PowMSP4 (92.3%). Furthermore, rPocMSP4 and rPowMSP4 cross-reacted with anti-PowMSP4-specific or anti-PocMSP4-specific antibodies. Additionally, anti-PoMSP4 IgG antibodies showed broad immuno-specificity in reacting against rPoMSP1 and rPoAMA1. Lastly, PocMSP4- and PowMSP4-immunized mice induced cellular immune responses with PocMSP4 (36%) and PowMSP4 cells (15.8%) during splenocyte proliferation assays.

Conclusion

Findings from this study suggest conservation in PoMSP4 protein sequences and high immunogenicity was observed in rPoMSP4. Furthermore, induction of immune responses in PocMSP4- and PowMSP4-immunized mice informed that both humoral and cellular immune responses play crucial roles for PoMSP4 in protection.

Infection-Induced Resistance to Experimental Cerebral Malaria Is Dependent Upon Secreted Antibody-Mediated Inhibition of Pathogenic CD8+ T Cell Responses

Cerebral malaria (CM) is one of the most severe complications of Plasmodium falciparum infection. There is evidence that repeated parasite exposure promotes resistance against CM. However, the immunological basis of this infection-induced resistance remains poorly understood. Here, utilizing the Plasmodium berghei ANKA (PbA) model of experimental cerebral malaria (ECM), we show that three rounds of infection and drug-cure protects against the development of ECM during a subsequent fourth (4X) infection. Exposure-induced resistance was associated with specific suppression of CD8+ T cell activation and CTL-related pathways, which corresponded with the development of heterogeneous atypical B cell populations as well as the gradual infection-induced generation and maintenance of high levels of anti-parasite IgG. Mechanistically, transfer of high-titer anti-parasite IgG did not protect 1X infected mice against ECM and depletion of atypical and regulatory B cells during 4X infection failed to abrogate infection-induced resistance to ECM. However, IgMi mice that were unable to produce secreted antibody, or undergo class switching, during the repeated rounds of infection failed to develop resistance against ECM. The failure of infection-induced protection in IgMi mice was associated with impaired development of atypical B cell populations and the inability to suppress pathogenic CD8+ T cell responses. Our results, therefore, suggest the importance of anti-parasite antibody responses, gradually acquired, and maintained through repeated Plasmodium infections, for modulating the B cell compartment and eventually suppressing memory CD8+ T cell reactivation to establish infection-induced resistance to ECM.

Reverse immunodynamics: a new method for identifying targets of protective immunity

Despite a dramatic increase in our ability to catalogue variation among pathogen genomes, we have made far fewer advances in using this information to identify targets of protective immunity. Epidemiological models predict that strong immune selection can cause antigenic variants to structure into genetically discordant sets of antigenic types (e.g. serotypes). A corollary of this theory is that targets of immunity may be identified by searching for non-overlapping associations of amino acids among co-circulating antigenic variants. We propose a novel population genetics methodology that combines such predictions with phylogenetic analyses to identify genetic loci (epitopes) under strong immune selection. We apply this concept to the AMA-1 protein of the malaria parasite Plasmodium falciparum and find evidence of epitopes among certain regions of low variability which could render them ideal vaccine candidates. The proposed method can be applied to a myriad of multi-strain pathogens for which vast amounts of genetic data has been collected in recent years.

Recombinant modified vaccinia virus Ankara-based malaria vaccines

A safe and effective malaria vaccine is a crucial part of the roadmap to malaria elimination/eradication by the year 2050. Viral-vectored vaccines based on adenoviruses and modified vaccinia virus Ankara (MVA) expressing malaria immunogens are currently being used in heterologous prime-boost regimes in clinical trials for induction of strong antigen-specific T-cell responses and high-titer antibodies. Recombinant MVA is a safe and well-tolerated attenuated vector that has consistently shown significant boosting potential. Advances have been made in large-scale MVA manufacture as high-yield producer cell lines and high-throughput purification processes have recently been developed. This review describes the use of MVA as malaria vaccine vector in both preclinical and clinical studies in the past 5 years.

A PfRH5-based vaccine is efficacious against heterologous strain blood-stage Plasmodium falciparum infection in aotus monkeys

Antigenic diversity has posed a critical barrier to vaccine development against the pathogenic blood-stage infection of the human malaria parasite Plasmodium falciparum. To date, only strain-specific protection has been reported by trials of such vaccines in nonhuman primates. We recently showed that P. falciparum reticulocyte binding protein homolog 5 (PfRH5), a merozoite adhesin required for erythrocyte invasion, is highly susceptible to vaccine-inducible strain-transcending parasite-neutralizing antibody. In vivo efficacy of PfRH5-based vaccines has not previously been evaluated. Here, we demonstrate that PfRH5-based vaccines can protect Aotus monkeys against a virulent vaccine-heterologous P. falciparum challenge and show that such protection can be achieved by a human-compatible vaccine formulation. Protection was associated with anti-PfRH5 antibody concentration and in vitro parasite-neutralizing activity, supporting the use of this in vitro assay to predict the in vivo efficacy of future vaccine candidates. These data suggest that PfRH5-based vaccines have potential to achieve strain-transcending efficacy in humans.

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Vaccines based on the P. falciparum merozoite antigen PfRH5 were tested in Aotus monkeys

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PfRH5-based vaccines afforded protection against heterologous strains of P. falciparum

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Protection correlated with anti-PfRH5 IgG concentration and in vivo neutralization

Recent developments in malaria vaccinology

The development of a highly effective malaria vaccine remains a key goal to aid in the control and eventual eradication of this devastating parasitic disease. The field has made huge strides in recent years, with the first-generation vaccine RTS,S showing modest efficacy in a Phase III clinical trial. The updated 2030 Malaria Vaccine Technology Roadmap calls for a second generation vaccine to achieve 75% efficacy over two years for both Plasmodium falciparum and Plasmodium vivax, and for a vaccine that can prevent malaria transmission. Whole-parasite immunisation approaches and combinations of pre-erythrocytic subunit vaccines are now reporting high-level efficacy, whilst exciting new approaches to the development of blood-stage and transmission-blocking vaccine subunit components are entering clinical development. The development of a highly effective multi-component multi-stage subunit vaccine now appears to be a realistic ambition. This review will cover these recent developments in malaria vaccinology.

Strategies for designing and monitoring malaria vaccines targeting diverse antigens

After more than 50 years of intensive research and development, only one malaria vaccine candidate, “RTS,S,” has progressed to Phase 3 clinical trials. Despite only partial efficacy, this candidate is now forecast to become the first licensed malaria vaccine. Hence, more efficacious second-generation malaria vaccines that can significantly reduce transmission are urgently needed. This review will focus on a major obstacle hindering development of effective malaria vaccines: parasite antigenic diversity. Despite extensive genetic diversity in leading candidate antigens, vaccines have been and continue to be formulated using recombinant antigens representing only one or two strains. These vaccine strains represent only a small fraction of the diversity circulating in natural parasite populations, leading to escape of non-vaccine strains and challenging investigators’ abilities to measure strain-specific efficacy in vaccine trials. Novel strategies are needed to overcome antigenic diversity in order for vaccine development to succeed. Many studies have now cataloged the global diversity of leading Plasmodium falciparum and Plasmodium vivax vaccine antigens. In this review, we describe how population genetic approaches can be applied to this rich data source to predict the alleles that best represent antigenic diversity, polymorphisms that contribute to it, and to identify key polymorphisms associated with antigenic escape. We also suggest an approach to summarize the known global diversity of a given antigen to predict antigenic diversity, how to select variants that best represent the strains circulating in natural parasite populations and how to investigate the strain-specific efficacy of vaccine trials. Use of these strategies in the design and monitoring of vaccine trials will not only shed light on the contribution of genetic diversity to the antigenic diversity of malaria, but will also maximize the potential of future malaria vaccine candidates.

Analysis of human B-cell responses following ChAd63-MVA MSP1 and AMA1 immunization and controlled malaria infection

Acquisition of non-sterilizing natural immunity to Plasmodium falciparum malaria has been shown in low transmission areas following multiple exposures. However, conflicting data from endemic areas suggest that the parasite may interfere with the induction of effective B-cell responses. To date, the impact of blood-stage parasite exposure on antigen-specific B cells has not been reported following controlled human malaria infection (CHMI). Here we analysed human B-cell responses in a series of Phase I/IIa clinical trials, which include CHMI, using candidate virus-vectored vaccines encoding two blood-stage antigens: merozoite surface protein 1 (MSP1) and apical membrane antigen 1 (AMA1). Previously vaccinated volunteers show boosting of pre-existing antigen-specific memory B-cell (mBC) responses following CHMI. In contrast, unvaccinated malaria-naive control volunteers developed an mBC response against MSP1 but not AMA1. Serum IgG correlated with the mBC response after booster vaccination but this relationship was less well maintained following CHMI. A significant reduction in peripheral MSP1-specific mBC was observed at the point of diagnosis of blood-stage infection. This was coincident with a reduction in peripheral blood B-cell subsets expressing CXCR3 and elevated serum levels of interferon-γ and CXCL9, suggesting migration away from the periphery. These CHMI data confirm that mBC and antibody responses can be induced and boosted by blood-stage parasite exposure, in support of epidemiological studies on low-level parasite exposure.

Immunization with apical membrane antigen 1 confers sterile infection-blocking immunity against Plasmodium sporozoite challenge in a rodent model

Apical membrane antigen 1 (AMA-1) is a leading blood-stage malaria vaccine candidate. Consistent with a key role in erythrocytic invasion, AMA-1-specific antibodies have been implicated in AMA-1-induced protective immunity. AMA-1 is also expressed in sporozoites and in mature liver schizonts where it may be a target of protective cell-mediated immunity. Here, we demonstrate for the first time that immunization with AMA-1 can induce sterile infection-blocking immunity against Plasmodium sporozoite challenge in 80% of immunized mice. Significantly higher levels of gamma interferon (IFN-γ)/interleukin-2 (IL-2)/tumor necrosis factor (TNF) multifunctional T cells were noted in immunized mice than in control mice. We also report the first identification of minimal CD8(+) and CD4(+) T cell epitopes on Plasmodium yoelii AMA-1. These data establish AMA-1 as a target of both preerythrocytic- and erythrocytic-stage protective immune responses and validate vaccine approaches designed to induce both cellular and humoral immunity.

Challenges of assessing the clinical efficacy of asexual blood-stage Plasmodium falciparum malaria vaccines

In the absence of any highly effective vaccine candidate against Plasmodium falciparum malaria, it remains imperative for the field to pursue all avenues that may lead to the successful development of such a formulation. The development of a subunit vaccine targeting the asexual blood-stage of Plasmodium falciparum malaria infection has proven particularly challenging with only limited success to date in clinical trials. However, only a fraction of potential blood-stage vaccine antigens have been evaluated as targets, and a number of new promising candidate antigen formulations and delivery platforms are approaching clinical development. It is therefore essential that reliable and sensitive methods of detecting, or ruling out, even modest efficacy of blood-stage vaccines in small clinical trials be established. In this article we evaluate the challenges facing blood-stage vaccine developers, assess the appropriateness and limitations of various in vivo approaches for efficacy assessment and suggest future directions for the field.

Utilizing poxviral vectored vaccines for antibody induction-progress and prospects

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Poxviral vectors are now regarded as robust tools for B cell and antibody induction.

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Antibody responses can be induced against the vector as well as a transgene.

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Increasing application is seen in heterologous prime–boost immunization regimes.

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Effective veterinary poxviral vaccine products are now licensed.

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Promising results of antibody induction are being reported in human clinical trials.

Over the last decade, poxviral vectors emerged as a mainstay approach for the induction of T cell-mediated immunity by vaccination, and their suitability for human use has led to widespread clinical testing of candidate vectors against infectious intracellular pathogens and cancer. In contrast, poxviruses have been widely perceived in the vaccine field as a poor choice of vector for the induction of humoral immunity. However, a growing body of data, from both animal models and recent clinical trials, now suggests that these vectors can be successfully utilized to prime and boost B cells and effective antibody responses. Significant progress has been made in the context of heterologous prime–boost immunization regimes, whereby poxviruses are able to boost responses primed by other vectors, leading to the induction of high-titre antigen-specific antibody responses. In other cases, poxviral vectors have been shown to stimulate humoral immunity against both themselves and encoded transgenes, in particular viral surface proteins such as influenza haemagglutinin. In the veterinary field, recombinant poxviral vectors have made a significant impact with numerous vectors licensed for use against a variety of animal viruses. On-going studies continue to explore the potential of poxviral vectors to modulate qualitative aspects of the humoral response, as well as their amenability to adjuvantation seeking to improve quantitative antibody immunogenicity. Nevertheless, the underlying mechanisms of B cell induction by recombinant poxviruses remain poorly defined, and further work is necessary to help guide the rational optimization of future poxviral vaccine candidates aiming to induce antibodies.

Assessment of immune interference, antagonism, and diversion following human immunization with biallelic blood-stage malaria viral-vectored vaccines and controlled malaria infection

Overcoming antigenic variation is one of the major challenges in the development of an effective vaccine against Plasmodium falciparum, a causative agent of human malaria. Inclusion of multiple Ag variants in subunit vaccine candidates is one strategy that has aimed to overcome this problem for the leading blood-stage malaria vaccine targets, that is, merozoite surface protein 1 (MSP1) and apical membrane Ag 1 (AMA1). However, previous studies, utilizing malaria Ags, have concluded that inclusion of multiple allelic variants, encoding altered peptide ligands, in such a vaccine may be detrimental to both the priming and in vivo restimulation of Ag-experienced T cells. In this study, we analyze the T cell responses to two alleles of MSP1 and AMA1 induced by vaccination of malaria-naive adult volunteers with bivalent viral-vectored vaccine candidates. We show a significant bias to the 3D7/MAD20 allele compared with the Wellcome allele for the 33 kDa region of MSP1, but not for the 19 kDa fragment or the AMA1 Ag. Although this bias could be caused by "immune interference" at priming, the data do not support a significant role for "immune antagonism" during memory T cell restimulation, despite observation of the latter at a minimal epitope level in vitro. A lack of class I HLA epitopes in the Wellcome allele that are recognized by vaccinated volunteers may in fact contribute to the observed bias. We also show that controlled infection with 3D7 strain P. falciparum parasites neither boosts existing 3D7-specific T cell responses nor appears to "immune divert" cellular responses toward the Wellcome allele.

The utility of Plasmodium berghei as a rodent model for anti-merozoite malaria vaccine assessment

Rodent malaria species Plasmodium yoelii and P. chabaudi have been widely used to validate vaccine approaches targeting blood-stage merozoite antigens. However, increasing data suggest the P. berghei rodent malaria may be able to circumvent vaccine-induced anti-merozoite responses. Here we confirm a failure to protect against P. berghei, despite successful antibody induction against leading merozoite antigens using protein-in-adjuvant or viral vectored vaccine delivery. No subunit vaccine approach showed efficacy in mice following immunization and challenge with the wild-type P. berghei strains ANKA or NK65, or against a chimeric parasite line encoding a merozoite antigen from P. falciparum. Protection was not improved in knockout mice lacking the inhibitory Fc receptor CD32b, nor against a Δsmac P. berghei parasite line with a non-sequestering phenotype. An improved understanding of the mechanisms responsible for protection, or failure of protection, against P. berghei merozoites could guide the development of an efficacious vaccine against P. falciparum.

ChAd63-MVA-vectored blood-stage malaria vaccines targeting MSP1 and AMA1: assessment of efficacy against mosquito bite challenge in humans

The induction of cellular immunity, in conjunction with antibodies, may be essential for vaccines to protect against blood-stage infection with the human malaria parasite Plasmodium falciparum. We have shown that prime-boost delivery of P. falciparum blood-stage antigens by chimpanzee adenovirus 63 (ChAd63) followed by the attenuated orthopoxvirus MVA is safe and immunogenic in healthy adults. Here, we report on vaccine efficacy against controlled human malaria infection delivered by mosquito bites. The blood-stage malaria vaccines were administered alone, or together (MSP1+AMA1), or with a pre-erythrocytic malaria vaccine candidate (MSP1+ME-TRAP). In this first human use of coadministered ChAd63-MVA regimes, we demonstrate immune interference whereby responses against merozoite surface protein 1 (MSP1) are dominant over apical membrane antigen 1 (AMA1) and ME-TRAP. We also show that induction of strong cellular immunity against MSP1 and AMA1 is safe, but does not impact on parasite growth rates in the blood. In a subset of vaccinated volunteers, a delay in time to diagnosis was observed and sterilizing protection was observed in one volunteer coimmunized with MSP1+AMA1-results consistent with vaccine-induced pre-erythrocytic, rather than blood-stage, immunity. These data call into question the utility of T cell-inducing blood-stage malaria vaccines and suggest that the focus should remain on high-titer antibody induction against susceptible antigen targets.